Effect of Alkyl Chain Length on Shale Hydration Inhibitive Performance

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Applied Chemistry

Effect of alkyl chain length on shale hydration inhibitive performance of vinylimidazolium-based ionic liquids Lili Yang, Xiao Yang, Tengda Wang, Guancheng Jiang, Paul Frederick Luckham, Xinliang Li, He Shi, and Jiansheng Luo Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.9b01016 • Publication Date (Web): 29 Apr 2019 Downloaded from http://pubs.acs.org on May 7, 2019

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Industrial & Engineering Chemistry Research

Effect of alkyl chain length on shale hydration inhibitive performance of vinylimidazolium-based ionic liquids Lili Yanga,b, *, Xiao Yanga, Tengda Wanga, Guancheng Jianga,*, Paul F. Luckhamb, Xinliang Lia, He Shia, Jiansheng Luoc a

MOE Key Laboratory of Petroleum Engineering, State Key Laboratory of Petroleum

Resources and Prospecting, China University of Petroleum (Beijing), Changping District, Beijing 102249, China b

Department of Chemical Engineering, Imperial College London, Prince Consort

Road, London SW7 2AZ, UK c

Oilfield Chemicals Division, China Oilfield Services Limited, Yanjiao Town, Hebei

065201, China

* Corresponding

author. Tel.: +86 10 89732239; fax: +86 10 8973 2196; E-mail address: [email protected] (L. Yang) Tel.: +86 10 897321969; fax: +86 10 8973 2196; E-mail address: [email protected] ( G. Jiang)

ACS Paragon Plus Environment

Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT Previously, our group developed a vinylimidazolium-based ionic liquid (IL) as an excellent shale hydration inhibitor for water-based drilling fluids (WBDFs). Herein, several ILs with different alkyl-chain lengths on the vinylimidazolium group were successfully synthesized by adjusting the cation composition to study their influence on inhibition performance. The results indicated that the IL with an ethyl group (C2) showed the strongest inhibitory effects for bentonite swelling, shale-cutting dispersion and rheological properties of bentonite suspension. Furthermore, the IL inhibition performance decreased with increasing alkyl-chain length. Accordingly, we concluded that as alkyl-chain length increased, the IL molecular volume increased, while the IL hydrophilicity and solubility decreased; minimizing the interlayer space and decreasing the water activity became more difficult, thus decreasing their inhibiting performance. Simultaneously, the reduction in inhibition performance has little relationship with the ability to suppress the double electron layers. All these findings can serve as a basis for designing ILs for high-performance shale hydration inhibition in WBDFs.

1. INTRODUCTION Mitigating shale hydration is the key to a successful drilling. The severe hydration and swelling of shale can narrow the bore diameter, alter the stress distribution around the borehole, reduce the shale mechanical strength, and cause borehole instability1, 2. Furthermore, shale dispersion can accelerate wellbore


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sloughing and deteriorate the rheological property of drilling fluids3, 4. Eventually, the drilling process is delayed, and oil-well construction costs increase significantly due to severe shale hydration, swelling and dispersion. In some extremely easily hydrated areas, such as Peiling in China, oil-based drilling fluids (OBDFs) have often been applied to address the well instability issue in shale, especially in long horizontal sections. However, OBDFs are expensive, disadvantageous for well logging, environmentally incompatible, and considerably troublesome for disposal. Therefore, OBDFs must be abandoned and replaced in the future by improved water-based drilling fluids (WBDFs) containing effective inhibiting additives. WBDFs with comparable performance are inexpensive, have no influence on well logging, are environmentally friendly, and are easily treated5-7. Obtaining insights into the characteristics of shale swelling has been an intriguing area of research for decades. Shale inevitably takes up water from drilling fluids, completion fluids, and fracturing fluids, although many additives, including nanoparticles, asphaltene sulfonates, gel, and sodium silicate8-12, are used to control the invasion of fluids by forming a physical plug. However, ascribing shale swelling to water entering the shale is far from satisfying. Shale formations contain a high fraction of layered montmorillonite (MMT), which is classified among the phyllosilicates. MMT easily adsorbs water by crystalline hydration and osmosis hydration of cations, mainly Na+ and Ca2+, existing in the interlayer region. The swelling includes at least two processes, namely, hydration and expansion, i.e., the increase in interlayer space and interparticle void, which together finally result in



increased total volume13. For the interlayer swelling, crystalline swelling and osmotic swelling should be considered successively. First, the crystalline swelling can increase the interlayer space from 10 Å to 20 Å by a stepwise progression from one to four layers of water, which has been widely studied by X-ray diffraction (XRD) and adsorption measurements. Given that the interlamellar space is completely filled, an osmotic swelling regime then sets in. This phenomenon has been extensively studied by small-angle X-ray and neutron scattering14-16. Osmosis steeply increases the space to 35–40 Å with the development of electrical double layers and continuously increases to several hundred Angstroms depending on the ionic concentration. Notably, the swelling of mesopores and macropores occurs before the complete filling of the interlayer space, and the interlayer is the predominant space where water is adsorbed compared with mesopores17. The increase in interlamellar space and mesopores causes the shale to swell, and the resultant magnitude of the swelling is a function of water content, cation properties (mainly charge and radius), temperature, pressure, and exposure time18, 19. Remarkably, the total degree of shale swelling also depends on the percentage of clay, the complex texture and orientation13. Various chemicals have also been added to the drilling fluids to control the hydration, swelling, and even dispersion of MMT. Compared with a physical plug, chemicals can interact with the shale surface and enter the interlayer void, imparting the shale itself with resistance from hydration with the intruded water. Potassium salts are widely used and can suppress the swelling of clay minerals by the ion exchange with sodium cations in the interlayer region. The weak ability for hydration and


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suitable size for forming coordination cages with siloxane oxygen atoms together contribute to forming only one monolayer of water molecules, similar to illite. Potassium salts have been a field-proven technology in combination with a variety of polymers although proven to be disadvantageous to the environment to date20, Alternative

organic

cationic

ions

behaving

like

potassium

ions,

21.

