Effect of saturated fluid on the failure mode of brittle gas shale

Abstract

To understand the influence of fluid saturation on the failure mode of gas shale, a series of triaxial compression tests under the quasi-static loading conditions were conducted using the GCTS (Geotechnical Consulting and Testing System) under different conditions (dry, water saturation, and slickwater saturation). The results showed that the rupture of dry shale samples was accompanied by a transient release of energy, which formed into the main splitting crack. Simultaneously, high-energy tension-type acoustic emission (AE) events were detected around the peak stress. Shale saturated with fluid decreased the strength and increased deformation. The presence of a “step” disturbance was clear in the expansion stage before rupture, and low-energy shear rupture events were detected repeatedly. This indicates that there were internal microslips and distributed micro-cracks to form, which maintained the overall integrity of the cores. Based on the analysis above, two failure modes were discovered: transient main crack failure mode for dry shale and distributed micro-failure mode for shale saturated with fluid. The effect of fluid saturation on the failure mode of shale was clear. This effect can increase the pore pressure; reduce fracture-surface energy; enhance the softening effect caused by water-absorbing clay, which reduced the effective stress on crack surfaces; promoted rock shear slip inside the rock; and changed the energy release characteristic and fracture mode of the shale under a stress condition, which were conducive to the formation of a crack network. The experimental results showed that saturated slickwater had a stronger effect on the failure mode of brittleness gas shale than that of saturated water.