Micro-cracking behavior of shale matrix during thermal recovery: Insights from phase-field modeling

Abstract

Thermal stimulation for shale gas reservoirs may cause water to evaporate, accelerate desorption of adsorbed gas and induce micro-scale cracking in shale matrix, which will increase shale gas productivity. However, few researches have been focused on the thermal micro-cracking behavior of shale matrix. In this paper, a novel energy functional considering the effect of thermal contribution and initial stress field is established, which then is used to formulate coupled thermo-mechanical phase-field method by the variational principle. The validations with analytical, numerical and experimental results show reliability and robustness of presented model. The quartet structure generation set is used to reconstruct the heterogeneous shale matrix, including quartz, feldspar and dolomite. The image function recognizes pixel points to assign fixed parameter values for different minerals in COMSOL. The three reconstructed shale matrixes subjected quasi-static thermal shock are simulated based presented model. The simulation results indicate that a higher initial stress ratio can selectively suppress or accelerate crack propagation. The thermal expansion coefficient of different minerals can significantly affect the crack initiation temperature, crack pattern and thermal cracking efficiency. Furthermore, the presented phase-field can be extended easily and applied to coupled thermo-hydro-mechanical-chemo fracture problem in underground rock engineering.