A Novel Environment-Friendly Natural Extract for Inhibiting Shale

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A Novel Environment-Friendly Natural Extract for Inhibiting Shale Hydration Fan Zhang,† Jinsheng Sun,*,† Xiaofeng Chang,† Zhe Xu,† Xianfa Zhang,† Xianbin Huang,† Jingping Liu,† and Kaihe Lv† †

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School of Petroleum Engineering, China University of Petroleum (East China), Qingdao, Shandong 266580, People’s Republic of China ABSTRACT: This paper reports the first use of Tribulus terrestris extract (TTS), a newly developed nonionic surfactant, as an inhibitor for shale hydration. The critical micelle concentration of TTS was determined using the surface tension method, while its inhibition properties were evaluated with a variety of inhibition assessment methods. The sodium bentonite inhibition tests revealed that TTS can significantly inhibit clay pulping. The scanning electron microscopy images of unmodified and TTSmodified shale samples were observed and discussed. The hydrogen bonds between the hydrophilic tail of the TTS molecules and the available oxygen atoms on the clay silica surface led to the formation of a hydrophobic shell on the clay surface, which is considered as the main inhibition mechanism of TTS. The results from the contact angle experiments also validated this conclusion. In addition, the environmental aspects of TTS were measured in this paper. The experimental results suggest that TTS has a better inhibitor performance than either of the more conventional potassium chloride or polyamine and show that it has a good application prospect as an environment-friendly shale inhibitor. operational mechanism is to embed K+ ions in clay wafers, which are then not easily separated. In addition, K+ has a lower hydration energy and can replace Ca2+ and Na+, which have larger hydration potentials, thus inhibiting the hydration expansion of the clay. However, the shortcomings of potassium chloride include relatively large concentration requirements, inconsistency with environmental protection requirements, the high cost of liquid waste disposal, and its corrosiveness to downhole drilling tools.11 Organic ammonium species are a kind of high-performance shale inhibitor, but their application is limited by several shortcomings, including their toxicity, incompatibility with anionic drilling fluid additives, and flocculation.12 The addition of polyether amine can increase the adsorption sites of ammonium inhibitors on clay and reduce their flocculation. Meanwhile, the molecular weight of the polyether amine additives is small, and they can easily enter the intergranular compression−diffusion double layer of clay to exert their inhibiting properties. However, their small molecular weight prevents them from being effectively encapsulated in the clay surface, resulting in their weak ability to inhibit the hydration and dispersion of clay.13 Copolymer inhibitors have excellent inhibitory effects, but still exhibit some problems, such as relatively expensive raw materials, high molecular weight, nondegradability, effects on the rheological properties of the shale, and compatibility with drilling fluids.14−16 Recently, with the increasing pressure of environmental protections, researchers are attempting to find improved and environmentally friendly inhibitors that intersect biology, medicine, chemistry, and drilling fluids. For example, Jiang

1. INTRODUCTION Statistical data reported show that drilling costs wasted by wellbore instabilities worldwide is US $1 billion per year.1 About 90% of wellbore instability problems occurs in shale formation. This is because shale is a kind of rock formed by dehydration and cementation of clay, which has obvious thin bedding structure. The main component of shale consists of clay minerals, which easily soften and expand after immersion and have a strong water sensitivity. The softening and expansion processes depend on the relative percentages of different clay minerals contained in the shale. Graf and Anderson2 suggested that the hydration of clay minerals can be divided into surface and permeation hydration. Wilson and Wilson3 proved that osmotic hydration is the main cause of hydration in montmorillonite-bearing shale. Tan et al.4 showed that after shale contacts water-based drilling fluids, fractures and extended fractures will form along the interface of the highly active clay minerals. Then, the water-based drilling fluids infiltrate along these fractures, which result in hydration expansion and the subsequent dispersion of the clay minerals. Therefore, the inhibition of clay hydration is the key to solving wellbore instability. Oil-based drilling fluids (OBDFs) have an excellent inhibition performance, which is one solution to water-sensitive shale. However, OBDFs cannot meet the environmental requirements and have high associated costs.5 In recent years, researchers have been looking for highperformance, water-based, drilling fluid inhibitors to solve the problem of shale instability without the use of oil-based fluid solutions. A series of shale inhibitors have been developed to meet these engineering needs, including inorganic ionic inhibitors, organic ammonium inhibitors, polyether amine inhibitors, and copolymer inhibitors.6−10 Potassium chloride is a commonly used inorganic ionic inhibitor that is low cost and has wide application. Its © XXXX American Chemical Society

