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Preparation and performance properties of polymer latex SDNL in water-based drilling fluids for drilling troublesome shale formations
Journal of Natural Gas Science and Engineering 37 (2017) 462e470
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Preparation and performance properties of polymer latex SDNL in water-based drilling fluids for drilling troublesome shale formations Jiangen Xu*, Zhengsong Qiu, Weian Huang, Xin Zhao School of Petroleum Engineering, China University of Petroleum, Qingdao, 266580, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 28 July 2016 Received in revised form 26 November 2016 Accepted 29 November 2016 Available online 13 December 2016
In this paper, a novel poly(styrene/n-butyl acrylate/acrylic acid) latex (SDNL) for water-based drilling fluids was successfully prepared and characterized. Emulsion polymerization was used to prepare the polymer latex (SDNL), which was characterized by transmission electron microscopy (TEM), particle size distribution (PSD) and thermal gravimetric analysis (TGA). Firstly, its effect on the rheological and filtration control performance of drilling fluids was analyzed through the compatible test. Secondly, its inhibition performance was observed with the linear swelling test, and the shale cuttings hot-rolling dispersion test. Then, its sealing performance was investigated through the pressure transmission test. Finally, its effect on the lubrication performance was revealed by measuring lubrication coefficients of the drilling fluids. The results indicated that the particle size distribution of the spherical polymer latex nanoparticles ranged from 50 nm to 190 nm with the D50 value of around 88 nm. SDNL could significantly improve the rheological performance and enhance the shear thinning property of the drilling fluids. SDNL had favorable filtration control performance and good temperature resistance at 120 C. The inhibition performance test demonstrated that SDNL had excellent ability to inhibit shale hydration swelling and dispersion. At the concentration of 2%, the linear expansion rate of the shale sample was only 4.9%, and the recovery rate was 93.7%. The pressure transmission test revealed that SDNL had outstanding sealing performance. A dense plugging film could form on the shale surface, the permeability of which was almost zero, and the pore pressure was difficult to continually transmit. Furthermore, the lubrication performance results confirmed that SDNL had good lubrication performance. After adding 2.0 w/v% of SDNL into the drilling fluids, the lubrication coefficient was reduced as high as 64.4%. The newly developed polymer latex SDNL is expected to be a superior additive in water-based drilling fluids for drilling troublesome shale formations. © 2016 Elsevier B.V. All rights reserved.
Keywords: Polymer latex Water-based drilling fluids Shale formations Inhibition performance Pressure transmission
1. Introduction Wellbore instability is often encountered during oil and gas drilling process, which results in higher drilling costs and delays in the drilling process (Hale et al., 1993; Bol et al., 2013; Van Oort, 1994). Approximately 75% of the formations in the drilling process are shale formations, and 90% of the borehole instability occurs in shale formations (Steiger and Leung., 1992). Thus, borehole instability in shale formations has attracted much attention (Chenevert and Sharma., 1993; Ma and Chen., 2015). Shale is mainly composed of clay minerals and suffers from hydration effect when
* Corresponding author. E-mail address: [email protected] (J. Xu). http://dx.doi.org/10.1016/j.jngse.2016.11.064 1875-5100/© 2016 Elsevier B.V. All rights reserved.
drilled with traditional water-based drilling fluids. This leads to a change in the stress state of the borehole wall and a decrease of the rock strength, which is not conducive to wellbore stability. In particular, as shale has extremely low permeability, the pressure transmission and filtrate invasion of drilling fluids will cause an increase in pore pressure and a reduction in the effective support of the liquid column pressure, thus resulting in wellbore instability (Van Oort, 1997; Stowe et al., 2001). Therefore, the inhibition and sealing performance of the traditional water-based drilling fluids should be enhanced (Qiu et al., 2007). Over the years, inorganic salt, silicate, formate, polyol and amine polymer have been introduced into drilling fluids as inhibitors, while asphalt, silicate and aluminum salts have been used as plugging agents (Caenn and Chillingar., 1996; Ramirez et al., 2007). The inhibition and sealing performance of the traditional water-
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based drilling fluids have been improved, but the problem of borehole instability still occurs (Witthayapanyanon et al., 2013; Stevens et al., 2013). Therefore, the search for an efficient shale stabilizer in water-based drilling fluids for drilling troublesome shale formations is a continuous endeavor. Nano materials have attracted widespread attention in the scientific community (Amanullah and Tahini., 2009; Hoelscher et al., 2012; Zou et al., 2008). The preparation technology of nano materials is one of the three major technologies in the modern era. It has been applied to many fields, such as electronics, chemical industry, and energy (Rao and Cheetham., 2001). Nano materials have at least one dimension in the nanometer scale (1e100 nm) in threedimensional space. Because the dimensions of nano materials are between the microscopic and macroscopic objects, their functionality is often different from conventional materials. Unique properties displayed by nano materials include the quantum size effect, surface effect and small size effect et al. At present, exploration and development of shale gas resources are in full swing, and borehole instability in the drilling process becomes a major problem. Conventional plugging agents are too large to provide full sealing performance, which limits their application. Nano materials have characteristics of the surface effect, small size effect and less consumption, in line with the development trend of drilling fluids to a low solid content. Nano materials also have excellent sealing and inhibition performance, so they are conducive to improving the comprehensive performance of drilling fluids. The application of nano materials in drilling fluids has already gained attention in recent years (Abdo and Haneef., 2012; Li et al., 2012; Amanullah et al., 2011). In the early stage, nanoparticles were introduced into water-based drilling fluids, and it was found that they could penetrate and block the pore throats of shale to form a permanent mud cake, thus reducing the permeability of shale formations (Sensoy et al., 2009). In addition, non-modified silica nanoparticles were introduced into the drilling fluids, and they not only helped to form a thin and compact mud cake, but also greatly improved the rheological and filtration control performance of drilling fluids (Cai et al., 2012; Jung, 2013). Furthermore, the modified silica nanoparticles were used to adjust the performance of water-based drilling fluids, which could enhance the filtration control and inhibition performance of drilling fluids (Jain et al., 2015). However, silica nanoparticles cannot solve the problem of wellbore instability completely, because they are very easy to aggregate and have poor dispersion in water-based drilling fluids (Baran and Cabrera., 2006; Kong et al., 2012). Additionally, the particles are rigid and have poor deformation capacity, so excellent performance is difficult to achieve. Conventional emulsion polymerization is a polymerization process using aqueous media and emulsifiers can help disperse the monomers. Emulsion polymerization system is often composed of monomers, water, an initiator and emulsifiers. Water is used as the medium, and the monomers are dispersed into an emulsion state. Compared with other methods to prepare polymer nanoparticles, conventional emulsion polymerization has the advantages of functioning at low viscosity, having quick heat dissipation, good environmental protection, low cost, and safety in production (Cao, 1997). Exploring the application of polymer latex prepared by emulsion polymerization in drilling fluids is a meaningful work (Liu et al., 2015; Bailey, 2004). On the one hand, the presence of emulsifiers ensures the dispersion of polymer latex particles in water-based drilling fluids. On the other hand, proper polymerization monomer can ensure that the polymer latex particles have both the ability of deformation and temperature resistance. On the basis of giving full play to their deformable sealing performance, the presence of specific functional groups is beneficial to the inhibition performance. Additionally, polymer latex can improve the