mainly,

amine-containing chemicals, were initially developed as shale inhibitors instead to care for the environment. Subsequently, quaternary ammonium salts22 were developed to improve the storage stability, minimize the odor, and avoid the influence of pH. Suter et al.23 also proposed that primary diamines or monoquaternary amines are particularly suitable cationic swelling inhibitors. Due to additional chemical adsorption of organic chemicals on MMT, their inhibition performance seems to be lasting, except for the low hydration ability similar to potassium cations. The organic chemicals can also be polymerized to prevent osmotic swelling by virtue of the larger molecular weight, coating the MMT with a protective layer.24 Adding these chemicals can preserve the initial appearance of the shale. However, many other amine-type chemicals are available other than diamines and quaternary amines. Ionic liquids (ILs) are composed of an organic cation and an anion, wherein the cation is always possessed of an amine-containing functionality, including imidazolium, piperidine, and pyridine. As an organic salt with a low melting temperature, ILs have been successfully applied in many chemical industrial fields,

especially

in

material

separation.

In

our

previous

research,

an

imidazolium-based IL named 1-vinyl-3-ethylimidazolium bromide (VeiBr) and its



corresponding homopolymers were studied and utilized as shale inhibitors.24 VeiBr showed better inhibition performance compared with KCl and a commonly used quaternary amine. By polymerization, the inhibition performance can be improved even further. Many other researchers realized their potential application as shale inhibitors in the oil and gas industries. Luo et al. found that 0.05 wt.% of an IL exhibited inhibition performance superior to that of 5 wt.% KCl and equal to that of 2 wt.% polyether diamine25. Gou et al. introduced ILs onto polyethylene glycol and prepared a biodegradable shale inhibitor26. However, the literature to date reports very little on the effect of IL composition on inhibition performance. Variations in the cationic or anionic moieties would accordingly influence the size, solubility, melting temperature, and hydrophilicity/hydrophobicity properties, all of which possibly affect the shale inhibition performance. Thus, the cation component on the inhibition performance of IL was systematically evaluated in our study to easily prepare an IL-based shale inhibitor27. Herein, we designed and synthesized a series of ILs composed of bromide anion and similar vinylimidazolium-based cations differing in alkyl-chain length on the vinylimidazolium cations. The impacts of these differences on inhibition performance were studied by linear swelling measurement, rolling recovery, and rheological properties. Interpretation and possible explanations of the observed effects were proposed, which are also beneficial to the preparation and study of IL-clay nanocomposite materials28, 29.

2. EXPERIMENTAL


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2.1. Materials 1-Vinyl-3-ethylimidazolium bromide (C2) and 1-vinyl-3-n-butylimidazolium bromide (C4) were supplied by the Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences (China). Other ILs were synthesized according to the literature30. The raw chemicals, including 1-vinylimidazole (VIM), n-hexyl bromide, n-octyl bromide, n-decyl bromide, n-dodecyl bromide, and n-tetradecyl bromide, were supplied by J&K Scientific (China). The sodium bentonite (Na–BT), consisting of 69.1% MMT (Table 1, Table 2 and Figure S1(a)), which is used for linear swelling height and rheological measurements and mechanism analysis, was a commercial product purchased from the Weifang Huawei New Materials Technology Co., Ltd. (China). The outcrop shale (Table 1, Table 2 and Figure S1(b)) employed for hot rolling recovery was obtained from China National Petroleum Corporation Chuanqing Drilling Engineering Co. Ltd. (China). All other chemicals (analytical grade) were supplied by the Beijing Chemical Reagent Factory (China) and were used without further purification. Table 1. Mineral compositions of Na–BT and the shale in our study Clay Quartz K-feldspar Albite Calcite Hematite Plagioclase Dolomite mineral Mineral (%) (%) (%) (%) (%) (%) content （%） (%) Na-BT 9.0 2.8 0.4 12.4 1.1 74.3 * Shale 34.9 0.7 7.1 29.1 1.7 26.5 *-:

indicates that this component is not included Table 2. Relative content of clay minerals in Na-BT and shale in our study smectite Illite kaolinite chlorite Chlorite/smectite Illite/smectite

Na-BT Shale

(%) 93 -

(%) -* 48

(%) 1 33

(%) 6 1


(%) 1

（%） 17


*-:

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indicates that this component is not included

2.2. Preparation of vinylimidazolium-based ionic liquids A general synthesis protocol for a series of ILs was as follows: 0.1 mol of VIM, 0.1 mol of n-alkyl bromide and 30 mL of methanol were loaded into a 100 mL reactor. The mixture was stirred at 60 °C for 48 h. After cooling, the reaction mixture was added dropwise into 500 mL of diethyl ether. The white precipitate was filtered to remove the unreacted reagents and dried in a vacuum desiccator in a 60 °C atmosphere until constant weight. With n-hexyl bromide and n-octyl bromide, a yellow liquid was obtained instead of a white powder, and this product was washed with excess diethyl ether. The solvent was removed using a rotary evaporator. The resultant

products

1-vinyl-3-hexylimidazolium

1-vinyl-3-n-octylimidazolium

bromide,

1-vinyl-3-n-decylimidazolium

bromide, bromide,

1-vinyl-3-n-dodecylimidazolium bromide, and 1-vinyl-3-n-tetradecylimidazolium bromide were named C6, C8, C10, C12, and C14 according to the alkyl number of the n-alkyl bromide, respectively. The synthesis route for the series of ILs is shown in Figure 1. 2.3. Characterization of ILs and Na–BT/IL composites The ILs were characterized by 1H NMR. The spectra were recorded with a Bruker AV 400 NMR apparatus by using dimethyl sulfoxide-d6 as a solvent. The signals were referenced to those of the residual protonated solvent at δ = 2.50 ppm. To determine the solubility of the IL in water, 2 g of IL was added to 100 g of water at 25 °C and stirred for 1 h. Then, the solubility was determined as soluble, partly soluble or insoluble by visual observation. The differential scanning calorimetry (DSC) measurements were performed on a TA Q200 instrument. The sample was heated at a heating rate of 10 °C min-1 in the


