Effect of CO2 injection on heterogeneously permeable coalbed reservoirs

Further, a reservoir with initial permeability heterogeneity was considered to explore the effect of CO2 injection on the evolution of permeability heterogeneity - whether heterogeneity increases or decreases. The evolution of permeability heterogeneity is investigated for the same two CO2 injection scenarios. For the specific parameters selected, the model results demonstrate that: (1) The injection of CO2 in coalbed reservoirs increases the production nearly 10-fold. (2) At higher injection pressures the recovery is rapid and the production increases dramatically - the production increases 2-fold on increasing the CO2 injection pressure from 4 MPa to 8 MPa. (3) However, CO2 breakthrough occurs earlier at higher injection pressures. (4) The permeability heterogeneity in the reservoir is reduced after a threshold time (∼500 days) although the overall heterogeneity is increased relative to the initial condition and is overall increased for both non-CO2 and CO2 injection scenarios. This indicates that the homogenizing influence of CO2-sorption-swelling is outpaced by CH4-desorption-shrinkage and effective stress influences. This leaves the reservoir open to short-circuiting and earlier breakthrough of CO2 rather than having this effect damped-out by the homogenizing influence of swelling. (5) The cumulative volume of CO2 produced and stored in the reservoir is proportional to the injection pressure.