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lubrication performance of aqueous solutions. If the prepared polymer latex by emulsion polymerization exhibits these advantages in water-based drilling fluids, it will be a better shale stabilizer to resolve the problem of wellbore instability. But at present the literature reports about the application of polymer latex in drilling fluids are also relatively fewer. In this study, we attempted to prepare the novel polymer latex (SDNL) by emulsion polymerization for water-based drilling fluids. The morphology and thermal characteristics of the polymer nanoparticles were investigated by conventional techniques. Furthermore, the compatible test, shale inhibition test, pressure transmission test and lubrication performance test were carried out to evaluate the comprehensive performance of SDNL in waterbased drilling fluids. Through the laboratory evaluation, it is found that the new additive has broad application prospects in water-based drilling fluids for drilling troublesome shale formations. 2. Materials and methods 2.1. Materials Styrene (St), n-butyl acrylate (BA) and acrylic acid (AA) monomers were purchased from Sinopharm Chemical Reagent Co. Ltd (China). The monomers were purified by vacuum distillation to remove the inhibitors. Octylphenol polyoxyethylene ether (OP-10) was purchased from Aladdin Reagent Co. Ltd (China). Sodium dodecyl sulfate (SDS), potassium persulfate (KPS) and sodium bicarbonate (NaHCO3) were purchased from Sinopharm Chemical Reagent Co. Ltd (China) and used as received. Deionized water was used throughout the work. Shale samples were collected from Turpan Hami Basin, China. The mineralogical composition of the shale samples is presented in Table 1. 2.2. Preparation of polymer latex Polymer latex (SDNL) was prepared by emulsion polymerization with potassium persulfate as an initiator. Polymerization was carried out in a four-necked flask equipped with a reflux condenser, a mechanical stirrer, a thermometer, and a feed inlet tube. Briefly, 32 g of St, 8 g of BA, 1.2 g of AA and 0.12 g of NaHCO3 were dispersed in 60 mL deionized water, in which a certain amount of emulsifiers SDS and OP-10 were dissolved. Then they were transferred into the flask, stirred and heated up to 80 C. After 30 min, KPS (0.24 g) was added to the homogeneous emulsion to induce polymerization. The polymerization was then continued in the flask for another 3.5 h at 80 C and finally cooled to room temperature. 2.3. Characterization of polymer latex The dispersing morphology of the polymer latex particles in aqueous solution was observed by JEOL JEM-2100UHR Transmission Electron Microscopy (TEM) at an accelerating voltage of 200 kV. The particle size distribution (PSD) of the polymer latex particles was measured with dynamic light scattering using the Zetasizer Nano ZS90 produced by Malvern Instruments Ltd. The thermal gravimetric analysis (TGA) was carried out using a TGA/ DSC1 thermal analyzer from METTLER-TOLEDO Inc. at a heating rate of 10 C/min under nitrogen atmosphere. 2.4. Compatible test The base fluid was prepared by mixing 4.0 w/v% pre-hydrated sodium bentonite slurry, 0.3 w/v% of xanthan gum and 1.0 w/v% low viscosity polyanionic celluloses at high speed. Then varied
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J. Xu et al. / Journal of Natural Gas Science and Engineering 37 (2017) 462e470 Table 1 Mineralogical composition of the shale samples. Component
Content (wt%)
Component of clay mineral
Content (wt%)
Quartz Potassium feldspar Plagioclase Calcite Iron dolomite Clay mineral
20 6 29 28 2 15
Kaolinite Chlorite Illite Illite/smectite mixed layer
3 3 12 82
amounts of SDNL were added into the base fluid to evaluate the rheological and filtration control performance of the homogenous drilling fluids. The rheological performance was tested using a model ZNN-D6 viscometer (Haitongda Company, China). The filtration control performance was analyzed with ZNS-2A filtration apparatus (Haitongda Company, China). The rheological parameters were calculated by the following formulas according to the API recommended practice of standard procedure for field testing of drilling fluids.
Apparent ViscosityðAVÞ ¼ F600 =2 ðmPa$sÞ Plastic ViscosityðPVÞ ¼ F600 F300 ðmPa$sÞ Yield PointðYPÞ ¼ ðF300 PVÞ=2 ðPaÞ The temperature resistance of SDNL in drilling fluids was conducted by aging test. The base fluid and the base fluid containing 2.0 w/v% SDNL were hot rolled at 120 C for 16 h in a XGRL-4 type rolling oven (Haitongda Company, China). The rheological and filtration control performance of the drilling fluids after aging were tested again as described above.
2.6. Pressure transmission test The pressure transmission test was performed using a simulation experiment device for hydra-mechanics coupling of shale (Xu and Qiu., 2006; Xu et al., 2005). The device was developed by China University of Petroleum (East China), and the test evaluated the sealing performance of the prepared novel polymer latex SDNL. Fig. 1 shows a schematic of the simulation experiment device. The experiment device consists of five parts, including a high temperature-pressure kettle, a hydraulic control system, a fluid circulation system, a temperature transfer and control system, and a data acquisition-processing system. The main technical indices include the axial compression and confining pressure ranging from 0 to 50 MPa, the testing fluid pressure between 0 and 35 MPa, the sensor pressure between 0 and 10 MPa, and the temperature between room temperature and 150 C. A shale sample was placed into the core holder, and made contact with the testing fluid through the upstream inlet. In the experimental process, the upstream pressure was maintained at 2.0 MPa while the initial downstream pressure was set to 1.0 MPa. Pressure transmission curves were obtained to evaluate the sealing performance of the testing fluid. The data from these curves demonstrated how the downstream pressure changed over time.
2.5. Shale inhibition test 2.5.1. Linear swelling test The linear swelling test was designed to measure the linear expansion rate of shale cores after making direct contact with testing fluid (Zhong et al., 2016). Firstly, the shale samples were crushed to fine powders <100 mesh, and oven-dried for four hours at 105 C. A shale core was prepared by pressing 10 g of crushed shale samples at 10 MPa for 5 min. The initial height of shale core was measured. Finally, the shale core was placed into the expansion instrument (NP-02A, Haitongda Company, China), and the swelling height with time was recorded after the shale core made contact with testing fluid. Then the linear expansion rate could be calculated using the swelling height recorded. Moreover, the smaller linear expansion rate indicated the better capacity of the testing fluid to inhibit shale hydration and swelling. 2.5.2. Shale cuttings hot-rolling dispersion test The shale cuttings hot-rolling dispersion test was designed for two purposes. It can determine the dispersion characteristics of shale cuttings and evaluate the testing fluid's ability to inhibit shale hydration and dispersion (Zhong et al., 2015). For the test, 50 g of shale cuttings with a particle size between 2.00 mm and 3.35 mm were placed into the sealed cell containing 350 mL testing fluid and hot rolled at 77 C for 16 h. After cooling to room temperature, the remaining shale cuttings were recovered with a 40 mesh sieve, washed with fresh water, and oven-dried at 105 C for 4 h. The recovery rate of shale cuttings was calculated using the measured weight of the recovered shale cuttings. Moreover, the higher recovery rate indicated better ability of the testing fluid to inhibit shale hydration and dispersion.
2.7. Evaluation of lubrication performance An extreme pressure lubrication apparatus (EP-B, Haitongda Company, China) was used to measure the lubrication performance of drilling fluids and evaluate the ability of lubricants to reduce torque. Varied amounts of polymer latex SDNL were added into the 4% sodium bentonite slurry, and their influence on the extreme pressure lubrication coefficient of drilling fluids were monitored. Firstly, the sample cell full of testing fluid was immobilized on the supporting plate, while the friction ring and block were completely immersed into testing fluid. The torque value was adjusted to 16.95 N m with a rapidly rotating pressurized handle, and the time was recorded with stopwatch. After five minutes at a speed of 60 rpm/min, the friction coefficient was obtained to calculate the extreme pressure lubrication coefficient.
