Simulating Coal Permeability Change as a Function of Effective Stress Using a Microscale Digital Rock Model

Investigating the coal microstructure under the influence of effective stress is vital for evaluating hydrocarbon gas production and CO2 geo-storage potential of deep coal seams. While several theories and approaches are reported in the past decade, the development of a representative dynamic model (e.g., using digital rock technology) is an attractive (although challenging) approach. The digital rock technology offers an effective way for investigating effective stress-cleat-permeability. In this research, we constructed a novel digital coal model, which included a microcleat system with a stress-strain function added to the coal matrix phase, and then simulated it under different effective stress conditions. Subsequently, the permeability of coal under different effective stress was calculated using the lattice Boltzmann method. Our results indicate an exponential correlation between coal permeability and effective stress. Moreover, the number of large microcleats on the digital coal samples decreased, while the small microcleats increased with a corresponding increase in effective stress. The simulation results were consistent with the experimental measurements. The results suggested that such novel digital core analysis methods offer an effective way of investigating the physical characteristics of the coal microstructure and simulating the permeability as a function of effective stress.