Nanoscale Laponite as a Potential Shale Inhibitor in Water-Based

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Nano laponite as a potential shale inhibitor in water based drilling fluid for stabilizing wellbore stability and mechanism study Xianbin Huang, Haokun Shen, Jinsheng Sun, Kaihe Lv, Jingping Liu, Xiaodong Dong, and Shaojie Luo ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b11419 • Publication Date (Web): 05 Sep 2018 Downloaded from http://pubs.acs.org on September 12, 2018

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ACS Applied Materials & Interfaces

Nano Laponite as a Potential Shale Inhibitor in Water Based Drilling Fluid for Stabilizing Wellbore Stability and Mechanism Study Xianbin Huanga*, Haokun Shena, Jinsheng Suna*, Kaihe Lva, Jingping Liua, Xiaodong Donga, Shaojie Luob a

Department of Petroleum Engineering, China University of Petroleum (East China), Qingdao,

Shandong, China 266580 b

PetroChina Jilin Oilfield Company, Jilin, Songyuan, China 138000

Corresponding Author *E-mail: [email protected] [email protected]

KEYWORDS: laponite; nanoparticle; shale inhibitor; water based drilling fluid; wellbore stability

ABSTRACT: Shale hydration is a main reason to cause wellbore instability in oil and gas drilling operations. In this study, nano laponite as the shale inhibitor was employed to stabilize the wellbores. The inhibition property of laponite suspensions was evaluated by immersing experiment, linear swelling measurement and shale recovery test. Then the shale inhibition mechanism was studied by using capillary suction time (CST) measurement, thixotropy study, 1

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plugging performance evaluation and related theoretical analysis. Evaluation experiment results showed that laponite had a better inhibition property than widely used inhibitors of KCl and polyester amine. The mechanism study revealed that integration of several factors strengthened the inhibition property of laponite suspensions. Laponite nanoparticles could plug interlayer spaces of clays by electrostatic interaction to reduce water invasion; the “house of cards” structure of laponite suspensions enables large CST values and low free water contents; the excellent thixotropy of laponite nanofluid could allow to form a nano film to reduce water invasion into formation; and the nanoscale laponite particles could substantially reduce shale permeability and form less porous surfaces. Furthermore, laponite could considerably decrease filtrate volume of drilling fluid while KCl and polyester amine had negative influences on the properties of drilling fluid. This approach described herein might provide an avenue to inhibit shale hydration.

1. INTRODUCTION Wellbore instability1,2 is a major problem encountered during oil and gas drilling process. It could easily lead to downhole complexities such as borehole collapse, tight hole, stuck pipe, etc. Shale formations account for about 75% of the drilled formations worldwide3,4. And about 90% of wellbore instability incidents occurs in shale formations3,5. Thus, the stability of shale formation is of great importance for safe, efficient, and economical drilling.

Clays are the important constituents of shales. The swelling of clays occurs when meeting

2


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with the filtrate of water based drilling fluid, which may lead to wellbore instability and significantly increase well construction cost. So minimizing clay swelling is the key to prevent wellbore instability. The mostly common method of minimizing clay swelling is the use of plugging materials6 and/or shale inhibitors7–9 in drilling fluid. The plugging materials in the water based drilling fluid, such as super-fine calcium carbonate and elastic particles, can form a thin layer of solids to reduce water invasion into formation to some extent. With the development of nano technology, recent years also showed an increasing amount of work on the application of nanomaterials10–18 to reduce wellbore instability. The shale inhibitors, such as KCl8,9, surfactants19,20, polymers21 and polyamines22–24, are widely used in drilling fluid to reduce clay swelling near the wellbore. The cooperation of plugging materials and shale inhibitors plays an important role in stabilizing wellbore. However, the wellbore instability caused by clay swelling is still facing great challenges in drilling complex formations. Laponite25 is a synthetic sodium magnesium silicate clay, the crystal of which has a disc-shaped structure with a diameter of 25 nm and a thickness of 0.92 nm26. The faces of the laponite particles are charged negatively and the edges are charged positively in an aqueous medium (pH＜11)27. Due to the unique structural and electrical nature, laponite is widely used in cosmetic, medicament, coating, etc. In the drilling fluid area, US. Pat. No. 4888120A28 firstly introduced the application of laponite as a high-temperature stable thixotropic thickening agent for water-based drilling fluids. Previous rheological studies29,30 indicated laponite suspensions 3



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could improve the suspending and cutting-carrying capability of drilling fluids. However, the shale inhibition property of laponite suspensions has never been explored.

