AN OVERVIEW OF THE POTENTIALS AND PROSPECTS OF COALBED METHANE EXPLORATION AND EXPLOITATION IN THE PERMO-CARBONIFEROUS COAL MEASURES OF THE BARAKAR FORMATION, JHARIA BASIN, INDIA

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Abstract

The Permian Barakar coal formation of the Jharia Basin, Damodar valley, India has a very active role to play in the future energy scenario of India. However, in general coal seams are poorly understood with respect to their behaviour as a gas reservoir. Gas is predominantly stored in an adsorbed and a compressed state. For the exploration and assessment of coalbed methane it is important to understand the mechanism of adsorption, retention and accumulation of methane gas within seams, during the coalification. The gas storage capacity of a saturated coal seam varies non-linearly as a function of pressure, as described by Langmuir (1916). The Langmuir constants, that define the gas storage as a function of pressure for saturated coals, are measured in the laboratory providing an adsorption isotherm. At low pressure, the relationship between storage capacity and pressure is linear as in Henry’s Law isotherm. It provides predictive information into “Gas Storage Capacity�? and “Recovery Factor�?. A study on the sorption characteristics of seams from the Jharia basin, India, led to the generation of an adsorption isotherm. The approach adopted here for CBM reserve prediction, uses a modified mass balance and a field example of the coal seams of Barakar Formation. The methods presented here use the well-documented gas storage records and production history of the reservoir, to estimate the ultimate reserve recovery for the next twenty years. The objective can be achieved by optimising completion design, well spacing and simulation designs. Each of these optimisations requires accurate prediction of long-term well production. In this work two techniques are discussed, to estimate coalbed methane well production; a production decline technique and a material balance and flow equation calculation. The usefulness depends on the data available for analysis and the required accuracy of the production forecast. In general, both methods satisfy for producing wells within established well patterns, which have production profiles with a consistent decline trend. Reservoir simulation is applicable to all stages of the well life. However, it is most useful in areas where an abundance of core, log, and well test data are available. Molecular diffusion of methane in a coal matrix has been quantified by determining a sorption time, t (days), which is related to cleat spacing (ft) and the diffusion coeffi cient (ft2/day). An effort is made to use a diffusion coefficient or diffusivity as a tool for seam-to-seam correlation.

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A propos de : Saikat MAZUMDER

A propos de : Karl-Heinz A.A. WOLF