Experimental investigation on the failure and acoustic emission characteristics of shale, sandstone and coal under gas fracturing

Gas fracturing plays an increasingly important role in the stimulation of unconventional gas reservoirs. However, previous studies on the use of gas as stimulants in the reservoir focus on field applications. The studies on the fracturing mechanism of the reservoir are limited. In this study, the gas fracturing experiments were conducted using shale, sandstone and coal at the laboratory scale. The results are as follows. (1) The lateral deformations of the specimens are greater than the axial deformations in gas fracturing. The rock samples exhibit a low elasticity modulus, high Poisson's ratio, low tensile strength, low compressive strength and medium porosity, which means deformations of the rock samples occur easily. A high breakdown pressure and a high stiffness are more likely to come from rock specimens with both high brittleness and low porosity. (2) The acoustic emission (AE) and the fracture patterns of the specimens indicate that the failure of the rock with low porosity and high strength is the most violent during gas fracturing. The AE information and micro-fracture surface of the specimen also reveal that the pore gas pressure has an obvious effect on the micro-structure of the rock with low porosity and low strength. (3) The bedding direction of the shale is considered in this study. The mean values of the breakdown pressure and stiffness for the shale with the axis vertical to the bedding plane are 4.20 and 2.06 times as high as those of the shale with the axis parallel to the bedding plane because of the difference of the failure mechanism, respectively. For the shale with the axis vertical to the bedding plane, the special orientation bunch structures with high strength in the bedding plane fail resulting in violent failure and a complex fracture surface. Moreover, the average breakdown pressure of the rock under the installed sealing device in the sides of the samples is only 37.1% of the average breakdown pressure under the uninstalled condition and the fracture patterns of the specimens under the installed sealing device in the sides of the samples are more complex which indicates that gas filtration cannot be ignored.