Borehole Image Analysis and Geological Interpretation and Modeling of Selected Features in Well DP 27-15 at Desert Peak, Nevada: Pre-Stimulation Evaluation of an Enhanced Geothermal System

K. M. Kovac¹, S. J. Lutz², P. S. Drakos³, J. Byersdorfer⁴, and A. Robertson-Tait^5
1Schlumberger, Data and Consulting Services, Bakersfield, CA, [email protected]
²TerraTek, A Schlumberger Company, Salt Lake City, UT, [email protected]
³ORMAT Nevada Inc., Reno NV, [email protected]
⁴Schlumberger, Data and Consulting Services, Greenwood Village, CO, [email protected]
⁵GeothermEx Inc., Richmond, CA, [email protected]

An industry-DOE cost-shared project is underway to evaluate the technical feasibility of developing an Enhanced Geothermal System (EGS) power generation project in Desert Peak (Nevada) geothermal field. As part of pre-stimulation analysis, a non-commercial well (DP 27-15) in the hydrothermal portion of the field has been image-logged to aid in evaluating lithologies and stress and fracture characterization of potential reservoir units. Borehole images, gamma ray, caliper, and cable tension were logged. The wellbore image log obtained from deeper portions of the well (3057.5-5620 ft) has been analyzed. Features identified from these resistivity-contrast generated images include bedding planes, lithologic contacts, foliations, conductive mineral grains, drilling induced fractures, and natural fractures. This paper describes selected geologic features seen on the image logs that may influence the design and success of stimulation activities in the potential reservoir, as part of a multi-disciplinary approach to understanding EGS systems.

The logged interval of the well consists of three parts. From 3057.5 to 3300 ft the altered Tertiary rhyolitic tuffs interval is encountered. Below this, the pre-Tertiary Unit 1, consisting largely of weakly metamorphosed hematitic and dolomitic mudstones occurs between 3300 and 4800 feet. Pre-Tertiary Unit 2 from 4800 to 5620 ft includes strongly propylitically-altered and metamorphosed diorites, foliated phyllites, and hornfels.

The drilling induced fractures present suggest a maximum horizontal stress orientation of NE-SW, consistent with the nearby Rhyolite Ridge Fault System. The largest natural fracture population identified is striking NE-SW consistent with the identified maximum horizontal stress orientation and the Rhyolite Ridge Fault System. A statistical fracture spacing of approximately one fracture every 20 ft seems to be consistent through each of the major lithologic units, suggesting this is a young fracture population. Single well fracture network modeling was used to better visualize the fracture populations identified and characterized in this study.

AAPG Search and Discovery Article #90088©2009 Pacific Section Meeting, Ventura, California, May 3-5, 2009