Poly(2-acrylamide-2-methylpropanesulfonic acid) - American

Dec 20, 2016 - Polymers Research Group, Institute for Frontier Materials, Deakin University, Locked Bag 20000, Geelong, Victoria 3220, Australia. §...
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Poly (2-acrylamide-2-methylpropanesulfonic acid) modified SiO2 nanoparticles for water based drilling fluids Yuanpeng Wu, Zhihao Wang, Zhu Yan, Tao Zhang, Yang Bai, Pingquan Wang, Pingya Luo, Shaohua Gou, and Qipeng Guo Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b03450 • Publication Date (Web): 20 Dec 2016 Downloaded from http://pubs.acs.org on December 24, 2016

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Poly (2-acrylamide-2-methylpropanesulfonic acid) modified SiO2 nanoparticles for water based muds Yuanpeng Wu,1,2* Zhihao Wang,1 Zhu Yan,1 Tao Zhang,4 Yang Bai,2 Pingquan Wang,2 Pingya Luo,2 Shaohua Gou,2 Qipeng Guo3* 1

The Center of New Energy Materials and Technology, School of Materials Science and Engineering,

Southwest Petroleum University, Chengdu, Sichuan Province 610500, PR China; 2

State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation (Southwest Petroleum

University), Chengdu, Sichuan Province 610500, PR China; 3

Polymers Research Group, Institute for Frontier Materials, Deakin University, Locked Bag 20000,

Geelong, Victoria 3220, Australia; 4

Department of Materials Science and Engineering, Technion–Israel Institute of Technology, Haifa,

32000, Israel.

Abstract: We report the fabrication of poly(2-acrylamide-2-methylpropanesulfonic acid) (PAMPS) modified SiO2 (SiO2/PAMPS) nanoparticles as plugging agents in water based muds (WMDs). SiO2/PAMPS nanoparticles were prepared by copolymerization of 3-methacryloxy propyltrimethoxysilane functionalized SiO2 nanoparticles and 2-acrylamide-2-methylpropanesulfonic acid. These SiO2/PAMPS nanoparticles show excellent salt tolerance, thermal stability and good compatibilities with commonly used polymeric additives in WMDs. The plugging capabilities of the nanoparticles were tested by filter cakes on high temperature high pressure filtration press. The results indicate that SiO2/PAMPS nanoparticles can effectively block the nano-pores in filter cake. And the WMDs containing SiO2/PAMPS nanoparticles exhibit excellent plugging property both at ambient temperature and 80°C, demonstrating that these nanoparticles are promising plugging agents in WMDs for drilling in shale formations. Keywords: SiO2 nanoparticles; poly(2-acrylamide-2-methylpropanesulfonic acid); plugging agents; fluid loss

1. Introduction 

Correspondence author: Yuanpeng Wu, E-mail: [email protected]; Pingquan Wang, E-mail:

[email protected]; Fax: +86 28 83037409, Tel: +86 28 83037409. Qipeng Guo, E-mail: [email protected], Fax: +61 3 5227 1103, Tel: +61 3 5227 2802. 1

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Drilling fluids play an important role in oil and gas drilling operations. Their mainly functionalization include carrying and suspending cuttings, stabilizing wellbore, avoiding formation collapse, cooling and lubricating drilling pipes and bits.1 Drilling fluids are classified into three categories: water based mud (WMD), oil based drilling fluids and synthetic-based drilling fluids. WMD is widely used due to their advantages of cost effective, environment-friendly and safety.2 WMD is typically composed of water, clay, drilling fluid additives such as fluid loss controller and plugging materials. These additives are used to prevent fluid penetration invading from WMD into shale formations which may lead to wellbore instability and formation collapse.1 Recently, developments of unconventional energy including shale gas and shale oil have attracted much attention due to the dramatically growing consumption and relatively shortage of the conventional oil and gas. Usually, shale gas and shale oil mainly stored in shale formations. Different from traditional sandstone or carbonate reservoirs, shale formations are featured with size of holes in nanometer scale. That means the pores in shale formations are much smaller than those in traditional formations whose size probably in micrometer or millimeter scale.3 While drilling in shale formation for developing shale gas, WMD is optimal drilling fluid because of their advantages. However, it is known that water penetration from WMD into shale formations results in swelling and subsequent wellbore instability. The best possible method of avoiding these failures is to prevent contact of shale with water in drilling mud. One of the effective ways is to seal off the pores in shale formations which are drilled.4 When drilling in shale formation, conventional plugging materials with size in micrometer or millimeter scale are not effective for plugging the pores of wellbore because the size of the holes in shale formation is smaller than other formations.5 The size of these holes is mainly in nanometer scale. For obtaining better plugging performance when WMDs are used in shale formations, smaller plugging materials such as nano-plugging materials are inquired.6 Nanotechnology has been explored and developed for improving the materials’ performances in many areas, such as paints and coatings, electronics, medical, cosmetics, energy materials and etc.7 Nanoparticles with size from 1 to 100 nanometers are promising materials for oil and gas industry. Due to their unique properties, nanoparticles will find as important and potential candidates for applying in WMDs. Nowadays, several kinds of nanoparticles have been introduced into 2

