A multi-physics coupled multi-scale transport model for CO2 sequestration and enhanced recovery in shale formation with fractal fracture networks

CO2 sequestration and enhanced gas recovery (CS-EGR) of shale formation involve complex multi-physics couplings, which are still not precisely captured by recent models. In this regard, a modified model fully considering the fractal characterization of fracture networks and multi-physics coupled transport behaviors in multi-scale shale formation is established. The fractal fracture networks are depicted based on the L-system theory with microseismic events (MSE), and embedded into the numerical modeling by an automatic fitting algorithm. As a development from the traditional transport model, the formulas among the pressure field, thermal field, and the molecular diffusion of binary gases are constructed based on the Chapman-Enskog theory. The effect of tortuosity and surface diffusion on gas transport efficiency can reach up to 5 %. The morphology parameters of induced fractures affect gas migration mainly by changing the size of SRV and the space between adjacent induced fractures, with their influence on CH4 production potentially as high as 20 %. Meanwhile, there are improved CH4 production, energy efficiency, and CO2 storage amounts, when the injection parameters are optimized. By optimizing the injection conditions (temperature, pressure), CH4 production can be increased by about 8 %.