CBM Dewatering

The coalbed methane industry has become an important source of natural gas production. Proper dewatering of coalbed methane (CBM) wells is the key to efficient gas production from these reservoirs.



Coalbeds are naturally fractured, low pressure, water saturated gas reservoirs. The mechanism by which gas is stored and produced in coalbed reservoirs is quite different from sandstone reservoirs. In a conventional sandstone reservoir, gas is stored in the pore space and flows through the pores into the fractures and/or the wellbore In a coal-seam reservoir, while some free gas may exist in the coal deposits, the majority of the gas is adsorbed on the surface of the coal matrix. To produce this gas, the reservoir pressure must be reduced so that the gas can be desorbed and released from the matrix into the fractures. The gas can then migrate through the fractures and coal cleat system and flow into the wellbore.


Initially the natural fractures of the coal are typically water saturated. This water has to be removed in order to achieve any significant gas production. Dewatering of the coal seam reduces the hydrostatic pressure of the reservoir, which allows the gas to be desorbed from the coal matrix. At the same time, lowering the water saturation level of the reservoir increases the relative permeability of gas, thereby permitting the desorbed gas to flow to the wellbore. The maximum gas production is achieved when the BHP is minimized.


A proper dewatering system is essential for the production of methane from most coalbeds.