Experimental Study Of Triaxial Compression By CO₂ Migration On Shale

The technology of supercritical CO2 fracturing and displacement for shale gas has been proved to be an effective mining method.However,the law of the seepage mechanism and seepage-stress coupling for shale under supercritical C02 has not been clearly understood.In this paper,the Silurian dragon Maxi formation and the Ordovician five peak shales in the Changning Weiyuan shale gas field are selected as the research object.Based on the TRXC100-200 three-axis test machine of the Earth Science System Laboratory of Chinese University Hong Kong,the three axis compression and seepage tests of shale under the action of CO2 are carried out to explore the seepage stress coupling cooperation of the shale under this condition.The effects of mineral composition and microstructure on the mechanical properties of shale and the coupling of seepage in shale were analyzed with the results of X ray diffraction(XRD)and scanning electron microscopy(SEM).The main works and finding sare as following:(1)The XRD test,scanning electron microscopy,porosity,and permeability tests of the initial state of shale outcrops in the target block were carried out to evaluate the effects of shale microstructure and organic matter content on the pore and seepage;(2)The permeability effective stress experiment was conducted using transient-pressure pulses for shale.Response surface method was used to process the experimental data,and the variation law between shale effective stress and permeability was analyzed,and the shale stress sensitivity was evaluated;(3)After saturated soaking of shale with CO2 and N2 fluids and three phases,a full stress-strain test and continuous compression in the post-failure zone were conducted.The mechanical properties of shale were compared between different fluid media and different fluid phase states.The characteristics of the impact.On this basis,the shale failure mode was analyzed from the perspective of bedding and fracture mechanics;(4)The surface micro-morphology and mineral composition of shale after coupling were tested.The micro-pore structure and macro-mechanical properties of shale before and after the coupling of shale and C02 were investigated.The microscopic mechanism of supercritical C02 in shale seepage was analyzed.Fluid flow in the process of seepage and the influence of fluid-solid coupling;(5)According to the flow characteristics of different fluids in shale reservoirs,based on the existing gas flow model,the flow type and state of CO2 in shale pores are optimized through parameter optimization;(6)Based on the content equation and flow model of CO2 in shale,the retained amount and flow type in shale are obtained,and it is concluded that the supercritical flow is a slip flow and transition flow during the experimental process;This study analyses the influence of effective stress on shale permeability,the mechanics and seepage parameters in the process of CO2 fracturing shale were obtained,and the seepage-stress coupling mechanism of shale under CO2 was explored.Development provides certain experimental parameters.