ABSTRACT
The conductivity of rocks in unconventional oil and gas reservoirs (such as sandstone) directly leads to the production capacity of oil and gas. Carbon dioxide (CO2) can react with some minerals in the sandstone and effectively improve the production of reservoirs. Studying the mechanical properties and fracture behavior of sandstone with different bedding plane under the effect of CO2 are very important for field development. In this paper, specimens were cut from a sandstone with bedding plane, then they were treated by CO2. Brazilian tests were conducted with different loading angles and strain field were monitored with Digital Image Correlation (DIC) system during the process. The study showed that: (1) the loading displacement decreased when loading angles reduce; (2) the tensile strength of the specimens with bedding plane was lower than that of the specimens without bedding. The smaller of the loading angle, the larger of the slope in load-displacement curve was; (3) among the maximum tensile strain with different loading angles, the maximum tensile strain at 60° was the smallest, then the 30° specimen, and the maximum tensile strain of 0° was the largest. This experiment is helpful for understanding the differences of lateral strain field, mechanical properties and fracture behavior of sandstone with different angles under the action of CO2.
In the process of hydraulic fracturing, the tensile strength of rocks is one of the main factors affecting fracture propagation, so it is the focus of research in the process of unconventional oil and gas development. The rock material is a very complex heterogeneous solid medium. Many scholars conducted a large number of experiments on the tensile strength of various types of rocks, and also studied the anisotropic properties of the rocks. Cho (Cho et al., 2012) studied the tensile strength anisotropy of gneiss, shale and schist. Lisjak (Lisjak et al., 2014) performed a Brazilian test on shale to analyze the effect of bedding direction on their tensile strength. Chen (Chen et al., 1998) established a functional relationship between the tensile strength of anisotropic rocks and rock parameters. Yang (Yang et al., 2019) analyzed the shale in the Brazilian test through PFC simulation and found that the tensile and shear fractures of the rock matrix along the bedding plane are main fracture modes of shale disc specimens. Yin (Yin et al., 2018) studied the sandstone at different bedding angles in the Brazilian test and found that as the bedding angle increases, the normal stress on the bedding plane changes from compression to tensile. Liu (Liu et al., 2012) studied the influence of different bedding orientations on the anisotropic tensile strength of slate. Deng (Deng et al., 2018) designed uniaxial and triaxial compression tests at different bedding angles for layered sandstone. The study found that as the bedding angle increases (0 ° to 90 °), the elastic modulus increases, while the deformation modulus, compressive strength, cohesion, and friction angle decrease first then increase. Zhang (Zhang et al., 2018) performed Brazilian test on bedding sandstone specimens, and studied the anisotropic failure mode of sandstone. These studies showed that rock bedding plane has obvious influence on tensile strength and failure mode.
