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Oct 17, 2017 - DOI: 10.1039/C9RA00038K. Zhiwen Dai, Jinsheng Sun, Yan Wang. A Polymer-based Drilling Fluid with High Temperature, Salt and Calcium ...
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Novel Acrylamide/2-Acrylamide-2-methylpropanesulfonic Acid/4Vinylpyridine Terpolymer as an Anti-calcium Contamination FluidLoss Additive for Water-Based Drilling Fluids Jie Cao,*,†,‡ Lingwei Meng,†,‡ Yuping Yang,§ Yuejun Zhu,∥ Xiaoqiang Wang,†,‡ Chengyan Yao,†,‡ Mingbo Sun,*,†,‡ and Hanyi Zhong†,‡ †

School of Petroleum Engineering, China University of Petroleum (East China), Qingdao, Shandong 266580, People’s Republic of China ‡ National Engineering Laboratory for Testing and Detection Technology of Subsea Equipment, Qingdao, Shandong 266580, People’s Republic of China § CNPC Drilling Research Institute, Beijing 102206, People’s Republic of China ∥ CNOOC Research Institute, Beijing 100027, People’s Republic of China ABSTRACT: A novel terpolymer of acrylamide (AM), 4-vinylpyridine (VP), and 2-acrylamide-2-methylpropanesulfonic acid (AMPS) was synthesized through free radical polymerization and characterized by proton nuclear magnetic resonance, Fourier transform infrared spectroscopy, elemental analysis, and static light scattering measurement. The monomer ratio was shown to be the predominant factor to the fluid-loss control performance of this polymer in drilling fluids. The terpolymer under optimal polymerization conditions (PAAV) was prepared, and the dipolymer of AM and AMPS (PAA) was synthesized as a contrast sample. In an American Petroleum Institute (API) filtration test of bentonite-based mud with 10% CaCl2 contamination after a 16 h aging at 150 °C, mud with 1% PAAV maintained an API filtrate volume (FLAPI) of 4.8 mL, whereas mud with 1% PAA reached a FLAPI of 96.0 mL. The fluid-loss control mechanism of PAAV was investigated through adsorption experiments, ζ potential measurements, and particle size distribution analysis. The results illustrate that the introduction of VP units into a polymer molecule greatly improves the temperature resistance performance of the polymer and enhances the interaction between the polymer and bentonite, which improves colloidal properties of bentonite particles, and these make PAAV a pronounced fluidloss control agent in deep gypsum drilling operations.

1. INTRODUCTION Water-based drilling fluids (WDFs) are widely used in the petroleum industry. They are important for oil well drilling because of their various functions, such as transporting rock cuttings to the surface, lubricating drill bits, applying hydrostatic pressure to wellbores to ensure well safety, and minimizing fluid-loss across permeable formations by forming a filter cake on the wellbore.1−4 Sodium montmorillonite (Mt), a major component of bentonite, is a dioctahedral smectite (called swelling clay mineral group) and has excellent adsorption, ion-exchange properties, and unique colloidal and rheological properties. Thus, bentonite is used as the major additive in WDFs to improve the viscosity via edge-to-face attraction and form a compact filter cake, which prevents deep filtration invasion.5,6 The ability of bentonite to carry out the aforementioned functions is strongly linked to the chemical environment in which bentonite is placed. When bentonite is exposed to harsh conditions (e.g., high-salinity water), its ability to hydrate, swell, and disperse in aqueous solution is suppressed greatly, especially in calcium contamination.7 Ca2+ has a much larger impact on bentonite than other ions. For instance, the ratio of the coagulation value between Ca2+ and Na+ is approximately 100:1.6 according to the Derjaguin, Landau, Verwey, and Overbeek (DLVO) theory and the Schulze−Hardy rule.8−10 Ca2+ can adsorb on the surface of bentonite and greatly © XXXX American Chemical Society