Fig. 1. The schematic of the simulation experiment device for pressure transmission test.
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3. Results and discussion 3.1. Characterization of polymer latex The specific surface area of the polymer latex nanoparticles was found to be 26.03 m2/g, which was very high. Fig. 2 presents the particle size distribution of the polymer latex SDNL. It can be seen that the prepared polymer latex had a narrow particle size distribution, which ranged from 50 nm to 190 nm, and D50 (medium particle size) was 88 nm with the D10 value of 62 nm and D90 value of 129 nm. Fig. 3 presents the TEM micrograph of diluted SDNL. The micrograph displayed uniformly dispersed, spherical polymer latex nanoparticles. Moreover, the average particle size was about 85 nm, which coincided with the D50 value of around 88 nm from the test results of particle size distribution. The TGA curve of the polymer latex nanoparticles is shown in Fig. 4. It can be seen that when the temperature was above 270 C, the polymer latex nanoparticles began to decompose. The weight loss of the polymer latex nanoparticles increased dramatically as the temperature was increased to 375 C, at which point the TGA curve descended almost vertically. This indicated that the newly prepared polymer latex nanoparticles maintained good thermal stability. 3.2. Compatible test The influence of polymer latex SDNL on the rheological and filtration control performance of the base fluid is shown in Table 2. It can be concluded that with the increase of the SDNL concentration in drilling fluids, the apparent viscosity, plastic viscosity, yield point, and the ratio of dynamic shear force of the drilling fluids gradually increased. This indicated that SDNL could enhance the shear thinning property of drilling fluids, and thus more conducive for drilling fluids to effectively breaking rock at high shear rate and efficiently carrying cuttings at low shear rate. In addition, when adding 2.0 w/v% of SDNL into drilling fluids, the API filtration loss was reduced from 8.6 mL to 5.4 mL, illustrating that SDNL had favorable filtration control performance. The aging test was performed to evaluate the temperature
Fig. 3. TEM micrograph of diluted polymer latex SDNL.
resistance of SDNL. Table 3 shows the results of aging test. It can be seen that SDNL could improve the rheological performance of drilling fluids due to the increase of the ratio of dynamic shear force. In addition, compared to base fluid without SDNL, the filtration loss decreased obviously from 9.8 mL to 6.0 mL, which indicated that SDNL still retained excellent filtration control performance. Thus, SDNL possessed good temperature resistance in drilling fluids at 120 C.
3.3. Evaluation of inhibition performance Hydration occurs when shale is in contact with water, which can lead to an increase in the hydration stress, a change in the shale strength and shale deformation such as swelling and dispersion (Anderson et al., 2010). Therefore, the shale inhibition performance of SDNL can be directly reflected by means of the linear swelling test and the shale cuttings hot-rolling dispersion test. Experiments were carried out to evaluate the shale inhibition performance of SDNL by comparing frequently used shale inhibitors, including inorganic salts (KCl) and polymeric alcohol (JHC).
Fig. 2. Particle size distribution of diluted polymer latex SDNL.
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Fig. 4. TGA curve of the polymer latex nanoparticles.
Table 2 Effect of SDNL on the rheological and filtration control performance of the base fluid. Formula Base Base Base Base
fluid fluidþ0.5% SDNL fluid þ1% SDNL fluid þ2% SDNL
AV (mPa.s)
PV (mPa.s)
YP (Pa)
YP/PV
API FL(mL)
41 49 51 55
31 32 33 35
10 17 18 20
0.32 0.53 0.55 0.57
8.6 6.8 6.2 5.4
inferred that SDNL had outstanding capacity to prevent shale hydration and dispersion. There are some factors contributing to the excellent inhibition ability of SDNL. The polymer latex particles have the characteristics of small particle size, large specific surface area and high surface energy, which can be easily adsorbed onto the surface of shale. And the polar hydrophilic carboxyl groups are introduced onto the
Table 3 Temperature resistance of SDNL in drilling fluids at 120 C for 16 h. Formula
Experimental condition
AV (mPa.s)
PV (mPa.s)
YP (Pa)
YP/PV
API FL (mL)
Base fluid Base fluid þ2% SDNL
120 C/16 h 120 C/16 h
29 40
24 28
5 12
0.21 0.43
9.8 6.0
3.3.1. Linear swelling test Results of the linear swelling test for different concentrations of SDNL, KCl, and JHC are presented in Fig. 5. The linear expansion rate of shale sample in tap water was 12.5%, demonstrating that the shale had strong hydration swelling capacity. Besides, it can be clearly seen that the inhibition performance of SDNL was better than KCl and JHC. At a concentration of 2% SDNL, the linear expansion rate was only 4.9%, which was 2.55 times lower than that of tap water, and also significantly lower than that of KCl and JHC at the same concentration. It can be inferred that SDNL had excellent ability to prevent shale hydration and swelling.
particle surface via the monomer of acrylic acid (AA). When shale is in contact with an aqueous solution of SDNL, the polymer particles are adsorbed onto the surface of the clay particles by hydrogen bonds (Burchill et al., 1983). The clay particles bridge together by multi-point adsorption, thereby maintaining shale integrity. Moreover, the adsorption of polymer particles can form a hydrophobic isolation film, which can further prevent water molecules into clay mineral particles clearance, and therefore effectively inhibiting clay hydration swelling and dispersion.
3.3.2. Shale cuttings hot-rolling dispersion test Fig. 6 presents the results of the shale cuttings hot-rolling dispersion test for various concentrations of SDNL, KCl and JHC. Results indicated that the recovery rate of shale cuttings in tap water was 40.2%, demonstrating that the shale had strong hydration dispersion capacity. It can be clearly concluded that the inhibition performance of SDNL was better than KCl and JHC. When the concentration of SDNL was 2%, the recovery rate was 93.7%, which was 2.33 times higher than that of tap water, and also significantly higher than that of KCl and JHC at the same concentration. It can be
Retarding pore pressure transmission was a key measure to maintain shale stability (Ewy and Morton., 2009). In this test, 4.0 w/ v% pre-hydrated sodium bentonite slurry was used as the base fluid. The curves of the pressure transmission test are depicted in Fig. 7. Pore pressure increased from initially 1 MPae2 MPa after 2400 s for the shale sample, and 7700 s for the base fluid. After interacting with the base fluid containing 2.0 w/v% SDNL, the pore pressure was only 1.8 MPa after 16,300 s. The pressure transmission time was much longer than the shale sample and the base fluid. More importantly, when the pore pressure was increased to
3.4. Pressure transmission test
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Fig. 5. Effect of different shale inhibitors on shale linear expansion rate.
Fig. 6. Effect of different shale inhibitors on shale cuttings recovery.
1.8 MPa, it was almost no longer increased. This indicated that the shale surface was covered with a dense plugging film. It could be concluded that SDNL had strong plugging capacity, which could effectively retard pore pressure transmission, and therefore maintain shale stability. A schematic diagram of the plugging mechanism is shown in Fig. 8. As shown in Fig. 8, the polymer nanoparticles under differential pressure rely on its elastic deformation, bridging and sealing the pore throats and micro cracks of the shale. Over time, a dense plugging film gradually forms on the shale surface, the permeability of which is almost zero, and the pore pressure is difficult to continually transmit. Therefore, in shale
formation drilling process, SDNL can retard pore pressure transmission and reduce filtrate invasion, thereby stabilizing shale formation. It can be regarded as an efficient plugging agent. 3.5. Evaluation of lubrication performance The influence of SDNL on the extreme pressure lubrication coefficient of drilling fluids is shown in Table 4. As the concentration of SDNL was increased, the lubrication coefficients of drilling fluids gradually decreased. Adding 2.0 w/v% of SDNL into the drilling fluids, the lubrication coefficient was reduced by 64.4%, indicating
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Fig. 7. Pressure transmission test curves.
