Accepted Manuscript Title: Improving the Performance of Oil Based Mud and Water Based Mud in a High Temperature Hole using Nanosilica Nanoparticles Author: Allan Katende Natalie V. Boyou Issham Ismail Derek Z. Chung Farad Sagala Norhafizuddin Hussein Muhamad S. Ismail PII: S0927-7757(19)30509-6 DOI: https://doi.org/doi:10.1016/j.colsurfa.2019.05.088 Reference: COLSUA 23527 To appear in: Colloids and Surfaces A: Physicochem. Eng. Aspects Received date: 28 March 2019 Revised date: 29 May 2019 Accepted date: 30 May 2019 Please cite this article as: Allan Katende, Natalie V. Boyou, Issham Ismail, Derek Z. Chung, Farad Sagala, Norhafizuddin Hussein, Muhamad S. Ismail, Improving the Performance of Oil Based Mud and Water Based Mud in a High Temperature Hole using Nanosilica Nanoparticles, (2019), https://doi.org/10.1016/j.colsurfa.2019.05.088 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. https://doi.org/doi:10.1016/j.colsurfa.2019.05.088 https://doi.org/10.1016/j.colsurfa.2019.05.088 Page 1 of 42 Acc ep te d M an us cr ip t Improving the Performance of Oil Based Mud and Water Based Mud in a High Temperature Hole using Nanosilica Nanoparticles Allan Katendea,c,e,∗, Natalie V. Boyoub, Issham Ismailb, Derek Z. Chungb, Farad Sagalad, Norhafizuddin Husseinb, Muhamad S. Ismailb aDepartment of Energy, Minerals and Petroleum Engineering. Mbarara University of Science and Technology(MUST), Uganda bDepartment of Petroleum Engineering, Faculty of Chemical and Energy Engineering, Universiti Teknologi Malaysia. cDepartment of Mechanical and Industrial Engineering. Mbarara University of Science and Technology(MUST), Uganda dDepartment of Chemical and Petroleum Engineering, University of Calgary(UC), Canada eDepartment of Geoscience and Petroleum, Norwegian University of Science and Technology(NTNU), Norway Abstract Oil-based mud (OBM), a non-Newtonian fluid, is known for its superior performance in drilling complex wells as well as combating potential drilling complications. However, the good performance may degrade under certain circumstances especially because of the impact of chemical instability atan elevated temperature. The same phenomenon occurs for water- based mud (WBM) when it is used in drilling under high temperature conditions. To prevent this degradation from occurring, numerous studies on utilizing nanoparticles to formulate smart fluids for drilling operations are being conducted worldwide. Hence, this study aims to evaluate the performance of nanosilica (NS) as a fluid loss reducer and a rheological property improver in both OBM and WBM systems at high temperature conditions. This study focuses on the impacts of different nanosilica concentrations, varying from 0.5 ppb to 1.5 ppb,and different mud weights of 9 ppg and 12 pg as well as different aging temperatures, ranging from ambient temperature to 300 ◦F, on the rheological performance of OBM and WBM. All the rheological properties are measured at ambient temperature, and additionally tests, including lubricity, electrical stability, and high-pressure high-temperature filtration measurements, are conducted, and rheological models are obtained. Theperfor- mance of nanosilica is then studied by comparing each of the nanosilica-enhanced mud systems with the corresponding basic mud system, taking the fluid loss and rheological propertiesas the benchmark parameters. Nanosilica shows a positive impact on OBM and WBM, as the presence of nanosilica in the mud systemscan effectively improve almost all their rheological properties. Keywords: Nanosilica, Oil-Based Mud, Water-Based Mud, High PressureHigh Temperature, Degradation, Rheological properties Contents 1 Introduction 1 2 Methodology 2 2.1 Dispersion of Nanoparticles. . . . . . . . . . . . . 2 2.2 Mud Formulation. . . . . . . . . . . . . . . . . . . 3 2.3 Rheological Property Test. . . . . . . . . . . . . . . 4 2.4 Rheological Model . . . . . . . . . . . . . . . . . . 4 2.5 Lubricity Test . . . . . . . . . . . . . . . . . . . . . 4 2.6 Electrical Stability(ES) Test . . . . . . . . . . . . . 4 2.7 Transmission Electron Microscop(TEM) and Zeta potential characterization of Nanosilica. . . . . . . 4 2.8 Field Emission Scanning Electron Micro- scope(FESEM) analysis. . . . . . . . . . . . . . . . 5 3 Results and Discussions 5 3.1 Rheological Properties of OBM Samples. . . . . . . 5 3.2 Rheological Properties of WBM Samples. . . . . . 18 4 Conclusions 31 Nomenclature 31 Acknowledgements 31 5 Appendix A 31 Bibliography 31 1.Introduction D RILLING fluid, more commonly known as drilling mud, is often used during drilling of subterranean wells. It aids drilling operations by cooling the drill bits and lubricat- ing the process. This role makes the mixture fluid as vital to the development of petroleum resources as blood is to the hu- man body [1; 2; 3] because almost 25% of expenditures for oilfield exploration are spent on drilling [4; 5; 6; 7; 8], and drilling without drilling fluid is barely practical. Drilling fluid has an extensive history in the oil and gas industry, which can be dated back to the third millennium [9; 10]. Based on the history of drilling, water was the first drilling fluid utilized by the French almost two centuries ago [11], and today, water is ∗Corresponding author Email address: allan_katende@hotmail.com, akatende@must.ac.ug, allank@alumni.ntnu.no (Allan Katende) Received: March 28, 2019; Revised: May 29, 2019 Colloids and Surfaces A: Physicochemical and Engineering Aspects. Page 2 of 42 Acc ep te d M an us cr ip t COLLOIDS AND SURFACES A: PHYSICOCHEMICAL AND ENGINEERING ASPECTS 2 still an important component [3; 6] primarily used in drilling fluid formulation. Generally, several types of drilling fluids are currently widely used, namely, oil-based drilling fluid, synthetic-based drilling fluid and water-based drilling fluid [3; 12]. Because water-based drilling fluids are less expensive and more envi- ronmentally friendly [3; 13; 14], they are always preferred in drilling operations. However, under more complicated drilling conditions with the presence of shale in the formation, the drilling mud must maintain a high pressure and be able to tolerant a high temperature; thus, oil-based drilling fluids are often desired due to their superior drilling performance [15]. With the challenges and in the era of low oil prices, pre- cise selection of the drilling fluid type and properties is es- sential to optimizing the drilling time and expenditure. Im- proved formulation and engineering design of drilling mud is constantly under development to improve water-based mud (WBM) for application in environmentally sensitive areas [3; 12; 15; 16] and to prevent OBM from degradation, which will compromise its superior performance, due to the temperature effect. In recent years, nanotechnology has aroused attention in the oil industry due to its vast applicability[17; 18; 19; 20; 21; 22; 23] and is being used in formulating drilling fluid, which is more often known as a nano-mud, to combat stand- ing challenges and enhance the drilling performance [1; 24; 25; 26; 27; 28]. A nano-mud can be simply defined as any drilling fluid whose composition includes at least one type of mud additive in the nanoparticle size range of 1-100 nanome- ters [15; 29]. Due to the extremely tiny particle size, which results in an exceptionally high surface area to volume ra- tio [30], nanoparticles are often stronger and more reactive than non-nanoparticles [31; 32; 33; 34]. These special characteristics of nanoparticles make them very unique, sensitive and chemically as well as physically reactive agents. Due to the tiny size of nanoparticles, they can act as great bridging agents, as they can effectively plug nanosized pores and prevent fluid loss especially to the shale formation, through which wellbore instability can be eventu- ally prevented [35; 36]. In addition, nanoparticles that have high sensitivity and reactivity with bentonite particles can im- prove and stabilize the rheological properties of drillingflu- ids, which will lead to better hole cleaning and cuttings sus- pension [15; 25; 37; 38]. Because of the superior performance of nanoparticles, they are broadly applied in drilling fluid formulations, such as in WBM in order to enhance the environmentally and cost friendly drilling fluid to combat problems such as shale in- hibition, fluid loss [39], lubricity and thermal stability [40], and in oil-based mud (OBM) to enhance its thermal stabil- ity and preserve or even enhance its properties at an elevated temperature. Scholars have been trying to formulate enhanced WBM and OBM by using numerous special additives, includ- ing nanoparticles, to formulate a commercially inexpensive and environmentally suitable WBM and a more thermally sta- ble and rheologically better OBM [15; 41; 42; 43]. Therefore, considering the potential superior properties that nanoparti- cles can offer, nanosilica, a type of hydrophilic nanoparticle, is used in both WBM and OBM to study the effect of nanosil- ica on the rheological properties of both muds. Drilling problems are the potential complications that arise during a drilling operation of a complicated well, which may be a deviated well or one in which a formation with the presence of shale layers or rock layers with different hard- nesses has to be drilled through, which may result in a stuck pipe, shale swelling and well bore instability. Unsurprisingly, the root of all drilling problems is inseparable from the in- capability of the current conventional drilling fluids usedto fulfill particular functional tasks that are very vital in today’s challenging environments involving both drilling and produc- tion [31; 32; 44; 45; 46; 47]. Although OBM is often preferred for its superior perfor- mance in drilling a complicated well, as it can easily com- bat these drilling problems [48]; the macroscopic and micro- scopic types of additives are still very likely insufficient, as their properties can be altered under extreme temperature or pressure conditions either physically or chemically [31; 32]. This issue has led the industry to switch to more attractive alternatives [12; 49]: the application of WBM and OBM en- hanced by nanotechnology. Thus, in this study, nanosilica is used as an enhancing nanosized additive to design water- based and oil-based nano-muds in order to achieve their func- tionalities from the commencement of the drilling process to its cessation. 2.Methodology Start Formulating drilling !uid Dispersion and addition of NS with di"erent concentration Determination of ESV, rheological properties, Lubricity, Rheology Model Aging at di"erent temperature (ambient -300 0F) Determination of ESV, rheological properties, HPHT, Filtrate, Lubricity, Rheology ModelResults and analysis End Aging Mud weight NO YES 9 ppg 12 ppg Figure 1: Experimental methodology. The methodologies in this study are based solely on labo- ratory experimental work. The rheological property tests are conducted according to the recommended practice of API RP 13B-1 [50] for examining a drilling fluid. The experimental plan is as shown in Figure1 2.1.Dispersion of Nanoparticles Any nanoparticles to be used as an additive of a drilling fluid need to be thoroughly dispersed prior to adding them to the drilling fluid [51; 52] so that they work optimally. Riley et al. (2012)[28] assert that the dispersion of nanoparticles has Page 3 of 42 Acc ep te d M an us cr ip t COLLOIDS AND SURFACES A: PHYSICOCHEMICAL AND ENGINEERING ASPECTS 3 to be stabilized to prevent the particles from agglomerating and precipitating. Nanosilica (NS) is dispersed under a high pH condition so that anionic charge is acquired on the native silica. Charging the nanosilica’s surfaces, as shown in Figure2, enables it to be self-repulsive, which leads to a good dispersion of nanosilica particles [53; 54; 55]. The working condition is then main- tained at a pH above 9 to ensure that the nanosilica dispersion is stabilized. Shear plane Electric double layer Si-O+ Si-O- Si-O- Si-O+ Particle surface OH- anions Positive ions (eg. Na+) Figure 2: Charging of the nanosilica surface to enable self- repulsion. 