compact the electrical double layer, which makes bentonite obviously coagulate and greatly worsen the rheology property of the suspension. As a result, the filtrate loss volume will increase dramatically and thick and loose mud cakes will form, as evidenced by sticking of the pipes and wellbore instability.8 Nowadays, the search for petroleum resources has moved out of these “easy to get” shallow pay zones and partly moved into some deeply buried reservoirs. High salinity, high aquatic hardness, and high temperature are usually met and cause severe water loss of the drilling fluid. Therefore, novel drilling fluids with improved filtration properties have received considerable attention. In recent years, various types of polymer-based fluid-loss additives, such as starch, lignite, xanthan gum, polyanionic cellulose, chitosan, and synthetic polymers, have been applied to improve the filtration property of WDFs.6,11−17 Generally speaking, a polymer molecule as a fluid-loss additive contains at least two types of functional groups. One is the adsorption group, such as a hydroxyl group, an amide group, and a cationic group, for enhancing the interaction between the polymer and bentonite. The polymeric fluid-loss additive can therefore better adsorb on the bentonite surface by hydrogen and coordination bonding to raise the ζ Received: August 10, 2017 Revised: September 24, 2017 Published: October 17, 2017 A

DOI: 10.1021/acs.energyfuels.7b02354 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels

analysis was purified from sodium bentonite, which was from Xiazijie Bentonite Company, China. The bentonite was dispersed in deionized water for 24 h to make 80 g/L suspensions. The suspensions were centrifuged at 8000 revolutions/min for 30 min, and the upper ocher part of the precipitation was recovered. The purified drilling fluid bentonite (Bent) sample was dried at 105 °C for 24 h and sieved by a 200-mesh sieve.21,22 The cation-exchange capacity of Mt was determined to be 118 cmol/kg by the ammonium acetate method. The Bent was supplied by Weifang Huawei Bentonite Group Co., Ltd., China, following the American Petroleum Institute (API) standard. 2.2. Synthesis of Poly(AM−AMPS−VP) and Poly(AM−AMPS). Poly(AM−AMPS−VP) was prepared through radical polymerization. Orthogonal tests were used to optimize the four main factors to determine the best formulation in reducing fluid loss under calcium contamination, and these four factors are the mole ratio between adsorption groups (AM + VP) and hydration groups (AMPS), the mole ratio between AM and VP, the reaction pH, and the initiator amount. A total of 20 g of monomers with a certain mole ratio of AM, AMPS, and VP was dissolved in 80 g of distilled water at room temperature. After the pH was adjusted to a certain value, the solution was added to a three-neck flask and stirred with an electric stir bar under an inert N2 atmosphere for 30 min. Then, APS was added to the solution to trigger the reaction. The system was maintained at 70 °C for 6 h to obtain a crude product and then dialyzed through a semipermeable membrane [molecular weight cut-off (MWCO) of 6000− 8000] in distilled water for 24 h. The dialyzed solution was dried at 80 °C under a vacuum condition, and the dried product was ground to powder. The poly(AM−AMPS−VP) under the optimal polymerization conditions was named as PAAV. A contrast sample poly(AM− AMPS), named as PAA, was prepared in the same conditions as PAAV. 2.3. Characterization of PAAV and PAA. A Bruker AVANCE 400 NMR spectrometer was used to measure the proton nuclear magnetic resonance (1H NMR) spectra. D2O was used for fieldfrequency lock, and the observed 1H chemical shifts were reported in parts per million (ppm). Before 1H NMR analysis, the pH value of the polymer solution was adjusted to around 9.0 by dilute NaOH/D2O solution. Elemental analysis (C, N, S, and H) was performed on an Elementar Vario E1 III analyzer (Germany). Fourier transform infrared spectroscopy (FTIR) was carried out on a Tensor 27 spectrometer (Bruker, Switzerland) with samples prepared as KBr pellets. The spectra in the frequency range of 4000−400 cm−1 were acquired at a resolution of 4 cm−1 with a total of 16 scans. The static light scattering (SLS) measurement was conducted by a Wyatt Technology DAWN HELEOS 18 angle (from 15° to 165°) light scattering detector using a Ga−As laser (658 nm and 40 mW). 2.4. Performance Evaluation of PAAV and PAA. 2.4.1. Mud Preparation and Aging Test. A total of 16 g of Bent was added to 400 mL of distilled water, and the mixture was stirred at 2000 rpm for 2 h, followed by static standing for 24 h. A certain amount (w/v) of polymer was dissolved in the Bent mixture with 10 000 rpm stirring for 20 min. Then, a certain amount (w/v) of CaCl2 was added with 10 000 rpm stirring for 20 min, followed by static standing for 12 h, to ensure thorough contact between Bent and calcium ions. Aging experiments of Bent/polymer mud were carried out in a GH-3-type rolling oven through hot rolling at 150 °C for 16 h. 2.4.2. API Static Filtration Test. The API filtrate volume of Bent/ polymer mud was measured using a ZNZ-D3-type medium-pressure filtration apparatus (Qingdao Haitongda Special Instrument Co., Ltd., China). A volume of mud was loaded on the filter press equipped with a filter paper under a fixed pressure of 0.7 MPa. The filtrate volume (FLAPI) was recorded after 30 min, which was recommended by API.23 2.4.3. Rheology Test. The rheological property of Bent/polymer mud was measured using the ZNN-D6-type six-speed rotating viscometer (Qingdao Haitongda Special Instrument Co., Ltd., China). The apparent viscosity (AV), plastic viscosity (PV), and yield point (YP) were calculated from 300 and 600 rpm readings (θ300 and θ600) using the API recommended procedure for field testing of drilling fluids.