Fig. 8. Schematic diagram of plugging mechanism, (a) compression and deformation of polymer nanoparticles and (b) formation of a dense plugging film.
Table 4 Influence of SDNL on lubrication performance. Concentration(wt/v%)
Lubrication coefficient
Reduced rate of the lubrication coefficient (%)
0 0.5 1 2
0.432 0.245 0.207 0.154
e 43.3 52.1 64.4
that SDNL had good lubrication performance. The lubrication mechanism of SDNL was different from traditional lubricants, which depended on the micro “rolling” and “film forming” (Zhao and Zhou., 1999; Zhang et al., 1994; Xue et al., 1997). A schematic diagram of the lubrication mechanism is presented in Fig. 9. As shown in Fig. 9, the small nanoparticles are spherical and can be freely rolled between the friction pairs. They act as “rolling bearing”, which can make the sliding friction into rolling friction, thereby reducing torque. In addition, the polymer nanoparticles dispersed in the drilling fluids can be easily squeezed and deformed
during the friction process, and eventually form a thin and dense boundary lubrication film on the friction surface, which plays the role of friction-reducing and anti-wear. Therefore, in shale formation drilling process, SDNL can effectively improve the lubrication performance of drilling fluids, which will help reduce the drilling torque and resistance, thus reduce the wear of drilling tools. 4. Conclusion In summary, a novel polymer latex (SDNL) was successfully
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Fig. 9. Schematic diagram of lubrication mechanism, (a) “rolling bearing” of polymer nanoparticles and (b) “lubrication film forming” of polymer nanoparticles.
prepared by emulsion polymerization as a multifunctional additive for water-based drilling fluids. The particle size distribution of spherical polymer latex particles was between 50 and 190 nm with D50 value of 88 nm. SDNL could significantly improve the rheological performance and enhance the shear thinning property of drilling fluids. SDNL had excellent filtration control performance and good temperature resistance at 120 C. The inhibition performance test showed that SDNL had excellent ability to prevent shale hydration swelling and dispersion. At the concentration of 2%, the linear expansion rate of the shale sample was only 4.9%, and the recovery rate was 93.7%, indicating that it possessed a significantly better inhibition performance than that of KCl and JHC at the same concentration. Based on the pressure transmission test results, the base fluid containing 2.0 w/v% of SDNL required more time to transfer the same pressure difference compared with shale and base fluid. Furthermore, a dense plugging film finally formed on the shale surface, the permeability of which was almost zero, making the pore pressure difficult to continually transmit. The lubrication performance results illustrated that SDNL had good lubrication performance. After adding 2.0 w/v% of SDNL into the drilling fluids, the lubrication coefficient was reduced by 64.4%, indicating that SDNL could effectively improve the lubrication performance of drilling fluids. Hence, the newly developed polymer latex SDNL is expected to be a superior additive in water-based drilling fluids for drilling troublesome shale formations. Acknowledgments This work was financially supported by National Natural Science Foundation of China (51474236), the National Key Basic Research Special Foundation of China (2015CB251205), and China Postdoctoral Science Foundation (2015M580618; 2016T90658). References Abdo, J., Haneef, M.D., 2012. Nano-enhanced drilling fluids: pioneering approach to overcome uncompromising drilling problems. J. Energy Resour. Technol. 134 (1), 014501. Amanullah, M., Al-Tahini, A.M., 2009. Nano-technology-its significance in smart fluid development for oil and gas field application. In: SPE Saudi Arabia Section Technical Symposium. Society of Petroleum Engineers. Amanullah, M., AlArfaj, M.K., Al-abdullatif, Z.A., 2011. Preliminary test results of nano-based drilling fluids for oil and gas field application. In: SPE/IADC Drilling Conference and Exhibition. Society of Petroleum Engineers. Anderson, R.L., Ratcliffe, I., Greenwell, H.C., Williams, P.A., Cliffe, S., Coveney, P.V., 2010. Clay swellingda challenge in the oilfield. Earth-Science Rev. 98 (3), 201e216. Bailey, L., 2004. Latex additive for water-based drilling fluids. US, US 6715568 B1. Baran, J.R., Cabrera, O.J., 2006. U.S. Patent No. 7,033,975. U.S. Patent and Trademark Office, Washington, DC. Bol, G.M., Wong, S.W., Davidson, C.J., Woodland, D.C., 2013. Borehole stability in shales. SPE Drill. Complet. 9 (2), 87e94. Burchill, S., Hall, P.L., Harrison, R., Hayes, M.H.B., Langford, J.I., Livingston, W.R., Tuck, J.J., 1983. Smectite-polymer interactions in aqueous systems. Clay Min. 18 (4), 373e397.
Caenn, R., Chillingar, G.V., 1996. Drilling fluids: state of the art. J. Petroleum Sci. Eng. 14 (3e4), 221e230. Cai, J.H., Chenevert, M.E., Sharma, M.M., Friedheim, J.E., 2012. Decreasing water invasion into Atoka shale using nonmodified silica nanoparticles. SPE Drill. Complet. 27 (01), 103e112. Cao, T.Y., 1997. Synthesis Principles, Properties and Applications of Polymer Emulsion. Chemical Industry Press, Beijing. Chenevert, M.E., Sharma, A.K., 1993. Permeability and effective pore pressure of shales, 8 SPE Drill. Eng. 1 (8), 28e34. Ewy, R.T., Morton, E.K., 2009. Wellbore-stability performance of water-based mud additives. SPE Drill. Complet. 24 (3), 390e397. Hale, A.H., Mody, F.K., Salisbury, D.P., 1993. The influence of chemical potential on wellbore stability. SPE Drill. Complet. 8 (03), 207e216. Hoelscher, K.P., De Stefano, G., Riley, M., Young, S., 2012. Application of nanotechnology in drilling fluids. In: SPE International Oilfield Nanotechnology Conference and Exhibition. Society of Petroleum Engineers. Jain, R., Mahto, V., Sharma, V.P., 2015. Evaluation of polyacrylamideegrafted -polyethylene glycol/silica nanocomposite as potential additive in water based drilling mud for reactive shale formation. J. Nat. Gas Sci. Eng. 26, 526e537. Jung, C.M., 2013. High-performance water-based mud using nanoparticles for shale reservoirs. In: Unconventional Resources Technology Conference (URTEC). Kong, D.Y., Du, X.F., Wei, S., Zhang, H., Yang, Y., Shah, S.P., 2012. Influence of nanosilica agglomeration on microstructure and properties of the hardened cementbased materials. Constr. Build. Mater. 