2.2.Mud Formulation 2.2.1.WBM Formulation The WBM in this study was prepared according to the Recommended Practice for Field Testing Water-Based Drilling Fluid (API RP 13B-1, 2009[50]). The practice clearly states that 350 cc of WBM is equivalent to a labo- ratory barrel, and 15 lb/bbl of bentonite is used per laboratory barrel. The standard WBM formulations for mud weights of 9 ppg and 12 ppg are tabulated in Tables1 and 2. Table 1: Standard water-based mud formulation for 9 ppg mud weight Additives Function Composition Fresh water Base fluid 332cc Soda ash Treat water for con- tamination 0.25cc Bentonite Viscosifier 15cc PAC-HV Fluid loss control agent 0.2cc Hydroxypropyl Starch(HS) Fluid loss control agent 1.5g Caustic soda pH controller 0.25g Barite Weighting agent 27.99g Table 2: Standard water-based mud formulation for 12 ppg mud weight Additives Function Composition Fresh water Base fluid 293cc Soda ash Treat water for con- tamination 0.25cc Bentonite Viscosifier 15cc PAC-HV Fluid loss control agent 0.2cc HS Fluid loss control agent 1.5g Caustic soda pH controller 0.25g Barite Weighting agent 193.43g 2.2.2.WBM Formulation with Nanosilica In this study, rheological property, low-pressure low-temperature (LPLT) filtration, high-pressure high- temperature (HPHT) filtration and lubricity tests are per- formed, and rheological models are obtained. The perfor- mance of WBM with nanosilica is compared with that of its basic mud. The procedure for formulating the nano-WBM is as follows: 1. Nanosilica of respective concentrations (0.5, 1.0, and 1.5 ppb) are dispersed into 100 cc of distilled wa- ter with 0.25 g of caustic soda mixture (alkaline so- lution). Transmission Electron Microscope(TEM) im- ages of these mixtures are portrayed in Figure3. Re- sults showed that they are well dispersed, and this is proven with zeta potential values in Table 7, as the ab- solute values for all three nanosilica concentration in the alkaline solution are well over 30 mV. 2. Calculated amount of water – 100 cc of water is weighed using an electronic balance and then poured into a mixer cup; in this study, 193 cc (293 cc – 100 cc) of water is used. 3. The water is stirred using a multi-mixer while 0.25 g of soda ash is added into the water. Two minutes are counted from the beginning of the stirring. 4. At the end of the two minutes, 15 g of bentonite is added, and the solution is then stirred for five minutes. 5. A total of 0.2 g of high viscosity polyanionic cellulose (PAC-HV) is added, and the solution is stirred for three minutes. 6. Next, 1.5 g of hydroxypropyl starch is added, and the mixture is stirred for three minutes. 7. The nanosilica solution is added into the mixture and then stirred for 10 minutes. 8. The stirring is continued until, at the 35th minute, the calculated amount of barite is added in order to formu- late a mud sample of 9 or 12 ppg; the mixture is then stirred throughout the remaining time. The entire mix- ing process of the mud takes 45 minutes. 2.2.3.OBM Formulation Standardized OBM with an oil-to-water ratio of 80:20 is formulated for all tests throughout the study. The composi- Page 4 of 42 Acc ep te d M an us cr ip t COLLOIDS AND SURFACES A: PHYSICOCHEMICAL AND ENGINEERING ASPECTS 4 tions of the OBM are illustrated in Table3 and Table4. Table 3: Standard oil-based mud formulation for 9 ppg mud weight Additives Function Composition Sarapar 147 Base oil 182.09cc Confimul P Primary emulsifier 5cc Confimul S Secondary emulsifier 5cc Distilled water Liquid phase 59cc Calcium chloride Reactive clay stabil- liser 8.65g Confi-gel Viscosifier and gelling agent 5g Confi-trol Fluid loss control agent 7g Lime pH controller 4g Barite Weighting agent 103.53g Table 4: Standard oil-based mud formulation for 12 ppg mud weight Additives Function Composition Sarapar 147 Base oil 159.77cc Confimul P Primary emulsifier 5cc Confimul S Secondary emulsifier 5cc Distilled water Liquid phase 52cc Calcium chloride Reactive clay stabil- liser 7.58g Confi-gel Viscosifier and gelling agent 5g Confi-trol Fluid loss control agent 7g Lime pH controller 4g Barite Weighting agent 260.22g 2.3.Rheological Property Test The rheological properties of a mud sample, such as the plastic viscosity (PV), yield point (YP), 10 second gel strength (10 s GS), 10 minute gel strength (10 m GS) , are all measured with the aid of a FANN Viscometer. All these tests are repeated before and after aging at a temperature range from 77 to 300◦F for two different mud weights (9 ppg and 12 ppg). Table 5 shows the recommended rheological properties, which were utilized to evaluate the performance of all the for- mulated samples. Table 5: Recommended rheological properties suggested by Li et al(2016) [56; 57] Rheological property Specification Electrical stability, V 400 , as high as practicable Plastic viscosity, cp 10-60, preferably 15-40 Yield point, lb/100 sq. ft 2.5-20, preferably 5-12.5 Gel strength (10 second) 4-10 Gel strength (10 minutes) 4-15 HPHT fluid loss, cc <10 2.4.Rheological Model Rheological models of mud samples were obtained using a Brookfield RST TouchT M Rheometer both before and after aging to investigate the effect of temperature on the rheologi- cal model of the mud samples and study the effect of nanosil- ica on both WBM and OBM. 2.5.Lubricity Test The lubricity test was conducted on mud samples both be- fore and after aging using an OFITE Lubricity Tester to in- vestigate the effect of temperature on the lubricity of the mud samples. The lubricity of a mud sample can be determined by measuring the coefficient of friction (CoF) of the sample with a lubricity test. This test utilizes the concept of metal-to-metal contact to simulate the drill string and wellbore condition. Lu- bricity is calculated by the following equations. Coe f icient f actor = Meter reading using deionised water 34 (1) Lubricity Coe f f icient CoF = CF(Meter reading f rom a mud sample) 100 (2) Relative CoF reduction = CoFbasic mud −CoFnanosilica CoFbasic mud (3) 2.6.Electrical Stability(ES) Test The ES test is carried out to examine the OBM emulsion and oil-wetting qualities of mud samples. ES is measured by generating a gradually increasing sinusoidal alternatingvolt- age across a pair of electrodes submerged in the OBM sam- ples. 2.7.Transmission Electron Microscop(TEM) and Zeta poten- tial characterization of Nanosilica Table 6: Properties of Nanosilica Properties Specifications Appearance White powder Density 2.4 g/cm3 Purity of SiO2 99.90% Particle size 14 nm pH (5 % suspension) 4.5 Heating loss (105oC for 2 hr.) 0.90% Ignition loss (1000oC for 2 hr.) 1.20% Absorption value 230 ml/100g Specific surface area 202 m2/g Heavy metals < 0.001 % Sodium sulphate < 0.02 % Lead content < 0.0001 % Iron(Fe) 149 mg/kg Manganese(Mn) 3 mg/kg Copper 1 mg/kg Arsenic < 0.00001 % The nanosilica used in this study was procured and pre- pared by Shanghai Honest Chem Co., Ltd, China. No further modifications were made prior to testing. The properties of Page 5 of 42 Acc ep te d M an us cr ip t COLLOIDS AND SURFACES A: PHYSICOCHEMICAL AND ENGINEERING ASPECTS 5 nanosilica are listed in Table6. The particle size of nanosil- ica is approximately 14 nm and round in shape as observed in TEM images (Figure3). Figure 3: TEM Images of nanosilica in alkaline solution with different concentrations: (a) 0.5 ppb, (b) 1.0 ppb, (c) 1.5 ppb. Table 7: Average zeta potential values with different nanosil- ica concentrations Sample type T Zeta Potential(ζ) [oC] [mV] Water + caustic soda + Nano 0.5ppb 25.1 -42.2 Water + caustic soda + Nano 1.0ppb 25 -43.4 Water + caustic soda + Nano 1.5ppb 25 -43.9 Table7 shows the Zeta potential measurement and Fig- ure 3 shows the Transmission Electron Microscopy (TEM) images of the 14 nm nanosilica tested in an alkaline solu- tion of 100 ml distilled water with 0.25 ppb of caustic soda. The nanosilica used in this study was spherical in shape and these images showed well dispersions of nanosilica without the need for long ultrasonication. According to Wissing et al. (2004)[58] and Kavitha et al. (2014)[59], absolute zeta potential values above 30 mV provide good dispersion and stability. Meanwhile, zeta po- tential values of above 60 mV and around 20 mV provide ex- cellent stability and short-term stability respectively (Honary & Zahir, 2013[60]). This study shows that different concen- trations of nanosilica produced almost similar zeta potential values. There was a slight increase in zeta potential values when nanosilica increases. All samples in Table7 generated high zeta potential values (>30 mV) which suggests good and stable dispersions. 2.8.Field Emission Scanning Electron Microscope(FESEM) analysis The mud cake is thicker for muds with nanosilica because as nanosilica plugged the pore spaces of the filter paper, water was still able to seep through the hydrophilic layer of nanosil- ica [61]. Thus, resulted in thicker mud cake. This also ex- plained why filtrate volumes of muds with nanosilica was slightly higher than muds without nanosilica. FESEM results in Figure4 show that 9 ppg WBM and OBM produces slightly smother surfaces compared to 9 ppg WBM and OBM with nanosilica. Based on the images, nanosilica was still in their nanosized state after they were mixed in the fluids. Figure 4: FESEM images of (a) WBM 9 ppg, (b) WBM 9 ppg with 1.0 ppb nanosilica, (c) OBM 9 ppg, (d) OBM 9 ppg with 1.0 ppb nanosilica 3.Results and Discussions 3.1.Rheological Properties of OBM Samples 3.1.1.ES Test Figure 5: ES of 9 ppg OBM samples with and without nanosilica Figure5 shows the ES of the 9 ppg OBM samples with and without nanosilica, with the concentration ranging from 0 – 1.5 ppb. The samples were tested before the hot rolling process and after the hot rolling process under 100 psi at re- spective temperatures ranging from 77◦F to 300◦F (API RP 13B-1, 2009[50]). Sarapar 147(see Table18), the base oil used in formulat- ing the OBM, consists of 96% (minimum) n-paraffins,<5% iso-paraffins,<0.1% napthanics, and<0.01% aromatics. It has density of 775 kg/m3 with boiling range from 258◦C to Page 6 of 42 Acc ep te d M an us cr ip t COLLOIDS AND SURFACES A: PHYSICOCHEMICAL AND ENGINEERING ASPECTS 6 293◦C. As observed from Figure5, all the OBM samples ex- hibit ES values equal to or greater than 400 V. According to the guidelines, these samples are practical, as the ES values imply that the emulsion system is stable. Additionally, other trends can be observed from the figure: all the ES values increase after aging and increase with increasing nanosilica concentration. This phenomenon occurs because the emul- sifiers were protected (Growcock et al., 1994 [62]) even af- ter hot rolling with the addition of nanosilica due to its high thermal stability. According to Growcock (1994) [62], the dispersed nanosilica will wrap and form an insulating film around the emulsifiers, protecting them from thermal degra- dation. This is true for OBM samples which were hot rolled from 77◦F till 250◦F. In accordance with kinetic theory [63], when mud sam- ples undergo hot rolling at a high temperature range of 250 ◦F to 300◦F (API RP 13B-1, 2009[50]), additive particles, in- cluding nanosilica, will be more kinetically active, resulting in poor formation of an insulating film, which causes a slight decrease in the ES values obtained. But Figure5 shows that all OBM samples increase after they were hot rolled at 300◦F. The theory is valid as long as the OBM sample is still in liquid form. As Sarapar 147s(see Table18) boiling point is 293◦F, and above that point it appears as vapour. Coupled with con- tinuous hot–rolling process, this allows the insulation process to happen in a more positive form for all the OBM samples and subsequently results in a better ES reading as shown in Figure5. This new finding opens a window for further exper- imental investigation. 