Study on the damage characteristics of gas-bearing shale under different unloading stress paths

In order to understand the influence of unloading on the mechanical properties of shale rock, triaxial unloading tests under different stress paths were conducted. In this paper, three types of tests are completed, including: 1) Conventional triaxial compression test;2) Pre-peak constant maximum principal stress-unloading confining pressure test with different initial confining pressures and rates;3) Increasing axial stress-unloading confining pressure test. The deformation and rupture modes characteristics of shale sample under different unloading stress paths were obtained. Research results show that: 1) The confining pressure effect is obvious and the peak strength increases with the increase of initial confining pressure, under conventional triaxial compression test, the samples show obvious elastic-plastic characteristics; Under unloading confining pressure test, it shows obvious elastic brittleness characteristics.2) Compared with conventional triaxial compression test, unloading confining pressure is more prone to deformation and rupture, and the damage is more serious. Under same initial stress level, the brittle characteristics in unloading confining pressure are more obvious and the expansion is more intense. 3) Under same unloading stress path, the higher the initial confining pressure is, the more severe the sample failure is. With the increase of unloading rate, the rupture degree of the sample becomes more complex.4) The brittle rupture characteristic increases with the increase of unloading rate and initial confining pressure. Increasing axial stress-unloading confining pressure, various types of tensile and shear fractures with different mechanisms are well developed. These conclusions reveal loading and unloading mechanical properties of gas-bearing shale under different stress paths; it provides theoretical basis for horizontal drilling, fracturing design and long-term fracturing effect analysis of shale gas reservoirs.