The ability to produce low cost, pipeline quality gas from low permeability deep coal reservoirs depends largely on thorough integration of exploration methods with appropriate drilling, completion and production strategies. In the extreme case, areas that are not economic with current completion technologies should be avoided with adherence to prudent exploration practices. Therefore, reliable predictive geologic methods to specify coal reservoir conditions need to precede drilling and completion decisions.
Dominant coal reservoir mechanisms include: permeability, saturation, reservoir pressure and gas-in-place. Production characteristics from low permeability coal reservoirs are most sensitive to the interaction between type of saturation and reservoir pressure. The geologic processes responsible for these reservoir conditions have been examined in the Piceance Basin. This basin, known for low permeability reservoirs, was selected for geologic evaluation due to the large coalbed methane resource and large data base. Also located in the basin is the Deep Coal Seam Project, a multiyear, multi-well, field laboratory joint venture by the Gas Research Institute and Resource Enterprises, Inc., providing fully integrated reservoir and geologic engineering data on deep coal reservoirs.
Reservoir diagnostics and modeling suggests that reservoir pressure and type of saturation demonstrate an interaction between catagenesis and permeability. Thick, thermally mature coal deposits actively generate more gas than can be adsorbed by the coal or be diffused through a low permeability system. In these regions over-pressuring occurs. Ultimately, pore pressure will exceed insitu stresses resulting in tensional fractures through the coal bearing sequences. While the coal seams remain in the active gas generation phase, the fractures and pore spaces become gas saturated. Eventual temperature reduction through erosion will under-pressured reservoir. Imbibition of water into these reservoir is unlikely in areas of low permeability.
In summary, source rock evaluation techniques have been applied to characterize and predict coal reservoir mechanisms in low permeability basins.
Introduction
A portion of the extensive gas accumulations found in the San Juan Basin, the Green River Basin, and the Alberta Basin has been sourced largely by coal seams. Despite the amount of data collected on coal reservoir characteristics and coal as a source rock, little work has been done integrating the two sciences. A coupled understanding of coal reservoir mechanisms and coal maturation will assist the explorationist in his pursuit of coalbed gas resources. This work was sponsored by the Gas Research Institute under Contract No. 5083-214-0844 with Resource Enterprises, Inc.
A geologic model is presented which incorporates the coal's inability to transfer heat with its ability to generate large volumes of gas over a specific temperature and time sequence. In basins where low reservoir permeability prohibits crossformational fluid flow, gas generation can exceed the quantity of gas that can migrate through the geologic system. This results in high pore pressure within the coal reservoirs. Conversely, where gas migration exceeds the rate of gas generation, low pore pressure will be observed. These stages of reservoir disequilibrium have been observed in various deep coal basins of the western United States.
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