3.1.2.PV of 9 ppg OBM Samples with and without Nanosilica 0 0.5 1 1.5 Concentration of Nanosilica (ppb) 0 5 10 15 20 25 30 35 40 45 50 P la s ti c V is c o s it y ( c p ) BHR Figure 6: PV of 9 ppg OBM samples before aging Figure6 portrays the PV of the 9 ppg OBM samples with nanosilica as an additive before aging. According to Salih (2017) [15], the PV of a mud should decrease with the ad- dition of nanoparticles, as the nanosized particles will act as ”ball bearings” and be arranged among the larger particles of other additives such that less friction occurs. Figure6 shows that with the addition of 0.5 ppb, 1 ppb and 1.5 ppb nanosilica, the PV decreases from 28 cp to 20 cp, 22 cp and 24 cp, respectively. The greatest reduction of the PV is observed with the addition of 0.5 ppb nanosilica. At 1.0 and 1.5 ppb nanosilica, the mud samples experience a lower reduction because more solid content is present in the mud systems, and less reduction of fiction is obtained because of the constant collision of the mud particles within the system (Salih, 2017 [15]). Although all the reduced PV values obtained are all in the recommended range when nanosilica is used as an enhancing additive, the mud sample with the lowest PV value within the recommended range should be adopted because the minimum PV is often preferred in drilling operations in order to esca- late the rate of penetration (ROP), reduce the energy needed for mud circulation, provide better cooling and lubrication functions to the downhole equipment and eventually dimin- ish the mud loss during the circulation process resulting from the unexpected excessive equivalent circulation density that may give rise to formation fractures [15; 64]. A reduction in PV is observed as soon as 0.5 ppb of nanosilica was added into the mud. This reduction is due to the ability of nanosilica to disrupt gel formations betweenpar- ticles in the mud [61]. The presence of nanosilica could help reduce PV for low and high mud weights and enables rapid drilling. Results show that as concentration of nanosilicain- creases to 1.0 and 1.5 ppb, PV increases as well. This increase was due to an increase in solid particles in the mud. In low mud weights, the attractive forces of the particles in the mud are not as high as in high mud weights. Thus, in low mud weights, nanosilica helps increase gel formations which increase YP readings. In high mud weights however, the presence of 0.5 and 1.0 ppb concentration of nanosilica disrupts gel formations of other particles in the mud. Thus,a higher concentration of nanosilica is required for higher mud weights. In this case, 12 ppg mud requires a minimum con- centration of 1.5 ppb of nanosilica for optimum YP perfor- mance. Figure7 shows the PVs of 9 ppg OBM samples after ag- ing at their respective temperature under 100 psi for 16 hours. Figure7 shows that the PVs of the OBM samples are high after aging, and higher PV values are obtained when the sam- ples are aged at a higher temperature. This increase in the PV occurs because when the samples are aged at a higher temper- ature, the particles in the mud will be more kinetically active, which will result in more shear within the mud system itself (Paramasivam, 2016 [65]) and eventually lead to an increase of the PV with increasing aging temperature. All the OBM samples exhibit the same trends before ag- ing and after aging. For all the samples, both before and after aging at their respective temperature, the PVs decrease when nanosilica is added. After aging at 77◦F, the OBM sample with the addition of 0.5 ppb nanosilica shows a reduction in PV value from 30 cp to 21 cp, while the addition of 1.0 ppb nanosilica results in a decrease from 30 cp to 23 cp, and with 1.5 ppb nanosilica usage, a decrease from 30 cp to 25 cp is obtained. As for the PVs obtained after aging at 150◦F, the usage of 0.5 ppb, 1.0 ppb and 1.5 ppb nanosilica results in a decrease of the PV from 34 cp to 24 cp, 27 cp and 32 cp, respectively. Similarly, the samples that undergo aging at 250◦F demon- Page 7 of 42 Acc ep te d M an us cr ip t COLLOIDS AND SURFACES A: PHYSICOCHEMICAL AND ENGINEERING ASPECTS 7 strate a decline in the PV when nanosilica is added. When 0.5 ppb, 1.0 ppb and 1.5 ppb nanosilica is added into the OBM samples, a reduction from 35 cp to 25 cp, 28 cp and 33 cp is obtained, respectively. When the OBM samples are aged at 300◦F, with the ad- dition of 0.5 ppb, 1.0 ppb and 1.5 ppb nanosilica, a reduction of PV from 36 cp to 26 cp, 28 cp and 34 cp is obtained, re- spectively. All the samples show the greatest reduction in PV when 0.5 ppb nanosilica is used, which shows that 0.5 ppb is the optimum concentration for the PV. Agglomeration of nanosilica occurs above this concentration, leading to an in- crease in the PV. 0 0.5 1 1.5 Concentration of Nanosilica (ppb) 0 5 10 15 20 25 30 35 40 45 50 P la st ic V is co si ty ( cp ) BHR AHR at 77oF AHR at 15oF AHR at 250oF AHR at 300oF Figure 7: PV of 9 ppg OBM samples after aging 3.1.3.YP of 9 ppg OBM Samples with and without Nanosilica Figure8 shows the YP of the 9 ppg OBM samples when nanosilica is used as the enhancing additive with concentra- tions of 0.5 to 1.5 ppb before aging. According to the recom- mended specification from previous studies [56; 57], the ideal YP for an OBM should be in the range of 5 – 12.5 lb/100 sq ft. The YP is defined as the minimum force required to trans- form drilling fluid to the gel condition to the flowing condi- tion as soon as it has become motionless [9] and is precisely referred to as the cuttings carrying capacity of the drilling fluid [6; 7; 9; 61; 66; 67; 68]. The YP of a drilling mud must be within the recommended range, as when a mud exceeds this range and reaches a fairly high YP, flocculation occurs, and a high YP further results in the unnecessary loss of pump pressure, a decrease in the ROP and an increase in the surge and swap pressure [15; 69; 70]. Unfortunately, an excessively low YP is also undesirable, as it will lead to sagging of barite. 0 0.5 1 1.5 Concentration of Nanosilica (ppb) 0 2 4 6 8 10 12 14 16 18 20 Y ie ld P o in t (l b /1 00 ft 2 ) BHR Figure 8: YP of 9 ppg OBM samples before aging Referring to Figure8, all the nanosilica-enhanced samples are within the recommended range except for one sample, the basic mud, as it has a low YP below the recommended range. Sagging of barite may be encountered if the basic mud sample is put into practice. When 0.5 ppb nanosilica is added, the YP value rises from 4 lb/100 sq ft to 8 lb/100 sq ft. As the con- centration of nanosilica increases up to 1.0 ppb and 1.5 ppb, the YP values increase from 4 to 9 lb/100 sq ft and 12 lb/100 sq ft, respectively. The increase of the YP after the addition of nanosilica and the directly proportional trend can be explained by the theory that there is a higher attractive force in the mud system with nanosilica. This higher attractive force is contributed bythe huge surface area of nanosilica, as this will lead to a superior chemical reactivity, which eventually increases the YP of the nanosilica-enhanced mud samples [26; 71; 72; 73; 74]. Figure9 presents the YPs obtained for the 9 ppg OBM samples after the mud samples with and without nanosilica have gone through the aging process for 16 hours under 100 psi at their respective temperature. The YP values obtained before aging are also plotted for a clearer comparison. 0 0.5 1 1.5 Concentration of Nanosilica (ppb) 0 2 4 6 8 10 12 14 16 18 20 Y ie ld P o in t (l b /1 00 ft 2 ) BHR AHR at 77oF AHR at 15oF AHR at 250oF AHR at 300oF Figure 9: YP of 9 ppg OBM samples after aging From Figure9, a trend of increasing YP values is obtained with increasing nanosilica concentration. This result is simi- lar to the trend observed for the OBM samples before aging. Page 8 of 42 Acc ep te d M an us cr ip t COLLOIDS AND SURFACES A: PHYSICOCHEMICAL AND ENGINEERING ASPECTS 8 After the samples are aged at 77◦F, a few samples lie within the recommended range of the ideal YP: the basic mud sam- ple and OBM samples with 0.5 and 1.0 ppb nanosilica; the mud sample with 1.5 ppb nanosilica unfortunately no longer falls in the range. The YP values obtained when 0.5 ppb, 1.0 ppb and 1.5 ppb nanosilica is used as an additive show an in- crease from 5 lb/100 sq ft to 9 lb/100 sq ft, 11 lb/100 sq ft and 15 lb/100 sq ft, respectively. When the aging temperature is increased to 150◦F, 3 sam- ples also fall into the ideal range of the YP: the basic mud sample and OBM samples with 0.5 and 1.0 ppb nanosilica. In contrast, the sample with 1.5 ppb nanosilica has a YP that exceeds the recommended specification. When 0.5 ppb, 1.0 ppb and 1.5 pbb nanosilica is used as an additive, the YP of the basic mud is increased from 5 lb/100 sq ft to 11 lb/100 sq ft, 12 lb/100 sq ft and 16 lb/100 sq ft. When the OBM samples undergo aging at 250◦F, only two samples remain in the ideal range, namely, the basic mud sample and the sample with 0.5 ppb nanosilica. With the ad- dition of 0.5 ppb nanosilica, the YP value of the basic mud increases from 6 lb/100 sq ft to 12 lb/100 sq ft. However, a further increase in the nanosilica concentration promotes a further increase of the YP of the mud samples such that it no longer falls within the ideal range. The mud sample with 1.0 ppb nanosilica has a YP of 14 lb/100 sq ft, while that with 1.5 ppb nanosilica has a YP of 17 lb/100 sq ft. A similar scenario occurs when the OBM samples are tested after aging at 300◦F for 16 hours. The YP range recom- mended by the previous researchers [56; 57] is from 5 – 12.5 lb/100 sq ft; however, the precision of a viscometer only goes to 1 unit, so 12.5 is rounded off to 13 lb/100 sq ft. For this range, only two samples fall within the specification, the ba- sic mud sample and the OBM sample with 0.5 ppb nanosilica, as their YPs are 7 lb/100 sq ft and 13 lb/100 sq ft, respectively. As for the samples with nanosilica concentrations of 1.0 and 1.5 ppb, their YP values of 15 lb/100 sq ft and 18 lb/100 sq ft exceed the recommended range. Paramasivam (2016) [65] claimed that a larger minimum force will be required at higher temperature conditions. This phenomenon occurs when OBM samples are tested after ag- ing, as the YP values obtained are higher than those before aging. Additionally, the samples exhibit a pattern in whichthe YP values are higher when their aging temperature is higher. Overall, the highest YP within the range is always desirable because it promotes the efficient cuttings lifting of a drilling fluid, while a lower value may result in sagging of barite, and sagging is a very complicated issue [56; 57]. In our case, 0.5 ppb nanosilica is the optimum concentration for obtaining the highest YP value within the recommended range. 3.1.4.GS at 10 s of 9 ppg OBM samples with and without Nanosilica Basic OBM OBM with 0.5 ppb NS OBM with 1.0 ppb NS OBM with 1.5 ppb NS Mud Sample 0 2 4 6 8 10 G e ls S tr e n g th ,( lb /1 0 0 ft 2 ) BHR AHR at 77oF AHR at 150oF AHR at 250oF AHR at 300oF Figure 10: GS at 10 s of 9 ppg OBM samples with and with- out nanosilica The GS is defined as the capability of a drilling fluid to develop and maintain a gel structure as soon as the drilling operation comes to a halt (Paramasivam, 2016 [65]). The GS is a measurement of the shear stress after the gel has set qui- escently for some time [75; 76; 77]. A good GS is always required, as it would maintain the excessive circulation pres- sure needed to restart drilling operations [29; 78; 79]. Figure10shows that the 10 s GSs of all OBM samples are within the recommended specification. No specific trend can be observed regarding the impact of temperature on the 10 s GS of the mud samples. 