Thermal pyrolysis-induced shale micro-cracks: 3D characterization and implication for reservoir stimulation

Abstract

The geometrical characteristics of thermal cracks at the micro-scale contain valuable information for subsurface enhancing the production of shale oil and gas via in-situ pyrolysis. In this study, six organic-rich shale samples from the Junggar Basin, western China, were thermally pyrolyzed to 250–500 °C, respectively, which were then imaged using X-ray micro-CT. A novel image-processing technique was employed to separate and identify individual cracks that intersect and form 3D networks. Unprecedentedly, over thousands of cracks in each of the volumes were individually characterized by their length, width, thickness, and orientation and then statistically analyzed. It is shown that crack orientations are predominantly parallel to the bedding structures of the shale, different crack networks of the samples represent the heterogeneity of the shale, and the fractal dimension of the crack length ranges from 1.50 to 1.85. All parameters, including porosity, specific surface area, percolation, and the fractal dimension of crack length, show a distinct increase from 350 °C, which is consistent with the threshold temperature for gas generation during thermal pyrolysis. The analysis quantifies the efficacy of pyrolytic stimulation methods and paves the way for developing pyrolysis stimulations for producing shale oil and gas.