Experimental evaluation of gas flow characteristics in fractured siltstone under various reservoir and injection conditions: an application to CO2-based fracturing

CO2 fracturing can perform better in enhancing the unconventional gas production than water-based fracturing by avoiding serious reservoir permeability damage. The evolution of the fracture permeability of created and natural fractures during exposure to different fracturing fluids is critical to the effectiveness of CO2 fracturing. Therefore, a comprehensive steady-state permeability tests by using various fracturing fluids in fractured siltstone under tri-axial conditions was conducted. According to the experimental results, the decrease of the uniaxial compressive strength of tested samples after water saturation was greater than 23% due to the increase of pore pressure. The flow capacity of the fracture system can suffer a great loss by the irreversible deformation of fracture surfaces caused by the high confinements and water softening effects. The water–rock reactions can accelerate the reductions of fracture hydraulic aperture at confining pressures higher than 20 MPa. The fracture gas conductivity decreased by 80–90% at fully or partially water-saturated rock formations due to the water softening and capillary effects.