Experimental Study on Mechanical Properties of Rock in Water-Sensitive Oil and Gas Reservoirs Under High Confining Pressure

Abstract

Injecting high-pressure fluid into a reservoir rock mass will change the mechanical properties of the rock; the strength and safety of a shale well wall are also extremely critical. In order to investigate the law of variation in water-sensitive shale strength during fracturing, an experimental study on the mechanical properties of shale under high confining pressure and water–rock coupling was carried out. Taking water-sensitive shale rock as the research object, the effects of high confining pressure and water content on the mechanical properties, residual strength, and macroscopic and microscopic failure modes of shale were analyzed. The test results show that the stress–strain curve of the shale gradually shortened with the decrease in the water content in the stage of void compaction and plastic yield, and the peak of the stress–strain curve was continuously enhanced. The water content and the peak intensity exhibited a negative linear correlation. The elastic modulus and water content showed an exponentially decreasing distribution. However, as the water content increased, the decreasing rate became slower, the softening coefficient increased, and the plastic deformation increased. The research results provide basic load parameters for the strength and safety of the casing of an oil layer under fracturing conditions.

1. Introduction

Shale wall instability is a prominent problem in the exploration and development of western oil and gas resources, and the change in casing external pressure under fracturing conditions is also a problem that cannot be ignored. Oil and gas drilling is mostly carried out in sedimentary rock formations, and more than 70% of sedimentary rock is mud shale [1]. During the drilling process, about 90% of borehole collapse problems are related to shale instability [2]. In addition, under fracturing conditions, injecting a large amount of fracturing fluid will change the rock water content of the reservoir, affecting the rock mechanical elastic modulus, Poisson’s ratio, and other key parameters of the rock, and re-distributing the ground stress, changing the casing external pressure and affecting the casing safety. Therefore, the in-depth study of the water sensitivity of shale is of great significance for efficient, safe, and low-cost drilling and can accelerate the exploration and development process of oil and gas fields.

Due to the comprehensive effects of high confining pressure and seepage water in the depth of the formation, water-sensitive shale’s mechanical properties and failure modes are quite complex. Experts and scholars at home and abroad have conducted a series of studies and explorations. Zhu Kuanliang et al. [3] studied the physical and chemical properties, mechanical properties, macro- and microstructural characteristics, coupling rock mechanics with drilling fluid, and the instability of shale rock. The problem of the wellbore stability of shale has been effectively solved. Hao Yunqing et al. [4] studied the influence of shale characteristics on compressibility. They found that the factors affecting the compressibility of shale are complex, depending on petrological characteristics, confining pressure conditions, and differences in ground stress, and the fracturing effect is related to the selection of a fracturing fluid system and construction parameters. Zhao Kai et al. [5] compared three kinds of borehole stability analysis methods, namely, pure mechanics, seepage force coupling, and chemical coupling, according to the differences in the composition and structure of hard and brittle shale. Four kinds of mechanical instability mechanisms of hard and brittle shale have been studied. You M, Li Rong, et al. [6,7] used a triaxial rock mechanic testing machine to conduct a triaxial test of mud shale. According to the test, a static elastic modulus, cohesion, an internal friction angle, Poisson’s ratio, and compressive strength characteristic parameters were obtained; mechanical analysis was carried out on shale well walls; and safety windows of drilling fluid density, including collapse pressure and rupture pressure, were obtained. Guang Xinjun et al. [8] explored the borehole instability mechanism of layered mud shale and studied the borehole stability prediction model of layered mud shale formation considering the presence of drilling fluid under a real three-way stress state. Ignoring the influence of intermediate principal stress, it is impossible to reasonably predict the possibility of the collapse of the layered shale formation. The seepage of drilling fluid filtrate along the bedding surface will reduce the cohesion and internal friction angle of the bedding surface. The improved analysis model of the wellbore stability of the layered shale formation can truly reflect the state of the in situ stress and the mechanical properties of the wellbore rock. Yin Shuai et al. [9] comprehensively analyzed shale fracture characteristics and factors influencing UCS. The sliding of micro-fractures, the generation and expansion of new micro-fractures, and the collapse of pores will affect the deformation mechanism of the inelastic section of the uniaxial loading curve of shale. The uniaxial compressive fracture of mud shale is tensile or shear. The shear fracture can easily occur in the part with low stiffness and a weak consolidation degree, and the tensile fractures can occur in the part with high stiffness and a strong consolidation degree. Yu Huichang et al. [10] analyzed the relationship between rock yield strength, ultimate strength, residual strength, and confining pressure and obtained rock’s peak shear strength parameters and residual shear strength. According to the theory of elastic–plastic mechanics, a bilinear elastic–linear softening–residual ideal plastic model considering the strain-softening of rock was established, the constitutive equations were set up, and the parameters of each stage were determined. Tian Hongming et al. [11] analyzed the change law of damage dissipation energy in the relaxation process, established the relaxation damage evolution equation, introduced the damage factors into the Nishihara model, established a nonlinear relaxation damage model, and verified the rationality of the established model. Diao Haiyan [12] studied the mechanical properties of shale. You Mingqing [13] studied the strength and failure criteria of full-size rocks. Zhou Shunlin et al. [14] experimentally analyzed the influence of stress on the brittleness of mud shale and concluded that, with an increase in confining pressure, its elastic and plastic strain increased, and its brittleness index decreased. Zhao Jinzhou et al. [15] studied the compressibility of shale gas reservoir rocks. Yuan Huayu et al. [16] analyzed mudstone shaft wall collapse, and the results show that water weakened the strength of the mud shale. According to the study of Duan Tianzhu et al. [17], under uniaxial compression, the peak strength and elastic modulus of sandstone gradually decrease with the increase in water content, while the change in peak strain shows an opposite trend. The total work and elastic energy decreased with the increase in water content, and the dissipative energy increased with the increase in water content. Zhang Wei et al. [18,19] characterized the micromechanical properties of shale after it encountered water, and a new water-sensitivity-based upscaling model was constructed. Alouhali Raed et al. [20] evaluated the shale fluid sensitivity of target unconventional shale formations by conducting two shale fluid sensitivity tests on samples using four different water-based fluids, as well as fresh water and diesel oil.