Herein, we evaluated the inhibition property of laponite suspensions and analyzed the inhibition mechanism comprehensively. Interestingly, we found laponite suspensions have an excellent shale inhibition property and the inhibition mechanism is different from that of commonly used shale inhibitors. This paper might provide an avenue to inhibit shale hydration.

2. EXPERIMENTAL SECTION 2.1 Materials Laponite RD was obtained from Southern Clay Products, UK. Potassium chloride (KCl, 99.5wt%), polyether amine (PA, 99wt%) and sodium based bentonite (Na-bt) were obtained from J&K Scientific Ltd., Beijing. Outcrop shale samples were from Sichuan Province, China. The mineral compositions of shale sample analyzed by X-ray diffraction were shown in Table 1.

Table 1. Mineral Compositions of shale sample by X-ray diffraction analysis. Mineral Compositions

Content (%)

Quartz K feldspar Na feldspar Calcite Dolomite Kaolinite Illite Clay minerals Chlorite Illite-Smectite mixed layer 4


41.3 2.1 3.8 6.9 4.2 0.8 12.5 6.7 21.7

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2.2 Preparation of laponite suspensions Laponite suspensions with different concentrations were prepared by mixing different amount of laponite into distilled water and aged for 24 h at room temperature. The continuous mixing speed was 500 r/min. 2.3 Inhibition property evaluation (1) Immersing test Immersing test31,32 is a simple and intuitive evaluation method. Bentonite pellets were prepared by compressing 10.0 g Na-bentonite into compact round cakes with a diameter of 2.5 cm using a hydraulic press. The compressing pressure was 10 MPa, and the compressing time was 10 min. Then the bentonite pellets were immersed into 200 mL tap water, 5wt% KCl water solution, 1wt% polyester amine, 2wt% polyester amine, 0.5wt% laponite, 1wt% laponite and 2wt% laponite, respectively. The statuses of the immsered bentonite pellets were observed after 12 h. (2) Linear swelling test The linear swelling tests22,24,32 of the Na-bentonite pellets were conducted by a CPZ-II type dual-channel linear swell meter (Qingdao Tongchun, China) at 20℃ at atmosphere pressure. After placed the bentonite pellets into metal cylinders and immersing into different inhibitive solutions, the changes in the length (mm) of the pellets as a function of time were recorded by the equipment. (3) Shale recovery test 5



Shale recovery tests22–24 were performed on shale samples using various inhibitive solutions. First, the shale sample was grounded and sieved using one 6-mesh sieve and one 10-mesh sieve. 6 to 10-mesh shale fragments were selected for use in this test. 20 g shale fragments and 300 mL one of the inhibitive solutions were added into a stainless steel aging cell. Then the cells were placed in a BGRL-5 type hot roller oven (Qingdao Tongchun, China) and hot rolled at 150℃ for 16 h. After hot rolling was complete, the cells were taken out and cooled to room temperature. Finally, the retained shale fragments in each cell were filtered out, washed with tap water and dried in an oven at 100℃ for 24 h. The dried shale fragments were seived by a 40-mesh sieve and the residue on sieve was weighted as M1.

Recovery=

M1 ×100% 20

2.4 Rheology measurement Rheological performances of the laponite suspensions with different concentrations were carried out on a rotational type rheometer (Fann 50SL, Fann, USA) at 20℃. (1) Shear rate vs. viscosity

The viscosities of laponite suspensions were measured as a function of shear rate. The shear rate varied from 0 to 510 s−1 (600 r/min).

