Deep Geothermal Projects Research Articles

Siting deep geothermal energy: Acceptance of various risk and benefit scenarios in a Swiss-German cross-national study

Deep geothermal energy projects offer low-carbon, renewable base-load resources for electricity and heat production. Siting such projects can be challenging because past projects have induced seismicity. This suggests siting projects in remote areas away from populated areas and infrastructure, with minimal seismic risks. However, deep geothermal projects are most viable when they use residual heat, which requires proximity to heat consumers and thus, ideally, a rather urban environment. Hence, siting options carry various risks and benefits. It is informative to see how the public responds to these risks and benefits. This study investigates how well the public accepts various heat benefits when induced seismic risks are comparatively high or low. Respondents rated their acceptance of four deep geothermal energy scenarios in an online survey (N = 814) conducted in Switzerland and Germany. Conjoint and mixed multivariate statistical analyses show that the public prefers projects sited in remote areas and using residual heat for industrial applications. The results in Switzerland and Germany were rather similar, but the Swiss public was generally more positive. Importantly, induced seismic risks affected acceptance ratings most strongly. Thus, policies to reduce the risk of induced seismicity must be given the highest priority to enable an open dialogue.

Social shaping of deep geothermal projects in Alsace: politics, stakeholder attitudes and local democracy

This paper examines the social, cultural and political factors that favor or disrupt deep geothermal energy projects in Alsace. The research was conducted in the Risk Governance package of the H2020 DESTRESS program, aimed at comparing public perceptions of deep geothermal energy in different contexts. The French case studies focus on two different contexts: one in Northern Alsace, where geothermal energy is fairly well accepted, and another in the Eurometropolis of Strasbourg, where some projects have raised substantial controversy. Several conceptual tools are used to understand variations in the public perceptions of geothermal projects. First, the distinction between “locally anchored” projects and “unbound” or exogenous projects account for the way these projects take shape, based on dialog or facilitated by a favorable economic and national political context, ignoring local specificities. Second, the concepts of social identity and social worlds allow us to yield insights into the acceptability of a project on a more sociological and cultural level, instead of adopting a limited risk perception focus. By considering the social and cultural contexts, we can better explain the weight that opponents place in their discourses on induced risks, local politics, or environmental issues during a technoscientific controversy. This ultimately leads us to underline the limitations of the communication models for project governance that are chiefly aimed at reassuring the local population.

THM modeling of hydrothermal circulation at Rittershoffen geothermal site, France

BackgroundThe Rittershoffen deep geothermal project located 6 km east from Soultz-sous-Forts EGS site (France) includes a doublet GRT-1 and GRT-2 to exploit the geothermal resource at the sediments–granite transition where higher temperatures than those of Soultz-sous-Forêts have been measured. Detailed stratigraphic and geophysical data, temperature logs, and tracer surveys have been collected. However, no reservoir model, integrating large-scale geophysical measurements, exists for this site.MethodsWe developed a reservoir model in two dimensions (10 km × 5 km) based on a finite element method. It includes thermo–hydro–mechanical (THM) coupling and extended brine properties. A representative elementary volume of 100 m is assumed to homogenize the fault network complexity at small scales. A back analysis is performed to obtain large-scale rock properties using GRT-1 temperature log and regional stress-depth profiles.ResultsThe inverted large-scale properties are consistent with their counterparts measured at the laboratory scale. The bottom of the hydraulic cap rock is 1.2 km ± 0.1 km deep. It is shallower than the discontinuity of the thermal gradient. Hydrothermal convection cells are 2.7 km high which is larger than that previously proposed.ConclusionsA very good fit of the GRT-1 temperature log is obtained using our simplified two-dimensional THM model with four homogenized units at a 100 m scale. The comparison between Rittershoffen and Soultz-sous-Forêts models highlights many similarities in terms of rock properties, decoupling of hydraulic and thermal cap rocks and temperature spatial variability (about 50 °C). Predictions of the relationship between reservoir temperature and surface thermal gradients are proposed for future explorations.

Reservoir Imaging Using Ambient Noise Correlation From a Dense Seismic Network

AbstractIn September 2014, a dense temporary seismic network (EstOF) including 288 single‐component geophones was deployed during 1 month in the Outre‐Forêt region of the Upper Rhine Graben (France), where two deep geothermal projects (Soultz‐sous‐Forêts and Rittershoffen) are currently in operation. We apply ambient seismic noise correlation to estimate the empirical Green's function of the medium between the ~41,200 station pairs in the network. The noise correlation functions obtained are comparable to those from previous studies based on the sparse long‐term networks settled in the area mostly to monitor the induced seismic activity. However, the dense spatial coverage of the EstOF network improves our ability to identify the main phases of the Green's function. Both the fundamental mode and the first overtone of the Rayleigh waves are identified between most station pairs. P waves are also evidenced. We analyze the statistical distribution of the Rayleigh wave group velocity between station pairs as a function of the period (between 0.8 and 5 s), the station pair orientation, the distance over wavelength ratio and the signal‐to‐noise ratio. From these observations, we build a high‐resolution three‐dimensional S wave velocity model of the upper crust (down to 3 km deep) around the regional deep geothermal reservoirs. This model is consistent with local geological structures but also evidences nonlithological variations, particularly at depth in the basement. These variations are interpreted as large‐scale temperature anomalies related to deep hydrothermal circulation.

