The Effects of Injection and Production Well Arrangement on Carbon Dioxide Sequestration in Deep, Unmineable Coal Seams: A Numerical Study

Abstract

Carbon dioxide (CO2) sequestration in deep, unmineable coal seams provides a promising opportunity for the reduction of anthropogenic greenhouse gas emissions. However, little is known about optimal design features of the required in-ground infrastructure. Here, we report on a numerical study investigating the influence of the number of injection (and production) wells and their arrangement on the CO2 storage capacity of a coal-seam reservoir. The results of the modelling reveal a decline in injection rates and practical storage capacities with continued CO2 injection. This is related to coal matrix swelling (and associated permeability decrease) caused by adsorption of injected CO2 on coal, coupled with increasing reservoir pore pressures during injection, which reduce the pressure gradient from well to reservoir and thus the impetus for flow. Modelling carried out for scenarios with multiple wells showed an increase in storage capacity for the two-well scenario but a reduction in storage capacity for the three- and four-well models. This was related to the relative timing of interaction between zones of influence (swollen, high-pore-pressure zones) for neighboring injection wells and their limitations on additional CO2 injection. Models incorporating a distal water-production well demonstrated that reduction in pore pressures by water production can help increase injection rates and practical storage capacities. However, modelling showed that if the production well is placed too close to the injection well, methane accumulating in the vicinity of the production well can mix with injected CO2, causing significant changes in CO2 densities, pore pressures, and thus storage capacities. The results of the modelling confirm the influence of well arrangement on coal-seam CO2 storage capacity.