Study on mechanical properties and energy evolution characteristics of proppant-containing sandstone under unloading conditions

To investigate the effects of unloading on the failure mechanical properties and energy evolution characteristics of proppant-containing sandstone, triaxial compression contrast tests were carried out on proppant-containing sandstone specimens using a TAJW-3000 microcomputer-controlled rock triaxial test system, with unloading rates set at 0 (no unloading), 0.025, 0.050, and 0.200 MPa/s, respectively. By acquiring the deviatoric stress-strain curves, the test results were analyzed from the dual perspectives of mechanical properties and energy characteristics. The results show that the peak deviatoric stress and peak axial strain of the specimens under the unloading path are both lower than those under the loading path; with the increase of unloading rate, the unloading stabilization time presents a power function decreasing trend, the peak axial strain decreases rapidly first and then slowly, while the peak radial strain shows a linear increasing trend; under the condition of the same effective unloading amount, the radial strain increment is slightly smaller than the axial strain increment; the dilatancy angle increases with the increase of effective unloading amount, and the higher the unloading rate, the larger the variation range of the dilatancy angle. Under triaxial loading conditions, the total strain energy, elastic energy, and dissipated energy corresponding to the peak deviatoric stress are all higher than those under unloading conditions, and all energy indexes decrease with the increase of unloading rate; as the unloading rate increases, the energy variation in the unloading stage shows a decreasing trend, while the conversion rates of both elastic energy and dissipated energy gradually accelerate; the conversion rate of dissipated energy is faster than that of elastic energy, and the difference between the two rates further widens with the increase of unloading rate. In terms of failure modes, the proppant-containing sandstone specimens under loading conditions mainly exhibit a single shear failure mode; in contrast, under unloading conditions, the internal microcracks of the specimens propagate, secondary cracks increase, and the failure mode is dominated by shear failure accompanied by a small amount of tensile cracks. Moreover, the shear failure characteristics become more prominent with the increase of unloading rate.