Modeling and Upscaling of Binary Gas Coal Interactions in CO2 Enhanced Coalbed Methane Recovery

When CO2 injected into the unmineable coal seam, complicated binary gas CH4-CO2 coal interactions affect coal porosity and permeability, which is one of the bottleneck scientific issues faced in enhancing production of coalbed methane and the geological sequestration of carbon dioxide. The issue is addressed through the development and application of a novel fully coupled coal deformation, gas transport and gas adsorption/desorption finite element (FE) model with COMSOL Multiphysics. The COMSOL FE model considers the combined net effects on coal permeability among the coal matrix swelling/shrinking due to gas displacement, pore pressure and in-situ stress. These combined effects are quantified through solving a set of coupled field equations which govern the coal deformation, prescribe the transport and interaction of gas flow in a similar way to poroelastic theory, and define CH4-CO2 counter diffusion and flow in a coal seam. Numerical models were verified with the experiment data. The established COMSOL FE simulator was applied to simulate the CO2 injection performance in Qinshui Basin field under in-situ size and conditions, to address in-situ spatial-temporal evolutions of coal deformation and permeability. Simulation results suggest that net change of coal permeability accompanying binary gas dispersion is controlled competitively by the influence of effective stresses and differential swelling of coal. 1.75×104t CO2 can be sequestrated in 300×300 m2 area of Qinshui Basin within 10 years. During the simulation, coalbed methane recovery was promoted by 1.44 times.