Tracer Tests in the Enhanced Geothermal System (EGS) of Soultz-sous-Forêts. What Does the Stratified Medium Approach Tell Us About the Fracture Permeability in the Reservoir?

Abstract

We present a new interpretation of tracer tests performed in 2005 in the French EGS located at Soultz-sous-Forêts. A previous analysis of fluorescein tests (Sanjuan et al., 2006) revealed substantial differences in terms of connectivity between the injection well (GPK3) and the two production wells (GPK2 and GPK4). The tracer concentration in the fluid discharged from the production wells was measured and partially modeled using a code based on a model of dispersive transfer. The results suggested the existence of at least three fluid circulation loops. Two of them (loops 1 and 2) associated to GPK2, and the third one (loop 3) associated to GPK4.

Our new analysis is based on the equivalent stratified porous medium model. In this approach, each circulation loop, which corresponds to a flow path through the rock formation, is regarded as a stratified porous medium having a log-normal permeability distribution. The heterogeneity of this equivalent porous medium is quantified by the stratification factor defined as the standard deviation to the mean permeability ratio.

The results of this work allowed a very accurate modeling of the concentration curves for both production wells. The stratification factors show that the three circulation loops have different levels of heterogeneity. In addition, the tracer mean arrival times are in a ratio of 4.3 between loop 2 and loop 1, and 13.6 between loop 3 and 1 thus giving some insight on the mean permeabilities of the equivalent stratified media.

This tracer test model is compared to a new deterministic 3D model of the fracture zones observed in the Soultz granitic reservoir. The major fracture zones encountered around 6 wells (4550, EPS1, GPK1, GPK2, GPK3 and GPK4) are located and characterized in terms of size, orientation, extensions and widths: fracture zones, microseismic structures and structures derived from vertical seismic profiles are modeled using Discrete Fracture Network tools of the gOcad modeling platform (Paradigm, Earth Decision Sciences). The objective is to match the permeability results derived from the tracer test modeling and from the geometry of the Soultz fracture network to try to better understand the geothermal fluid flow pathways in the reservoir.