Received: April 15, 2019 Revised: July 23, 2019 Published: August 1, 2019 A

DOI: 10.1021/acs.energyfuels.9b01166 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels and co-workers17 found biodegradable poly(L-lysine) to be a shale inhibitor, which inhibits the crystallization expansion of montmorillonite and weakens the electric double layer exclusion between clay particles, showing excellent shale hydration swelling inhibition. In addition, Zhong et al.18 prepared an asphalt shale inhibitor from oil sludge. The inhibitor exhibited good inhibition ability on shale hydration and expansion and physical sealing ability. Moslemizadeh et al.19 found that extract from henna leaves, which is a natural dye, has inhibitory properties. Shadizadeh and co-workers20,21 obtained an environmentally friendly nonionic surfactant and clay hydration inhibitor, as extracted from Ziziphus spina-christi leaves. Barati et al.22 studied the inhibiting effects of Astragalus negundo extract on the expansion of shale due to hydration. The introduction of these types of nontoxic and cost-effective shale inhibitors is currently very important for both research and industrial sectors. The properties of Tribulus terrestris extract (TTS) as an inhibitor have been studied for the first time in the literature. TTS has many advantages, such as being friendly to the environment and low cost (only $5.7/kg). The critical micelle concentration (cmc) of TTS was determined by the surface tension method and conductivity method, and a wide range of characterization methods were implemented to evaluate the inhibition performance and environmental aspects of TTS.

sonication dispersion, the samples were centrifuged (4 h, 200 rpm) and injected for HPLC (injection volume 10 mL). 2.2.3. Critical Micelle Concentration Measurements. There are several methods to determine the cmc of surfactants in aqueous solutions, including examining the surface tension, conductivity, interfacial tension, thermal conductivity, and pH or using the gravimetric method. In this study, the most classical surface tension method was used to evaluate the cmc of TTS in an aqueous solution. For this purpose, TTS solutions with different concentrations were prepared using a homogeneous dispersion of TTS powder in deionized water followed by mixing with a magnetic stirring device for 2 h. Then, the surface tension (DCAT21-German Dataphysics, using the Wilhelmy plate method) and conductivity (Nano S90Malvern) of different concentrations were measured by continuous dilution from high concentration to low concentration, and the results were plotted. The cmc of the TTS was calculated from the associated plots as the concentration corresponding to the inflection point of the curve. 2.2.4. Linear Swelling Tests. A mass of 10 g of sodium bentonite (100 mesh sieve, 103 °C drying for 12 h) was placed in the mold. A manual hydraulic pump was used to pressurize 10 MPa for 10 min to make artificial particles. The thickness of the core was measured and recorded as the initial thickness (H0). The prepared artificial particles were placed in an HTP-C4-type double-channel shale dilatometer (Qingdao Tongchun Co., Ltd.) and soaked in different solutions. The expansion height (h) of the artificial particles with time was recorded continuously for 16 h.26,27 2.2.5. Sodium Bentonite Inhibition Tests. A sodium bentonite inhibition test is a classical experiment to evaluate the inhibition of additive agent. Generally speaking, the load of bentonite in WBDF is limited. However, the hydration and expansion of sodium bentonite were significantly inhibited by inhibitors, and the load of the waterbased drilling fluid bentonite was further increased, which can be revealed by measuring the rheological properties of the suspension. Ten grams of sodium bentonite was added to the test solutions and they were aged for 16 h at 80 °C. The rheological properties were measured at 60 °C. A 10 g portion of sodium bentonite was further added, the samples were aged for 16 h at 80 °C, and the rheological properties were determined until the samples were too thick to be measured by a six-speed rheometer (ZNN-D6-type, Qingdao Haitongda Special Instrument Co., Ltd.). The apparent viscosity (AV), plastic viscosity (PV), and yield point (YP) were calculated according to the API-recommended practice of the standard procedure for the field testing of drilling fluids28−30 using the formulas shown.

