Changes in geothermal reservoirs due to hydrothermal eruptions and fault reactivation

Abstract

Geothermal reservoirs naturally change over time as hot upflows shift within the subsurface. The causes of such evolving flow paths are often speculated upon. Of the many possible mechanisms, large hydrothermal eruptions (focal depth > 100 m) and faulting (new or reactivation) may cause sudden changes in the pressure and flow field locally, leading to changes in temperature and pressure in the deeper subsurface. The extent and magnitude of such changes are poorly understood.
We employ a numerical reservoir simulator to investigate how fluid flow throughout the geothermal system is affected by a sizable hydrothermal eruption or permeable fault zone. By using simplified models, we identify scenarios in which these mechanisms have greatest impact and set bounds on the extent and magnitude of change. We show that the permeable zone created by a hydrothermal eruption has strong impacts on the pressure, temperature, and flow regime in the shallow
reservoir (<1 km), but their effect on the deeper reservoir (>1 km) can be limited in the presence of deeper flow barriers (i.e., a deep clay cap). By contrast, fault development extending into the deeper reservoir and penetrating such flow barriers has the capacity to alter the system by slightly shifting where hot upflows move into the shallow system. Our findings can be applied to other reservoirs, providing a first-order assessment of whether hydrothermal eruptions and/or faulting could be responsible for the changes in the temperature, pressure, and hydrologic structure of the geothermal system.