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2 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. Swift 2 1 The University of Texas of the Permian Basin, Center for Energy and Economic Diversification, 4901 E. University, Odessa, TX Swift-Arrow Geological Consulting, 203 W. Wall St., Suite 609, Midland, TX Keywords 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 150 o C. 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 30 o C/km or higher are common. With proven porosity and permeability, this region has the potential for future significant geothermal production. 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 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

3 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 36 o C/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 45 o to 59 o C/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 30 o C. Both Hoffer and Henry followed this definition in their thermal studies. Hoffer collected 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 125 o C. 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 (30 o to 40 o C/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. 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 143 o C. A histogram of their Permian Basin data showed that thermal gradients ranged from around 11.6 o C/km to over 28 o C/km, with a mean gradient value of about 19 o C/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

4 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 temperatures encountered, due to borehole cooling during drilling. Formation temperatures may be 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 180 o C 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 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

5 calculating the determination coefficient (R 2 ) for each line, and maximizing the value Rs 2 Rd 2, s and d being shallow and deep respectively. This optimized line intersection at 72 o C, 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 58 o C/km (Table 1). These deep gradients exceed the ausgs/aapg verage Permian Basin gradient of 19 o C/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 W/m-oK for limestone and W/m-oK for dolomite (dolostone). Using these values, a 50/50 limestone/dolostone composition results in an average thermal conductivity of 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 115 o C to >160 o C, temperatures clearly within the binary plant operational optimum. Chapman Deep, in northern Reeves County, has an average temperature of 145 o C. Toro and Worsham Bayer fields, in the eastern part of Reeves County, have BHTs of 158 o C and 127 o C, respectively. Gomez (155 o C) and Brown Bassett (115 o C) are in Pecos and Terrell Counties, in that order. 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 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 ( o C) 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 330

6 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. 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. 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. 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. 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 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. Gretener, P. E., 1981, Geothermics: Using temperature in hydrocarbon exploration: AAPG Education Course Note Series #17, 156 p. 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. 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 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. 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 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 , p Valenza, J., 1995, Geothermal energy, in Faidley, R., ed., Texas Renewable Energy Resources Assessment: Survey, Overview and Recommendations: Virtus Energy Research Associates, 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. 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 , 145 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 , v. 1, 248p, v. 2, Appendices A D, v. 3, Appendices E - H. 331

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