However, there are few experimental studies on the mechanical properties of shale under high confining pressure and water–rock coupling conditions. In this paper, triaxial tests of water-sensitive shale under 60 MPa, 70 MPa, and 80 MPa confining pressure and different water contents are carried out. The damage and failure modes of shale under different confining pressures and different water contents are discussed, and the changes in its mechanical properties, residual strength, and softening coefficient are analyzed. The experimental results are of great value for guiding the exploration and development of oil and gas in our country and for other engineering projects related to water-sensitive rocks under high confining pressure.

2. Experimental Device and Sample Preparation

2.1. Experimental Equipment and Methods

The whole experiment was carried out in the Geotechnical Engineering GCTS Laboratory of Xi’an University of Science and Technology using the electro-hydraulic servo-controlled high–low-temperature and high-pressure dynamic geotechnical triaxial test system GCTS (GCTS Testing Systems, Tempe, AZ, USA) (see Figure 1). The system can be loaded up to 1500 kN, with a confining pressure of up to 140 MPa, a loading accuracy of up to 0.25%, and a confining pressure and pore pressure accuracy of 0.005 MPa. Strain or stress control can be performed, and the test system meets the requirements of the International Society of Rock Mechanics for rock triaxial testing.

2.2. Sample Preparation and Experimental Scheme

The shale rock samples were collected via man–machine collaboration, and the sampling site was Dongnihe Village, Yijun County, Tongchuan City. Through the on-site collection, uniform shale rock blocks of the same strata were obtained. The shale rock blocks had good integrity and a stable structure. The shale rock blocks, without obvious damage, were transported to the geotechnical Laboratory of Xi’an University of Science and Technology. According to the “Engineering Rock Mass Test Method Standard”, the shale rock blocks were processed via core drilling, cutting, and grinding. Then, standard shale rock samples with a diameter of 5 cm and a height of 10 cm were obtained. The shale rock samples here were massive and formed by the cementation of rock mineral particles. They were argillaceous siltstone samples with a dark red color. They were heterogeneous bodies composed of clay and clastic minerals from the Yanchang Formation of the Upper Triassic Series. Different mineral particles were bonded to each other by cementation to compose these samples. Through a D/max-2500 X-ray diffractometer (Rigaku, Tokyo, Japan), we determined that the mineral composition of the shale rock samples was mainly clay minerals and quartz, in which the average mass fraction of clay minerals was 35.28% and the average mass fraction of quartz was 40.56%. Organic pores, inorganic pores (intergranular pores and intragranular pores), and micro-fractures were developed in the test rock samples, and the pores were mainly micro- and medium-sized pores. According to the experimental procedures of the International Society of Rock Mechanics, samples with similar longitudinal wave velocities were selected with an acoustic test system (Crystal Instruments, Santa Clara, CA, USA) (see Figure 2), and the samples were vacuumized and saturated by the vacuum saturator (Vinci Technologies, Nanterre, France) for 24 h.

