Implication of petrography and structure of a rock mass for geomechanical processes associated with EGS projects

Abstract

Effective and low-risk stimulation of geothermal reservoirs requires profound understanding of the lithology and structure of the target rock mass, which are key factors for the geomechanical response to high-pressure injection. The characterization of a rock volume is generally accomplished on the basis of geophysical borehole logging, hydraulic measurements, and cutting analysis. Very rarely, drill core material is available. Novel methods are required to retrieve a maximum amount of information from often limited data.
The Enhanced Geothermal System (EGS) in Soultz-sous-Forêts provides a large database of both, geophysical and hydraulic data, to develop and test new analysing techniques giving insight into processes during hydraulic stimulation. Recent studies [Meller and Kohl, 2014, 2015] already demonstrated a significant impact of hydrothermal alteration zones in granitic reservoir rock on the mechanical processes related to stimulation and deep water circulation.
This study aims at integrating various different datasets towards a petrophysical and structural description of the reservoir rock. We use borehole and hydraulic data of the Soultz-sous-Forêts EGS field to characterize the deep crystalline reservoir with respect to its role in affecting the failure processes during stimulation. Neural network analysis was applied to identify alteration zones and their related clay content. Combination of borehole data revealed the existence of mainly three geological units, which differ in lithology, alteration grade, and fracture content: the porphyritic granite, a transition zone, and a two-mica granite.
The occurrence of an abundance of fractures and breakouts in the two-mica granite indicate that this zone is weaker than the massive porphyritic granite. The interjacent transition zone is identified by its high clay content. A basically different response to fluid injection is observed for the three rock units. The study reveals an episodic fracture reactivation from two-mica to porphyritic granite, an anti-correlation between clay content and seismicity, and a significantly higher number of events in the two-mica granite. As the crystalline basement is the main target for EGS in the Upper Rhine Graben ab EGS projects world-wide, the understanding of the lithological key factors controlling seismicity in granitic rock is crucial for the future development of EGS.