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 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 shrinkage happens in the 0.66 nm pore, which provides most to the CH4 recovery. Besides, the maximum swelling and shrinkage is generally not affected by the CO2 ratio except the deformation at low pressures and even a small amount of CO2 injection could induce the maximum swelling for the corresponding pores in shale or coal. The bulk pressure has a more significant effect on the deformation of the 0.75-1.05 nm pores with the increase of CO2 ratio, and the pore width for the maximum swelling decreases with the increase of pressure. At 100 MPa, a second minor peak of swelling and shrinkage occurs in the 0.85-0.9 and 0.95-1.05 nm pores, respectively. Furthermore, temperature has no effect on the maximum swelling at 100 MPa,but the overall deformation generally decreases with the increase of temperature including the maximum shrinkage. The 1.4-2.0 nm pores only have slight deformation regardless of the CO2 ratio, pressure, and temperature. It is also found that the solvation pressure is the driving force for the deformation irrespective of the adsorbed gas species. However, the adsorbed CH4 and CO2 molecules exert different solvation pressures to the pores during the competitive adsorption. The local solvation pressure is heterogeneous across the pore space for both CH4 and CO2. The positive pressures are close to the pore walls, which tend to swell the pores, but negative pressures are in the pore interior, which incline to contract the pore.