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Characterization of Opening-Mode Fracture Systems in the Austin Chalk
The Upper Cretaceous Austin Chalk is a low-permeability, fractured reservoir that has been the target of numerous horizontal wells in Texas. For wells to be successful, parameters such as well-bore azimuth, target horizon, and length must be optimized. Furthermore, information on fracture height and the storage volume potentially connected to the well bore is required. Two case studies are presented in which the height, aspect ratio, and vertical persistence of opening-mode fractures in the Austin Chalk have been examined, together with previously known attributes of fracture population attributes of orientation, aperture, effective aperture, spacing, aspect ratio, fracture fill, and fracture permeability. The studies are of an outcrop of the upper Austin Chalk near Waxahachie, North Central Texas (Grove Creek), and of a vertical segment and two horizontal laterals of a core in the Atco Member in Frio County, Pearsall Field (Kinlaw core). Large, potentially open fractures are commonly clustered, the distance between clusters ranging from ~1 to ~50 m. Aperture-size distributions follow power laws, and spacing-size distributions are negative-logarithmic or lognormal. The aperture size at which fractures are open to fluids is variable (0.14 to 11 mm). Fracture permeability, which is scale dependent, has been determined at 7.1 D (for 18 m of lower Austin Chalk core) and 286 D (for 300 m of upper Austin Chalk outcrop). Fractures may terminate at chalk-marl contacts, tip out within the marl, or pass through the marl layers. Fracture height is governed by truncation effects at marl horizons and pressure-solution seams, which in turn partly depend on fracture aperture and thickness of the inhibiting layer.
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