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.

Abstract

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.