THREE-DIMENSIONAL MODELS OF FRACTURE ARCHITECTURE IN THE COAL-BEARING POTTSVILLE FORMATION, BLACK WARRIOR BASIN, ALABAMA

Leakage risks associated with coalbed methane and carbon sequestration operations in the Black Warrior Basin are related to cross-formational flow along natural fractures. To assess these risks, fracture networks have been modeled stochastically on the basis of core and outcrop data using discrete fracture network modeling software. Systematic fractures in shale and sandstone are more weakly aligned than those in coal, and spacing and the degree of alignment control fracture length. Cross fractures tend to terminate at systematic fractures with high frequency in all rock types. Fractures in all rock types are dominantly strata-bound, thus fracture height is approximately equal to bed thickness. Joint spacing follows log-normal distributions, and joint aperture follows exponential distributions. In coal, cleat spacing and aperture follow Gaussian scaling laws.

Realization of fracture networks using OpenGL visualization software indicates that the great length of systematic fractures relative to that of cross fractures gives rise to significant permeability anisotropy. Adjacency analysis indicates that strata-bound fracture networks tend to be self-compartmentalizing, which helps limit cross-formational flow. Joints enable localized hydraulic communication between closely spaced coal beds, but thick intervals of interbedded shale and sandstone restrict communication among widely spaced coal zones, thus limiting environmental risk.

Contrasting styles of joint spacing and aperture distribution give rise to fundamentally different transmissivity patterns in siliciclastic strata and coal. The exponential distribution of kinematic aperture in siliciclastic strata indicates that a minor percentage of the fracture population is capable of transmitting large volumes of fluid. By comparison, the Gaussian nature of cleat spacing and kinematic aperture indicates that flow is more uniformly distributed in coal than in the other rock types. However, large-aperture cleats can form major flow pathways and are thus a source of significant reservoir heterogeneity. Furthermore, understanding the frequency of large-aperture cleats can facilitate understanding of reservoir drainage and can provide critical information on the fate of carbon dioxide injected during sequestration operations.