Deformation of Shale and Coal Organic Carbon Slit Micropores Induced by CO2 Enhanced Gas Recovery: A Monte Carlo Simulation Study

The swelling of shale and coal induced by the CO2 enhanced gas recovery (CO2-EGR) has proved to reduce the reservoir permeability and the CH4 production. In this work, we have studied the adsorption-induced deformation of the organic carbon slit micropores during the displacement of CH4 by the injected CO2 using grand canonical Monte Carlo simulation. Particularly, we have investigated the effect of the injected CO2 ratio on the deformation strain for each pore width from 0.5 nm to 2.0 nm under a series of pressures and temperatures. The results showed that the pore deformation is distinct depending on the pore size and the injected CO2 ratio, which generally includes monotonic swelling and shrinkage followed by swelling with bulk pressure. The pores below 0.55 nm have no deformation, as these pores are too narrow for both CH4 and CO2. The maximum swelling in CO2-EGR occurs in the 0.55-0.59 nm pores, which contributes most in terms of the CO2 storage, but has no contribution to CH4 recovery. The maximum s...