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Coalbed methane

Introduction

Coalbed methane (CBM) is a hydrocarbon gas that is chiefly composed of methane and is generated and stored in coal beds in the adsorbed state. Itis a kind of unconventional gas that occurs in coal beds. Here, coal is both the source rock and the reservoir, resulting in no obvious trap definition. CBMis composed of methane (more than 95%), minor amounts of heavier hydrocarbons (mostly ethane and propane), and nonhydrocarbon gases (i.e., N2 and CO2). CBM is mainly distributed at the grain surfaces of coal as the absorbed gas also occurs in cleats and fractures as free gas or gas dissolved in water. As a general rule, the lower the coal rank, the more free gas in the coal bed, and the higher the coal rank, the more adsorbed gas in the coal bed.

Extraction of the CBM

Most CBM wells are relatively low-cost shallow (250–800m) vertical wells commonly run with a 5.5′′ or 7′′ production casing size. CBM wells are commonly tested for the permeability of the target zone in open hole by using packers or a drill stem test (DST) in the open hole before running casing.

To carry out CBM production, a steel-cased hole is drilled into the coal seam and the underground (produced) water is pumped out through tubing. Removal of produced water helps reduce the hydrostatic pressure within the coal bed, causing the gas to be desorbed from its surface. As production occurs, the change in pressure alters the porosity and permeability of the coal bed. The recovered gas is sent into a natural gas pipeline or air compressor system.

Controversy surrounding coal bed methane extraction

The controversy surrounding coal bed methane extraction has to do with the use of water during the process and its impact on global climate change. Coal bed methane exploration involves pumping large volumes of water out of coal seams to reduce thehydrostatic pressure and liberate the gas.

The produced water from coal bed methane wells has a fairly high salinity (due to dissolved sodium bicarbonate ions or chlorides) making it unsuitable for certain agricultural activities due to adverse and long-term chemical or physical effects on soil structure.

Production forecasting

CBM production behavior is complex and difficult to predict or analyze especially at the early stages of the recovery. This is because gas production from CBM reservoirs is governed by complex interaction of single-phasegas diffusion through micropore (matrix) system and two-phase gas and water flow through macropore(cleat) system.

that are coupled through desorption process. Therefore, the conventional reservoi rengineering techniques cannot be used to predict CBM production behavior. The best tool to predict performance CBM reservoirs is a numerical reservoir simulator that incorporates the unique flow and storage characteristics of CBM reservoirs and accounts for various mechanisms that control CBM production. In addition, history matching with simulator is one of the key tools for determining reservoir parameters that are often difficult to obtain by other techniques.