A New Approach to Fluid-Flow Modeling of Directional Permeability in a Faulted Anticline, Little Sand Draw Field, Big Horn Basin, Wyoming.

Abstract

Abstract Little Sand Draw field, Hot Springs County, Wyoming, is a fractured and faulted asymmetric anticlinal oil reservoir. The main producing formation is the Permian Phosphoria Formation. The goal of this project1 was to apply a new geological and engineering approach to simulate directional permeability in a faulted and fractured oil reservoir. The hypothesis to be tested in this study was whether reservoir models with or without tear faults provide a good history match. Tear faults, which have apparent strike slip offset and occur at high angles to the fold axis, have been quantified at Thermopolis anticline, an analogous structure 6 mi (10 km) to the south. Tear faults are largely unrecognized in subsurface reservoirs and are commonly overlooked in simulations. However, they may have important effects on directional permeability. Anisotropic directional permeabilities, roughly perpendicular to fold axes, are known from pressure-interference tests in the Little Sand Draw field. The spacing of faults in outcrop was used as input for faults in the reservoir model. A 3-D geological model that correctly honors the structural geology was built using EarthVision (Dynamic Graphics) software. Using the ECLIPSE black-oil simulator (Geoquest, Schlumberger), three fluid-flow models were generated: (1) an unfaulted-unfractured model, (2) a faulted-unfractured model, and (3) a faulted-fractured model which simulated high permeability conduits along the tear faults. Fractures were simulated by assigning permeability anisotropy to the matrix blocks. History matching performed on these models tested which one of them best resembles actual field performance. The unfaulted-unfractured model could not produce enough total fluids to match historical data, and reservoir pressures were too low. The faulted-unfractured model improved the history match, but could not match production data. The faulted-fractured model does the best job of matching the observed past performance at Little Sand Draw.