A Case Study from FORGE Well 21-31, Fallon, Nevada, USA

Targeting structurally controlled permeability in geothermal fields remains a challenge because of the difficulties in characterizing subsurface structures in terms of their hydraulic behavior within the reservoir. Measurement sensitivity in combination with approximately 80% circumferential coverage (in an 8-1/2-inch wellbore) makes the FMI™ fullbore formation microimager useful for identifying subsurface structures and providing direct data on in-situ stress orientations in the formation intersected in the logged interval. Accurate, high-resolution wellbore images paired with skilled image interpretation enables fractures and faults to be characterized as open (hydraulically conductive) or closed (non-conductive, typically due to mineral deposition). Knowledge of the depths and orientations of permeable structures is beneficial at the local scale (within the logged well) and at the reservoir scale. The improved understanding of permeability distribution is used to optimize operations and improve decisions about future well targets in conventional hydrothermal projects and particularly in Enhanced Geothermal Systems (EGS) projects.

FORGE well 21-31 in the Fallon geothermal area of Nevada was logged on behalf of the U.S. Department of Energy’s Geothermal Technologies office using Schlumberger’s FMI, Platform Express Triple Combo, Sonic Scanner™ acoustic scanning platform, and Temperature and Pressure Survey tools. These logs were run in the 8-1/2-inch diameter production interval of the well (6,080-8,139 feet-MD), and the results have been analyzed and interpreted. This has enabled improved classification of fractures, including basic properties (direction, density/intensity, and aperture), and determination of whether the fractures are open or healed. With respect to the last, the sensitivity of the measurement of borehole imaging plus the broad range of Stoneley-mode frequencies have been particularly valuable. Electrically-conductive fractures are observed in much of the logged interval; they show a dominant strike orientation of NNE-SSW, with dip magnitudes mostly in the range of 60°-88°. Drilling-induced fractures and borehole breakouts are also observed, with dip statistics showing a dominant strike orientation of NNE-SSW for drilling induced fractures and ESE-WNW for borehole breakouts. Well 21-31 is vertical; thus the strikes of drilling induced fractures and borehole breakouts tend to align with the trends of maximum and minimum horizontal stresses, respectively. The interpretation of the logging results is consistent with the predominant structural / tectonic trend in the area (most mapped faults are oriented N-S to NNE-SSW).