--> ABSTRACT: Gas-In-Place Determination for Coal Gas Reservoirs, by Matthew J. Mavor, Timothy J. Pratt, Charles R. Nelson, and Tom-Ann Casey; #91019 (1996)

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Gas-In-Place Determination for Coal Gas Reservoirs

Matthew J. Mavor, Timothy J. Pratt, Charles R. Nelson, and Tom-Ann Casey

The Upper Cretaceous Fruitland Formation of the San Juan Basin has been a very active natural gas play in recent years. Case studies of coal natural gas-in-place reassessments have revealed that gains of up to 74% are possible based upon methods recently developed by the Gas Research Institute. The greater gas-in-place estimates were consistent with production history and provide a new geologic perspective upon the producible coal gas resources.

Coal gas-in-place is proportional to the thickness, average density, and average gas content. The GRI procedure evaluates each of these properties with a combination of core and density log data for specific reservoirs. The

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procedure follows eight steps: (1) Perform desorption measurements at reservoir temperature on conventional core samples; (2) Estimate total gas content of each sample using the Direct Method lost gas content procedure; (3) Relate total gas content to sample composition; (4) Relate sample composition to density; (5) Determine the in-situ moisture content from equilibrium moisture content measurements; (6) Determine the gross thickness and average in-situ density from log data; (7) Estimate the in-situ gas content at the average reservoir density and moisture content; (8) Compute the gas-in-place volume.

Quantitative errors and causes of errors in the three parameters have been determined. Errors are often caused by determining gross thickness with low density cut-off limits, by basing average density estimates upon "rules of thumb" or bounding rock density, by performing desorption measurements at ambient temperature, and by the use of drill cuttings rather than core samples.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California