• Experimental scheme: In order to test the mechanical properties of shale rock, the effects of water sensitivity characteristics under different confining pressures and different water contents on its mechanical properties were analyzed. The samples of shale rock with water contents of 0%, 2%, 4%, 6%, and 8% were subjected to triaxial compression tests under confining pressures of 60 MPa, 70 MPa, and 80 MPa.
• Experimental procedure: After the shale rock sample had been wrapped with rubber film, it was fixed on the pressurized platform, and the anisotropic strain gauge was installed. We then started the system to set the control parameters, installed the pressure chamber cover, and filled the oil to discharge the gas. After the oil had been filled, the confining pressure was applied, and the setting confining pressure was loaded at the rate of 2 MPa/min, after which the axial strain rate was set as 10⁻⁶/s for the loading experiment. After the sample had been damaged, the system automatically generated the test data. The physical parameters of the tested shale rock samples are shown in

  Table 1. Determination of numbers and sizes of tested shale rocks.

  ID	Altitude (cm)	Diameter (cm)	ID	Altitude (cm)	Diameter (cm)	ID	Altitude (cm)	Diameter (cm)
  A-1	10.23	4.85	B-1	9.86	4.85	C-1	9.85	4.85
  A-1	10.12	4.92	B-1	10.03	4.92	C-1	9.64	5.08
  A-1	10.32	5.05	B-1	9.85	4.88	C-1	10.12	4.85
  A-2	10.10	5.12	B-2	10.12	5.01	C-2	10.03	5.04
  A-2	10.30	4.87	B-2	10.06	5.10	C-2	10.25	5.13
  A-2	10.11	4.85	B-2	10.21	5.07	C-2	10.08	5.09
  A-3	9.88	4.87	B-3	9.88	4.89	C-3	10.10	4.98
  A-3	10.30	5.01	B-3	9.96	4.92	C-3	10.05	5.12
  A-3	10.10	5.06	B-3	10.22	4.83	C-3	10.18	4.98
  A-4	9.85	4.90	B-4	10.14	4.88	C-4	9.96	5.06
  A-4	10.28	5.14	B-4	10.30	4.85	C-4	10.05	5.08
  A-4	10.13	5.02	B-4	10.11	4.85	C-4	9.85	5.11
  A-5	10.06	4.85	B-5	9.88	4.90	C-5	10.12	5.01
  A-5	10.03	4.98	B-5	9.85	4.89	C-5	10.28	5.12
  A-5	10.16	4.85	B-5	9.75	5.01	C-5	10.13	5.06

  .

3. Experimental Results and Analysis

Through triaxial compressive strength tests of all shale rock samples, the compressive strengths of each shale rock sample could be obtained (see

Table 2. Test results of three-axial compressive strength.

ID	Intensity (MPa)	Average Compressive Strength (MPa)	ID	Intensity (MPa)	Average Compressive Strength (MPa)	ID	Intensity (MPa)	Average Compressive Strength (MPa)
A-1	35.092	41.659	B-1	49.527	46.932	C-1	57.839	50.491
A-1	43.317		B-1	49.223		C-1	59.245
A-1	46.568		B-1	42.046		C-1	34.389
A-2	37.853	34.552	B-2	39.417	37.577	C-2	48.196	44.273
A-2	33.891		B-2	37.481		C-2	38.362
A-2	31.912		B-2	35.833		C-2	46.261
A-3	35.124	29.945	B-3	27.573	33.819	C-3	30.253	37.666
A-3	29.348		B-3	37.118		C-3	39.368
A-3	25.363		B-3	36.766		C-3	43.377
A-4	26.367	23.034	B-4	27.851	24.049	C-4	30.687	30.991
A-4	25.142		B-4	26.416		C-4	28.421
A-4	17.593		B-4	17.88		C-4	33.865
A-5	12.463	10.347	B-5	11.646	10.512	C-5	14.147	12.038
A-5	9.857		B-5	13.422		C-5	11.736
A-5	8.721		B-5	6.468		C-5	10.231

