A cost-shared industry-USDOE project is underway to evaluate the technical
feasibility of developing an EGS power generation project on the eastern side of the
Desert Peak geothermal field. An existing well (DP 23-1) is the focus of much of the
Phase I investigation, including re-interpretation of lithology, acquisition and
analysis of a wellbore imaging log, and conducting and analyzing a step-rate
injection test. In addition, 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 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 TD (2,939 m or 9,641 feet) and is likely to have
considerable lateral extent. A wellbore image log obtained over a significant
portion of the open hole has been analyzed in terms of the distribution and
orientation of natural fractures and borehole failure phenomena (tensile fractures
and breakouts). The features analyzed from the image log have been used to evaluate
the orientation of the stress field and constrain the magnitudes of the principal
stresses. These analyses permit an evaluation of the effects of pore pressure
increase on pre-existing fractures, and, in conjunction with lithology, mineralogy,
drilling rate and geophysical log data, have been used to identify the most
prospective interval for stimulation. Future plans in Phase II include undertaking a
“mini-frac,” re-completing the well in preparation for hydraulic stimulation, and
planning, conducting, monitoring and evaluating a massive hydraulic stimulation.
Should the stimulation result in the creation of a large enough reservoir, a second
and perhaps a third well would be drilled and stimulated, and the system would be
tested for several months to determine its capacity. In Phase III, a 2-5 MW
stand-alone binary power plant would be designed and constructed at Desert Peak East,
and in Phase IV, the power would be either sold to a utility customer or used to
supply the parasitic power needs of the existing Desert Peak hydrothermal power
plant. 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
(DP 27-15).
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