Coal-Gas Desorption Measurements: Cuttings Samples Compared with Conventional Core Samples

Abstract

Comparable conventional core and cuttings samples were collected from wells in the coal-bearing strata of the Fruitland Formation in the central San Juan basin, Rio Arriba and San Juan counties, New Mexico. Both sets of samples were handled similarly by reputable, commercial contractors and desorbed to atmospheric pressure from sealed canisters to determine total gas content. Although there has been considerable, and occasionally heated, discussion as to the merits and shortcomings of the various methods used to calculate lost gas in coal-gas desorption studies (USBM direct method, Smith and Williams Unipore method, decline curve method), the various total gas values within each sample set - either core or cuttings - are readily correlative, regardless of which lost gas calculation was applied. The results from the cuttings desorption tests, however, are not correlatable with the conventional core results. The data set derived from the cuttings samples has a lower mean total gas content and a larger standard deviation from that mean. The total gas content values determined from the conventional cores are significantly correlatable with wireline log bulk density, which is an indirect measure of coal grade (ash content). Despite their shortcomings, the desorption data derived from cuttings are often themore » only such data available from a particular well or prospect. Caution should be used in applying cuttings-derived desorption data. Useful application of this data depends upon a working knowledge of factors that may affect the results of desorption tests on cuttings samples.« less