Characterization of Fracture Connectivity and Fluid Flow Pathways Derived from Geological Interpretation and 3D Modelling of the Deep Seated EGS Reservoir of Soultz (France)

Abstract

This study presents a 3D interpretation of the fracture connectivity and the resulting permeability derived from the study of the geophysical logs run after the drill of three wells at 5 km depth in the framework of the Soultz European EGS geothermal project.

The major fracture zones encountered in these wells are located through examination of borehole image logs, classical geophysical well logs and cutting samples. These large-scale fracture zones could be 10m thickness and are characterized by sealed core and a peripheral damage zone that is highly porous and/or fractured and therefore highly permeable. However, two scales of fracture networks are present in the granite: a highly connected network consisting of fractures with small apertures that maybe represents the far field reservoir, and another network that contains these isolated and wide fracture zones which develops an anisotropic permeability in the rock and allow the hydraulic connection of the wells. This hierarchy of flow is strongly dependant on the petrophysical properties of the rock and especially is directly linked to the intensity of the granite hydrothermal alteration.

A 3D model of the Soultz exchanger is constructed with the gOcad code (Earth Decision, Paradigm). The model is enclosed in a 3D regular grid extending in the open part of the heat exchanger between 4000 and 5200 m true vertical depth. The grid axes fit on the directions of the maximal anisotropy of the microseismic cloud observed during the different stimulation tests. The grid is aligned with a global N170