37, 707e715. Li, G.R., Zhang, J.H., Zhao, H.Z., Hou, Y.G., 2012. Nanotechnology to improve sealing ability of drilling fluids for shale with micro-cracks during drilling. In: SPE International Oilfield Nanotechnology Conference and Exhibition. Society of Petroleum Engineers. Liu, J.Y., Qiu, Z.S., Huang, W.A., 2015. Novel latex particles and aluminum complexes as potential shale stabilizers in water-based drilling fluids. J. Petroleum Sci. Eng. 135, 433e441. Ma, T., Chen, P., 2015. A wellbore stability analysis model with chemical- mechanical coupling for shale gas reservoirs. J. Nat. Gas Sci. Eng. 26, 72e98. Qiu, Z.S., Xu, J.F., Lyu, K.H., Yu, L.X., Huang, W.A., Wang, Z.M., 2007. A multivariate cooperation principle for well-bore stabilization. Acta Pet. Sin. 28 (2), 117e119. Ramirez, M.A., Benaissa, S., Ragnes, G., Almaraz, A.A., 2007. Aluminum-based HPWBM Successfully Replaces Oil-based Mud to Drill Exploratory Well in the Magellan Strait. Society of Petroleum Engineers, Argentina. Rao, C.N.R., Cheetham, A.K., 2001. Science and technology of nanomaterials: current status and future prospects. J. Mater. Chem. 11 (12), 2887e2894. Sensoy, T., Chenevert, M.E., Sharma, M.M., 2009. Minimizing Water Invasion in Shales Using Nanoparticles. SPE Annual Technical Conference and Exhibition. Society of Petroleum Engineers. Steiger, R.P., Leung, P.K., 1992. Quantitative determination of the mechanical properties of shales. SPE Drill. Eng. 7 (3), 181e185. Stevens, S.H., Moodhe, K.D., Kuuskraa, V.A., 2013. China shale Gas and Shale Oil Resource Evaluation and Technical Challenges[C]//SPE Asia Pacific Oil and Gas Conference and Exhibition. Society of Petroleum Engineers. Stowe, C., Halliday, W., Xiang, T., Clapper, D., Morton, K., Hartman, S., 2001. Laboratory pore pressure transmission testing of shale. In: AADE National Drilling Technical Conference, pp. 1e10. Van Oort, E., 1994. A novel technique for the investigation of drilling fluid induced borehole instability in shales. In: Rock Mechanics in Petroleum Engineering. Society of Petroleum Engineers. Van Oort, E., 1997. Physico-chemical stabilization of shales. In: International Symposium on Oilfield Chemistry. Society of Petroleum Engineers. Witthayapanyanon, A., Leleux, J., Vuillemet, J., Morvan, R., Pomian, A., Denax, A., Bland, R., 2013. High performance water-based drilling fluids-an environmentally friendly fluid system achieving superior shale stabilization while meeting discharge requirement offshore Cameroon. In: SPE/IADC Drilling Conference. Society of Petroleum Engineers. Xu, J.F., Qiu, Z.S., Lyu, K.H., 2005. Pressure transmission testing technology and simulation equipment for hydra-mechanics coupling of shale. Aca Pet. Sin. 27 (2), 26e27. Xu, J.F., Qiu, Z.S., 2006. Simulation test equipment of coupled hydra-mechanics of shales (Edition of Natural Science). J. Chin. Univ. Pet. 30 (3), 63e66.
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Preparation and performance properties of polymer latex SDNL in water-based drilling fluids for drilling troublesome shale formations Jiangen Xu*, Zhengsong Qiu, Weian Huang, Xin Zhao School of Petroleum Engineering, China University of Petroleum, Qingdao, 266580, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 28 July 2016 Received in revised form 26 November 2016 Accepted 29 November 2016 Available online 13 December 2016
In this paper, a novel poly(styrene/n-butyl acrylate/acrylic acid) latex (SDNL) for water-based drilling fluids was successfully prepared and characterized. Emulsion polymerization was used to prepare the polymer latex (SDNL), which was characterized by transmission electron microscopy (TEM), particle size distribution (PSD) and thermal gravimetric analysis (TGA). Firstly, its effect on the rheological and filtration control performance of drilling fluids was analyzed through the compatible test. Secondly, its inhibition performance was observed with the linear swelling test, and the shale cuttings hot-rolling dispersion test. Then, its sealing performance was investigated through the pressure transmission test. Finally, its effect on the lubrication performance was revealed by measuring lubrication coefficients of the drilling fluids. The results indicated that the particle size distribution of the spherical polymer latex nanoparticles ranged from 50 nm to 190 nm with the D50 value of around 88 nm. SDNL could significantly improve the rheological performance and enhance the shear thinning property of the drilling fluids. SDNL had favorable filtration control performance and good temperature resistance at 120 C. The inhibition performance test demonstrated that SDNL had excellent ability to inhibit shale hydration swelling and dispersion. At the concentration of 2%, the linear expansion rate of the shale sample was only 4.9%, and the recovery rate was 93.7%. The pressure transmission test revealed that SDNL had outstanding sealing performance. A dense plugging film could form on the shale surface, the permeability of which was almost zero, and the pore pressure was difficult to continually transmit. Furthermore, the lubrication performance results confirmed that SDNL had good lubrication performance. After adding 2.0 w/v% of SDNL into the drilling fluids, the lubrication coefficient was reduced as high as 64.4%. The newly developed polymer latex SDNL is expected to be a superior additive in water-based drilling fluids for drilling troublesome shale formations. © 2016 Elsevier B.V. All rights reserved.
Keywords: Polymer latex Water-based drilling fluids Shale formations Inhibition performance Pressure transmission
1. Introduction Wellbore instability is often encountered during oil and gas drilling process, which results in higher drilling costs and delays in the drilling process (Hale et al., 1993; Bol et al., 2013; Van Oort, 1994). Approximately 75% of the formations in the drilling process are shale formations, and 90% of the borehole instability occurs in shale formations (Steiger and Leung., 1992). Thus, borehole instability in shale formations has attracted much attention (Chenevert and Sharma., 1993; Ma and Chen., 2015). Shale is mainly composed of clay minerals and suffers from hydration effect when
* Corresponding author. E-mail address: [email protected] (J. Xu). http://dx.doi.org/10.1016/j.jngse.2016.11.064 1875-5100/© 2016 Elsevier B.V. All rights reserved.
