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Scientific Paper Elevated Crustal Temperatures in West Virginia: David Blackwell, Zachary Frone, and Maria Richards Abstract A large area in eastern West Virginia has been found to have elevated heat flow and upper crustal temperatures compared to the rest of the eastern United States. The high heat flow has been recognized based on interpretation of bottom-hole temperature (BHT) data from oil and gas drilling in the area. The high heat flow is located within the western part of the Appalachian Mountains. There are several possible explanations for the observed thermal regime given the existing data that can only be resolved by more accurate determination of the temperature and heat flow in this area. The temperatures are high enough to make this the most attractive area for Geothermal Energy development in the eastern 1/3 of the country and the heat in place is sufficient to support large scale development of Enhanced Geothermal Systems. 1. Introduction A significant area of high temperature at depth due to high heat loss from the interior of the Earth has been identified in the eastern part of West Virginia. The finding is the result of detailed mapping and interpretation of bottom-hole temperature (BHT) data from oil and gas wells conducted as part of an ongoing project to update the Geothermal Map of North America and to revise previously calculated stored heat resource values for the United States. There are several geologic situations that could be responsible for the West Virginia elevated temperatures. The data suggest that West Virginia possesses a significantly higher thermal profile than previously estimated and at such quantities and temperatures as to be capable of supporting commercial geothermal energy production. As a result of the new data, the previous estimate of West Virginia’s geothermal resources between depths of 3-10 km is revised upwards to 113,300 EJ, a 78% increase from the Future of Geothermal Energy Report (Tester et al., 2006) calculation of stored heat. The revised estimate of geothermal potential from this stored energy is 18,800 MWe (electrical power) at a 2% recovery factor. 2. Updating the Geothermal Map of North America In 2008, the SMU Geothermal Laboratory began a multi-year project to update the Geothermal Map of North America (GMNA) (Blackwell and Richards, 2004) and estimates of U.S. Enhanced Geothermal Systems (EGS) resources. The 2004 GMNA and thermal data set have been used by SMU and others to estimate stored heat content, crustal heat flow, and EGS resource values (Tester et al., 2006, referred to subsequently as FGE2006). EGS resources are high-temperature (>150 ºC) geologic formations where development of power systems requires the enhancement or creation of a subsurface reservoir for fluid circulation. The GMNA was developed from roughly 3,600 heat flow and 12,000 BHT data measurements along with regional thermal conductivity models. However, large areas of the Central and Eastern United States contain few data points and have been under sampled in all previous national geothermal resource assessments. Since the previous GMNA data sets were completed, approximately 7,500 new data points have been analyzed and currently more are being processed for this project. The data was collected from oil, gas, water, and thermal gradient wells from New York, Pennsylvania, West Virginia, Ohio, Indiana, Illinois, Kentucky, Tennessee, and Michigan. As a result of the new heat flow determinations, estimates of heat content and MWe potential for Michigan, Pennsylvania and West Virginia are substantially increased. 2.1 Data Sources, Manipulation, and Density This study focused on aggregating the BHT data and developing new thermal conductivity models for estimating crustal heat flow for portions of the Eastern and Midwestern U.S. The parameters considered include: BHT, surface temperature, heat flow, thermal conductivity of the rocks, and heat production of the rocks at depth. The depths of measurements used to update the West Virginia geothermal resource range from 600 m to as deep as 6.15 km. These temperatures have been corrected for the disturbance due to the drilling process to represent the in situ Earth temperature at the point of measurement using the Harrison correction (Harrison et al., 1983). Thermal conductivities have been estimated from published geologic sections and thermal conductivity information for the rock types encountered in the wells (see Joyner, 1960). Geologic sections from the AAPG Correlation of Stratigraphic Units of North America were used to generate a thermal conductivity model for the eastern United States (AAPG-COSUNA, 1994). Thermal conductivity and temperature gradient were combined to calculate updated estimates of stored heat, heat flow, temperature-at-depth, and technical geothermal MWe generating potential for these states. The mapping has identified a large area in eastern West Virginia with elevated heat flow (Figure 1).