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range from 20 °C to 100 °C, then held for 5 min, and cooled from 100 °C to 20 °C at a cooling rate of 10 °C min-1 and held for 5 min under N2 atmosphere. 2.4. Inhibition performance of ILs To appreciate the swelling behavior of a compacted Na–BT pellet in contact with different inhibitor solutions at atmospheric temperature (25 °C) and pressure (0.1 MPa), dried Na–BT (5 g) was initially compressed into a pellet by a cylindrical device under 10 MPa pressure for 5 min. Then, the compacted pellet (with a height of 4.0 mm) was installed on a CPZ-2 dual channel linear swell meter (Qingdao, China), wherein a 15 mL inhibitor solution was added to the immersed Na–BT pellet. The pellet height was recorded every 30 s through a transducer for 24 h. The swelling behavior of the Na–BT pellet (with a height of 10.0 mm) was also evaluated at 60 ºC, 80 ºC and 120 ºC using a WT-HTHP-2A automatic high-temperature and high-pressure linear swell meter (Beijing, China) with an airtight jar. The pressure was set to 1.0 MPa for measurement at 120 ºC to avoid boiling. The pellet height was also recorded every 30 s for 16 h. The linear swelling rate is calculated according to the following equation: Linear swelling rate =

ℎ𝑡 ― ℎ0 ℎ0

(1)

× 100%

where h0 is the initial height of Na–BT pellet, and ht is the final height after immersing in pure deionized (DI) water and IL aqueous solutions for a certain time. The inhibition performance measurement of ILs on the hydrated disintegration or erosion of shale cuttings at different temperatures was carried out by hot rolling tests. The 20 g shale cuttings with a diameter of 2.0–3.2 mm screened by 6–10 mesh were hot-rolled in 300 mL of DI water or IL aqueous solution at 25 ºC, 60 ºC, 80 ºC and 120 °C in a roller oven for 16 h. After cooling, the remaining cuttings were collected after passing through a 40-mesh sieve and washed gently with DI water to remove the shale fragments with a diameter less than 425 μm. Subsequently, the cuttings retained on the sieve were dried for 5 h at 105 °C. The recovery percentage during hot rolling was calculated according to the following equation: Recovery percentage =

𝑚 𝑚0

× 100%,


(2)


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where m0 is the initial mass weight of shale cuttings before hot rolling and m is the final mass weight of the shale cuttings remaining on the sieve after hot rolling and washing. The inhibition effect of ILs on the hydrated dispersion of Na–BT in water was evaluated by measuring the rheological properties by using a ZNN-D6L rotational viscometer (Qingdao, China). Na–BT (12 g) was suspended in 300 g DI water or inhibitor solution at 25 °C. After high-speed stirring at 12 000 rpm for 30 min, the apparent viscosity (AV), plastic viscosity (PV), and yield point (YP) of the Na– BT/inhibitor suspension were determined. Then, another 12 g Na–BT was added and stirred for 30 min for the next measurement. The Na–BT was added continuously, and the rheological data were recorded until the AV was not measurable with the viscometer. The viscosity measurements were conducted at fixed rates of 600, 300, 100, 20, 6, and 3 rpm. AV, PV, YP were calculated according to the following formulas from the American Petroleum Institute (API)-recommended practice for field testing drilling fluids: AV = 𝜃600 2

(3)

PV = 𝜃600 ― 𝜃300

(4)

YP = 0.511(𝜃300 ― PV)

(5)

where θ600 and θ300 are the dial reading of the viscometer at 600 and 300 rpm, respectively. The influence of the IL on the morphology of the Na–BT pellet was also observed. The as-prepared pellet was immersed in 150 mL DI water or 2.0 wt.% IL aqueous solution. Morphology images of the pellet were obtained at 5 min, 24 h, and 10 days, respectively. 2.5. Microstructural analysis The thermogravimetric analysis (TGA) curves of ILs and Na–BT/IL composites were constructed using a PE Pyris 1 instrument at a temperature range of 30 °C to 700 °C, a heating rate of 10 °C/min and a N2 gas flow of 20 mL min−1. In this technique, the residual mass of the substance and mass percentage of the residual


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sample were determined as a function of temperature. The wettability of Na–BT/IL composite membranes was characterized by measuring the contact angle. First, glass substrates were separately immersed in a Na– BT suspension with 2 wt.% IL added and then dried for 12 h. Then, the contact angle of the Na–BT/IL surface was immediately measured 3 s after the water droplets (10 µL) had dropped and recorded by a CCD camera. Each measurement was repeated at least five times for reproducibility. Na–BT (8 g) was suspended in 200 mL DI water and stirred for 24 h for complete hydration. Inhibitor solution (4 mL, 2.0 wt.%) was added to 4 mL freshly prepared supernatant Na–BT suspensions. The Na–BT/IL mixture suspensions were obtained. Finally, the inhibition mechanism of different inhibitors was characterized by XRD, water activity, zeta potential and particle distribution analysis and discussed. The mixture dispersion was stirred for 24 h, and the resulting precipitates were collected by centrifugation for 30 min. The wet precipitates were measured directly by a wide-angle XRD measurement to analyze the influence of IL inhibitor on the MM Na–BT interlayer swelling at 25 ºC, 60 ºC and 80 ºC. XRD analysis was performed by an Empyrean X-ray generator with Cu-Kα radiation, λ = 1.5406 Å at a voltage of 40 kV, and a current of 40 mA. The basal spacing (d-spacing) was analyzed using Bragg’s equation. The particle-size distribution of neat Na–BT suspension and the suspensions of the Na–BT/IL mixture was determined using a Malvern Mastersizer 2000 particle size analyzer at room temperature. The prepared samples were used directly, and the concentration of IL in all samples was approximately 10.0 g·L-1. The zeta potential was measured by Malvern Zetasizer Nano series at room temperature. The prepared samples were used directly, and the concentration of IL in all samples was approximately 1.0 g·L-1. The water activity of pure water and inhibitor solutions was evaluated by Rotronic hydrolab C1 at room temperature. The prepared suspensions of Na–BT/IL mixture were used directly, and the concentration of IL in all samples was approximately 10.0 g·L-1.



