Expl New Fields.indd - GRC Geothermal Library

NOTICE CONCERNING COPYRIGHT
RESTRICTIONS
This document may contain copyrighted materials. These materials have
been made available for use in research, teaching, and private study, but
may not be used for any commercial purpose. Users may not otherwise
copy, reproduce, retransmit, distribute, publish, commercially exploit or
otherwise transfer any material.
The copyright law of the United States (Title 17, United States Code)
governs the making of photocopies or other reproductions of copyrighted
material.
Under certain conditions specified in the law, libraries and archives are
authorized to furnish a photocopy or other reproduction. One of these
specific conditions is that the photocopy or reproduction is not to be "used
for any purpose other than private study, scholarship, or research." If a
user makes a request for, or later uses, a photocopy or reproduction for
purposes in excess of "fair use," that user may be liable for copyright
infringement.
This institution reserves the right to refuse to accept a copying order if, in
its judgment, fulfillment of the order would involve violation of copyright
law.
Geothermal Resources Council Transactions, Vol. 28, August 29 - September 1, 2004
Deep Permeable Strata Geothermal Energy (DPSGE):
Tapping Giant Heat Reservoirs within Deep Sedimentary Basins—
An Example From Permian Basin Carbonate Strata
Richard J. Erdlac, Jr.1 and Douglas B. Swift2
1
The University of Texas of the Permian Basin, Center for Energy and Economic Diversification,
4901 E. University, Odessa, TX 79762-0001
2
Swift-Arrow Geological Consulting, 203 W. Wall St., Suite 609, Midland, TX 79701
Keywords
geothermal fields are only a few 10’s of square km in size,
limited by the extent of the plume. Finally, geothermal energy
fields are less than 4 km depth, due to drilling costs associated
with the rock type being drilled, safety considerations, and the
proximity to water sources.
This paper addresses the potential for tapping vast reserves
of heat stored within permeable strata of sedimentary basins
at depths greater than the present 4 km range, utilizing high
volumes of in place brine water. The deep regions of the
Permian Basin (Delaware and Val Verde Basins) have been
Exploration, geothermal, electric power, sedimentary basin,
geothermal gradient
ABSTRACT
Increased electric power production from geothermal
energy sources will require expansion of the geological environments in which such energy can be developed. The oil
and gas industry has long been aware of regions of both high
subsurface temperature and abundant brine water, generally
considered by that industry as a liability. However, these two
commodities are of great importance for geothermal energy
production. The deepest parts of the Permian Basin (Delaware
and Val Verde Basins) have bottom hole temperatures in excess
of 150oC. These regions display a shallow log-normal and a
deep linear temperature gradient that is quite different from
those found by the few past investigations of the area. Deep
gradients on the order of 30oC/km or higher are common.
With proven porosity and permeability, this region has the
potential for future significant geothermal production.
Introduction
Worldwide development of geothermal electric power generation has focused on regions where the geothermal gradient
is anomalously high and where hot water or dry steam can be
produced from relatively shallow depths. The sources of this
heat are active surface volcanism or near surface heat sources
from magmatic plutons that heat the surrounding rock and
associated water. Thus, most nations developing geothermal
energy fall within the region of the world known as “the ring
of fire”, or within other areas where local volcanic activity is
apparent.
Reliance upon these types of heat sources restricts future
development of geothermal energy for electric production.
Existing plant sites are highly site specific, being built over
heat plumes related to volcanic and geyser activity. Existing
Figure 1. A schematic map of the Permian Basin in western Texas and
southeastern New Mexico. The Delaware and Val Verde Basins represent
the deepest parts of the entire Permian Basin, and are of most interest for
geothermal energy extraction as described in this paper. BHT readings
from the four counties displayed in gray (Loving, Reeves, Pecos, Terrell)
were used to conduct preliminary analyses of subsurface temperature and
thermal gradient conditions within these two basins.
327
Erdlac and Swift
groundwater samples throughout the Trans-Pecos region as
part of a 4-year study (El Paso, Culberson, Hudspeth, Jeff
Davis, Presidio, and Brewster Counties). Hoffer concluded,
from silica geothermometry, that seven areas in the Trans-Pecos
existed with subsurface waters above 125oC. These included
areas in northeast El Paso, western and southeastern Jeff Davis,
western and northern Presidio, and southern Brewster Counties. Henry (1979) identified areas in the Presidio and Hueco
Bolsons, and parts of the Big Bend region, with hot springs
whose heat comes from abnormally high (30o to 40oC/km)
geothermal gradients. Henry proposed that thin crust from
Basin and Range extension enhanced heat flow. He posited
that Presidio and Hueco Bolsons represent the best potential
for future geothermal development in the region. No other
research has been undertaken towards more detailed investigation that would lead to geothermal electrical heat production
from this region. Valenza thus concluded that Texas was not
prospective for geothermal development.
prolific natural gas provinces, demonstrating extensive, highly
permeable strata, along with high volumes of subsurface brine.
