Evolution Of Anisotropic Thermophysical,Hydraulic,Mechanical Characteristics Under High Temperature And Its Application

Oil shale,as a sedimentary rock,present strong anisotropy in thermal-physical,hydraulic and mechanical properties due to its orderly arrangement of clay minerals and natural deposition structure.In the process of in situ thermal recovery of oil shale,the physico-mechanical anisotropy of oil shale changes with temperature,which will affect the temperature conduction law,seepage law,contaminations migration law and wellbore stability of in-situ thermal recovery of oil shale.Therefore,it is of great significance to reveal the evolution of the thermophysical,hydraulic,mechanical and deformable anisotropy of oil shale under real-time high temperature.The evolution law of anisotropic thermophysical,hydraulic and mechanical characteristics of oil shale under high temperature are studied by laboratory experiments.A Thermal-hydraulic-mechanical?THM?coupling mathematical model considering the anisotropy of oil shale was established.Combined with the anisotropic thermal,hydraulic and mechanical parameters of oil shale under high temperature obtained in laboratory experiment,the numerical simulation of in situ superheated steam injection pyrolysis oil shale process was carried out by COMSOL software.Then,the distribution of temperature field,seepage field,stress field and deformation field of oil shale reservoir during steam injection process and the law of oil-gas production were analyzed.The specific research contents and main conclusions are as follows:?1?Through NETZSCH LFA 457 laser thermal conductivity analyzer,SYC-2 ultrasonic rock parameter tester and CT,the anisotropy of thermal conductivity,wave velocity and crack propagation of oil shale under temperature were studied respectively.The results are as follows:the thermal conductivity and wave velocity in the direction of perpendicular to bedding direction linearly from temperature to 350?,and decrease rapidly from 350?to 500?and decrease obviously at 400?,and gradually stabilize from 500?to 600?.The thermal conductivity and wave velocity in the direction of parallel to bedding plane decrease linearly from 20?to 500?,and gradually stabilize from 500?to 600?.At different temperatures,the thermal conductivity and the wave velocity are consistent in the same bedding direction,and the relationship between them is linear..?2?The high-temperature triaxial rock steady-state permeability test system and the high-temperature triaxial pulse-method low-permeability test system independently developed by Taiyuan University of Technology were used to study the variation of permeability in the direction of parallel and perpendicular to bedding directions of oil shale from 20?to 600?.The conclusions are as follows:The permeability in the direction of parallel to bedding directions(k_par)increases with the increase of temperature from 20?to300?and decreases from 300?to 350?due to fracture closure.The permeability at 20?to 350?is between 2.3×10^-19and 2.9×10^-18m²,and at low permeability stage.When the temperature rises to 400?,the fractures in parallel bedding direction leads to a sudden increase of permeability to6.4×10^-17m²,which is 440 times higher than that at 20?.400?is the threshold temperature of k_parchanging with temperature.With the increase of temperature,the permeability continues to increase rapidly,and increases slowly until 550?.The permeability in the perpendicular to bedding directions(k_per)is in the ultra-low permeability stage at 20?to 450?,The permeability level is10^-20m²or less.At 450?,the permeability suddenly increases to 2.77×10^-19m²due to the connectivity of macropore,450?can be called the threshold temperature of k_perchanging with temperature.As the temperature continues to rise,k_percontinues to increase and decreases slightly after 550?.?3?Based on the high-temperature uniaxial rock testing machine independently developed by Taiyuan University of Technology,combined with the acoustic emission test system,the mechanical characteristics and evolution of acoustic emission characteristics with temperature of oil shale in the perpendicular and parallel to bedding directions were studied 20?to 600?.The results show:The elastic modulus and compressive strength of oil shale in the direction of perpendicular to bedding plane first decrease and then increase with the increase of temperature,and the compressive strength and elastic modulus reach the minimum at 400?.Free water evaporation at 100?resulted in a large number of parallel cracks,and the compressive strength and elastic modulus of the parallel bedding direction showed the first sudden drop,and then it was in the stable phase.When the temperature rises to 400?,a large number of cracks are generated due to pyrolysis of the kerogen,a second sudden drop in strength occurs,and the modulus of elasticity rises slightly,and then rises slightly as the temperature continues to rise.Under high temperature,the acoustic emission phenomena of oil shale in parallel bedding direction and vertical bedding direction show obvious anisotropy.The peak energy rate first increases and then decreases when the specimen in vertical bedding direction is damaged;the peak energy rate gradually decreases and then increases slightly when the specimen in parallel bedding direction is damaged.?4?The high-temperature triaxial rock experimental system independently developed by Taiyuan University of Technology was used to study the thermal deformation and creep variation of oil shale in the perpendicular and parallel to bedding directions under different stress constraints.The results show that:In the vertical stratification direction of oil shale,the oil shale specimens are expanded with temperature under unconstrained conditions;the specimens exhibit compressive deformation with temperature at a constant axial pressure of 2 MPa,the temperature point of compression deformation drastically changes is 200?;while under axial compression of 5 MPa,the deformation law is similar,but the value is different,the temperature point of drastic change of deformation is still 200?;and under constant axial compression of 10 MPa,the temperature point of dramatic change of compression deformation becomes100?.In the direction of parallel lamination,the specimen is expanded and deformed in the unconstrained state;when the axial pressure is 2 MPa and 5MPa,the deformation law is first expansion,compression,expansion and compression deformation;when the axial pressure is 10 MPa,it is compression deformation.Creep law:creep deformation in perpendicular and parallel to bedding directions is compressed with time and logarithmically with time,and the creep rate is maximum at 200?and 350?,respectively.?5?A Thermal-hydraulic-mechanical?THM?coupling mathematical model considering the anisotropy of oil shale was established.Combined with the anisotropic thermal,hydraulic and mechanical parameters of oil shale under high temperature obtained in laboratory experiment,the numerical simulation of in situ superheated steam injection pyrolysis oil shale process was carried out by COMSOL software.The main results show that:the temperature field distribution of oil shale reservoir is closely related to the migration of superheated steam in the reservoir,and a rapid temperature reduction zone is formed at the edge of the temperature field;The anisotropy of permeability has the greatest impact on oil and gas production,followed by the anisotropy of mechanical parameters,while the anisotropy of heat transfer coefficient has little effect on oil and gas production.