Micro Experiment Research On The Effect Of Supercritical Carbon Dioxide On The Physical Properties Of Shale

Shale oil and gas efficient development has a significant strategic significance for protecting national energy security.But due to its characteristics such as low porosity,low permeability and high clay minerals,conventional hydraulic fracturing results in problems such as clay swelling and low recovery rate.Supercritical carbon dioxide(SC-CO2),as non-aqueous fracturing fluid,has gradually gained attention.It is usually injected into the reservoir as pre-fluid to acidified reservoir and enhances the fracturing effect to increase the oil and gas recovery.The shale organic matter(OM)has strong adsorption capacity of SC-CO2,and the physical properties of OM affect the way shales deform and fracture.However,little is known about the physical properties of OM in fine-grained rocks,mainly because it often occupies tiny nanometer-scale voids between the mineral grains,which cannot be accessed using conventional macro testing techniques.Therefore,characterize the effect of SC-CO2 immersion on shale OM and reveal the SC-CO2 interaction mechanism are of great significance to the SC-CO2 fracturing plan.This paper along with a key scientific issue of interaction mechanism of microstructure and mechanical properties of SC-CO2 immersion on shale components.The shale of Longmaxi formation in Sichuan is selected as the research object.The shale is exposed in SC-CO2 environment of 10 MPa and 40 ℃.The effects of SC-CO2 on the mineral composition content and pore structure of shale are investigated by X-ray diffraction(XRD),scanning electron microscopy(SEM)and energy dispersive(EDS)experiments.The effects of SC-CO2 on the mechanical properties of shale components are studied from the micro-nano using atomic force microscopy(AFM)and nanoindentation tests.Finally,through the upscaling model of mechanical parameters,the micro-mechanical parameters are upscaled to the macro.The research results are as follows:(1)XRD and EDS measurement results show that after SC-CO2 exposure,quartz content increased significantly while carbonate and clay content decreased by5.6 %,4.7 % and 2.9 %,respectively.In the shale target area,C and Al are basically unchanged.O,Si,S,Mg and Fe are unnoticeable.While Ca is decreased significantly by 30 %;(2)Experimental analysis of SEM,AFM and nanoindentation show that after SC-CO2 immersion,OM and clay show a swelling trend while calcite becomes rougher.The average elastic modulus of OM,clay and quartz increase and the elastic modulus of kerogen increases by 124.39%.Calcite and pyrite are decreased by 5.88 % and 7.40 %,respectively.All areas are increased by 4.90 % and the positive correlation between elastic modulus and hardness becomes stronger;(3)Among the four shale mechanical properties homogenization algorithms,the dilute method performed the best,followed by Mori-Tanaka,and Reuss and Voigt performed the worst.This paper studies the interaction mechanism between SC-CO2 and shale components from a microscopic perspective and provides theoretical guidance for the SC-CO2 fracturing and carbon capture,utilization and storage(CCUS)in oil fields.