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3. RESULTS AND DISCUSSION 3.1 Characterization of ILs In general, ILs have been extensively utilized as green solvents due to their low melting points, negligible vapor pressures at room temperature, high thermal and chemical stability, high solvation ability and easy recycling. In this research, ILs were utilized as a shale inhibitor, mainly taking advantage of their natural ionic properties and thermal stability. The final properties of ILs can easily be managed by adjusting combinations of cations or anions31. Additionally, vinylimidazolium-based ILs were selected because they are polymerizable and expected to prepare high performance additives

by

copolymerization

with

other

monomers.

A

series

of

vinylimidazolium-based ILs in this study was designed by the VIM alkylation with different lengths of alkyl chain on the nitrogen atom apart from the vinyl group (Figure 1), and the NMR spectra (Figure S2) were similar to the NMR spectra obtained in other research30. These differences allowed the manipulation of different properties, namely, molecular size, solubility, hydrophilic/hydrophobic property, hydration ability, interaction with shale, melting point and decomposition temperature. These properties might have a great influence on their shale inhibition performance32.

Figure 1. Synthesis route of vinylimidazolium-based ionic liquids with different


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lengths of the alkyl chain on the nitrogen atom apart from the vinyl group. The alkyl-chain length was expected to affect the solubility. Table 3 shows that all ILs can be dissolved completely in dimethyl sulfoxide (DMSO), and only C2 cannot be dissolved in dichloromethane (DCM) at 25 ºC. This result indicated that lipophilicity increased when the ethyl group was replaced by a longer alkyl chain. The solubility of IL in water is critical for utilization as a shale inhibitor for WBDFs. The short-chain length alkyl-substituted ILs (C2 and C4) were readily soluble in water. When the chain length increased to 6 and 8 (C6 and C8), the aqueous solutions became cloudy, indicating the formation of micelles. This outcome was in accordance with the literature that long alkyl chain imidazolium ILs have a high degree of self-organization30, 31. With the continuous increment in chain length, dissolving in water became increasingly difficult for ILs. Apparently, the poor solubility of C10, C12, and C14 would have a great impact on the inhibition performance of the IL. This condition will be illuminated further in the following section. Table 3. Solubility of vinylimidazolium-based ILs with different lengths of alkyl chains at 25 ºC in common solventsa IL water Solubility DCMb DMSOc a+:

C2

C4

C6

C8

C10

C12

C14

+

+

±

±

±

-

-

-

+

+

+

+

+

+

+

+

+

+

+

+

+

completely soluble; −: insoluble; ±: partly soluble.

bdichloromethane



cdimethyl

sulfoxide

Increasing temperature would influence the state of the IL in water. On the one hand, the solubility had a temperature dependence. Increasing temperature would improve the solubility of IL in water and might have a great impact on inhibition performance for the partly soluble and insoluble ILs. On the other hand, insoluble ILs began to soften and then adhered onto the shale more easily, although they are still not soluble in WBDFs when the temperature is over the melting point. For ILs, the diversity in size and interactions of anions and cations led to the different accumulations of ions and determined their melting point. As the DSC curves show in Figure S3, there are no peaks in the range of 20-100 ºC for C2, C4, C6, C8, and C10. In fact, C2 and C4 are liquid at room temperature, and increasing temperature would not influence the phase while C6, C8 and C10 are quite sticky and appear more solid. C12 and C14 are solid at room temperature, and their melting points as read from DSC curves are approximately 48 °C and 58 °C, respectively. Increasing temperature would soften C12 and C14, which might influence the inhibition property at high temperature. Therefore, the inhibition performance should be studied at high temperature to fully understand the influence of alkylation chain length on shale inhibition performance due to improved solubility and flow state. 3.2 Inhibition performance When the WBDFs with no effective inhibitors are used, water is likely to enter into water-sensitive clay formations and induce swelling of shale. Thus, wellbore instability occurs. The linear swelling test is a method that assesses the swelling


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inhibition performance of Na–BT as a function of time by the shale inhibitor. As shown in Figure 2, the swelling increment of Na–BT pellets and the swelling rate decreased progressively with increased immersing time during the first 24 h in all IL aqueous solutions of different concentrations (0.5 wt.%, 1.0 wt.%, 1.5 wt.%, and 2.0 wt.%) at 25 °C. It is obvious that C2 exhibited the best inhibition performance at all concentrations. The swelling rate of the Na–BT pellet in 0.5 wt.%, 1.0 wt.%, 1.5 wt.%, and 2.0 wt.% is 127.8%, 119.0% 116.5% and 107.3%, corresponding to the inhibition efficiency of 15.1%, 20.9%, 22.6% and 28.7% compared with that in pure water. The inhibition efficacy of different ILs indicated by the linear swelling of Na– BT pellet follows such an order: C2 > C4 > C6 > C8 > C10 > C12 > C14. To clearly illustrate the effectiveness of ILs, the performance of the VIM was also measured for comparison. Apparently, less inhibition performance was observed for VIM at all concentrations, indicating that the alkylation contributed to the improvement of the shale inhibition effect for all the prepared ILs.