2. EXPERIMENTAL SECTION 2.1. Materials. 2.1.1. T. terrestris Extract. In this study, TTS was provided by Xi‘an Realin Biotechnology Co., Ltd, which was extracted by spray-drying.23−25 The properties of TTS are shown in Table 1.

Table 1. Properties of T. terrestris Extract product

result

used part color odor solubility in water solubility in alcohol loss on drying (105 °C/6 h) total ash (550 °C/4 h) origin major applications

dry fruit of T. terrestris brown powder slight fragrance and bitter taste soluble soluble 2.14% 1.13% Henan, Hebei, Shandong, and Anhui, China drugs and medicine

AV = θ600/2 (mPa· s) PV = θ600 − θ300 (mPa· s) YP = (θ300 − PV)/2 (Pa)

2.1.2. Minerals. Shale cuttings are provided by Chuanqing Drilling Co., and sodium bentonite is provided by Bohai Drilling Co. The mineral composition of shale cuttings and clay mineral analysis of sodium bentonite were determined by X-ray diffraction (XRD-6100, Shimadzu Corp.). 2.1.3. WBDF Additive. Potassium chloride, sodium chloride, and sodium carbonate were provided by Aladdin Pharmaceutical Co., Ltd., and the polyamine inhibitor SDJA was made by China University of Petroleum. 2.2. Methods. 2.2.1. Characterization of TTS. TTS was characterized using an IRTracer-100 FTIR manufactured by Shimadzu Corp. A small amount of the MTMS monomer was placed in a mortar and then ground uniformly with potassium bromide and compressed using a tableting machine. The spectra range of infrared scanning was 4000−500 cm−1, and the scanning resolution was 4 cm−1. 2.2.2. Sample Preparations and High-Performance Liquid Chromatography−Mass Spectrometry (HPLC−MS) Analysis. A 2 g portion of TTS powder was dissolved in 20 mL of EtOH/H2O (1:4), and the solution was extracted by sonication for 15 min. After

2.2.6. Immersion Test. The immersion test is a simple and intuitive method to evaluate the performance of inhibitors. The mud balls were prepared using a certain proportion of the constituents (bentonite:distilled water = 2:1). These mud balls were immersed in different solutions (40 mL of deionized water, 5.0 wt % KCl solution, 4.0 wt % polyamine, 1.0 wt % KPAM, 1.0 wt % TTS, and 4.0 wt % TTS). After 24 h of immersion, the immersion state of the mud balls was photographed and mud balls were respectively weighed. 2.2.7. Contact Angle Measurements. The image of water droplets on the surface of the samples was captured using an OCA-25 optical contact angle measuring instrument (German Data Co.).31,32 The original shale samples and modified shale samples used for contact angle were used as control experiments. The modified shale samples were obtained by immersing in 4.0 wt % TTS solution for 16 h and drying at 70 °C for 6 h. The sampler was used to drop 2 μL of distilled water on the surface of the samples, which stabilized for 10 s. Then the droplet is photographed and the contact angle is measured. The contact angles of modified and unmodified shale samples were measured separately. The contact angles of modified and unmodified B

