Twin Shear Unified Strength Solution of Shale Gas Reservoir Collapse Deformation in the Process of Shale Gas Exploitation

The collapse deformation of shale has a significant influence on the exploitation process. Experimental analysis has indicated a correlation coefficient range from 0.9814 to 0.9981 and the established sample regression formula could be used to express the relationship between the dynamic elastic modulus and static elastic modulus of shale specimens. Based on the twin shear unified-strength theory, where coefficient b was considered to express the effect of intermediate principal stress, with the deduced regression formula, the unified solution of major principal strains describing a critical collapse of the shale shaft wall was derived. The results showed that the intermediate principal stress had a significant influence on the major principal strain, describing the critical collapse of the shale shaft wall. At the same depth, the critical collapse major principal strain increased with the increase in the b values. With the change in b value from 0 to 1, the calculated difference in critical collapse major principal strain with the same wellbore depth would change from 22.1% to 45.5%. With the change in b value from 0 to 1, the calculated difference in critical collapse major principal strain with the same wellbore temperature would change from 22.1% to 45.6%. The unified solution formula of the major principal strain, describing the critical collapse of the shale shaft wall expressed by the dynamic elastic modulus, could adjust the contribution of intermediate principal stress by changing the values of b, while considering the influence of temperature and confining pressure. The twin shear unified-strength solution of the shale gas reservoir collapse deformation could be used to effectively evaluate the shale gas reservoir stability during shale gas exploitation.