(2) Three interval thixotropy test

Three interval thixotropy tests (3ITT) were used to determine the internal structure of laponite suspensions. In the first section, the samples were subjected to a low shear rate (3 r/min) for 3

6


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min. Then a high shear rate (600 r/min) was applied for 3 min in the second section. In the third section, the low shear rate (3 r/min) was applied for 5 min. 2.5 Plugging tests (1) Determination of gas permeability of shale samples Pulse decay method was used to measure permeability of shale samples according to American Petroleum Institute standard API RP-40. Firstly, initial gas permeabilities (K0) of shale samples were determined using a Pulse Decay Permeameter (ULP-713, Beijing Yineng, China). The confining pressure was 12 MPa and pore pressure was 8 MPa. Secondly, one ends of the shale samples were plugged with different plugging fluids at room temperature for 5 h. The plugging pressure is 3.5 MPa and the confining pressure is 5 MPa. Finally, the shale samples were air-dried at room temperature till constant weight, and gas permeabilities after plugging (K1) were determined. (2) Scanning electron microscopy (SEM) observation After plugging, the 0.5-cm end of the shale sample contacting with 1wt% laponite was cut down. SEM observation of the shale slice was conducted by a scanning electron microscope (SEM, Ultra 55, Zeiss). 2.6 Property measurement of the base drilling fluid 4wt% bentonite suspension (i.e. base drilling fluid) was prepared by adding bentonite into water using an electrical mixer. The base drilling fluid was then aged for 24 h at room temperature. The apparent viscosity and filtrate volume were determined accorrding to the 7



American Petroleum Institute (API) Recommended Practice 13B-1：Recommended Practice Standard Procedure for Testing Drilling Fluids. 3. RESULTS AND DISCUSSIONS 3.1 Physcial properties of laponite nanoparticles

Figure 1. The formation of “house of cards” structure is because of edge (+) to face (-) electrostatic attractions between disc-shaped laponite nanoparticles As shown in Figure 1, laponite consists of disc-shaped nanoparticles with a diameter of 25 nm and a layer thickness of 0.92 nm. In the dry state, isomorphic substitutions of the divalent magnesium atoms by monovalent lithium generate negative charges on the faces balanced by the sodium ions in the interlayer. In the aqueous media, the dissociation of Na+ results in permanent negative charges on the faces. The edge of the laponite particle, which contains predominantly MgOH, is positive at pH below 10~1133. Thus edge (+) to face (-) electrostatic attractions lead to the formation of “house of cards” structure25,26, as shown in Figure 1. 3.2 Inhibition property evaluation of laponite suspensions 8


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The inhibition property of laponite suspensions was evaluated by three different methods: immersing test, linear swelling test and shale recovery test. Moreover, comparisons were made with water, KCl and polyester amine.

(1) Immersing test

Clay minerals in the shale formation mainly are illite, montmorillonite and kaolinite. Kaolinite is non-swelling in water; montmorillonite is more swelling in nature; and illite falls in between. Sodium-based bentonite with a montmorillonite content of 85 to 90% was used to evaluate the inhibition property in immersing test.

Figure 2. The images of sodium bentonite pellets after immersed in water (b), 5wt% KCl (c), 1wt% polyester amine (d), 2wt% polyester amine (e), 0.5wt% laponite (f), 1wt% laponite (g) and 2wt% laponite (h) for 12 hours. Image (a) showed the original state of the sodium bentonite pellet.

9



The immersing test results are shown in Figure 2. Figure 2a shows the original state of the sodium bentonite pellet. After immersed into different inhibitive solutions for 12 h, the images of the sodium bentonite pellets are shown in Figure 2b to 1h. It can be seen that after the bentonite pellet was immersed in water for 12 h, it could not maintain structural integrity due to an excess swelling in water (Figure 2b) and the size of the pellet increased several times of its original state. When immersed in 5wt% KCl, the bentonite pellet dispersed quickly and became fully dispersed in less than 1h. But the volume change of the bentonite pellet was little (Figure 2c). For polyester amine (Figure 2d and Figure 2e), both the swelling and dispersion were serious. Apparently, 0.5~2wt% laponite had a good inhibition property. After immersed for 12 h, the bentonite pellets remained almost their original form (Figure 2f-2h). The sizes of the pellets were slightly increased. Moreover, based on our observation, the inhibition property of laponite suspensions increased as laponite concentration increased.