potential and hydrated shell thickness of the bentonite particle. The other is the hydration group, such as a carboxylate group and a sulfonate group, for improving the dispersion property of bentonite. It reduces the attractions between bentonite particles by electrostatic stabilization, strengthens the bentonite structure, and plugs the filter cake holes.17 Hence, the quality of the filter cake is improved, and the coefficient of penetration and filtration is reduced. Additionally, inflexibility groups, such as big side groups, are usually introduced into the polymer molecule for a better temperature resistance ability.18 Therefore, a reduced filtrate volume of drilling fluid can be expected under a high-salinity and high-temperature drilling environment.19,20 In this study, a novel polymeric fluid-loss additive for WDFs was prepared by co-polymerization of acrylamide (AM), 2acrylamide-2-methylpropanesulfonic acid (AMPS), and 4vinylpyridine (VP). In this polymer, AM and VP units act as adsorption groups and AMPS units act as an anti-calcium hydration group. VP units with inflexible pyridyl groups also help to improve the temperature resistance of the polymer. Another advantage is that, as shown in Figure 1, the state of VP

Figure 1. Cationization and deionization of VP units in PAAV with the change of pH.

units (cationization or deionization) changes at different pH values. For common bentonite-based WDFs in normal drilling (alkaline environment), the terpolymer under optimal polymerization conditions (PAAV) exists as an anionic polymer and has negligible impact on the integrate performance of WDFs compared to other amphoteric drilling fluid additives. When a calcium contamination occurs, the pH value of the drilling fluid system obviously decreases and the positively charged VP unit increases the interaction between PAAV and bentonite and, thus, improves colloidal properties of bentonite particles and reduces the filtrate volume of drilling fluid. As far as we know, this paper is the first investigation on the addition of a pyridyl unit into an AM-based polymer for the application of a fluidloss additive in WDFs. PAAV might be especially suitable for the fluid-loss control when the invasion of formation water containing a high Ca2+ concentration into WDFs occurs during the drilling process.

