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Georg-August-UniversitätGöttingen
Tracer techniques for the characterisation of geothermal
reservoirs
I. Ghergut, M. Lodemann, M. Sauter
Angewandte Geologie,
Geowissenschaftliches Zentrum,
Universität Göttingen
Angewandte GeologieGeowissenschaftliches Zentrum
Overview
• General concepts for determination of volumes and surface areas
• Single-well method: dual-tracer push-pull• Application in three crystalline reservoirs in
Germany (Urach, Lindau, KTB)• A special flow path tracing application using a
single well - example from the sedimentary reservoir near Hannover
Tracer techniques to determine:
• travel times (residence times, contact times) reservoir volume or porosity
• temporal changes in reservoir properties from cooling (deformation) (e.g., coupled thermo-hydro-mechanical changes)
• contact surface between fractures and rock matrix (heat exchange surface)
• reservoir temperature (volume averaged)
Contact surface between fractures and rock matrix
same void-space volume
( Area / Vol ) is high ( Area / Vol ) is low
(fracture-dominated reservoir)
(pore-space dominated reservoir)
Hydraulically equivalent reservoirs can be distinguished
Tracer candidates (soluble)
• traced water molecules (HTO, D and 18O less suitable)
• fluorescent dyes (e.g. Uranine)
• food dyes / additives (e.g. Tartracine, E...)
• Naphthaline-Sulphonates (z.B. 1,5-NDS)
• sulphonated Naphthalene Formaldehyde condensates (SNFC)
‚Colouring our food in the last and the next millennium‘ (Downham & Collins, 1999)
Some well characterised food additives
further criteria : Analytics Price EU-admission etc
Forward model: concentration evolution in the fracture
Tracer separation by diffusion/sorption coefficients reverts monotonicity upon transition from peak to tailing phases; it is advisable to use the latter in fitting the surface area parameter.
note: concentrations can be measured only during the WITHDRAWAL phase (unless some in-situ detector available)
Forward model: concentration evolution in the fracture
Push-pull test in hydrothermally-altered granite formation (Southern Black Forest)
(~ free outflow phase)100-300 m2/m3
Push-pull test in 4-km deep crystalline geothermal reservoir at Bad Urach
(~ free outflow phase) 1-10 m2/m3