Investigation on the physical–mechanical response characteristics and failure mechanisms of shale under the laser thermal field

Abstract The mainstream method for extracting shale gas involves hydraulic fracturing to create fracture networks. However, as extraction depth increases, notable issues such as rapid production decline, low recovery rates, high water consumption, and resource waste become apparent. Identifying new and efficient auxiliary rock-breaking technologies is crucial for overcoming these challenges. The laser, successfully utilized in industrial production, medical treatment, and technological research, offers unique features such as good directionality, coherence, and high energy density, providing novel possibilities for addressing the limitations of existing deep reservoir transformation. This research focuses on a novel laser-assisted rock-breaking technology, with shale featuring different bedding angles as the subject of investigation. The investigation methodically explored how shale responded to thermal fracture at high temperatures when exposed to laser irradiation with different spot diameter. It investigates the spatiotemporal evolution characteristics of the shale temperature field under laser irradiation, the propagation features of cracks on shale surface, and the physicochemical fracture mechanisms. The research yields the following results: (1) The region of thermal influence of the irradiation surface can be divided into three regions based on the change of rise curve of temperature in the shale surface. (2) Based on the scanning electron microscopy (SEM) testing, combined with the macroscopic and microscopic morphological characteristics of shale fracture surfaces, it reveals significantly distinct zoning characteristics in the roughness of the rock sample’s fracture surfaces after laser irradiation. (3) The thermal fracturing process of shale under laser irradiation involves chemical reactions of constituent minerals and stress generated by the thermal expansion of shale oil in the reservoir. (4) The damage and fracture of shale under the irradiation of laser show significant bedding effect, and there are three modes of rock sample failure: Pattern T (thermal failure), Pattern T-B (thermal and bedding synergistic failure), and Pattern B (bedding failure). The research findings presented in this article serve as a foundation and reference for the theory and technology of laser-assisted shale gas extraction.