Fracture model of the Upper Freeport coal: Marshall County West Virginia pilot ECBMR and CO2 sequestration site

Abstract

A model discrete fracture or cleat network was developed for the Pennsylvanian Upper Freeport coal seam at a carbon sequestration pilot site in southeastern Marshall Co., West Virginia, U. S. A. The model reservoir cleat network was developed using 3D seismic post-stack processing and interpretation workflows combined with observations of fracture systems in the surrounding region. The Upper Freeport coal is unminable in this area and served as a test for the feasibility of combined enhanced coalbed methane recovery (ECBMR) operations with CO2 storage in unminable coals. Well log-derived subsurface structure maps were used to calibrate 3D seismic time-to-depth conversion. Post-stack processing was used to enhance and identify subtle discontinuities in the seismic data that could be related to field-scale faults, fracture zones or stratigraphic heterogeneity. The cleat model was developed using power law fracture length and aperture distributions. Spatial variations of cleat intensity in the model were controlled using measures of localized structural discontinuity extracted from 3D seismic attribute workflows. The cleat model identifies a permeable fracture facies within the reservoir most likely to be impacted by CO2 injection and migration. Areas of concentrated seismic discontinuity bear some spatial association to areas of positive vertical deformation observed in a tiltmeter survey of the site and attributed to CO2 injection. The model could help focus time-lapse processing and interpretation in regions estimated to have higher permeability and porosity and higher probability for change of acoustic properties in response to CO2 injection and methane production. The approach developed here has general applicability to identification of fracture facies and development of unconventional naturally fractured reservoirs in other lithologies and basins.