WHOLESCALE - Characterization of Conductive Fractured Zones Based on Borehole Data at San Emidio Geothermal Field, Nevada


Hiroki SONE, Oddy MUDATSIR, Zirou JIN, Matt FOLSOM, Gabrielle RAMIREZ, and Kurt L. FEIGL

Key Words:

WHOLESCALE, San Emidio, wireline logging, mud logging, damage zone


Stanford Geothermal Workshop




Enhanced Geothermal Systems



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4948 KB

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Successful heat production from the San Emidio Geothermal Field, Nevada, operated by Ormat Technologies Inc., highlights the existence of conductive pathways for subsurface fluid flow between the injection and production wells. These zones of highly permeable rock formations are identified mainly from drilling records that indicate zones where drilling breaks and lost circulation occur. Interpolation between the high-permeability zones identified in each well allow us to estimate the approximate location and orientation of the first-order planar structure (i.e., fault zone) that constitute the conductive pathway in the subsurface. However, the detailed structural nature of these permeable zones (e.g., fracture distribution, fracture orientation, gouge fill, thickness, and aperture) are still unknown. Such information is essential for conducting a geomechanical analysis to calculate the mechanical response of the permeable zone to injection and production activities. We integrate lithological, structural, petrophysical information from mud, image, and sonic logs to characterize the permeable zones within the reservoir. Lithological boundaries identified in mud logs are used to infer fault planes necessary to match known permeable zones and offsets in lithology. Resistivity image logs reveal abundant natural fractures, potential fault zones (some of which are thicker than tens of feet) that host numerous open fractures and conductive rock units, as well as some potential drilling-induced tensile fractures. Sonic log data also shows low-velocity zones correlated with potential fault zones identified from the image logs. Sonic reflections also reveal the presence and clustering of reflective fracture planes in the vicinity of the borehole. In this paper, we provide a snapshot of work in progress, focusing on the geologic characterization of the permeable zones in the reservoir. Observations show that the most permeable fractures are not always found within the major fault zone structure, suggesting the importance of dissolution/precipitation processes in establishing feed zones. The work presented herein has been funded in part by the Office of Energy Efficiency and Renewable Energy (EERE), U.S. Department of Energy, under Award Number DE-EE0009032.

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