2. EXPERIMENTAL SECTION 2.1. Materials. AM (99%), AMPS (99%), and VP (95%) were purchased from J&K Chemical, Ltd. (Beijing, China) and used as received. Ammonium persulfate (APS, 98%), CaCl2 (anhydrous, 96%), NaOH (97%), and HCl (36%) were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Sodium montmorillonite (Mt) was used in adsorption experiments, and ζ potential B

DOI: 10.1021/acs.energyfuels.7b02354 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels AV = θ600/2

(mPa s)

PV = θ600 − θ300 YP = θ300 − θ600/2

Table 2. Orthogonal Experiment Results for Poly(AM− AMPS−VP)

(mPa s) (Pa)

2.5. Fluid-Loss Control Mechanism Analysis. 2.5.1. Adsorption Experiments. The isothermal adsorption experiments of PAAV onto Mt were carried out in batch mode at different pH values (9.1, 6.2, and 5.2). PAAV solutions had varying concentrations between 20 and 1250 mg/L. For the adsorption process, 0.1 g of Mt was added to 20 mL of PAAV solution, and the solution pH was adjusted by NaOH (0.1 mol/ L) or HCl (0.1 mol/L). Subsequently, the suspension was continuously shaken in an incubator shaker at 200 rpm and 25 °C for 24 h. Then, the suspension was centrifuged at 6000 rpm for 10 min, and the adsorbed amount of PAAV was determined from the difference between the initial polymer concentration and the concentration in the supernatant, measured by a total organic carbon analyzer (Shimatsu TOC 5050). The adsorption capacities of Mt for PAAV and PAA at different pH values were also carried out in a similar process with an initial polymer concentration of 1000 mg/L. 2.5.2. ζ Potential Measurements. The effects of pH and CaCl2 concentration on the ζ potential of Mt, Mt/PAAV, and Mt/PAA suspensions were investigated using a Zeta Meter 3.0 (Zeta Meter, Inc., Staunton, VA, U.S.A.) equipped with a microprocessor unit, which can automatically calculate the electrophoretic mobility of the particles and convert it to the ζ potential. The average of five measurements was taken to represent the measured ζ potential. 2.5.3. Particle Size Distribution Analysis. Particle size distributions of Bent/polymer suspensions before and after aging were analyzed using a Baxter Bettersize 2000 laser (Dandong, China). The Bent/ polymer mud contained Bent (4%, w/v), PAA or PAAV (1%, w/v), and CaCl2 (10%, w/v). Aging experiments were carried out at 150 °C for 16 h. The analyzed sample was obtained by adding 2 g of Bent/ polymer mud to 30 g of distilled water.

mole ratio of (AM + VP)/AMPS

reaction pH

initiator amount (wt %)a

1 2 3

1:1 3:1 5:1

6:4 5:5 4:6

5 7 9

0.1 0.2 0.3

mole ratio of (AM + VP)/AMPS

reaction pH

initiator amount

FLAPI (mL)

1 2 3 4 5 6 7 8 9 K1 K2 K3 R

1:1 1:1 1:1 3:1 3:1 3:1 5:1 5:1 5:1 42.6 33.8 14.1 28.5

6:4 5:5 4:6 6:4 5:5 4:6 6:4 5:5 4:6 21.3 28.3 40.9 19.6

5 7 9 7 9 5 9 5 7 24.7 30.7 35.0 10.3

0.1 0.2 0.3 0.3 0.1 0.2 0.2 0.3 0.1 29.9 26.4 34.2 7.8

28.0 37.5 62.2 29.5 36.6 35.3 6.3 10.8 25.1

Figure 2. 1H NMR spectra of PAA and PAAV.

