Active Seismics to Determine Reservoir Characteristics of a Hot Dry Rock Geothermal System Page: 4 of 10
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PROCEEDINGS, Twelfth Workshop on Geothermal Reservoir Engineering
Stanford University, Stanford, California, January 20-22, 1987
ACTIVE SEISMICS TO DETERMINE RESERVOIR CHARACTERISTICS
OF A HOT DRY ROCK GEOTHERMAL SYSTEM.
A S P Green and R Baria
Camborne School of Mines Geothermal Energy Project
Rosemanowes Quarry, Penryn, Cornwall, England
Since 1981 three wells have been drilled to
depths of between 2.0 and 2.6 km in the
Carnmenellis granite, Cornwall, England in
order to create a HDR geothermal system.
These wells are separated by between 150 and
300 m and have been hydraulically connected by
massive injections of both water and viscous
gel (50 cpoise). Passive microseismic monit-
oring of the hydraulic stimulation and circu-
lation experiments has been used since 1982 to
determine the size and structure of the
reservoir, and monitor its growth.
The active seismic survey techniques of
cross-hole seismics and vertical seismic
profiling (VSP) have been introduced to comp-
lement the passive microseismic monitoring in
characterising the reservoir.
The cross-hole seismic surveys indicate that
the microseismicity defines the area of joint
dilation. The attenuation of high frequencies
in the region of microseismicity suggests that
the reservoir is composed of a complex zone of
cracks rather than a single large fracture.
VSP surveys also show a good agreement between
the the microseismically defined reservoir and
seismic signal attenuation.
Recent improvements in hardware, computer
processing and interpretation indicate that
active seismics will play an increasingly
important part in mapping and understanding
The Camborne School of Mines (CSM) has been
researching into hot dry rock (HDR) geothermal
energy since the mid 1970's. The development
of an HDR reservoir at 2 km depth has been
underway since 1980. This has been designated
Phase 2, Phase 1 being a 300 m system dev-
eloped in the late 1970's. Phase 2 is split
into Phase 2A -and Phase 28. Phase 2A
comprised the drilling of two wells (RHll and
RH12) and the subsequent circulation of water
between these two wells. Phase 2B commenced
with the drilling of a third well (RH15)
followed by the fracturing of this and the
circulation of water between RH12 and RH15.
Several aspects of the project have been
presented at previous Stanford Workshops
(Batchelor 1986, Pine 1984). These details
will not be repeated here.
Phase 2A work started in 1981 with the
drilling of wells RHll and RH12 in a Hercynian
granite to true vertical depths (TVD) of
2038 m and 2058 m. The wells have a maximum
deviation of 30* at the base and a closest
approach of 150 m in the uncased sections.
During 18 months of fracturing and cir-
culation, 300 000 m3 of water was injected
into the system and only 100 000 m3 recovered.
In the period in which 200 000 m3 of water was
lost 30 000 microseismic events were detected
of which over 5000 were located. (Baria et
al, 1985). The microseismic locations showed
a systematic downward growth and suggest that
this was where the lost water went (Pine and
Batchelor 1984). This phase of the project
was completed in 1983.
Phase 2B started in 1983 and involved the
drilling of a third well.
Based on the analysis of the microseismic data
a highly deviated third well (RH15) was
drilled to a depth of 2653 m TVD, beneath RHll
and RH12 to intersect the region of seis-
micity. RH15 was drilled in 1984 with a gas
(nitrogen) lift test run immediately. Flow
logs run during the test showed that the only
flowing zones were in regions where the well
intersected microseismicity (Baria and Green
The hydraulic connection between RH15 and RH12
was still not adequate for a prolonged
circulation to take place. A massive viscous
injection of RH15 was designed to improve the
hydraulic connection. The bottom 365 m of
RH15 was sanded off so the hydraulic energy
could be concentrated on a 146 m section of
open well between the casing shoe and the top
of sand. The injection took place on
4 July 1985 when 5500 m3 of viscous gel
(50 cpoise) was pumped into RH15 over an
8 hour period at an average flow rate of
198.2 1/s. This was followed by 200 m3 of
water. The peak wellhead pressure was 15 MPa
during the viscous injection and 16.3 MPa
during the water injection.
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Green, A.S.P. & Baria, R. Active Seismics to Determine Reservoir Characteristics of a Hot Dry Rock Geothermal System, article, January 20, 1987; United States. (https://digital.library.unt.edu/ark:/67531/metadc874928/m1/4/: accessed April 20, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.