Experimental Research Of Effect Of Supercritical Carbon Dioxide On Mechanical Characteristics Of Shale

Shale oil and gas is an unconventional reservoir with great development potential.Due to characteristics of low porosity,low permeability and high clay,the way of conventional drilling and development is prone to clay swelling and low recovery.Supercritical carbon dioxide（SC-CO₂）,free of water and solids,has proven to be feasible for oil drilling and fracturing stimulation.In the process of SC-CO₂exploitation of shale oil and gas reservoir,borehole stability,fracturing stimulation and CO₂ geological storage are the key research issues.When SC-CO₂ comes into contact with shale,shale mechanical properties changes will have an impact on borehole stability of SC-CO₂ drilling,crack growth in fracturing engineering and geological storage evaluation of CO2,etc.,and then affect the development of shale oil and gas reservoir.It is of great significance to study the interaction between SC-CO₂ and shale for the development of shale oil and gas with SC-CO₂.According to the anisotropic characteristics of shale mechanical parameters,the shale of different bedding angles was drilled by adjusting the core angle,and then processed into standard cylindrical specimens.The compressive test was carried out in a self-made SC-CO₂immersion test device to study the effect of bedding plane dig angle（α）on the mechanical properties of shale in CO₂ immersion environment,and a comparative experiment in helium immersion was conducted.The shale was immersed in SC-CO₂ environment with different conditions to investigate the effect of SC-CO₂ on the tensile strength of shale.Nano-indentation technology was used to study the effect of SC-CO₂ on the mechanical properties of shale on a micro scale.Combined with electron microscopy（SEM）,MLA mineral analysis,X-ray diffraction（XRD）and low-pressure nitrogen adsorption experiment about shale matrix and bedding plane,the effects of SC-CO₂ on the pore structure and chemical properties of shale were studied.And the mechanism of SC-CO₂ on the mechanical properties of shale was explored.The results show:in the compression experiment of shale,the compressive strength decreases first and then increases with the increase of bedding angle.The compressive strength after SC-CO₂ immersion decreases by 11 MPa～15 MPa.Elastic modulus of unsatutated shale increases with the increase of bedding plane dig angle,whenα=0°,the average value is about26.2 GPa,and whenα=90°,the average value is about 35.2 GPa;elastic modulus increases in SC-CO₂ immersion environment,and elastic modulus decreases with the increase of the bedding dig angle.Whenα=0°,the average value is 80.7 GPa,and whenα=90°,the average value is 48.8 GPa.With the increase of bedding inclination,poisson’s ratio shows an upward trend,and poisson’s ratio increases in soaking environment.The average Poisson’s ratio of unsoakedα=0°is 0.18,the average Poisson’s ratio ofα=90°is 0.24;the average Poisson’s ratio ofα=0°after soaking is 0.24,Poisson of ratio ofα=90°is 0.32.In the helium immersion environment,the shale mechanical parameters value are between that of unsoaked and SC-CO₂soaking.By comparing the shale failure pattern,it is found that SC-CO₂ deteriorates the layered surface and changes the shale failure morphology.The Brazilian splitting experiment shows that the tensile strength of shale decreases with increase of SC-CO₂ immersion pressure,temperature and time.The shale minerals withα=0°are disorderly arranged and the pores are more developed.The shale minerals withα=90°are arranged neatly and the pores are less developed.The results show that the anisotropy of shale elastic parameters is determined by the fabric.After SC-CO₂ immersion,some minerals in shale were dissolved.The results of MLA and XRD showed that the content of quartz increased and the content of clay mineral decreased.After soaking,the pore volume of shale bedding plane increased by 19.4%,and the specific surface area decreased by 46.4%.The pore volume of matrix increased by 8.1%and the specific surface area decreased by 12.2%.Nanoindentation results show that SC-CO₂ soaking reduces shale hardness,elastic modulus and fracture toughness.Combined with all experiments,it can be proved that the high permeability characteristics of bedding plane,SC-CO₂ pressure,adsorption and dissolution are the reasons for SC-CO₂ to change the shale mechanical parameters.This article provides data guidance and theoretical basis for wellbore stability of SC-CO₂ drilling,fracturing operations with SC-CO₂.