the polymer backbone) appears at 1.9−2.3 ppm, methylene proton (−CH2− in the polymer backbone) appears at 1.0−1.8 ppm, and methylene proton of the AMPS unit (−CH2− of −CH2SO3−) contributes to the signal at around 3.0−3.6 ppm. Additionally, the peaks at 7.3−7.5 and 8.2−8.5 ppm for PAAV consist of the contribution of two different protons in pyridyl units. The actual mole ratio of AM/AMPS/VP was calculated via 1H NMR spectra, as listed in Table 3. Figure 3 shows FTIR spectra of PAAV and PAA. The characteristic peaks around 1186 and 1041 cm−1 at PAAV and PAA are identified as the absorption band of sulfonic groups. The characteristic peaks around 1550 and 771 cm−1 at PAAV are assigned to the absorption band of pyridyl groups.24 The weight-average molecular weight (Mw) was obtained from SLS analysis in 0.1 mol/L NaCl solution. The results of Mw are listed in Table 3, where C ranges from 3.5 × 10−5 to 2.3 × 10−4 g/mL. This indicates that PAAV and PAA have similar Mw. 3.3. Fluid-Loss Control Property Measurements. Figure 4 shows FLAPI as a function of the CaCl2 concentration for Bent-based mud with 1% PAA or PAAV. PAA and PAAV present similar properties for controlling FLAPI without calcium contamination, and both filtrate volumes are less than 10 mL. The filtrate volume of PAA mud increased sharply in calcium contamination. When the CaCl2 concentration increases from 2

Table 1. Factors and Levels of the Polymerization Experiment mole ratio of AM/VP

mole ratio of AM/VP

3.2. Characterization of PAA and PAAV. The structure of polymer was confirmed by 1H NMR analysis, and the spectra are shown in Figure 2. As shown, methine proton (−CH− in

3. RESULTS AND DISCUSSION 3.1. Orthogonal Test of Poly(AM−AMPS−VP). According to the synthesis principle of the polymer fluid-loss additive, dominant factors of the polymer performance include the monomer mole ratio, pH, and initiator amount. The orthogonal tests designed with three levels and four factors are shown in Table 1, and the results are shown in Table 2. The fluid-loss

factor level

sample

a

Initiator amount is the weight percentage of initiator in total monomer.

control properties of polymer samples were evaluated in Bentbased mud after aging, where the sample and CaCl 2 concentrations were 1 and 10%, respectively. The monomer mole ratio has the most pronounced effect on filtration under calcium contamination, which means that the functional group ratio is crucial to the fluid-loss control performance of additives in drilling fluids.8 PAAV was synthesized under the following optimal conditions determined by FLAPI: mole ratio of 5:1:4 between AM, VP, and AMPS, reaction pH of 5, and initiator concentration of 0.2 wt %. PAA was synthesized under the same conditions, except 6:4 was used for the mole ratio between AM and AMPS. C

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Energy & Fuels Table 3. Composition and Molecular Weight of PAA and PAAV elemental analysis

a

sample

C (wt %)

H (wt %)

N (wt %)

S (wt %)

PAA PAAV

37.49 36.07

6.137 6.265

8.97 8.45

8.899 8.205

actual mole ratio in the polymer (AM/AMPS/VP) 56.6:43.4:0a 50.6:42.4:7.0a

57.5:42.5:0b 51.6:41.3:7.1b

Mw (×106, g/mol) 1.69c 1.16c

Determined via elemental analysis. bDetermined via 1H NMR. cDetermined via SLS.

Figure 3. FTIR spectra of PAA and PAAV.

Figure 5. FLAPI of Bent/PAA and Bent/PAAV under different polymer concentrations after aging at 150 °C for 16 h.

Table 4. API Filtration Tests of Different Polymers in Mud after Aging at 150 °C for 16 h sample

FLAPI (mL)a

partially hydrolyzed polyacrylamide sodium carboxymethyl cellulose sulfonated phenol formaldehyde resin Driscal D polymer PAA PAAV

138 132 79.6 16.2 96.0 4.8

a

The CaCl2 concentration is 10%, and the polymer concentration is 1%. Figure 4. FLAPI of Bent, Bent/PAA, and Bent/PAAV under different CaCl2 contaminations after aging at 150 °C for 16 h.

