Interpreting tracer tests in the enhanced geothermal system (EGS) ofSoultz-sous-Forêts using the equivalent stratified medium approach

Abstract : A new interpretation of tracer tests performed in 2005 in the French EGS located at Soultz-sous-Forêts is presented. A previous analysis of fluorescein tests revealed significant differences in terms of connectivity between the injection well (GPK3) and the two production wells (GPK2 and GPK4). The results suggested the existence of at least three fluid circulation loops. Two of them (loops 1 and 2) connecting GPK3 to GPK2, and the third one (loop 3) connecting GPK3 to GPK4. The present analysis is based on the equivalent stratified porous medium model, in which each circulation loop corresponds to a flow path through the rock formation and 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. A very accurate fit of the concentration curves for both production wells was obtained. The stratification factors show that the three circulation loops have different levels of heterogeneity. The wellhead pressure recorded during the tests was used to calculate the mean permeability of the loops. Loop 1 appears to be very permeable and narrow while loops 2 and 3 are less permeable and wider. This tracer test model is compared to a new deterministic 3D model of the fracture zones observed in the Soultz granitic reservoir. A well-known major fault in the Soultz Graben intersecting GPK2 and GPK3 could explain the rapid tracer circulation in loop 1.