Mechanisms of induced seismicity due to geothermal operations in the Dinantian fractured carbonates in Mol, Belgium

The geothermal project at the VITO Sustainability Park in Mol targets the fractured carbonates of the Dinantian formation. During balanced geothermal operations, several earthquakes were recorded, with a maximum magnitude of M_L 2.2. To better understand the mechanisms driving this seismicity, we developed a thermo-hydro-mechanical model to simulate pressure, temperature and stress changes on a fault near the injection well, which hosts most of the seismic events. Given the observations of a low stress drop and low seismic moment release, the fault is represented in the model as comprising a few isolated asperities embedded within a broader aseismic zone. Although the model geometry is simplified, and both flow and mechanical behavior are not fully constrained, it shows potential to reproduce the main characteristics of field observations of seismicity at the geothermal site. Our simulations indicate that thermal effects had limited influence on fault stress, primarily due to the relatively small injected volumes. Rate effects - through rapid poroelastic unloading immediately after shut-in – may have facilitated seismogenic slip after shut-in. Aseismic slip, primarily driven by pressure increases, appears to have played a significant role in fault reactivation. The model results suggest that stress transfer from aseismic slip to fault asperities may have been a key driver of seismicity, particularly for larger events occurring at greater depths and farther from the injection well.