Experimental Study on the Difference of Shale Mechanical Properties

Abstract

This paper studies the anisotropic characteristics of shale and the difference in mechanical performance between deep shale and outcrop shale. The outcrop shale was collected from the Shuanghe section in Changning County, southern Sichuan, and the deep shale was collected from the Wells Yi201 and Lu202. Study their basic mechanical parameters, failure modes, and wave velocity responses through laboratory tests. Research shows that with the increase of bedding angle, the deformation mode has the trend from elastic deformation to plastic deformation in high-stress state. When the bedding angles are 0°, 30°, and 45°, the weak bedding surface plays a leading role in the formation of the failure surface trend. As the bedding angle increases to 60° and 90°, its influence is weakened. The tensile strength, elastic modulus, and wave velocity decrease with the increase of bedding angle. The compressive strength and Poisson’s ratio have the law of U-type change, there are higher values at 0° and 90°, and the lowest values are at 30°. The brittleness index first increases and then decreases with the increase of the bedding angle. The tensile strength and Poisson’s ratio of outcrop shale and deep shale are close, but the compressive strength of deep shale is only 1/3 of outcrop shale, the elastic modulus is only 3/4 of outcrop shale, and the failure of deep shale is accompanied by instability failure.

Experimental Study on Hydraulic Fracturing of Bedding Shale Considering Anisotropy Effects

“…According to the author’s previous research, the least-square fitting is made for the tensile strength of the outcrop shale sample in the 0–90° splitting direction of the Longmaxi formation, as shown in Figure 12 . 42 The tensile strength of the shale as a function of the splitting angle α can be obtained as follows: From eq 8 , the first-order coefficient of the function is low, and the function value is less affected by the first-order term; therefore, the first-order term is omitted. According to the symmetry, the tensile strength eq 9 of the shale under different splitting angles can be obtained.…”

“…Before the hydraulic fracturing experiment, the basic mechanical parameters of the shale were measured. The specific parameters are given in the author’s previous research …”

“…From eq , it can be found that σ 3 is most likely the maximum tensile principal stress, that is, the maximum tensile stress at a point on the wellbore wall is a function of θ. The sedimentary characteristics lead to the development of the weak bedding plane of the shale, and the tensile strength along the bedding plane at different angles (different splitting angles) shows anisotropic characteristics. According to the author’s previous research, the least-square fitting is made for the tensile strength of the outcrop shale sample in the 0–90° splitting direction of the Longmaxi formation, as shown in Figure . The tensile strength of the shale as a function of the splitting angle α can be obtained as follows: From eq , the first-order coefficient of the function is low, and the function value is less affected by the first-order term; therefore, the first-order term is omitted.…”

“…In addition, most of the above studies were based on experimental and theoretical analyses of isotropic shale. In practice, the bedding plane formed by sedimentation makes shale anisotropic . Li concluded that the crack damage stress, crack initiation strain, and crack damage strain strongly depend on the bedding angle.…”

“…In practice, the bedding plane formed by sedimentation makes shale anisotropic. 42 Li 43 concluded that the crack damage stress, crack initiation strain, and crack damage strain strongly depend on the bedding angle. Through a Brazilian splitting test and a three-point bending test, Jin et al 44 discussed the anisotropy in the fracture behavior of shale with respect to the bedding angle.…”