High bottom hole temperature (BHT) readings have been
known by the oil and gas industry for many decades. These
temperatures and shallower brine zones have always been considered liabilities to drilling and production. However these
oil and gas liabilities are considered assets when geothermal
energy development is considered. Because the Permian Basin
has been seen strictly as an oil and gas province, no rigorous
effort has been mounted to investigate and evaluate the deep
geothermal potential of the region. This paper presents preliminary data that documents a huge store of subsurface heat
capable of being tapped by converting deep, depleted gas wells
into heat extraction wells.
Previous Texas Geothermal Investigations
Evaluation of Texas geothermal potential has focused in
two directions: hydrothermal and geopressured areas (Valenza,
1995). These studies identified three main geothermal regions
within the state. Hydrothermal areas of Central Texas have
documented geothermal resources within Cretaceous aquifers,
stretching in a band from Val Verde County to Red River
County. Temperature gradients often reach 36oC/km at specific
locations (Sorey et al, 1983; Woodruff et al, 1979, 1982). A
Department of Energy (DOE) demonstration project in Marlin, Texas (Falls County) used geothermal hot water for space
and water heating at the Torbett-Hutchings-Smith Memorial
Hospital. The Marlin hot springs was used for balinological
purposes since the late 1800s. None of these hydrothermal
sources have been utilized for electrical power generation.
Geopressure-geothermal potential of Gulf Coast sands
was investigated in the late 70’s and early 80’s. It is considered
non-renewable in nature (Bebout et al, 1978, 1982; Dorfman
and Morton, 1985; Seni and Walter, 1993; Valenza, 1995). A
DOE program gathered data on the feasibility of obtaining
geothermal energy from wells in these Gulf Coast geothermal
zones. But, liabilities and uncertainties about reservoir drive
mechanisms, aquifer capability for extended long-term brine
production, brine disposal, the amount of energy to be recovered, and possible subsidence issues were not answered. No
commercial electrical power generation was established.
Though hydrothermal and geopressure-geothermal have
been the focus of activity in Texas, the hot dry rock (HDR)
potential was briefly suggested for counties in East Texas, where
geothermal gradients are between 45o to 59oC/km (Valenza,
1995). However, no substantial exploration or experimentation
on hot dry rock geothermal resources in Texas has occurred.
The USGS HDR program at Las Valles caldera in New Mexico
could not induce sufficient permeability in the host rock, resulting in premature cooling along the fracture faces.
While most efforts focused on eastern Texas, investigations of the Trans-Pecos hydrothermal region documented
hot springs or water wells with elevated temperatures at or
near the surface (Hoffer, 1979; Henry, 1979). Waring (1965)
defined thermal springs, in West Texas, as those with water
temperatures at or above 30oC. Both Hoffer and Henry followed this definition in their thermal studies. Hoffer collected
Deep Permian Basin Temperatures
Past geothermal work in the Permian Basin is confined to a
1976 study of North America conducted by the USGS and the
AAPG, using well bottom hole temperature (BHT), and to a
more recent preliminary survey conducted within four counties
in the deeper part of the Permian Basin (Figure 1) (Delaware
and Val Verde Basins) by Swift and Erdlac (1999).
The USGS study was highly generalized, with regional interpretation of subsurface temperatures and thermal gradient
distributions calculated from limited BHT readings. Gradient
calculations assumed a linear function from average surface
temperature to a corrected temperature reading from the deepest part of the hole. These data demonstrate that a number of
wells in the Delaware and Val Verde Basins had temperatures
equal to or greater than 143oC. A histogram of their Permian
Basin data showed that thermal gradients ranged from around
11.6oC/km to over 28oC/km, with a mean gradient value of
about 19oC/km (Figure 2). However, Swift and Erdlac (1999)
Figure 2. Histogram of thermal gradients from Permian Basin BHT
readings used by the USGS and the AAPG in the 1976 heat study of North
America. Note range and average (~19°C/km) gradient for entire Permian
Basin. Delaware-Val Verde Basin data from 1976 study are in same range
as all Permian Basin data. Newer analyses do not support this conclusion.
