The Desert Peak East EGS Project: A progress report

Enhanced Geothermal System” (EGS) power generation project on the eastern side
of the Desert Peak geothermal field. An existing well (23-1) is the focus of much of
the investigation, including re-interpretation of lithology, acquisition and analysis of
a wellbore image log, injection testing, performing a “mini-frac” to determine the
magnitude of the least principal stress, and re-completing the well in preparation for
later hydraulic stimulation. Numerical modeling has been undertaken to estimate
heat recovery and make generation forecasts for various stimulated volumes and
well configurations.
The target formations for hydraulic stimulation in well 23-1 lie below an unstable
phyllite which bottoms at about 1,740 m (5,700 feet). The formations beneath this
unit include a section of Jurassic/Triassic metamorphic rocks (of which the phyllite is
a part) and an underlying, younger (Cretaceous?), massive granodiorite that
intrudes the older rocks above. This granodiorite unit extends from 2,140 m (7,020
feet) to total depth of 23-1 (2,939 m or 9,641 feet), and is likely to have considerable
lateral extent. Analysis of the distribution and orientation of natural fractures and
borehole failure features (tensile fractures and breakouts) from an image log in well
23-1 indicate that the direction of the maximum horizontal stress is between N28°E
and N38°E and a predominance of NNE-striking fractures. Both observations reflect
the current stress field as indicated by regional and local geologic data. A stress
model was developed to permit an evaluation of the pore pressure increase required
to initiate shear failure on optimally oriented pre-existing fractures. The stress field
analysis, in conjunction with lithology, mineralogy, drilling rate and geophysical log
data, have been used to identify the deeper section of the granodiorite as the most
prospective interval for stimulation.
Work remaining to be completed in Phase I includes the “mini-frac” and re-completion
of well 23-1 in preparation for hydraulic stimulation. Future plans for the
project in Phase II focus on planning, conducting, monitoring and evaluating a massive
hydraulic stimulation of well 23-1. Should the stimulation result in the creation of
a large enough reservoir, additional drilling, stimulation and testing would be conducted.
A recent additional focus of the project involves evaluating the feasibility of
stimulating a non-commercial well drilled in the hydrothermal portion of the field.