Description of a CO2 enhanced coal bed methane field trial using a multi-lateral horizontal well

Enhanced recovery of coal bed methane by CO2 injection (CO2-ECBM) offers the potential of increasing recovery of the gas in place relative to primary recovery methods while storing CO2. This paper describes a CO2-ECBM field trial that used a multi-lateral horizontal well with 2.3 km in seam length for injection. The trial, performed in China's Shanxi basin, involved transport of liquid CO2 to the injection site and pumping of this directly into the injection well. A u-tube sampling system was installed in a monitoring well 25 m from the main horizontal branch close to the vertical section of the injection well. This u-tube system comprised three intervals separated by inflatable packers from which gas and water samples were automatically collected and recovered to an on-site field laboratory for gas analysis. The middle interval of this packer assembly was positioned to collect fluid samples from the coal seam targeted for ECBM. At the start and towards the end of the period of CO2 injection, a pulse of non-adsorbing tracer gas was added to the CO2. There was clear breakthrough of the tracer in the middle packer interval of the monitoring well, demonstrating the good connection between the injection and monitoring wells. The CO2 composition of the gas sample from the coal seam gradually increased over time as injected CO2 migrated to the monitoring well and reached a maximum of 12%. Coal permeability, and thus CO2 injectivity, has been observed to decline during other CO2-ECBM field trials as higher adsorbing CO2 displaces reservoir methane. However in this trial there was no clear trend of decreasing injectivity with time, possibly due to the flow behaviour in the long horizontal well and the short periods of CO2 injection.