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WMDs for improving their performances.3,8-10 Typically, Fe2O3 nanoparticles were incorporated into WMDs for decreasing the fluid loss by blocking the micro- and nano-pores in the filter cakes.11 Silica nanoparticles were added into WMDs to plug the nano-pores in shale formation and forms low permeability sealing zones for restricting the further invading of drilling mud.12 So the wellbore stability and thermal stability in shale were obviously improved.9 However, the pristine nanoparticles in WMD may easily aggregate and lead to bigger particles, making the nano-plugging invalidly. For inhibiting the aggregation of the pure nanoparticles, our previous study modified the surface of organic nanoparticles by poly(acrylic acid) and the enhanced plugging effects were obtained.13 Herein, in order to improve the dispersion stability of SiO2 nanoparticles in WMD at high temperatures and high salt concentration, poly(2-acrylamide-2methylpropanesulfonic acid) (PAMPS) was used to modify

the surface of SiO2

nanoparticles, and as a result, hybrid nanoparticles were formed. The hybrid nanoparticles show high salt tolerance, temperature resistance and compatibility with polymeric additives in WMDs. Furthermore, the hybrid nanoparticles exhibit excellent plugging properties both at ambient temperature and at a high temperature of 80°C. 2. Experimental 2.1 Material Tetraethyl orthosilicate (TEOS), 3-methacryloxypropyltrimethoxysilane (MPS), 2-acrylamide-2-methylpropanesulfonic acid (AMPS) and potassium persulfate (K2S2O8) were bought from Sigma-Aldrich and used as received. NaCl, ammonia (NH3•H2O,

25%),

ethanol,

sodium

carboxymethyl

cellulose

(CMC)

and

polyacrylamide (PAM) were purchased from J&K Chemical and used without further treatment. Sulfomethylated phenolic resin (SMP) was obtained from Sichuan Zhengrong Industrial Co., LTD. 2.2 Preparation 2.2.1 Synthesis of MPS modified SiO2 nanoparticles (SiO2/MPS) SiO2/MPS nanoparticles were prepared as previous works with modification.14,15 TEOS (2.5 mL) was dissolved in ethanol (24 mL) at 25 °C under stirring. Then a mixture of NH3•H2O (0.7 mL), deionized water (3 mL) and ethanol (20 mL) was added dropwise into the above solution. The reaction was maintained at 25 °C for 24 h. Subsequently, MPS (0.5 mL) was added and the surface functionalization of SiO2 3

