On the numerical approach of hydraulic stimulation modeling in geothermal resources: a two-scale characterization
Résumé
Stimulation techniques in the context of Enhanced Geothermal Systems (EGS) are aimed at improving overall well injectivity/productivity. This is particularly true of hydraulic stimulation, which consists in injecting highly pressurized water into the reservoir’s existing fault network so that, under the anisotropic in situ stress state, the fault zones slide and open due to their dilatant nature. The effectiveness of this kind of stimulation depends on the hydromechanical (HM) response of the system. This paper deals with the numerical modeling of fault zones under stimulation, and aims at highlighting the HM mechanisms involved in the irreversible increase of well injectivity. To be able to numerically simulate the interactions of a fault network using 3DEC© software, the fault zones must be approximated as 2D joints. A complementary model based on a more realistic geometry is proposed for simulating the stimulation of a single fault zone, and is used to highlight the phenomena that cannot be reproduced by a simplified 2D model. The results are based on the Soultz-sous-Forêts EGS case study. First, the results at fault-network scale highlight the complementary role of the fault zones; the so-called favorably oriented fault zones drive the stimulation by sliding, while the less favorably oriented fault zones tend to increase the overall network connectivity. In addition, sliding of the favorably oriented faults can trigger induced microseismicity, whilst the less favorably oriented zones can mitigate it. Second, the impact of the geometry on the HM response of a fault zone is tested at single fault zone scale where the single fault zone is modeled as a 3D object. The results obtained here highlight complex displacements that so far are not reproduced by 2D models. The increase in fault zone aperture is found to result both from the sliding of discontinuities and their subsequent dilation, and from block rotations occurring within the fault zone. Being aseismic, this mechanism is particularly of interest for stimulation purposes.