Quantitative Investigation on the Fracture Evolution of Shale at High Temperatures up to 500°C via In-situ X-Ray Computed Tomography

The formation of fracture networks plays a critical role in enhancing shale reservoir permeability and improving the efficiency of in-situ conversion processes. However, the mechanisms governing the formation and evolution of complex fracture networks under high-temperature conditions remain poorly understood. In this study, high-temperature in-situ X-ray computed tomography (CT) was employed to investigate the fracture evolution behavior of shale samples with different bedding orientations under stepwise heating. Based on three-dimensional reconstructions, fracture structures at successive temperature stages were extracted, and key structural parameters were quantitatively analyzed to systematically characterize fracture network evolution. X-ray diffraction (XRD) and scanning electron microscopy (SEM) were integrated to examine fracture evolution from the perspectives of mineralogical changes and microstructural development. The results demonstrate that shale fracture evolution is governed by coupled thermal–mechanical–chemical processes. With increasing temperature, the dominant mechanism transitions from physically induced deformation to organic matter pyrolysis. Quantitative tracking of individual fractures shows that fracture network development is initially dominated by pre-existing natural fractures. At elevated temperatures, thermally induced fractures increase markedly in number and density and progressively connect with natural fractures, leading to the formation of a highly connected fracture network. This cooperative evolution fundamentally enhances shale reservoir permeability and underpins the effectiveness of in-situ conversion processes. These findings can guide in-situ heating operations, helping to optimize heating rates and fracture layout to improve fracture network control and overall process efficiency.