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nanoparticles was allowed to proceed for another 12 h. Then the obtained SiO2/MPS nanoparticles were purified by several centrifugation and re-dispersion cycles with the supernatant replaced by ethanol during each cycle. 2.2.2 Preparation of Poly (2-acrylamide-2-methylpropanesulfonic acid) modified SiO2 (SiO2/PAMPS) nanoparticles SiO2/PAMPS nanoparticles were prepared by copolymerization of SiO2/MPS nanoparticles and AMPS monomer.16 The SiO2/MPS ethanol solution (10 mL) was dispersed in deionized water (20 mL) and ultra-sonicated for 1.5 h. Then AMPS aqueous solution (10 mL, 0.1g/mL) and K2S2O8 aqueous solution (10 mL, 10 mg/mL) were injected into the above solution under mechanical stirring. Nitrogen was bubbled into the solution and the temperature was elevated to 80 °C and reacted for 6 h. The obtained SiO2/PAMPS hybrid nanoparticles were purified by several centrifugation and re-dispersion cycles with the supernatant replaced by deionized water during each cycle. 2.2.3 Preparation of water based drilling fluid containing SiO2/PAMPS nanoparticles The water based fluids with nanoparticles were made of deionized water and as-prepared SiO2/PAMPS nanoparticles at a concentration of 0.8, 1.6 and 2.4 wt%. 2.3 Characterization 2.3.1 Instrument and characterization Transmission electron microscope (TEM) images were obtained using an H-600 transmission electron microscope. The samples were prepared by placing a drop of sample solution on a copper grid for observation. Thermogravimetric analysis (TGA) measurements were conducted under nitrogen atmosphere with a TGA/SDTA85 thermalanalyzer at a scanning rate of 10°C/min. Fourier-transform infrared (FTIR) spectra were taken on a Nicolet 6700 Fourier transform infrared spectrometer. Zeta potential of SiO2 and SiO2/PAMPS nanoparticles was carried out by NanoBrook ZetaPlus Zeta Potential Analyzer. Ultraviolet–visible spectrophotometer (UV–vis) absorptions were measured on Shimadzu UV 2450. Plugging properties tests were carried out by using the GGS42-2 high temperature high pressure filtration press. 2.3.2 Salt tolerance of SiO2/PAMPS nanoparticles SiO2/PAMPS dispersion stability analysis was conducted by time of stably dispersed in NaCl solution.13 Typically, the SiO2/PAMPS nanoparticles were dispersed into NaCl aqueous solution with concentrations of 0, 0.5, 1, 2, 4, 6, 8 or 10 4

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wt%. The coagulations of the nanoparticles in the solution were recorded by photos at different times. And the stable times these nanoparticles were also tested. UV–vis was employed to analyze optical absorbance of SiO2/PAMPS dispersions at 400 nm which is related to their stable times. The aggregation of SiO2/PAMPS was studied via the absorbance of nanoparticles dispersed in solution with time. 2.4.3 Thermal stability of SiO2/PAMPS nanoparticles SiO2/PAMPS nanoparticles were dispersed into NaCl aqueous solution with the concentration of NaCl equal to 0 wt%, 0.5 wt%, 1 wt%, 2 wt%, 18 wt%, 20 wt% and 22 wt%. Then these dispersions were placed into an oven and heated to 90, 100, 110, 120,130, 140, 150, 160, 170, 180, 190, 200, 210 or 220 °C. The nanoparticles dispersions were kept at each temperature for 16 h and the coagulation behaviors were recorded to measure the thermal stability of nanoparticles dispersions. 2.4.4 Compatibilities of the SiO2/PAMPS nanoparticles with PAM, CMC, and SMP SiO2/PAMPS nanoparticles were dispersed into PAM aqueous solution with concentration of PAM equal to 0 wt%, 1 wt%, 2 wt% and 3 wt%. Then these dispersions were set into oven and kept at different temperature for 16 h for measuring the compatibility of nanoparticles with PAM. The test temperature changed between 120 to 220°C with an interval of 10 °C and the aggregation behaviors of the nanoparticles dispersions were recorded. The compatibility of SiO2/PAMPS nanoparticles with CMC or SMP was investigated similarly. 2.4.5 Filtration test The plugging performance of SiO2/PAMPS nanoparticles was tested by filtration of water based mud containing nanoparticles on filter cakes. The filter cakes were formed by fresh-water fluid on high temperature and high pressure filter press with an area of 22.6 cm2 and a pressure difference of 3.5 MPa. The fresh-water fluid was prepared by dissolving barite (120 g) and bentonite (4g) into deionized water (100 mL) under vigorously stirring at room temperature. Then the fluid losses of water based fluid containing 0.8, 1.6 and 2.4wt% SiO2/PAMPS nanoparticles were tested on the as-prepared filter cakes at room temperature and 80°C respectively. The volumes of fluid loss were recorded every minute. The permeability of WMD containing nanoparticles on filter cake before and after plugging by nanoparticles was calculated by Darcy's Law from total filtrate volumes of 30 min. 3. Results and Discussion 5

