Micromechanical characteristics and controlling mechanism of deep shale: a case study of well JYA in Pingqiao block, Fuling area

The deep shale gas in the Fuling area is characterized by complex structures, high crustal stress, significant stress differences, high formation temperatures, high compaction levels, low porosity, low permeability, and complex porosity-permeability variation patterns. One reason for the significant production differences between wells is the insufficient understanding of the geomechanical features and controlling mechanisms of deep shale gas reservoirs, and the inaccurate identification of sweet spots for hydraulic fracturing. This study focuses on the marine shale of the Wufeng-Longmaxi formations in the Fuling area, investigating the micromechanical characteristics and controlling mechanisms of deep shale gas reservoirs through five series of experiments: micro rock mechanics experiments, digital speckle experiments, X-ray diffraction, total organic carbon content measurement, and scanning electron microscopy (SEM). Coupled with digital image processing technology, the changes in stress field and displacement field, and microcrack propagation processes in the Longmaxi shale under loading conditions were meticulously depicted. The deformation and fracture characteristics of the Longmaxi shale were analyzed. Experimental results indicated that the total organic carbon content in the deep shale is approximately 4.2%, with quartz content at 55.4% and clay mineral at 26.9%. The study identified five stages of microdamage evolution in deep shale: compaction, elasticity, uniform crack propagation, crack propagation failure, and brittle failure. Under the influence of brittle minerals such as quartz, and soft components such as organic matter, the microcracks in deep shale exhibit various propagation modes, including transgranular, intergranular, and laminated layer cracks. Additionally, the fracture toughness indices of the deep shale samples were calculated, with Mode Ⅰ index being 8.279 $\mathrm{MPa} \cdot \sqrt{\mathrm{m}}$ and Mode Ⅱ index being 1.243 $\mathrm{MPa} \cdot \sqrt{\mathrm{m}}$. These experimentally obtained fracture toughness indices can be applied to evaluate the brittleness of deep shale, providing guidance for deep shale fracturing modification.