Assessment of the Enhanced Geothermal System Resource Base of the United States

Abstract

This paper describes an assessment of the enhanced geothermal system (EGS) resource base of the conterminous United States, using constructed temperature at depth maps. The temperature at depth maps were computed from 3 to 10 km, for every km. The methodology is described. Factors included are sediment thickness, thermal conductivity variations, distribution of the radioactive heat generation and surface temperature based on several geologic models of the upper 10 km of the crust. EGS systems are extended in this paper to include coproduced geothermal energy, and geopressured resources.

A table is provided that summarizes the resource base estimates for all components of the EGS geothermal resource. By far, the conduction-dominated components of EGS represent the largest component of the U.S. resource. Nonetheless, the coproduced resources and geopressured resources are large and significant targets for short and intermediate term development. There is a huge resource base between the depths of 3 and 8 km, where the temperature reaches 150–250°C. Even if only 2% of the conventional EGS resource is developed, the energy recovered would be equivalent to roughly 2,500 times the annual consumption of primary energy in the U.S. in 2006. Temperatures above 150°C at those depths are more common in the active tectonic regions of the western conterminous U.S., but are not confined to those areas. In the central and eastern U.S. there are identified areas of moderate size that are of reasonable grade and probably small areas of much higher grade than predicted by this analyses. However because of the regional (the grid size is 5′ × 5′) scale of this study such potentially promising sites remain to be identified.

Several possible scenarios for EGS development are discussed. The most promising and least costly may to be developments in abandoned or shut-in oil and gas fields, where the temperatures are high enough. Because thousands of wells are already drilled in those locations, the cost of producing energy from such fields could be significantly lowered. In addition many hydrocarbon fields are producing large amounts of co-produced water, which is necessary for geothermal development. Although sustainability is not addressed in this study, the resource is so large that in at least some scenarios of development the geothermal resource is sustainable for long periods of time.

Appendix A: Details of the Temperature-at-Depth Calculations

Several models of thermal conductivity and radioactive heat generation of the upper 10 km were used for the temperature at depth calculations. Shown in Figure A.1 are the geologic distributions over depth scale over which the temperature at depth was calculated.

Case A is in the simplest possible tectonic distribution. From surface to the 10 km the geology is represented by basement, with an assumed thermal conductivity of 2.7 W/m/K and a radioactive heat generation of A _b . For such a situation the temperature at any depth X is given by:

$$ T = \frac{{Q_m }}{K} - A_b b^2 \frac{{1 - e^{-\frac{X}{b}} }}{K},\!\!\!\quad {\rm where}\!\!\!\quad A_b = (Q_0 - Q_m )/b. $$

The quantities involved in this equation are: surface heat flow (Q ₀), mantle heat flow (Q _m ), thermal conductivity (K), and the scale depth of heat generation (b=10 km).

Case B is for the areas represented by a layer of young volcanics/basin fill overlying the basement. Geographically such an area would be represented by Basin and Range for example. In this area a thermal conductivity of 2 W/m/K was assumed to a depth of 2 km, with the basement value of 2.7 W/m/K used below 2 km. The heat generation was assumed to be constant at 1 μW/m³ for the upper 2 km, whereas below 2 km the distribution was assumed to be exponential as described in the text. The equations involved in the calculations are:

For X=0–2 km

$$ T_{2\,{\rm km}} = \frac{{Q_0 X}}{K} - A_S \frac{{X^2 }}{K}, $$

where the quantities involved are: surface heat flow (Q ₀), heat generation (A _s =1 μW/m³) and thermal conductivity K=2 W/m/K.

For X>2 km the temperature equation can be written as:

$$ T = T_{2\,{\rm km}} + \frac{{Q_m }}{K} - A_b b^2 \frac{{1 - e^{-(\frac{{X - 2}}{b})} }}{K}, $$

where the quantities are similar to those involved in Case A.

Case C is represented by a shallow X<3 km sedimentary section overlying the basement. The conductivity of the sedimentary section is variable, but the basement conductivity is 2.7 W/m/K. In such a situation the equations necessary to compute the temperature at a specific depth are:

For X=0–3 km

$$ T_S = \frac{{Q_0 X}}{K} - A_S \frac{{X^2 }}{K} $$

(note that X=S, in this particular example), where the thermal conductivity K is variable, and A _s =1 μW/m³.

For X>3 km

$$ T = T_S + \frac{{Q_m }}{K} - A_b b^2 \frac{{1 - e^{-(\frac{{X - S}}{b})} }}{K}. $$

Case D is relatively similar to the Case C. In this case the geology is represented by a sedimentary layer of thickness 3 to 4 km, overlying basement rocks. Again the conductivity distribution in variable for the sedimentary section, while the basement rocks have a conductivity of 2.7 W/m/K. The only difference from Case C is the depth scale of the heat generation (the value of b), which is variable. The value of b is selected so that b=13−S. The reasoning for this approach (and the similar one for Case E) is the assumption that a thick sedimentary basin would form only over attenuated or eroded continental crust. Thus the radioactive layer in the basement is assumed to be thinned in the situation of a thick sediment cover. The equation for the temperature up to 4 km is similar to Case C:

For X=0–4 km

$$ T_S = \frac{{Q_0 X}}{K} - A_S \frac{{X^2 }}{K}({\rm again}\,X = S) $$

For X>4 km

$$ T = T_S + \frac{{Q_m }}{K} - A_b b^2 \frac{{1 - e^{-(\frac{{X - S}}{b})} }}{K}, $$

where b is variable (b=13­S)

Case E is the most complicated. Geologically it is represented by a layer of sedimentary thickness larger that 4 km, overlying basement rocks. The conductivity of the rocks is variable for the upper 4 km, depending on the geological properties of the various sedimentary basins, whereas below 4 km the thermal conductivity was assumed to be 2.7 W/m/K no matter whether the rocks at the depths were the temperature is computed are basement or sediments. The equations involved in the computation are:

For X=0–4 km

$$ T_{4\,{\rm km}} = \frac{{Q_0 X}}{K} - A_S \frac{{X^2 }}{K}, $$

where K is variable, As=1 (in this situation X≠ S)

For X=4 To S km (S being the bottom of sediments)

$$ T_S = T_{4\,{\rm km}} + \frac{{Q_0 - 4A_S }}{K} - A_S \frac{{X^2 }}{K}, $$

where K=2.7 W/m/K

For X>S

$$ T = T_S + \frac{{Q_m }}{K} - A_b b^2 \frac{{1 - e^{-(\frac{{X - S}}{b})} }}{K}, $$

where b is variable (b=13−S).

Figure A.1.

Thermal conductivity and radioactivity models for temperature calculation.