Coal Bed Methane (CBM)
Coal Bed Methane, commonly known as CBM, is the entrapped methane gas found in underground coal seams. Methane is produced when organic material is geologically turned into coal and found in the adsorbed form in the coal itself. The gas is extracted using a pressure drop technique and is widely used for industrial and utility purposes.
Understanding CBM in Detail:
CBM is an unconventional gas as it is different from the conventionally found natural gas based on the nature of accumulation. Natural gas is accumulated into the pores of naturally occurring reservoir rocks whereas, CBM is found on the surfaces of coal seams in the adsorbed state.
The gas is formed as a result of coalification which is a process of conversion of plants into coal. During coalification, the coal becomes saturated with water trapping methane gas into the facets of coal. CBM mainly contains methane gas (CH4) along with some traceable amounts of ethane (C2H6), carbon dioxide (CO2) and water (H2O).
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As per a report published by USGS, a sample of CBM in Virginia contains around 96.6 per cent of methane and has a calorific value of around 990 Btu/cubic feet.
How is CBM formed?
CBM is formed along with the formation of coal. The organic matter gets deposited in swampy lakes and undergoes various stages of bacterial and chemical alterations to form peat. Induced pressure and temperature on buried peat further converts it to lignite, sub-bituminous, bituminous and anthracite, over a period of time. The entire conversion process may take millions of years to convert Peat into Anthracite. During the transformation process, the decomposition of organic matter gives rise to methane gas along with nitrogen and carbon dioxide.
Where is CBM found?
CBM is found as an associated gas with Coal deposits along the facets of the coal seams. Previously, due to limited technological support, the extraction and usage of CBM were not possible, which was the leading cause of undermining explosions and accidents. With the gradual advancement in technology, the gas first started to be vented out from the mines. More recently, with further advanced technology CBM is targeted as an unconventional source of energy to fulfil the rising energy demands worldwide.
CBM Extraction Process:
Due to the challenging extraction process in contrast to conventionally deposited Natural gas, CBM is considered as an unconventional source of energy. The method of extraction of CBM is much similar to the extraction of shale gas.
A horizontal well is drilled conventionally as applicable in the case of oil extraction. Casing, cementation followed by perforation is carried out to allow methane gas flow into the well and ultimately to the surface. Usually, coal seams are also covered in water which must be pumped out to obtain the methane. Water removal lowers the reservoir pressure, and entrapped methane gas tends to follow the path of the water to the surface. The gas mixed in water is separated at the surface and transported to gathering station and then shipped to storage facilities through pipelines or tankers.
Uses of CBM:
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CBM can be used in for various purposes like in CNG station for fueling vehicles, in power plants for the generation of electricity, as feedstock for the manufacturing of fertilizers, in petrochemical product generation, in rolling mills and for cement production.
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Environmental Concerns in CBM Production:
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The issue around coal bed methane extraction is related to the water drawn out during the CBM extraction process and its effect on global climate change. CBM extraction incorporates removal of a huge amount of water present along the coal seam, which is responsible for maintaining the hydrostatic pressure and holding CBM within the seams. Removal of water releases the pressure and gas starts flowing out.
The pumped water from CBM wells has high salt content making it unacceptable for certain agrarian use due to long term adverse effects on soil. Generally extracted water is re-injected into subsurface formations, and sometimes it is allowed to flow to the surface to drain or put in ponds to evaporate. Produced water contaminates and deteriorates the soil & surface water quality along with groundwater table.