Coalbed Methane Reservoir Characterization

ARC Group’s Expertise in Coalbed Reservoir Characterization

ARC Group’s coalbed gas team has developed a basin-scale coalbed gas exploration and producibility model based on more than a decade of research and development in the:

• San Juan, Sand Wash (Greater Green River), Piceance, Powder River, Raton, and Uinta Basins of the Rocky Mountain Foreland Province;
• Frontier Alaskan coal basins, including the North Slope of Alaska;
• Texas Gulf Coast coal basins; and in
• Reconnaissance studies of several other producing and prospective coal basins in the United States, China, Botswana, South Africa and world-wide.

Coalbed Reservoir Characterization Programs

Approaches to Coalbed Reservoir Characterization

The best potential for the discovery of coalbed gas resources lies in conventional and hydrodynamic traps basinward of where outcrop and subsurface coals are in good reservoir and hydraulic communication and in areas of vertical flow potential and fracture enhanced permeability. Exploration and development for migrated conventionally trapped gases, in situ-generated secondary biogenic gases, and solution gases are required to achieve coalbed gas production in many coal basins.

The ARC Group emphasizes a multidisciplinary team approach to meeting the current challenges of discovering, evaluating, and developing coalbed methane resources both in the United States and internationally.

Development of a Coalbed Gas Exploration and Producibility Model

Our coalbed gas producibility model, that can be applied to evaluation of coalbed gas exploration and producibility in coal basins throughout the world, indicates that tectonic/structural setting, depositional systems and coal distribution, coal rank, gas content, permeability, and hydrodynamics are controls critical to coalbed gas producibility.

However, simply knowing a basin’s geologic and hydrologic characteristics will not lead to a conclusion about coalbed gas producibility because it is the interplay among geologic and hydrologic controls on production and their spatial relation that governs producibility. High producibility requires that the geologic and hydrologic controls be synergistically combined. That synergism is present in the prolific producing San Juan Basin. As predicted from our producibility model, significant coal gas production (wells greater than 1 MMcf/d) are found in the high productivity fairway of the San Juan Basin.

Geologic and Hydrologic Characteristics Critical to Coalbed Gas Producibility

Key Elements of the Producibility Model

In frontier exploration, geologic and hydrologic controls on coalbed gas producibility that can be used to predict high productivity fairways, will be governed by:

• Thick, laterally continuous coals of high thermal maturity and adequate permeability;
• Basinward flow of ground water through coals of high rank and high gas content orthogonally toward no-flow boundaries (regional hingelines, fault systems, facies changes, and/or discharge areas);
• and Conventional trapping along those boundaries to provide additional gas beyond that generated during coalification

This hydrogeological producibilty model provides a framework for exploration and development strategies in coal basins throughout the world.

Hydrologic and Geologic Model for Coalbed Gas Exploration

Calculating Coalbed Gas Resources

Accurately assessing coal and coal gas resources and delineating areas within basins containing the largest resources and exploration targets are important aspects of resource development.

In basins where coal analysis data are sparse, coal gas resources are best calculated on an ash-free basis. The density contrast between ash-forming minerals and organic matter is large enough that the weight percent ash is much larger than the corresponding volume percent. Therefore, ARC Group uses a correction factor relating weight percent ash-free coal and ash yield (determined from proximate analysis) to ash-free coal volume for accurate coal gas resource calculations. Rather than using one density value these calculations require that bulk coal density (including mineral matter) be distinguished from ash-free coal density.

Coal and coalbed methane resources are then calculated from digitized structure, topographic, and net-coal-thickness maps on a 3.5-sq. mi. grid, using plots of gas content versus depth, density, and coal volume.

ARC Group CBM courses have been sponsored by the United Nations, the Gas Research Institute, the US Department of Energy, many universities, and national and local institutions and societies

Coalbed Gas Resource Assessment and Play Analysis of the Guasare Subbasin, Zulia State, Western Venezuela

• Dr. Noel Tyler and Mr. Roger Tyler
• The ARC Group
• Ms. Vania Savian and Mr. Ulneiver Canonico
• Mr. Rafael Tocco, Project Manager
• PDVSA-INTEVEP
• December, 2002