Coal Bed Methane
Published on Feb 21, 2020
To meet the rapidly increasing demand for energy and faster depletion of conventional energy resources, India with other countries is madly searching for alternate resources like coal bed methane (CBM), shale gas, gas hydrate. CBM is considered to be the most viable resource of these. The present paper discussed about the prospect of CBM as a clean energy source, difficulty involved in production of CBM, enhanced recovery techniques. In this regards, one Indian coal field is selected and gas content is determined by analyzing the collected samples.
Depletion of conventional resources, and increasing demand for clean energy, forces India to hunt for alternatives to conventional energy resources. Intense importance has been given for finding out more and more energy resources; specifically non-conventional ones like CBM, shale gas & gas hydrates, as gas is less polluting compared to oil or coal. CBM is considered to be one of the most viable alternatives to combat the situation . With growing demand and rising oil and gas prices, CBM is definitely a feasible alternative supplementary energy source. Coalbed methane is generated during coalification process which gets adsorbed on coal at higher pressure.
However, it is a mining hazard. Presence of CBM in underground mine not only makes mining works difficult and risky, but also makes it costly. Even, its ventilation to atmosphere adds green house gas causing global warming. However, CBM is a remarkably clean fuel if utilized efficiently. CBM is a clean gas having heating value of approximately 8500 KCal/kg compared to 9000 KCal/kg of natural gas. It is of pipe line quality; hence can be fed directly to national pipeline grid without much treatment. Production of methane gas from coalbed would lead to de-methanation of coal beds and avoidance of methane emissions into the atmosphere, thus turning an environmental hazard into a clean energy resource. As the third largest coal producer in the world, India has good prospects for commercial production of coal bed methane.
Methane may be a possible alternative to compressed natural gas (CNG) and its use as automotive fuel will certainly help reducing pollution levels. India is one of the select countries which have undertaken steps through a transparent policy to harness domestic CBM resources. The Government of India has received overwhelming responses from prospective producers with several big players starting operations on exploration and development of CBM in India and set to become the fourth after US, Australia and China in terms of exploration and production of coal bed methane. However, in order to fully develop India's CBM potential, delineation of prospective CBM blocks is necessary. There are other measures like provision of technical training, promotion of research and development, and transfer of CBM development technologies that can further the growth of the sector.
PRODUCTION OF GAS FROM COALBED.
A. Gas Transportation mechanism in reservoir: Production of gas is controlled by a three step process
(i) desorption of gas from the coal matrix,
(ii) diffusion to the cleat system, and (iii)flow through fractures  as shown in Fig 5..
Many coal reservoirs are water saturated, and water provides the reservoir pressure that holds gas in the adsorbed state. Flow of coalbed methane involves movement of methane molecules along a pressure gradient. The diffusion through the matrix pore structure, and steps include desorption from the micropores, finally fluid flows (Darcy) through the coal fracture (cleat) system. Coal seams have two sets of mode; breaking in tension joints or fractures that run perpendicular to one another.
The predominant set, face cleats, is continuous, while the butt cleat often terminates into the face cleats. Cleat systems usually become better developed with increasing rank, and they are typically consistent with local and regional stress fields. The size, spacing, and continuity of the cleat system control the rate of fluid flow once the methane molecules have diffused through the matrix pore structure. These properties of the coal seams vary widely during production as the pressure declines. Coal, being brittle in nature, cannot resist the overburden pressure with reduction in pore pressure during dewatering; and fractures are developed. In addition, hydraulic fracturing is done to increase the permeability of coal. Because, permeability and porosity of coal is extremely low for which production rate is also low.
The basic petrophysical properties of coal responsible for production of methane, e.g. porosity, permeability vary widely with change in the pore pressure during dewatering as well as gas production period. Hence, efficient production of methane from coal bed needs continuous monitoring of variation in porosity, permeability and compressibility of coal. The unique features of the coal are that coals are extremely friable; i.e., they crumble and break easily. Therefore, it is nearly impossible to recover a “whole” core. Direct measurement of intrusive properties like permeability, porosity, compressibility, relative permeability measurements are very difficult and must rely on indirect measurement.
B. Enhanced recovery techniques:
The main hurdle associated with the production of CBM is the requirement of long dewatering of coal bed before production. This difficulty may be resolved to some extent with implementing the CO2 sequestration technology. Due to higher adsorption affinity of CO2 to coal surface , methane will be forced to desorb from the coal surface at comparatively high pressure and can reduce the dewatering time and hence the total project period. Also the problem associated with variation in coal properties related to pressure depletion may be alleviated. China, Australia, USA have been started to implement this technology for enhanced recovery of CBM gases.
CBM technology is proceeding with good space to prove itself as a cleaner energy security to India as well as the World. However, production strategy of methane from CBM is very much different from conventional gas reservoir. The study revealed that the coal type, rank, volatile matter and fixed carbon are strongly influence the adsorption capacity of methane into the coal bed. With increasing depth maturation of coal increases and generation of methane gas also increases. Gondwana basin as the most prospective CBM field is being developed now. From the studies, it is observed that Singareni coal field under Gandowana basin contains low gas Hence, presently it is not considered for CBM exctraction.
However, in future this field may be considered for methane extraction using advanced technology and in emergency condition. Sequestration of CO2 helps in mitigation of global warming, at the same time helps in recovery of methane gas from coal bed unveiled otherwise. However, detailed and intensive studies are required for efficient and economic production of coal bed methane. India with ~4.6 TCM of methane reserves in coal bed can enrich its per capita energy demand by successful exploitation of CBM.
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