Have you heard about Coal Bed Methane (CBM)? Do you already know it? Well, CBM is simply defined as a form of natural gas collected from drilling to unmined coal bed. It mainly consists of 90% methane so that it called Coal Bed Methane rather than Coal Bed Gas. CBM is quite different with Coal Mine Methane (CMM), because CMM is gas that occurs in ‘disturbed’ coal bed and released as the side effect of coal mining process.
Why does CBM exist?
Methane can occur in coal bed by two steps: biological processes and thermal process. Biological process involves microbacterial activities under low oxygen condition. In this process, anaerobe bacterias are using hydrogen to transform acetate acid and carbon dioxide into methane. The result of this process is biogenic gas. The second process need rising of pressure and temperature. Those condition can be obtained by increasing of burial depth. At the end, this process produces thermogenic gas.
Where does CBM take place?
The occurrence of CBM in coal bed is different based on its conditions. The conditions are also related to migration process of CBM from tiny pore into fracture network (see Fig 1).
Fig 1. Migration process that is also related with CBM occurences
- Free Gas
Gas generated in excess of what which can be adsorbed on the coal surfaces will be ‘free’ gas within the porosity of the coal, commonly in the fracture porosity. This gas is available to be dissolved in groundwater moving through the coal. The CBM saturation of large volumes of water can remove large volumes of gas from the coal seam(s) which will be lost to the system and possibly vented to the atmosphere. The fracture porosity in coal is primarily due to the formation of fracture called cleat. Cleat is a joint or set of joints perpendicular to the top and bottom of the coal seam (see Fig 2). Usually there are two cleat sets developed in an orthogonal pattern. Cleat is a major control on the directional permeability of coals (Thomas, 2002).
Fig 2. Coal cleat terminology
- Adsorbed Molecules
The primary mechanism of CBM retention in coal beds is adsorption on the coal surface within the matrix pore structure. Adsorption is on an open surface, and there is no resistance to gas access to adsorption sites. The most commonly used equation to describe the adsorption of gases on a solid is that of Langmuir, who developed the theory in 1918. The major assumptions in deriving the equation are as follows:
- One gas molecule is adsorbed at a single adsorption site.
- An adsorbed molecule does not affect the molecule on the neighboring site.
- Sites are indistinguishable by the gas molecules.
Pressure is one of the important role in adsorption. As pressures in coalseams increase with depth or with the hydrostatic head of water, the capacity of the coal for adsorbing more methane improves. It is also evident that present gas content of a coal may have been set by some previously lower or higher pressure in geologic time and that current depth may be misleading in estimating gas content. A number of fractures influence the ability to utilize pore surface for CBM adsorption. The size of the aperture accessing the pore surfaces, the variation in moisture content and/or the degree of coalification may alter the surface areas of a coal. Surface area is also related to the carbon content. Porosity may also depend on the maceral content in high volatile bituminous coals.
- Dissolved in Groundwater within the Coal
Free gas which occurs in fractures will be dissolved in the groundwater that moving through the coal seam. Large volume of groundwater that contains CBM will remove large volume of gas in coal seam.
CBM Potential in Indonesia
Based on Indonesian Sediment Basin Map published by the Indonesian Geological Survey in 2009, the territory of Indonesia has about 128 Tertiary and Pre-Tertiary sedimentary basin. Based on the stratigraphic of Tertiary sedimentary basin, there are 27 basins considered as coal basin, Potential coal basins are mostly found in the western region of Indonesia, especially in Sumatra and Kalimantan. In 2003, Advanced Resources International, Inc. (ARII) Arlington, Virginia, USA, conducted a study on 11 coal basins. ARII estimate the potential of CBM in Indonesia (11 basins) is about 453.3 Tcf (see Fig 3).
Fig 3. CBM Potential Resources in Indonesia (Advanced Resources International, Inc., 2003)
Stevens, S. H. & Hadiyanto. 2004. Indonesia: Coalbed Methane Indicators and Basin Evaluation. Australia: SPE Asia pacific Oil and Gas Conference.
Sukhyar R., et al. 2012. Potensi dan Pengembangan CBM Indonesia. Jakarta: Badan Geologi, Kementerian Energi dan Sumber Daya Mineral.
Thomas, Larry. 2002. Coal Geology. West Sussex: John Wiley & Sons Ltd. 261p
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- Khairani Al Katiri
- Syayidu Guntur Ma’arif