Gas saturation and CO₂ enhancement potential of coalbed methane reservoirs as a function of depth

The influence of moisture, temperature, coal rank, and differential enthalpy on the methane (CH₄) and carbon dioxide (CO₂) sorption capacity of coals of different rank has been investigated by using high-pressure sorption isotherms at 303, 318, and 333 K (CH₄) and 318, 333, and 348 K (CO₂), respectively. The variation of sorption capacity was studied as a function of burial depth of coal seams using the corresponding Langmuir parameters in combination with a geothermal gradient of 0.03 K/m and a normal hydrostatic pressure gradient. Taking the gas content corresponding to 100% gas saturation at maximum burial depth as a reference value, the theoretical CH₄ saturation after the uplift of the coal seam was computed as a function of depth. According to these calculations, the change in sorption capacity caused by changing pressure, temperature conditions during uplift will lead consistently to high saturation values. Therefore, the commonly observed undersaturation of coal seams is most likely related to dismigration (losses into adjacent formations and atmosphere). Finally, we attempt to identify sweet spots for CO₂-enhanced coalbed methane (CO₂-ECBM) production. The CO₂-ECBM is expected to become less effective with increasing depth because the CO₂-to-CH ₄ sorption capacity ratio decreases with increasing temperature and pressure. Furthermore, CO₂-ECBM efficiency will decrease with increasing maturity because of the highest geochemistry of petroleum and coal sorption capacity ratio and affinity difference between CO₂ and CH₄ for low mature coals.