EGU22-6133, updated on 28 Mar 2022
EGU General Assembly 2022
© Author(s) 2022. This work is distributed under
the Creative Commons Attribution 4.0 License.

Do tracer tests enable model-independent predictions of georeservoir output? two examples from Southern Germany, involving thermal drawdown and solute co-production

Horst Behrens Julia Ghergut, Bettina Wiegand, Bianca Wagner, and Martin Sauter
Horst Behrens Julia Ghergut et al.
  • Applied Geoscience Dept., University of Göttingen, Germany (

For geothermal reservoirs operated by production/re-injection wells, thermal lifetime is usually defined in terms of a temperature drop threshold, and estimated as a function of fluid turnover time and heat exchange surface-area-per-volume,

Theat  =  R · Tfluid  +  D · σ2 · Tfluid2  ,

with Tfluid supposed to be measurable by means of a tracer test; 
         σ is rather difficult to infer from tracer signals alone.

For ‘aquifer’-like reservoirs, the linear term prevails:

       R > (>>) 1 ,    D · σ2 · Tfluid << 1

For fracture-dominated (‘petrothermal’) reservoirs, the quadratic term prevails:

       R ≈ 1 ,           D · σ2 · Tfluid >> 1

Deriving Tfluid from artificial-tracer signals looks 'model-independent', but is subject to large-time extrapolation uncertainty (which 'restores' model-dependence).

Unlike thermal forecasting, tracer-based prognosis of solute co-production (more precisely, of its lower-bound level, assuming conservative transport by fluid turnover only, non-'replenished' from adjacent rocks) isn't impeded by large-time extrapolation uncertainty, nor by reservoir model/parameter ambiguity, since mass output prediction as a function of time,

Mout (t)   =   (Cini – Cresid)  [ VOLout(t) – ∫otot' Q(t’) Q(t’’) g(t’’) dt’’ dt’  ]

requires just knowledge of conservative-tracer fluxes within the forecasting time horizon.

Once a tracer test was conducted in accordance with the rules of the art [usually including observance of flux-type B.C. for tracer input and fluid sampling, cf. Kreft_and_Zuber_1978], the reservoir can be treated like a 'black box' with 'response function' (Green’s kernel surrogate) g.

This approach is adequate for (conservative) solute co-production, but not for heat transport.



Tracer test results from a particular Upper-Jurassic (Malm) carbonate aquifer near Munich illustrate the issue with Theat  as a 'function' of Tfluid. Tracer signals available to date yield Tfluid in the range of months (still subject to extrapolation uncertainty), and are compatible with both fracture-dominated and ‘aquifer’-like representations of reservoir structure; ‘compatible’ σ values span four(!) magnitude orders.

By contrast, tracer signals from a fractured-porous reservoir, Eastern side of the Upper Rhine rift could be used to predict 'geothermal lithium' output (and its gradual depletion in reservoir fluid turnover), irrespective of reservoir model availability/parametrization. The non-trivial challenge, however, is to foresee and quantify overall WRI effects of ‘spent fluid’ re-injection, the latter being depleted of its particular micro-constituent (albeit at trace levels only), but likely acidized / 'unreliably' buffered at major-ion levels. WRI cannot be told from conservative-tracer signals; hydrogeochemical modeling (Kölbel_et_al._2020, Maier_et_al._2021) becomes indispensable.

We gratefully acknowledge long-term support from Germany’s Federal Ministry for Economic Affairs and Energy (BMWi) within applied research projects “LOGRO”, “TRENDS”, “UnLimiteD”, funded under grant nos.  0325111B, 0325515, 03EE4023E (,,


Kölbel L, Kölbel T, Maier U, Sauter M, Schäfer T, Wiegand B (2020) Water-rock interactions in the Bruchsal geothermal system by U-Th series radionuclides. GeoThermalEnergy, 8:24

Kreft A, Zuber A (1978) On the physical meaning of the dispersion equation and its solutions for different IBC. Chem Eng Sci, 33:1471–1480

Maier U, Tatomir A, Sauter M (2021) Hydrogeochemical modeling of mineral alterations following CO2 injection. Appl Geochem, 136:10515

How to cite: Ghergut, H. B. J., Wiegand, B., Wagner, B., and Sauter, M.: Do tracer tests enable model-independent predictions of georeservoir output? two examples from Southern Germany, involving thermal drawdown and solute co-production, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6133,, 2022.

Comments on the display material

to access the discussion