328
Erdlac and Swift
temperatures encountered, due to borehole cooling during
drilling. Formation temperatures may be 12- 40°C higher than
temperatures reported on electric log headers (Bullard, 1947;
Gretner, 1981; Jam et. al., 1969).
Preliminary work developed a well data file containing
information from 2,758 narrow and large-scale logs, representing 11% of the 24,000 wells drilled in the Delaware-Val Verde
region. Many of these wells did not exist at the time of the
1976 USGS/AAPG study. BHT readings from log headers
covered a depth range of a few hundred meters to over 8,000
meters. These wells are located within Reeves (573), Loving
(13), Pecos (2,058), and Terrell (114) Counties (Figure 1). The
number of BHT readings available for this study was 3,623,
being divided between Pecos, at 2,558, Reeves, at 833, Terrell,
at 196, and Loving, at 36.
Graphing BHT data against depth (Figure 3A, B, C) delineates a pronounced clustering of 95% of the data within
well-defined temperature-depth boundaries. Many BHT
readings were found into the 180oC range, with a few higher
readings reported. Preliminary analyses of gradient values
used three sets of assumptions: 1) a logarithmic temperature
gradient; 2) multiple linear functions; and 3) a combination of
logarithmic and linear functions (Figure 3A, B, C).
Temperature-depth plots demonstrate that these data
do not follow a single linear function from surface to BHT
depth. A substantial change in temperature gradient occurs
at moderate depth, separating shallow and deep thermal gradient regimes. This is readily seen when the data is graphed
log-normally. A well-defined log-normal curve was calculated
that fits most of the data. The fit for shallower temperature
data coincides well with this distribution, but deeper well
BHT readings tend to be lower for a specific depth than that
predicted by the log-normal curve.
A second approach assumes that two linear functions
characterize the data. Line intersection was established by
demonstrated that an assumed linear distribution was invalid
in the Permian Basin.
The deepest part of the Permian Basin is within the Delaware and Val Verde sub-basins. Penetrations reach 9,046 m.
Preliminary investigations begun in 1999 (Swift and Erdlac,
1999) and continued to date, focus on BHT data reported
on electric logs. These BHT readings represent minimum
Figure 3. Point plots of bottom hole temperature (°C) as a function of
depth (m). A. This plot displays BHT data as a log-normal distribution.
The black line represents a logarithmic curve of data from all four
counties. BHT readings in Pecos and Reeves Counties lie on top of each
other. Terrell County displays somewhat hotter temperatures while Loving
County is slightly cooler. B. This normal plot of BHT data shows the log
curve calculated from A as well as an interpretation of data using two
linear functions to describe shallow and deep data respectively. These
lines were used to calculate thermal gradients as outlined in Table 1. C.
This plot shows both a logarithmic fit for shallow data and a linear fit for
the deeper data. A temperature cut off of 72° was chosen for best fit of
data to the lines.
Table 1. Thermal gradient calculations based upon a linear function
interpretation of shallow and deep temprature-depth plots. Note that deep
gradient is usually twice that for the shallow gradient. Data for Loving
County is an exception, however only 13 wells were available for study
during this preliminary investigation.
329
Erdlac and Swift
calculating the determination coefficient (R2) for each line,
and maximizing the value Rs2Rd2, s and d being shallow and
deep respectively. This optimized line intersection at 72oC,
or nearly 3,300 m. Total shallow amd deep thermal gradient values were found to be 15.5 and 28.9°C/km respectively.
Interpretive analysis of the data in each county gave gradient
values in the 26 to 33°C/km range, and in one case possibly
reaching 58oC/km (Table 1). These deep gradients exceed the
aUSGS/AAPG verage Permian Basin gradient of 19oC/km.
Deep data in the second analysis display a good fit to the
straight-line assumption, but shallower data do not have as
good a fit as that obtained with a log-normal function. A third
option is a combination of these two approaches, optimizing
the change from shallow to deep in the same manner as in
the dual linear approach. Shallow data is described by a lognormal distribution while deep data are described by a linear
function, with a thermal gradient of 28.9°C/km. The reason
for the shallow log-normal distribution is unclear. Evaporite
strata, thick shale, and highly porous Delaware sands that
transport heat out of the system may all contribute to a variable thermal gradient. Further detailed study is necessary to
clarify this observation.
