Comparison of Lost Gas Projections in Coalbed Methane

Waechter, Noel¹, George Hampton¹, John Seidle^2
1 Hampton, Waechter & Associates, LLC, Englewood, CO
² Sproule Associates Inc, Denver, CO

Lost gas is determined by projecting the first couple of hours of desorption measurements back to time zero (the time core retrieval is at the half-way point coming out of the drill hole). Industry standard has been to project lost gas using a linear fit to cumulative-gas vs. square-root-of-time data. We use a polynomial fit for lost gas projections, in addition to reporting gas content using a linear fit for lost gas. A polynomial projection for lost gas generally gives a better fit to the data. The cumulative-gas vs. square-root-of-time curve is just that – a curve, not a straight line. In one example, we examined the gas content of a wireline core sample using 36 minutes of lost-gas time. Using the same data, we discarded an hour of desorbed gas measurements to simulate a conventional-core-retrieval time of 90 minutes. When using a polynomial projection for lost gas, the difference between total-gas content determined using a wireline-core-retrieval time of 36 minutes and total-gas content determined using a simulated conventional-core-retrieval time of 90 minutes was negligible (3.8%). These differences were significantly greater (12.8%) using a linear projection for lost gas. Furthermore, with linear projections, these differences increase progressively as lost-gas time increases, but do not with polynomial projections. The observed linearity to early parts of some desorption curves may be related to temperature disequilibrium (and concomitant increases in diffusion rates) as coals warm back up to reservoir temperature inside the canisters.