Influence of sorption time in CO_2-ECBM process in Indian coals using coupled numerical simulation

摘要

Production of coal-bed methane from a reservoir is a function of several parameters, including in situ gas content, the permeability of the coal, and the thickness of the coal seams. Such coal seams usually have low sorption time and there is an easy release of methane from coal upon pressure depletion due to water extraction. Coals with high sorption time are usually not suitable from an economical point of view. This study investigates the role of sorption time in the production behavior of coal under carbon dioxide injection using numerical simulation. A thick coal seam at an intermediate depth of 1600 ft was modeled with two production wells and one injection well between them. Sorption time was varied and the water/ methane production and CO_2 injection behavior were monitored up to a period of 4000 days. It was found that coals with non-equilibrium sorption time have high CO_2 adsorption capacity. Therefore, they can be considered for the enhanced recovery of methane with gas injection. A large quantity of water is released from this type of coal until the start of methane desorption, and despite CO_2 injection the onset of gas release remains delayed. At the end of the first year, a reduction of nearly 50% water production was computed for coal with sorption time τ= 0.1 day, while water release reduced by only 23.5% for coal with τ= 50 days. The rate of CO_2 injection after six months duration increased to 41.6 mscfd in the case of high sorption time coal, while it rose to only 20 mscfd for low sorption time coal, indicating almost double the rate of gas injection in the former case. The first year methane production from a coal with τ= 0.1 day was 90 mscf, and that for τ = 300 days was 42 mscf. At the end of the fifth year, the cumulative gas production was 842 mscf and 613 mscf respectively for the respective varieties, showing that the difference slowly reduced. Possible mechanisms to understand the behavior of coals with different sorption times are proposed. It is also established that coals with sorption time less than 10 days follow an equilibrium trend in typical Indian Gondwana settings.