Research On Mechanical Properties And Permeability Of Shale Under True Triaxial Stress Condition

Based on the experimental research,data,theoretical analysis and numerical simulation,taking the black shale of Longmaxi Formation in Shizhu Tujia Autonomous County of Chongqing as the research object,the mechanical properties and permeability characteristics of shale under true triaxial stress conditions were systematically developed.Firstly,a series of fundamental experiments on physical mechanical properties of shale have been completed;then,based on the self-developed multifunctional true triaxial test system of fluid-solid coupling,loading-unloading tests under true triaxial stress conditions of different intermediate principal stresses,bedding directions and different intermediate principal stress coefficients,gas pressure gradients were carried out;and finally using the multi-physics coupling analysis software COMSOL Multiphysics?based on finite element method,the shale fluid-solid coupling in the loading path of intermediate principal stress?₂ under true triaxial stress conditions was simulated.Based on the above experimental researches,the following main conclusions can be obtained:?1?The uniaxial compressive strength?UCS?of this shale is 98.33 MPa.In the nitrogen adsorption pore test,the shale belongs to the type IV isotherm,the H3 type hysteresis loop,and the BET specific surface area is 9.92 m²/g.The average pore size of the shale is 16.84 nm and has a wide distribution of nanopores and strong non-uniformity.In the pore test of mercury porosimetry,the nano-pores of the shale developed,and the pore size was mainly distributed between 10 and 220 nm.The pore volume at 10¹00 nm is about 60%in the specimen,about 30%at 101²00 nm,and about 10%at more than 200 nm.?2?The effect of intermediate principal stress on permeability of shale is greatest when intermediate principal stress is perpendicular to the bedding plane,lower when minor principal stress is perpendicular to the bedding plane,and least when maximum principal stress is perpendicular to the bedding plane.When the intermediate principal stress is perpendicular to the bedding plane or parallel to the bedding plane,the intermediate principal stress is negatively correlated with the permeability.The permeability is more sensitive to the intermediate principal stress when the intermediate principal stress is perpendicular to the bedding plane than parallel to the bedding plane?the difference is 1 or 2 orders of magnitude?.During the loading process,the slope of the damage variable-intermediate principal stress curve does not change too much,while the slope changes obviously during the unloading process.The damage variable is bigger when the intermediate principal stress is perpendicular to the bedding plane than parallel to the bedding plane during the whole process.?3?The permeability of shale decreases with the increasing intermediate principal stress coefficient,and the influence of intermediate principal stress coefficient on the permeability of shale increases with the increase of stress difference.The permeability of shale is nonlinearly related to the gas pressure gradient and decreases with increasing gas pressure gradient.Under the same stress difference,the change in permeability decreases as the gas pressure gradient increases.The permeability of shale decreases with the increasing effective stress,and the greater the stress difference,the greater the effect of effective stress on permeability.In the condition of true triaxial stress,using the exponential expression to predict the permeability of shale is better than using the power law,and the fitting effect is getting better with the increase of the stress difference.?4?When the intermediate principal stress?₂ is gradually loaded,the shale model is stressed in the direction of middle principal stress while pulled in the direction of maximum and minimum principal stress.The Darcy velocity field of CO₂ in the X and Y directions shows a free flow with an irregular distribution.With the continuous increase of the intermediate principal stress?₂,the gas velocity decreases continuously in the X-direction and Y-direction,and the control effect of the stress field on the seepage field is numerically verified.The numerical solution of permeability using COMSOL can be qualitatively consistent with the experimental results and has a good consistency,indicating that the numerical model and its finite element program implementation are correct.