Abstract: Differential Compaction and Shifts in Coal Fracture Patterns: Field Example and Finite Element Model

S. E. Laubach, D. D. Schultz-Ela, R. Tyler

The flow of gas and water in coal beds is controlled by fractures. Efficient coalbed methane development needs to target areas where the density of open fractures and coal transmissivity are high while avoiding areas where fractures are closed and impermeable. Coal exposures in the Upper Cretaceous Rock Springs Formation, Mesaverde Group, in southwest Wyoming, show that significant variations in coalbed fracture style exist over distances of tens to hundreds of feet. Typical joint (cleat) patterns are replaced by arrays of

closely spaced faults where cleat-forming conditions were perturbed by differential compaction under sandstone lenses. Because densely mineralized compaction faults have little or no porosity, the coal that contains them is likely to have low permeability compared to that of coal having typical cleat patterns. Finite element modeling of coal deformation shows that shear stress is augmented in coal layers below abruptly tapering edges of sandstone lenses, favoring normal fault development. Reconnaissance of coal beds in the western U.S. associated with broadly lenticular sandstones suggests that shifts in fracture style are rare; in these rocks, shear stresses are not sufficiently enhanced. Shifts in fracture style may be more common in depositional facies having abruptly tapering sand tone lenses. Replacement of joints by faults could interfere with the success of well completions, compartmentalize and channelize gas and water flow, and create traps for gas accumulation.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994