Investigation of microwave-induced cracking behavior of shale matrix by a novel phase-field method

Abstract

Microwave heating is a potential and effective technology for enhancing shale gas recovery. The microwave radiation may induce cracking of the shale matrix, which further improves the permeability of reservoirs. Therefore, it is important to study the cracking characteristics of shale matrices to evaluate the permeability enhancement effect of microwave radiation on shale. This study proposes a novel dual-scale phase-field model to investigate the mechanisms of intergranular and transgranular fractures within a shale matrix subjected to microwave radiation. The model combines microwave heating modeling at the macro-scale and fracture modeling at the micro-scale with shale microstructures created through Voronoi tessellation. Each mineral grain has an elastic isotropic behavior, and multiphase aggregates (quartz, feldspar, pyrite, clay, calcite, and dolomite) are considered. Intergranular fractures are modeled using a lower-dimensional interfacial element formulation. The responses of temperature and cracking are analyzed based on combined simulations at the macro- and micro -scales. The simulations showed that the microwave heating efficiencies of clay and pyrite are significantly higher than those of the other minerals. Microwave-induced cracking within the shale matrix preferentially occurs in quartz particles, resulting from the lower critical energy release rate and mismatched thermal expansion of the various components of the shale matrix. The fracture modes are highly dependent on the shape and distribution characteristics of the mineral particles.