Thermo-hydro-mechanical simulation of cooling-induced fault reactivation in Dutch geothermal reservoirs

Abstract

Geothermal energy is one of the most viable sources of renewable heat. However, the potential risk of induced seismicity associated with geothermal operations may slow down the growth of the geothermal sector. Previous research has led to significant progress in understanding fluid-injection-induced seismicity in geothermal reservoirs. However, an in-depth assessment of thermal effects on the seismic risk was generally considered to be of secondary importance. This study aims to investigate the relative influence of temperature and key geological and operational parameters on the slip tendency of pre-existing faults. This is done through coupled thermo-hydro-mechanical simulations of the injection and production processes in synthetic geothermal reservoir models of the most utilized and potentially exploitable Dutch geothermal reservoir formations: Slochteren sandstone, Delft sandstone and Dinantian limestone.

In our study, changes in the slip tendency of a fault can largely be attributed to thermo-elastic effects, which confirms the findings of recent studies linking thermal stresses to induced seismicity. While the direct pore pressure effect on slip tendency tends to dominate over the early phase of the operations, once pore pressure equilibrium is established in a doublet system, it is the additional stress change associated with the growing cold-water front around the injection well that has the greatest influence. Therefore, the most significant increase in the slip tendency was observed when this low-temperature front reached the fault zone. The distance between an injection well and a pre-existing fault thus plays a pivotal role in determining the mechanical stability of a fault. A careful selection of a suitable target formation together with an appropriate planning of the operational parameters is also crucial to mitigate the risk of induced seismicity. Besides the well-known relevance of the in situ stress field and local fault geometry, rock-mechanical properties and operation conditions exert a major influence on induced stress changes and therefore on the fault (re)activation potential during geothermal operations.