Enhanced Gas Recovery and CO2 Storage in Coal Bed Methane Reservoirs with N2 Co-Injection

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Field demonstrations of carbon dioxide (CO2) storage in coal seam reservoirs have suffered greatly from lost injectivity due to coal swelling. The swelling of the coal matrix, due to preferential adsorption of CO2 as compared to methane (CH4), causes a reduction in the coal cleat porosity and a corresponding exponential reduction in cleat permeability, which negatively impacts CO2 injection and, as a result, CO2 storage. This might be the greatest technical hurdle to the commercial deployment of CO2-enhanced coal bed methane (ECBM) and storage operations. To mitigate the impact of CO2 swelling, this work will demonstrate that the co-injection of nitrogen (N2) may offset the swelling effects presented by the CO2 and consequently improve the utilization of injected gas in terms of incremental methane recovery, and determine the overall storage potential that these coal bed reservoirs may provide.Previous work employing Monte Carlo probabilistic techniques emphasized the impact of pressure dependent permeability which controls the flow, and Langmuir isotherms which control the coal storage capacity, on the storage and ECBM processes. These key parameters were thoroughly studied and defined on a coal rank basis. A parametric study was employed and conducted through reservoir simulation of various injection cases. The results show that low rank coals, thanks to their high porosity, need a minimum amount of N2 to reach optimum conditions. On the other hand, high rank coals, due to their high pore and matrix compressibility, require much more N2 to reach optimum conditions. The optimum methane recovery and sequestration conditions for medium rank coals were found to be highly sensitive to initial porosity values.Economic assessments were conducted only for the medium rank coals to explore the impact of costs on the engineered injection stream. This was done for the low (0.25%) and high (1%) porosity cases. For high porosity cases, the economics confirmed the findings from the simulations requiring less N2 in the optimized injection stream (0 to 20%) to achieve a positive net present value. For the low porosity cases, the gas price had to be set to $6/Mcf to force any case to screen economic. At this setting, only a 60% N2/40% CO2 composition was found to be economic.