Efficiency of Carbon Dioxide Storage and Enhanced Methane Recovery in a High Rank Coal

The high affinity and adsorption capacity of coal to carbon dioxide provides an alternative approach for the enhanced recovery of methane from unminable coalfields (CO₂-ECBM) by which a potential solution for long-term CO₂ sequestration in deep geological formations can also be achieved. However, due to chemomechanical effects induced by the interactions between CO₂ and coal, the effective methane production and carbon dioxide storage can be degraded which has caused uncertainties about the techno-economic feasibility of the CO₂-ECBM process. This study presents an experimental investigation that aims to address key knowledge gaps related to the efficiency of CO₂ storage and CH₄ recovery in high rank coals for which a comprehensive experimental data set and analysis are largely missing. Competitive displacements of CH₄ with N₂ or CO₂ in an anthracite coal sample from a South Wales coalfield have been studies, based on a series of core flooding experiments. The results show that the N₂ breakthrough time (the time at which 1% of the total gas injected was recovered) was almost spontaneous whereas a considerably delayed breakthrough time was observed for the case of the CO₂-ECBM experiment. In addition it was observed that for the CO₂-ECBM experiment, the ratios of CH₄ recovery with respect to the total amount of gas injected and gas stored were higher by factors of 10 and 2.4, respectively. The results also show that 90% of the total N₂ injected was produced in the outflow gas, whereas for the case of the CO₂ experiment, only 63% of the total injected CO₂ was produced. The presence of a high amount of N₂ in the outflow may lead to additional challenges in order to separate N₂ from CH₄ and thus affect the efficiency of the N₂-ECBM method. Under the conditions of the experiments, the total CH₄ displacement ratio and breakthrough for the case CO₂-ECBM were found to be more favorable compared to those obtained from N₂-ECBM. This study provides new insights into the efficiency of the CO₂-ECBM process and offers a comprehensive experimental data set that can be used for testing the accuracy of predictive models.