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Energy & Fuels shale samples were measured respectively, and the wettability of the TTS solution to shale samples was analyzed by comparing the changes of contact angles. 2.2.8. Shale Cuttings Recovery Tests. The shale cuttings recovery test is a traditional method to determine the dispersion characteristics of shale cuttings, which is used to evaluate the ability of a test solution to inhibit shale hydration and dispersion.33 In the experiment, shale samples were crushed in a mineral crusher, and core particles were collected between 10 mesh and 6 mesh sieves. Samples collected were placed in a constant-temperature oven and dried for 10 h at 103 °C. Then, 50 g of shale cuttings were placed in a sealed pressure vessel containing 350 mL of test fluid and rolled for 16 h at 120 °C. The remaining shale cuttings are cooled to ambient temperature, washed with tap water, sifted with 40 meshes, and dried at 103 °C for 4 h before weighing. The recovery rate of shale cuttings is calculated by using the weight of samples before and after shale cuttings recovery tests. 2.2.9. Scanning Electron Microscopy (SEM) Observations. Scanning electron microscopy (Zeiss EVO MA 15/LS) was used to study the micromorphology of the original shale surface and the shale surface modified by TTS solution. Modified shale samples were obtained by immersing the subion polished shale samples in TTS solution of 4.0 wt % concentration for 20 h and then taking out the shale samples and drying them in a vacuum drying chamber at 60 °C. Before the analysis, in order to improve the imaging quality, the samples were placed on an ion sputtering device for gold spraying to eliminate the charging effect. 2.2.10. Environmental Aspects. The methods to evaluate the concentration for 50% of maximal effect (EC50), chemical oxygen demand (COD), and biochemical oxygen demand (BOD5) for TTS are shown in the Table 2 below.34

Figure 1. FTIR spectrum of TTS.

identified as in-plane and out-of-plane bending vibration characteristic peaks of −OH bonds on aromatic rings.35−38 3.2. HPLC−MS Analysis. The chemical components present in the TTS were analyzed by HPLC−MS. The HPLC chromatogram of TTS in EtOH/H2O is shown in Figure 2. The MS spectra profiles of TTS are shown in Figure

Table 2. Methods of Evaluating Environmental Indicators indicator

method

national standard

EC50

luminescent bacteria

GB/T 18420.2-2009

COD

fast digestionspectrophotometric method microbial electrode method

HJ/T 399-2007

BOD

HJ557-2010HJ/T 86-2002

measuring instrument Modern Water DeltaTox II LEICI-571 COD detector LY-05 BOD rapid measuring instrument

3. RESULTS AND DISCUSSION 3.1. Characterization of TTS. The main components of T. terrestris fruit extract were saponins, which are also a kind of biosurfactant. The molecular structure of saponin is amphiphilic: one end is a hydrophilic group, and the other end is a hydrophobic group. The hydrophobic part consists of triterpenoids and steroids or steroid-alkaloids, while the hydrophilic part consists of carbohydrate residues. The hydrophilic part is connected with the hydrophobic part through a glycoside bond.20 Figure 1 shows the FTIR spectra of TTS. The characteristic peaks around 3377 cm−1 of TTS were identified as the stretching vibration absorption peaks of −OH. The characteristic peaks around 2924 cm−1 for TTS were the C−H bond stretching vibration peak in alkanes. The characteristic peaks around 1629 cm−1 were identified as the stretching vibration peak of saturated fatty acid ester CO. The characteristic peaks around 1415 cm−1 were the bending vibration peak of C−H bond in alkanes. The characteristic peaks around 1155 and 1026 cm−1 were identified as the stretching vibration characteristic peak of C−O on the ether bond. Characteristic peaks at 707, 765, and 856 cm−1 were

Figure 2. HPLC chromatogram of TTS in EtOH/H2O.

3. As shown in Figures 2 and 3, the chromatograms possessed a high and diverse number of peaks. The HPLC profile shows the occurrence of four main peaks (a, b, c, and d). Molecular mass corresponding to different retention times is shown in Table 3. In this research, T. terrestris, ext;39 D-glucaric acid, 1,4lactone, ion (1−); spirost-5-ene-1,3-diol, (1β,3β,25R)-;40 spirostan-2,3-diol, (2α,3β,5α,25R)-;41 L-glycero-β-L-gluco-heptopyranosylamine, 4-deoxy-4-[[2-[(10-methyl-1-oxoundecyl)amino]acetyl]amino]-N-9H-purin-6-yl-;42 and 4H-1-benzopyran-4-one, 5,7-dihydroxy-2-(4-hydroxyphenyl)-3-[[6-O-[3-(4hydroxyphenyl)-1-oxo-2-propen-1-yl]-β-D-glucopyranosyl]C

