A History of Geothermal Energy Research and Development in the ...

EXPLORATION
In the early 1980s, the U.S. Air Force (USAF) asked that DOE undertake
exploration of Ascension Island to determine whether geothermal energy could
generate some or all of its electrical-power requirements. The USAF was generating
power by burning jet fuel in diesel generators. DOE provided supplementary
support for research on the volcanic island. The exploration project included
geologic mapping, geophysical and geochemical surveys, developing an exploration
model, and drilling thermal-gradient and test wells. 37
Geologic mapping documented the presence of young felsic volcanic rocks,
indicating the possibility of a shallow magmatic heat source. 38 Due to the
presence of young volcanic cover, aeromagnetic and electrical-resistivity surveys
were conducted to locate buried faults that might be conduits for hydrothermal
convection. The aeromagnetic survey identified east- and northwest-trending
magnetic sources interpreted as mafic dikes emplaced along structures that fed
the volcanic centers peripheral to the central core of the island. 39 Northeast-,
northwest-, and north-trending magnetic signatures and low electrical resistivities
(5-10 ohm-meters) were observed in the weakly magnetic central part of the
island. 40 These geophysical signatures were interpreted to reflect the presence of
altered rocks and possibly geothermal fluids at depth.
Seven core holes were drilled to depths of about 500 meters (1,600 feet) to obtain
subsurface samples, measure temperature gradient and heat flow, and site a deep
exploration test well. Test well Ascension #1 was drilled at production diameter to
a depth of 3,126 meters (10,256 feet). 38/41 The well encountered a temperature of
247°C (477°F) in propylitically altered rocks at total depth (TD). A subsequent
flow test showed that flow rates were sub-commercial and production could not be
sustained. Temperature, pressure, gamma-ray, sonic, and dipmeter logs were run
in the well. The well encountered acidic fluids at TD, indicated by high hydrogen
sulfide (H 2
S), carbon dioxide (CO 2
), hydrogen-2 (H 2
), and methane (CH 4
)
in the return line. As a result, the bit and bottom-hole assembly were severely
corroded. The well was plugged back for a sidetrack but was lost in the sidetracking
effort. The exploration program was terminated at that point by the USAF.
Many reports documented the exploration of the interesting environment on
Ascension Island. The core and cuttings as well as original data are housed at EGI.
The exploration project discovered and documented the existence of temperatures
high enough for power generation. There remains the possibility that a permeable
part of the heat source could be found by further exploration. The area would
also seem to be a potential candidate for Enhanced Geothermal System (EGS)
development. Further exploration may be warranted if and when development
of a local energy source for baseload power generation becomes a higher priority.
36 A History of Geothermal Energy Research and Development in the United States | Exploration