ASSESSMENT OF GEOTHERMAL RESOURCES OF THE UNITED STATES-1975* SUMMARY AND CONCLUSIONS (By D. E. White and D. L. Williams) The appraisal of the geothermal resources of the United States presented here is as factual as we can provide from available data. Much effort has been made in each individual chapter to specify the uncertainties and assumptions involved in each estimate; we urge that these uncertainties be kept in mind. The estimates should be regarded as first attempts that will need to be updated as new information becomes available. This assessment consists of two major parts: (1) estimates of total heat in the ground to a depth of 10 km and (2) estimates of the part of this total heat that is recoverable with present technology, regardless of price. No attempt has been made to consider most aspects of the legal, environmental, and institutional limitations in exploiting these resources. Definitions Resource-related terms used in this circular are defined as follows: Geothermal resource base includes all of the stored heat above 15° C. to 10 kilometers depth (under all 50 States). Geothermal resources are defined as the stored heat, both identified and undiscovered, that is recoverable using current or near-current technology, regardless of cost. Geothermal resources are further divided into three categories based on cost of recovery: (1) submarginal geothermal resources, recoverable only at a cost that is more than two times the current price of competitive energy systems; (2) paramarginal geothermal resources, recoverable at a cost between 1 and 2 times the current price of competitive energy; and (3) geothermal reserves are those identified resources recoverable at a cost that is competitive now with other energy resources. The distinction between resource base and resources is technologic, in contrast to the distinctions between submarginal resources, paramarginal resources, and reserves, which are economic. Resource Base The three major categories of the resource base are shown in table 26. The hydrothermal convection systems of category 1 (Renner and others, this circular) occur where circulating water and steam are transferring heat from depth to the near surface; they tend to occur in areas of unusually great heat supply and favorable hydrology. These systems are relatively favorable for geothermal development because Department of the Interior. Geological Survey Circular 726. “Assessment of Geothermal Resources in the United States-1975," D. F. White and D. L. Williams. high temperatures occur near the ground surface and drilling costs are low. We have a sound basis for optimism that many concealed. hydrothermal systems exist, and that they can be discovered (see Renner and others, this circular). TABLE 26.-ESTIMATED HEAT CONTENT OF GEOTHERMAL RESOURCE BASE OF THE UNITED STATES (HEAT IN THE GROUND, WITHOUT REGARD TO RECOVERABILITY) 1 1018 calories equivalent to heat of combustion of ~690,000,000 barrels of petroleum or ~154,000,000 short tons of coal. The hot, young igneous (volcanic) systems of category 2 (Smith and Shaw, this circular) occur in regions where molten magma has been generated deep in the Earth's crust or mantle and has intruded upward into the shallow crust. Silicic magma (equivalent to granite where crystallized) commonly comes to rest as large masses at depths of a few kilometers, thus conserving its heat; basaltic magma, being much more fluid, is commonly erupted at the surface, where its heat is rapidly dispersed. Many young igneous systems and a few older stillhot systems are identified but are not yet evaluated in detail. Estimates of the heat content of hot igneous systems, both evaluated and unevaluated, are included in table 26. All these are favorable target areas in exploring for concealed hydrothermal convection systems. The stored heat of the conduction-dominated environments, category 3, is huge in quantity, even though temperatures are low, because so much area and volume are involved (Diment and others, this circular). Most of the heat is transferred from the deep, hot interior by thermal conduction through solid rocks, but some is generated by normal radioactivity of rocks, mainly in the upper crust. The entire United States is subdivided into 19 heat-flow provinces that, with present limited data, are classified into three basic types, each with characteristic trends in temperature with depth. The Basin and Range type has the highest temperature gradients; the eastern and Sierra Nevada types have much lower gradients except in special areas, such as the gulf coast, which constitutes a special part of the resource base. Three kinds of potential energy are available from the geopressured pore fluids, including geothermal energy, mechanical energy from the overpres sured fluids, and methane dissolved in the pore waters. Heat flows of the gulf coast are presumed to be similar to the eastern type, but adequate data are lacking. Temperature gradients however, are higher than in most of the eastern region because the high-porosity sediments of the gulf coast have low thermal conductivities. In general, the average heat content of rocks is considerably higher in the Western United States than in the East. This also helps to explain why the most favorable hydrothermal convection systems and the hot young igneous systems also occur in the West. The anamolous heat of the hydrothermal convection and the hot igneous systems can be considered as "hotspots" superimposed on regional conduction-dominated environments. About 0.01 percent of the total heat stored beneath the United States to a depth of 10 km is in identified hydrothermal convection systems, and about 0.3 percent is in the best known of the hot igneous systems. If our estimates of the undiscovered and unevaluated "hotspots" are valid (table 26), the corresponding percentages are 0.04 and 1.2. Recoverability The useful heat recoverable from identified systems with present or near-current technology and prices (= reserves) and at as much as double present prices (=paramarginal resources) exists almost entirely in the hydrothermal convection systems of the Western States (table 27) and the geopressured sedimentary environment of the gulf coast (table 28). TABLE 27.