|Schematic of a conceptual two-well Enhanced Geothermal System in hot rock in a low-permeability crystalline basement formation. Click to enlarge.|
A comprehensive new MIT-led study of the potential for geothermal energy within the United States has found that Enhanced Geothermal System (EGS) technology could supply a substantial portion of US electricity well into the future, probably at competitive prices and with minimal environmental impact.
Overall, the panel concluded that EGS can likely deliver cumulative capacity of more than 100,000 MWe within 50 years with a modest, multiyear federal investment for RD&D. The panel estimated the total EGS resource base to be more than 13 million exajoules (EJ), with an estimated extractable portion to exceed 200,000 EJ—about 2,000 times the annual consumption of primary energy in the United States in 2005.
An 18-member panel led by MIT prepared the 400-plus page study, titled The Future of Geothermal Energy. Sponsored by the US Department of Energy, it is the first study in some 30 years to take a new look at geothermal.
The goal of the study was to assess the feasibility, potential environmental impacts and economic viability of using EGS technology to greatly increase the fraction of the US geothermal resource that could be recovered commercially.
The Department of Energy defines Enhanced (or engineered) Geothermal Systems (EGS) as engineered reservoirs that have been created to extract economical amounts of heat from low permeability and/or porosity geothermal resources. EGS recovers thermal energy contained in subsurface rocks by creating or accessing a system of open, connected fractures through which water can be circulated down injection wells, heated by contact with the rocks, and returned to the surface in production wells to form a closed loop.
In its assessment, the panel adapted that definition to include all geothermal resources that are currently not in commercial production and require stimulation or enhancement. In addition, it added coproduced hot water from oil and gas production as an unconventional EGS resource type that could be developed in the short term and possibly provide a first step to more classical EGS exploitation.
The study viewed the quality of a geothermal resource as a continuum in several dimensions: temperature-depth relationship (i.e., geothermal gradient), the reservoir rock’s permeability and porosity, and the amount of fluid saturation.
High-grade hydrothermal resources have high average thermal gradients, high rock permeability and porosity, sufficient fluids in place, and an adequate reservoir recharge of fluids.
All EGS resources lack at least one of these, according to the study. For example, reservoir rock may be hot enough but not produce sufficient fluid for viable heat extraction, either because of low formation permeability/connectivity and insufficient reservoir volume, and/or the absence of naturally contained fluids.
The analysis considered three main components:
Resource: estimating the magnitude and distribution of the US EGS resource.
Technology: establishing requirements for extracting and utilizing energy from EGS reservoirs including drilling, reservoir design and stimulation, and thermal energy conversion to electricity.
Economics: estimating costs for EGS-supplied electricity on a national scale using newly developed methods for mining heat from the earth. Developing levelized energy costs and supply curves as a function of invested R&D and deployment levels in evolving US energy markets.
Specific findings of the report include:
EGS is one of the few renewable energy resources that can provide continuous base-load power with minimal visual and other environmental impacts. Geothermal systems have a small footprint and virtually no emissions, including carbon dioxide. Geothermal energy has significant base-load potential, requires no storage, and, thus, it complements other renewables—solar (CSP and PV), wind, hydropower—in a lower-carbon energy future. In the shorter term, EGS would provide a buffer against the instabilities of gas price fluctuations and supply disruptions, as well as nuclear plant retirements.
The accessible geothermal resource, based on existing extractive technology, is large and contained in a continuum of grades ranging from today’s hydrothermal, convective systems through high- and mid-grade EGS resources (located primarily in the western United States) to the very large, conduction-dominated contributions in the deep basement and sedimentary rock formations throughout the country. The panel estimated the total EGS resource base to be more than 13 million exajoules (EJ), with an estimated extractable portion to exceed 200,000 EJ—about 2,000 times the annual consumption of primary energy in the United States in 2005.
With technology improvements, the economically extractable amount of useful energy could increase by a factor of 10 or more, thus making EGS sustainable for centuries.
Ongoing work on both hydrothermal and EGS resource development complement each other. Improvements to drilling and power conversion technologies, as well as better understanding of fractured rock structure and flow properties, benefit all geothermal energy development scenarios.