used additive in WDFs, Driscal D polymer, synthesized via addition polymerization, was supplied by Chevron Phillips Chemical Company. PAAV presented the best fluid-loss control performance in all investigated polymers. Partially hydrolyzed polyacrylamide and sodium carboxymethyl cellulose represented worse performance than PAA. The hydration groups in these two polymers were both carboxylate groups, which could be partial precipitation as a result of the bridging effect of Ca2+. Sulfonated phenol formaldehyde resin was an important antisalt and anti-temperature fluid-loss control additive in WDFs, especially for drilling in China. However, as a result its low molecular weight and weak interaction between bentonite particles and hydroxyl groups on this polymer, the proper concentration in WDFs for this polymer was generally more than 4%. Table 5 shows rheological properties (AV, PV, and YP) of different systems before and after aging tests. For a 4% Bent system, the increase of AV, PV, and YP was observed because the delamination of Mt particles was promoted by the elevating temperature, resulting in a higher surface area and, accordingly, more resistance to flow. A descending trend of AV, PV, and YP was observed for all calcium-contaminated systems after aging. Divalent cations can obviously compress the electric double layer of Mt particles; as a result, the network structure and faceto-face associations are disrupted.25,26 It is noteworthy that the

to 20%, FLAPI increases from 56 to 107 mL. However, the filtrate volume curve of PAAV mud is nearly flat as the CaCl2 concentration increases. FLAPI is only 8.2 mL when the CaCl2 concentration is 20%. These results clearly demonstrate that PAAV exhibited much better anti-calcium contamination performance than PAA at a high temperature and could resist a 20% concentration of calcium contamination. Figure 5 shows FLAPI as a function of the polymer concentration for Bent-based mud with 10% CaCl2. FLAPI of Bent/PAA is virtually unchanged when the polymer concentration is less than 1%. FLAPI decreases to 67 mL when the polymer concentration is 1.5%. FLAPI of Bent/PAAV obviously decreases with the increase of the polymer concentration. When the polymer concentration is high enough (0.8%), FLAPI changes very slowly and could be as low as 7 mL. These results indicate that PAAV is an efficient anti-calcium contamination fluid-loss additive for WDFs under a high temperature. The anti-calcium contamination properties for other frequently used fluid-loss control additives in WDFs, including partially hydrolyzed polyacrylamide, sodium carboxymethyl cellulose, sulfonated phenol formaldehyde resin, and Driscal D polymer, were also studied, as shown in Table 4. As a widely D

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Energy & Fuels Table 5. Rheological Behaviors of PAAV and PAA in Mud after aging at 150 °C for 16 h

before aging sample 4% 4% 4% 4% 4% 4% 4%

Bent Bent Bent Bent Bent Bent Bent

+ + + + + +

4% CaCl2 + 1% PAA 4% CaCl2 + 1% PAAV 10% CaCl2 + 1% PAA 10% CaCl2 + 1% PAAV 20% CaCl2 + 1% PAA 20% CaCl2 + 1% PAAV

AV (mPa s)

PV (mPa s)

YP (Pa)

AV (mPa s)

PV (mPa s)

YP (Pa)

10 45.5 40.5 48 45.5 69 71.5

7 20 24 21 31 51 50

3 25.5 16.5 27 14.5 18 21.5

26 14 15 12 18 11.5 23.5

16 12 12 10 13 9 16

10 2 3 2 5 2.5 7.5

contaminated PAAV system exhibited better rheological properties than the contaminated PAA system at the same aging conditions, meaning that the number and surface area of Mt particles are larger in the contaminated PAAV system. This indicates that PAAV has better deflocculation performance for Bent mud during the calcium contamination process.27,28 3.4. Fluid-Loss Control Mechanism Analysis. 3.4.1. Adsorption Behavior. The adsorption capacities (qe, mg/g, per unit weight of Mt) for PAAV and PAA at different pH values were measured to investigate the interaction intensity between Mt and different polymers, and the results are shown in Figure 6. The adsorption capacity for PAAV is higher than that of PAA Figure 7. Adsorption isotherms for PAAV on Mt at different pH values. The solid lines are the Freundlich model prediction.

