Thermal Energy Recovery from Enhanced Geothermal Systems – Evaluating the Potential from Deep, High-temperature Resources

A variety of mechanical, chemical and thermal approaches to reservoir stimulation have been proposed and tested over more than three decades of research on Enhanced Geothermal Systems (EGS) technology, with the primary focus at present on enhancing fracture permeability by elevating fluid pressure sufficiently to induce shear failure along pre-existing natural fractures. A critical issue in assessing the potential EGS resource is quantifying Rg, the geothermal recovery factor, which is defined as the ratio of produced thermal energy to the thermal energy contained in the fractured volume comprising the reservoir. One approach to EGS resource assessments incorporates the assumption that a constant amount of thermal energy is recovered during the life of a project, regardless of the temperature of the reservoir, thereby concluding that there is a decrease in Rg with increasing reservoir temperature and a reduced potential associated with deep, higher temperature resources. By contrast, production experience and simulations of thermal energy recovery from naturally fractured geothermal reservoirs indicate that Rg, which typically falls in the range from 0.05 to 0.2, is primarily a function of internal reservoir structure, not temperature. Because the thermal energy content of the crust increases linearly with increasing temperature, if the characteristics of Rg for naturally fractured reservoirs apply to EGS reservoirs, proportionally greater resource potential is associated with the deeper, hotter portions of the Earth’s crust, despite the costs and challenges associated with creating and exploiting reservoirs at greater depths and higher temperatures. However, other aspects of production from deep, hot EGS reservoirs need further evaluation, such as the relative effects on productivity of declining fluid viscosity with increasing temperature, fracture closure at higher levels of effective stress, and the increased rates of mineral precipitation and dissolution at higher temperatures. These aspects may limit the viability of deep EGS resources. INTRODUCTION Conventional geothermal resources depend upon hydrothermal fluid circulation that results from the convergence of high temperatures and high permeability, typically fracture permeability produced as a result of recent or active faulting. Enhanced Geothermal Systems (EGS) are geothermal resources that require some form of engineering to develop the permeability necessary for the circulation of hot water or steam and the recovery of heat for commercial applications (DOE, 2008). Because exploitation of EGS resources incorporates the augmentation or creation of permeability in situ, the presence of elevated temperatures at drillable depths is the dominant factor controlling the quality of the resource. Under the assumption of successful implementation of EGS technology, provisional estimates of EGS electric power resource potential in the western United States, where high crustal heat flow is most favorable for EGS development (Figure 1), were included in the recent USGS geothermal resource assessment (Williams, et al., 2008a). In this assessment, models for the extension of geothermal thermal energy recovery techniques into regions of hot but low permeability crust down to a depth of 6km yield an estimated mean electric power resource on private and accessible public land of approximately 520,000 MWe. This is nearly half of the current installed electric power generating capacity in the United States and an order of magnitude larger than the conventional geothermal resource. Another recent EGS resource assessment was produced by a panel of experts convened by the Massachusetts Institute of Technology (MIT) under Department of Energy (DOE) sponsorship (Tester et al., 2006). In their report, Tester and others estimate the EGS potential for entire continental United States to a depth of 10km. The portion of this assessment covering the same western states as the USGS assessment and over approximately the same 6km depth range varies between 200,000 and 2,000,000 MWe, depending on the assumptions applied for the recoverability of heat from the Earth’s crust. Although the mean USGS estimate lies within the range of values produced by the MIT panel, the wide variation highlights significant uncertainties in the potential recovery of useful heat from the Earth’s upper crust. Understanding and reducing these uncertainties is of critical importance to the successful development of the EGS resource. Figure 1: Map showing an estimated distribution of temperature at a depth of 6km in the western United States. THE RECOVERY OF HEAT FROM GEOTHERMAL RESERVOIRS The potential energy recovery from a geothermal reservoir depends on the thermal energy, qR, present in the reservoir, the amount of thermal energy that can be extracted from the reservoir at the wellhead, qWH, and the efficiency with which that wellhead thermal energy can be converted to electric power. Once the reservoir fluid is available at the wellhead, the thermodynamic and economic constraints on conversion to electric power are well known (for example, DiPippo, 2005). The challenge in geothermal resource assessment lies in quantifying the size and thermal energy of a reservoir as well as the constraints on extracting that thermal energy. In the approach applied in USGS assessments, the reservoir thermal energy is calculated as 0 ( ) R R q CV T T ρ = − (1) where ρC is the volumetric specific heat of the reservoir rock, V is the volume of the reservoir, TR is the characteristic reservoir temperature, and T0 is a reference, or dead-state, temperature. The thermal energy that can be extracted at the wellhead is given by 0 ( ) WH WH WH q m h h = − (2) where mWH is the extractable mass, hWH is the enthalpy of the produced fluid, and h0 is the enthalpy at some reference temperature (typically 15C). The wellhead thermal energy is then related to the reservoir thermal energy by the recovery factor, Rg, which is defined as / g WH R R q q = (3) Inherent in these equations is a geometrical concept of the reservoir that allows calculation of a volume and an estimate of the ability to extract hot fluid from the volume. In ideal cases values for Rg as large as 0.5 to 0.6 have been derived from analytical and numerical models of heat extraction from a geothermal reservoirs through a “cold sweep” process, in which the hot reservoir water is gradually replaced by colder water through natural recharge and/or artificial injection (e.g., Nathenson, 1975; Muffler and Cataldi, 1978; Muffler, 1979; Sanyal and Butler, 2004). Analyses of production data from naturally fractured reservoirs indicate that Rg typically varies between 0.05 and 0.2 (e.g., Lovekin, 2004; Williams, 2004). These lower values for Rg reflect the thermal effects of heterogeneities in the spatial distribution and flow characteristics of permeable fractures in the reservoirs (Bodvarsson and Tsang, 1982; Pruess and Bodvarsson, 1984; Williams, et al., 2007, 2008b). From estimates of Rg and measurements of reservoir volume and properties, the exergy, E, (DiPippo, 2005) for a geothermal reservoir can be determined as 0 0 0 [ ( )] WH WH WH E m h h T s s = − − − (4) where sWH is the entropy of the produced fluid and s0 is the entropy at the reference temperature. The electric energy, e W& , for a given period of time (typically 30 years) is then determined through multiplying the exergy over the same period of time by a utilization efficiency, ηu, which is generally well-constrained for a reservoir of a specified fluid state and temperature (Muffler and others, 1979; DiPippo, 2005; Williams et al., 2007, 2008), as