Energy Studies, Volume 9, Number 2, November/December 1983 Page: 2
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This process is under way for the
three geothermal test wells, Pleas-
ant Bayou Nos. 1 and 2 in Texas
and Amoco Fee No. 1 in Louisiana.
Dr. Gray said that the UT studies
of rock behavior have turned up
nonlinear variations in compressibil-
ity, creep, permeability, resistivity,
and other factors related to flow
rates and reservoir characterization.
"We haven't found a thing about
a rock that is constant-not a
thing," he said.
The main studies have dealt with
compressibility and creep. In a typ-
ical lab test of compressibility, salt
water and a sample of sandstone
are placed in a pressure vessel
that simulates geopressured condi-
tions. When the rock has stabilized
under the pressure, the fluid is re-
moved systematically, and the
changes in the compaction of the
sample are measured precisely.
Testing creep. In a test of creep-
how much a rock distorts when
pressure is reduced-the sample is
again put under pressure and al-
lowed to stabilize. Then the pres-
sure is reduced and changes in
the rock are measured over time.
Many rocks creep quite a lot in the
first few hours and less and less
as time goes by, but not all rocks
follow this pattern.
For compressibility, reservoir en-
gineers have often used a linear,i.e., one-to-one, relationship be-
tween pressure and compressibility:
for each unit of pressure applied to
a rock, the rock will compress one
(tiny) unit of volume-no matter the
depth, no matter the pressure al-
ready on the rock.
Dr. Gray said that geopressured
rocks compress significantly less
than was anticipated. The initial
compressibility for a geopressured
reservoir (before production starts)
will be about 1 to 10 x 10-6 psi-,
about one-tenth the values com-
monly assumed.
The studies also indicate that,
while the compressibility will start
high, it will decline during produc-
tion to approximately one-third of
the initial value. Permeability of the
rock is also likely to decline over
the lifetime of the well, to a final
level of 30 to 50 percent of the
original value.
The researchers are now doing
rock compaction studies at
elevated temperatures (385*F) to
see how compressibility and other
rock behavior are affected by
geopressured temperature.
Subsidence is another concern in
the decision to drill and produce a
geopressured well.
"If the surface were to subside
one inch out in a cow pasture, that
wouldn't matter," Dr. Gray said, but
if a manmade facility such as a re-
finery were nearby, one inch ofsubsidence would cause problems.
Removing tons of salt water from
beneath the ground will result in
creep and may result in subsi-
dence. Some of the subsidence ef-
fect can be offset if the water is
reinjected as planned into a shal-
lower, more porous formation.
Creep and subsidence. The creep
tests done so far have given unex-
pected results.
"We knew it would creep-we
were astounded at how much," Dr.
Gray said. The creep will help sus-
tain the pressure, but overall will
have a negative effect on flow be-
cause it will decrease the permea-
bility and porosity of the rock. It
probably will cause long-term sub-
sidence also.
"We may produce the reservoir
for 50 years. It may subside for
200. The truth is, we don't know
yet how long or how much it will
subside."
More creep studies are needed
to follow up on the few that have
been done, he said. "Until creep
behavior can be predicted over
hours, weeks, and months, predic-
tions 50 years and beyond are
subject to substantial error."
At present all three geothermal
test wells are shut in, with full test-
ing yet to be completed. Congress
and the Department of Energy will
decide in the next few months
whether to do so.CES Publications
Geopressured Geothermal
Bibliography, Second Edi-
tion, Vol. 1, Citation Ab-
stracts, Vol. 2, Geopressure
Thesaurus
by Kamy Sepehrnoori,
Frances Carter, Robert
Schneider, Steve Street, and
Kira McGill, Vol. 1, 761
pages, Vol. 2, 204 pages.
A computerized data base, the
Geopressured Geothermal Bibliog-
raphy is primarily a literature search
tool for geopressured-geothermal
researchers. About 300 citations
have been added in the second
edition of the bibliography, bringing
the total to more than 1,100.
The bibliography's citations de-
scribe journal articles, research re-ports, conference proceedings, and
other publications related to geo-
pressured energy resources. The
years covered are 1963 to 1981.
Most of the citations contain an ab-
stract, or summary, of the publica-
tion cited, and other information,
such as how to obtain it.
Quick and accurate searches of
the bibliography are made possible
by six indices and a thesaurus,
which is a list of 3,800 key terms.
Each citation is coded for appropri-
ate key terms, and a user can
search the data base by means of
the key term system.
The development of the Geopres-
sured Geothermal Bibliography was
funded by the US DOE Division of
Energy Technology.
An unbound, computer printout
version ($20) is available from the
Center for Energy Studies. Bound
2paper versions (Vol. 1, $54; Vol. 2,
$18) and microfiche versions ($4
per volume) are available from
NTIS as described in the accom-
panying box.
Publications List Available
A list of Center for Energy
Studies publications can be ob-
tained by writing Publications,
Center for Energy Studies, ENS
143, The University of Texas at
Austin, Austin, Texas 78712.
Many of these publications are
available free from the center (in
single copies only). Most out-of-
print reports can be ordered
from the National Technical Infor-
mation Service, P. O. Box 1552,
Springfield, Virginia 22151
(202/487-4600).0
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University of Texas at Austin. Center for Energy Studies. Energy Studies, Volume 9, Number 2, November/December 1983, periodical, November 1983; Austin, Texas. (https://texashistory.unt.edu/ark:/67531/metapth1032319/m1/2/: accessed July 1, 2024), University of North Texas Libraries, The Portal to Texas History, https://texashistory.unt.edu; crediting UNT Libraries Government Documents Department.