Enhanced Geothermal Systems Technology Research Articles

Laboratory hydraulic shearing of granitic fractures with surface roughness under stress states of EGS: Permeability changes and energy balance

To offer key parameters and constitutive laws required for field-scale multiphysics simulations that can accurately predict created fracture network structures in enhanced geothermal systems (EGS) and resulting energy extraction, we explore the full spectral of hydraulic shear process of granite fracture and revisits the linkage between hydraulic and mechanical properties during shear slips. Key results from our novel lab-experiments include the followings: (1) fracture permeability of granite increases due to the hydraulic shear slip even at an effective normal stress of over 50 MPa, (2) shear slip and stress drop are in proportion and increment of the fracture permeability increases with increasing total shear slip displacement, and (3) although hydraulic shear slip tends to make fracture surfaces slightly smoother, the factual characteristics of surface are maintained after slip. By combining our experimental results with the seismological analysis, we first explore energy balance during hydraulic shearing of the preexisting rock fracture and point out the crucial role of the elastic potential energy stored in the surrounding bulk rock masses. Subsequently, we derive a constitutive model related to permeability change of granite fracture during hydraulic shearing under the typical crustal stress of EGS and presume that the maximum change in fracture permeability due to shear dilation is ∼20-fold, although scale effect is not considered. Finally, via the high resolution 2-D mapping of the surface damages, we illuminate the formation of preferential flow paths within rock fracture during hydraulic shearing and its possible role on the improvement of fracture permeability. Thus, we successfully demonstrate both new and sophisticated insights in hydro-mechanical coupled processes during hydraulic shearing to improve the accuracy of fracture network designs in EGS technology.

Seismogenic potential of withdrawal-reinjection cycles: Numerical modelling and implication on induced seismicity

Induced seismicity can be associated to the activity of fluid withdrawal and injection from/into the shallow crust (fracking, wastewater disposal into the deep crust, Enhanced Geothermal Systems technology, fluid extraction in oil fields and geothermal power plants). Long-term injection of large volumes of fluids is normally associated with induced seismicity, but the effect of withdrawal-reinjection in the same reservoir is less known, at least regarding its relation to simple injection. However, it is common experience worldwide that small (i.e. 10 MW or less) geothermal plants with withdrawal and re-injection of fluids in the same reservoir are mostly not associated with significant induced/triggered seismicity. This paper aims at understanding how to discriminate, on a numerical modelling basis, the seismogenic potential of withdrawal-reinjection with respect to injection only. With this aim, we analysed the induced pressure changes, the perturbed volumes of rocks and the potential for induced seismicity due to these operations. A set of simulations of injection or withdrawal-reinjection cycles, obtained by using the numerical code TOUGH2®, is applied to simple models of geothermal reservoirs, with varying permeability and lateral boundary constraints. For each permeability model, we then compare the time growth of perturbed volumes obtained with withdrawal-reinjection cycles to those obtained during simple injection, using the same flow rates. The size of perturbed volumes is then related to the maximum magnitude of induced/triggered seismicity, using models accredited in recent literature. Our results show that, for all models, withdrawal-reinjection is by far less critical than simple injection, because the perturbed volumes are remarkably smaller and remain constant over the simulated time, so minimizing the likelihood of interference with seismogenic faults. These results have significant implications for geothermal projects, and in the assessment of the potential risk related to fluid stimulation and induced seismicity.

Integrated Research as Key to the Development of a Sustainable Geothermal Energy Technology

AbstractAs estimated by the International Energy Agency, geothermal power can contribute to 3.5 % of worldwide power and 3.9 % to heat production by 2050. This includes the development of enhanced geothermal systems (EGSs) in low‐enthalpy systems. EGS technology is still in an early stage of development. Pushing EGS technologies towards market maturity requires a long‐term strategic approach and massive investments in research and development. Comprehensive multidisciplinary research programs that combine fundamental and applied concepts to tackle technological, economic, ecological, and safety challenges along the EGS process chain are needed. The Karlsruhe Institute of Technology (KIT) has defined a broad research program on EGS technology development following the necessity of a transdisciplinary approach. The research concept is embedded in the national research program of the Helmholtz Association and is structured in four clusters: reservoir characterization and engineering, thermal water circuit, materials and geoprocesses, and power plant operation. The proximity to industry, closely interlinked with fundamental research, forms the basis of a target‐orientated concept. The present paper aims to give an overview of geothermal research at KIT and emphasizes the need for concerted research efforts at the international level to accelerate technological breakthrough of EGS as an essential part of a future sustainable energy system.

The transformative potential of geothermal heating in the U.S. energy market: A regional study of New York and Pennsylvania

Enhanced Geothermal Systems (EGS) could supply a significant fraction of the low-temperature (<125°C) thermal energy used in the United States through Geothermal District Heating (GDH). In this study we develop a regional model to evaluate the potential for EGS district heating in the states of New York and Pennsylvania by simulating an EGS district heating network at each population center within the study region and estimating the levelized cost of heat (LCOH) from GDH for each community. LCOHs were then compiled into a supply curve from which several conclusions could be drawn.Our evaluation revealed that EGS district heating has the potential to supply cost-effective energy for space and water heating in several New York and Pennsylvania communities in the near future. To realize wider deployment, modest improvements in EGS technology, escalation of natural gas prices, and/or government incentives will likely be required to enable GDH to compete with other heating alternatives today. EGS reservoir flow rates, drilling costs, system lifetimes, and fluid return temperatures have significant effects on the LCOH of GDH and thus will provide the highest return on R&D investment, while creative implementation strategies can help EGS district heating overcome initial cost barriers that exist today.

Economic Optimization of a CO2-Based EGS Power Plant

CO2-based enhanced geothermal systems (EGSs) have been examined from a reservoir-oriented perspective, and as a result thermodynamic performance is well explored. Economics of the system are still not well understood, however. In this study, the economics of the CO2-based EGS technology is explored for an optimized power plant design and best-available cost estimation data. We demonstrate that near-optimum turbine exhaust pressure can be estimated from surface temperature. We identify that achievable cooling temperature is an important economic site consideration alongside resource temperature. The impact of time required to sufficiently dry the reservoir prior to power generation is also addressed. The role of sequestration as part of CO2-based EGS is also examined, and we conclude that if fluid losses occur, the economic viability of the concept depends strongly on the price associated with CO2.

Emerging geothermal energy technologies

Geothermal energy, whether as a source of electricity or to heat or cool buildings, has an enormous potential as a renewable energy source. This paper presents a broad overview of geothermal energy, with a focus on the emerging technologies of Enhanced Geothermal Systems (EGS) and Ground Source Heat Pumps (GSHPs). EGS and GSHP systems are important because of their potential for widespread use, as opposed to traditional geothermal energy systems which are generally limited to tectonically active regions. Commercial application of EGS technology is some years off but its enormous potential demands investment for further research. GSHP heat pump technology is available now — policy makers need to be educated and the potential of GSHPs demonstrated. Research should focus on optimising the design of the in-ground component of GSHP systems to maximise their economic, as well as environmental, benefits.