Gas flow characteristics in shale fractures after supercritical CO2 soaking

Using supercritical CO2 as fracturing medium to develop shale gas not only improve the stimulation effect of reservoir, but also reduce CO2 emissions. To optimise the flow of shale gas into a wellbore, it is important to understand the flow characteristics of gas in the shale fracture system after reservoir reconstruction. To date, no research has been conducted on the effect of supercritical CO2 on the flow characteristics of gas in shale fractures. Hence, in this study, experiments were conducted to investigate the effect of supercritical CO2 on the conductivity of shale fractures, for different effective stress and gas pressure conditions. Also, the changes in shale fracture surface roughness, hydraulic aperture, Reynolds number, and friction factor, before and after soaking with supercritical CO2 have been studied; the results show an increase in shale fracture conductivity for low effective stress conditions, and a decrease in conductivity for high effective stress conditions. Further, the relationship between the hydraulic aperture of shale fractures and the effective stress is found to be hyperbolic. The changes in the conductivity of shale fractures due to the influence of supercritical CO2 are primarily because of an increase in shale fracture surface roughness and a decrease in the resistance to deformation of the fractures. Nonlinear gas flow behaviour in shale fractures has been observed, which can be described by the Forchheimer equation. A new friction factor model based on Reynolds number and relative roughness is proposed, that can more accurately reflect the flow pattern of gas in shale fractures than the classical Lomize model.