(2) Linear swelling test

Figure 3. The results of linear swelling tests (a) and shale recovery tests (b) for water, 2wt% 10


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polyester amine (PA), 5wt% potassium chloride (KCl), 0.5wt% laponite (Lap), 1wt% laponite and 2.0wt% laponite Linear swelling test is the most commonly used quantitative evaluation method of clay swelling. The experimental results are shown in Figure 3a. In different solutions, the bentonite pellets had different swelling degrees. The swelling increments of bentonite pellets immersed in water, 5wt% KCl, 2wt% polyester amine, 0.5wt% laponite, 1wt% laponite and 2wt% laponite were 11.03, 4.82, 3.31, 5.51, 3.17, 2.73 mm, respectively. Compared with the sample in water, the swelling degrees of the inhibitive solutions were significantly reduced. In particular, 2wt% laponite suspension achieved the lowest swelling increment, which indicates that 2wt% laponite was effective to inhibit hydration swelling of shale.

(3) Shale recovery test

The shale recovery test is a standard method to evaluate the hydration dispersion. As shown in Figure 3b, the recovery value of water was the lowest (19.83%), which indicated that the used shale samples had a relatively strong water sensitivity. The shale recovery values of 5wt% KCl and 2wt% polyester amine were 52.83% and 60.18%, respectively. The recovery value obtained in 2wt% laponite was apparently higher (85.27%) than those of other inhibitive solutions, which indicated that laponite had a better capability of inhibiting hydration of shale.

(4) Inhibition property analysis

The above three inhibition evaluation tests emphasized differently. The linear swelling test 11



only considers the hydration swelling. The rolling recovery test mainly focuses on the hydration dispersion that is also caused by hydration swelling. The immersing test is more visual.

In aqueous media, the bentonite will firstly undergo surface hydration by adsorbing water. The expansions of crystal layers will double the volume of clay minerals. Then, osmotic hydration will make the swelling a couple of times due to osmotic pressure and electric double layer repulsion. The hydration swelling of clay minerals weakens rock strength and leads to hydration dispersion, especially for illite-smectite mixed-layer minerals. Apparently, water achieved the lowest inhibition property in all the three inhibition evaluation tests. KCl is a most commonly used clay inhibitors in various drilling fluid systems such as KCl/polymer drilling fluid. K+ has a low hydration energy compared to Na+, Ca2+, Al3+, etc. In aqueous solution, K+ ions can migrate to interlayer spaces, remain bound to the clay surface34,35, leading to decreased swelling. However, KCl solution with a high cation concentration may cause edge-edge repulsion and lead to fast dispersion as shown in Figure 2c. KCl can inhibit shale swelling to some degree, but cannot meet the high inhibition requirements in the drilling of the complex formations. Polyester amine is an effective shale inhibitor and obtained intensive studies worldwide. It can exchange Na+ and Ca2+, and adsorb on layers of clays through electrostatic interaction and hydrogen bonding21,24, decreasing clay swelling. In comparison with KCl and polyester amine, laponite has the best inhibition property in all the three inhibition evaluation tests, indicating laponite could considerably reduce wellbore collapse, tight hole, etc. 3.3 Inhibition mechanism analysis 12


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In this section, the role of laponite in inhibiting shale swelling and stabilizing wellbore was analyzed from the following four aspects.

(1) Plugging effect of interlayer spaces of clays

Figure 4. The schematic diagram of plugging interlayer spaces of bentonite by laponite. Laponite nanoparticles could plug interlayer spaces of clays by electrostatic attraction. Due to the unique microstructure, the faces of the laponite particles are charged negative in an aqueous medium. The charge on the edge of bentonite particles is pH dependent because the Al−OH has an isoelectric point (pH=7~9)36. The bentonite particles have positively charged edges at a low pH and negative charged edges at a high pH. Thus, at an appropriate pH, electrostatic interaction exists between bentonite particle edges (+) and nanoscale laponite faces (-) in aqueous media as displayed in Figure 4. The laponite faces could absorb on the edges of bentonite particles, plugging interlayer spaces and slowing down the water invasion into interlayers.