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3.1 Characterization of SiO2/PAMPS nanoparticles Fig. 1a shows a typical TEM image of the as-synthesized SiO2/PAMPS nanoparticles. It can be seen that the nanoparticles are well dispersed and their average sizes are probably 60 nm. The core-shell structure of polymer modified inorganic nanoparticles are demonstrated by the high resolution TEM photograph of SiO2/PAMPS nanoparticles, as shown in the insert of Fig. 1 b.

Fig. 1 (a) TEM image of SiO2/PAMPS nanoparticles and (b) TGA curves of SiO2/MPS and SiO2/PAMPS nanoparticles. Insert show the high resolution TEM image of SiO2/PAMPS nanoparticles.

TGA curves of the as-prepared SiO2/MPS and SiO2/PAMPS nanoparticles are showed in Fig. 1b. It is obvious that both SiO2/MPS and SiO2/PAMPS nanoparticles exhibit two weight loss stages. For SiO2/MPS nanoparticles, the first weight loss process (about 7.11 wt%) from room temperature to 200 °C can be ascribed to the release of adsorbed water. The second weight loss process (about 6.3 wt%) in the range of 200°C to 650 °C may result from the decomposition of MPS molecules bonded on the surface of the SiO2 nanoparticles. For SiO2/PAMPS nanoparticles, the first weight loss process (about 6.1 wt%) from room temperature to 200 °C is also due to adsorbed water. The second weight loss process (about 18.9 wt%) in the range of 200°C to 800 °C is because of thermal decomposition of MPS and PAMPS molecules grafted on the surface of SiO2 nanoparticles. These results confirm that PAMPS has been grafted on the silica nanoparticles.

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Fig. 2 FTIR spectra of SiO2/MPS (a) and SiO2/PAMPS (b) nanoparticles.

The FTIR spectra of SiO2/MPS and SiO2/PAMPS nanoparticles are shown in Fig. 2. The peak located at 1708 cm-1 in Fig. 2a can be assigned to the stretching vibration of –C=O in MPS, indicating MPS is modified on the surface of SiO2 nanoparticles.17 As shown in Fig. 2b, the bands from 3600 to 3100 cm-1 are due to stretching vibration of –SO3H and –NH in AMPS, respectively. The peak at 1710 cm-1 is attributed to the stretching vibration of –C=O in PAMPS and the band dominated at 1094 cm-1 is related to the stretching vibration of Si–O–Si.18 Additionally, other peaks such as 948, 800 and 469 cm-1 are probability due to scissor vibration of Si–OH, stretching vibration of Si–O and scissor vibration of Si–O group, respectively. These peaks indicate that PAMPS is modified on the surface of SiO2 nanoparticles through the coupling of MPS.19 The grafting density of polymers on SiO2 nanoparticles was calculated to be 5.98 chains/nm2 based on the results of TEM and TGA, according to the methods described in literatures.20,21 3.2 Salt tolerance of the SiO2/PAMPS nanoparticles Zeta potential can be used to estimate dispersed stability of nanoparticles in solution. Herein, zeta potential of SiO2 nanoparticles is -31.6 mV, while the value changed to -66.7 mV for SiO2/PAMPS nanoparticles, indicating that PAMPS with electronegative ions enhances the dispersion of SiO2 nanoparticles. 22 Photos of SiO2/MPS and SiO2/PAMPS nanoparticles dispersed in aqueous NaCl solutions with different concentrations are shown in Fig. 3. The concentrations of NaCl in the solutions are 0.5, 1, 2, 4, 6, 8 and 10 wt%. Typically, 2 h has been selected as the stable time to study salt tolerance because it is the common time of drilling fluid working in well. It can be seen that SiO2/MPS nanoparticles are precipitated in 7

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NaCl aqueous solution quickly. In contrast, SiO2/PAMPS nanoparticles are dispersed stably even in 10 wt% of NaCl aqueous solution. These nanoparticles can be dispersed stably in NaCl aqueous solution for more than 2 h, showing that PAMPS on the surface of SiO2 nanoparticles improves the dispersion stability effectively. In other words, salt tolerance of these nanoparticles was enhanced.23 This may be explained by the effect of steric hindrance from the polymer layer and the electrostatic repulsion which are greatly enhanced, leading to a stable SiO2/PAMPS dispersions at higher NaCl concentration in comparison with SiO2/MPS.

