Simulation of CO2 sequestration in coal beds: The effects of sorption isotherms

For over 30 years, horizontal wells have been drilled into coal seams to release trapped methane and improve mine safety. For more than two decades, significant quantities of gas sorbed in coal seams have been collected as a relatively environmentally friendly fossil fuel energy resource. Laboratory experiments have shown that coals preferentially sorb carbon dioxide. Thus, concomitant enhanced coal bed methane production and carbon dioxide sequestration in unminable coal seams is a promising technology being developed as a win-win process to reduce global warming and produce a valuable energy resource. However, because CO2 will not reach all portions of the seam, not all of the in situ methane will be produced and not all of the “theoretical” sequestration capacity will be utilized. For sequestration, the amount of carbon dioxide that could be stored in the coal seam was found to be between 50% and 70% of the thermodynamic limit. The fraction of methane produced was much higher, between 80% and 97%. Reservoir simulations were used to predict how the well pattern and operating conditions can be modified to maximize the amounts of CO2 stored and CH4 recovered. For this study, we used the PSU-COALCOMP compositional coal bed methane reservoir simulator and measured sorption isotherms to predict the maximum amount of carbon dioxide that could be sequestered in a coal seam and show how coal seam characteristics and injection practices will reduce the actual amount sequestered.