Estimating Resources and Reserves in Coalbed Methane and Shale Gas Reservoirs

Creties Jenkins
DeGolyer and MacNaughton, Dallas, TX

Coalbed methane and shale gas reservoirs are continuous gas accumulations characterized by extreme heterogeneity and multiple gas producing mechanisms. Factors controlling the distribution of gas resources include organic richness, maturity, gas content, and gas saturation state in addition to conventional elements such as porosity and water saturation. Understanding the core and log data needed to quantify these parameters, as well as their variability and underlying controls, are critical for making reasonable estimates of gas resources. Factors controlling gas reserves include the orientation and frequency of natural fractures, proximity to water-bearing intervals, and stimulation techniques. Effective permeabilities are highly-variable, ranging from nanodarcies to tens of millidarcies, depending upon fracture development. Faults and karst features serve as conduits for water production in some reservoirs and need to be located using 3-D seismic. Stimulation techniques continue to evolve rapidly and include low viscosity fluids, lightweight proppants, and multi-stage frac jobs. Reserves estimation techniques include analogs, numerical simulation, and decline curve analysis. The choice of techniques to use depends upon reservoir complexity, data quality, and project maturity. The assignment of reserves ultimately requires demonstrated gas production which can be highly-variable depending on whether the reservoir is wet or dry, underpressured or overpressured, and saturated or undersaturated with gas. Throughout the estimation process, care must be taken to avoid common pitfalls that result in erroneous values and bad decisions regarding which projects to develop or divest. It is particularly important to realize that limited data will result in a high degree of uncertainty, and that a large gas resource may have little reserves potential.

AAPG International Conference and Exhibition, Cape Town, South Africa 2008 © AAPG Search and Discovery