Coalbed Gas Exploration and Resource Development

ARC Group’s coalbed gas team has developed a basin-scale coalbed gas producibility and exploration model based on more than a decade of research in the San Juan, Sand Wash (Greater Green River), Piceance. Powder River, Raton, and Uinta Basins, Rocky Mountain Foreland; Alaskan coal basins, including the North Slope; the Texas Gulf Coast coal basins; and reconnaissance studies of several other producing and prospective coal basins in the United States and world-wide. This model, that can be applied to evaluation of coalbed gas 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. The best potential for coalbed gas production lie 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. Therefore, The ARC Group emphasizes a multidisciplinary team approach to meeting the current challenges of assessing and developing coalbed methane resources both in the United States and internationally.

Coalbed Gas Producibility Model

ARC researchers have developed a producibility model that explains the prolific and marginal coalbed gas production in frontier exploration basins and fairways. To delineate the presence, origin and extent of coalbed gas producibility requires an understanding of the interplay among tectonic and structural setting, depositional systems and coal distribution, coal rank, gas content, hydrodynamics, and permeability. Tectonic and structural setting control the distribution of coal resources and determine their location with respect to the thermally mature parts of a basin and their orientation relative to ground-water flow direction. Knowledge of depositional framework enables prediction of coalbed thickness, geometry, and continuity, especially when data are sparse. Coal rank is a direct indicator of gas generation but not of gas content. Maximum generation of thermogenic gas occurs at ranks of medium-volatile to low-volatile bituminous (Ro of 1.3 to 1.8%). Although gas content generally increases with rank, it is not determined by rank alone, but reflects permeability contrasts and hydrodynamics. Gas content is influenced by conventional trapping of gas, reservoir pressure, generation of secondary biogenic gases, and long-distance migration of gas. Thus, gas content in coals can be much higher than that predicted from rank alone. Permeability of a coal bed is determined by its fracture (cleat) system, which in turn is largely controlled by the tectonic/structural setting. Coals are commonly aquifers with permeabilities that may be orders of magnitude larger than those of associated sandstones, and coal beds act as conduits for gas migration. Basinward flow of ground water requires recharge of laterally continuous permeable coals at the elevated structurally defined basin margins.

High coalbed gas productivity requires dynamic ground-water flow through coals of high thermal maturity (rank) and high gas content orthogonally toward flow barriers, accompanied by generation of secondary biogenic gas and conventional trapping of migrated and solution gases along those barriers. The resulting interplay leads to high gas contents or even fully gas-saturated coals for consequent high productivity. Our approach to developing a frontier coalbed gas exploration play includes advanced reservoir characterization of:

Depositional Systems and Coal Distribution

Coal beds are the source and reservoir for gas, indicating that their widespread distribution within a basin is critical to establishing a significant coalbed gas resource. Coal distribution is closely tied to the tectonic, structural, and depositional settings because peat accumulation and preservation as coal require a delicately balanced subsidence rate that maintains optimum water-table levels but excludes disruptive clastic sediment influx. The depositional systems define the substrate upon which peat growth is initiated and within which the peat swamps proliferate. Knowledge of depositional framework enables predication of coalbed thickness, geometry, and continuity and, therefore, areas of potential coalbed gas resources.

Coal Rank and Gas Generation

Coals must reach a certain threshold of thermal maturity (vitrinite reflectance values between 0.8 and 1.0 percent; high-volatile A bituminous) before significant volumes of thermogenic gases are generated. The amount and types of coal gases generated during coalification are a function of burial history, geothermal gradient, maceral composition, and coal distribution within the thermally mature parts of a basin.

Gas Content

Higher rank coals generally have higher gas contents but gas content is not determined by coal rank alone. Gas content changes when equilibrium conditions within the reservoir is disrupted. Gas content of coals can be increased by generation of secondary biogenic gases or by diffusion and long-distance migration of gases to no-flow boundaries (facies changes, structural hingelines, or faults) for resorption and conventional trapping. Gas content enhancement generally requires laterally extensive, permeable coals to serve as conduits for gas migration, and dynamic ground-water flow to transport gases.

Permeability

Permeability and hydrodynamics (ground-water flow) are intimately related to coal distribution and tectonic/structural setting because basinward flow of ground water through coal beds requires recharge of laterally continuous permeable coals at the structurally defined basin margins. Permeability in coal beds is determined by its fracture (cleat) system. The coals, therefore, act not only as conduits for gas migration but also are commonly aquifers having permeabilities that are orders of magnitude larger than associated sandstones.

Tectonic and Structural Setting

Permeability in coal beds is determined by its fracture (cleat) system, which is in turn controlled by the tectonic/structural regime. Cleats are the permeability pathways for migration of gas and water to the well head, and cleat attributes may either enhance or retard the success of the coalbed gas completion. Moreover, aquifer pinchout or faults may serve as permeability barriers (no-flow boundaries) where migrated gas can be trapped.

Calculating 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. Many resource studies may have overestimated or underestimated coal gas resources because the ash and density terms of resource equations were inconsistently and/or inappropriately considered. 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 uses a correction factor relating weight percent ash-free coal and ash yield (determined from proximate analysis) to ash-free coal volume is required 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-mi2 grid, using plots of gas content versus depth, density, and coal volume as described by Scott (1995).

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;

• 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);

• Generation of secondary biogenic gases; and

• Conventional trapping along those boundaries to provide additional gas beyond that generated during coalification

This producibilty model provides a framework for exploration and development strategies in coal basins, worldwide, and the multidisciplinary team approach has been applied by ARC to basins, both locally and internationally, to meet the challenges and demands of coalbed methane research and development.