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Stress, Fracture, and Fluid-flow Analysis Using Acoustic and Electrical Image Logs in Hot Fractured Granites of the Coso Geothermal Field, California, U.S.A.
Nicholas C. Davatzes,1 Stephen H. Hickman2
1U.S. Geological Survey, Menlo Park, California, U.S.A.; present address: Temple University, Philadelphia, Pennsylvania, U.S.A.
2U.S. Geological Survey, Menlo Park, California, U.S.A.
Steve Bjornstad and Frank Monastero of the Navy Geothermal Program Office provided data, insight, and review that greatly helped with this study. Other collaborators include the Coso Operating Company and the Energy and Geoscience Institute (Univeristy of Utah). Funding for N.C. Davatzes was provided by a U.S. Geological Survey Mendenhall Fellowship and for S. H. Hickman was provided by the U.S. Geological Survey Energy and Earthquake Hazards Programs. Judith Sheridan and Dan Moos of GeoMechanics International provided input on previous image log interpretations at Coso.
Development of the Advanced Logic Technology (ALT) ABI85 borehole televiewer was funded by the U.S. Department of Energy and Navy Geothermal Program Office, with engineering oversight and contract management provided by Scott Kuszmaul of Sandia National Laboratory. Valuable assistance in tool testing and field operations at Coso was provided by Joe Svitek of the U.S. Geological Survey, Jean-Luc Deltombe and Jean-Marc Naisse of ALT, and John Stowell of Mt. Sopris Instruments.
Finally, we wish to thank Phil Nelson and Colin Williams at the U.S. Geological Survey; Steve Rogers; and editor Michael Poppelreiter for thoughtful reviews of the manuscript.
Acoustic and electrical image logs in fractured granitic rocks penetrated by U.S. Navy well 58A-10, Coso Wash," in the eastern margin of the Coso geothermal field, California, were compared to evaluate their relative ability to characterize fractures and fault rock textures and to measure stress orientations from borehole failure. Electrical image logs are sensitive to variations in mineralogy or porosity, which affect conductivity. Thus, they capture both open and healed natural fractures as well as rock foliation. In acoustic image logs, fractures and faults are principally revealed by increased roughness of the borehole wall and acoustic impedance contrasts caused by increased microcrack density or hydrothermally altered fault rock. Thus, they reveal rock fabric and healed fractures relatively poorly while favoring open fractures and well-developed fault zones. These tools are thus complementary and fracture characterization benefits from using both. Drilling-induced structures such as breakouts and tensile fractures that form at the borehole wall and petal-centerline fractures that form just ahead of the borehole floor record the orientation of the principal stresses. Although both types of logs produce good images of drilling-induced tensile fractures, acoustic logs are superior to electrical logs in recording the distribution and geometry of borehole breakouts and petal-centerline fractures because they produce a full 360 image of borehole wall reflectivity and radius. Analyses of repeat temperature logs reveal that zones of localized fluid flow coincide with large faults visible in both types of image logs. These faults are characterized by distinctive brittle fracture texture and are well oriented for slip.
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