), as well as the stress–strain curves (see Figure 3).

3.1. Stress–Strain Curves of Shale Rock Samples

Under the same confining pressure and different water contents, it can be seen that the stress–strain curves of the shale rock undergo great changes from the beginning of loading to the four stages of failure with increasing water content. The void compaction stage is gradually shortened with the decrease in water content; that is, under the action of external load, the microcracks in the shale rock sample are closed, and the greater the water content, the longer the drainage and compression time of the void-weakened material. The plastic yield stage decreases with the decrease in water content, while the peak value of the stress–strain curve increases and the brittle deformation decreases. With the increase in confining pressure, the strain decreases gradually when the sample reaches peak strength.

When the water content is high, the shale is softened to a large extent, and the drying strength is nearly five times that of the saturated water strength; the softening results in large plastic deformation, and vice versa. At a high water content, due to serious plastic deformation, a large number of micro-cracks appear and gradually expand, and the shale rock sample appears slightly swollen.

3.2. Strength Characteristic Analysis

When the shale is softened by water, aging deformation occurs, which leads to the accumulation of internal water, causing physical and chemical damage, and this leads to the spread, propagation, penetration, and collapse of initial defects in the shale rock mass. During the micro-deformation process, the chemical interaction between water and shale composition significantly affects its strength. By fitting the test data, the relationship between the triaxial compressive strength and water content of the shale rock under different confining pressures can be obtained.

At the confining pressure of 60 MPa,

σ = 3.7071 W + 42.7358,

(1)

Formula: σ is the triaxial compressive strength (MPa) of shale; W is the moisture content (%); the correlation coefficient R = −0.98244.

At the confining pressure of 70 MPa,

σ = −4.3184 W + 47.8514,

(2)

where σ is the triaxial compressive strength (MPa) of shale; W is the moisture content (%); the correlation coefficient R = −0.98284.

At the confining pressure of 80 MPa,

σ = −4.5093 W + 53.1294,

(3)

where σ is the triaxial compressive strength (MPa) of shale; W is the moisture content (%); the correlation coefficient R = −0.96341.

Equations (1)–(3) indicate that, under the same confining pressure, the triaxial compressive strength of shale is closely related to the water content and the former decreases as the latter increases. The compressive strength decreases linearly with the increase in water content. With the same water content, the triaxial compressive strength of shale rock increases as the confining pressure increases.

From the strength fitting curve relationship diagram reflecting different confining pressures and different water contents (as shown in Figure 4), the following can be seen:

• When the dry shale rock sample increases from 60 MPa to 80 MPa, the strength increases from 41.659 MPa to 50.491 MPa, an increase of 21.2%. When the saturated shale rock sample increases from 60 MPa to 80 MPa, the strength increases from 9.872 MPa to 11.746 MPa, an increase of 18.98%. It can be seen that the strength of the shale rock clearly increases under the action of high confining pressure;
• When the confining pressure is 60 MPa, the compressive strength of shale with a water content from 0 to 8% decreases from 41.659 MPa to 10.347 MPa, a decrease of 75.16%. When the confining pressure is 70 MPa, the compressive strength of shale with a water content from 0 to 8% decreases from 46.932 MPa to 10.512 MPa, a decrease of 77.60%. When the confining pressure is 80 MPa, the compressive strength of the shale rock decreases from 50.491 MPa to 12.038 MPa, a decrease of 76.16%. It can be seen that hydration significantly degrades the compressive strength of shale.

