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, Stephen H. Hickman, 2010. "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.", Dipmeter and Borehole Image Log Technology, M. Pöppelreiter, C. García-Carballido, M. Kraaijveld
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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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Borehole imaging is among the fastest and most accurate methods for collecting high resolution subsurface data. Recent breakthroughs in acquisition, tool design, and modeling software provide real-time subsurface images of incredible detail, from the drill bit straight to a workstation. Associated interpretation workflows offer the high level of detail that is needed to make operational decision and to increase the predictability of subsurface models. Many exploration and production companies have acquired a wealth of dipmeter and image log data. The data are readily available and provide, for example the orientation of fractures and fluvial channels in space. Further applications of borehole imaging technology include matrix and fracture characterization, pore-type partitioning, geosteering, and in-situ stress determination. Exciting new applications are found in enhanced oil recovery, carbon dioxide sequestration, and geothermal projects. In addition, borehole image data are paramount to unlocking unconventional plays such as shale gas and coal-bed methane. AAPG Memoir 92 portrays key applications of dipmeter and image log data across the exploration and production life cycle. (Continued)