Mining Publication: Measuring Formation Pressures and the Degree of Gas Drainage in a Large Coalbed Gas Drainage Field

The Bureau of Mines and United States Steel Corp. are conducting a joint project to monitor formation pressures at a large (23-well) coalbed gas drainage field near Oak Grove, Alabama. Three monitor holes were drilled in late 1981, and pressure monitoring began in December 1981. The Bureau of Mines direct method was used to obtain gas content data from cores taken in the monitor holes. Comparison of the 1981 gas content data from the monitor holes with initial gas content values obtained from the production wells in 1977 indicates a 50-pct reduction in adsorbed gas content inside the pattern and a 29-pct reduction at one point 500 ft outside the pattern. The combination of pressure and gas content data has also allowed an in situ isotherm curve, relating the formation pressure to adsorbed gas content, to be developed for the area of the production wells. From the current rate of pressure decline (1 lbf/in2 per month), the observed changes in gas content, and the isotherm curve, it appears that most of the early gas production came from within the pattern, but since 1981 (and possibly earlier) most of the gas produced has been migrating to the wells from outside the pattern area.