A total of 1,455 wells with BHT data are included in the updated West Virginia heat flow and temperature data set. From those 55 wells have high thermal gradients and heat flows, i.e., above 30°C/km and 60 mW/m2 respectively. In comparison, the 2004 GMNA contours in West Virginia are based on only four data points. Interpretations of temperature contours are more constrained where there are multiple wells in proximity to each other (central portion of WV, Figure 1). There is significant scatter in the BHT data, even within a close proximity, due to the marginal conditions under which they are measured and recorded. Working with equilibrium temperature data is the most accurate method of resolving these differences and is one of the next steps for refining the state’s geothermal assessment. 2.2 Methodology of Temperature and Heat Calculations The BHT data from hydrocarbon wells are measured as part of the drilling - logging process. A plot of the corrected BHTs from the hydrocarbon wells is shown in Figure 2. The highest corrected temperature in the data set was in western West Virginia with 152°C at 6.15 km. This site is located outside the area of highest estimated temperatures, but useful for calibrating the deeper temperatures. The solid lines on Figure 2 are individual wells with multiple BHT values representing single well temperature-depth curves.
The methodology for calculating the EGS stored heat resource follows the process used in the FGE2006. In order to determine thermal energy (or “heat”) in place, calculations use lithology, thermal conductivity, gradient, and thickness of sedimentary rock (depth to basement). Temperatures were calculated for the depths of 3.5 to 9.5 km at an interval of 1 km (Figure 3). Values, averaged for 1 km intervals, were used in the recoverable resource analysis in the FGE2006 (Chapter 2 Appendix). In this study the West Virginia heat-in-place was calculated based on 1 km x 1 km x 1 km blocks centered at depths of 3.5 to 9.5 km using the same assumptions and equations shown in FGE2006 and Blackwell et al. (2006). Synthesis of the new data yields a geothermal resource estimate of 113,300 EJ of stored energy between depths of 3 and 10 km. This is an increase of 78% over the previous estimate, based on limited data, in the FGE2006. 3. Results 3.1 Temperature-at-Depth A series of maps of calculated temperature-at-depth for West Virginia for depths of 4.5, 5.5, 6.5, and 7.5 km are shown in Figure 3. These maps show the increasing temperatures and the significantly large area over which the temperatures higher than 150 °C occur. In the hottest area, predicted temperatures are as high as 200°C at 5 km in discrete locations, and reach 175°C conservatively over a large area (about 110 km by 170 km). Several wells have been drilled in the depth range of 4.5 to 5.5 km in western and central West Virginia. This information is helpful in calibrating the temperatures to these depths across the state, determining the extent of the significant geothermal resources, as well as determining drilling conditions at depths similar to those needed for geothermal development.
3.2 Location of High Temperatures in West Virginia Thermal Area The area of high temperatures lies in eastern West Virginia at the edge of the Appalachian fold belt and the Appalachian Sedimentary Basin (Figure 1), which formed during the Appalachian orogeny 300-350 million years ago. The basin is bounded on the east by the structures of the Allegheny Mountains. The basin is deepest along the western edge of the Appalachians. The basin depths in the area of high temperatures are 5 to 7 km. Thus, the lithology of the rocks at high temperature may be both sedimentary and basement (igneous and metamorphic). The uppermost basement lithology is dominantly granite or gneiss based on a few basement samples from West Virginia, including a granite sample outside the thermal area of interest from the Rome Trough at 5,971 m depth.
Geologic details of the thermal area are shown in Figure 4 with contours of temperature overlaid on a geologic cross-section located at the position shown on Figure 1. The primary basement structural features of West Virginia are identified in Figure 1: the Rome Trough (or graben, i.e. a down-dropped block); and the Upland Horst (or up-thrown block) bounded on the east by a buried normal fault (Shumaker and Wilson, 1996). These structural features underlie the west edge of the thin skinned deformation associated with the Appalachian Valley and Ridge province. The folds are related to near horizontal faults that translate into near vertical faults beneath the folds as shown near the east (right side of the section). Along the section, the thermal anomaly seems to coincide in part at least with the uplifted basement block (Upland Horst) geographically related to the area east of the Rome Trough. Note that the section is highly vertically exaggerated (about 20:1) so that the thermal anomaly is much broader in respect to the relief than appears on the section. The high heat flow values are primarily located in the West Virginia counties of Tucker, Randolph, Pocahontas, and Greenbrier. The trend of the highest heat flow area is almost N/S. To the west and north the temperature mapping is controlled by additional BHT data to depths of at least 2-3 km. To the east