Fig. 3 Photos of (a) SiO2/MPS and (b) SiO2/PAMPS nanoparticles in water and aqueous NaCl solutions with different concentrations from 0.5 to 10 wt % at 0 and 2 h.

UV–vis measurements were conducted to determine the critical salt concentration (CSC) for NaCl above which SiO2/PAMPS nanoparticles become unstable and precipitate within 2 h.24 The absorbance of SiO2/PAMPS aqueous dispersions at 400 nm as a function of time for high concentrations of NaCl (20, 21, 22 and 24 wt%) are depicted in Fig. 4. The decrease of absorbance reflects the precipitation of the SiO2/PAMPS nanoparticles in NaCl solution. When the NaCl concentration is above 21 wt%, the sedimentation of nanoparticles is clearly observed after around 2 h, suggesting that SiO2/PAMPS nanoparticles possess a CSC for NaCl is 21 wt%. The concentration for SiO2/PAMPS nanoparticles is much higher than that of SiO2 or SiO2/MPS nanoparticles. These results further confirm that PAMPS have been successfully grafted on the surface of SiO2 nanoparticles and the salt tolerance of the pristine nanoparticles is greatly enhanced.

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Fig. 4 UV–vis absorption (at 400 nm) of SiO2/PAMPS nanoparticles in different concentration of NaCl solution as a function of corresponding time.

3.3 Thermal resistance of SiO2/PAMPS nanoparticles The thermal resistances of SiO2/PAMPS nanoparticles were investigated by the dispersion stability of the nanoparticles in salt solution at high temperature because the temperature of formation is higher than that of surface.25 When drilling in shale formations, the concentration of salt is usually lower than 20 wt%. The temperature of formations which are drilled increases with respect to increasing depth in the Earth's interior by a temperature gradient of 30 °C/km. So the drilling muds working in the deep formations should keep dispersed stable at high temperature. As shown in Table 1, with the concentration of NaCl increasing from 0 to 20 wt%, the aqueous solution of SiO2/PAMPS nanoparticles are stable from 90 to 220°C. However, by increasing the concentration of NaCl to 22 wt%, the nanoparticles deposit from the solution when the temperature rises to above 120°C. These results show that the SiO2/PAMPS nanoparticles possess excellent thermal resistance, salt tolerance and dispersion stability in WMDs. Table 1 Thermal stability of the SiO2/PAMPS nanoparticles at different concentrations of NaCl aqueous solutions (U=undeposited, D=deposited, the concentration of NaCl is wt%)

Temperature (°C) 0 % NaCl

90 100 U

U

U

U

U

U

0.5 %NaCl

U

U

U

U

U

1 % NaCl

U

U

U

U

2 % NaCl

U

U

U

18 % NaCl

U

U

20 % NaCl

U

22 % NaCl

U

110

120 130 140 150

160

180

200

220

U

U

U

U

U

U

U

U

U

U

U

U

U

U

U

U

U

U

U

U

U

U

U

U

U

U

U

U

U

U

U

U

U

U

U

U

U

U

U

U

U

U

U

U

U

U

U

D

D

D











9

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3.4 Compatibilities of SiO2/PAMPS nanoparticles with PAM, CMC and SMP The compatibility of SiO2/PAMPS nanoparticles was evaluated by the dispersion stability of nanoparticles in the solution of PAM, CMC or SMP at high temperature.13 PAM, CMC and SMP were chosen because they are the most commonly used additives in the WMDs. As shown in Table 2, for 1 wt%, 2 wt%, 3 wt% PAM, the critical temperature at which precipitation nanoparticles occurs is 220, 210, 200 °C, respectively. With the concentration of CMC increasing from 1 to 3 wt%, critical temperature decreased from 190 °C to 160 °C. While the dispersions of SiO2/PAMPS nanoparticles maintained stable from 120 to 190°C at different SMP concentrations. These results indicate that the SiO2/PAMPS nanoparticles have excellent compatibilities with PAM, CMC, SMP and can be used in WMDs for developing shale gas.26 Table 2 Compatibilities of SiO2/PAMPS nanoparticles with PAM, CMC and SMP (U=undeposited, D=deposited, the concentration of polymeric additives is wt%)