-GEOTHERMAL RESOURCES OF HYDROTHERMAL CONVECTION SYSTEMS ASSUMED RECOVERABLE WITH PRESENT AND Near-current TECHNOLOGY AND WITHOUT REGARD TO COST (NATHENSON AND MUFFLER, THIS CIRCULAR) 1 1018 cal (a billion-billion calories) is equivalent to heat of combustion of 690, 000, 000 barrels of oil or 154,000,000 short tons of coal. 2 Assumed recovery factor 0.25 for convective resources. • Thermal energy applied directly to its intended thermal (nonelectrical) use; 1018 cal of beneficial heat, if supplied by electrical energy, would require at least 1,330 MW cent (or 4,400 MW for 30 years); however, a user of this geothermal energy must be located or must relocate close to the potential supply; insufficient data available to predict demand or to subdivide into reserves, paramarginal, and submarginal resources. Unit of electrical energy; 1 MW cent is equivalent to 1,000 KW produced continuously for 100 yr. 6 Small because of exclusion of systems with temperatures below 150° C. 7 Perhaps as much as 60 percent will be reserves and paramarginal resources; costs of discovery and development are more speculative than for identified resources. TABLE 28.-GEOTHERMAL RESOURCES OF GEOPRESSURED SEDIMENTARY ENVIRONMENTS ASSUMED RECOVERABLE WITH PRESENT AND NEAR-CURRENT TECHNOLOGY AND WITHOUT REGARD TO COST (PAPADOPOULOS AND OTHERS, THIS CIRCULAR) 1 Thermal energy only; 101 cal is equivalent to heat of combustion of 690,000,000 barrels of oil 2 All plans assume 0.15 m/sec. flow rate per well and saturation of water with methane, but reliable data lacking. 3 Unit of electrical energy; 1 MW-cent is equivalent to 1,000 kw produced continuously for 100 yr. 4 Estimates made for 20 yi production period; converted to 30 yr to be consistent with other estimates of this circular. 5 Methane assumed recovered but not used locally for electricity. Perhaps in part reserves but mostly paramarginal, depending on environmental and other costs. Thermal equivalent of methane included in heat at well head but excluded from electrical energy; recoverable part highly speculative because of unknown porosities and permeabilities, but probably largely submarginal. * No detailed assessment but considered likely to exist in California and other States. Resources of the most attractive identified convection systems (excluding national parks) with predicted reservoir temperatures above 150° C (~300° F) have an estimated electrical production potential of about 8,000 MW cent, or about 26,000 MW for 30 years (Nathenson and Muller, this circular).' Assumptions in this conversion are: (1) one-half of the volume of the heat reservoirs is porous and permeable, (2) one-half of the heat of the porous, permeable parts is recoverable in fluids at the wellheads, and (3) the conversion efficiency of heat in wellhead fluids to electricity ranges from about 8 to 20 percent, 1A megawatt century of electricity is a unit of energy equivalent to 1 MW (1,000 kW) of power being produced for 100 years (or 3.33 MW for 30 years). Approximately 1,000 MW (the capacity of many modern nuclear powerplants) is required to satisfy the electrical needs of an average city of 1 million people. depending on temperature and kind of fluid (hot water or steam). The estimated overall efficiency of conversion of heat in the ground to electrical energy generally ranges from less than 2 to 5 percent, depending on type of system and reservoir temperature. In order to divide the resources of the high-temperature convection systems into reserves and paramarginal and submarginal resources, each system should have been analyzed individually for economic and physical recoverability. In general, the necessary physical data are not available; few systems have been drilled or tested extensively, and the necessary economic data are not well known. No hot-water system in the United States has yet been produced extensively. Thus, in lieu of an objective analysis, subjective evaluations were made for the three resource categories. The most important single factor is temperature; reservoirs above 200° C are most likely to contain reserves. Other utilized data include indicated magnitude of the reservoir and indicated lack of severe problems, such as high salinity and inadequate fluid supply. Nearly one-half of the production potential from the identified systems (3,500 MW cent or nearly 12,000 MW for 30 years) is considered to be reserves, recoverable with present prices and technology. Paramarginal resources recoverable at as much as twice present prices and with existing and near-current technology are also estimated to be 3,500 MW cent or about 12,000 MW for 30 years. In addition, hightemperature resources in undiscovered convection systems, using the estimates of Renner, White, and Williams (this circular) and the conversion efficiencies expected of these systems (Nathenson and Muffler, this circular), are estimated to be 38,000 MW cent or about five times that of the identified hot-water systems, excluding the national parks. Of the undiscovered resources, a considerable fraction is likely to be recoverable at present prices and technology, but a larger part will probably be paramarginal. All of the intermediate-temperature convective resources (90° to 150° C) are submarginal for the generation of electricity, but, under favorable conditions, some are utilizable now for space heating and industrial uses. The potential for nonelectrical uses may attract new industry in many places because the supply is relatively dependable and because the overall efficiency of the direct use of the geothermal energy for heating is greater than for generating electricity for the same purposes (Nathenson and Muffler, this circular). The beneficial heat that can be recovered in favorable circumstances, assuming that a need occurs near the same locality as the potential supply, totals 20.7 × 1018 cal in identified systems (table 27); this is equivalent to about 14.3 billion barrels of oil. The heat content of pore fluids of the assessed onshore geopressured parts of the gulf coast to depths up to 7 km (Papadopulos and others. this circular) is shown in table 28, This heat component excludes all heat contained in rocks and minerals and also excludes the potential energy of dissolved methane and the mechanical energy from excess pressure. The recoverable part of the total fluid resource base depends critically on the specific plan (or plans) selected for reservoir development. Factors that can be emphasized include: (1) maximizing total recovery from the reservoirs, (2) maximizing production from individual wells, (3) establishing some minimum pressure decline that |