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Figure 3. MS spectra profiles of TTS measured by HPLC−MS.

oxy]-39 from TTS were characterized tentatively from the literature. 3.3. Cmc Measurements. The surface tensions of the TTS solutions with the different concentrations are shown in Figure 4a. The cmc measurements help bring an understanding of the active surface chemistry for TTS in solution. Saponin is a kind of natural nonionic surfactant. When the concentration of the surfactant molecules increases beyond the cmc, the surfactant molecules self-aggregate in solution, the hydrophobic groups aggregate to form an inner core, and the hydrophilic groups contact with water to form a shell to create a micelle, which cannot be adsorbed on the clay surface. Therefore, it is necessary to determine the cmc through experiments.43,44 It is observed that the interfacial tension of the TTS solution decreases with its increasing concentration. However, when the concentration exceeds 4.0 wt %, the surface tension of the TTS solution decreases to its lowest value. This is because when the solution reaches a critical micelle concentration, the concentration of the TTS starts to increases, and the surface tension increases due to the formation of a large number of micelles. Meanwhile, as shown in Figure 4b, the conductivity of TTS at different concentrations was measured. The cmc of TTS obtained by the conductivity method is 4.15 wt %, which is basically consistent with that measured by the surface tension method. 3.4. XRD Analysis of Shale and Sodium Bentonite. The mineral composition of shale cuttings and the results of

Figure 4. Cmc determination via (a) the surface tension method and (b) the conductivity method.

clay mineral analysis of sodium bentonite are shown in Tables 4 and 5. 3.5. Linear Swelling Tests. A linear swelling test can quantitatively evaluate the swelling of clay. Because of the different inhibition properties, the linear expansion rate of bentonite particles was different in the test solutions.45−47 The experimental results are presented in Figure 5. First, the linear expansion rate of artificial particles in tap water was measured to be 83.37%. The linear expansion rates of 1.0, 3.0, and 4.0 wt % TTS solutions were 57.03%, 36.67%, and 32.32%, respectively. The experimental results show that the linear expansion rate decreases with the increase of TTS concen-

Table 3. Chemical Compositions of TTS peak

retention time (min)

molecular mass

formula

CAS registry number

a b c c d

2.08 2.56 4.35 4.35 5.69

326.1272 191.0199 414.6260 432.3234 565.3226

C10H21N3O9 C6H7O7 C27H42O3 C25H42O3N3 C26H43O7N7

90131-68-3 929706-31-0 472-11-7 511-96-6 1613024-04-6

d

5.69

594.1368

C30H26O13

22153-44-2

proposed molecule T. terrestris, ext39 D-glucaric acid, 1,4-lactone, ion (1−) spirost-5-ene-1,3-diol, (1β,3β,25R)-40 spirostan-2,3-diol, (2α,3β,5α,25R)-41 L-glycero-β-L-gluco-heptopyranosylamine, 4-deoxy-4-[[2-[(10-methyl-1-oxoundecyl)amino] acetyl]amino]-N-9H-purin-6-yl-42 4H-1-benzopyran-4-one, 5,7-dihydroxy-2-(4-hydroxyphenyl)-3-[[6-O-[3-(4-hydroxyphenyl)1-oxo-2-propen-1-yl]-β-D-glucopyranosyl]oxy]-39 D

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Energy & Fuels Table 4. Mineralogical Composition Analysis of Shale and Sodium Bentonite Determined by XRD Analysis minerals

shale (mass %)

sodium bentonite (mass %)

quartz potassium feldspar plagioclase hematite calcite siderite iron dolomite clay mineral

