CO2 storage and enhanced coalbed methane recovery: Reservoir characterization and fluid flow simulations of the Big George coal, Powder River Basin, Wyoming, USA

Coalbeds are an attractive geological environment for storage of carbon dioxide (CO2) because CO2 is retained in the coal as an adsorbed phase and the cost of injection can be offset by enhanced coalbed methane (ECBM) production. This paper presents the findings of a CO2 storage feasibility study on coalbeds in the Wyodak-Anderson coal zone of the Powder River Basin, Wyoming, USA, using reservoir characterization and fluid flow simulations. A 3D numerical model of the Big George coal was constructed using geostatistical techniques, with values of cleat and matrix permeability and porosity constrained through history-matching of production data from coalbed methane (CBM) wells in the field area.Following history-matching, several ECBM and CO2 storage scenarios were investigated: shrinkage and swelling of the coal was either allowed or disallowed, a horizontal hydraulic fracture was either placed at the injection well or removed from the model, the number of model layers was varied between 1 and 24, and the permeability and porosity fields were constructed to be either homogeneous or heterogeneous in accordance with geostatistical models of regional variability. All simulations assumed that the injected gas was 100% CO2 and that the coalbed was overlain by an impermeable caprock. Depending on the scenario, the simulations predicted that after 13 years of CO2 injection, the cumulative methane production would be enhanced by a factor of 1.5–5. Including coal matrix shrinkage and swelling in the model predicted swelling near the injection well, which resulted in a slight reduction (10%) in injection rate. However, including a horizontal hydraulic fracture in the model at the base of the injection well helped mitigate the negative effect of swelling on injection rate. It was also found that six model layers were needed to have sufficient resolution in the vertical direction to account for the buoyancy effects between the gas and resident water, and that capturing the heterogeneous nature of the coal permeability and porosity fields predicted lower estimates of the storage capacity of the Wyodak-Anderson coal zone.After noting that gravity and buoyancy were the major driving forces behind gas flow within the Big George coal, several leakage scenarios were also investigated, in an effort to better understand the interplay between diffusion and flow properties on the transport and storage of CO2. The modeling predicted that the upward migration of gas due to the buoyancy effect was faster than the diffusion of CO2 and therefore the gas rapidly rose to the top of the coalbed and migrated into overlying strata when an impermeable caprock was not included in the model.