Figure 2. Linear swelling of Na–BT pellet in pure water and aqueous solutions of VIM (a) and IL inhibitors (C2 (b), C4 (c), C6 (d), C8 (e), C10 (f), C12 (g) and C14 (h)) at a concentration of 0.5 wt.%, 1.0 wt.%, 1.5 wt.% and 2.0 wt.% at 25 ºC. As the temperature was increased to 60 °C, 80 °C and 120 °C (Figure 3(b), 3(c) and 3(d)), the swelling rate of the Na–BT pellet in IL aqueous solutions increased with temperature increasing, and the ILs displayed much more discriminated inhibition performance, wherein C2 always performed the best, and C14 tended to be the worst. For example, adding 1.0 wt.% C2 at 80 ºC can reduce the swelling rate to


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88.2%, whereas C14 aggravated the pellet swelling to 112.4% even higher than in pure water (Figure 3(b)). The inhibition efficacy on the linear swelling of the Na–BT pellet of other ILs fell between C2 and C14 in the sequence of C2 > C4 > C6 > C8 > C10 > C12 > C14, irrespective of temperature. In addition, the swelling height of the Na–BT pellet in IL-aqueous solutions always decreased with increased concentration.

Figure 3. Linear swelling of Na–BT pellet in pure water and aqueous solutions of VIM and IL inhibitors at a concentration of 1.0 wt.% at different temperatures and different pressures (a) 60 ºC, 0.1 MPa, (b) 80 ºC, 0.1 MPa and (c) 120 ºC, 1.0 MPa. This phenomenon was inconsistent with the reported method for preparing high-performance shale inhibitors by improving hydrophobicity that adding additives with more hydrophobic groups helped increase the surface hydrophobicity of the



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shale. This condition can impede water penetration into the pellet, and many researchers also designed several proficient inhibitors using this concept33,

34.

Although ILs with an alkyl-chain length over six displayed poor inhibition performance, probably due to limited solubility and high melting temperature, C4 was completely dissolved in water and may be expected to have a higher shale swelling inhibition performance. The interesting results mentioned above infer that other inhibiting mechanisms must be occurring other than hydrophilicity–hydrophobicity characteristics. The shale recovery test is another common method in the oil industry to assess the inhibitive properties of drilling fluid in the disintegration of drilled cuttings transported from the drill bit to the surface. Without the help of a shale inhibitor, the drill cuttings probably disperse easily in drilling fluids in the case of high clay content. This condition would cause deterioration of the rheological properties of the drilling fluid and adversely affect the safe, effective, and efficient drilling. As shown in Figure 4, the remaining shale in fresh water accounted for approximately 3.0– 15.0% after hot rolling at 25 °C, 60 °C, 80 °C, 120 °C atmosphere for 16 h. Similar to the results of linear swelling inhibition, C2 turned out to be the best inhibitor on shale dispersion that increased the recovery percentage to 95.0% independent of the hot rolling temperature. Meanwhile, other ILs displayed poorer inhibition in this measurement. Accordingly, the recovery percentage decreased with increasing alkyl-chain length. We can conclude that ILs seemed to be inhibitive on shale dispersion with respect to alkyl-chain length close to two at all temperatures,


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combined with the results in linear swelling. Therefore, IL composition has a great impact on inhibiting Na–BT swelling and dispersion, while increasing temperature cannot change this trend.

Figure 4. Recovery percentages of shale after being hot-rolled at 25 °C, 60 °C, 80 °C and 120 °C in inhibitor solutions at a concentration of 2.0 wt.% for 16 h. The edge-to-face (EF) attraction between the negatively charged flat surface and positively charged edge form a “house-of-cards” structure responsible for the high viscosity of Na–BT suspensions. Optimization of rheological properties is important to suspend the cuttings, recycle the fluid, and maintain a high rate of penetration. With shale cuttings, especially those containing a high percentage of Na–BT dispersed in drilling fluids, the rheological properties might deteriorate dramatically.



An effective shale inhibitor should have the ability to avoid the rheological properties of drilling fluids being affected by the increase in the concentration of dispersed shale. In this study, the rheological parameters AV, PV, and YP of Na–BT dispersions at room temperature were calculated to investigate the resistance of Na–BT-based drilling fluids added with varying ILs from Na–BT addition (Figure 5). This test provided the information as to what extent the AV, PV, and YP of drilling fluids can be recorded by a viscometer. By adding Na–BT continuously, the pure Na–BT suspensions can merely endure totally 12 wt.% Na–BT, as shown in Figure 5(a). However, AV maintained a relatively slow increasing rate even over 12.0 wt.%, provided that the IL was added in advance. In detail, AV can be measured by the viscometer until the Na–BT concentration reached 36.0 wt.%, 32.0 wt.%, and 20.0 wt.% by adding C2, C4, and C6, respectively, whereas C8–C14 can resist no more than 16.0 wt.% of Na–BT. This result reflected the different hydration effects of Na–BT in different inhibitor solutions. Two main reasons explained the IL influence on the rheological properties of Na–BT suspensions. First, the high concentration of vinylimidazolium cations can change continuity and decrease the friction among solvent, IL, and Na–BT particles, therefore decreasing the PV of the Na–BT suspension. Second, the vinylimidazolium cation of the IL can adsorb on the negative surface by ionic bond or hydrophobic interaction and prevent the EF attraction of Na–BT, which can disassemble the microstructure of the Na–BT dispersions, which decreases the YP of the Na–BT suspension. PV and YP can both be effectively inhibited by the presence of ILs. In


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addition, PV and YP in Na–BT/IL had a trend similar to AV when adding Na–BT continually (Figure 5(b) and Figure 5(c)). As the alkylation chain length increased, IL becomes more hydrophobic and less soluble in water, having a great impact on their homogeneity of distribution and interaction with Na–BT particles. These eventually influence their ability to decrease the friction among solvent, IL, and Na–BT particles and to broke the microstructure of Na–BT dispersions. Therefore, C2 has the best inhibition performance on PV and YP of the Na–BT suspension, and increasing alkylation chain length is not advantageous. The IL inhibition of viscosity also displayed a similar trend with linear swelling performance and the hot rolling test, in which C2 had the best inhibition ability among all the series of ILs.