When the laponite faces and edges of bentonite particles have opposite charges, the electrostatic interaction between bentonite particle edges and nanoscale laponite faces is stronger,

13



and the kinetics of plugging process are faster. In the linear swelling tests (Figure 3a), the linear swelling value of 1wt% laponite (pH≈7.0) was smaller than that of 2wt% laponite (pH=9.8) within the first 4 hours, which indicates the absorption speed is advantageous for inhibition.

(2) Low free water content

In addition to the plugging effect, free water content is an important factor affecting the hydration of clay. The inhibitive solution with a high free water content is not conductive to the inhibition of clay hydration. The free water content was assessed by measuring the capillary suction time (CST) in this paper. As shown in Figure 5, 5wt% KCl, 2wt% polyester amine and water had similar CST values. Laponite could increase CST substantially. When laponite concentration in water was 1.0wt%, the CST was approximately 21.8 times higher than that of water. And when laponite concentration was 2.0wt%, the CST is more than 1000 times higher than that of water. Thus, laponite suspensions had relatively higher CST values, lower free water contents than commonly used inhibitor solutions such as polyester amine and KCl solution.

14


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Figure 5. Capillary suction times of pure water, 5wt% KCl, 2.0% polyester amine (PA) and laponite (Lap) water suspensions with various concentrations Laponite suspensions are able to establish “house of cards” structure and maintain gel states at low concentrations, which enhances trapping effect of water within these aggregate particulate structures and benefits to reduce the free water content. In linear swelling tests, although 1wt% laponite suspension could plug bentonite interlayer spaces faster than 2wt% laponite suspension in the first 4 h, 2wt% laponite suspension had a much lower free water content, the inhibition property of 2wt% laponite suspension was better in the long run.

(3) Excellent thixotropy

Figure 6a shows the plots of viscosity versus shear rate. The viscosity increased tremendously with increasing concentration of laponite. As the shear rate increased, viscosity decreased dramatically and the laponite suspensions had a good shear thinning behavior. The three interval thixotropy tests (3ITT) were used to investigate thixotropic behavior of laponite suspensions. As shown in Figure 6b, the viscosity was considerably high at the low shear rate (3 r/min), became substantially low under the high shear rate (600 r/min), and recovered rapidly at the third interval when the low shear rate was applied again.

At low shear rate, the gel structure of laponite suspensions formed, resulting in a high viscosity that is good to suspend weighting materials, especially when circulation is ceased such as in tripping process for changing drill bit. A better suspending property prevents settling down

15



of weighting materials, which are critical to keep a constant density of drilling fluid and balance the formation pressure.

At high shear rate, the gel structure was broken down, leading to a dramatic decrease in viscosity (Figure 6a). A good shear thinning property is helpful to reduce equivalent circulation density (ECD) of the drilling fluid and ECD induced wellbore instability. In addition, low viscosity of drilling fluid could be helpful to reduce energy consumption and enhance the rate of penetration (ROP).

When the same low shear condition was applied, the gel structure can be re-established within a short time, providing enough suspension capability.

Figure 6. Viscosity vs. shear rate plots (a) and three interval thixotropy test results (b) of laponite suspensions with various concentrations. During normal circulation of drilling fluid, the shear rate is highest near the drilling pipe and lowest near the wellbore wall. The shear rate close to wellbore wall is nearly zero, so the viscosity will be extremely large near the wellbore wall. Thus a film of nano particles could form 16


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on the wellbore wall (refer to Figure 7), which could theoretically reduce water invasion, stabilize water activity of formation, prevent clay swelling and stabilize wellbore.

(4) Pore plugging of shale formation

Laponite can not only inhibit the hydration swelling and hydration dispersion of clay minerals in formation, but also play a role in plugging the nano pores of shale formations during drilling process30. The plugging prevents water invasion, thus effectively weakens the hydration of shale and helps to stabilize the wellbore.

The shale cores were treated by plugging fluids (water, 0.5%~2.0% laponite) for 5 h. As shown in Table 2, after treated by water, the gas permeability increased from 72.24×10-6 µm2 to 114.95×10-6 µm2 due to the swelling of clay minerals. After treated by laponite suspensions, the permeability recovery (K1/K0) decreased substantially, which indicated that laponite has an

effective shale plugging performance. The experimental results are consistent with other literature30.