EGS technology has advanced since its infancy in the 1970s. Field studies conducted worldwide for more than 30 years have shown that EGS is technically feasible in terms of producing net thermal energy by circulating water through stimulated regions of rock at depths ranging from 3 to 5 km. Current technology can now stimulate large rock volumes (more than 2 km3), drill into these stimulated regions to establish connected reservoirs, generate connectivity in a controlled way if needed, circulate fluid without large pressure losses at near commercial rates, and generate power using the thermal energy produced at the surface from the created EGS system.
Initial concerns regarding five key issues—flow short circuiting, a need for high injection pressures, water losses, geochemical impacts, and induced seismicity—appear to be either fully resolved or manageable with proper monitoring and operational changes.
At this point, the main constraint is creating sufficient connectivity within the injection and production well system in the stimulated region of the EGS reservoir to allow for high per-well production rates without reducing reservoir life by rapid cooling. US field demonstrations have been constrained by many external issues, which have limited further stimulation and development efforts and circulation testing times—and, as a result, risks and uncertainties have not been reduced to a point where private investments would completely support the commercial deployment of EGS in the United States.
Research, Development, and Demonstration (RD&D) in certain critical areas could greatly enhance the overall competitiveness of geothermal in two ways. First, it would lead to generally lower development costs for all grade systems. Second, it could substantially lower power plant, drilling, and stimulation costs, which increases accessibility to lower-grade EGS areas at depths of 6 km or more.
In a manner similar to the technologies developed for oil and gas and mineral extraction, the investments made in research to develop extractive technology for EGS would follow a natural learning curve that lowers development costs and increases reserves along a continuum of geothermal resource grades.
The report presents the examples of impacts that would result from research-driven improvements in three areas:
- Drilling technology: both evolutionary improvements building on conventional approaches to drilling such as more robust drill bits, innovative casing methods, better cementing techniques for high temperatures, improved sensors, and electronics capable of operating at higher temperature in downhole tools; and revolutionary improvements utilizing new methods of rock penetration to lower production costs. These improvements will enable access to deeper, hotter regions in highgrade formations or to economically acceptable temperatures in lower-grade formations.
- Power conversion technology: improving heat-transfer performance for lower-temperature fluids, and developing plant designs for higher resource temperatures to the supercritical water region would lead to an order of magnitude (or more) gain in both reservoir performance and heat-to-power conversion efficiency.
- Reservoir technology: increasing production flow rates by targeting specific zones for stimulation and improving downhole lift systems for higher temperatures, and increasing swept areas and volumes to improve heat-removal efficiencies in fractured rock systems, will lead to immediate cost reductions by increasing output per well and extending reservoir lifetimes.
For the longer term, using CO2 as a reservoir heat-transfer fluid for EGS could lead to improved reservoir performance as a result of its low viscosity and high density at supercritical conditions. In addition, using CO2 in EGS may provide an alternative means to sequester large amounts of carbon in stable formations.
EGS systems are versatile, inherently modular, and scalable from 1 to 50 MWe for distributed applications to large power parks, which could provide thousands of MWe of base-load capacity.
- Using coproduced hot water, available in large quantities at temperatures up to 100°C or more from existing oil and gas operations, it is possible to generate up to 11,000 MWe of new generating capacity with standard binary-cycle technology, and increase hydrocarbon production by partially offsetting parasitic losses consumed during production.
- A cumulative capacity of more than 100,000 MWe from EGS can be achieved in the United States within 50 years with a modest, multiyear federal investment for RD&D in several field projects in the United States.
We’ve determined that heat mining can be economical in the short term, based on a global analysis of existing geothermal systems, an assessment of the total US resource and continuing improvements in deep-drilling and reservoir stimulation technology.
EGS technology has already been proven to work in the few areas where underground heat has been successfully extracted. And further technological improvements can be expected.—Jefferson W. Tester, the H. P. Meissner Professor of Chemical Engineering at MIT, panel-leader
In its report, the panel recommends that:
More detailed and site-specific assessments of the US geothermal energy resource should be conducted.
Field trials running three to five years at several sites should be done to demonstrate commercial-scale engineered geothermal systems.
The shallow, extra-hot, high-grade deposits in the west should be explored and tested first.
Other geothermal resources such as co-produced hot water associated with oil and gas production and geopressured resources should also be pursued as short-term options.
On a longer time scale, deeper, lower-grade geothermal deposits should be explored and tested.
Local and national policies should be enacted that encourage geothermal development.
A multiyear research program exploring subsurface science and geothermal drilling and energy conversion should be started, backed by constant analysis of results.