Experimental study on the dynamic fracture response law of shale under the impact of methane combustion fracturing

The methane in-situ combustion fracturing is a technology to realize in-situ combustion and fracturing on the target layer section in the shale gas wellbore. The technology can realize the enhanced fractured reservoir modification by means of the combustion-supporting agents injected from the ground and the methane gas released from the formation, which has the advantages of low cost, high safety, and convenient repeatability. Among them, the understanding on the dynamic damage law of reservoir rock during the fracturing is the basis for ensuring desirable fracturing results and wellbore safety. Based on this, this study conducted the quasi triaxial and true triaxial rock dynamic impact damage simulation experiments to analyze the dynamic failure response mechanism of shale rocks under the condition of explosive impact. And the dynamic fracture strength and fracture morphology of shale were evaluated quantitatively, which are influenced by the impact mode, loading rate, reservoir stress, and shale bedding. The results show that the high energy fluid penetrating impact reduces the dynamic fracture strength of shale due to the “wedge fracture effect” of the fluid. And the positive correlation with the pressurization rate is weaker than that of pure stress impact. Parallel-layer shale and lower in-situ stress level can reduce the dynamic fracture strength of shale. Single stress impact is more likely to generate more complex fracture network, and the fracture surface is highly concave and convex. Comparatively, the high energy fluid penetrating impact is more inclined to produce straight radial fractures. Besides, the number and complexity of multiple fractures as well as the peeling and fragmentation of fractures surface are also enhanced as the loading rate increase. More specifically, the post-fracturing shale samples show three damage modes gradually including the collaborative fracturing by stress tension and fracturing-fluid wedging (27 MPa/ms), the further fracturing by the fluid erosion of crack surface after stress induced cracking (52 MPa/ms), and the fluid shock cracking after stress induced fracturing (116 MPa/ms). Based on the experimental results, the relationship between the three damage modes and the pressure loading rate and reservoir stress was plotted, which can provide certain theoretical support for the design of methane combustion fracturing.