Temperatu re (°C) 1% PAM

120

140

150

160

170

180

190

200

210

220

U

U

U

U

U

U

U

U

U

D

2% PAM

U

U

U

U

U

U

U

U

D

D

3% PAM

U

U

U

U

U

U

U

D

D

D

1% CMC

U

U

U

U

U

U

D

D

D

D

2% CMC

U

U

U

U

D

D

D







3% CMC

U

U

U

D

D

D

D







1% SMP

U

U

U

U

U

U

U

U

D

D

2% SMP

U

U

U

U

U

U

U

U

D

D

3% SMP

U

U

U

U

U

U

U

D

D

D

3.5 Plugging property of SiO2/PAMPS nanoparticles

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Secheme 1 Illustration of plugging performance of water based fluid containing nanoparticles.

The filtration of water fluid under 3.5 MPa was used to assess the plugging property of nanoparticles (Scheme 1). The fluid loss of WMDs containing different concentrations of SiO2/PAMPS nanoparticles on the barite filter cakes at room temperature and 80°C are depicted in Fig. 5. The fluid loss of pure water fluid on the filter cake within 30 min is 29.4 mL at room temperature. With increasing the amount of SiO2/PAMPS nanoparticles from 0.8, 1.6 to 2.4 wt% in water fluid, the fluid loss with 30 min decrease from 18.5, 7.9 to 7.8 mL, respectively. When the temperature is elevated to 80°C, the fluid loss of pure water fluid on filtrate cake reaches to 35.3 mL. By adding SiO2/PAMPS nanoparticles with concentration of 0.8, 1.6 and 2.4 wt%, the fluid loss within 30 min obviously decrease to 29.1, 7.5 and 6.8 mL at 80°C, respectively. These results indicate that the nano-pores in the filtrate cake are blocked by the SiO2/PAMPS nanoparticles and thus the fluid loss as well as permeability of filtrate cake is decreased obviously.

Fig. 5 Fluid loss as a function of time at (a) room temperature and (b) 80°C under 3.5 MPa. 11

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The permeability of the filter cake before and after plugging with nanoparticles can be calculated by Darcy's Law27 from volume of fluid loss. Darcy's Law is expressed as dV KAp  dt h

(1)

which describes fluid loss dV as a function of time dt, cross-sectional area A, permeability K, pressure differential Δp, viscosity μ and filter cake thickness h. The permeability of filter cake before plugging is calculated to be 0.000413 μm2 from the control experiment, in which the cross-sectional area A is 22.6 cm2, the pressure differential Δp is controlled at 3.5 MPa , the viscosity μ and filter cake thickness h are measured to be 1 mPa·s and 2.1 cm, respectively. The filter cake permeability values calculated by Equation (1) from fluid filtrate volumes after plugging with SiO2/PAMPS nanoparticles under different conditions are provided in Table 3. Table 3 Permeability (K) calculated by Darcy's Law from Fig. 5.

Concentration of SiO2/PAMPS Room temperature

80 °C

0 wt%

0.8 wt%

1.6 wt%

2.4 wt%

V30 (mL)

29.4

18.5

7.9

7.8

K (10-4μm2)

4.13

2.6

1.11

1.1

-37 %

-73.1 %

-73.4 %

% change



V30 (mL)

35.3

29.1

7.5

6.8

K (10-4μm2)

4.96

4.09

1.05

0.96

-17.5%

-78.8 %

-80.6 %

% change



V30, volume of fluid loss from 0 to 30 min.