3.1.5.GS at 10 m of 9 ppg OBM samples with and without Nanosilica Basic OBM OBM with 0.5 ppb NS OBM with 1.0 ppb NS OBM with 1.5 ppb NS Mud Sample 0 2 4 6 8 10 12 14 16 18 G e ls S tr e n g th ,( lb /1 0 0 ft 2 ) BHR AHR at 77oF AHR at 150oF AHR at 250oF AHR at 300oF Figure 11: GS at 10 m of 9 ppg OBM samples with and with- out nanosilica Figure 11 demonstrates the 10 m GSs of OBM sample with and without nanosilica both before and after aging at Page 9 of 42 Acc ep te d M an us cr ip t COLLOIDS AND SURFACES A: PHYSICOCHEMICAL AND ENGINEERING ASPECTS 9 their respective aging temperature. The ideal range of the 10 m GS suggested by previous researchers [56; 57] is from 4 to 15 lb/100 sq ft. Referring to Figure11, the 10 m GSs of almost all samples are in the recommended range except for the OBM samples with 1.0 and 1.5 ppb nanosilica that have undergone aging at 150 ◦F and the sample with 1.5 ppb nanosilica that has been aged at 300◦F; these samples have a 10 m GS of 16 lb/100 sq ft. Additionally, no specific pattern can be observed for the10 m GS, and the impact of temperature is not significant enough to produce a trend in the 10 m GS. The mud samples should have both the 10 s and 10 m GSs within the recommended ranges because excessive GSs can often result in swabbing and surging during run in hole and pull out of hole. Excessive GSs can also make the process of running in logging tools difficult and result in a tendency to- ward retaining and trapping sands, cuttings and air in the mud during drilling operations [75; 76; 77]. On the other hand, GS values lower than the ideal range will result in the formation of a too fragile gel [15], which is undesirable, as these mud samples will not have enough gel to suspend cuttings when the mud circulation or drilling operation is brought to a halt. The cuttings or even barite will suffer from sagging. Basic OBM OBM with 0.5 ppb NS OBM with 1.0 ppb NS OBM with 1.5 ppb NS Concentration of nanosilica (ppb) 0 2 4 6 8 10 12 14 G el s St re ng th ,(l b/ 10 0f t2 ) BHR 10s BHR 10m (a) Basic OBM OBM with 0.5 ppb NS OBM with 1.0 ppb NS OBM with 1.5 ppb NS Concentration of nanosilica (ppb) 0 2 4 6 8 10 12 14 16 G el s St re ng th ,(l b/ 10 0f t2 ) 10s AHR at 77oF 10m AHR at 77oF (b) Basic OBM OBM with 0.5 ppb NS OBM with 1.0 ppb NS OBM with 1.5 ppb NS Concentration of nanosilica (ppb) 0 2 4 6 8 10 12 14 16 G el s St re ng th ,(l b/ 10 0f t2 ) 10s AHR at 150oF 10m AHR at 150oF (c) Basic OBM OBM with 0.5 ppb NS OBM with 1.0 ppb NS OBM with 1.5 ppb NS Concentration of nanosilica (ppb) 0 2 4 6 8 10 12 14 16 G el s St re ng th ,(l b/ 10 0f t2 ) 10s AHR at 250oF 10m AHR at 250oF (d) Basic OBM OBM with 0.5 ppb NS OBM with 1.0 ppb NS OBM with 1.5 ppb NS Concentration of nanosilica (ppb) 0 2 4 6 8 10 12 14 16 18 G el s St re ng th ,(l b/ 10 0f t2 ) 10s AHR at 300oF 10m AHR at 300oF (e) Figure 12: Comparison of the 10 s and 10 m GSs of 9 ppg OBM samples with different aging temperatures: (a) before aging, (b) after aging at 77◦F, (c) after aging at 150◦F, (d) after aging at 250◦F and (e) after aging at 300◦F Page 10 of 42 Acc ep te d M an us cr ip t COLLOIDS AND SURFACES A: PHYSICOCHEMICAL AND ENGINEERING ASPECTS 10 Figure12 shows a comparison of the 10 s and 10 m GSs of each set of OBM samples before and after aging at their respective temperature. The purpose of this comparison is to investigative if a particular OBM sample can form a progres- sive gel such that the ability to suspend cuttings when thereis a pause in drilling would be promoted and the ability to break the gel and start flowing when drilling is resumed would be realized. Progressive gels are formed when there is a reason- ably wide range between the initial (10 second) and 10 minute gel readings [2; 26]. Referring to Figure12 (a) – (e), a trend can be observed, in which the 10 m GS is almost twice the 10 s GS. This result is desirable and proves the theory of Amani et al. (2012) [80]. In addition to ensuring the formation of progressive gels, this comparison is significant for ensuring that the developmentof high-flat gels or low-flat gels, for which both the 10 s GS and 10 m GS are high or low with no appreciable difference in the range between them, is avoided. 3.1.6.HPHT Filtration of 9 ppg OBM Samples with and with- out Nanosilica Figure13 displays the results of the HPHT filtration test conducted on the 9 ppg OBM samples with nanosilica con- centration from 0.5 ppb to 1.5 ppb; however, the fluid loss after aging is only determined for those samples that have un- dergone high temperature aging (from 250◦F to 300◦F). The recommended specification for an HPHT filtration test by researchers [56; 57] is that the filtrate volume be less than or equal to 10 cc after 30 minutes of testing. 0 0.5 1 1.5 Concentration of Nanosilica (ppb) 0 1 2 3 4 5 6 H P H T F ilt ra ti o n L o ss ( cc ) AHR at 250oF AHR at 300oF Figure 13: HPHT filtration of 9 ppg OBM samples with and without nanosilica From Figure13, a pattern can be observed in which when nanosilica is added, the filtration increases with increasing nanosilica concentration. All the samples fulfill the ideal specification of having an HPHT filtration loss of less 10 cc. Referring to the OBM samples aged at 250◦F, when 0.5 ppb nanosilica is added, the HPHT filtrate volume obtained at the end of 30 minutes is 2.2 cc, while the filtrate obtained from the basic mud is only 1.8 cc. A further increase in the nanosilica concentration leads to a further escalation of the HPHT fluid loss, reaching 3.0 cc and 3.4 cc when 1.0 ppb and 1.5 ppb nanosilica is added, respectively. The same trend can be observed for the OBM samples aged at 300◦F. The usage of nanosilica causes a greater HPHT fluid loss, in which the basic mud only experiences a 3.0 cc fluid loss, while the mud samples with 0.5 ppb, 1.0 ppb and 1.5 ppb nanosilica undergo a filtration loss of 3.4 cc, 4.4 cc and 4.8 cc, respectively. An increase in the temperature also increases the fluid loss of the mud samples; for instance, the OBM samples aged at 300◦F have greater fluid loss than those aged at 250◦F. This increase may occur because irregular flocculation of solid par- ticles occurs at an elevated temperature [1; 81; 82], which worsens the fluid loss of the samples. According to Wahid et al. (2015) [83], an increase in the nanosilica concentration will result in an adverse effect on the filtration properties. Nanosilica’s inability to prevent fluid loss may be due to its overly small size [84], as the nanosilica used in this study is only 14 nm in size, while the pore size of the filter paper is 60 m; hence, the nanosilica will flow out to- gether with the filtrate, resulting in almost no positive impact on combating fluid loss. Salih (2017) [15] stated that flocculation promotes link- ing of the solid particles in mud systems in an edge-to-edge as well as edge-to-face manner, which will result in the forma- tion of an open network structure that favors fluid loss. This theory matches the results in this study because the usage of nanosilica as well as a higher temperature generally increases the YP of a sample, which may lead to a very minor floccula- tion process. 3.1.7.Mud Cake Thickness of 9 ppg OBM Samples with and without Nanosilica Basic OBM OBM with 0.5 ppb NS OBM with 1.0 ppb NS OBM with 1.5 ppb NS Mud Sample 0 1 2 3 4 5 M u d c a k e t h ic k n e s s ,( /3 2 i n c h ) AHR at 250oF AHR at 300oF Figure 14: Mud cake thickness of 9 ppg OBM samples with and without nanosilica A filter cake or a mud cake will form at the wellbore across the permeable zone when filtrate loss occurs from the wellbore into the formation through the permeable zone. Ex- cess filtration loss may result in formation damage. Thus, for any practical mud formulation, formation of a thin strong im- Page 11 of 42 Acc ep te d M an us cr ip t COLLOIDS AND SURFACES A: PHYSICOCHEMICAL AND ENGINEERING ASPECTS 11 permeable filter cake as filtration occurs is necessary to avoid further formation contamination resulting from filtrate loss and a blocked pipe due to an uneven or overly thick mud cake (Paramasivam, 2016 [65]). Figure14portrays the thickness of the mud cakes formed during filtration for the OBM samples that have been aged at the high temperature of 250◦F or 300◦F. The figure shows that the thickness of the mud cake obtained is closely related or even directly proportional to the filtration volume. At a higher aging temperature, a higher filtrate loss volume is ob- tained, which contributes to thicker mud cake formation. With increasing nanosilica concentration, the YP of the mud sam- ples increases, leading to very minor flocculation, which re- sults in failure of impermeable mud cake formation and of fluid loss retainment. 3.1.8.Lubricity of 9 ppg OBM Samples with and without Nanosilica The lubricity of a drilling mud is one of the impor- tant properties that should be governed during mud formu- lation, as the mud helps lubricate the drill string as drilling progresses [85; 86]. The CoF is defined as the ratio of the frictional forces of two bodies that press against each other [9; 78; 87; 88]. Figure15 shows the CoF values and Figure16 shows the reductions in the CoF relative to the basic mud of the 9 ppg OBM samples with and without nanosilica before and after aging at the respective aging temperature. An obvious trend can be observed when nanosilica is added into the mud sam- ples. With the addition of nanosilica, the nano-muds exhibit lower CoF values compared to the basic mud, both before and after aging. The reduction in the CoF values relative to the ba- sic mud is directly related to the nanosilica concentration; a lower CoF value is obtained when a higher concentration of nanosilica is added into the mud. For example, before aging, with the addition of 0.5 ppb, 1.0 ppb and 1.5 ppb nanosilica to the OBM samples, relative CoF reductions of 1.4%, 2.8% and 3.3% are obtained, respec- tively. A similar trend is witnessed for the samples aged at ambient temperature, in which 4.6%, 6.6% and 7.9% reduc- tions in the CoF relative to the basic mud are obtained when 0.5 ppb, 1.0 ppb and 1.5 ppb nanosilica is used. After aging at 150◦F, the OBM samples with nanosilica concentrations of 0.5 ppb, 1.0 ppb and 1.5 ppb show relative CoF reductions of 4.4%, 5.3% and 6.8%, respectively. The highest reduction in the CoF is obtained for the sam- ples that have undergone the 250◦F aging process. The high- est reduction of 10.2% is obtained with the addition of 1.5 ppb nanosilica to the mud samples. As for the samples that have undergone aging at 300◦F, slight relative CoF reductions of 2.9%, 3.5% and 4.1% are obtained with the addition of 0.5 ppb, 1.0 ppb and 1.5 ppb nanosilica, respectively. The lubricity of the drilling mud samples solely depends on the nanosilica concentration, while the temperature does not have an impact on it. Nanosilica has the ability to increase the lubricity of a mud by forming a boundary type of lubrica- tion via physisorption [89; 90]. The dispersed nanosilica will form a thin film and completely cover the metal surface of the lubricity tester. An increase in the concentration will favor the formation of the thin film, which will lubricate surfaces like ball bearings [15]. 0 0.5 1 1.5 Concentration of Nanosilica (ppb) 0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2 A b s o lu te C o e ff ic ie n t o f fr ic ti o n , C o F BHR AHR at 77oF AHR at 15oF AHR at 250oF AHR at 300oF Figure 15: CoF of 9 ppg OBM samples with and without nanosilica 0 0.5 1 1.5 Concentration of Nanosilica (ppb) 0 2 4 6 8 10 12 R e la ti v e C o e ff ic ie n t o f fr ic ti o n , C o F ( % ) BHR AHR at 77oF AHR at 15oF AHR at 250oF AHR at 300oF Figure 16: Relative CoF reduction of 9 ppg OBM samples with and without nanosilica 3.1.9.Electrical Stability(ES) of 12 ppg OBM Samples with and without Nanosilica Figure17 shows the results of the ES test conducted on the 12 ppg OBM samples with and without nanosilica both before and after aging at their respective temperature, ranging from 77 ◦F to 300◦F, for 16 hours under a backpressure of 100 psi. The specification recommended by [56; 57] for an ES test to indicate the stability of an emulsion is that the ES value be greater than or equal to 400 V. Page 12 of 42 Acc ep te d M an us cr ip t COLLOIDS AND SURFACES A: PHYSICOCHEMICAL AND ENGINEERING ASPECTS 12 Basic OBM OBM with 0.5 ppb NS OBM with 1.0 ppb NS OBM with 1.5 ppb NS Mud Sample 0 100 200 300 400 500 600 700 800 E le c tr ic a l S ta b il it y (E S ) BHR AHR at 77oF AHR at 150oF AHR at 250oF AHR at 300oF Figure 17: ES of 12 ppg OBM samples with and without nanosilica All the OBM samples satisfy the ideal specifications sug- gested, exhibiting ES values greater than 400 V. Additionally, the ES values of the mud samples after aging are maintained above the desired value, which signifies that there is no degra- dation of the emulsifier within the systems. Both the 9 ppg OBM and 12 ppg OBM samples dis- play similar trends in terms of being able to maintain a sta- ble emulsion throughout the study by satisfying the recom- mended specification. Moreover, the 12 ppg OBM samples have higher ES values than the 9 ppg OBM samples, which might occur because heavy muds contain less brine within their systems since the ES values signify the maximum volt- age that an OBM can sustain before it starts to conduct cur- rent [10; 62]. 