41 4 12 2 15 1 − 25

21 7 10 − 2 − − 60

Table 5. Clay Mineral Analysis of Sodium Bentonite chemical compositions

mass %

kaolinite (K) chlorite (Ch) illite (I) montmorillonite (S) illite/montmorillonite interlayer (I/S) illite/montmorillonite interlayer ratio (% S)

0 0 1 99 0 100

Figure 5. Linear expansion rate of inhibitors.

tration, which reveals the dependence of TTS inhibition on concentration. The linear expansion rate of artificial particles is 48.09% in 5.0 wt % potassium chloride solution, 38.55% in 3.0 wt % polyamine solution, and 32.32% in 4.0 wt % TTS solution. The experimental results show that TTS has good inhibition performance and can effectively inhibit clay expansion, and its inhibition performance is better than that of polyamines and KCl. 3.6. Sodium Bentonite Inhibition Tests. Evaluations of the TTS inhibitor performance were performed through sodium bentonite inhibition tests to study the effect of sodium bentonite on the rheological properties of different inhibitor systems. In a strong inhibitive system, the inhibitor prevents the hydration and swelling of sodium bentonite, resulting in a lower rheological property and higher sodium bentonite loading. On the contrary, in a weak inhibitive system, a higher rheological property and lower sodium bentonite loading will appear. Results including the AV, PV, and YP as a function of sodium bentonite loading are shown in Figure 6. With the increase of sodium bentonite content, the AV, PV, and YP of the tap water system increase rapidly. This is because sodium

Figure 6. Sodium bentonite inhibition test results after being rolled at 80 °C for 16 h and reading at 60 °C: (a) alteration of the apparent viscosity with sodium bentonite concentration, (b) alteration of plastic viscosity with sodium bentonite concentration, and (c) alteration of yield point with sodium bentonite concentration.

bentonite is easy to hydrate, swell, and disperse and can form a grid structure in the system. When the addition of sodium bentonite reaches 10 wt % in tap water, the system becomes too thick to measure its rheological readings. However, in the inhibitor system, the maximum amounts of loaded sodium bentonite for the 4.0 wt % TTS, 3.0 wt % polyamine, and 3.0 E

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Energy & Fuels wt % potassium chloride are 27.5, 20.0, and 22.5 mass %, respectively. The results show that TTS, compared to polyamine and potassium chloride, has the remarkable ability to inhibit clay hydrating and swelling. 3.7. Immersion Test. The immersion test results are shown in Figure 7. Figure 7a shows the control group status of

Table 6. Mud Ball Weight after Immersing in Different Types of Solutions no.

solutions

weight after immersion (g)

1 2 3 4 5 6

deionized water 5.0 wt % KCl solution 3.0 wt % polyamine 1.0 wt % KPAM 1.0 wt % TTS 4.0 wt % TTS

21.55 18.53 17.12 17.45 17.97 16.82

(Figure 8a) and that of the modified clay sample is 23.5° (Figure 8b). The shapes of the water droplets on the surface of

Figure 7. Images of the mud balls after immersion in (a) deionized water, (b) 5.0 wt % KCl, (c) 3.0 wt % polyamine, (d) 1.0 wt % KPAM, (e) 1.0 wt % TTS, and (f) 4.0 wt % TTS for 24 h.

