Development patterns of structural fractures in reservoirs under different stress states: An experimental investigation of true triaxial mechanics

Under the influence of multiple periods of tectonic movement, different types of structural fractures have widely developed in basins. These fractures effectively improve the quality of low-permeability reservoirs and are key to geothermal, oil, and gas resource utilization; carbon dioxide sequestration; and gas storage. Most oil and gas reservoirs in the world experience different stress fields, such as compression, strike-slip and transition stress fields, resulting in different fracture development modes in reservoirs with different burial depths. Traditionally, tensile fractures form mostly near surfaces or under high pore pressure conditions. However, the genetic mechanism of tensile fractures in deep reservoirs is unclear. To address these problems, by collecting tight sandstone samples from the field, 13 groups of 100*100*100 mm cubic rock samples were prepared, and true triaxial rock mechanics experiments were conducted to reveal the fracture development patterns under different stress states and confining pressures. The experimental results revealed that under in situ stress, the rock elastic parameters increased as the intermediate principal stress ( σ ₂ ) increased, and σ ₂ had a positive effect on the ability of a sample to resist deformation. With increasing horizontal minimum principal stress ( σ _hmin ), the Young’s modulus and compressive strength increased, whereas σ _hmin had little effect on the residual stress and Poisson’s ratio. Mechanics experiments based on different stress states revealed the geomechanical mechanism underlying the differential development of structural fractures in tight sandstones in the Tarim Basin and Ordos Basin. The transformation of the stress state was found to be the main genetic mechanism for the complexity of the occurrence of fractures in the Tarim Basin. Under strong compression, i.e., early tectonic movement, tensile fractures perpendicular to the orientation of the horizontal maximum principal stress ( σ _Hmax ) developed, and the larger the difference between the σ _hmin and vertical stress ( σ _v ) was, the greater the development of tensile fractures. These results increase the understanding of the formation mechanisms of tensile fractures. Models of fracture development under different stress states were established, confirming that tensile fractures could develop in deep and strong compression environments. The research results have reference value for fracture cause analysis, carbon capture and sequestration, gas storage construction, and deep oil and gas exploration and development.