3.1.10.PV of 12 ppg OBM Samples with and without Nanosil- ica 0 0.5 1 1.5 Concentration of Nanosilica (ppb) 0 5 10 15 20 25 30 35 40 45 P la s ti c V is c o s it y ( c p ) BHR AHR at 77oF AHR at 15oF AHR at 250oF AHR at 300oF Figure 18: PV of 12 ppg OBM samples with and without nanosilica Figure18 displays the PVs of the 12 ppg OBM samples with and without nanosilica at concentrations of 0.5 ppb to 1.5 ppb both before and after aging at a temperature rang- ing from 77◦F to 300◦F. The recommended range suggested by [56; 57] is from 10 to 60 cp for any heavy mud sample. Referring to Figure18, the usage of nanosilica in OBM samples reduces the PVs of each set of samples relative to the basic mud, both before and after aging. The greatest reduc- tion in the PV is observed when 0.5 ppb nanosilica is used for every set of samples. Above 0.5 ppb, a further increase in the nanosilica concentration does not reduce the PV further; instead, the use of a higher nanosilica concentration results in a smaller PV reduction, making the PVs of these nano-muds closer to the basic mud’s PV. Before aging, when 0.5 ppb, 1.0 ppb and 1.5 ppb nanosil- ica is used as the additive, the PV of the basic mud is reduced from 30 cp to 23 cp, 26 cp and 29 cp, respectively. After aging at 77◦F, the 0.5 ppb, 1.0 ppb and 1.5 ppb nanosilica-enhanced muds exhibit reductions in the PV compared to the basic mud from 30 cp to 21 cp, 21 cp and 25 cp, respectively. Reductions in the PVs from that of the basic mud of 34 cp to 24 cp, 27 cp and 32 cp are observed for the OBM samples containing 0.5 ppb, 1.0 ppb and 1.5 ppb nanosilica, respec- tively, aged at 150◦F. When the OBM samples have been aged at 250◦F, the samples with 0.5 ppb, 1.0 ppb and 1.5 ppb nanosilica exhibit reduced PVs compared to the value of 35 cp for the basic mud of 25 cp, 28 cp and 33 cp, respectively. A similar trend occurs after the samples are aged at 300◦F, where reductions in the basic mud PV from 36 cp to 26 cp, 26 cp and 34 cp occur when the muds contains 0.5 ppb, 1.0 ppb and 1.5 ppb nanosilica, respectively. As for the effect of the mud weight on the PV, the 12 ppg OBM samples generally have higher PV values than those of the 9 ppg OBM samples. This increase occurs because heavy mud has a higher solid content than normal mud weight sam- ples [91], and a higher solid content often results in a higher PV [15]. 3.1.11.YP of 12 ppg OBM Samples with and without Nanosil- ica Figure 19 displays the YP of the 12 ppg OBM sam- ples with and without nanosilica both before and after ag- ing. The ideal range of YP recommended by previous re- searchers [56; 57] is 2.5 – 20 lb/100 sq ft for a heavy OBM mud. Figure19 shows a pattern in which the YP of the OBM samples decreases when nanosilica is added as the mud ad- ditive. The greatest decrease of YP can be noted when the nanosilica concentration is 0.5 ppb both before and after ag- ing. A further increase of the concentration above 0.5 ppb does not decrease the YP value further but instead results ina smaller reduction, bringing the YP value closer to that of the basic mud. Since a higher YP value will result in floccula- tion, a relatively moderate YP within the range is desired so that the mud does not flocculate but is strong enough to hold the drill cuttings. Before aging, all the mud samples are within the sug- gested range of the ideal YP, with values of 14, 12, 14 and 15 lb/100 sq ft for the basic mud and 0.5, 1.0 and 1.5 ppb Page 13 of 42 Acc ep te d M an us cr ip t COLLOIDS AND SURFACES A: PHYSICOCHEMICAL AND ENGINEERING ASPECTS 13 nanosilica-enhanced muds. However, after aging at 77◦F, only one mud sample is within the ideal YP range, the OBM sample with 0.5 ppb nanosilica. At this aging temperature, the basic mud has a value of 23 lb/100 sq ft, while the 1.0 ppb and 1.5 ppb nanosilica muds have YP values of 21 and 24 lb/100 sq ft, respectively. When the samples are aged at 150◦F, similarly, only the 0.5 ppb nanosilica mud is within the suggested range, while the basic mud achieves a YP value of 25 lb/100 sq ft and the 1.0 ppb and 1.5 ppb nanosilica muds obtain YP values of 22 and 25 lb/100 sq ft, respectively. For the OBM samples aged at a high temperature of 250 ◦F or 300◦F, only one sample from each case satisfies the ideal YP range, both of which are the 0.5 ppb nanosilica mud, with YP values of 19 and 20 lb/100 sq ft, respectively. Comparing the 9 ppg and 12 ppg OBM samples, a differ- ence in the patterns is observed. The YP of the 9 ppg OBM samples increases proportionally with increasing nanosilica concentration, which implies that the basic mud samples both before and after aging have the lowest YP value. In contrast, the YP values of the basic muds for the 12 ppg samples are usually the highest among all the muds in each condition. This difference in patterns occurs because of the difference in the solid content at each mud weight. According to Salih (2017) [15], the YP is largely affected by the electrochemical attractive force and the frictionalforce between particles, and these two forces depend on the dis- tance between solid particles; with an increase in the solid content within the same volume, the distance between solid particles is reduced, resulting in greater forces. In the ba- sic mud, no nanosilica exists to rearrange the solid particles; hence, the greater forces directly result in higher YP values. 0 0.5 1 1.5 Concentration of Nanosilica (ppb) 0 5 10 15 20 25 30 Y ie ld P o in t (l b /1 0 0 ft 2 ) BHR AHR at 77oF AHR at 15oF AHR at 250oF AHR at 300oF Figure 19: YP of 12 ppg OBM samples with and without nanosilica 3.1.12.GS at 10 s of 12 ppg OBM Samples with and without Nanosilica Basic OBM OBM with 0.5 ppb NS OBM with 1.0 ppb NS OBM with 1.5 ppb NS Mud Sample 0 2 4 6 8 10 12 14 16 18 G el s S tr en gt h, (lb /1 00 ft 2 ) BHR AHR at 77oF AHR at 150oF AHR at 250oF AHR at 300oF Figure 20: GS at 10 s of 12 ppg OBM samples with and with- out nanosilica Figure20 shows the 10 s GSs of the 12 ppg OBM sam- ples with and without nanosilica both before and after ag- ing. The range of the 10 s GS recommended by previous researchers [56; 57] is from 4 to 10 lb/100 sq ft. From Figure20, under the conditions of before aging and low temperature aging in the temperature range from 77◦F to 150 ◦F, an increase in the nanosilica concentration generally increase the 10 s GS of the samples. However, only a few samples are in the ideal 10 s GS range, while the others ex- hibits undesirable 10 s GSs higher than the ideal range. The ideal samples include the basic mud sample, 0.5 ppb nanosil- ica OBM sample and 1.0 ppb nanosilica OBM sample before aging. After aging at a high temperature range of 250◦F to 300 ◦F, the OBM samples exhibit a different pattern, in which the 10 s GSs of the basic muds are actually reduced by the ad- dition of nanosilica. This decrease might be due to the solid particles in the basic mud samples undergoing degradation at higher temperature, while the mud samples with nanosilica have lower 10 s GSs because of the protective effect from the nanosilica [75]. The 0.5 ppb nanosilica OBM samples for each aging temperature condition are within the ideal range. At lower temperature conditions (77◦F to 150◦F), an in- crease in the nanosilica concentration linearly increasesthe 10 s GS because of the increase in the solid content within the mud system. However, at higher temperature conditions (250 ◦F to 300◦F), the basic mud samples exhibit the high- est values of the 10 s GS. The presence of nanosilica protects the mud particles, hence resulting in lower 10 s GSs. With increasing nanosilica concentration, the solid content and the 10 s GSs increase. Page 14 of 42 Acc ep te d M an us cr ip t COLLOIDS AND SURFACES A: PHYSICOCHEMICAL AND ENGINEERING ASPECTS 14 3.1.13.GS at 10 m of 12 ppg OBM Samples with and without Nanosilica Basic OBM OBM with 0.5 ppb NS OBM with 1.0 ppb NS OBM with 1.5 ppb NS Mud Sample 0 5 10 15 20 25 30 G el s St re ng th ,(l b/ 10 0f t2 ) BHR AHR at 77oF AHR at 150oF AHR at 250oF AHR at 300oF Figure 21: GS at 10 m of 12 ppg OBM samples with and without nanosilica Figure 21 demonstrates the 10 m GSs of 12 ppg OBM samples with and without nanosilica both before and after ag- ing. The range of the 10 s GS recommended by previous re- searchers [56; 57] is from 4 to 15 lb/100 sq ft. Figure21 shows that the 10 m GS has a similar pattern to the 10 s GS. Before aging and after aging at low temper- ature, the 10 m GS exhibits an increase with the addition of nanosilica and with increasing nanosilica concentration.Un- fortunately, only two samples are within the recommended specifications: the basic mud sample and the OBM sample with 0.5 ppb nanosilica before aging. After aging at a higher temperature ranging from 250◦F to 300◦F, a similar trend to that exhibited by the 10 s GS is observed because the basic mud samples generally have higher 10 m GS values, which may result from flocculation. Only one sample is within the ideal specification, the OBM sample with 0.5 ppb nanosilica aged at 300◦F. Basic OBM OBM with 0.5 ppb NS OBM with 1.0 ppb NS OBM with 1.5 ppb NS Concentration of nanosilica (ppb) 0 2 4 6 8 10 12 14 16 18 G el s St re ng th ,(l b/ 10 0f t2 ) BHR 10s BHR 10m (a) Basic OBM OBM with 0.5 ppb NS OBM with 1.0 ppb NS OBM with 1.5 ppb NS Concentration of nanosilica (ppb) 0 5 10 15 20 25 G el s St re ng th ,(l b/ 10 0f t2 ) 10s AHR at 77oF 10m AHR at 77oF (b) Basic OBM OBM with 0.5 ppb NS OBM with 1.0 ppb NS OBM with 1.5 ppb NS Concentration of nanosilica (ppb) 0 5 10 15 20 25 30 G el s St re ng th ,(l b/ 10 0f t2 ) 10s AHR at 150oF 10m AHR at 150oF (c) Basic OBM OBM with 0.5 ppb NS OBM with 1.0 ppb NS OBM with 1.5 ppb NS Concentration of nanosilica (ppb) 0 5 10 15 20 25 G el s St re ng th ,(l b/ 10 0f t2 ) 10s AHR at 250oF 10m AHR at 250oF (d) Basic OBM OBM with 0.5 ppb NS OBM with 1.0 ppb NS OBM with 1.5 ppb NS Concentration of nanosilica (ppb) 0 5 10 15 20 25 G el s St re ng th ,(l b/ 10 0f t2 ) 10s AHR at 300oF 10m AHR at 300oF (e) Figure 22: Comparison of 10 s and 10 m GSs of 12 ppg OBM samples with different aging temperatures: (a) before aging, (b) after aging at 77◦F, (c) after aging at 150◦F, (d) after aging at 250◦F and (e) after aging at 300◦F Page 15 of 42 Acc ep te d M an us cr ip t COLLOIDS AND SURFACES A: PHYSICOCHEMICAL AND ENGINEERING ASPECTS 15 Figure22shows the comparison of the 10 s and 10 m GSs of each set of 12 ppg OBM samples before aging and after aging at their respective temperature. The aim of this com- parison is to investigative if a particular OBM sample has an appreciable range of differences between its 10 s and 10 m GSs to ensure the formation of a progressive gel for cuttings suspension. Although some of the 10 s and 10 m GSs are not within the ideal specifications, appreciable differences still exist be- tween the 10 s and 10 m GSs of each sample. This result signifies that progressive gels are successfully formed, which is a very desirable property for any mud sample. An increase in temperature generally increases the 10 s and 10 m GSs for a sample [92], as can be seen in both Fig- ures20, 21and 22, in which the 10 s and 10 m GSs of the samples after aging at any temperature are higher than those of their respective samples before aging. The effect of the mud weight on the 10 s and 10 m GSs of the OBM samples can also be investigated by comparing the results obtained for the 9 ppg OBM and 12 ppg OBM. Based on the comparison, OBM samples of higher mud weight have higher 10 s and 10 m GS values due to the higher amount of solid content present in the heavy mud system. 