The Influence Of Supercritical CO₂ On The Conductivity Of Shale Fractures

China has made progress in shale gas exploration and development.However,the technology of hydraulic fracturing cannot meet the long-term demand for shale gas exploitation due to the particularity of domestic shale reservoirs,which makes it valuable for domestic shale gas development to fracture shale reservoirs with supercritical CO₂instead of water.The surface structure,hydraulic aperture and gas nonlinear flow characteristics of shale fractures change with the injection of CO₂,which will affect the conductivity of shale fractures,thereby changing shale gas production.Therefore,it is of great significance to study the change of conductivity of shale fractures after supercritical CO₂ saturation for evaluating the effect of reservoir reconstruction and accurately predicting the production of shale gas wells.In this paper,the Longmaxi Formation shales in the southeastern Sichuan Basin of China were chosen as samples,artificial tensile fractures were used as the research object,and a self-designed high-temperature and high-pressure shale saturation device was employed to saturate shale fractures with supercritical CO₂.The microstructure,mineral contents and roughness characteristics of shale fractures were characterized by scanning electron microscopes(SEM),X-ray diffraction analysis(XRD)and three-dimensional morphology scanning.In addition,the hydraulic aperture and gas nonlinear flow characteristics of shale fractures were studied by performing conductivity experiments under different effective stresses and gas pressure gradients with a coal-rock triaxle rheological experiment equipment.The change law of surface structure,hydraulic aperture and gas nonlinear flow characteristics of shale fractures after supercritical CO₂saturation were comprehensively discussed,and the influence mechanism of supercritical CO₂ on conductivity of shale fractures was fully revealed.The main research results are as follows:(1)The effect of supercritical CO₂ on the surface structure of shale fractures was analyzed.For shale fractures with supercritical CO₂ saturation,the microstructure was destroyed,the surface became rougher and the initial average aperture increased.At the micro-scale,new dissolution pores or collapses were seen to appear on the surface of shale fractures,and the degree of cementation between mineral particles was weakened.The contents of carbonate minerals such as calcite and dolomite decreased,while the content of other minerals increased.At the meso-scale,the geometric and statistical parameters and the joint roughness coefficient(JRC)of the surface of shale fractures increased,and the greater the JRC before supercritical CO₂ saturation,the greater the range of change.At the same time,the initial average aperture increased and the distribution was more dispersed.(2)The evolution characteristic of the hydraulic aperture of shale fractures after supercritical CO₂ saturation was clarified,and based on the fracture roughness parameters and effective stress,a new hydraulic aperture model was proposed.Supercritical CO₂ did not change the behavior of gas nonlinear flow in shale fractures,but it caused the curve of hydraulic aperture and effective stress to cross at the critical stress.Analysis showed that the critical stress was proportional to the decrease rate of the initial strength to resist deformation and inversely proportional to the increase rate of the initial hydraulic aperture.It was also found that there was a linear positive correlation between the initial hydraulic aperture and the roughness coefficient JRC of shale fractures.Based on the hyperbolic function relationship,a more comprehensive hydraulic aperture model was established,which fitted the experimental results well.(3)The influence mechanism of supercritical CO₂ on the gas nonlinear flow of shale fractures was revealed,and the flow friction resistance model was proposed based on the relative roughness(R_r)and Reynolds number(Re).When the flow velocity was low,the gas flow in shale fractures was mainly linear laminar flow,and with the increase of flow velocity,nonlinear flow appeared.The critical Reynolds number for gas flow regimes to change was 5-25 and the value became larger after supercritical CO₂ saturation,which means the decrease of the degree of nonlinearity.In the process of gas flow regimes changing from laminar flow to transitional flow and turbulent flow,the influencing factors transformed from Re-dominant to Re&R_r jointly impact to R_r-dominant.It was thought that the new flow friction resistance model had a good applicability to explain the gas nonlinear flow of shale fractures after supercritical CO₂ saturation.