Coalbed methane is an unconventional gas resource that consists of methane production from the coal seams. The key parameters for the evaluation of coalbed methane (CBM) prospects are the gas resources, reserves and deliverability. Coalbed methane reservoirs are dual-porosity media where the vast majority of the gas is stored in the low permeability coal matrix (primary porosity) by sorption. The flow to production wells, however, occurs through the co a l"s n a tu ra l fra ctu re syste m (secondary porosity), which stores relatively small amounts of gas, because coal matrix practically has no permeability. For the gas to be released from the coal, its partial pressure must be reduced, and this is done by removing water from the coalbed fractures. During the dewatering process, the gas desorbs from the coal matrix, thereby gas rate increases and the water saturation decreases. The water production declines rapidly until the gas rate attains a peak value and water saturation approaches the irreducible water saturation i.e., reaches connate water saturation. Once the peak gas rate is attained, CBM reservoirs act like a conventional reservoir. Reservoir engineers usually use production decline curves in order to predict well performance.
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