3.1.14.HPHT Filtration of 12 ppg OBM Samples with and without Nanosilica 0 0.5 1 1.5 Concentration of Nanosilica (ppb) 0 1 2 3 4 5 6 H P H T F il tr a ti o n L o s s ( c c ) AHR at 250oF AHR at 300oF Figure 23: HPHT filtration of 12 ppg OBM samples with and without nanosilica Figure23 shows the results of HPHT filtration conducted on the 12 ppg OBM samples with nanosilica concentration from 0.5 ppb to 1.5 ppb aged at a high temperature ranging from 250◦F to 300◦F. The ideal specification for an HPHT filtration test recommended by previous researchers [56; 57] is that the filtrate volume be less than or equal to 10 cc after 30 minutes of testing. In Figure23, a trend is observed in which, with the ad- dition of nanosilica, the filtration volume tends to increase with increasing nanosilica concentration. Nevertheless,all the samples still fulfill the ideal specification of having an HPHT filtration loss of less 10 cc. The OBM samples aged at 250◦F experience a greater fluid loss when a higher concentration of nanosilica is used as an additive. The basic mud sample only experiences a fluid loss of 2.0 cc, while the samples with 0.5 ppb, 1.0 ppb and 1.5 ppb nanosilica experience losses of 2.6 cc, 3.4 cc and 3.8 cc, respectively. A similar pattern is observed for the samples aged at 300 ◦F, in which the basic mud sample has the lowest filtration loss value of only 3.2 cc, while the other samples with nanosilica have greater fluid loss compared to the basic mud. The 0.5 ppb, 1.0 ppb and 1.5 ppb nanosilica-enhanced muds experi- ence fluid losses of 3.8 cc, 4.8 cc and 5.2 cc, respectively. A comparison between the HPHT filtration losses of the 9 ppg and 12 ppg OBM samples is performed to study the impact of the mud weight on the fluid loss of the mud sam- ples. From the comparison, the filtration losses experienced by the 12 ppg OBM samples are generally higher than those by the 9 ppg OBM samples. This higher loss occurs because the 12 ppg OBM samples contain a higher solid content and have higher PV values compared to the 9 ppg samples. The comparison results verify the theory of Salih (2017) [15] that the flocculation effect is more severe in samples with a higher solid content, as flocculation creates a pathway that favorsa higher fluid loss. 3.1.15.Mud Cake Thickness of 12 ppg OBM Samples with and without Nanosilica Basic OBM OBM with 0.5 ppb NS OBM with 1.0 ppb NS Mud Sample 0 1 2 3 4 5 M u d c a k e t h ic k n e s s ,( /3 2 i n c h ) AHR at 250oF AHR at 300oF Figure 24: Mud cake thickness of 12 ppg OBM samples with and without nanosilica Figure24 shows the mud cakes thicknesses of the 12 ppg OBM samples tested in the HPHT filtration test, which were aged at the temperatures of 250◦F and 300◦F. From Figure24, the thickness of the mud cakes obtained is directly proportional to the filtration loss experiencedby the OBM samples. The thickest mud cakes of 4/32 inches are obtained for the samples aged at 300◦F with 1.0 ppb and 1.5 ppb nanosilica concentrations. Meanwhile the thinnest mud Page 16 of 42 Acc ep te d M an us cr ip t COLLOIDS AND SURFACES A: PHYSICOCHEMICAL AND ENGINEERING ASPECTS 16 cake of only 2/32 inches is obtained for the basic mud sample aged at 250◦F. Increases in the temperature and mud weight increase the filtration loss of the OBM samples tested; hence, the mud cake thickness is indirectly affected since it is only directly impacted by the fluid loss volume of the studied samples. 3.1.16.Lubricity of 12 ppg OBM Samples with and without Nanosilica 0 0.5 1 1.5 Concentration of Nanosilica (ppb) 0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2 A b s o lu te C o e ff ic ie n t o f fr ic ti o n , C o F BHR AHR at 77oF AHR at 15oF AHR at 250oF AHR at 300oF Figure 25: CoF of 12 ppg OBM samples with and without nanosilica 0 0.5 1 1.5 Concentration of Nanosilica (ppb) 0 1 2 3 4 5 6 7 8 9 R el at iv e C o ef fi ci en t o f fr ic ti o n , C o F ( % ) BHR AHR at 77oF AHR at 15oF AHR at 250oF AHR at 300oF Figure 26: Relative CoF reduction of 12 ppg OBM samples with and without nanosilica Figure 25 demonstrates the CoF values of the 12 ppg OBM samples with and without nanosilica both before and after aging, while Figure26 displays the CoF reductions of every nanosilica-enhanced sample relative to its basic mudat the respective condition. From Figure25, a pattern of decreasing CoF with increas- ing nanosilica concentration is obtained, both before aging and after aging. This result implies that the 1.5 ppb nanosil- ica muds can provide the lowest CoF values regardless of the condition. For instance, with the usage of 1.5 ppb nanosilica, before aging, a 2.2% relative CoF reduction is obtained. The high- est relative reduction among all the samples is exhibited by the 1.5 ppb nano-mud post-77◦F aging, with a relative CoF reduction of up to 8%. When the OBM samples are aged at 150◦F, 250◦F and 300◦F, the 1.5 ppb nanosilica muds exhibit relative CoF reductions of 5.2%, 6.9% and 7.2%, respectively. 3.1.17.Rheological Modeling of OBM Samples with and with- out Nanosilica 0 20 40 60 80 100 120 140 160 180 200 Shear rate (s-1) 0 0.5 1 1.5 2 2.5 3 3.5 4 S h e a r s tr e s s ( P a ) Measurement of 9pp OBM -basic mud Newtonian Bingham Ostwald-deWaele Power Law Herschel-Bulkley Figure 27: Rheological modeling of 9 ppg OBM basic mud 0 20 40 60 80 100 120 140 160 180 200 Shear rate (s-1) 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 S h e a r s tr e s s ( P a ) Measurement of 9pp OBM -basic mud Newtonian Bingham Ostwald-deWaele Power Law Herschel-Bulkley Figure 28: Rheological modeling of 9 ppg OBM with 0.5 ppb nanosilica Rheological modeling is often conducted to investigate which rheological model best describes the selected drilling fluid sample A few rheological models are obtained, and the shear stress and shear rate measurements of the studied mud samples are compared with the parameters generated by all the rheological models. Trend lines are generated from both Page 17 of 42 Acc ep te d M an us cr ip t COLLOIDS AND SURFACES A: PHYSICOCHEMICAL AND ENGINEERING ASPECTS 17 the studied mud samples and rheological models, and com- parison and matching of the lines are performed in order to determine which law of fluids the mud samples follow. The deviation error is used to help identify the correct rheological model. The selected samples for this rheological modeling are the basic mud of each mud weight as well as the mud sam- ples that contain 0.5 ppb nanosilica because, overall, the 0.5 ppb nanosilica OBM samples have performed optimally in terms of almost all the rheological properties investigated. The types of rheological models investigated are the Newto- nian [9], Bingham [93], Herschel–Bulkley [94] and Ostwald power law models [95]. The results obtained from the modeling of the 9 ppg OBM basic mud sample are tabulated and plotted in Table8 and Fig- ure27, respectively. From Figure27, the largest deviation is observed for the Newtonian model, with an error of 9.53%. Additionally, the trend line for the 9 ppg OBM basic mud most closely matches that of the Herschel–Bulkley model, which is confirmed by the deviation error of only 1.16%. Hence, the Herschel– Bulkley [94] model is the most suitable model to describe the 9 ppg basic OBM sample. The modeling results of the 9 ppg OBM sample with 0.5 ppb nanosilica are tabulated and plotted in Table9 and Fig- ure28, respectively. Table 8: Modelled equations for 9 ppg OBM basic mud Model Equation Parameters Error (%) τo,τy k n µp,µ Newtonian [9] τ = 0.0193γ 0.0193 9.53 Bingham [93] τ = 0.0112·γ +1.2286 0.0112 2.94 1.2286 HerschelBulkley [94] τ = 1.0869+ 0.0426(·γ0.7483) 1.0869 0.0426 0.7483 1.16 OstwaldPower Law [95] τ = 1·γ0.1988 1 0.1988 3.81 Table 9: Modelled equations for 9 ppg OBM with 0.5 ppb of nanosilica Model Equation Parameters Error (%) τo,τy k n µp,µ Newtonian [9] τ = 0.0201γ 0.0201 5.98 Bingham [93] τ = 0.0159·γ +0.642 0.642 0.0159 3.69 HerschelBulkley [94] τ = 0.7458+ 0.0319(·γ0.8502) 0.7458 0.0319 0.8502 0.84 OstwaldPower Law [95] τ = 1·γ0.1948 1 0.1948 6.70 Table 10: Modelled equations for 12 ppg OBM basic mud Model Equation Parameters Error (%) τo,τy k n µp,µ Newtonian [9] Undefined Bingham [93] τ = 0.0239·γ +3.3252 3.3252 0.0239 7.17 HerschelBulkley [94] τ = 3.7762 + 0.0109(·γ0.1.1261) 3.7762 0.0109 1.1261 1.00 OstwaldPower Law [95] τ = 2.0544·γ0.2339 2.0544 0.2339 6.53 Page 18 of 42 Acc ep te d M an us cr ip t COLLOIDS AND SURFACES A: PHYSICOCHEMICAL AND ENGINEERING ASPECTS 18 Table 11: Modelled equations for 12 ppg OBM with 0.5 ppb of nanosilica Model Equation Parameters Error (%) τo,τy k n µp,µ Newtonian [9] Undefined Bingham [93] τ = 0.024·γ +3.4522 3.4522 0.024 7.46 HerschelBulkley [94] τ = 3.9418 + 0.0096(·γ1.1497) 3.9418 0.0096 1.1497 1.10 OstwaldPower Law [95] τ = 2.1514·γ0.229 2.1514 0.229 6.54 As illustrated in Figure28 and Table9, the highest de- viation is observed for the Ostwald model [95], with a de- viation error of nearly 7%, while the other models exhibit deviation errors of greater than 3% except for the Herschel– Bulkley [94] model. The error of this model is only 0.84%, making it the most appropriate rheological model for the 9 ppg OBM sample with 0.5 ppb nanosilica. The modeling results of the 12 ppg OBM basic mud are tabulated and plotted in Table10and Figure29, respectively. As demonstrated in Figure29, the Newtonian model is undefined in the modeling test of the 12 ppg OBM basic mud sample, which is in line with the theory that drilling fluids do not behave like Newtonian fluids (Forthun, 2016 [96]). According to the figure, the trend line of the mud sample matches the lines of both the Bingham plastic model [93] and Herschel–Bulkley model [94]; however, the deviation er- ror percentage from Table10 indicates that the 12 ppg ba- sic mud sample behaves according to the Herschel–Bulkley model [94], with an error value of only 1%. 0 20 40 60 80 100 120 140 160 180 200 Shear rate (s-1) 0 1 2 3 4 5 6 7 8 9 S h e a r s tr e s s ( P a ) Measurement of 12 ppg OBM -basic mud Bingham Herschel-Bulkley Ostwald-deWaele Power Law Figure 29: Rheological modeling of 12 ppg OBM basic mud The modeling results of the 12 ppg OBM containing 0.5 ppb nanosilica are tabulated and plotted in Table11 and Fig- ure30, respectively. A similar pattern is observed in Figure30: the Newtonian model is also undefined, and the trend line of the 12 ppg OBM with 0.5 ppb nanosilica again falls between the trend lines of the Bingham plastic [93] and Herschel–Bulkley [94] models. Nevertheless, the deviation error from Table11 indicates that the most accurate model to describe the 12 ppg OBM with 0.5 ppb nanosilica is the Herschel–Bulkley [94] model, with an error value of only 1.1%. 0 20 40 60 80 100 120 140 160 180 200 Shear rate (s-1) 0 1 2 3 4 5 6 7 8 9 S h e a r s tr e s s ( P a ) Measurement of 12 ppg OBM -basic mud Bingham Herschel-Bulkley Ostwald-deWaele Power Law Figure 30: Rheological modeling of 12 ppg OBM with 0.5 ppb nanosilica 3.2.Rheological Properties of WBM Samples Table 12: Recommended rheological properties for WBM at low temperature conditions (SCOMI Oil Tools,2017 [97]) Rheological property Specification Plastic viscosity, cp < 30 Yield point, lb/100 sq. ft < 50 Gel strength (10 second) < 15 Gel strength (10 minutes) < 35 API fluid loss, cc/30 min < 15 API mud cake thickness, /32 inch <3 The rheological properties investigated for WBM are the PV, apparent viscosity, YP, gel strength of each mud sample at both 10 seconds and 10 minutes, API filtration and HPHT filtration. Mud cake thickness and lubricity tests are also con- ducted, and rheological models are obtained. Page 19 of 42 Acc ep te d M an us cr ip t COLLOIDS AND SURFACES A: PHYSICOCHEMICAL AND ENGINEERING ASPECTS 19 Since WBMs of two mud weights are studied, two differ- ent guidelines, i.e., Table12 and Table13, are obtained from SCOMI Oil Tools,(2017) [97] as benchmark specifications to evaluate the performance of all the formulated WBM samples. Table 13: Recommended rheological properties for WBM at high temperature conditions (SCOMI Oil Tools, 2017 [97]) Rheological property Specification Plastic viscosity, cp < 30 Yield point, lb/100 sq. ft 10 - 25 Gel strength (10 second) < 15 Gel strength (10 minutes) < 35 API fluid loss, cc/30 min < 15 API mud cake thickness, /32 inch <3 HPHT fluid loss, cc/30 min 25 HPHT mud cake thickness, /32 inch < 10 3.2.1.PV of 9 ppg WBM Samples with and without Nanosilica Figure31 represents the PVs of the 9 ppg WBM samples with and without nanosilica before and after aging at their respective aging temperatures. The recommended range for PVs is less than or equal to 30 cp at both low and high tem- perature conditions. 