Seismic hazard related to deep geothermal operations (Part I): identification of key criteria for hazard assessment

Deep geothermal projects can trigger seismic events depending on geological context and operations. This seismicity is generally of low magnitude but, in some cases, larger events may occur, which could lead to geothermal project abandonment and could present risk to neighboring populations. Thus, development of deep geothermal projects requires the management of induced seismicity to control it and to avoid any surface disturbance. It is from this perspective that, in 2023, Ineris and BRGM published a guide of good practices and recommendations for operators and French administration involved in deep geothermal energy.

The guide provides recommendations for assessing geothermal-induced seismic hazard, depending on the type of geothermal system and its intrinsic and operational characteristics, at each key step of a project (i.e. from the exploration phase until the end of the project). A worldwide review of deep geothermal projects, carried out with the aim of identifying key factors triggering induced seismicity, has enabled the definition of the most relevant criteria to take into account in the hazard assessment. In this review, geothermal projects were chosen to be representative of different types of geothermal systems (e.g. deep sedimentary aquifers, volcanic and plutonic regions, deep dry crystalline basements, etc.) and operating conditions (e.g. well configuration, type of operation, etc.). Moreover, the review includes projects associated with several episodes of induced seismicity, ranging in magnitude from microseismicity (M < 2) to large seismic events (M > 5), as well as projects marked by the absence of induced seismic activity.

From the 53 projects and 77 seismic episodes analyzed in this review, we can state that not all geothermal projects are equally prone to seismic events. The occurrence and the intensity of induced seismicity are the results of interactions between several natural (intrinsic) and anthropogenic (operational) factors, often concomitant and dependent on each other. Seismic response of analyzed projects appears to be largely different depending on the type of geothermal system. Indeed, the type of geothermal system characterizes reservoir porosity, as well as heat transfer and fluid circulation modes in the reservoir. Other key factors include the presence of faults that can be critically loaded and/or connected with the basement, the use of EGS technologies, and situations where injected and produced volumes are highly unbalanced. These results allowed defining key criteria for seismic hazard assessment methodology proposed within the good practice guide.