Carola Meller

The significance of hydrothermal alteration zones for the mechanical behavior of a geothermal reservoir

BackgroundThe 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.MethodsThe 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.ResultsIt 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.ConclusionsSecondary 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.