Bureau of Economic Geology, The University of Texas at Austin (www.beg.utexas.edu).
West Texas Geological Society Fall Symposium, Midland, Texas, October 25-26, 2001
Characterization Using Rotary-Drilled Sidewall Cores:
Gomez1, Julia F. W. Gale2, Stephen C. Ruppel2,
The understanding of fluid flow in fractured hydrocarbon reservoirs is hindered by low data density that prevents effective fracture-attribute mapping. Sparse sampling of large fractures is unavoidable, and collection of meaningful, systematic data at the wellbore and extrapolation into the interwell volume are significant challenges. New techniques that use microstructures to predict orientation and fill of macrofracture sets have been successfully applied using rotary-drilled sidewall cores.
The Ellenburger dolomite in West Texas has been a successful play for many years, but recovery of original oil in place may be only 17% in some fields. Because fractures are thought to play a significant role in the permeability of the Ellenburger Formation, a fracture characterization project was undertaken as part of a joint advanced recovery initiative between Goldrus Production Company and The University of Texas System. Fracture characterization commenced using 75 sidewall cores, drilled from open-hole sections in two ³ 45-year-old wells, A and B. Core azimuths from well A were measured on an image log run after the cores were taken. The wellbores, which had received several acid treatments, were badly altered, however, resulting in poor-quality image logs. Additionally, the log in well B was run prior to coring, so these cores could not be oriented. Core tops from well A were oriented, and orientation certainty was ranked using methodologies developed at the University of Texas. Of the 75 sidewall cores, 38 could be completely oriented. Of these, 26 cores had natural fractures, visible with a hand lens. Fracture azimuth and dip were measured in 14 cores that had a core-top-certainty rank of 3 or higher (out of a maximum possible rank of 24). Fractures in cores having low orientation certainty, or unorientable cores, were examined petrographically but were not included in the orientation analysis.
Two sets of steeply dipping fractures, striking NW-SE and NE-SW, are recognized on a stereographic projection of the 17 oriented fractures. This is in marked contrast with the four moderately dipping fractures discernible on 116 ft of image logs, which in isolation could give no clear indication of dominant fracture orientation. Fractures observed on image logs of a recently drilled vertical well are consistent with the two orthogonal fracture sets. Petrographic analysis and SEM-based cathodoluminescence observations of horizontal and vertical thin sections from the cores indicate early migration of hydrocarbons through the fracture system, followed by later precipitation of dolomite cement. This information would not be obtainable from image logs.
of Geological Sciences, The University of Texas at Austin
F. W. Stowell, Research Associate