Maelle Bremaud, Neil Murray Burnside, Zoe Shipton, Cees Jean Leonardus Willems, Martin Pujol, Claire Bossennec

Hot sedimentary aquifers (HSAs) have huge potential for low-carbon energy supply but remain a relatively untapped resource. For example, HSAs could meet 100 years of UK national heat demand. The main technical barriers to HSA deployment are subsurface risks and associated well completion requirement. Numerous studies and policies have attempted to tackle these hurdles, but the sluggish implementation of HSA projects underscores the need for a deeper understanding of what works and what does not. Embracing a "learning from failure" ethos, we compiled a comprehensive database of key performance indicators (KPIs) through a systematic review of publicly available information from 256 HSA projects across eight countries where data were widely available: Australia, Croatia, Denmark, France, Germany, Poland, the Netherlands, and the UK. This database encompasses project specifics, borehole details, geological and hydrogeological parameters, associated risks, and mitigation strategies. Analysis reveals that 26% of HSA projects failed, mainly due to geological and hydrogeological (39% of all reasons), financial (26%) and technical (25%) issues. Mitigation or remediation strategies were implemented by 53% of both failed and running projects, resulting in a general decrease in failure rate over time. Successful projects emphasise the importance of robust pre-drilling site characterisation and ongoing system monitoring. We recommend the adoption of international standards for geothermal play classification and data reporting to enhance appraisal of HSA prospects. By quantifying KPIs for project failure and success, we hope to derisk and inform better budgeting of HSA endeavours, thereby bolstering the success and viability of future HSA projects.