A Modified Approach to Coalbed Methane Exploration – What Have We Learned in the Past 20 Years?

Charles Morris Boyer and Stephen W. Lambert
Schlumberger, Pittsburgh, PA

Historically, coalbed methane (CBM) exploration programs have relied upon methodologies, such as identification of maximum coal thickness, optimum coal rank, and maximum gas resources in place. These programs also adopted a methodology of reducing exploration and financial risk by relying upon a phased exploration program that often began with low-cost exploration core holes sited in what was believed to be the optimum prospect location. Production tests only occurred if certain exploration hurdles were achieved, allowing the project to grow into a higher-cost multiple well pilot program. However, this approach overlooks the consequence of reservoir heterogeneities, especially permeability, which ultimately dominate later large-scale project performance. Due to these heterogeneities, commercial coalbed methane production areas may be bypassed or prematurely condemned. Evaluation of numerous CBM production fields in the USA showed significant production heterogeneity that could not be attributed to static reservoir conditions or well completion/production methods. These data were analyzed using statistical sampling methods to determine minimum well requirements (number and location of wells) for understanding production variability. Based on the results of this analysis, a modified exploration model is proposed that addresses the need to understand reservoir permeability heterogeneity early in a CBM exploration project. Total Reservoir Assessment (TRA) targets the range in reservoir types and conditions of any prospect using single-well production tests, coupled with standard CBM reservoir evaluation techniques. Application of this model may result in a more realistic approach to the exploration process and increases the probability of large-scale technical and financial success.