TRIAXIAL STRESS DYNAMICS DURING HYDRAULIC FRACTURING AND THE CORRELATION WITH FRACTURE DISTRIBUTION

The fluid pressure within the shale formation can trigger the initiation and propagation of fractures during the fluid injection process of hydraulic fracturing in reservoirs. This fracturing process can induce mechanical responses in the shale and disturb the triaxial stress state. The resulting triaxial stress dynamics may lead to fault failure and seismic activities. Given the critical role of in-situ stress in hydraulic fracturing operations,this study employs cubic shale specimens with dimensions of 300 mm and conducts hydraulic fracturing experiments using a true triaxial rock mechanics testing system. The experimental procedure involves gradually loading the specimen to the target triaxial stress state,followed by water injection through a borehole drilled at the center of the specimen. The triaxial stress dynamics and corresponding deformations along the principal stress were comprehensively analyzed in conjunction with the fluid pressure. Furthermore,computer tomography(CT)scans were performed on the post-fracturing shale samples to obtain the configuration and volume distribution of fractures generated by hydraulic fracturing. The research results indicate that during hydraulic fracturing,fracture initiation and propagation induce deformation in the surrounding rock and create stress perturbations along the triaxial principal stress directions. The most significant deformation occurs in the direction of the minimum horizontal principal stress,manifesting as expansion,while the deformation in the vertical principal stress direction is minimal,showing contraction. The deformation of specimens 1 to 3 along the minimum horizontal stress direction was 3.81 mm,4.25 mm,and 5.71 mm,respectively,showing an increasing trend consistent with the rise in applied triaxial stress. The triaxial principal stress perturbations induced by hydraulic fracturing generally exhibited a decreasing trend with increasing triaxial principal stress levels. The perturbation range of the vertical stress was 0.090~0.112 MPa,that of the maximum horizontal principal stress was 0.355~0.951 MPa,and that of the minimum horizontal principal stress was 0.941~1.236 MPa. The perturbation of the minimum horizontal principal stress was the most pronounced and was consistent with the deformation characteristics observed along the triaxial principal stress directions. As the triaxial stress increases,the number of hydraulically induced fractures decreases,while reactivated natural fractures dominate. The fracture volume distribution exhibits two distinct characteristics: fractures with small apertures but large volumes,and fractures with large apertures but small volumes. Under stress conditions,hydraulic fracturing produces fewer but larger-aperture fractures. However,the increase in fracture aperture does not lead to an increase in fracture volume; the fracture volume decreases from 276,300 mm³ to 199,300 mm³. An increase in in-situ stress actually reduces the fracture volume.