Enhanced Coalbed Methane Recovery Using CO2 Injection: Worldwide Resource and CO2 Sequestration Potential

Injeetion of carbon dioxide into deep coal seams has the potential to enhance coalbed methane recovery, while simultaneously sequestering a greenhouse gas. Analysis of production operations from the world’s fwst carbon dioxide-enhanced coalbed methrme (CO,-ECBM) pilot a 4-injector/7-producer pattern in the San Juan Basin, indicates that the process is technically and economically feasible. To date, over 2 Bcf of CO, has been sequestered with negligible breakthrough. Enhancement of gas production can be as high as 1500/0over conventional pressure-depletion methods. Dewatering of the reservoir is also improved. ECBM development may be profitable in the San Juan basin at wellhead gas prices above $1.75/Met adding as much as 13 Tcf of additional methane resource potential within this mature basin. The key reservoir screening criteria for successful application of C02-ECBM include lateralIy continuous and permeable coal seams, concentrated seam geomeby, and minimal faulting and reservoir compartmentalization. Operational practices for CO,-ECBMreccwery are stiIl being refined. Injection wells should be completed unstimulated, while production wells can be cavitated or hydraulically stimulated. COa injection should be continuous and concurrent with methane production to prevent lateral water encroachment. Apart from the San Juan basin, many other coal basins have signi.tlcant C02-ECBM potential. In the U. S., the Uinta and Raton basins are geologically most favorable, while additional potential exists in the Greater Green River, Appalachian and other coal basins. Coal basins in Australia, Russia, China, India, Indonesia and other countries also have large COZ-ECBM potential. When viewed fi-om a commercial project viewpoint, the total worldwide potential for C02-ECBM is estimated at approximately 68 Tcf, with about 7.1 billion metric tons of associated COZsequestration potential. If viewed purely as a non-commercial COZ sequestration technology, the worldwide sequestration potential of deep coal seams maybe 20 to 50 times greater. Introduction Coalbed methane (CBM) has beeome a significant component of U.S. natural gas supplies. CBM production grew to 2.9 Bcfd of gas supply during 1997, accounting for about 6’70of total U.S. natural gas production. 1 Essentially all CBM operations still employ prhmiry recovery methods, generally by pumping off large volumes of formation water to lower reservoir pressure and elicit methane deso@ion fi-om the coal. Primary production of coalbed methane recovers onIy 20°/0 to 60°/0 of original gas-in-place, depending on coal seam permeability, gas saturation, and other reservoir properties. Well spacing and other operational practices also will tiect recovery efficiency. Primary recovery thus bypasses a sizeable gas resource. For example, we estimate that primary production in developed areas of the San Juan basin alone may leave behind as much as 10 Tcf of natural gas in areas with completed coal seams. New technologies have been proposed for enhanced coalbed methane recovery (ECBM) to recover a larger &action of gas in place. The two principle variants of ECBM are 1) inert gas stripping using nitrogen injection and 2) displacement resorption employing carbon dioxide injeetion. Simulation and early demonstration projects indicate that N2-ECBM is capable of recovering 90% or more of gas in place, at an average incremental capital and operating cost of about $ 1.00/Mcf. 2 The C02-ECBM process is less well documented but likewise shows signit3cant promise for enhanced coalbed methane recovery. For the past three years, Burlington Resources, the world’s larges~ producer of eoalbed metlume, has been operating an 11-well COZ-ECBM pilot in the San Juan basin. Initial results show improvement in methane recovery in some wells with minimal breakthrough of COZ. However, due to the complex operational history of this pilot, this