According to Table 3, the filter cakes exhibit lower permeability after plugging with SiO2/PAMPS nanoparticles both at room temperature and 80°C, showing that these nanoparticles are effectively plugging agents for nano-pores. Herein, the as-prepared filter cakes are just as the models of shale formations. The water based fluid containing 0.8, 1.6 and 2.4 wt% SiO2/PAMPS nanoparticles blocks the nano-pores and leads to an obvious reduction in K compared to the control fluid, demonstrating that SiO2/PAMPS nanoparticles modified WMDs are potential 12

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candidates in drilling shale gas formation.28

Fig. 6 SEM images of (a) filter cake and (b) SiO2/PAMPS nanoparticles plugged filter cake.

Based on the results of permeability improved by SiO2/PAMPS nanoparticles, a possible mechanism is proposed. Under the pressure of 3.5 MPa, the nanoparticles are squeezed into nano-, micro-pores or cracks of the filter cake, resulting in tightly blocking at room temperature.29,30 This can be demonstrated by SEM of filter cake, as depicted in Fig. 6. There are many nano size pores in the man-made filter cake prepared from the fresh-water fluid (Fig. 6a). And these nano pores can be effective blocked by nanoparticles in the WBD (Fig. 6b). Thus the permeability and fluid loss decreased with the increasing amount of nanoparticles.8 Water fluid containing SiO2/PAMPS nanoparticles exhibits a similar filtration behavior at 80°C, demonstrating

that

SiO2/PAMPS

nanoparticles

maintain

efficient

plugging

performance even at temperature up to 80°C. This can be attributed to the excellent dispersion stability of SiO2/PAMPS nanoparticles in water fluid.31 The dissociation of PAMPS molecules on the surface of SiO2 nanoparticles induces negative charges on the outside of nanoparticles, providing considerable electrostatic repulsion between nanoparticles.32 The electrostatic repulsion prevents nanoparticles from aggregating together and makes the nanoparticles plug the nano-pores effectively even at high temperature. Thus small amount of SiO2/PAMPS nanoparticles (1.6 wt%) can effectively improve the plug property of filtrate cake, while such result can be obtained by pure SiO2 nanoparticles at a very high concentration of 10 wt%.9 Therefore, these SiO2/PAMPS nanoparticles prepared in this work are effective plugging agents for WMDs in drilling and developing shale gas and shale oil in shale formations. 13

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4. Conclusions SiO2/PAMPS nanoparticles were successfully prepared as plugging agents. These nanoparticles exhibit excellent salt tolerance, thermal stability as well as compatibility with common additives in WMDs, and these nanoparticles can block the nano-pores in filter cakes effectively. The fluid loss of water based fluids containing SiO2/PAMPS nanoparticles are tremendously improved at both room temperature and 80°C. Therefore, SiO2/PAMPS can be an excellent additive for WMDs when they are used during drilling in shale formations.

Acknowledgements This work was financially supported by the National Natural Science Foundation of China (No. 51304166) and Foundation of Department of Education of Sichuan Province (No. 16CZ0007).

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Table of Contents (TOC) Graphic Poly (2-acrylamide-2-methylpropanesulfonic acid) modified SiO2 nanoparticles for water based mud Yuanpeng Wu,1,2* Zhihao Wang,1 Zhu Yan,1 Tao Zhang,4 Yang Bai,2 Pingquan Wang,2 Pingya Luo,2 Shaohua Gou,2 Qipeng Guo3* 1

The Center of New Energy Materials and Technology, School of Materials Science and Engineering,

Southwest Petroleum University, Chengdu, Sichuan Province 610500, PR China; 2

State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation (Southwest Petroleum

University), Chengdu, Sichuan Province 610500, PR China; 3

Polymers Research Group, Institute for Frontier Materials, Deakin University, Locked Bag 20000,

Geelong, Victoria 3220, Australia; 4

Department of Materials Science and Engineering, Technion–Israel Institute of Technology, Haifa,

32000, Israel.



Correspondence author: Yuanpeng Wu, E-mail: [email protected]; Pingquan Wang, E-mail:

[email protected]; Fax: +86 28 83037409, Tel: +86 28 83037409. Qipeng Guo, E-mail: [email protected], Fax: +61 3 5227 1103, Tel: +61 3 5227 2802. 17

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