Underground rock formations, especially the deep surrounding rock, are affected by the surrounding ground stress, and a triaxial mechanical characteristic experiment with high confining pressure can reflect the mechanical characteristics of the deep rock. The results of this experiment show that the peak strength of shale (the maximum shear stress that rocks can resist) will be enhanced, and the degree of enhancement with 70 MPa to 80 MPa is slightly greater than that with 60 MPa to 70 MPa. In the dry state, the strength under confining pressures from 60 MPa to 70 MPa and from 70 MPa to 80 MPa is increased by 5 MPa. With the increase in water content, the increase in strength under confining pressure decreases from 60 MPa to 70 MPa, and the strength strops increase with confining pressure when the water content is full. However, when the confining pressure increases from 70 MPa to 80 MPa under each different level of moisture content, the strength increases significantly by about 5 MPa.

The residual strength (the residual resistance to external load after the deformation and failure of the sample depends on the friction between the blocks and particles) is significantly improved due to the increase in interparticle friction caused by high confining pressure. The triaxial compressive strength test results of water-sensitive shale with different water contents under confining pressures of 60 MPa, 70 MPa, and 80 MPa show (see

Table 3. Residual strength/peak strength percentage and softening coefficient (%) of triaxial shale.

Confining Pressure/MPa	The Ratio of Residual Strength to Peak Strength Under Different Water Contents %					Softening Coefficient η
	0	2	4	6	8
60	65.12	55.57	52.45	49.71	41.66	24.89
70	66.85	59.74	59.23	58.54	58.31	26.84
80	67.48	65.89	64.58	63.89	62.76	27.31

) that the percentages of residual strength and peak strength of water-sensitive shale with the same water content increase with an increase in confining pressure, except for the small increase under dry conditions. Under confining pressures ranging from 60 MPa to 80 MPa, the increase rate was 2.36%, and the rate was large under the water-containing state. The increase rate was increased under conditions of 2%, 4%, 6%, and 8%, to 10.32%, 12.13%, 14.18%, and 21.1%, under confining pressures ranging from 60 MPa to 80 MPa. The greater the water content, the smaller the void, and the greater the effective stress produced by confining pressure transformation. The softening coefficient of shale under high confining pressure increases with the increase in pressure. However, due to the interactions of other factors in the compound state of high confining pressure, from 60 MPa to 70 MPa, the softening coefficient did not increase by 1.95%. From 70 MPa to 80 MPa confining pressure, the increase in softening coefficient was 0.47%. It can be seen that the increase in softening coefficient decreases with the increase in confining pressure.

3.3. Deformation Characteristic Analysis

Water undergoes a complex physical, chemical, and mechanical interaction with the minerals in rock. The seepage stress of water reduces the effective stress of rock mass and causes the shear strength to decrease. The coupling effect of water and rock affects the stress distribution of rock mass, thus affecting its strength and deformation. The elastic modulus increases with the increase in confining pressure, and the elastic modulus and water content of shale rocks show an exponentially decreasing distribution under the same confining pressure (see Figure 5). When the water content is 0%, the original micro-fissure intergranular cement is packed tightly, and the mechanical integrity is strong. However, with the increase in water content, the slope of the decline of the elastic modulus curve gradually decreases; that is, with the increase in water content, the rate of decrease in the elastic modulus of shale slows down. From Figure 5, it can be seen that as the water content increases, the gradient of decrease of the elastic modulus curve gradually decreases, which means that the rate of decrease in the elastic modulus of shale slows down. This is due to the increase in water content, which causes more of the clay particles in the shale to expand. Under hydrolysis, the cementation of rock particles in shale is gradually undone, leading to a looser structure in the entire rock sample.

When saturated with water, due to the dissolution effect of water, the degree of cementation between particles is quite poor, the particle fullness is significant, the particles are arranged in a turbulent manner, the initial micro-cracks are fully developed, and many small holes emerge to be used as channels by which water can form micro-cracks, giving rise to a rather loose mesostructure. Thus, the elastic modulus of the saturated shale rock is reduced. However, the elastic modulus of shale increases with the increase in confining pressure because the confining pressure causes the cracks inside the shale rock to close, thus increasing the stiffness of the shale rock.