Assessment of deep geothermal energy exploitation methods: The need for novel single-well solutions

Geothermal energy is a constant and independent form of renewable energy and plays a key role towards the world's future energy balance. In particular, deep geothermal resources are largely available across continents and can help countries become less dependent on energy imports and build a broader base in their future energy mix.However, despite its significant potential, the total contribution of the geothermal sector to global power generation remains relatively small. The International Energy Agency has recommended devising plans to address technology-specific challenges to achieve faster growth and improving policies tackling pre-development risks for geothermal energy. Reaching considerable depths is a requirement to exploit deep geothermal resources, but experience gained to date from the implementation of complex, engineered deep geothermal projects has unveiled technical and economic challenges, lower-than-expected performance and poor public image. There is therefore an urgent need for alternative, more sustainable well designs.This paper critically assesses conventional and unconventional deep geothermal well concepts, focusing on the basic Borehole Heat Exchanger (BHE) concept. The discussions are supported by numerical simulations of a BHE design that includes heat conductive fillers to enhance the heat exchange with the surrounding formation, while avoiding direct fluid interaction with the latter.

The induced earthquake sequence related to the St. Gallen deep geothermal project (Switzerland): Fault reactivation and fluid interactions imaged by microseismicity

AbstractIn July 2013, a sequence of more than 340 earthquakes was induced by reservoir stimulations and well‐control procedures following a gas kick at a deep geothermal drilling project close to the city of St. Gallen, Switzerland. The sequence culminated in anML3.5 earthquake, which was felt within 10–15 km from the epicenter. High‐quality earthquake locations and 3‐D reflection seismic data acquired in the St. Gallen project provide a unique data set, which allows high‐resolution studies of earthquake triggering related to the injection of fluids into macroscopic fault zones. In this study, we present a high‐precision earthquake catalog of the induced sequence. Absolute locations are constrained by a coupled hypocenter‐velocity inversion, and subsequent double‐difference relocations image the geometry of theML3.5 rupture and resolve the spatiotemporal evolution of seismicity. A joint interpretation of earthquake and seismic data shows that the majority of the seismicity occurred in the pre‐Mesozoic basement, hundreds of meters below the borehole and the targeted Mesozoic sequence. We propose a hydraulic connectivity between the reactivated fault and the borehole, likely through faults mapped by seismic data. Despite the excellent quality of the seismic data, the association of seismicity with mapped faults remains ambiguous. In summary, our results document that the actual hydraulic properties of a fault system and hydraulic connections between its fault segments are complex and may not be predictable upfront. Incomplete knowledge of fault structures and stress heterogeneities within highly complex fault systems additionally challenge the degree of predictability of induced seismicity related to underground fluid injections.

Overcoming challenges in the classification of deep geothermal potential

<p><strong>Abstract.</strong> The geothermal community lacks a universal definition of deep geothermal systems. A minimum depth of 400 m is often assumed, with a further sub-classification into middle-deep geothermal systems for reservoirs found between 400 and 1000 m. Yet, the simplistic use of a depth cut-off is insufficient to uniquely determine the type of resource and its associated potential. Different definitions and criteria have been proposed in the past to frame deep geothermal systems. However, although they have valid assumptions, these frameworks lack systematic integration of correlated factors. To further complicate matters, new definitions such as hot dry rock (HDR), enhanced or engineered geothermal systems (EGSs) or deep heat mining have been introduced over the years. A clear and transparent approach is needed to estimate the potential of deep geothermal systems and be capable of distinguishing between resources of a different nature. In order to overcome the ambiguity associated with some past definitions such as EGS, this paper proposes the return to a more rigorous petrothermal versus hydrothermal classification. This would be superimposed with numerical criteria for the following: depth and temperature; predominance of conduction, convection or advection; formation type; rock properties; heat source type; requirement for formation stimulation and corresponding efficiency; requirement to provide the carrier fluid; well productivity (or injectivity); production (or circulation) flow rate; and heat recharge mode. Using the results from data mining of past and present deep geothermal projects worldwide, a classification of the same, according to the aforementioned criteria is proposed.</p>

Seismic monitoring and analysis of deep geothermal projects in St Gallen and Basel, Switzerland

Seismic monitoring and analysis of deep geothermal projects in St Gallen and Basel, Switzerland

Inferring the in situ stress regime in deep sediments: an example from the Bruchsal geothermal site

Knowledge of the ambient state of stress is of crucial importance for understanding tectonic processes and an important parameter in reservoir engineering. In the framework of the 2,500-m deep geothermal project of Bruchsal in the central part of the Upper Rhine Graben, new evidence is presented for the stress field in deep-seated sedimentary rocks. With a sophisticated data analysis based on the concept of critical stress ratios, we evaluate the quality and uncertainty range of earlier stress field models in the Bruchsal area. New data from borehole logging and leak- off tests in deep sediments are used to propose an alternative stress profile for this part of the Upper Rhine Graben. The revised stress field model for the Bruchsal area predicts a normal with transition to strike-slip faulting regime. Stress field perturbations and potential decoupling process within specific clay-, salt-, and anhydrite-bearing units of the Keuper can be observed. By comparison with other models, we can show a regional consistency of our stress field model that is reliable throughout the central Upper Rhine Graben extending from Bruchsal in the East to the Soultz-sous-Forêts EGS site in the West.