Figure 5. Na–BT inhibition test at 25 °C by comparing the apparent viscosity (a), plastic viscosity (b), and yield point (c) of IL aqueous solutions and base Na–BT WBDFs as a function of Na–BT content. Associated water moving into the shale accompanying ionic diffusion may change the ionic concentration and composition of the shale pore fluid. This phenomenon can unfavorably affect the shale mechanical properties and cause shale swelling, cohesion degradation, cementing bond weakening, and overall rock-strength reduction. The compressive strength alteration of shale is greatly influenced by the



type and concentration of the salt solution. Potassium ion has been proven to have a strengthening effect on shale, whereas sodium and calcium ions have a weakening effect on shale35. In our study, after immersing in pure water and 2.0 wt.% VIM aqueous solution, the Na–BT pellet spread all around after 24 h (Figure 6(b1)) and even overspread the bottom of the container after 240 h (Figure 6(b2)). Instead, the Na–BT pellet kept the initial morphology for 24 h (Figure 6(c1-i1)) and split after being immersed for 240 h (Figure 6(c2-i2)) in all IL aqueous solutions. The Na–BT bits formed seemed to be harder and stronger than the ones in VIM and KCl aqueous solutions. This result showed the advantage and potential of IL used as a shale inhibitor for WBDFs. To further stabilize the shape of Na–BT pellet, polymers should be added36.

Figure 6. Morphology of the Na–BT pellet immersed in pure water, VIM aqueous solution, and IL aqueous solutions. (a) Pure water, (b) 2.0 wt.% VIM (c) 2.0 wt.% C2, (d) 2.0 wt.% C4, (e) 2.0 wt.% C6, (f) 2.0 wt.% C8, (g) 2.0 wt.% C10, (h) 2.0 wt.% C12, and (i) 2.0 wt.% C14. Panels a1–i1 and a2–i2 are those of panels a–g after immersing for 24 and 240 h, respectively. Thermal stability is important for additives in drilling fluids. Inorganic inhibitors are stable at high temperature, whereas organic inhibitors may undergo thermal degradation. Figure 7(a) and Table 4 show that these vinylimidazolium-based ILs can


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withstand an elevated temperature between 250 and 290 °C. In detail, C2 was the best temperature resistant IL with an initial decomposition temperature of 282.69 °C, whereas the decomposition temperature of C10 was 256.55 °C, displaying the worst thermal stability. Besides, all ILs almost decomposed completely that the remaining weight percentage is merely between 0.3 and 4.1%, which is because ILs are organic salts. However, no apparent relationship between decomposition temperature and alkyl-chain length of ILs was found. Therefore, IL-based materials may be considered a suitable candidate inhibitor for oil and gas formations with severe high temperature conditions. The thermal decomposition temperature of Na–BT/IL composites increased compared with the pure IL (Figure 7(b) and Table 4), further improving the thermal stability and showing the possibility of ILs being used as a shale inhibitor at high temperatures. The composites with ILs of alkyl-chain length less than eight can withstand the influence of high temperature over 300 °C, whereas the thermal decomposition temperature of those ILs with alkyl-chain length more than eight increased slightly compared to the pure IL. These phenomena were probably attributed to ILs with short alkyl-chain length that can be effectively protected by Na– BT, which impeded their thermal decomposition. By comparison, ILs with longer alkyl-chain lengths were too large to interact with the Na–BT and probably exist separately as IL and Na–BT. Thus, the temperature resistance was not greatly improved. The quantities of inhibitor in Na–BT/inhibitor were compared by the difference in the weight loss determined from Na–BT and the Na–BT/inhibitor at



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700 °C, neglecting the trace of adsorbed water. The composites of Na–BT with C2, C4, and C6 had the similar residual weight percentage, inferring only 15% ILs penetrating into the interspace of Na–BT. For C8, C10, C12, and C14, the residual weight percentage increased with increasing alkyl-chain length, signifying that the IL residue is difficult to clean with increased hydrophobic property.

Figure 7. Thermal stability of pure IL (a), pure Na–BT, and Na–BT/IL composites (b) Table 4. Onset thermal decomposition temperature of pure IL and Na–BT/IL composites IL

C2

C4

C6

C8

C10

C12

C14

Pure IL

282.69 268.08 270.42 261.95 256.55 263.35 263.28

Na–BT/IL

303.13 357.92 371.24 374.44 266.67 265.49 269.46

Taonset/°C

aT onset

is the onset thermal decomposition temperature.

3.3 Microstructural analysis Wettability is one of the important characteristics of a solid surface, which can control the pore pressure transmission6, 37 and water invasion into shale. Each IL has a hydrophilic anion (Br−) and displays considerable hydrophilicity, whereas the organic


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cation induces ILs with different hydrophobicity relying on the length of the aliphatic tails. As shown in Figure 8, water spread on the films of Na–BT/IL when the alkyl-chain length was no longer than C6. The static contact angle was approximately 0°, inferring the excellent hydrophilicity of these ILs. When the alkyl-chain length was longer than C6, the contact angle varied from 50° (C8) to 90° (C14). These results show that a series of ILs with different wettability had readily been prepared by adjusting the length of the aliphatic tails. When the IL adsorbs onto the Na–BT surface, it would change the surface wettability accordingly. Generally, a high contact angle would increase the difficulty for water ingress into the shales, resulting in improved wellbore stability. According to Suter23, increasing the length of the organic backbone increases the amount of organic atomic density near the clay surface, which prevents the water hydrogen from bonding to the clay surface, and hence inhibits the MMT dispersion, whereas an appropriate size cation and a strong interaction with MMT would benefit to decrease the interlayer distance and inhibit the shale swelling. Remarkably, the effective structure for IL was not the same with amine and quaternary amine. In other words, the rule-based design criteria that Suter raised for amine- and quaternary amine-based clay-swelling inhibitors were not applied to the IL in our study.