the mud balls immersed in distilled water for 24 h. Figure 7b−f shows the mud balls immersed in different inhibitory solutions for 24 h. Furthermore, we found that the mud balls were obviously hydrated and completely dispersed (Figure 7a). However, when the mud balls were immersed in 5.0 wt % KCl, their volume varied considerably with significant osmotic hydration, swelling, and dispersion (Figure 7b). When the mud balls were immersed in 3.0 wt % polyamine and 1.0 wt % KPAM (Figure 7c,d), their volume changed slightly, and their surface was obviously soft and cracked; the particle size increased by several times the original state. After 24 h immersion, the size of the clay ball in 1.0 wt % TTS solution increased (Figure 7e), while that in 4.0 wt % TTS solution remained almost unchanged (Figure 7f). The results show that TTS solution inhibits the expansion of clay balls due to hydration to a certain extent, which is basically consistent with the experimental results of the linear expansion test. The mechanism may be that the hydrophilic part of the TTS saponin molecule is adsorbed on the clay surface, while the hydrophobic part is toward the water phase. The hydrophobic part of the TTS saponin molecule surrounds the clay surface and becomes more hydrophobic, which slows down more water molecules from entering the clay sphere, thus inhibiting the clay expansion due to hydration. The mass of each clay ball before immersion is 10 g, and the mass after immersion is weighed again (Table 6). These results are also consistent with the observed clay ball. 3.8. Contact Angle Measurements. Shale instability and clay swelling are the direct result of the strong hydrophilicity of active clay. This paper studied the influence of TTS on the hydrophilicity of clay surfaces. The photographing time of the contact angle experiment was 10 s after the water droplets fell on the surface of the sample and stabilized. Here, the change of hydrophilicity is explained by measuring the contact angle of the water droplets on the surface of both modified and unmodified clay samples. It can be seen that the water contact angle on the surface of the unmodified clay sample is 13.9°

Figure 8. Contact angle between deionized water and (a) sodium bentonite and (b) TTS-modified sodium bentonite.

the unmodified sample are wider than those for the modified sample, and the contact angle for the water on the unmodified clay is smaller than on the modified sample. The results show that the clay sample modified with TTS has a larger contact angle, suggesting it has poor wetting. Therefore, it is concluded that the tendency of the samples modified with TTS to adsorb less water and consequently have a lower swell effect indicates that the adsorption of TTS can significantly reduce the hydrophilicity of clay surfaces. 3.9. Shale Cuttings Recovery Tests. The inhibiting property of TTS is further evaluated by measuring the shale cuttings recovery rate of shale. The results are shown in Figure 9. The shale cuttings recovery rate in deionized water is 8.82%, which indicates the high hydration and dispersion potential of shale cuttings. After adding 1.0 wt % TTS, 3.0 wt % TTS, 4.0 wt % TTS, 3.0 wt % KCl, and 3.0 wt % polyamines, the shale cuttings recovery rate of the measurements increased to 15.25%, 23.3%, 27.3%, 22.14%, and 21.25%, respectively. The experimental results show that TTS can inhibit the hydration and dispersion of shale cuttings. In addition, TTS has better inhibition performance compared with two commonly used inhibitors (KCl and polyamines). 3.10. Scanning Electron Microscopy (SEM) Observations. As shown in Figure 10, the micromorphology of unmodified and TTS-modified shale samples was observed by SEM at different magnifications. This was done to explain the mechanism of TTS inhibition properties by observing the morphological changes of samples. Parts a and c of Figure 10 show SEM images of unmodified shale samples at 1000 and F

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Table 7. Evaluation Results of Environmental Indicators indicator

measured value

standard value

classification

EC50(mg/L) COD (mg/L) BOD5 (mg/L) BOD5:COD (%)

36000 93.6 13.0 13.89

≥30000 60−100 ≤20 10

nontoxicity qualified qualified biodegradable

3.12. Probable Inhibition Mechanism. The content of montmorillonite in shale is the main factor affecting hydration and expansion. The main characteristics of montmorillonite are wide unit spacing and weak bonding, which leads to a significant trend of hydration and expansion.12 Montmorillonite is a layered mineral that consists of very fine waterbearing aluminosilicate. It contains alumina−oxygen octahedron in the middle layer and silica−oxygen tetrahedron in the upper and lower layers. Its crystal structure contains water and some exchange cations, and it also has a high ion exchange capacity and high affinity for water absorption and expansion. In contrast, TTS contains a large number of saponins. The molecules in the chemical structure of saponins are composed of both hydrophilic and hydrophobic parts. Among these, the aglycons are hydrophobic and have different degrees of lipophilicity. These molecules are composed of triterpenes, steroids, or steroid alkaloids. The hydrophilic part is composed of sugar residues, where the sugar chains have strong hydrophilicity. The hydrophilic and hydrophobic parts are linked through glycoside bonds, which allows saponins to act as a surfactant. The possible underlying mechanism for the use of TTS as a shale inhibitor is that the hydrophilic hydroxyl ends of the saponins combine with the Si−O bond on clay surfaces via a hydrogen bond, which causes the saponins to adsorb onto the clay surface. If the concentration of saponin molecules is sufficient to encapsulate the entire clay surface, the hydrophobic ends of the saponin molecules form a hydrophobic membrane layer on the clay surface. This prevents water molecules from contacting the clay, which reduces its hydration and expansion potential.19,20,48

Figure 9. Shale cuttings recovery rate of different inhibitors.

