• journal_title：AAPG Bulletin
• Contributor：Xiaojun Cui ; R. Marc Bustin ; Gregory Dipple
• Publisher：American Association of Petroleum Geologists (AAPG)
• Date：2004-
• Format：text/html
• Language：en
• Identifier：10.1306/02230403047
• journal_abbrev：AAPG Bulletin
• issn：0149-1423
• volume：88
• issue：8
• firstpage：1149
• section：Articles

Coalbed gas content and composition are critical for the successful exploration and production of coalbed methane. We investigated the differential transport of a preadsorbed CO₂-CH₄ gas mixture along a coalbed aquifer by groundwater flow or across a coal bed by upward diffusion through pore water. Consistent analytical approximations and numerical solutions were obtained for typical cases. The results suggest that differential transport of CH₄ and CO₂ in a water-saturated coal seam is mainly controlled by their adsorption equilibrium in coals and solubilities in water. Although CO₂ is about 20 times more soluble than CH₄ in water at temperatures lower than 50°C, the transport of CO₂ from a coal seam is only several times more efficient than that of CH₄ because of the stronger adsorption of CO₂ than CH₄ in coals. Preferential transport of CO₂ over CH₄ by groundwater advection can substantially accumulate more gas rich in CO₂ at downstream or near discharge zones if gas-trapping structures exist, and it may cause low total gas content of CH₄ near recharge zones, which may explain the CO₂ and CH₄ distribution in the San Juan and Powder River basins and the undersaturation of gas in many other coals. The separation and redistribution of coalbed gas by transport processes may also be complicated by late-stage biogenic gas generation with infiltration of meteoric water with nutrients and gas-producing microbes into the coal beds as suggested for some basins. Overall, hydrogeologic systems strongly influence the distribution of coalbed gas content and composition in coal seams.