The role of stress-dependent permeability in enhanced geothermal systems modelling

Abstract

We present a procedure for testing the interpretation of the induced seismicity. The procedure is based on Coulomb stress changes induced by deep fluid injection during well stimulation, providing a way to estimate how the potential for seismic failure in different volumes of a geothermal reservoir might change due to the water injection. It was successfully applied to reproduce the observations collected during and after the complex stimulation cycle of the GPK2 and GPK3 wells at the Soultz sous Foret Enhanced Geothermal System EGS site (Alsace, France). In this paper, a conceptual model that links the induced stress tensor and the permeability modifications is considered, with the aim to estimate the permeability change induced during the water injection. In this way, we can adapt the medium behaviour to mechanical changes, in order to better evaluate the effectiveness of the stimulation process for the enhancement of the reservoir permeability, while also refining the reconstruction of the Coulomb stress change patterns. Numerical tests have been developed that consider a physical medium comparable with the granitic basement of the Soultz EGS site, and a geometry of the system that is compatible with that of the GPK2-GPK3 wells. In such a way, the forecasting of the areas of higher likelihood for induced seismicity results in a good agreement between Coulomb stress change patterns and induced seismicity.