Bedding-Parallel Fractures in Shales: Characterization, Prediction and Importance

Abstract

Bedding-parallel fractures are common although not ubiquitous in shale. Several lines of evidence (mine-back experiments, microseismic data and tiltmeter data) suggest that hydraulic fractures used to stimulate wells in hydrocarbon reservoirs sometimes have a horizontal (bedding-parallel) component, even at significant depth. Natural, sealed horizontal fractures may facilitate horizontal growth of hydraulic fractures by acting as planes of weakness that enhance the already marked strength anisotropy due to bedding-parallel laminae and planar fabric. Impacts on hydraulic fracture growth might include height growth inhibition and horizontal propagation. Intensity and morphology of bedding-parallel fractures vary; planar, lens-shaped fractures, and complex, branching geometries are all found. Cement fills are typically calcite or sulphates, sometimes with hydrocarbon inclusions, and may be fibrous or show crack-seal texture. Quartz also occurs, and pyrite is a common accessory mineral. Scaling of faults and subvertical opening-mode fractures has been well documented, with populations following power-law distributions. We investigate whether similar scaling laws apply to bedding-parallel fracture sets. One of the challenges of collecting systematic data for bedding-parallel fractures is the difficulty of distinguishing between fibrous beef-filled fractures and fossils with a fibrous structure, e.g. the Inoceramidae bivalves present in many Mesozoic shales. We show criteria to distinguish these during core-description work, and demonstrate stable isotope and petrographic differences. Different mechanisms may be responsible for fracture generation and more than one mechanism may have operated in the history of a given shale. We will attempt to use stable isotope geochemistry and fluid inclusion analysis to constrain conditions of formation, and thereby narrow down the possible mechanisms for each case. This approach, along with tying the occurrence to lithotype, will allow us to predict bedding-parallel fracture occurrences.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015