Sample News Story
How Coal-Bed Methane is Found and Produced - Gas Matters
28/12/1999
Except for a small amount of free gas occasionally found in the cleat or fracture system of a coal seam, coal-bed methane (CBM) exists as a layer adsorbed on the surface of coal and held there by pressure. The sorption process results from an attraction between the surface area of coal micropores and gas molecules, and coals vary in their adsorptive capacity.
Methane is desorbed and released from coal surfaces following the drilling of a well which decreases coal-bed pressure, and the continuous removal of water from the fracture system of the coal bed serves to lower pressure further. The freed, desorbed methane moves along existing and newly created coalbed fractures until it reaches production wells which provide a conduit by which it can rise to the surface.
Wells generally are produced for a month or two to drain water from the region near the well bore before beginning hydraulic fracturing – the injection of water or other fluids under very high pressure and speed to induce new fractures in the coal for the passage of methane. In some sites, such as those in the central San Juan Basin, this case-and-frac method does not work well and cavitation, an open-hole completion method, is preferred. Some 80% of the cumulative production of a CBM well occurs within 10 years, though it is highest in the first few years.
Enhanced recovery techniques are being tested in some mature CBM operations. A BP Amoco nitrogen injection enhanced CBM recovery pilot project launched in 1998 in the San Juan Basin resulted in a five-fold increase in production levels within 14 months. Another technique under trial is the injection of carbon dioxide.