Thermo-hydro-mechanical couplings controlling gas migration in heterogeneous and elastically-deformed coal

摘要

Revealing gas field evolution and distribution in coal seams is of great significance to coalbed methane (CBM) recovery. It is believed that gas migration in coal involves multifield coupling processes. In this study, an improved thermo-hydro-mechanical (THM) coupling model is developed to investigate the physical fields evolution during gas flow in coal. In this model, the effect of internal swelling deformation is considered to modify the conventional coal deformation equation and coal permeability model. A dynamic diffusion model is introduced to accurately characterize the dynamic evolution of gas diffusion in matrix. Besides, a new matrix porosity model is developed based on the assumption that the pore volume change equals to the matrix volume change. To reveal the gas field evolution and distribution in coal core during coal permeability test in the lab, we build two heterogeneous coal models based on numerically reconstructed method and CT scanning. With the THM model, the gas pressure distribution in the coal core is quantitatively characterized. It is found that in the heterogeneous coal core, gas streamline fluctuates in space. The gas pressure nonlinearly distributes in the coal core, rather than the linear distribution assumed when testing permeability in the lab. Aiming at the confliction between the theoretical result and the assumption used in permeability testing, we propose to improve the accuracy of lab testing result by reducing the gas pressure difference between the gas inlet and outlet.