EXPERIMENTAL STUDY ON PERMEABILITY ANISOTROPY OF DEEP RESERVOIR SANDSTONE UNDER TRUE TRIAXIAL STRESS

Permeability anisotropy is a very typical phenomenon in sedimentary bedding structure. On one hand, it is determined by the primary sedimentary structure (i.e. primary anisotropy) and on the other hand, it is affected by stress and pore pressure (i.e. induced anisotropy). In order to study the primary and induced anisotropy of reservoir sandstone permeability under true triaxial stress, the reservoir sandstone of S6 gas storage in northeast China was taken as the research object. The true triaxial stress-seepage coupling device of hard rock independently developed by Northeastern University was adopted to carry out seepage experiment on reservoir sandstone, and the permeability test of the same sandstone in three mutually vertical directions was completed by steady-state method. The test results show that: in the range of applied stress and pore pressure, permeability of sandstone in parallel bedding directionk_x is 100.94 mD ~ 113.98 mD, k_y is 98.34 mD ~ 111.41 mD, and permeability in vertical bedding direction k_z is 54.98 mD ~ 63.29 mD. The permeability of sandstone in three orthogonal directions decreases with the increase of principal stress and increases with the load of pore pressure. The effect of stress perpendicular to the gas seepage direction on permeability is greater than that of stress parallel to the gas seepage direction. When the direction of external stress is perpendicular to the direction of gas flow, the effect of stress perpendicular to bedding on permeability is greater than that of stress parallel to bedding. The linear elastic response of pore pressure to reservoir sandstone permeability is not isotropic. The linear permeability increment generated by pore pressure to horizontal bedding direction is higher than that in vertical bedding direction. The research results provide a reference basis for accurate prediction of sandstone permeability of underground gas storage and a new petrophysical property data for operation and management of underground gas storage.