Review on Coal Bed Methane as Natural Gas

Abstract

Coal Bed Natural Gas (CBNG) also referred to as Coal Bed Methane (CBM) is an unconventional natural gas resource of energy. Coal plays the role of source rock as well as reservoir for coal bed methane. CBNG is the gas found in coal deposits. It consists mostly of methane but may also contain trace amounts of carbon dioxide and/or nitrogen. Most coalbeds are permeated with methane, and a cubic foot of coal can contain six or seven times the volume of natural gas that exists in a cubic foot of a conventional sandstone reservoir. Within coal seams, methane is present on the surface of the solid material. Hydrostatic pressure causes the methane to adhere to the coal surface via a phenomenon termed adsorption. Whenever reservoir pressure is reduced, the methane desorbs off of coal surfaces, diffuses through the matrix material, and then flows through a system of natural fractures (cleats) and into a well for delivery to the surface. Coalbed natural gas is either biogenic or thermogenic in origin. Biogenic methane is generated from bacteria in organic matter and is typically a dry gas. Thermogenic methane forms when heat and pressure transform organic matter in coal into methane. This type of methane is typically a wet gas. CBNG-related research focuses on the potential for enhanced gas recovery and carbon sequestration as an integrated operation. CO2 storage is feasible because coal preferentially adsorbs CO2 at twice the volume that it stores methane. The net result would be less CO2 in the atmosphere and additional recovery of sorely needed natural gas.