NI: DEEPHEAT: Digging deep Earth for heat to promote environmental sustainability

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Geothermal energy provides an important alternative to fossil fuels, both for heating and for electricity generation. EGS (enhanced geothermal systems) enables the targeting of deep rock formations, at ~2 to 5 km depth for heat extraction. However, few attempts at EGS development have reached the commercial stage. A recent review identifies ~30 EGS sites in granites or other crystalline rocks worldwide, a large proportion of which have failed. One main reason is difficulty in developing EGS without generating unwanted seismicity. In the UK, the unsuccessful Rosemanowes project, in the Carnmenellis granite pluton in west Cornwall, was shut down in the early 1990s, after years of hydraulic fracturing failed to establish any significant inter-well hydraulic connection. This failure killed UK EGS R&D for a generation. Most recently, starting with drilling in 2019, a second project - at the United Downs site - has proceeded in the Carnmenellis granite. However, although the developer has not yet made any official announcement, for months the UK geothermal community 'grapevine' has discussed reasons why this project is in trouble, involving both seismicity and the lack of hydraulic connection between wells. This latest failure, involving the loss of a ~£20 million investment, highlights the need for greater expertise in EGS. Despite the body of research on reservoir stimulation, the general processes that govern the evolution of in-situ stress during reservoir stimulation, and the associated anthropogenic seismicity, still remain poorly understood. For example, how does chemical stimulation change the mechanical state of a fault surface? Will chemical reactions, creating new secondary minerals, alter the frictional properties of a fault in a manner that favours instability? How does the traction on a fault evolve as material is removed by dissolution? How do we manage fluid injection rates and pressures to avoid anthropogenic seismicity? This project aims to create a new multidisciplinary environment and identify key scientific questions that need to be addressed to mitigate risks of failure for future EGS projects. We have assembled a team of enthusiastic early-career and more senior researchers with high international standing and expertise in geoscience, geomechanics, and geophysics, from University of Glasgow (UG) in the UK, University of Wisconsin-Madison (UW) and Lawrence Berkeley National Laboratory (LBNL) in the USA, and Sinopec Research Institute of Petroleum Engineering (SRIPE) in China. Only by working together, can we use our complementary expertise, advanced laboratory facilities, unique field resources and site data to cover multiple scales and aspects that cannot be achieved by individual institutions. We will apply integrated laboratory, modelling and field approaches to develop new scientific understanding of how anthropogenic seismicity caused by geothermal reservoir stimulation can be controlled and eliminated. UW and LBNL will lead the experimental study using their laboratory facilities. The laboratory study will provide data for coupled modeling, which will be led by UG. SRIPE will lead field study and bring in unique resources and data from their Gonghe EGS site (the first and the most important EGS site in China). The field study at the unique Gonghe EGS site will provide vast future collaboration opportunities. We have also designed outreach and partnership activities to facilitate interaction and collaboration between researchers, and to develop long-term sustainable collaborations. These activities include two site visits (to Gonghe EGS site), annual 2-day workshops (in 2022 at UW and in 2023 at UG), 6 online smaller group meetings, and a project website. We expect this project will have significant impact on public and governmental attitudes to EGS in the UK and worldwide by contributing to evidence-based seismicity control and thus to breaking the existing pattern of EGS project failure.