“Natural Fracture Systems in the Spraberry Formation, Permian Basin”
Tuesday, November 15th, 2016 – Midland College Carrasco Room 11:30am
Engineering tests in the 1950s and 1960s showed that Spraberry reservoirs in the Permian basin of West Texas generally have a strong, fracture-controlled, NE-SW maximum horizontal permeability trend, but fracture data from vertical and horizontal cores show that the Spraberry natural-fracture system has remarkable variability. Fracture patterns change both vertically and laterally, and fracture permeability is dynamic, changing with variations in reservoir pressure. Horizontal core from a US DOE project in the 1990s showed that at one location, the 1U Spraberry sandstone contains a partially mineralized, NE-SW striking set of extension fractures, where only 140 ft below, the 5U Spraberry sandstone contains a conjugate pair of NNE-SSW and ENE-WSW striking, un-mineralized, minimal-displacement, strike-slip shear fractures. Engineering tests indicate that units containing the conjugate fractures have less anisotropic drainage than units containing the set of parallel extension fractures. Moreover, they demonstrated that fracturecontrolled permeability increased, and unexpected off-trend interference occurred, during injection into the underpressured reservoirs. Core from the interbedded shales is also naturally fractured but more heavily near the bottom than at the top of the shale beds. Examination of the natural fracture populations in more recent horizontal and vertical Spraberry cores has reinforced the conceptual model of strain partitioning, where extension fractures in one reservoir can be dynamically compatible with shear fractures in adjacent reservoirs. Parallel shear fractures can also form in isolation where a fault acts in place of the complimentary shear set of the ideal conjugate pair.