1. Tracer techniques for the characterisation of geothermal reservoirs
Georg-August-Universität Göttingen
Angewandte Geologie Geowissenschaftliches Zentrum
Tracer techniques for the characterisation of geothermal reservoirs
I. Ghergut, M. Lodemann, M. Sauter
Angewandte Geologie,
Geowissenschaftliches Zentrum,
Universität Göttingen

2. 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

3. 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)

4. Motivation VOLUMES or SURFACES Assessing equivalent
of a (geothermal) reservoir

5. Contact surface between fractures and rock matrix
Hydraulically equivalent reservoirs can be distinguished
(fracture-dominated reservoir)
(pore-space dominated reservoir)
same void-space volume
( Area / Vol ) is low
( Area / Vol ) is high

6. Push-pull tracer experimental concept

7. 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)

8. ‚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

9. Determination of diffusion coefficients

10. Tracer method to assess contact surfaces in a hybrid (fractured-porous) system
influence of surface- non-related processes (advection-dispersion, large-scale fracture heterogeneity) on measured tracer signals:
single-well method
‚push-pull‘
(single-well, injection-withdrawal)
No direct determination of contact surfaces; use tracers with different diffusion and/or sorption properties):
‚dual-tracer‘ multi-tracer

11. Push-pull tracer experimental concept

12. 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.

13. Forward model: concentration evolution in the fracture
note: concentrations can be measured only during the WITHDRAWAL phase (unless some in-situ detector available)

14. mid-late slopes of tracer BTCs from a push-pull test are independent upon early transport details, and they contain the desired contact surface information
At mid-late tailing times when advection-dispersion effects becomes negligible, the surface-area estimation from tracer BTCs reduces to fitting the parameter m for the simple integral equation (in which f represents the first time-derivative of tracer conc.):
with
with numeric coefficients an and n (having known analytical expressions) in the approximation for g() describing the geometry and size of matrix blocks.
The contact surface parameter (specific area m = A/V) multiplies different tracer parameter combinations (Dm/Rm, Dm/Rf), thus its ambiguity of determination can be reduced using several tracers with (known) different sorption and diffusion properties.

15. Stimulation / Push-Pull-Test (Horstberg)
Photo: BGR

16. Push-pull test in hydrothermally-altered granite formation (Southern Black Forest)
m2/m3
(~ free outflow phase)

17. Push-pull test in 4-km deep crystalline geothermal reservoir at Bad Urach
(~ free outflow phase)
1-10 m2/m3

18. Push-pull test in 4-km deep crystalline formation at the KTB-pilot hole
30 – 100 m2/m3

19. (details of KTB production phase)

20. KTB-2005, after injection of 100000m³ of water
 Change in volume and contact surface

21. some experiences in crystalline reservoirs, a review (1)

22. some experiences in crystalline reservoirs, a review (2)

23. some experiences in crystalline reservoirs, a review (3)

24. Flow path tracing application using a single well: sedimentary reservoir (Horstberg)

25. Single-well flow-path tracing in sedimentary reservoir

26. Single-well flow-path tracing in sedimentary reservoir: tracer recovery

27. single-well flow-path tracing in sedimentary reservoir around Hannover: details of hydraulic test design and physico-chemical parameters

28. Heat transport experiments

29. Longterm cooling in an HDR-Reservoir: Temperature development in the matrix

30. Temperature push-pull tracertest KTB-pilot hole

32. Flow path tracing at KTB-boreholes: circulation test – experimental design

33. nach Arrhenius‘ Gesetz erwartet:
Thermal Degradation
nach Arrhenius‘ Gesetz erwartet:
Rose et al. (2001)

34. Flow path tracing at KTB-boreholes: test design, and thermal / flow-path scenarios

35. Flow path tracing at KTB-boreholes: test design, and thermal / flow-path scenarios

36. Overview: tracers used
(final selection pending) tritiated water, inert gases, naphthalene-sulfonic, further tracer candidates under evaluation
uranine tritiated water NDS
flow-path tracing (as of 2005): uranine NDS
uranine NDS
push-pull tests (2004, 2005): heat (injected cold water) uranine tritiated water krypton NDS, PTS (Behrens)
naphthionate Lithium uranine Bromide NDS

37. Acknowlegements The German Research Foundation (DFG) H. Behrens
P. Rose (EGI Utah)
J. Orzol (Hannover)
R. Jung (Hannover)
O. Kolditz (Tübingen)
C. McDermott
M. Herfort (ETH)
S. Fischer,
J. Brinkmann,
O. Stückrad,
M. Armenat
M. Lambert (Karlsruhe),
KTB and Urach personnel (GFZ)

38. (following Y. Tsang up to 100 x) 10-4 m / (200 m) = 5 x 10-5
(assumptions regarding void-space structures that control hydraulics, geomechanics and fluid transport in the KTB reservoir)
W. Kessels
nf = 2 x 10-4
C. McDermott
ehy = 7 x10-5 m
(following Y. Tsang up to 100 x) 10-4 m / (200 m) = 5 x 10-5