three pH values could be described by the Freundlich equation (all with R2 > 0.95). The adsorption capacity is limited by the surface available and the amount of surface occupied by each polymer molecule. It also depends upon the polymer concentration because the polymer concentration could affect polymer coil dimensions and knottiness.31,32 The adsorption isotherm at pH 5.2 has the steepest initial slopes, indicating that PAAV has the strongest affinity to the clay surface at acidic conditions.29 3.4.2. ζ Potential Measurements. Several studies have reported that the adsorption of polymer on the bentonite surface can raise the ζ potential of bentonite particles, which increases the hydration shell thickness of bentonite particles by electrostatic stabilization, strengthens the bentonite gel structure, plugs the filter cake holes, keeps multiple dispersion of bentonite particles, improves the quality of the filter cake, and reduces the coefficient of penetration and filtration.33,34 ζ potentials of Mt/PAA and Mt/PAAV at different pH values or different CaCl2 concentrations were measured, and the results are shown in Figures 8 and 9. For the Mt suspension system, a noticeable increase of ζ potential is observed when the pH value decreases from 9.1 to 4.0. The increase in ζ potential is mainly due to the weakening interaction between OH− ions and the edges of the clay particle (positive charge at pH lower than 6−7), which are rendered neutral or negative by the adsorption of the hydroxyl ions.35 Larger absolute values of ζ potential could be observed after the addition of PAA or PAAV, which indicates that these suspensions are more dispersed. This phenomenon is attained because the screening effect of edge charges of clay minerals with an anionic polymer makes these negative charges repel each other.36 The absolute value of ζ potential of Mt/PAAV is greater than that of Mt/PAA in the whole pH range investigated, which indicates a better dispersion stability performance of PAAV for Mt particles in aqueous solution. Additionally, when the pH value decreases

Figure 6. Effect of initial pH on the adsorption capacity of PAA or PAAV on Mt.

in the whole pH range investigated, which indicates a stronger interaction between PAAV and Mt. The adsorption capacities of the two polymers show a variation trend similar to pH changes. The minimum value for adsorption capacity occurs at pH around 7 for both polymers. With pH increasing over 7, a slight increase of adsorption capacity is observed, which is mainly the contribution of improved dispersion and accessibility property of Mt particles at high pH.29 When pH decreases from 6 to 4, adsorption capacity sharply increases. This can be explained through the electric properties of the Mt edge surfaces, the electrostatic charge, and the structural shape of the polymer.30 Moreover, the adsorption capacity difference between PAA and PAAV is much greater in acidic conditions, which indicates that the introduction of VP units into the polymer enhance the interaction between Mt and polymer in an acidic environment remarkably. In conclusion, a much better performance of PAAV can be expected in calcium contamination. Adsorption isotherms for PAAV under three pH values are shown in Figure 7. The adsorption capacity increases as the polymer concentration increases. The adsorption isotherms at E

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Figure 8. Effect of initial pH on the ζ potentials of Mt, Mt/PAA, and Mt/PAAV.

Figure 10. Particle size distribution of Bent/PAA and Bent/PAAV (a) before and (b) after aging at 150 °C for 16 h.

larger particles increases evidently for both suspensions. The reason is that, a high temperature induces dehydration of Bent platelets and reduces the energy barrier that one particle must overcome to come into contact and agglomerate with other particles.41 Additionally, the amide groups in AM and AMPS units, which enhance the interaction between Bent particles and polymer molecules and increase the thickness of the electric double layer around Bent particles, might go through partial hydrolyzation and thermal degradation at a high temperature.20,42 In comparison to Bent/PAA, the proportion of smaller particles for Bent/PAAV is much larger. The particle diameters are mainly in the range of 2.4−188 μm for Bent/PAA and 0.72−91.1 μm for Bent/PAAV. The mean particle diameter was 51.1 μm for Bent/PAA and 8.8 μm for Bent/PAAV after aging. This indicates that the introduction of VP units in the copolymer of AM and AMPS improves the temperature resistance of the polymer and explains why the Bent/PAAV system exhibits much better colloidal dispersion property than the Bent/PAA system during calcium contamination at a high temperature.