The deep Permian Basin has a high percentage of dolostone
and limestone, especially within Devonian, Fusselman, and Ellenburger strata. The 1989 EPRI Soil and Rock Classification
Field Manual reports thermal conductivity ranges of 1.1-5.2
W/m-oK for limestone and 2.25-6.35 W/m-oK for dolomite
(dolostone). Using these values, a 50/50 limestone/dolostone
composition results in an average thermal conductivity of
3.725 W/m-oK. Thus a deep subsurface heat flux of over 100
mW/m2 is possible, a value in excess of heat flux values (40-50
mW/m2) formerly reported in the region. The 100 mW/m2
heat flux comparable to other western areas where geothermal
energy is or will be developed. It is important to acquire direct
conductivity measurements for these deep strata before a final
heat flux value can be accurately determined.
In addition to plotting BHT readings versus depth, several
deep gas fields were reconnaissance surveyed to determine
average BHT temperatures. These fields include Toro, Gomez, Chapman Deep, Worsham Bayer, and Brown Bassett.
They display BHTs ranging from 115oC to >160oC, temperatures clearly within the binary plant operational optimum.
Chapman Deep, in northern Reeves County, has an average
temperature of 145oC. Toro and Worsham Bayer fields, in the
eastern part of Reeves County, have BHTs of 158oC and 127oC,
respectively. Gomez (155oC) and Brown Bassett (115oC) are
in Pecos and Terrell Counties, in that order.
basins, as recognized by the oil and gas industry. Similarly the
oil and gas industry has not seen the abundance of hot brine
as an asset in energy production.
In 1999, the National Renewable Energy Laboratory indicated the cost for constructing a 10 Mw plant at $15 million
(~$1,500 per kw). This cost was for temperatures suitable for
binary plant development (110-193oC) and included exploration and drilling. The importance of the DPSGE approach
is the potential for greatly reducing the cost of geothermal
energy with a ‘meme’ change in the economic structure for
energy acquisition.
Cost savings for geothermal energy production will be
reduced along three lines. First, existing infrastructure of billions of dollars worth of existing well bores can be utilized to
vector heat to binary plants. Depleted wells can be converted
into heat production wells, greatly reducing or even eliminating
the need for major drilling costs. Second, the vast amount of
subsurface data can be used for site-specific geothermal development. Mature basins already have a wealth of subsurface
data acquired during oil and gas development and waiting to
be used for geothermal development. Detailed subsurface
temperature and gradient profiles can be established, as well
as subsurface structure, reservoir geometry and aerial extent,
stratigraphy, permeability, fluid migration, brine flow rates, and
subsurface pressure distributions. This greatly reduces cost of
exploration and development for subsurface heat production.
Finally, a successful application of this approach would change
the face of both the geothermal and the oil and gas industries.
Presently, these industries act independent of each other, with
economics based solely on the commodity being developed.
Heat energy production from sedimentary basins will foster
a coordinated exploration program by both industries, with
national and international implications.
A successful move into deep permeable strata geothermal
energy requires greater involvement and support of both the oil
and gas and geothermal industries to obtain, analyze, and act
upon the findings of subsurface heat research. It will, for the
first time, tap the geothermal gradient found below the existing 4 km economic limit, greatly expanding geographic areas
where geothermal energy can be harnessed. A more thorough
and detailed investigation must be undertaken to identify
specific areas, fields, and wells that can be converted into heat
extraction/water injection systems for electrical power generation. Such support must come from the industry itself.
A binary plant developed within a deep sedimentary basin,
either through conventional technology of water transport to
the surface or through more unconventional methods presently
being developed (i.e. Power Tube, Inc.), will have far reaching
consequences for improving our nation’s energy future. An
increase in the aerial coverage of geothermal power plants will
address important homeland energy security by reducing the
use of fossil fuels for electrical power generation. It will free
fossil resources for other crucial purposes, including product
manufacturing, and will function as a true non-interruptible
energy resource.
Human development began through harnessing and using
renewable resources (wind, water, wood, food). This renewable
Relevance And Conclusions
The oil and gas and the geothermal industries are like
brothers who rarely speak to each other. Although many of
the same subsurface engineering and geoscience techniques are
used in both, each industry has focused upon specific geological environments for entirely different energy goals. Thus the
geothermal industry tends to be unaware of high temperatures
and thermal gradients, documented permeability, porosity, and
abundant formation brine that exist within deep sedimentary
330
Erdlac and Swift
Henry, C. D., 1979, Geologic setting and geochemistry of thermal water
and geothermal assessment, Trans-Pecos Texas: The University of
Texas Bureau of Economic Geology, Report of Investigation No.
