Pore-Fracture Modification Mechanism in Transitional Shale of the Shanxi Formation under Varying Deflagration Pressures

Abstract

The strong water sensitivity of marine-continental transitional shale renders conventional hydraulic fracturing techniques inadequately applicable, while methane in situ deflagration fracturing represents a transformative waterless stimulation technology. However, the response of the pore-fracture system in the Shanxi Formation marine-continental transitional shale to varying deflagration pressures remains poorly understood. Therefore, to investigate the alteration characteristics of pore-fracture systems within these reservoirs under different deflagration pressure gradients, this study selected marine-continental transitional shale from the Shanxi Formation in the North China Basin. Deflagration fracturing experiments were conducted across a pressure range of 25 to 91 MPa. Coupled with computed tomography and low-field nuclear magnetic resonance analyses, the pore-fracture responses of the shale under different deflagration pressures were systematically characterized. Research findings indicate that deflagration-induced fracture development in shale exhibits localized and preferential characteristics. Fractures preferentially initiate and propagate along weak planes such as bedding surfaces and preexisting microcracks, and are primarily concentrated nearer to the deflagration source. With increasing deflagration pressure, fracture density rises and apertures widen. Porosity demonstrates a nonlinear growth trend, characterized by a decreasing proportion of adsorption pores and an increasing proportion of seepage pores, although the growth rates of both gradually diminish. After deflagration, all samples exhibit consistently improved interpeak coupling. As the deflagration pressure escalates, the connected porosity increases, mainly due to the formation of large through-going fractures parallel to bedding planes that connect with smaller fractures oriented in various directions. The fracturing process is primarily driven by the combined effects of gas shock stress waves, high-pressure gases generated during deflagration, and thermal action, which collectively modify the pore-fracture system. The research findings provide a theoretical basis for selecting the pressure required for deflagration fracturing in marine-continental transitional shales.