Evolution Law Of Mechanical Properties And Microstructure Characteristics Of Shale Under Temperature

With the increasing demand for mineral resources and increasing crisis of global energy supply,shale oil-gas,as unconventional oil-gas resources,have become an important new type of oil-gas reservoir,and their development and utilization have attracted more and more attention.The pore structure of shale is dominated by nano and micro pores.The complexity of micro pore structure determines its very low porosity.The micro pore structure of shale controls the occurrence state,migration mode,flow mechanism and enrichment law of oil-gas,and also the change characteristics of its macro mechanical properties.Therefore,it is of great significance to study the mechanical properties and pore structure characteristics of shale under high temperature for solving the problem of wellbore stability in deep drilling engineering and developing unconventional reservoirs.In this paper,taking the deep shale of Well SK-2 as the research object,standard shale samples are selected to study the mechanical properties and micro pore structure change characteristics of shale under high temperature,to explores the influence of temperature on the pore structure of shale,and to control to analyze the influence and control of micro pore structures on the macro fractures of shale.Firstly,the change laws of shale compressive strength,tensile strength,elastic modulus and Poisson's ratio are studied by using electronic material universal testing machine and ultrasonic velocimeter;secondly,the micro pore structure characteristics of shale are studied by using field emission scanning electron microscopy(SEM)and high pressure mercury injection,to analyze the pore size distribution laws of shale;Finally,based on the CT scanning technology,the 3D reconstruction technology is used to build the accurate and real 3D models of the rock,pore network structure models of the rock was build,the porosity of the rock was calculated,and the pore distribution characteristics of the rock was analyzed.The results show that the tensile strength and uniaxial compressive strength of shale samples decrease with the increase of temperature,the elastic modulus has a negative correlation with temperature,but Poisson's ratio has no obvious change,and the brittleness coefficient increases with the increase of uniaxial compressive strength.There are mainly intergranular pores,intragranular pores and microfractures in shale,but organic pores are not developed.There are a large number of micro and nano fractures between flaky clay minerals,mainly distributed between 20 nm and 400 nm,which provide the main space for shale oil-gas adsorption and play a major role in shale oil and gas storage and migration.The effect of temperature on the micro pore structure of shale is obvious,and the degree of impact increases with the increase of temperature.The porosity of shales increases with the increase of temperature,and the proportion of pores larger than 10 um decreases after heating.The pore size is mainly distributed between 100-900 nm,and the proportion of micropores in this range is more than 70%.Most of them are connected pores,and slit shaped.The porosity larger than 50 um decreases,and the connectivity of pores is general.Finally,the 3D pore models of shale are reconstructed by using CT and computer software,the pore structures of rock are observed and analyzed intuitively and stereoscopically.The porosity of rock is calculated by combining with the indoor tests.