--> Abstract: Methods Of Calculating In-Place and Recoverable Coal-Bed Methane Reserves, by L. Lottman-Craigg; #91012 (1992).

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ABSTRACT: Methods Of Calculating In-Place and Recoverable Coal-Bed Methane Reserves

LOTTMAN-CRAIGG, LINDA, GeoMet, Inc., Ashburn, VA

Over the past decade, coal-bed methane has evolved from a hazard encountered in underground coal mines to a major unconventional energy source. Owing to the relatively brief history of the coal-bed methane industry, methods of calculating reserves have likewise evolved recently. All reserve calculations depend primarily on gas content and thickness of coals. Several methods are currently used to determine in-place coal-bed methane reserves, including the volumetric method, the contour method, and decline analysis. The optimal method for a given coal basin or field within a basin depends on factors such as required accuracy and the quantity and quality of the available database.

The percentage of the in-place reserves that is ultimately recoverable is even more complex and problematic owing to the lack of long-term, producing coal-bed methane wells. The basic data needed to accurately determine the recoverable reserves of most wells or prospects may be unavailable. Also, each area differs geologically, and fields and/or wells may be developed by different techniques. This is reflected by the wide range of recovery factors published in the literature.

Several methods are currently used to determine recoverable coal-bed methane reserves. The most accurate recovery factor results from an engineering evaluation based on formation pressure, well spacing, gas content, permeability, and stimulation design. An economic analysis should be included because operating costs and the price of natural gas directly affect economic gas recoverability. A second method is use of adsorption isotherms to predict the quantity of remaining coal-bed gas based on the expected reservoir pressures at abandonment. A third method is prediction of gas production using coal-bed methane reservoir simulators, the accuracy of which is highly dependent on the database and on assumptions made in generating the model. Each of these methods has its own advantages and isadvantages, and the best method depends on the specific coal basin and database.

 

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)