drilled with traditional water-based drilling fluids. This leads to a change in the stress state of the borehole wall and a decrease of the rock strength, which is not conducive to wellbore stability. In particular, as shale has extremely low permeability, the pressure transmission and filtrate invasion of drilling fluids will cause an increase in pore pressure and a reduction in the effective support of the liquid column pressure, thus resulting in wellbore instability (Van Oort, 1997; Stowe et al., 2001). Therefore, the inhibition and sealing performance of the traditional water-based drilling fluids should be enhanced (Qiu et al., 2007). Over the years, inorganic salt, silicate, formate, polyol and amine polymer have been introduced into drilling fluids as inhibitors, while asphalt, silicate and aluminum salts have been used as plugging agents (Caenn and Chillingar., 1996; Ramirez et al., 2007). The inhibition and sealing performance of the traditional water-
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based drilling fluids have been improved, but the problem of borehole instability still occurs (Witthayapanyanon et al., 2013; Stevens et al., 2013). Therefore, the search for an efficient shale stabilizer in water-based drilling fluids for drilling troublesome shale formations is a continuous endeavor. Nano materials have attracted widespread attention in the scientific community (Amanullah and Tahini., 2009; Hoelscher et al., 2012; Zou et al., 2008). The preparation technology of nano materials is one of the three major technologies in the modern era. It has been applied to many fields, such as electronics, chemical industry, and energy (Rao and Cheetham., 2001). Nano materials have at least one dimension in the nanometer scale (1e100 nm) in threedimensional space. Because the dimensions of nano materials are between the microscopic and macroscopic objects, their functionality is often different from conventional materials. Unique properties displayed by nano materials include the quantum size effect, surface effect and small size effect et al. At present, exploration and development of shale gas resources are in full swing, and borehole instability in the drilling process becomes a major problem. Conventional plugging agents are too large to provide full sealing performance, which limits their application. Nano materials have characteristics of the surface effect, small size effect and less consumption, in line with the development trend of drilling fluids to a low solid content. Nano materials also have excellent sealing and inhibition performance, so they are conducive to improving the comprehensive performance of drilling fluids. The application of nano materials in drilling fluids has already gained attention in recent years (Abdo and Haneef., 2012; Li et al., 2012; Amanullah et al., 2011). In the early stage, nanoparticles were introduced into water-based drilling fluids, and it was found that they could penetrate and block the pore throats of shale to form a permanent mud cake, thus reducing the permeability of shale formations (Sensoy et al., 2009). In addition, non-modified silica nanoparticles were introduced into the drilling fluids, and they not only helped to form a thin and compact mud cake, but also greatly improved the rheological and filtration control performance of drilling fluids (Cai et al., 2012; Jung, 2013). Furthermore, the modified silica nanoparticles were used to adjust the performance of water-based drilling fluids, which could enhance the filtration control and inhibition performance of drilling fluids (Jain et al., 2015). However, silica nanoparticles cannot solve the problem of wellbore instability completely, because they are very easy to aggregate and have poor dispersion in water-based drilling fluids (Baran and Cabrera., 2006; Kong et al., 2012). Additionally, the particles are rigid and have poor deformation capacity, so excellent performance is difficult to achieve. Conventional emulsion polymerization is a polymerization process using aqueous media and emulsifiers can help disperse the monomers. Emulsion polymerization system is often composed of monomers, water, an initiator and emulsifiers. Water is used as the medium, and the monomers are dispersed into an emulsion state. Compared with other methods to prepare polymer nanoparticles, conventional emulsion polymerization has the advantages of functioning at low viscosity, having quick heat dissipation, good environmental protection, low cost, and safety in production (Cao, 1997). Exploring the application of polymer latex prepared by emulsion polymerization in drilling fluids is a meaningful work (Liu et al., 2015; Bailey, 2004). On the one hand, the presence of emulsifiers ensures the dispersion of polymer latex particles in water-based drilling fluids. On the other hand, proper polymerization monomer can ensure that the polymer latex particles have both the ability of deformation and temperature resistance. On the basis of giving full play to their deformable sealing performance, the presence of specific functional groups is beneficial to the inhibition performance. Additionally, polymer latex can improve the
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lubrication performance of aqueous solutions. If the prepared polymer latex by emulsion polymerization exhibits these advantages in water-based drilling fluids, it will be a better shale stabilizer to resolve the problem of wellbore instability. But at present the literature reports about the application of polymer latex in drilling fluids are also relatively fewer. In this study, we attempted to prepare the novel polymer latex (SDNL) by emulsion polymerization for water-based drilling fluids. The morphology and thermal characteristics of the polymer nanoparticles were investigated by conventional techniques. Furthermore, the compatible test, shale inhibition test, pressure transmission test and lubrication performance test were carried out to evaluate the comprehensive performance of SDNL in waterbased drilling fluids. Through the laboratory evaluation, it is found that the new additive has broad application prospects in water-based drilling fluids for drilling troublesome shale formations. 2. Materials and methods 2.1. Materials Styrene (St), n-butyl acrylate (BA) and acrylic acid (AA) monomers were purchased from Sinopharm Chemical Reagent Co. Ltd (China). The monomers were purified by vacuum distillation to remove the inhibitors. Octylphenol polyoxyethylene ether (OP-10) was purchased from Aladdin Reagent Co. Ltd (China). Sodium dodecyl sulfate (SDS), potassium persulfate (KPS) and sodium bicarbonate (NaHCO3) were purchased from Sinopharm Chemical Reagent Co. Ltd (China) and used as received. Deionized water was used throughout the work. Shale samples were collected from Turpan Hami Basin, China. The mineralogical composition of the shale samples is presented in Table 1. 2.2. Preparation of polymer latex Polymer latex (SDNL) was prepared by emulsion polymerization with potassium persulfate as an initiator. Polymerization was carried out in a four-necked flask equipped with a reflux condenser, a mechanical stirrer, a thermometer, and a feed inlet tube. Briefly, 32 g of St, 8 g of BA, 1.2 g of AA and 0.12 g of NaHCO3 were dispersed in 60 mL deionized water, in which a certain amount of emulsifiers SDS and OP-10 were dissolved. Then they were transferred into the flask, stirred and heated up to 80 C. After 30 min, KPS (0.24 g) was added to the homogeneous emulsion to induce polymerization. The polymerization was then continued in the flask for another 3.5 h at 80 C and finally cooled to room temperature. 2.3. Characterization of polymer latex The dispersing morphology of the polymer latex particles in aqueous solution was observed by JEOL JEM-2100UHR Transmission Electron Microscopy (TEM) at an accelerating voltage of 200 kV. The particle size distribution (PSD) of the polymer latex particles was measured with dynamic light scattering using the Zetasizer Nano ZS90 produced by Malvern Instruments Ltd. The thermal gravimetric analysis (TGA) was carried out using a TGA/ DSC1 thermal analyzer from METTLER-TOLEDO Inc. at a heating rate of 10 C/min under nitrogen atmosphere. 2.4. Compatible test The base fluid was prepared by mixing 4.0 w/v% pre-hydrated sodium bentonite slurry, 0.3 w/v% of xanthan gum and 1.0 w/v% low viscosity polyanionic celluloses at high speed. Then varied
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J. Xu et al. / Journal of Natural Gas Science and Engineering 37 (2017) 462e470 Table 1 Mineralogical composition of the shale samples. Component
Content (wt%)
Component of clay mineral
Content (wt%)
Quartz Potassium feldspar Plagioclase Calcite Iron dolomite Clay mineral
20 6 29 28 2 15
Kaolinite Chlorite Illite Illite/smectite mixed layer
3 3 12 82
amounts of SDNL were added into the base fluid to evaluate the rheological and filtration control performance of the homogenous drilling fluids. The rheological performance was tested using a model ZNN-D6 viscometer (Haitongda Company, China). The filtration control performance was analyzed with ZNS-2A filtration apparatus (Haitongda Company, China). The rheological parameters were calculated by the following formulas according to the API recommended practice of standard procedure for field testing of drilling fluids.
Apparent ViscosityðAVÞ ¼ F600 =2 ðmPa$sÞ Plastic ViscosityðPVÞ ¼ F600 F300 ðmPa$sÞ Yield PointðYPÞ ¼ ðF300 PVÞ=2 ðPaÞ The temperature resistance of SDNL in drilling fluids was conducted by aging test. The base fluid and the base fluid containing 2.0 w/v% SDNL were hot rolled at 120 C for 16 h in a XGRL-4 type rolling oven (Haitongda Company, China). The rheological and filtration control performance of the drilling fluids after aging were tested again as described above.
2.6. Pressure transmission test The pressure transmission test was performed using a simulation experiment device for hydra-mechanics coupling of shale (Xu and Qiu., 2006; Xu et al., 2005). The device was developed by China University of Petroleum (East China), and the test evaluated the sealing performance of the prepared novel polymer latex SDNL. Fig. 1 shows a schematic of the simulation experiment device. The experiment device consists of five parts, including a high temperature-pressure kettle, a hydraulic control system, a fluid circulation system, a temperature transfer and control system, and a data acquisition-processing system. The main technical indices include the axial compression and confining pressure ranging from 0 to 50 MPa, the testing fluid pressure between 0 and 35 MPa, the sensor pressure between 0 and 10 MPa, and the temperature between room temperature and 150 C. A shale sample was placed into the core holder, and made contact with the testing fluid through the upstream inlet. In the experimental process, the upstream pressure was maintained at 2.0 MPa while the initial downstream pressure was set to 1.0 MPa. Pressure transmission curves were obtained to evaluate the sealing performance of the testing fluid. The data from these curves demonstrated how the downstream pressure changed over time.
