Anisotropic Evolution Law Of Physical And Mechanical Properties Of Sandy Mudstone Under High Temperatures And Its Application

As a potential energy source,oil shale can be used as a supplementary and alternative energy source for oil and natural gas in the future.In order to investigate the stability of sandy mudstone caprock in the process of in-situ thermal injection of oil shale,thermal dilatometer,thermal conductivity tester,low-permeability measurement device,high-temperature triaxial rock permeability measurement equipment,high-temperature rock press and other equipment were used to study how the thermal expansion coefficient,thermal conductivity,permeability and mechanical parameters(elastic modulus E,compressive strength?_p,tensile strength?_t,cohesion c and friction angle?,etc.)of anisotropic sandy mudstone varied with temperature.Considering that the physical and mechanical properties of sandy mudstone have significant anisotropy,the permeability evolution law of anisotropic sandy mudstone in the full stress-strain process under high temperature triaxial stress were investigated,and the changes in the structure and composition of sandy mudstone minerals under the action of high temperature were analyzed by XRD and TG testing and other microscopic methods.Lastly,based on the transversely isotropic model,a mathematical model of fluid-solid-heat coupling is established and a finite element solution method is given by inrtoducing the anisotropy of rock thermophysics(thermal expansion coefficient and thermal conductivity)and permeability properties.Taking in-situ heat injection production of oil shale as the engineering background,the temperature field,displacement field,seepage field and physical parameters of the sandy mudstone caprock during the heat injection process are simulated in time and space.It is expected to provide theoretical support for the research on the stability of the caprock of oil shale in-situ heat injection mining engineering.The main conclusions obtained in this paper are as follows:(1)The thermal expansion coefficient and thermal conductivity of sandy mudstone were obviously affected by temperature,and showed strong anisotropy.The thermal conductivity of sandy mudstone in the parallel and vertical bedding directions decreased with increasing temperature,but the thermal conductivity in the parallel bedding direction was always greater than that of the vertical bedding,and the ratio of the two was maintained at 1.246.The longitudinal wave velocity of sandy mudstone in the parallel and vertical bedding directions decreased with the increase of temperature,but the specific laws were different,and the ratio of the two increased with the increase of temperature,showing three-stage characteristics:I slow increase stage(20??200?),II rapid increase stage(200??500?),III slow increase stage(200??500?).In the vertical bedding direction,the permeability of sandy mudstone continued to increase with the increase of temperature,showing three-stage characteristics:20??200?,the permeability increased by about 63 times although the increase in permeability was small;200??400?,the permeability kept basically stable;400??600?,the permeability increased by about 6.96 times.In the direction of parallel bedding,the permeability of sandy mudstone increased exponentially with the increase of temperature,with the highest increase of about 23 times.Under the same temperature,volume stress and osmotic pressure conditions,the permeability of sandy mudstone in the parallel bedding direction was1 to 2 orders of magnitude greater than that in the vertical bedding direction,and the permeability ratio k₂/k₁ decreased first as the temperature increased,rised again,and finally falled.Lastly,based on the empirical formulas of permeability,volume stress and pore pressure,an anisotropic sandy mudstone permeability formula considering temperature,volume stress and pore pressure was established.(2)Through the experimental study of the permeability evolution law of sandy mudstone in the vertical/parallel bedding direction under different temperatures,the mechanical properties,failure mechanism and permeability of sandy mudstones such as temperature and bedding were studied.The following conclusions were drawn:In the vertical and parallel bedding directions,the full stress-strain loading deformation and failure laws at high temperatures of 400?,500?,and 600? were basically the same,and the permeability change law and damage evolution law were consistent on the whole.In the vertical bedding direction,the permeability showed a downward trend at the crack compaction stage;at the linear elastic stage and the stable propagation stage of the crack,the permeability showed a steady increase trend;at the unstable propagation stage of the crack,the permeability appeared sharply increase;In the parallel bedding direction,the permeability increased steadily during the crack compaction stage,the linear elastic stage and the stable expansion stage of the crack,and the permeability increased rapidly during the unstable crack propagation stage.In the vertical or parallel bedding direction,as the temperature increased,the permeability corresponding to the same axial stress point gradually increased;under the same temperature conditions,during the loading process,the permeability in the parallel bedding direction was much greater than that in the vertical bedding direction at the same axial stress point.The higher the temperature of sandy mudstone,the greater the strain value corresponding to the same stress point,but the strength of sandy mudstone decreased.The fracture modes of sandy mudstone in the vertical and parallel bedding directions are mainly single-shear plane failure,but there were still some differences in the failure mechanism:in the parallel bedding direction,the main shear plane was accompanied by a vertical direction(bedding Direction)multiple groups of fracture surfaces,and the higher the temperature,the more developed the fracture surface.(3)In order to study the deformation characteristics,failure mechanism and acoustic emission characteristics of anisotropic sandy mudstone under real-time temperature(20?