The paper was presented at the SPE/DOE Unconventional Gas Recovery Symposium of the Society of Petroleum Engineers held in Pittsburgh, PA. May 16–18, 1982. The material is subject to correction PA. May 16–18, 1982. The material is subject to correction by the author. Permission to copy is restricted to an abstract of not more than 300 words Write: 6200 N. Central Expwy., Dallas, TX 75206.

Abstract

The potential methane resource for the Fruitland coal formation of the San Juan Basin of northwest New Mexico has been analyzed. Gas contents ranging from 1 to 8 cm3/g were found for 33 samples from 14 wells which were at depths from 300 to 1000 m. Gas contents were determined using the "chip desorption" method. This method allows the use of drill cuttings obtained from conventional drilling operations as the coal seam is penetrated. The use of core samples which is required in the Bureau of Mines method is eliminated.

Contrary to past basin studies which used a limited number of data points, no direct correlation between depth and in-place gas content was evident. In order to explain the lack of a depth/gas content relationship, the physical properties of the coal relevant to gas adsorption were studied. Surface area measurements using low-pressure CO2, CH4, and neopentane adsorption have been conducted. Differences in adsorption capacity between the various samples is small and cannot explain the order of magnitude variation in gas content. In summary, it appears that local geological factors (i.e., the nature of the over- and underburden) play a much larger role in the in-place gas content of coal than previously thought. This implies that a large previously thought. This implies that a large number of data points is required to accurately characterize the total quantity of coalbed methane in a particular basin. particular basin

Introduction

Ever since the methane which is trapped in coalbeds has been recognized as a natural gas resource, considerable effort has gone into delineation studies. For the most part, and especially in the western coalbeds of the United States, these studies have been based on a few direct measurements and extensive application of trends established for methane adsorption on the internal surfaces of coal. Direct measurement of coalbed methane was developed by Kissel, McCulloch, and Elder based on desorption of gas from core samples. The indirect method proposed by Kim can be used to extrapolate between gas contents determined by the direct method at known locations or to estimate gas content in coalbeds where the content has not been determined. Because of the high costs associated with direct measurements, limited data has been obtained.

During the last two years, Smith and Williams have developed a technique for measuring coalbed methane that does not require coring. Furthermore, the technique can sample coalbed methane during ongoing drilling operations that are financed for other reasons. As a result of their development studies, coalbed gas contents at fourteen locations in the San Juan Basin of New Mexico have been published. The data from the thirty-three samples published. The data from the thirty-three samples comprises a concentrated basis for studying the trends in gas contents across the basin. In this paper, we have corrected the field data to a wet paper, we have corrected the field data to a wet coal basis to see if the gas content follows trends predicted by the indirect method. In addition, we predicted by the indirect method. In addition, we have determined the surface properties of the coal samples to see if the variation in coalbed gas content could be attributed to changes in intrinsic adsorption capacity.

EXPERIMENTAL

Eleven representative coal samples of Fruitland coal from the San Juan Basin of New Mexico were selected from among the thirty-three samples collected as part of the chip desorption field work. The samples were chosen so that three widely different parts of the basin would be represented. Consideration was also given to the treatment of the coal samples after desorption had been completed. The coals used in the surface studies were all kept in sealed containers, wet, and as collected until they were needed for analyses. Previous studies have shown that the surface area of these coals does not change after six months storage in that manner. In addition to coal samples, two well-characterized carbon samples were used for the purpose of standardization and comparison.

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