2.5. Shale inhibition test 2.5.1. Linear swelling test The linear swelling test was designed to measure the linear expansion rate of shale cores after making direct contact with testing fluid (Zhong et al., 2016). Firstly, the shale samples were crushed to fine powders <100 mesh, and oven-dried for four hours at 105 C. A shale core was prepared by pressing 10 g of crushed shale samples at 10 MPa for 5 min. The initial height of shale core was measured. Finally, the shale core was placed into the expansion instrument (NP-02A, Haitongda Company, China), and the swelling height with time was recorded after the shale core made contact with testing fluid. Then the linear expansion rate could be calculated using the swelling height recorded. Moreover, the smaller linear expansion rate indicated the better capacity of the testing fluid to inhibit shale hydration and swelling. 2.5.2. Shale cuttings hot-rolling dispersion test The shale cuttings hot-rolling dispersion test was designed for two purposes. It can determine the dispersion characteristics of shale cuttings and evaluate the testing fluid's ability to inhibit shale hydration and dispersion (Zhong et al., 2015). For the test, 50 g of shale cuttings with a particle size between 2.00 mm and 3.35 mm were placed into the sealed cell containing 350 mL testing fluid and hot rolled at 77 C for 16 h. After cooling to room temperature, the remaining shale cuttings were recovered with a 40 mesh sieve, washed with fresh water, and oven-dried at 105 C for 4 h. The recovery rate of shale cuttings was calculated using the measured weight of the recovered shale cuttings. Moreover, the higher recovery rate indicated better ability of the testing fluid to inhibit shale hydration and dispersion.
2.7. Evaluation of lubrication performance An extreme pressure lubrication apparatus (EP-B, Haitongda Company, China) was used to measure the lubrication performance of drilling fluids and evaluate the ability of lubricants to reduce torque. Varied amounts of polymer latex SDNL were added into the 4% sodium bentonite slurry, and their influence on the extreme pressure lubrication coefficient of drilling fluids were monitored. Firstly, the sample cell full of testing fluid was immobilized on the supporting plate, while the friction ring and block were completely immersed into testing fluid. The torque value was adjusted to 16.95 N m with a rapidly rotating pressurized handle, and the time was recorded with stopwatch. After five minutes at a speed of 60 rpm/min, the friction coefficient was obtained to calculate the extreme pressure lubrication coefficient.
Fig. 1. The schematic of the simulation experiment device for pressure transmission test.
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3. Results and discussion 3.1. Characterization of polymer latex The specific surface area of the polymer latex nanoparticles was found to be 26.03 m2/g, which was very high. Fig. 2 presents the particle size distribution of the polymer latex SDNL. It can be seen that the prepared polymer latex had a narrow particle size distribution, which ranged from 50 nm to 190 nm, and D50 (medium particle size) was 88 nm with the D10 value of 62 nm and D90 value of 129 nm. Fig. 3 presents the TEM micrograph of diluted SDNL. The micrograph displayed uniformly dispersed, spherical polymer latex nanoparticles. Moreover, the average particle size was about 85 nm, which coincided with the D50 value of around 88 nm from the test results of particle size distribution. The TGA curve of the polymer latex nanoparticles is shown in Fig. 4. It can be seen that when the temperature was above 270 C, the polymer latex nanoparticles began to decompose. The weight loss of the polymer latex nanoparticles increased dramatically as the temperature was increased to 375 C, at which point the TGA curve descended almost vertically. This indicated that the newly prepared polymer latex nanoparticles maintained good thermal stability. 3.2. Compatible test The influence of polymer latex SDNL on the rheological and filtration control performance of the base fluid is shown in Table 2. It can be concluded that with the increase of the SDNL concentration in drilling fluids, the apparent viscosity, plastic viscosity, yield point, and the ratio of dynamic shear force of the drilling fluids gradually increased. This indicated that SDNL could enhance the shear thinning property of drilling fluids, and thus more conducive for drilling fluids to effectively breaking rock at high shear rate and efficiently carrying cuttings at low shear rate. In addition, when adding 2.0 w/v% of SDNL into drilling fluids, the API filtration loss was reduced from 8.6 mL to 5.4 mL, illustrating that SDNL had favorable filtration control performance. The aging test was performed to evaluate the temperature
Fig. 3. TEM micrograph of diluted polymer latex SDNL.
resistance of SDNL. Table 3 shows the results of aging test. It can be seen that SDNL could improve the rheological performance of drilling fluids due to the increase of the ratio of dynamic shear force. In addition, compared to base fluid without SDNL, the filtration loss decreased obviously from 9.8 mL to 6.0 mL, which indicated that SDNL still retained excellent filtration control performance. Thus, SDNL possessed good temperature resistance in drilling fluids at 120 C.
3.3. Evaluation of inhibition performance Hydration occurs when shale is in contact with water, which can lead to an increase in the hydration stress, a change in the shale strength and shale deformation such as swelling and dispersion (Anderson et al., 2010). Therefore, the shale inhibition performance of SDNL can be directly reflected by means of the linear swelling test and the shale cuttings hot-rolling dispersion test. Experiments were carried out to evaluate the shale inhibition performance of SDNL by comparing frequently used shale inhibitors, including inorganic salts (KCl) and polymeric alcohol (JHC).
Fig. 2. Particle size distribution of diluted polymer latex SDNL.
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Fig. 4. TGA curve of the polymer latex nanoparticles.
Table 2 Effect of SDNL on the rheological and filtration control performance of the base fluid. Formula Base Base Base Base
fluid fluidþ0.5% SDNL fluid þ1% SDNL fluid þ2% SDNL
AV (mPa.s)
PV (mPa.s)
YP (Pa)
YP/PV
API FL(mL)
41 49 51 55
31 32 33 35
10 17 18 20
0.32 0.53 0.55 0.57
8.6 6.8 6.2 5.4
inferred that SDNL had outstanding capacity to prevent shale hydration and dispersion. There are some factors contributing to the excellent inhibition ability of SDNL. The polymer latex particles have the characteristics of small particle size, large specific surface area and high surface energy, which can be easily adsorbed onto the surface of shale. And the polar hydrophilic carboxyl groups are introduced onto the
Table 3 Temperature resistance of SDNL in drilling fluids at 120 C for 16 h. Formula
Experimental condition
AV (mPa.s)
PV (mPa.s)
YP (Pa)
YP/PV
API FL (mL)
Base fluid Base fluid þ2% SDNL
120 C/16 h 120 C/16 h
29 40
24 28
5 12
0.21 0.43
9.8 6.0
3.3.1. Linear swelling test Results of the linear swelling test for different concentrations of SDNL, KCl, and JHC are presented in Fig. 5. The linear expansion rate of shale sample in tap water was 12.5%, demonstrating that the shale had strong hydration swelling capacity. Besides, it can be clearly seen that the inhibition performance of SDNL was better than KCl and JHC. At a concentration of 2% SDNL, the linear expansion rate was only 4.9%, which was 2.55 times lower than that of tap water, and also significantly lower than that of KCl and JHC at the same concentration. It can be inferred that SDNL had excellent ability to prevent shale hydration and swelling.
particle surface via the monomer of acrylic acid (AA). When shale is in contact with an aqueous solution of SDNL, the polymer particles are adsorbed onto the surface of the clay particles by hydrogen bonds (Burchill et al., 1983). The clay particles bridge together by multi-point adsorption, thereby maintaining shale integrity. Moreover, the adsorption of polymer particles can form a hydrophobic isolation film, which can further prevent water molecules into clay mineral particles clearance, and therefore effectively inhibiting clay hydration swelling and dispersion.
