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Abstract
The occurrence of hydrothermally altered zones is a commonly observed phenomenon in brittle rock. The dissolution and transformation of primary minerals and the precipitation of secondary minerals affect rocks in terms of mechanics, stress conditions, and induced seismicity. The present study investigates commonly observed phenomena of hydrothermal alteration and observations at the geothermal site of Soultz-sous-Forêts, which are related to the occurrence of hydrothermal alteration. Geomechanical observations at Soultz are interpreted on the basis of synthetic clay content logs, which are created from borehole logging data, and which identify clay in hydrothermally altered zones. It is shown that hydrothermal alteration results in a reduction of the frictional strength of the reservoir rock. Weak zones can act as stress-decoupling horizons, which locally perturb the stress field and affect the evolution of the microseismic cloud. For the first time, it is shown on a reservoir scale that large magnitude seismic events are restricted to unaltered granites, whereas in clay zones, only small magnitudes are observed. It is demonstrated that clay-rich zones foster the occurrence of aseismic movements on fractures. Secondary mineral precipitation during hydrothermal alteration has a great effect on the geomechanical properties of a geothermal reservoir. The identification of such zones is a first step towards understanding the relation between alteration and mechanical processes inside a reservoir and can help in reducing induced seismicity during hydraulic stimulation of a reservoir.
Highlights
The occurrence of hydrothermally altered zones is a commonly observed phenomenon in brittle rock
Impacts of hydrothermal alteration on rock mechanics Due to their preferential formation in weak rocks, a cumulative occurrence of breakouts in altered zones in the Soultz granite could be indicative of the weakness of alteration zones
In order to optimally use the properties of hydrothermally altered zones, further effort has to be done on understanding of the processes affecting the geomechanical behavior of a geothermal reservoir
Summary
The occurrence of hydrothermally altered zones is a commonly observed phenomenon in brittle rock. The importance of clay zones for the geomechanical structure and the earthquake mechanics in brittle rock became an important issue in the framework of mitigation studies of natural and man-made disasters (Holmes et al 2013). In terms of mitigation of induced seismicity, while increasing the permeability of the geothermal reservoir, detailed understanding of hydraulic and mechanical processes of fractured rock is the key for the success of an EGS project. The significance of clay for geothermal projects The development of EGS in low-enthalpy regions like the Upper Rhine Graben in central Europe involves the application of hydraulic stimulation for permeability enhancement in the geothermal reservoir. The injection of fluid into the underground changes the effective stress, inducing slips on fractures and faults associated with seismic events in brittle rock. In order to predict or even control the seismic behavior of a geothermal reservoir, the geomechanical structures and the associated processes must be known
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