Simulation study of supercritical carbon dioxide jet fracturing for carbonate geothermal reservoir based on fluid-thermo-mechanical coupling model

Geothermal energy is a kind of renewable,sustainable and clean energy resource.Geothermal energy is abundant in carbonate reservoirs.However,low matrix permeability limits its exploitation.The super-critical carbon dioxide(SC-CO_(2))jet fracturing is expected to efficiently stimulate the carbonate geothermal reservoirs and achieve the storage of CO_(2) simultaneously.In this paper,we established a transient seepage and fluid-thermo-mechanical coupled model to analyze the impact performance of sc-CO_(2) jet fracturing.The mesh-based parallel code coupling interface was employed to couple the fluid and solid domains by exchanging the data through the mesh interface.The physical properties change of sC-CO_(2) with temperature were considered in the numerical model.Results showed that SC-CO_(2) jet frac-turing is superior to water-jet fracturing with respect to jetting velocity,particle trajectory and pene-trability.Besides,stress distribution on the carbonate rock showed that the tensile and shear failure would more easily occur by SC-CO_(2) jet than that by water jet.Moreover,pressure and temperature control the jet field and seepage field of sC-CO_(2) simultaneously.Increasing the jet temperature can effectively enhance the impingement effect and seepage process by decreasing the viscosity and density of SC-CO_(2).The key findings are expected to provide a theoretical basis and design reference for applying SC-CO_(2) jet fracturing in carbonate geothermal reservoirs.

Rock mechanical properties of coal in cryogenic condition

Liquid nitrogen(LN2)fracturing is a kind of non-aqueous fracturing technology,which is expected to provide a new and efficient way for coalbed methane(CBM)development.The mechanical properties of coal under LN_(2) freezing are very important for studying the mechanism of LN2 fracturing.However,most of the current research is limited to studying mechanical properties of rocks after being frozen by LN2 and returned to room temperature.In this paper,the effect of LN2 freezing on the mechanical properties of coal was studied.Uniaxial strength tests and Brazil tests were carried out for dry and water-saturated coal samples with different types and bedding directions.In addition,standard electron microscopy(standard SEM)and cryo-electron microscopy(Cryo-SEM)were used to compare the fracture morphology of coal samples at room temperature and LN_(2) temperature.The results showed that LN_(2) freezing can damage and improve the mechanical properties of coal simultaneously.The strength of saturated coal under freezing is higher than that of dry coal,and the filling of ice can enhance the mechanical strength of coal.In addition,the mechanical properties of coal with higher porosity are enhanced more than that of coal with lower porosity under LN_(2) freezing.The main findings of this study are the keys to the research of LN_(2) fracturing mechanisms in CBM reservoirs.