Experimental Study On True Triaxial Mechanical Response And Seepage Law Of Reservoir Rocks Based On Bedding Structure And Pore Fluid

Permeability of reservoir rock is a key parameter for the efficient exploitation of unconventional natural gas,such as coalbed methane(CBM),shale gas and tight sandstone gas).The permeability is not only affected by the joint fractures,bedding planes and other flaws of the rock,but also related to the stress state and deformation behavior of the rock.Reservoir rocks are mostly sedimentary rocks.The bedding structure is an important component of sedimentary rocks.It makes the mechanical behavior and permeability evolution of the reservoir rock show significant anisotropy,and also has a great influence on the crack growth and failure mode of such rocks.The coupling between pore pressure and in situ stress in reservoir rock affect inevitably the crack propagation and failure modes.The reservoir rocks represented by sandstone,raw coal and shale are taken as the research object,and the seepage and failure experiments of reservoir rocks under true triaxial stress path are performed.The principal stress and pore pressure on the mechanical behavior and fracture pattern of reservoir rocks containing bedding planes and the migration law of internal fluid are investigated.The research results of this paper are as follows:(1)Volumetric strain of sandstone decreases with increasing Lode angle in the elastic deformation stage under the condition of constant bulk stress and deviator stress,and it is basically not affected by the stress path.Shale has typical bedding planes,and its volumetric strain has a weak correlation with the Lode angle,i.e.,the stress path significantly affects the deformation behavior and permeability evolution of shale.(2)Sandstone permeability increases with increasing shear strain under the condition of constant bulk stress and deviator stress,but it is independent of volumetric strain.The increase of shear strain means that the frictional sliding between sandstone particles increases,and the shear displacement promotes the growth of sandstone permeability.Due to the mature development of shale beddings,the stress normal to the bedding plane plays an important role in shale permeability,and this effect is stronger than the frictional sliding effect between shale particles.The permeability evolution of coal lies between sandstone and shale,which is related to its unique cleat system.(3)Both in situ stress and bedding affect the permeability of coal and shale.Due to the different environmental stresses of the coal and shale,and the uncertainty of the direction of the principal stress relative to the bedding strike,there is a competitive relationship between the two in controlling permeability.Compared with coals,shales show more significant bedding effect.However,the face cleat of coal also plays an important role in permeability evolution of,and the bedding does not play a sole role in controlling permeability.The small bedding elastic modulus determines that the bedding effect cannot be ignored,especially when the stress normal to the bedding planes is small,the bedding has strong stress sensitivity.(4)Coals and shales are conceptually composed of bedding strips and non-bedding strips.Both of them are evenly distributed in intervals,and the bedding and non-bedding strip are equidistant respectively.The bedding coefficient is introduced to characterize the bedding effect.Based on the unique structure and permeability evolution difference of coals and shales,the analytical model of stress-strain relationship and new permeability model under true triaxial stress condition were established respectively.For coals,the bedding and non-bedding strains are separated by the bedding coefficient and bedding elastic modulus,and an analytical model of stress-strain under true triaxial stress conditions is constructed.It is assumed that both the bedding permeability and non-bedding permeability are exponentially related to their respective bedding and non-bedding volumetric strain.Based on the analytical model,a new permeability model which can reflect the different influence of bedding and non-bedding on coal permeability is proposed and verified.For shales,based on the power-law relationship between permeability and porosity,a stress-dependent permeability model under true triaxial stress condition is established by introducing characteristic functions and characteristic parameters that characterize the process of bedding mechanical properties to that of non-bedding during compression.Compared with the experimental permeability,the coal and shale permeability models got good fitting results.(5)Peak fluid pressure required for the failure of coal by injecting high-pressure fluid decreases with increasing intermediate principal stress.Compared with the injection of liquid CO₂ and N₂,the peak fluid pressure is the largest during the pressurized water injection.The deformation of coal in the direction of minimum principal stress always showed dilated with increasing fluid pressure.The coal mainly formed tensile fractures propagating along bedding planes(or near bedding planes)under lower intermediate principal stress.The main fractures exhibited shear failure characteristics obliquely through the bedding planes under higher intermediate principal stress,resulting in larger coal blocks.Further,under increasing pore pressure,the fracture propagation behavior includes:1)reversal of coal particles,2)translation of bedding caused by tensile fractures under high pressure fluid,3)formation of macroscopic shear slip plane caused by shear fractures under deviator stress,and 4)arrest of shear fracture propagation at tensile fractures.(6)Coals showed obvious effective stress anisotropy during injecting fluid,and the maximum deviator stress increases accordingly,resulting in the failure of the coal.The modified crack-sliding model is used to get the evolution of the crack density parameter before the coal failure.The crack density parameter increases with increasing fluid pressure under the condition of fixed far-field stress,which is consistent with the strain evolution of coal.Because of the higher viscosity of water,the crack density parameter induced by pressurized water injection was smaller than that in the case of liquid CO₂and N₂ injection.Besides,the crack density parameter decreases with increasing horizontal stress difference.The modified crack-sliding model can also better characterize the stress-strain nonlinear behavior of rocks in the accelerated dilatation stage.