Continuum-discontinuum analysis of permeability enhancement of shale with Supercritical CO2 fracturing

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The shale gas development in China and Germany is expected to have great potential. However, conventional water-based fracturing treatments may not work well for many shale gas reservoirs. The supercritical CO2 (SC-CO2) fracturing technology can overcome a series of shortcomings of hydraulic fracturing and has great industrial prospects. In this study, first of all, CT scanning of shale specimen is employed to characterize the meso-structure. Supercritical CO2 fracturing tests on cylindrical shale samples are conducted to investigate the effect of pre-existing crack on fracturing process. Secondly, based on the equation-of-state of SC-CO2 and the coupling effect among fluid flow and shale deformation and damage, the coupled flow-solid model for SC-CO2 fracturing process is established. Thirdly, we form a new continuum-discontinuum model for the whole fracturing process by combining the continuum mechanics based fluid-solid model developed by the Chinese team with the discontinuum mechanics based model developed by the German team. The new model is validated against experimental results. Finally, based on a practical engineering background, the mechanical model for multi-stage fracturing in horizontal wells is built, and the effects of the distribution density of natural fractures, degree of natural fracture development, mechanical parameters of fracture surface, and fracturing fluid injection rate on stimulated fractures and resultant permeability enhancement are analyzed numerically. The analysis of multiple-fractures interactions and the volumetric permeability enhancement induced by SC-CO2 fracturing may provide theoretical support for the design of fracturing process.