by Malti Goel
Financial Express; Monday, July 25, 2011
Clean energy fuel is much in demand today. Natural gas has become a topic of debate as it is a clean and green fuel. It is mainly methane recovered from underground reservoirs and oil fields as associated gas. Combustion of natural gas gives rise to least emission pollution among fossil fuels.
Earlier, gas pipeline infrastructure did not exist and it was not possible to utilise the gas associated with oil. Long after technology has developed to separate natural gas produced from oilfields, efforts have begun in India to recover methane from coal beds. Coal bed methane is methane recovered from un-mineable coal beds. A large amount of methane gas gets associated with coal during coalification, some of it gets released during mining activity.
The CBM resource is estimated to be more than 1,000 trillion cubic metres. Coal mine methane (CMM), which is recovered from active mines, and abandoned mine methane (AMM) recovered upon closure of an active mine form additional resources. Several techniques have been put forward to recover methane stored in coal beds. First commercial CBM recovery was made by Great Eastern Energy Corporation Ltd (GEECL) in Raniganj coal fields, near Burnpur in West Bengal. A 70-km CBM pipeline has been laid for industry units in the coal belt. Success has been achieved in recovery of CMM. Coal India has ventured into CMM recovery in a big way.
At the same time, there are international concerns for carbon accumulation in the atmosphere from the use of fossil fuels, mainly coal combustion, giving rise to global warming mitigation actions. Carbon capture & storage (CCS) is one such pathway to reduce emissions in the atmosphere. It is based on capturing carbon from its large point sources and safely storing it in underground reservoirs so that it is permanently removed from the atmosphere. CCS is considered significant for oil and gas industry for enhanced oil recovery. Worldwide, efforts have been made to test carbon sequestration for enhanced oil recovery from depleted oil fields. In the same way, un-mineable coal beds are considered for enhanced coal bed methane (ECBM) recovery.
In this process of carbon sequestration carbon dioxide molecules get attached to coal and the trapped methane molecule is released. Coal beds have absorption capacity for carbon dioxide which is three times that of methane. In unmineable coal beds or in an abandoned mine, carbon dioxide sequestration by pumping flue gas (containing carbon) from a power plant into coal seams under pressure is being considered a possibility. It is estimated that primary CBM recovers 20-40% gas of what is in place, while injection of carbon can lead to 70-90% recovery of gas. Theoretically, 7.1 billion tonnes of carbon stored can produce about two billion cubic meters of gas. This would certainly ease the energy situation.
Coal resources that are too deep or too thin for mining can be considered as a carbon sequestration target. Several medium-to-large scale field tests have been reported worldwide. San Juan field of the USA has shown a carbon:methane ratio of 3:1 i.e., three molecules of carbon would displace one molecule of methane.
India is coal rich country, third in resources and fifth in production. While a new thrust is mounted on harnessing renewable energy sources, dependence on coal continues to grow. The CCS research and capacity building efforts have begun. India has huge Gondwana resources mainly Permian – 99.5% and remaining Tertiary. A few institutions in India have carried out research in ECBM. Classification of potential coal beds has been done for carbon dioxide storage in unmineable coal beds, grey areas and concealed beds. According to their preliminary assessment, potential sites of carbon dioxide storage exist in several coal beds in unmineable coal beds and grey areas. The contribution of different absorption mechanisms and chemical parameters, however, remain poorly understood and field testing is yet to begin.
Even though limited field experience exists worldwide with ECBM, should not India consider tapping the existing ECBM potential? Questions like whether carbon dioxide will remain entrapped and for how long and what coal types are suitable need further investigation. More studies on geological, geo-morphological and physico-chemical studies of reservoirs and cap rocks are needed. Certainly, ECBM in unmineable coal beds can produce more fuel synergy with carbon sequestration. Moreover, its recovery is feasible at shallower depths as compared to shale gas, which requires cost-intensive deeper drilling technology coupled with horizontal cutting techniques. It would be relevant to make investment to test the efficacy of ECBM in coal fields simultaneously, as the capabilities in CBM recovery or in shale gas are being developed.