Why Injection in a Geothermal Sediment Reservoir Causes Seismicity in Crystalline Basement - It Is Not Just Hydraulics

Abstract

In recent years, cases of induced seismicity have been reported for geothermal wells in aseismic regions. The use of geothermal energy naturally influences the reservoir as heat and water are withdrawn. However, most geothermal plants reinject the water so that pressure levels within the reservoir remain more or less stable. Despite these constant pressure levels and low injection pressures, induced seismicity in the surroundings of the reinjection well is sometimes experienced. An example is the Unterhaching Gt2 well, close to Munich in Germany. Here, the reservoir is a karstified limestone layer of the Upper Jurassic, approximately 500 m thick, in which extraction and reinjection take place. Flow rates of more than 100 l/s have been established with reinjection pressures below 10 bar. Nevertheless, induced seismicity occurs at this well. On the Richter scale, most of the events are below 1.0, but some reach up to 2.4. Due to their location, they can undoubtedly be attributed to the reinjection process. However, the origin of the quakes is not within the reservoir but located in the crystalline basement several hundred meters below. As the reinjection well cuts through a steeply inclined fault, a hydraulic connection between reservoir, borehole and basement might be given and could be an explanation for the unexpected location of seismicity. However, this implies that the fault is hydraulically open well into the basement. Which processes lead to induced seismicity in the basement, while the reservoir remains unaffected, is the question which now has to be answered. So far, it has been impossible to find a correlation between the occurrence of induced seismicity and operating parameters of the geothermal plant such as flow rate, injection pressure, or temperature. Therefore, thermo-hydraulic-mechanical numerical models of the subsurface have been developed to understand the interaction between different parameters and to possibly identify critical thresholds for the initiation of induced seismicity. Due to the large scale of the model, several kilometers in each direction, an equivalent porosity approach has been chosen for the hydraulic modeling of the karstic limestone layer. Flow within the fault is also described by Darcy’s law as the fault is not assumed to be a surface but a volume. This assumption is based on the analysis of seismic data of this region, which indicate a zone of damaged rock several tens of meters wide. Because of this approach of modeling the hydraulic system, the pore pressure within the fault will most likely be the determining factor for the onset of induced seismicity. Therefore, it is of great interest to analyse the influence of the operating parameters of the geothermal plant on this parameter. So far, the results show clearly that flow into the fault is small even if a high permeability equal to that of the reservoir is assumed all the way into the basement. As this is most likely not the case in reality, sensitivity studies with depth-dependent permeabilities are performed. Analysis of thermal stresses shows that they are confined to the immediate surroundings of the well and that they are far too small to have direct consequences for rock stability. However, they might influence the local stress field and act as a trigger for the induced seismicity.