Table 2. Gas permeability of shale cores before and after treatment by water, 0.5wt% laponite, 1.0wt% laponite and 2.0wt% laponite. The plugging treatment of shale cores was carried out with a confining pressure of 12 MPa and a pore pressure of 8 MPa at 20℃. Sample

Initial gas permeability K0 (10-6µm2)

1#

Plugging fluid

Gas permeability after plugging K1 (10-6µm2)

K1/K0

72.24

Water

114.95

1.59

2#

42.89

0.5% laponite

15.67

0.365

3#

55.18

1.0% laponite

4.92

0.089

17



4#

52.61

2.0% laponite

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1.67

0.032

Based on previous study, shale has pores in nanoscale to microscale sizes, in which nanoscale pores had a dominant proportion37–39. Because of the ultra-low permeability of shale, no mud cake forms on the wellbore wall during drilling process. Thus, no barriers on the wellbore wall could not stop water invasion and reduce clay swelling. In addition, microscale materials and plugging agents such as bentonite particles, superfine calcium carbonate, asphalts and walnut shell powder cannot effectively plug nanoscale pores shale pores.

Figure 7. Shale pore plugging mechanism of laponite suspensions. Laponite nanoparticles could plug nanoscale pores of shale and form a nano film on the surface of wellbore wall to reduce water invasion into formation. Nanoscale laponite particles could be beneficial to plug shale pores as shown in Figure 7. Under positive differential pressure between drilling fluid (high) and formation (low), nanoscale laponite particles are pushed close to the wellbore wall and block the shale pores. Figure 8a is the 18


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original shale surface with a porous structure from SEM. While after plugged by 1wt% laponite, a seamless film (Figure 8b) was observed on the surface of the shale slice, which could be beneficial to reduce water invasion into formation.

Figure 8. SEM images of original shale surface (a) and shale surface plugged by 1wt% laponite suspension with the pressure of 3.5 MPa for 5 h at room temperature (b) 3.4 Influence on drilling fluid property

During the drilling process, it is important to make sure that drilling fluid additives do not affect the properties of drilling fluid negatively.

Bentonite powders are not prone to be dispersed and in mud form when added in inhibitive liquids such as KCl solution, polyester amine solution and laponite suspensions. However, in turn, when KCl, polyester amine and laponite are added into a well-aged bentonite suspension, the properties of drilling fluid will be affected positively or negatively. Figure 9 shows the effect

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of KCl, polyester amine and laponite on the filtration volume and apparent viscosity of base drilling fluid (4wt% bentonite). As shown in Figure 9a, KCl and polyester amine had a negative effect on the filtration volume of drilling fluid. Because inhibitive KCl and polyester amine solution not only confine the clay hydration of wellbore but also inhibit the mud making of bentonite in the drilling fluid, resulting in aggregation of bentonite particles, viscosity increase (Figure 9b) and a bad filtration property (Figure 9a). However, in the studied concentration ranges, laponite had a small effect on the apparent viscosity (Figure 9b), and more importantly, laponite could considerably reduce the filtration volume (Figure 9a). The clay inhibition mechanism of laponite is different from KCl and polyester amine. K+ could exchange for Na+ and Ca2+ in the interlayer, and generate more stable structures. Polyester amine could exchange Na+ and Ca2+, and adsorb on layers of clays through electrostatic interaction and hydrogen bonding. Therefore, KCl and Polyester amine have a negative effect on the stability of fully hydrated and dispersed clays in water suspension. While laponite inhibits clay hydration by plugging interlayer spaces, providing low free water and excellent thixotropy, etc. For fully hydrated clays in the drilling fluid, laponite is no longer working to inhibit bentonite swelling. Thus, laponite is not detrimental to the bentonite suspension. On the contrary, the synergistic effect of microscale bentonite and nanoscale laponite was beneficial to reduce filtration volume.