0 0.5 1 1.5 Concentration of Nanosilica (ppb) 0 5 10 15 20 25 30 35 40 45 P la s ti c V is c o s it y ( c p ) BHR AHR at 77oF AHR at 15oF AHR at 250oF AHR at 300oF Figure 31: PV of 9 ppg WBM samples with and without nanosilica As shown in Figure31, almost all the samples satisfy the ideal PV for a mud except the basic mud sample aged at 150 ◦F. The addition of nanosilica to 9 ppg WBM decreases the PV values, and the reduction of PV increases as the nanosilica concentration increases up to 1.0 ppb. This trend is the same for all conditions, both before and after hot rolling. Afterthe nanosilica concentration reaches 1.0 ppb, a further increase of the concentration does not further increase the reduction but instead brings the PV value closer to that of the basic mud for each condition. The trend of the impact of nanosilica can be explained by the fact that the nanosilica particles are well dispersed around and between the bentonite platelets and cation exchange oc- curs in which the negatively charged nanosilica neutralizes the positive charge on the bentonite surfaces, hence converting them to negatively charged bentonite platelets overall. This negative charge strengthens the repulsive force among ben- tonite platelets and leads to an increase in the distance among all the bentonite particles; as a result, less friction is experi- enced [15]. Additionally, nanosilica acts as ball bearings by lubricating the surface of the bentonite particles, thus ensur- ing the decrease in the frictional force and thereby giving rise to a reduction of the PV. For example, before hot rolling, the PV value of the ba- sic mud is reduced from 15 cp to 13 cp with the usage of 1.0 ppb nanosilica. Moreover, after hot rolling at the tempera- tures of 77◦F, 150◦F, 250◦F and 300◦F, the addition of 1.0 ppb nanosilica reduces the PV relative to that of the respective WBM basic mud sample, from 28 to 15 cp, from 31 to 21 cp, from 22 to 18 cp and from 22 cp to 15 cp, respectively. On the other hand, an increase in the aging temperature clearly results in a generally lower PV value compared to those obtained for the WBM samples that have undergone low temperature aging. This result occurs because at a higher temperature, the kinetic energy of the particles, especially the bentonite particles, is higher; hence, they tend to move faster and more vigorously [63], which leads to a large increase in the distance among the bentonite particles and a lower PV. The PV should be kept low, as this favors a higher ROP and better cooling and lubrication of the downhole equipment, saves energy in mud circulation and reduces the chances of mud circulation loss to the formation fractures formed by an excessive equivalent circulation density of the mud. 3.2.2.YP of 9 ppg WBM Samples with and without Nanosilica Figure 32 represents the YPs of the 9 ppg WBM sam- ples with and without nanosilica before and after hot rolling at their respective aging temperature. The ideal YP suggested by Scomi Oil Tools [97] is below 50 lb/100 sq ft for low tem- perature conditions and between 10 and 25 lb/100 sq ft for high temperature conditions. As observed from Figure32, at the low temperature con- ditions, almost all YP values of the samples are within the rec- ommended range, except for those of the basic mud samples hot rolled at 77◦F and 150◦F, which are 55 lb/100 sq ft and 52 lb/100 sq ft, respectively. On the other hand, the samples aged at high temperature conditions show a good performance by satisfying the ideal range. With the addition of nanosilica, the YP of WBM decreases with increasing nanosilica concentration up to 1.0 ppb; at this concentration, all the WBM samples exhibit the lowest YP value for their respective condition. A further increase ofthe nanosilica concentration only brings the YP value closer to that of the respective basic mud. The YP is closely related to the PV because the YP largely depends on both the electrochemical attractive and frictional forces, which are easily affected by the distance of the ben- tonite particles [15]. Hence, since the usage of nanosilica in- creases the distance among the bentonite particles, a lowerPV is obtained, as is a lower YP. Since the YP of a WBM is directly associated with its PV, the effect of temperature on the YP is similar to that on the Page 20 of 42 Acc ep te d M an us cr ip t COLLOIDS AND SURFACES A: PHYSICOCHEMICAL AND ENGINEERING ASPECTS 20 PV. 0 0.5 1 1.5 Concentration of Nanosilica (ppb) 0 10 20 30 40 50 60 Y ie ld P o in t (l b /1 0 o ft 2 ) BHR AHR at 77oF AHR at 15oF AHR at 250oF AHR at 300oF Figure 32: YP of 9 ppg WBM samples with and without nanosilica 3.2.3.Gel Strength at 10 s of 9 ppg WBM Samples with and without Nanosilica Basic OBM OBM with 0.5 ppb NS OBM with 1.0 ppb NS OBM with 1.5 ppb NS Mud Sample 0 5 10 15 20 25 30 G e ls S tr e n g th ,( lb /1 0 0 ft 2 ) BHR AHR at 77oF AHR at 150oF AHR at 250oF AHR at 300oF Figure 33: GS at 10 s of 9 ppg WBM samples with and with- out nanosilica Figure33represents the 10 s GSs of the 9 ppg WBM sam- ples with and without nanosilica before and after hot rolling at a temperature ranging from 77◦F to 300◦F. The suggested range for the 10 s GS of a WBM is less than or equal to 15 lb/100 sq ft for the samples before aging, the samples aged at low temperature conditions and those that have undergone hot rolling at high temperature conditions. Referring to Figure33, of all the samples, only two are within the range of the suggested specifications for the 10 s GS: the WBM samples containing 1.0 and 1.5 ppb nanosilica hot rolled at 150◦F. Conversely, all the WBMs that have undergone hot rolling at high temperature are within the recommended range. An increase in temperature extensively decreases the 10 s GS, as the gel strength is also a parameter that depends on the PV of a mud sample. Generally, an increase in the nanosilica concentration de- creases the 10 s GS of the WBM, as the usage of nanosilica can enable the development of a fragile gel [15]. A fragile gel is required, as it enables the mud system to be effortlessly resumed and to suspend cuttings as well as enables the easy removal of cuttings at the shale shakers. 3.2.4.Gel Strength at 10 m of 9 ppg WBM Samples with and without Nanosilica Basic OBM OBM with 0.5 ppb NS OBM with 1.0 ppb NS OBM with 1.5 ppb NS Mud Sample 0 10 20 30 40 50 60 G e ls S tr e n g th ,( lb /1 0 0 ft 2 ) BHR AHR at 77oF AHR at 150oF AHR at 250oF AHR at 300oF Figure 34: GS at 10 m of 9 ppg WBM samples with and with- out nanosilica Figure34displays the 10 m GSs of the 9 ppg WBM sam- ples with and without nanosilica both before and after aging. The ideal range for the 10 m GS of a WBM sample is less than or equal to 35 lb/100 sq ft. As demonstrated in Figure34, only two samples for the low temperature conditions manage to satisfy the recom- mended range: the samples with 1.0 ppb and 1.5 ppb nanosil- ica aged at 150◦F. Fortunately, all the samples that have un- dergone hot rolling at 250◦F and 300◦F successfully fulfill the specifications. A fragile gel is desired for the instanta- neous resumption of mud circulation. The lowest 10 m GS is obtained when 1.0 ppb nanosilica is used. However, a further increase in the nanosilica concentration results in an increase in the 10 m GS. This result occurs because an increase of the nanosilica concentration beyond the optimum concentration will result in nanosilica agglomeration. Page 21 of 42 Acc ep te d M an us cr ip t COLLOIDS AND SURFACES A: PHYSICOCHEMICAL AND ENGINEERING ASPECTS 21 Basic OBM OBM with 0.5 ppb NS OBM with 1.0 ppb NS OBM with 1.5 ppb NS Concentration of nanosilica (ppb) 0 10 20 30 40 50 G el s St re ng th ,(l b/ 10 0f t2 ) BHR 10s BHR 10m (a) Basic OBM OBM with 0.5 ppb NS OBM with 1.0 ppb NS OBM with 1.5 ppb NS Concentration of nanosilica (ppb) 0 10 20 30 40 50 G el s St re ng th ,(l b/ 10 0f t2 ) 10s AHR at 77oF 10m AHR at 77oF (b) Basic OBM OBM with 0.5 ppb NS OBM with 1.0 ppb NS OBM with 1.5 ppb NS Concentration of nanosilica (ppb) 0 5 10 15 20 25 30 35 40 45 G el s St re ng th ,(l b/ 10 0f t2 ) 10s AHR at 150oF 10m AHR at 150oF (c) Basic OBM OBM with 0.5 ppb NS OBM with 1.0 ppb NS OBM with 1.5 ppb NS Concentration of nanosilica (ppb) 0 2 4 6 8 10 12 G el s St re ng th ,(l b/ 10 0f t2 ) 10s AHR at 250oF 10m AHR at 250oF (d) Basic OBM OBM with 0.5 ppb NS OBM with 1.0 ppb NS OBM with 1.5 ppb NS Concentration of nanosilica (ppb) 0 1 2 3 4 5 6 7 8 G el s St re ng th ,(l b/ 10 0f t2 ) 10s AHR at 300oF 10m AHR at 300oF (e) Figure 35: Comparison of 10 s and 10 m GSs of 9 ppg WBM samples aged at different temperatures: (a) before aging, (b) after aging at 77◦F, (c) after aging at 150◦F, (d) after aging at 250◦F and (e) after aging at 300◦F Figure35 shows the comparison of the 10 s and 10 m GSs of each sample for each condition in order to inspect the formation of progressive gels, which are fairly vital in ensuring the ability to instantaneously resume mud circulation, suspend cuttings and prevent sagging of barite. Fortunately, almost all the WBM samples manage to form a progressive gel by having an appreciable range between the 10 s and 10 m GSs, except for the mud samples aged at 300◦F that contain 0.5 ppb and 1.0 ppb nanosilica. These two samples developed a low-flat gel instead. A low-flat gel is undesirable, as it would not be able to provide enough strength to suspend cuttings and even result in sagging of barite. Page 22 of 42 Acc ep te d M an us cr ip t COLLOIDS AND SURFACES A: PHYSICOCHEMICAL AND ENGINEERING ASPECTS 22 3.2.5.API Filtration of 9 ppg WBM Samples with and without Nanosilica Figure36illustrates the API filtration of 9 ppg WBM sam- ples with and without nanosilica. This filtration test is con- ducted using an LPLT filter press in order to test the filtration losses of all mud samples before and after aging. The ideal specification is that the fluid loss of a WBM be less than or equal to 15 cc for 30 minutes. Referring to Figure36, a pattern can be noted in which almost all the samples experience a greater fluid loss with the addition of nanosilica, as the fluid loss increases with increas- ing nanosilica concentration. All the samples exhibit the same trend except for those tested before hot rolling, as in this con- dition, the fluid losses experienced by the WBM samples are inversely proportional to an increase of the nanosilica concen- tration [73; 84]. Basic OBM OBM with 0.5 ppb NS OBM with 1.0 ppb NS OBM with 1.5 ppb NS Mud Sample 0 5 10 15 20 25 30 A P I F il tr a ti o n L o s s ,( c c ) BHR AHR at 77oF AHR at 150oF AHR at 250oF AHR at 300oF Figure 36: API filtration of 9 ppg WBM with and without nanosilica All the samples for the conditions of before hot rolling and after aging at 77◦F and 150◦F have fluid losses within the recommended range. Only one samples from the condition of after aging at 250◦F satisfies the specifications suggested: the basic mud sample. The reason for the increase of fluid losses with increas- ing nanosilica concentration is that nanosilica usage increases the distance among bentonite particles [15], which prevents the packing of solid particles on the filter paper to form an impermeable cake to reduce fluid loss. As mentioned by Li et al. (2017) [98], the surfaces of nanosilica are composed of a huge number of hydrophilic groups, and if nanosilica particles pack on a filter paper, they will bind the free water through a surface wetting action. However, this phenomenon does not occur in our case be- cause, as claimed by Salih (2017) [15], the nanosilica parti- cles are distributed among and around the bentonite platelets, increasing the distance among the platelets due to surface re- pulsion; this mechanism thus hinders the packing of solid par- ticles to form an impermeable mud cake to prevent the loss of fluid. Mud cakes formed for the high temperature samples are also not compact due to the dispersion of the bentonite particles. 