Dynamic Characteristics of Cyclic Impacts on Gas Shale Under Confining Pressure

The dynamic mechanical behaviors of gas shale at high confining pressure and high strain-rate are essential to understand well drilling and hydraulic fracturing mechanisms. This paper presents an experimental study on the dynamic mechanical properties and fracturing behaviors of the Longmaxi shale under a high confining pressure up to 40 MPa. A customized split Hopkinson pressure bar with radial confining pressure chamber was used to record the dynamic stress-strain relations under biaxial compression. Results indicate that Longmaxi shale requires 2–5 repeated impact loads to reach macro failure under the confining pressure. Tensile cracking becomes dominant at the biaxial compression, which presents a noticeable distinction from the shear-dominated failure by uniaxial compression. Furthermore, the peak strength of biaxial compression is only 30%-80% of that in uniaxial compression tests. However, the specimen under confined state consumes almost one order of magnitude of energy when comparing to that of uniaxial state. The induced crack surface area of confined state is only half of that under uniaxial state. This phenomenon attributes to two aspects: the confined state enhances rock strength that makes it hard to be broken; the tensile crack damages accumulated by the repeated impacts promotes the ultimate tensile failure.