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Compartmentalization Analysis of Coalbed Methane Reservoirs in the Black Warrior Basin: Using 3-D Computer Models to Balance Industrial and Environmental Concerns

Jack C. Pashin, Geological Survey of Alabama, Tuscaloosa, AL

Landowner concerns related to coalbed methane operations in the BlackWarrior basin of Alabama include contamination of shallow sources of domestic groundwater by hydraulic fracturing. Although no claim of contamination has been substantiated, litigation and new regulations intended to address this concern have placed a hardship on landowners and gas producers alike. Central to this issue is the degree of hydraulic communication between deep reservoir coal beds and shallow aquifers. To help determine if coalbed methane operations can affect shallow aquifers, discrete fracture network models of compartmentalization have been developed using FracMan and FracMax software. FracMan is a PC-based application that was used to make numerical models of the fracture networks, and FracMax is an OpenGL application that is being developed in-house for advanced visualization and compartmentalization analysis of the models.

Discrete fracture networks incorporate statistical scaling rules derived from analysis of fracture length, height, spacing, orientation, and kinematic aperture. Once statistical properties of fracture systems are derived, compartmentalization models can be developed based on adjacency analysis of transmissive fractures. Basic data on natural fracture systems were derived from core and outcrop, and data on hydraulic fractures were obtained from file data and from direct observations in underground coal mines. The natural fracture systems are mainly strata-bound; thus hydrologic connections between beds are mainly near the tip regions of the fractures. The kinematic apertures of fractures in coal, sandstone, and shale follow exponential distributions. These distributions dictate that flow is concentrated in relatively few fractures and that connectivity among beds is limited. Accordingly, compartmentalization analysis of the discrete network models indicates that multiple no-flow boundaries exist between reservoir zones and shallow aquifers, and that shallow supplies of domestic groundwater are shielded from hydraulic fracturing operations. Independent evidence, including reservoir pressure, fluid production, and well-test results, supports that a high degree of hydrologic compartmentalization exists in coalbed methane reservoirs of the Black Warrior basin.

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