3.3.2. Shale cuttings hot-rolling dispersion test Fig. 6 presents the results of the shale cuttings hot-rolling dispersion test for various concentrations of SDNL, KCl and JHC. Results indicated that the recovery rate of shale cuttings in tap water was 40.2%, demonstrating that the shale had strong hydration dispersion capacity. It can be clearly concluded that the inhibition performance of SDNL was better than KCl and JHC. When the concentration of SDNL was 2%, the recovery rate was 93.7%, which was 2.33 times higher than that of tap water, and also significantly higher than that of KCl and JHC at the same concentration. It can be
Retarding pore pressure transmission was a key measure to maintain shale stability (Ewy and Morton., 2009). In this test, 4.0 w/ v% pre-hydrated sodium bentonite slurry was used as the base fluid. The curves of the pressure transmission test are depicted in Fig. 7. Pore pressure increased from initially 1 MPae2 MPa after 2400 s for the shale sample, and 7700 s for the base fluid. After interacting with the base fluid containing 2.0 w/v% SDNL, the pore pressure was only 1.8 MPa after 16,300 s. The pressure transmission time was much longer than the shale sample and the base fluid. More importantly, when the pore pressure was increased to
3.4. Pressure transmission test
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Fig. 5. Effect of different shale inhibitors on shale linear expansion rate.
Fig. 6. Effect of different shale inhibitors on shale cuttings recovery.
1.8 MPa, it was almost no longer increased. This indicated that the shale surface was covered with a dense plugging film. It could be concluded that SDNL had strong plugging capacity, which could effectively retard pore pressure transmission, and therefore maintain shale stability. A schematic diagram of the plugging mechanism is shown in Fig. 8. As shown in Fig. 8, the polymer nanoparticles under differential pressure rely on its elastic deformation, bridging and sealing the pore throats and micro cracks of the shale. Over time, a dense plugging film gradually forms on the shale surface, the permeability of which is almost zero, and the pore pressure is difficult to continually transmit. Therefore, in shale
formation drilling process, SDNL can retard pore pressure transmission and reduce filtrate invasion, thereby stabilizing shale formation. It can be regarded as an efficient plugging agent. 3.5. Evaluation of lubrication performance The influence of SDNL on the extreme pressure lubrication coefficient of drilling fluids is shown in Table 4. As the concentration of SDNL was increased, the lubrication coefficients of drilling fluids gradually decreased. Adding 2.0 w/v% of SDNL into the drilling fluids, the lubrication coefficient was reduced by 64.4%, indicating
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Fig. 7. Pressure transmission test curves.
Fig. 8. Schematic diagram of plugging mechanism, (a) compression and deformation of polymer nanoparticles and (b) formation of a dense plugging film.
Table 4 Influence of SDNL on lubrication performance. Concentration(wt/v%)
Lubrication coefficient
Reduced rate of the lubrication coefficient (%)
0 0.5 1 2
0.432 0.245 0.207 0.154
e 43.3 52.1 64.4
that SDNL had good lubrication performance. The lubrication mechanism of SDNL was different from traditional lubricants, which depended on the micro “rolling” and “film forming” (Zhao and Zhou., 1999; Zhang et al., 1994; Xue et al., 1997). A schematic diagram of the lubrication mechanism is presented in Fig. 9. As shown in Fig. 9, the small nanoparticles are spherical and can be freely rolled between the friction pairs. They act as “rolling bearing”, which can make the sliding friction into rolling friction, thereby reducing torque. In addition, the polymer nanoparticles dispersed in the drilling fluids can be easily squeezed and deformed
during the friction process, and eventually form a thin and dense boundary lubrication film on the friction surface, which plays the role of friction-reducing and anti-wear. Therefore, in shale formation drilling process, SDNL can effectively improve the lubrication performance of drilling fluids, which will help reduce the drilling torque and resistance, thus reduce the wear of drilling tools. 4. Conclusion In summary, a novel polymer latex (SDNL) was successfully
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Fig. 9. Schematic diagram of lubrication mechanism, (a) “rolling bearing” of polymer nanoparticles and (b) “lubrication film forming” of polymer nanoparticles.
prepared by emulsion polymerization as a multifunctional additive for water-based drilling fluids. The particle size distribution of spherical polymer latex particles was between 50 and 190 nm with D50 value of 88 nm. SDNL could significantly improve the rheological performance and enhance the shear thinning property of drilling fluids. SDNL had excellent filtration control performance and good temperature resistance at 120 C. The inhibition performance test showed that SDNL had excellent ability to prevent shale hydration swelling and dispersion. At the concentration of 2%, the linear expansion rate of the shale sample was only 4.9%, and the recovery rate was 93.7%, indicating that it possessed a significantly better inhibition performance than that of KCl and JHC at the same concentration. Based on the pressure transmission test results, the base fluid containing 2.0 w/v% of SDNL required more time to transfer the same pressure difference compared with shale and base fluid. Furthermore, a dense plugging film finally formed on the shale surface, the permeability of which was almost zero, making the pore pressure difficult to continually transmit. The lubrication performance results illustrated that SDNL had good lubrication performance. After adding 2.0 w/v% of SDNL into the drilling fluids, the lubrication coefficient was reduced by 64.4%, indicating that SDNL could effectively improve the lubrication performance of drilling fluids. Hence, the newly developed polymer latex SDNL is expected to be a superior additive in water-based drilling fluids for drilling troublesome shale formations. Acknowledgments This work was financially supported by National Natural Science Foundation of China (51474236), the National Key Basic Research Special Foundation of China (2015CB251205), and China Postdoctoral Science Foundation (2015M580618; 2016T90658). References Abdo, J., Haneef, M.D., 2012. Nano-enhanced drilling fluids: pioneering approach to overcome uncompromising drilling problems. J. Energy Resour. Technol. 134 (1), 014501. Amanullah, M., Al-Tahini, A.M., 2009. Nano-technology-its significance in smart fluid development for oil and gas field application. In: SPE Saudi Arabia Section Technical Symposium. Society of Petroleum Engineers. Amanullah, M., AlArfaj, M.K., Al-abdullatif, Z.A., 2011. Preliminary test results of nano-based drilling fluids for oil and gas field application. In: SPE/IADC Drilling Conference and Exhibition. Society of Petroleum Engineers. Anderson, R.L., Ratcliffe, I., Greenwell, H.C., Williams, P.A., Cliffe, S., Coveney, P.V., 2010. Clay swellingda challenge in the oilfield. Earth-Science Rev. 98 (3), 201e216. Bailey, L., 2004. Latex additive for water-based drilling fluids. US, US 6715568 B1. Baran, J.R., Cabrera, O.J., 2006. U.S. Patent No. 7,033,975. U.S. Patent and Trademark Office, Washington, DC. Bol, G.M., Wong, S.W., Davidson, C.J., Woodland, D.C., 2013. Borehole stability in shales. SPE Drill. Complet. 9 (2), 87e94. Burchill, S., Hall, P.L., Harrison, R., Hayes, M.H.B., Langford, J.I., Livingston, W.R., Tuck, J.J., 1983. Smectite-polymer interactions in aqueous systems. Clay Min. 18 (4), 373e397.
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