Coalbed methane (CBM) reservoirs produced 1.25 Tcf of gas in 1999, comprising 7% of the total U.S. natural gas production. Cumulative CBM production is more than 8 Tcf and proven reserves exceed 13 Tcf. The total in-place CBM resource in the U.S. is estimated to exceed 700 Tcf, with about 100 Tcf being recoverable with current technology.
Methane gas is generated in coals by two distinct processes. Thermogenic gas is a natural by-product of the coalification process that converts humic organic matter into coal. This gas includes methane, carbon dioxide and occasionally, ethane and propane. Secondary biogenic gas is produced in recent geologic time by anaerobic microorganisms carried in an active groundwater system after the coalification process is complete. Secondary biogenic gas can be generated in coals of any rank if an anoxic, low temperature environment exists, such as in regional coal aquifers.
Both thermogenic and secondary biogenic methane are physically adsorbed as a monomolecular layer on the surface of the micropores within the coal matrix. The methane is held in place by the hydrostatic pressure of the water within the coals. Natural fractures (cleats) within the coal contain water and provide permeability. In a coalbed methane well, water is usually produced first, which results in a reduction of the reservoir pressure. This is the de-watering phase of a CBM well's life. As the pressure declines, methane desorbs from the coal matrix by diffusion, and flows through interconnected cleats by Darcy flow. Consequently, most CBM wells show a negative decline curve for gas with the gas rate increasing with time and the water rate decreasing. Productive life may exceed 40 years. Coals are unique reservoirs because they are the source rock, reservoir and seal (trap) all in one
Coals make excellent reservoirs because their internal surface area can exceed one billion square feet per ton of coal. A ton of high volatile bituminous coal is a cube approximately three feet on a side. In-place gas contents within these coals can range from 200 to 500 standard cubic feet per ton of coal. Reserves can range from one to five Bcf per 160-acre drill block. At reservoir pressures below 1600 psi, coals can hold almost three times as much gas as conventional sandstone reservoirs with 20% porosity and 30% water saturation.
Successful coalbed methane exploration programs must evaluate the following parameters: coal thickness and rank, gas content, permeability, hydrodynamics, gas quality, water quality and water disposal options, depth and potential completion techniques.