4. Macro and Micro Damage Analysis

4.1. Macro and Micro Analysis of the Influence of Water–Shale–Rock Interaction on the Strength of Shale

The effect of water on shale reflects a complex stress corrosion process, accompanied by the physical and mechanical behaviors of clay, such as slime expansion, water absorption, and disintegration. The main forms of water molecules in shale are as follows: (1) interlayer-bound water; (2) binding water between clay particles; (3) free water between pores and cracks. Among them, surface adsorption is the main process of cell–water interaction. The surface adsorption of water molecules reduces the surface energy of the cell, resulting in the expansion of the cell structure. During the interaction between water molecules and crystal cells, the energy, bonding morphology, and ion distribution characteristics of the crystal cells will change greatly. When water molecules enter the interlayer, the van der Waals force decreases, most notably in the cell energy system, and the interlayer binding water molecules deteriorate the interlayer bonding force.

The bonding force between the mineral components of the shale is mainly produced by the clay minerals content when the shale rock is under water; on the one hand, the clay minerals content forms a hydration film that adsorbs water molecules, and the original clay cement forms a water-binder link. This causes the bonding forces and friction between the particles to weaken, and the water molecules act to lubricate. On the other hand, the clay particles become negatively charged, and the cations in the aqueous solution are enriched on the surfaces of the clay particles to produce osmotic repulsion. This causes the distance between the clay particles to become larger, and the water molecules act to wedge. According to the theory of the diffusion of a double layer of water-saturated clay particles in soil mechanics, the bonding force of clay is a manifestation of the attraction and repulsion between particles under the action of water and shale rock. With the increase in water content, the interaction force between particles decreases, and the shale rock cementation strength and micromechanical parameters deteriorate in water. Therefore, the fracture toughness of water-containing shale rock samples is lower than that under dry conditions. With the increase in water content, the primary cracking speed in shale becomes faster; thus, the strength of the shale rock sample with a higher water content will be lower. This is consistent with the experimental results.

The effects of water on the macroscopic strength of the shale main rock are reflected in the following: (1) water absorption expansion; (2) cementing or disintegration; (3) solution/subduction; (4) pore pressure; (4) seepage dragging force; (6) deterioration of strength. The water-absorbing expansion of clay mineral particles and the water-absorbing expansion of clay mineral crystal cells are the intrinsic characteristics of the water-absorbing expansion of shale. When clay minerals encounter water, a hydration film is formed between particles. As water molecules are continuously embedded into clay particles, the gap between clay mineral particles increases, the microstructure is damaged, and the bonding force decreases. Macroscopically, the clay cement is argillated to form a fine slime. When the pore water shows fluidity under external loading, the small particles of pelletized clay are carried away, showing the phenomenon of latent erosion. In terms of mechanical properties, when the shale rock encounters cement, the internal argillaceous connection weakens to a hydrocolloidal connection, and the cohesion and internal friction angle of shale rock decrease. During the process of subduction, the bonding force of shale rock deteriorates, its strength decreases, and large deformation occurs. According to Terzaghi’s effective stress principle, pore water pressure can reduce the effective stress of the shale rock skeleton, and the shale rock microstructure is easily destroyed by the drag force of seepage. This continuously induces subduction and deteriorates the mechanical properties of shale rock.

4.2. Effect of Water on Macroscopic Failure Mode of Shale

The stratification, heterogeneity, and anisotropy of shale rock result in a different failure mechanism from that of ordinary rocks, and the weak interlayer has a great impact on the failure mode. In the triaxial test, the weak contact surface between the middle layers is first squeezed and deformed; then, the effective interface increases, the weak components of the large pores continue to weaken, and the total stress cannot reach the maximum. At this time, the shale rock only undergoes elastic deformation. With continuous pressure, the extrusion deformation of the weak material reaches the maximum, the shale rock approaches the elastic–plastic state, and the cracks expand to give rise to local deformation and failure in the form of a narrow shear zone. With the change in water content, the fracture processes of shale rock samples adopt a variety of models. In addition, confining pressure changes the stress state of the shale rock, leading to changes in its failure mode.