in Virginia thermal data are limited and from relatively shallow wells. East of the area of high temperature wells in western Virginia up to 600 m deep can have temperature gradients as low as 10-11°C/km. Curiously, the warm springs area of the Appalachians occurs just east of the high heat flow area and in the region where the measured heat flow in shallow wells is low. Whether these lower heat flows or the warm springs are characteristic of the deeper thermal regime is not known at this time. 4. Interpretation The collocation of the high temperature area and the area of folding and faulting suggests that the high temperatures are geologically related. However, the nature of the relationship is not proven and there are several possible associations that have different implications on the temperature-at-depth interpretations. Firstly, the high heat flow areas could be associated with an underlying change in the basement (igneous/metamorphic) rock type and thus the radioactive heat production of the crust. In this case a deep heat conduction related source for the thermal area is implied with wells drilled to basement in West Virginia forming some constraint. The granite and granite gneiss lithologies encountered are typical of a high basement heat flow area, thus this model is definitely a possibility. In this situation the predicted temperatures shown at depths of 5 to 10 kilometers would be valid. Secondly, the correlation could be due to some large scale inhomogeniety in the thermal properties of the rock at depth that disturbs the temperature in some complicated way. In this case, the projected temperatures on Figure 3 could be somewhat high below 4 to 6 km (below the depth of the data points), but the area still would have higher temperatures than the surrounding lower heat flow areas. A third possibility is that there is fluid flow along the thrust faults, the steep faults, or along dipping aquifers (permeable layers) in the deformed Appalachian Valley and Ridge province (Appalachian Mountain belt). It is interesting that the anomaly is somewhat along the line of the Allegheny Front (see location on Figure 1) - a fundamental geologic structure that seems to localize the warm springs locations in Virginia (see Figure 1). On the basis of the very limited data available there are no large area thermal anomalies associated with these hot springs (Blackwell and Richards, 2004). The geochemistry of these springs also is not suggestive of high temperature-deep circulation. If a fluid flow phenomenon of this sort gives rise to the observed thermal anomaly then the calculated temperatures at depths of 5+ km may be high. However, the minimum temperature in the moving fluid would have to be above the measured temperatures which are up to 140 °C (284 °F), definitely into the temperature range for binary power generation. Therefore the final conclusion is that, regardless of the interpretation, significantly elevated temperatures will occur in the depth range of 4 to 10 km. 5. Areas for Further Investigation 5.1 Questions to Still Consider The individual BHTs and estimated thermal conductivity data have a relatively high uncertainty. Because drilling makes up a significant percentage of the cost for geothermal development, the most favorable areas will be the highest temperatures at shallowest depths, making the most favorable areas somewhat site specific. Also site specific is the rock type within the depth of interest, somewhat below the depths of the current drilling. Much of the thermal energy resides in basement rocks below the sedimentary section. Since basement is usually defined as areas of metamorphic or igneous rocks, the composition and lithology of basement is actually extremely variable. The West Virginia basement lithology where present is as complicated as the surface exposures. Because the generic description “granite” is often used for basement, the lithology is not exactly specified. 5.2 Confidence Level in Current Analysis The accuracy of the current data and the temperature calculations is difficult to assess. Comparisons with detailed temperature and coinciding geologic sections indicate about a ±10% error associated with the temperature interpretations within the depths drilled. At depths below the drill holes the extrapolated temperatures are less certain. In the areas of hydrocarbon development, wells that have been drilled to 3 to 6 km depths, so the predicted temperatures can be checked against actual measurements. Where this has been done and the agreement is within ± 20 °C in the 3 to 6 km depth range, although this does not include the thermal area of interest in eastern West Virginia. This information has been compared to the calculated values with similar results to the BHT comparison. 5.3 Gaps in the Current Analysis Major gaps include low quality thermal data, potential errors in matching the thermal conductivity to the well lithology, and areas with little or no data coverage. Addressing these limitations will require measurement of equilibrium temperatures in wells in the thermal anomaly regions and specific matching of the geologic sections to the wells. In areas with low thermal data density, holes drilled specifically for heat flow might be necessary as part of the exploration stage of development. 