Figure 10. SEM Images of shale samples: (a) unmodified (1000×), (b) 4.0 wt % TTS modified (1000×), (c) unmodified (3000×), and (d) 4.0 wt % TTS modified (3000×).

4. CONCLUSIONS T. terrestris extract, as a natural plant extract, was shown to have a good shale inhibition performance, making it superior to the more commonly used polyamine and KCl inhibitors. In this study, TTS showed good inhibition performance based on the cmc value as measured using the surface tension method. The inhibitor properties were compared intuitively through a series of immersion experiments. The linear expansion tests showed that the TTS had a slow linear expansion rate. In addition, the shale samples had a high shale cuttings recovery rate when in the TTS solutions. The inhibition mechanism of TTS was analyzed using scanning electron microscopy. The hydrophilic ends of TTS adsorbed onto the clay surface through OH bonds, allowing the hydrophobic end of the TTS to face outward (water phase) to form a “hydrophobic membrane” layer on the clay surface. As a result, the hydrophobicity of the shale was improved. The competitive adsorption between the TTS and water molecules reduced the adsorption of water by the shale, thus inhibiting shale hydration. This phenomenon was indirectly verified by immersion experiments and contact angle experiments. In addition to the good inhibition performance, the TTS is a natural plant extract and has the advantages of adhering to environmental protection regula-

3000 times, respectively. From the SEM images, it can be seen that the structure of TTS-modified shale samples has changed compared with the original shale samples. Unmodified shale samples are flaky, uneven in surface, and often covered by exfoliated debris, with many small pores and cracks, which make water intrusion easier, and the contact between shale and water will lead to the expansion and collapse of shale. Figure 10b,d shows that after TTS modification, the clay layers are interconnected, and TTS adsorbs on the clay surface to form inclusions. Compared with the unmodified shale samples, the surface is relatively smooth and the porosity is less. The SEM images directly reflect that TTS has remarkable ability, which can inhibit the hydration and dispersion of shale and enhance wellbore stability. 3.11. Cost and Environmental Aspects. In order to evaluate the environmental protection performance of TTS, EC50, COD, and BOD5 values of TTS were measured. The results and classification of biological toxicity prescribed by the US EPA are shown in the table below. Table 7 shows that TTS, as an environmentally friendly inhibitor, meets the pollutant discharge standard GB-31571-2015 formulated by the People’s Republic of China for the petrochemical industry. G

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tions. This was validated by measuring the EC50, COD, and BOD5 as evaluation indicators.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Fan Zhang: 0000-0002-5520-3628 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was financially supported by Petro China Innovation Foundation (Grants 2018D-5007-0306), Joint Funds of the National Natural Science Foundation of China (No. U1762212), CNPC Science and Technology Project (No. 2018A-3907).

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NOMENCLATURE TTS = Tribulus terrestris extract cmc = critical micelle concentration WBDF = water-based drilling fluid SEM = scanning electron microscopy ppm = part per million XRD = X-ray diffraction EC50 = concentration for 50% of maximal effect COD = chemical oxygen demand BOD5 = biochemical oxygen demand HPLC−MS = high-performance liquid chromatography− mass spectrometry AV = apparent viscosity, cP PV = plastic viscosity, cP YP = yield point, Pa

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DOI: 10.1021/acs.energyfuels.9b01166 Energy Fuels XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.energyfuels.9b01166 Energy Fuels XXXX, XXX, XXX−XXX