Experiments show that the failure mods of the shale under triaxial compression with different water contents can be divided into three forms: shear failure, tensile fracture failure, and mixed failure of both forms (see Figure 6). The dry sample mainly undergoes a mixture of tensile and shear failure, with splitting cracks mainly accompanied by a few shear inclined cracks and shear cracks mainly accompanied by a few splitting cracks. When the shale is dry, a large amount of debris collapse occurs at the front end of the main fracture of the shale rock sample, and more small cracks expand and sprout before the main crack. The energy release is severe during crack development, leading to brittle failure. A sample with a moisture content of 4% will show large deformation before failure, and the final failure form will be typical “X” shear failure, comprising mixed tension and shear failure. The failure forms of cement-saturated shale are viscous failure and shear failure. With the increase in load, large plastic deformation occurs before the shale rock sample reaches failure load, which is closely related to the water content of water-sensitive shale. The presence of water will soften the shale, reduce the severity of the failure, release energy less violently during the failure process, and cause the shale rock to shift from brittleness to plasticity. Then, under the constraint of high confining pressure, a shear zone is formed in the shale rock sample, viscosity arises, and shear failure occurs.

4.3. Effect of Water on the Microstructure of Shale

The water content causes changes in the microstructure of the shale rock. Electron microscope scanning (SEM) of shale rock samples under different water content states was carried out. Microstructural images were obtained, as shown in Figure 7. The micro-fissures and particle structures of the depositional cemented shale change with different water contents. The dry shale rock samples show a natural-layered texture structure, the primary micro-cracks are clearly visible, and the sizes of the layered particles are different, but the interparticle bonding is close, the integrity is strong, the mechanical connectivity is good, and the physical and mechanical properties are better. The microstructure of the shale with a 4% moisture content is obviously different from that in the dry state. At this point, water has penetrated into the micro-cracks and pores of the shale, and the hydrophilic intergranular cement has begun to melt, accompanied by slight expansion. The internal stress increases, and the local tensile stress causes the originally tightly connected particles to gradually relax; the brittleness decreases, the plasticity increases, and some particles enter a free state, thus forming small holes. The bonding force between particles is slightly poor, and the hardness decreases. Shale with a saturated water content fully hydrates the cemented minerals and forms elliptic clumps wrapped around shale particles. The microcracks gradually expand and loosen the shale rock mass. The intergranular connection and friction forces are greatly reduced, while the plasticity reaches an extreme value; the integrity decreases, and the comprehensive mechanical properties decline. This is consistent with the strength variation, softening characteristics, and failure mode reflected in the experimental results.

5. Conclusions

The mechanical properties of water-sensitive shale under high confining pressures and water-rock coupling were here experimentally studied, and the strength variation law of water-sensitive shale during drilling was discussed, providing a scientific basis for addressing the mechanism of wellbore instability in shale and analyzing changes in casing external pressure. The main conclusions are as follows:

(1) Under the same confining pressure, the triaxial compressive strength of shale is closely related to its water content. As the water content increases, the triaxial compressive strength of shale decreases by up to 77.6%, so we can infer that hydration leads to significant deteriorations in the compressive strength of shale. Under the same water content, with the increase in confining pressure, the triaxial compressive strength of rocks increased by up to 22.1%, suggesting the strength of shale significantly increases under high confining pressures. The effect of confining pressure on the strength of saturated samples is more significant than under dry conditions;

(2) The water content has a significant impact on the failure mode of shale. With the increase in water content, the failure mode of shale under triaxial compression load gradually transitions from brittle failure to plastic failure. Under the constraint of high confining pressure, the rock sample generates shear bands, leading to viscosity and shear failure;

(3) The influence of the water–rock interaction on the strength of shale was analyzed from macro and micro perspectives. The main effects of the water–rock interaction on the macroscopic strength of shale are ① water absorption and expansion, ② cementation or disintegration, ③ dissolution/latent corrosion, ④ pore pressure, ⑤ seepage drag force, and ⑥ strength degradation. As a result of the water absorption and expansion of clay mineral particles, the microstructure is destroyed, the clay cementitious material becomes muddy, and the bonding force is reduced. At the same time, the pore water pressure also reduces the effective stress placed on the rock skeleton, and the water–rock interaction changes the internal structure of shale, ultimately leading to the deterioration of its mechanical properties.

(4) Through SEM experiments, it was found that the action of water causes changes in the microstructure of shale, which determines its macroscopic mechanical response and leads to the deterioration of its mechanical properties under external loads.