Compression characteristic distribution and anisotropic evolution of shale fracture with shale self-supporting particles

The in-situ methane deflagration fracturing technique utilizes the collaborative combustion and explosion of in-situ desorbed methane and oxygen to produce high-temperature and high-pressure gases that effectively fracture shale reservoirs. During the fracturing process, some shale debris particles are generated around the fracture, providing a degree of self-support to the fracture channels. Among these, the understanding on the compressive closure behavior and fracture aperture evolution of shale fractures with shale debris particles is a critical research focus. This study focused on four types of specimens: Parallel Bedding Unsupported (PBU), Parallel Bedding Self-Supported (PBS), Vertical Bedding Unsupported (VBU), and Vertical Bedding Self-Supported (VBS). Employing a non-contact full-field strain measurement device, the authors investigated the influence of self-supporting particles and bedding on the distribution of compression behavior and anisotropic characteristics for shale fracture during stress loading. The results revealed that all specimens can be divided into four typical regions along the loading direction: the fracture zone, near-matrix zone, far-matrix zone, and no-impact zone. In each region, the vertical displacement exhibits a linear decreasing trend along the loading direction, with decreasing rates in sequential order and approaching zero in the no-impact zone. Self-supporting particles mitigate the influence of bedding orientation on the distribution of fracture compression characteristics along the fracture direction. The compressible and incompressible components of self-supported fractures are 47−56 times and 11 times that of unsupported fractures, respectively. Meanwhile, the proportion of compressible component of fracture aperture in self-supported fractures is 4−5 times greater than that in unsupported fractures. The Bedding Anisotropy Index (I_BA) was introduced to quantitatively analyze the impact of self-supporting particles on the anisotropic evolution characteristics of shale fractures. During the stress loading process, all specimens sequentially experienced four phases: the I_BA descent zone, I_BA steep increase zone, I_BA oscillation zone, and I_BA stabilization zone. Self-supporting particles reduced the extent of the I_BA oscillation zone to (1−5 MPa) and increased the I_BA stabilization value to 5.6, eliminating the differences in I_BA stabilization values among different bedding orientations.