5.4 Favorability for Enhanced Geothermal Systems The calculated heat-in-place of 113,300 EJ is the starting point for the geothermal resource base and electrical generating capacity. The favorability for EGS development depends primarily on reservoir temperature and the suitability of target formations for the generation of reservoirs. The lithology, nature of the existing pore space, stress regime, and types and orientation of fractures are features that need to be determined in the depth range of interest for development. Geothermal exploitation in West Virginia could involve the development or enhancement of fracture systems to generate high temperature (>150 °C) reservoirs in the low permeability formations. There is also the potential for low temperature (65˚C-150˚C) geothermal development at shallower depths. Lastly, quantification of the most favorable rock composition and structure for EGS development remains to be accomplished. Most of the experimental EGS sites have been in granite (in a strict geologic sense) because of the expected homogeneity of the rock type. In fact there may be situations where layered sedimentary rocks might be equally or more favorable as the orientations of fractures might be easier to predict and the rock types may be more extensively fractured. Tight (low porosity) sandstones are often developed by fracturing into oil and gas reservoirs. As shown on the cross-section (Figure 4), there are thick sandstones at depth in the Appalachian Basin. 5.5 Recommended Areas for Future Research Further research on the relationship of the Appalachian Mountain belt as a geothermal structure should be completed. The edge of the Appalachian Mountain belt extends either at the surface or as a subsurface feature from the Canadian border to the Mexican border in a serpentine course that crosses the states of Alabama, Mississippi, Arkansas, and Texas in addition to the states crossed by the Appalachian Mountains. The area of the same structure buried beneath Gulf Coast sediments is a significant thermal feature of Texas, Louisiana and Mississippi (Negraru et al., 2008) that also could be amenable to geothermal development. 6. Conclusions This reconnaissance investigation of the thermal regime of the eastern U.S. has defined a significant thermal anomaly along the Appalachian Mountain trend in West Virginia and demonstrated that temperatures high enough for electrical power generation occur at depths greater than 4 to 5 km in large areas of eastern West Virginia. This finding opens the possibility of geothermal energy production near the heavily populated Eastern Seaboard. Further research is needed to refine estimates of the magnitude and distribution of West Virginia’s geothermal resource and to understand the cause of the high heat flow values. The presence of a large, baseload, carbon neutral, and sustainable energy resource in West Virginia could make an important contribution to enhancing the U.S. energy security and for decreasing CO2 emissions. 7. Acknowledgements This project would not have been possible without the support of Google.org and of Charles Baron and Dan Reicher. Students in the SMU Geothermal Lab were helpful in many aspects of the project, particularly Joseph Batir, who helped develop the temperature calculation process employed in this study. Maps of temperature at various depths for the conterminous U.S. from this study may be found at google.org/EGS. 8. References CitedAAPG - COSUNA (1994), American Association of Petroleum Geologist, CSDE, COSUNA, and Geothermal Survey Data_Rom. Blackwell, D. D., and Richards, M. (2004), Geothermal Map of North America, Amer. Assoc. Petrol. Geol. Map, scale 1:6,500,000. Blackwell, D.D., Negraru, P., Richards, M. (2006), Assessment of the enhanced geothermal system resource base of the United States. Natural Resources Research, Springer Netherlands, v 15/4, 283-308, DOI 10.1007/s11053-007-9028-7. Joyner, W. B. (1960), Heat flow in Pennsylvania and West Virginia, Geophys., 25, 1229-1241. Negraru, P., Blackwell, D. D., and Erkan, K. (2008), Heat Flow and Geothermal Potential in South-Central United States, Natural Resources Research, vol. 17., no. 4, 227-243, doi:10.1007/s11053-008-9081-x. Ryder, R.T., Swezey, C.S., Crangle, R.D., Jr., and Trippi, M.H. (2008), Geologic cross section E–E’ through the Appalachian basin from the Findlay arch, Wood County, Ohio, to the Valley and Ridge province, Pendleton County, West Virginia: U.S. Geological Survey Scientific Investigations Map 2985, 2 sheets, 48-p. pamphlet. Shumaker, R.C., and Wilson, T.H. (1996), Basement structure of the Appalachian foreland in West Virginia: Its style and effect on sedimentation, in van der Pluijm, B.A., and Catacosinos, P.A., eds., Basement and Basins of Eastern North America: Boulder, Colorado, Geological Society of America Special Paper 308. Tester, J. W., Anderson, B., Batchelor, A., Blackwell, D., DiPippo, R., Drake, E., Garnish, J., Livesay, B., Moore, M.C., Nichols, K., Petty, S., Toksoz, N., Veatch, R., Augustine, C., Baria, R., Murphy, E., Negraru, P., Richards, M. (2006), The future of geothermal energy: Impact of enhanced geothermal systems (EGS) on the United States in the 21st century. Massachusetts Institute of Technology, DOE Contract DE-AC07-05ID14517 Final Report, 209 p. (FGE2006)
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