SECONDARY RECOVERY FOR THE OPTIMIZATION OF COAL BED METHANE (CBM) PRODUCTION. A SIMULATION STUDY

The CBM industry is exploring new methods of enhancing the gas production. This is done by experimenting with the injection of nitrogen (N2) and carbon dioxide (CO2) into the coal bed in order to displace methane along the coal face cleats. The gas exchange results in incremental methane production rates as compared to just decreasing the hydrostatic pressure like the primary recovery process.
Coal can replace 25% to 50% of methane gas storage capacity with nitrogen and studies indicate that for each volume of nitrogen injected will result in two volumes of methane produced. While for carbon dioxide, laboratory studies shows that coal can adsorbs nearly twice volume of CO2 than methane. However, different rank of coal will have different acceptance towards gas injected as pressure dependent parameters also vary relative to the rank. Therefore, this paper will studied on the effect of different composition of both gases that can enhance the CBM recovery for different rank of coal.
The first part of the research shows that for all injection composition, medium rank gives the highest methane production which proves that medium rank coal is the best coal rank for CBM process. As the research goes by it shows that as coal rank increases, more nitrogen or low carbon dioxide is needed to enhance the CBM recovery which this is due to the pressure dependent parameter varies between coal ranks.