96, 48 p.
energy society expanded logarithmically by the discovery and
use of nonrenewable energy sources (fossil fuels). Now the age
of fossil fuels for energy is more uncertain due to significant
depletion. But heat is abundantly present in the subsurface,
and expansion of geothermal is needed. It will require the will
to acquire it and thinking in long-term energy needs rather
than sole focus on next quarter profits. Our energy future is
in our hands.
Hoffer, J. M., 1979, Geothermal exploration of western Trans-Pecos
Texas: Science Series No. 6, Texas Western Press, The University of
Texas at El Paso, 50 p.
Jam, P.L., P.A. Dickey, and E. Tryggvason, 1969, Subsurface temperatures
in South Louisiana: AAPG, vol. 53, no. 10, p. 2141-2149.
Seni, S.J., and T.G. Walter, 1993, Geothermal and heavy-oil resources in
Texas: Direct use of geothermal fluids to enhance recovery of heavy
oil: University of Texas Bureau of Economic Geology, Circular
GC9303, 52 p.
Acknowledgements
We are grateful to the data support shown to us by Jon
Olson while at Sul Ross. We also thank Paul Spielman of Coso
Operating Company for reviewing this paper. Any errors in
this paper are the sole responsibility of its authors.
Sorey, M.L., M.J. Reed, D. Foley, and J.L. Renner, 1983, Low-temperature
geothermal resources in the central and eastern United States, in Reed,
M. J., ed., Assessment of low-temperature geothermal resources of
the United States – 1982: U.S. Geological Circular 892, p. 51-65.
Swift, D.B., and R.J. Erdlac, Jr., 1999, Geothermal energy overview and
deep permeable strata geothermal energy (DPSGE) resources in the
Permian Basin, in Grace, D.T. and Hinterlong, G.D., ed., The Permian
Basin: Providing Energy For America: West Texas Geological Society
Fall Symposium, Publication 99-106, p. 113-118.
References
Bebout, D.G, R.G. Loucks, and A.R. Gregory, 1978 Frio sandstone reservoirs in the deep subsurface along the Texas Gulf coast – their potential for production of geopressured geothermal energy: University
of Texas Bureau of Economic Geology, reprinted 1983, RI91, 92 p.
Valenza, J., 1995, Geothermal energy, in Faidley, R., ed., Texas Renewable
Energy Resources Assessment: Survey, Overview and Recommendations: Virtus Energy Research Associates, p. 115-126.
Bebout, D.G., B.R. Weise, A.R. Gregory, and M.B. Edwards, 1982, Wilcox
sandstone reservoirs in the deep subsurface along the Texas Gulf
coast – their potential for production of geopressured geothermal
energy: The University of Texas, Bureau of Economic Geology,
RI117, 125 p.
Waring, G. A., 1965, Thermal springs of the United States and other
countries of the world – a summary: W. S. Geological Survey Professional Paper 492, 383 p.
Bullard, E.C., 1947, The time necessary for a bore hole to attain temperature equilibrium: Monthly Notices, Roy. Astr. Soc. London, Geophys.
Suppl., vol. 5, no. 5, pl 127-130.
Woodruff, C. M., Jr., and M.W. McBride, 1979, Regional assessment of
geothermal potential along the Balcones and Luling-Mexia-Talco
fault zones, Central Texas: The University of Texas at Austin, Bureau
of Economic Geology, Open-File Report OF-1979-0001, 145 p.
Dorfman, M. and R. Morton, eds., 1985, Geopressured geothermal
energy: Proceedings of the sixth U. S. Gulf Coast geopressured geothermal energy conference: Pergamon, New York, 344 p.
Woodruff, C. M., Jr., S.C. Caran, C. Gever, C.D. Henry, G.L. Macpherson, and M.W. McBride, 1982, Geothermal resource assessment for
the State of Texas: The University of Texas at Austin, Bureau of
Economic Geology, Open-File Report OF-1982-0001, v. 1, 248p, v.
2, Appendices A – D, v. 3, Appendices E - H.
Gretener, P. E., 1981, Geothermics: Using temperature in hydrocarbon
exploration: AAPG Education Course Note Series #17, 156 p.
331
332