Petrophysics and Permeability in the Soultz Granite

Abstract

This study presents a multidisciplinary approach to understand and describe types of fracture permeability in the Soultz-sous-ForÍts granite (Bas-Rhin, France). At Soultz, during the 1993 stimulation tests in the GPK1 well, it was shown that only a limited number of natural fractures contributed to flow whereas there are thousands of fractures embedded within the massive granite. In order to understand the past and actual flow hierarchy, a detailed comparison between petrophysics (log correlations), static (fracture apertures based on ARI raw curves) and dynamic data (hydraulic tests) is carried out. Two scales of fracture networks are present in the granite: a highly connected network consisting of fractures with a small apertures that maybe represents the far field reservoir, and another network that contains isolated and wide drains which develops an anisotropic permeability in the rock and could allow a hydraulic connection between the injection and production wells. This hierarchy of flow is strongly dependant of the petrophysical properties of the rock and especially is directly linked to the intensity of the granite hydrothermal alteration. Log charts are proposed to qualify the hydraulic behaviour of fractures. The final objective remains the matching of structural and hydraulic models and field data as flow, pressure, UBI image logs in GPK1 but also with data of injection tests in the other deep wells: GPK3 and future GPK4.