Figure 8. Contact angle of water on the films of Na–BT/IL. (a)VIM, (b) C2, (c) C4,



(d) C6, (e) C8, (f) C10, (g) C12, and (h) C14. When water approaches the Na–BT surface, water will be adsorbed into the interlayer region of the clay, initiating crystalline swelling and osmotic swelling. Na+ can form an outer-sphere surface complex with the interlayer water molecules strongly bound to the tetrahedral substitution sites of the clay layer. Therefore, Na–BT was a strong swelling clay. To interpret the role the inhibitor played in this process, XRD was used to measure the (001) interlayer spacing (d001) of the silicate layers. Typical interlayer spacing recorded in the crystalline swelling regime was in the range of 9 Å to 20 Å. Osmotic swelling can result in a significant increase in interlayer spacing from >20 Å to 130 Å. In our study, the Na–BT d-spacing in DI water was 19.97 Å at room temperature (Figure 9(a)), indicating that four main water layers existed between the silicate layers. Increasing temperature promoted the osmotic swelling so that the d-spacing was over 20 Å when the temperature was over 60 ºC. When C2, C4, and C6 were added at room temperature, the d-spacing decreased satisfactorily to 14.42, 14.48, and 15.00 Å, respectively, showing that water molecules were expelled from the interplanar space, and the water layers decreased. Notably, the d-spacing after adding C4 was larger than the d-spacing after adding C2, explaining further hydrophobicity although water solubility caused less inhibitive effect on the swelling height of the Na–BT pellet. In C8, C10, C12, and C14, the d-spacing was over 20 Å, showing the different impact on Na–BT hydration. Given that they are more hydrophobic and less water soluble than C2, C4, and C6, they should tend to be less


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hydrated and would adsorb the silicate layers together. However, the volume of cations in these ILs was too large, so the silicate layers could not approach each other, unlike the ILs with short alkyl-chain length. Therefore, a distinct peak above 20 Å in the XRD patterns was displayed. Hydrophobic ILs propped the clay sheet open, making it ineffective. Increasing the temperature did not change the influence of alkylation length on decreasing interlayer distance (Figure 9(b1), 8(b2) and 8(b3)), which is thought to be the main reason for the inhibition effect. Actually, the effect of hydrophobic ILs on MMT was usually utilized for MMT exfoliations such as organic MMT38 or preparing composites of MMT with organics.



Figure 9. XRD patterns of Na–BT wet samples with different IL inhibitors: C2–C6 (a1: 20 ºC, a2: 60 ºC and a3: 80 ºC) and C8–C14 (b1: 20 ºC, b2: 60 ºC and b3: 80 ºC) at a concentration of 2.0 wt.% The results of Na–BT dispersion inhibition showed that in the linear swelling inhibition test, immersing test, and rheological measurement, ILs displayed excellent inhibition performance on hydration, swelling, and dispersion of Na–BT, and the shale recovery test implied that IL played an important role in maintaining the


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integrity of the shale. To explain this phenomenon, electrokinetic potential was also measured for the inhibition performance related to stability of MMT after adding inhibitors. The zeta potential of Na–BT in DI water in our study was −43.2 mV, showing considerable stability (Figure 10): a colloidal system with absolute zeta potential value greater than 30 mV was considered stable39. After adding hydrophilic and amphiphilic inhibitors (C2, C4, C6, and C8), the zeta potential was reduced to −37.1, −36.2, −32.7, and −25.1 mV, respectively, inferring a reduced electric double layer and a stability reduction of the Na–BT colloid after adding IL as well. Generally, a low-magnitude zeta potential would accelerate coalescence and is more advantageous for shale inhibition, depending on the structure, added amount, and functionalities40. On the one hand, vinylimidazolium-based cations with short alkylation chain length can suppress the double electron layers in the manner similar to inorganic cations. On the other hand, these organic cations can adsorb onto Na–BT. Therefore, there is a decrease when adding C2, C4, C6, or C8 into Na–BT suspensions. However, the relationship between inhibition performance and the zeta potential results was not in accord with the literature that the ability to suppress double electron layers of Na–BT suspensions increased as alkylation chain length increases, but the inhibition ability did not increase. Additionally, hydrophobic ILs can reverse the zeta potential to +22.9, +27.7, and +38.4 mV, indicating the strong adsorption on the Na–BT external surface. Owing to the low solubility, C10, C12 and C14 act more like particles that contain many cations and hydrophobic groups on the surface. Under the strong electrostatic force and hydrophobic interaction between ILs



and Na–BT particles, the IL easily adsorbs on the Na–BT so that the zeta potential reversal happened. However, this phenomenon had little influence on the coalescence stability of Na–BT (Figure 10) or the IL inhibition performance. Therefore, the deviation of IL inhibition performance and zeta potential infers that decreasing the coalescence stability of Na–BT is not the reason for the shale inhibition ability.

Figure 10. Zeta potential measurement of Na–BT in various inhibitor systems. Figure 11 shows that the particle size increased dramatically after IL inhibitors were added. Apparently, C2 was not the best to prompt particle coalescence and aggregation, while C8 and C10 produced the largest ones instead. The particle size distribution is related mainly to the coalescence stability of the suspensions. Due to the zeta potential of C8 and C10 closest to zero, the suspensions tended to aggregate. The particle size distribution as alkylation chain length increased explains the inhibiting ability of C8 on Na–BT pellet swelling and shale dispersion despite increasing d-spacing and poor ability in reducing water activity. This phenomenon also demonstrated that colloidal stability was not the main reason for IL inhibition.


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Figure 11. Particle size distribution of Na–BT in different inhibitor aqueous solutions. The unfavorable transfer of water and ions into shale can impact the mechanical and physicochemical properties of shale, causing well instability problems, especially in deviated wells. This phenomenon is the most costly and troublesome problem in shale drilling. Although many additives can create a physical plug and minimize the diffusion and capillary effect, WBDFs, having invaded into the shale pore, transfer into the shale when the chemical potential of the drilling fluid is greater than the chemical potential of the shale. This condition causes shale swelling, increased pore pressure, and ultimate failure. Adding salts and other inhibitors to the drilling fluid to decrease water activity can achieve the desired osmotic potential between the shale and drilling fluids. As shown in Table 5, aqueous solutions of soluble ILs (C2 and C4) have a lower water activity compared with pure water and VIM aqueous solution.



