Quantitative Evaluation of Coal Seam Gas Content Estimate Accuracy
- Matthew J. Mavor (Tesseract Corp.) | Timothy J. Pratt (TICORA Geological Services) | Charles R. Nelson (Gas Research Institute)
- Document ID
- Society of Petroleum Engineers
- Low Permeability Reservoirs Symposium, 19-22 March, Denver, Colorado
- Publication Date
- Document Type
- Conference Paper
- 1995. Society of Petroleum Engineers
- 1.10 Drilling Equipment, 4.1.5 Processing Equipment, 1.6.9 Coring, Fishing, 1.6 Drilling Operations, 4.1.2 Separation and Treating, 5.6.1 Open hole/cased hole log analysis, 4.6 Natural Gas, 5.4.2 Gas Injection Methods, 5.8.3 Coal Seam Gas
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The proper measurement and analysis of gas desorption data from freshly cut coal samples is crucial to reliably predict the gas production performance and reserve potential of coal seam gas reservoirs. The technical literature offers may procedures for evaluating the total sorbed gas content of coal samples. This paper presents evaluations of the accuracy and comparability of gas content estimates for freshly cut San Juan Basin Fruitland Formation and Piceance Basin Cameo coal samples obtained using commonly practiced analysis procedures. Benchmark (or "ground truth") gas content values were assembled from pressure core and sorption isotherm data for comparison against gas content estimates obtained from conventional whole core, sidewall core, and drill cutting samples. Lost gas content estimate accuracy determined by the Direct (U.S. Bureau of Mines) Method, the Smith & Williams Method, and the Amoco Method was evaluated and compared. Assessments were also made of the sources and magnitudes of sampling and procedural method errors on the accuracy of the gas content estimates. The choices of sample type, gas desorption temperature conditions, and lost gas analysis method significantly affected the accuracy of the gas content values. The Direct Method analysis of conventional whole core, reservoir temperature gas desorption data provided the most accurate gas content values. Using ambient temperature gas desorption conditions rather than reservoir temperature conditions is the greatest source of error in gas content estimates.
Coal seam gas reservoirs hold a significant portion, approximately 14 percent (135 Tcf), of U.S. natural gas reserves and currently account for approximately 4 percent (800 Bcf) of annual, domestic gas production. This new gas resource still holds a significant, untapped gas deliverability potential. In the years to come, production from this new gas play will be critical to providing adequate gas supplies at reasonable prices.
Gas deliverability from coal seam reservoirs depends upon the amount of gas-in-place and the coal's gas sorption storage and production mechanisms. There are no standard, petroleum industry procedures for performing gas content analysis of coal seam reservoirs. Current methods used for determining coal seam reservoir gas content values require that coal samples be obtained at the time of drilling exploration or development wells. The freshly cut coal samples are used in gas desorption experiments that are started on location as soon as possible after sample collection. The need for on-site gas desorption experiments results from two limitations of available technology.
First, accurate coal seam gas content estimates cannot be obtained from wireline log data without calibration by core derived sorbed gas content data. This limitation occurs since the presence of gas has little affect upon the bulk properties of the coal. For example, an in- situ methane content of 400 scf/ton increases the density of a pure coal sample of density equal to 1.25 g/cm3 by 0.010 g/cm3 or 0.8%.
|File Size||1 MB||Number of Pages||14|