-600?)during uniaxial compression,the effect of bedding on peak strain and peak stress,Elastic modulus,and brittle ductility transformation characteristics was discussed,and the following conclusions were obtained:(1)When the load was perpendicular to the bedding direction,the elastic modulus of sandy mudstone first increased slightly with the increase of temperature(20??100?),and then linearly decreased(100??600?);When the load was parallel to the layering direction,the modulus of elasticity decreased approximately negatively exponentially with the increase of temperature.The elastic modulus ratio E₁/E₂ of sandy mudstone in the vertical and parallel bedding directions presented a three-stage change characteristic with increasing temperature:increase stage(20??100?),linear reduction stage(100??500?)and stable stage(500??600?),and the ratio E₁/E₂ was larger than 1 in the range of(20??600?).(2)When the load was perpendicular or parallel to the bedding direction,the compressive strength of sandy mudstone basically decreasesd linearly with the increase of temperature.When the load was perpendicular to the layering direction;and when the load was parallel to the layering direction.The compressive strength ratio?_p1/?_p2 of sandy mudstone in the vertical and parallel bedding directions increased approximately linearly with the increase of temperature.At 600?,the ratio?_p1/?_p2 increased from 1.73 at 20? to 2.76.(3)When the load was perpendicular to the bedding direction,the peak strain of sandy mudstone increased exponentially with the increase of temperature;When the load was parallel to the bedding direction,the peak strain of the sandy mudstone changed in stages with the increase of temperature,but the overall trend was decreasing.In the temperature range of 20? to 600?,the brittleness index of sandy mudstone in the vertical bedding direction was 2-3 orders of magnitude higher than the brittleness index in the parallel bedding direction.(4)When the load was perpendicular to the bedding direction,the failure type of sandy mudstone changed from tensile failure to tensile-shear failure;While the load was parallel to the bedding direction,the failure type of the sandy mudstone was mainly a"tensile"failure type.(4)The relationship between the tensile properties and the shear resistance of anisotropic sandy mudstone with temperature wre investigated,the conclusions were as follows:(1)When the loading was perpendicular,parallel or orthogonal to the bedding surface,the tensile strength continued to decrease with the increase of temperature,but the law and magnitude of the decline were different.Under the same temperature conditions,the tensile strength when the load was orthogonal to the bedding plane³>the tensile strength when the load was perpendicular to the bedding plane¹>the tensile strength when the load was parallel to the bedding plane²,but the relative magnitude relationship of the three was different with the increase of temperature.(2)when the shear plane was perpendicular,parallel and orthogonal to the bedding plane,the cohesion of sandy mudstone decreased approximately linearly with the increase of temperature,while the internal friction angle did not change much.Under the same temperature conditions,the cohesive force c₃ when the shear plane is orthogonal to the bedding plane>the cohesive force c₁ when the shear plane is perpendicular to the bedding plane>the cohesive force c₂ when the shear plane is parallel to the bedding plane.(3)The relative position of the shear plane and the bedding plane had an important influence on the failure mode of sandy mudstone.When the shear plane was perpendicular to the bedding plane,the shear failure plane was irregular,showing ductile failure characteristics;when the shear plane was parallel to the bedding plane,the shear failure plane was a single regular plane,showing brittle failure characteristics;When the shear plane was orthogonal to the bedding plane,the failure form of sandy mudstone is a"shear zone"with a certain width,which was characterized by ductile failure.(5)Based on the transversely isotropic model,the change law of the physical and mechanical parameters of oil shale and sandy mudstone(thermal conductivity,expansion coefficient,permeability coefficient,elastic modulus,etc.)with temperature were introduced,and the anisotropy of the physical and mechanical parameters were considered,the thermal-fluid-solid coupling mathematics that considers the relationship between the physical and mechanical parameters of oil shale and sandy mudstone and temperature was established.and then,the corresponding finite element format(total method)were given to solve the equation to study the temperature field,displacement field,seepage field and physical parameter dynamics in the sandy mudstone roof and floor during in-situ heat injection production of oil shale.The main conclusions obtained were as follows:(1)In the process of in-situ heat injection production of oil shale,the sandy mudstone(top and floor)was affected by temperature within 10m,and the temperature around the heat injection well was most affected by the temperature.It can be inferred that the sand around the heat injection well Mudstone(roof and floor)may be a dangerous area and should be paid attention to.(2)With the increase of the heat injection time,the temperature of the oil shale layer and the roof rock continued to rise under the effects of heat convection and heat conduction,and the weakening of the mechanical properties of the two increases,and the damage of the elastic modulus gradually increased,leading to constantincrease of the settlement of the formation.When the heat injection time was 60 months,the maximum subsidence was 3.80m,and the maximum displacement occured near the heat injection well,which was likely to induce surface subsidence,and the area around the heat injection well was a dangerous area,and the well wall should be prepared in advance.(3)With the increase of heat injection time,the influence length(horizontal direction)of pore pressure on sandy mudstone roof and floor rocks gradually increased,but the maximum influence depth of pore pressure did not change much.When the heat injection time was 60 months,the maximum impact depth of pore pressure on sandy mudstone was about 2.5m,which occured at the junction of the heat injection well and sandy mudstone.