Practices and Pitfalls in Estimating Coalbed Methane Resources and Reserves

Creties Jenkins
DeGolyer and MacNaughton, Dallas, TX

The estimation of coalbed methane (CBM) resources and reserves is a complex process due to the layered, fractured, and heterogeneous nature of these reservoirs. CBM resource estimation begins by quantifying variations in areal extent, thickness, gas content, and coal density. These then serve as input for determining the gas resource using probabilistic techniques. During this process, it is important to avoid pitfalls such as generating a resource range that is too narrow or failing to include non-coal lithologies. It is also important to realize, based on numerous examples, that resource estimates are commonly a poor predictor of gas production potential.

CBM reserves depend upon demonstrated production which is typically characterized by a dewatering and ramp-up period prior to attaining peak gas rates. Core and log data obtained from appraisal wells provide the gas content and isotherm data necessary to make the first estimates of gas-in-place and potential recovery. This information can be used to initialize screening-level numerical simulation models to understand the critical factors controlling gas production and recommend pilot well types and spacing.

Once economic rates are achieved, production forecasts can be generated and compared with gas-in-place estimates to ensure that the reserves numbers are reasonable. After the reservoir is dewatered, material balance and decline curve analyses can be used to estimate reserves in a manner similar to conventional reservoirs. Throughout this process, as shown by several case studies, care must be taken to avoid common technical and managerial pitfalls that result in erroneous reserves estimates and bad decisions regarding which projects to develop or divest.

AAPG Search and Discovery Article #90078©2008 AAPG Annual Convention, San Antonio, Texas