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Partly soluble IL aqueous solutions (C6, C8, and C10) have a comparably weak ability to reduce water activity, possibly due to the low solubility. Water activity of insoluble C12 and C14 does not decrease but increases instead. Accordingly, C2 and C4 perform best in inhibiting the swelling partly because of the greater suppression of the water activity. With increased alkyl-chain length, the water activity of the aqueous solutions increases, and it becomes easier for water to enter the shale. Therefore, shale hydration and swelling are considerably reduced with the C2 IL. Table 5. Water activity of pure water and IL aqueous solutions at a concentration of 2.0 wt.% Aqueous solutions

Pure IL water VIM

C2

C4

C6

C8

C10

C12

C14

91.6

88.0

88.9

90.5

91.0

91.5

91.7

92.1

Water 90.2

activity

3.4 Inhibition mechanism

Figure 12. Illustrative picture of IL inhibition mechanism on shale hydration, swelling and dispersion. IL-1: short-chain length alkyl-substituted ILs (C2, C4, C6);


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IL-2: long-chain length alkyl-substituted ILs (C8, C10, C12, C14). The increase in hydrophobicity promotes the separating-out of ILs, resulting in a solubility decrease, and the solubility decrease is disadvantageous for their inhibition performance in our study. For vinylimidazolium-based IL, the absolutely soluble and hydrophilic C2 performed the best shale inhibition, while increasing the hydrophobicity by changing the alkyl-chain length weakens the inhibition performance. It was thought the composition mainly influenced the ability to decrease the d-spacing (Figure 12) and water activity, not the coalescence stability, that resulted in the difference in shale inhibition. In detail, C2, C4, and C6 decrease the water activity and impede water entering the interlayer void. For the water existing in the interlayer space, C2, C4, and C6 can enter the inner-region and replace the sodium cations by cation exchange or chemical adsorption with layer surface adsorption, which decreased the hydration ability, therefore disposed of the water and displayed an inhibition effect on hydration, swelling and dispersion of MMT. C8, C10, C12 and C14 have long hydrophobic tails on the cations, which cannot decrease water activity. In addition, owing to the large size, they cannot affect d-spacing though they might have less hydration ability compared with C2, C4, and C6, and as a result, show poor shale inhibition ability.

4. CONCLUSIONS A series of vinylimidazolium-based ILs with different alkyl-chain lengths were prepared and evaluated as inhibitors in drilling fluids to inhibit the hydration, swelling, and dispersion of Na–BT. All ILs displayed good thermal stability,



beneficial to high-temperature drilling. For inhibition performance, ILs with the ethyl group showed superior ability in inhibiting hydration, minimizing the degree of linear swelling, abating the dispersion of Na–BT suspension at temperatures of 25 °C, 60 °C, 80 °C and 120 °C. Hydrophilic and amphiphilic ILs with short alkylation chain length can reduce the water activity, enter into silicate interlayer space, and decrease interlayer distance effectively, whereas amphiphilic and hydrophobic ILs with long alkylation chain length increase interlayer distance and display poor inhibition of water activity. These phenomena were different from the conventional knowledge that improving the hydrophobic property can improve the inhibition performance, beneficial for the preparation of effective shale inhibitors in the future. From this study, inhibitors should be designed by precisely studying the effect of the composition on inhibition performance. This work would inspire the efforts devoted to developing inhibitive WBDFs, radioactive matter isolation, and composites of MMT with organics.

ACKNOWLEDGEMENTS This work was supported by the National Natural Science Foundation of China (grant No. 51604290 and No.51874329); startup foundation of China University of Petroleum (Beijing) (grant No. ZX20150392); and National Science and Technology Major Project of China (2016ZX05020-004).

Associated content Supporting information: This file contains XRD patterns of MMT and shale iour study; NMR spectra of VIM, C2, C4, C6, C8, C10, C12, and C14; DSC curves of C6,


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C8, C10, C12 and C14; XRD pattern of Na-BT mineral analysis.



NOMENCLATURE AND ABBREVIATIONS: IL = ionic liquid C2 = 1-Vinyl-3-ethylimidazolium bromide C4 = 1-vinyl-3-n-butylimidazolium bromide C6 = 1-vinyl-3-n- hexylimidazolium bromide C8 = 1-vinyl-3- n-octylimidazolium bromide C10 = 1-vinyl-3- n-decylimidazolium bromide C12 = 1-vinyl-3- n-dodecylimidazolium bromide C14 = 1-vinyl-3- n-tetradecyllimidazolium bromide WBDFs = water-based drilling fluids OBDFs = oil-based drilling fluids MMT = montmorillonite VeiBr = 1-vinyl-3-ethylimidazolium bromide VIM = 1-vinylimidazole Na–BT = sodium bentonite

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Effect of alkyl chain length on shale hydration inhibitive performance of vinylimidazolium-based ionic liquids Lili Yanga,b, *, Xiao Yanga, Tengda Wanga, Guancheng Jianga,*, Paul F. Luckhamb, Xinliang Lia, He Shia, Jiansheng Luoc a

MOE Key Laboratory of Petroleum Engineering, State Key Laboratory of Petroleum

Resources and Prospecting, China University of Petroleum (Beijing), Changping District, Beijing 102249, China b

Department of Chemical Engineering, Imperial College London, Prince Consort

Road, London SW7 2AZ, UK c

Oilfield Chemicals Division, China Oilfield Services Limited, Yanjiao Town, Hebei

065201, China TOC

The alkyl-chain length of vinylimidazolium-based ionic liquid (IL) has a great influence on the shale inhibition performance. By comparison, IL with the ethyl group (C2) performs the best inhibiting ability on shale hydration, swelling and dispersion. * Corresponding

author. Tel.: +86 10 89732239; fax: +86 10 8973 2196; E-mail address: [email protected] (L. Yang) Tel.: +86 10 897321969; fax: +86 10 8973 2196; E-mail address: [email protected] ( G. Jiang)



As the alkyl-chain increases, the inhibition performance decreases accordingly.


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Effect of Alkyl Chain Length on Shale Hydration Inhibitive Performance

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