Coal Bed Methane Extraction - Basic Principles

Basic Principles

The methane is adsorbed into the solid coal matrix (coal macerals) and is released when the coal seam is depressurised. To economically retrieve reserves of methane, wells are drilled into the coal seam, the seam is dewatered, then the methane is extracted from the seam, compressed and piped to market. The goal is to decrease the water pressure by pumping water from the well. The decrease in pressure allows methane to desorb from the coal and flow as a gas up the well to the surface.

While dewatering is occurring, the operator should make sure that the pumpjack is not running too long. If the water level is pumped too low, this will allow the gas to travel up the tubing into the water line, causing the well to become "gassy". The main objective is not to put the gas in the water line, but to allow it to flow up the backside of the well (casing) and into the pipeline, where it can be transported to the compressor station and delivered to the customer for sales. Once the gas goes up the tubing, it is usually recovered in a water-gas separator at the surface. However, pumping water and gas is inefficient and can cause pump wear and breakdown.