3.2.6.API Mud Cake Thickness of 9 ppg WBM Samples with and without Nanosilica Figure37 demonstrates the API mud cake thicknesses of the 9 ppg WBM samples with and without nanosilica obtained from the API filtration test. The recommended specification of SCOMI [97] Oil Tools for an API mud cake is that its thick- ness should be less than or equal to 3/32 inches. Basic OBM OBM with 0.5 ppb NS OBM with 1.0 ppb NS OBM with 1.5 ppb NS Mud Sample 0 1 2 3 4 5 6 7 M u d C ak e T h ic kn es s, (/ 32 in ch ) BHR AHR at 77oF AHR at 150oF AHR at 250oF AHR at 300oF Figure 37: API mud cake thickness of 9 ppg WBM with and without nanosilica According to Figure37, a pattern can be found in which the mud cake thickness is closely associated with the API fluid loss experienced by each sample. All samples for low tem- perature conditions are successfully within the recommended range, while only two samples for the high temperature condi- tions are within the range: the basic mud and 0.5 ppb nanosil- ica samples aged at 250◦F. The samples that experienced greater fluid losses have rather thick mud cakes, as the failure of the formation of a thin impermeable cake leads to greater fluid loss. 3.2.7.HPHT Filtration of 9 ppg WBM Samples with and with- out Nanosilica Figure 38 illustrates the HPHT filtration of the 9 ppg WBM samples with and without nanosilica only for the sam- ples that have been aged at a high temperature ranging from 250◦F to 300◦F. The recommended value of the HPHT fluid loss is that it be less than or equal to 25 cc for 30 minutes of filtration. The trend observed from Figure38 is similar to that for API filtration, as an increase in the nanosilica concentration increases the HPHT fluid loss. Despite this trend, almost all fluid losses of the samples are below 25 cc after 30 minutes of HPHT filtration, except for that of the 1.5 ppb nanosilica WBM aged at 300◦F, with a value of 25.2 cc. However, the extra fluid loss of 0.2 cc should be tolerable due to its insignif- Page 23 of 42 Acc ep te d M an us cr ip t COLLOIDS AND SURFACES A: PHYSICOCHEMICAL AND ENGINEERING ASPECTS 23 icance. Basic OBM OBM with 0.5 ppb NS OBM with 1.0 ppb NS OBM with 1.5 ppb NS Mud Sample 0 5 10 15 20 25 30 H P H T F il tr a ti o n L o s s ,( c c ) AHR at 250oF AHR at 300oF Figure 38: HPHT filtration of 9 ppg WBM with and without nanosilica 3.2.8.HPHT Mud Cake Thickness of 9 ppg WBM Samples with and without Nanosilica Basic OBM OBM with 0.5 ppb NS OBM with 1.0 ppb NS OBM with 1.5 ppb NS Mud Sample 0 2 4 6 8 10 12 M u d C a k e T h ic k n e s s ,( /3 2 i n c h ) AHR at 250oF AHR at 300oF Figure 39: HPHT mud cake thickness of 9 ppg WBM with and without nanosilica Figure39portrays the HPHT mud cake thicknesses of the 9 ppg WBM samples with and without nanosilica after the samples have undergone the HPHT filtration test. The ideal range recommended for an HPHT mud cake is that its thick- ness be less than or equal to 10/32 inches. Referring to Figure39, similar to the mud cakes obtained from the API filtration test, the thicknesses of the HPHT mud cakes are directly impacted by the HPHT filtration volume loss. Only two samples do not fulfill the recommended speci- fications, the 1.0 and 1.5 ppb nanosilica samples, as they both have the same HPHT fluid losses of 11 cc after 30 minutes of filtration. 3.2.9.Lubricity of 9 ppg WBM Samples with and without Nanosilica Figure40demonstrates the CoF values of the 9 ppg WBM samples with and without nanosilica both before and after ag- ing, while Figure41 displays the reduction of the CoF of ev- ery nanosilica-enhanced sample relative to its basic mud at the respective condition. 0 0.5 1 1.5 Concentration of Nanosilica (ppb) 0.2 0.25 0.3 0.35 0.4 0.45 0.5 A b so lu te C o ef fi ci en t o f F ri ct io n , C o F BHR AHR at 77oF AHR at 15oF AHR at 250oF AHR at 300oF Figure 40: CoF of 9 ppg WBM samples with and without nanosilica 0 0.5 1 1.5 Concentration of Nanosilica (ppb) 0 1 2 3 4 5 6 7 8 R el at iv e C o ef fi ci en t o f F ri ct io n ( % ) BHR AHR at 77oF AHR at 15oF AHR at 250oF AHR at 300oF Figure 41: Relative CoF reduction of 9 ppg WBM samples with and without nanosilica According to Figure41, an obvious trend can be stated, that an increase in the nanosilica concentration increasesthe reduction of the CoF. This result occurs because the disper- sion of nanosilica enables it to act as ball bearings in lubricat- ing the surface of the equipment, resulting in the reductionof the CoF [15; 89]. The highest relative CoF reduction occurs when the nanosilica concentration for a particular condition is thehigh- est. The relative CoF reduction is the reduction in the CoF of a nano-mud sample relative to the CoF of its basic mud coun- Page 24 of 42 Acc ep te d M an us cr ip t COLLOIDS AND SURFACES A: PHYSICOCHEMICAL AND ENGINEERING ASPECTS 24 terpart. Before aging and after aging at 77◦F, 150 ◦F, 250 ◦F and 300◦F, the 1.5 ppb nanosilica WBM samples exhibit relative reductions of 7.4%, 6%, 5.6%, 3.6% and 3.2%. Interestingly, different from the OBM samples, the WBM samples’ lubricity is impacted by temperature. As seen from Figure41, the relative CoF reduction of each set of WBMs is higher for lower temperature conditions. Alshubbar et al. (2017) [89] mentioned that friction is reduced when nanosil- ica forms a thin film on the metal surface of the lubricity equipment, achieving a better lubricity. Meanwhile, Salih (2017) [15] stated that nanosilica particles are attached to ben- tonite particles in the mud systems, and the particles repel each other due to repulsion forces. At higher temperatures, according to kinetic theory [63], the bentonite particles are even further apart, which in turn leads to the inefficiency of nanosilica in forming a thin film that can cover the entire metal surface. Increasing the nanosilica concentration ata particular temperature condition enables more nanosilicato participate in the formation of a thin film lubricating the metal surface of the lubricity equipment. 3.2.10.PV of 12 ppg WBM Samples with and without Nanosil- ica Figure42displays the PVs obtained for the 12 ppg WBM samples with and without nanosilica both before and after ag- ing at their respective aging temperature. The ideal range pro- posed by SCOMI [97] Oil Tools is that the PV of a WBM sample be less than or equal to 30 cp. 0 0.5 1 1.5 Concentration of Nanosilica (ppb) 20 25 30 35 40 45 50 55 60 65 70 P la st ic V is co si ty ( cp ) BHR AHR at 77oF AHR at 15oF AHR at 250oF AHR at 300oF Figure 42: PV of 12 ppg WBM samples with and without nanosilica According to Figure42, only three samples satisfy the recommended specifications: the samples containing 1.0 ppb nanosilica before aging and after aging at 250◦F and 300◦F, with PV values of 27, 30 and 28 cp. The effects of temperature and nanosilica concentration on the PV of the 12 ppg WBM are similar to those on the 9 ppg WBM, as increases in the temperature and nanosilica concentration will both decrease the PV values. The effect of mud weight, in contrast, results in higher PV values. This re- sult is due to the higher solid content in a heavier mud, which leads to a decrease in the distance among bentonite particles and an increase in the frictional force [91]. 3.2.11.YP of 12 ppg WBM Samples with and without Nanosil- ica Figure43 shows the YPs obtained for the 12 ppg WBM samples with and without nanosilica before and after hot rolling. The recommended range for the YP of a WBM is less than or equal to 50 lb/100 sq ft for before hot rolling and low temperature hot rolling conditions and between 10 and 25 lb/100 sq ft for high temperature hot rolling conditions. 0 0.5 1 1.5 Concentration of Nanosilica (ppb) 0 20 40 60 80 100 120 140 160 Y ie ld P o in t (l b /1 00 ft 2 ) BHR AHR at 77oF AHR at 15oF AHR at 250oF AHR at 300oF Figure 43: YP of 12 ppg WBM samples with and without nanosilica As displayed in Figure43, only four samples are within the recommended range, and they are all samples that have been aged at high temperatures: the 1.0 ppb and 1.5 ppb nanosilica WBMs hot rolled at 250◦F and 300◦F. After 250 ◦F and 300◦F hot rolling, the YPs obtained for the 1.0 and 1.5 ppb nanosilica WBM are 18 and 22 lb/100 sq ft, respectively. The 1.0 ppb nanosilica sample shows the greatest reduction in the YP relative to its basic mud at all conditions. However, the samples before aging and after aging at low temperature exhibit rather high YPs, which are very undesir- able as a high YP leads to flocculation, decreases in the ROP and escalation of the surge and swap pressure. As mentioned earlier, the YP is a parameter that is closely related to the PV, and the YPs are higher for the 12 ppg WBMs because heavier muds have a higher solid content and a higher solid content will results in a higher PV and hence a higher YP [15]. 3.2.12.GS at 10 s of 12 ppg WBM Samples with and without Nanosilica Figure44 demonstrates the 10 s GSs of the 12 ppg WBM samples with and without nanosilica before and after aging. The ideal 10 s GS that a WBM should have is less than or Page 25 of 42 Acc ep te d M an us cr ip t COLLOIDS AND SURFACES A: PHYSICOCHEMICAL AND ENGINEERING ASPECTS 25 equal to 15 lb/100 sq ft. Basic OBM OBM with 0.5 ppb NS OBM with 1.0 ppb NS OBM with 1.5 ppb NS Mud Sample 0 10 20 30 40 50 60 70 G e ls S tr e n g th ,( lb /1 0 0 ft 2 ) BHR AHR at 77oF AHR at 150oF AHR at 250oF AHR at 300oF Figure 44: GS at 10 s of 12 ppg WBM samples with and with- out nanosilica As shown in Figure44, 6 samples are within the ideal specifications suggested. These samples are all nanosilica- enhanced WBM samples aged at high temperature. For the samples that have undergone aging at 250◦F, the 10 s GSs displayed by the 0.5 ppb, 1.0 ppb and 1.5 ppb nanosilica sam- ples are 15, 14 and 14 lb/100 sq ft, respectively. As for those aged at 300◦F, the 0.5 ppb, 1.0 ppb and 1.5 ppb nanosilica samples have 10 s GSs of 12 lb/100 sq ft, 10 lb/100 sq ft and 10 lb/100 sq ft, respectively. The concentration and temperature effects on the 12 ppg 10 s GS are similar to those on the 9 ppg 10 s GS. As for the impact of mud weight, since the GS of a drilling mud is closely associated with both its PV and YP, a higher PV mud sample will have a higher GS due to the higher solid content. 3.2.13.GS at 10 m of 12 ppg WBM Samples with and without Nanosilica Figure45 illustrates the 10 m GSs of 12 ppg WBM sam- ples with and without nanosilica both before aging and after aging. The recommended specification for the 10 m GS is that it be less than or equal to 35 lb/100 sq ft for both high temperature and low temperature conditions. Basic OBM OBM with 0.5 ppb NS OBM with 1.0 ppb NS OBM with 1.5 ppb NS Mud Sample 0 20 40 60 80 100 120 140 G e ls S tr e n g th ,( lb /1 0 0 ft 2 ) BHR AHR at 77oF AHR at 150oF AHR at 250oF AHR at 300oF Figure 45: GS at 10 m of 12 ppg WBM samples with and without nanosilica As portrayed in Figure45, all the samples aged at high temperature successfully satisfy the recommended specifica- tion. For these samples, an increase of the nanosilica con- centration increases the reduction of the 10 m GS of a mud sample relative to its basic mud. The 10 m GSs exhibited by the basic mud and 0.5 ppb, 1.0 ppb and 1.5 ppb nanosilica samples aged at 250◦F are 35 lb/100 sq ft, 28 lb/100 sq ft, 25 lb/100 sq ft and 19 lb/100 sq ft. Meanwhile, the basic mud and 0.5 ppb, 1.0 ppb and 1.5 ppb nanosilica samples that have undergone aging at 300◦F have 10 m GSs of 32 lb/100 sq ft, 28 lb/100 sq ft, 21 lb/100 sq ft and 22 lb/100 sq ft. The effects of nanosilica concentration, temperature and mud weight are similar to those for the 10 s GS. A comparison is performed between the 10 s and 10 m GSs of each sample in order to examine the formation of a progressive gel, which is relatively important in enablingthe instant resumption of mud circulation as well as suspension of cut