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Figure 9. The effect of KCl, polyester amine and laponite on the filtration volume (a) and apparent viscosity (b) of base drilling fluid (4wt% bentonite) 4. CONCLUSIONS

Nano laponite has an excellent shale inhibition property, which is better than that of widely used shale inhibitors of KCl and polyester amine. The better inhibition property of laponite manifested in three aspects: the bentonite pellet could keep a complete shape in the laponite suspensions; the bentonite pellets had lower linear swelling increments; the shale fragments got higher recovery after hot rolling in the laponite suspensions. According to mechanism study, the reasons of the better shale inhibition are listed as follows. (1) Laponite nanoparticle could plug interlayer spaces of clays by electrostatic interaction, slowing down the hydration of clays. (2) Laponite suspensions have low free water contents. (3) The excellent thixotropy enables the laponite suspensions to have a considerably high viscosity on the wellbore wall that might be helpful to form nano films, reducing water invasion. (4) The nanoscale particles are effective to plug shale pores and form seamless surfaces, preventing water invasion. In addition, laponite was 21



beneficial to reduce filtrate volume and had a slight influence on the apparent viscosity of the base drilling fluid, while KCl and polyester amine affected filtrate volume and apparent viscosity negatively. Acknowledgements The authors are thankful to the China Postdoctoral Science Foundation (2018M630812), the Fundamental Research Funds for the Central Universities (No.18CX02171A), Scientific Research Foundation for the Introduction of Talents (YJ20170014) and Joint Funds of the National Natural Science Foundation of China (U1762212). Notes Declarations of interest: none References (1)

Pašić, B.; Gaurina-Međimurec, N.; Matanović, D. Wellbore Instability: Causes and Consequences. Rud. Zb. 2007, 19, 87–98.

(2)

Yu, M.; Chenevert, M. E.; Sharma, M. M. Chemical-Mechanical Wellbore Instability Model for Shales: Accounting for Solute Diffusion. J. Pet. Sci. Eng. 2003, 38 (3–4), 131– 143.

(3)

Talabani, S.; Chukwu, G.; Hatzignatiou, D. Drilling Successfully through Deforming Shale Formations: Case Histories. In Rocky Mountain Regional Meeting/Low Permeability Reservoirs Symposium and Exhibition; Denver, Colorado, 1993.

(4)

Wilson, M. J.; Wilson, L. Clay Mineralogy and Shale Instability: An Alternative 22


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Figure 1. The formation of “house of cards” structure is because of edge (+) to face (-) electrostatic attractions between disc-shaped laponite nanoparticles 60x27mm (300 x 300 DPI)


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Figure 2. The images of sodium bentonite pellets after immersed in water (b), 5wt% KCl (c), 1wt% polyester amine (d), 2wt% polyester amine (e), 0.5wt% laponite (f), 1wt% laponite (g) and 2wt% laponite (h) for 12 hours. Image (a) showed the original state of the sodium bentonite pellet. 75x35mm (300 x 300 DPI)



Figure 3. The results of linear swelling tests (a) and shale recovery tests (b) for water, 2wt% polyester amine (PA), 5wt% potassium chloride (KCl), 0.5wt% laponite (Lap), 1wt% laponite and 2.0wt% laponite 61x23mm (300 x 300 DPI)


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Figure 4. The schematic diagram of plugging interlayer spaces of bentonite by laponite. Laponite nanoparticles could plug interlayer spaces of clays by electrostatic attraction. 38x11mm (300 x 300 DPI)



Figure 5. Capillary suction times of pure water, 5wt% KCl, 2.0% polyester amine (PA) and laponite (Lap) water suspensions with various concentrations 74x43mm (300 x 300 DPI)


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Figure 6. Viscosity vs. shear rate plots (a) and three interval thixotropy test results (b) of laponite suspensions with various concentrations. 56x20mm (300 x 300 DPI)



Figure 7. Shale pore plugging mechanism of laponite suspensions. Laponite nanoparticles could plug nanoscale pores of shale and form a nano film on the surface of wellbore wall to reduce water invasion into formation. 81x82mm (300 x 300 DPI)


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Figure 8. SEM images of original shale surface (a) and shale surface plugged by 1wt% laponite suspension with the pressure of 3.5 MPa for 5 h at room temperature (b)



Figure 9. The effect of KCl, polyester amine and laponite on the filtration volume (a) and apparent viscosity (b) of base drilling fluid (4wt% bentonite) 61x23mm (300 x 300 DPI)


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Nanoscale Laponite as a Potential Shale Inhibitor in Water-Based

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