Characterization of bedding-parallel fractures in shale : morphology, size distribution and spatial organization

Natural fracture systems are important for production in shale gas reservoirs as they may contribute to permeability of the reservoir, or they may reactivate during hydraulic fracture treatment. However, little is known about their size scaling and spatial distribution. Bed-parallel, calcite-filled fractures are common in shale. Knowing the aperture-size scaling and spatial organization of bed-parallel fractures may contribute to improved modeling of the combined fracture network (hydraulic and natural). Ten fracture data sets were collected from the Vaca Muerta (7), Marcellus (2) and Wolfcamp (1) shale formations. Bed-parallel fracture attributes such as strike, dip, aperture size, spacing, length and texture were collected from outcrops of the Vaca Muerta Formation in the Neuquén Basin, Argentina. Further fracture aperture-size and spacing data for the Vaca Muerta, and for the Marcellus and Wolfcamp, were collected through measurement direct from cores, and from photographic panels of slabbed core. A total of 1093 fractures were measured along 10 scanlines of total combined length of 629m. The aperture size of bed-parallel fractures ranges over 4 orders of magnitude, from 15 µm to 87 mm. Nine out of ten datasets follow a negative exponential distribution. Fracture attributes such as intensity and size range are different in the 3 studied shales. Even within the same shale formation, fracture intensity and size range can be variable. Aperture size ranges of bed-parallel and vertical fractures in these shales are comparable as are fracture intensities for the Marcellus examples. Bed-parallel fractures, however, have higher intensities than vertical fractures in the Vaca Muerta examples. Spatial organization of bed-parallel fractures is investigated using a normalized two-point correlation technique that allows distinction between clustering, regular spacing and a random distribution. The relationship between fracture spatial organization and stratigraphy and mechanical interfaces within the host rock is also investigated, with preliminary results suggesting that bed-parallel fractures are more intense in organic-rich layers in some cases, but not in others.