Coal Bed Methane
Coal Bed Methane (CBM) The methane gas that occurs within the coal beds lying underground. CBM is considered a non-conventional type of natural gas although it is virtually identical to conventional natural gas produced worldwide. The mode of occurrence of CBM differs from conventional natural gas in that it mainly occurs (90%) as adsorbed onto the surface of coal material with the remaining (10%) held as free within coal fractures. Thus the CBM is sourced and reservoired by the same rock ie coal. Historically, CBM has been considered a safety hazard in the underground coalmines, but due to current technological development and economics, it has now become a significant energy resource. CBM can be extracted through drilling into coal without the coal being mined, thus providing alternate natural gas reserves.
Till now the majority of the large scale commercial production centres are located in USA and these are being developed in some other countries as well, mainly Australia, Poland, China and Russia. In USA, CBM production has increased from about 0.30 TCF (trillion cubic feet) in 1991 to 1.1 TCF in 1997, the latter accounting for about 6% of the total annual natural gas production of the country. In eastern India, the CBM resource base for five Damodar valley coalfields (including Raniganj and Jharia in West Bengal-Bihar) has been estimated to be 40 TCF with 20-25% recoverability.
The most important parameters for CBM prospecting is the presence of large amounts of coal, high methane content in the coal and sufficient permeability within the coal seam to allow gas production. Geologic targeting of thick coal seams at appropriate depth and with optimum permeability is therefore crucial in any CBM exploration program.
Prospecting for CBM in the coalfields in the northwestern part of Bangladesh could be an option for closing the gap in energy availability between the eastern and western parts of the country. All the gasfields so far discovered in Bangladesh lie in the eastern part and the geological prospect of finding any significant conventional gasfield in northwestern Bangladesh is considered to be low. On the other hand the CBM prospect is high in some coalfields specially the ones not suitable for conventional coal mining for great depths.
The Jamalganj coalfield in the Joypurhat district is one, for example, which may well be taken as a test case. The positive factors for Jamalganj CBM development include large coal reserves (1,053 million ton), thick coal seams (as much as 46m thick individual coal seams and up to 64m cumulative coal in seven coal seams), optimum depth of burial (640 to 1,150m below surface) and indication of significant gas content as reported by the observation of gas bubbling from mud during drilling.
International CBM prospectors have shown interest in the feasibility studies for CBM development in Jamalganj coalfield. In early 1990,s BHP_UTAH International inc. submitted a report' to the government and proposed for undertaking' exploration and developemet of CBM in Jamalganj coalfield.but there was no report of a positive negotiation between the company and the government. Up until now there has been no investigation carried out on the jamalganj coal aimed for CBM development.Thaere are a number of unknown factors that are to be determined such as true gas content, coal permeability and in-seam pressure before the coal is taken for CBM development. However such prospecting could bring about a new era of non-conventional natural gas exploitation in Bangladesh. [Badrul Imam]