Coupled thermo-hydro-mechanical processes and induced seismicity in unconventional geothermal systems

The response of tight rock to harness geothermal energy in unconventional systems involves highly coupled processes. Deep fluid circulation induces thermo-hydro-mechanical coupled processes that may lead to induced seismicity. The traditional concept for harnessing unconventional geothermal systems consists in hydro-shearing pre-existing fractures to enhance permeability thanks to dilatancy. Such concept, known as enhanced geothermal systems (EGS), has been deployed over a few decades, inducing earthquakes on several occasions, giving rise to project cancellation in some instances, like Basel (Switzerland) and Pohang (Korea Republic). Induced seismicity may be controlled by using a favorable stimulation protocol (Tangirala et al., 2024). However, coupled processes acting across the stimulated fracture network may eventually lead to induced earthquakes regardless of the stimulation protocol (Kivi et al., 2024). To minimize the risk of induced seismicity, closed-loop geothermal systems have been proposed because no fluid is injected into fractured rock, reducing the risk of fracture instability. However, since the circulating fluid along the closed loop is heated up just by conduction, a rapid thermal decline occurs along the lateral (in a multilateral setup). Even drilling tens of horizontal multilaterals at depth to decrease the flow rate circulating in each tube cannot effectively limit thermal decline at the outlet, making closed-loop geothermal systems inefficient for scalable electricity generation (Tangirala and Vilarrasa, 2025). Another alternative is multi-stage stimulation of EGS and, in particular, hydraulic fracturing-based EGS. In this concept, rather than hydro-shearing pre-existing fractures, hydraulic fractures connecting a doublet are created, limiting the risk of reactivating a large patch of a pre-existing fracture or fault. Yet, early thermal breakthrough may occur because of positive feedback mechanisms if a fracture starts attracting more water than the others because cooling opens up fractures, enhancing its transmissivity and attracting even more water. We provide a detailed assessment of the coupled processes occurring in these three unconventional geothermal systems and discuss their potential and limitations.

 

REFERENCES

Kivi, I. R., Vilarrasa, V., Kim, K. I., Yoo, H., and Min, K. B. (2024). On the role of poroelastic stressing and pore pressure diffusion in discrete fracture and fault system in triggering post-injection seismicity in enhanced geothermal systems. International Journal of Rock Mechanics and Mining Sciences, 175, 105673.

Tangirala, S. K., and Vilarrasa, V. (2025). On the limitations of closed-loop geothermal systems for electricity generation outside high-geothermal gradient fields. Communications Engineering, 4(1), 116.

Tangirala, S. K., Parisio, F., Vaezi, I., and Vilarrasa, V. (2024). Effectiveness of injection protocols for hydraulic stimulation in enhanced geothermal systems. Geothermics, 120, 103018.