Figure 9. Effect of the CaCl2 concentration on the ζ potentials of Mt, Mt/PAA, and Mt/PAAV (initial pH of 7.2).

from 9.1 to 4.0, the decline in the absolute value of ζ potential is not as sharp in Mt/PAAV (from −65.4 to −51.3 mV) as in Mt/PAA (from −60.3 to −38.9 mV), which may be explained by the higher adsorption capacity of PAAV on the Mt surface.37 As shown in Figure 9, a significant decrease of the ζ potential for each suspension system can be observed in the presence of CaCl2. The degree of changes in the ζ potential of Mt particles could be predicted by the double layer theory, which predicts that increasing the electrolyte concentration suppresses the electric double layer thicknesses.38 The final plateau values of −7 and −11 mV for Mt and Mt/PAA, respectively, are reached at a CaCl2 concentration of 0.2%, and the final plateau value of −26 mV for Mt/PAAV is reached at a CaCl2 concentration of 0.4%. This indicates that PAAV exhibits much better performance for promoting dispersion stability of Mt particles than PAA at a high CaCl2 concentration, owing to a higher electrostatic repulsion between Mt particles and PAAV. This explains the superior capability of PAAV to disagglomerate Mt particles. 3.4.3. Particle Size Distribution Analysis. During calcium contamination, small particles are crucial for fluid-loss control because these colloidal particles will reduce the cake permeability by blocking pores of various sizes.39,40 The results from the particle size distribution analysis of Bent/PAA and Bent/PAAV suspensions are shown in Figure 10. Before aging, two suspensions have similar particle size distribution and the particle diameters are mainly in the range of 0.34−116 μm. The mean particle diameter was 10.3 μm for Bent/PAA and 7.1 μm for Bent/PAAV, indicating that Bent/PAAV was with a larger proportion of smaller particles. After aging, the proportion of

4. CONCLUSION A terpolymer of AM, AMPS, and VP was synthesized through free radical polymerization as a novel anti-calcium contamination fluid-loss control additive for WDFs. The optimized reaction conditions were obtained by the orthogonal test, and the corresponding polymer was characterized using 1H NMR, elemental analysis, FTIR, and SLS measurements. The API filtration test shows that bentonite-based mud with PAAV can resist 20% CaCl2 contamination at 150 °C. The fluid-loss control mechanisms were investigated by adsorption capacity measurement of the polymer on Mt particles, ζ potential measurement of Mt/polymer suspensions, and particle size distribution measurement of Mt/polymer. In comparison to PAA, PAAV exhibits a stronger interaction with Mt particles and better temperature resistance. PAAV could obviously improve the colloidal dispersion property for a bentonite suspension during calcium contamination at a high temperature. The fluid loss was reduced as a result of the larger proportion of small particles in the Bent/PAAV system. As a result of its superior rheological and filtration properties, PAAV can be a promising fluid-loss control additive in WDFs in deep gypsum drilling. F

DOI: 10.1021/acs.energyfuels.7b02354 Energy Fuels XXXX, XXX, XXX−XXX

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AUTHOR INFORMATION

Corresponding Authors

*Telephone: +86-532-86981190. Fax: +86-532-86981936. Email: [email protected]. *Telephone: +86-532-86981190. Fax: +86-532-86981936. Email: [email protected]. ORCID

Jie Cao: 0000-0002-3771-5806 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors appreciate the financial support from the National Natural Science Foundation of China (51203187), the Fundamental Research Funds for the Central Universities (17CX02053), the Open Fund of State Key Laboratory of Offshore Oil Exploitation (CCL2015RCPS0222RNN), and the Program for Changjiang Scholars and Innovative Research Team in University (IRT_14R58).

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DOI: 10.1021/acs.energyfuels.7b02354 Energy Fuels XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.energyfuels.7b02354 Energy Fuels XXXX, XXX, XXX−XXX