Probabilistic Estimate of Global Coalbed-Methane Recoverable Resources

Abstract

Summary Because today's oil and gas industry recognizes the value of coalbed-methane (CBM) resources, CBM exploration and development activities, once uniquely North American, are now under way on a global scale. Many publications address the technically recoverable resources (TRR) of CBM in small geographic areas. However, there is little publicly available information about the magnitude of TRR for CBM on a global scale. Uncertainty exists in geologic and engineering data and, consequently, in the results of calculations made with these data. The uncertainty of the problem begs for a probabilistic solution. Our objective was to determine distribution of CBM TRR worldwide. To achieve this goal, we developed a computer program: the unconventional gas-resource-assessment system (UGRAS). The program integrates a Monte Carlo technique with an analytical reservoir simulator. The combination provides an excellent means to predict production profiles for a wide variety of reservoir characteristics and producing conditions. By use of UGRAS, we established the probabilistic distribution of original gas in place, TRR, and recovery factor (RF) for selected coals in the US and derived a representative RF distribution for CBM reservoirs. CBM RFs follow a log-logistic distribution, with a P50 of 36%. We extended the distribution of gas RFs gained from our analyses of US coals to estimate global CBM TRR. Global technically recoverable CBM is estimated to be 500 (P90) to 3,000 (P10) Tcf, with a P50 value of 1,500 Tcf. UGRAS is a robust tool that may be used to evaluate and rank CBM resources worldwide. Results of this work verify the existence of significant technically recoverable CBM resources worldwide and can guide the industry to better target its exploitation efforts in coals around the world.