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

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)

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

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

Push-pull tracer experimental concept

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

Determination of diffusion coefficients

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

Push-pull tracer experimental concept

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.

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

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.

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

Push-pull test in hydrothermally-altered granite formation (Southern Black Forest) 100-300 m2/m3 (~ free outflow phase)

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

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

(details of KTB production phase)

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

some experiences in crystalline reservoirs, a review (1) 0.0001 - 0.001 0.0001 - 0.001

some experiences in crystalline reservoirs, a review (2)

some experiences in crystalline reservoirs, a review (3)

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

Single-well flow-path tracing in sedimentary reservoir

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

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

Heat transport experiments

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

Temperature push-pull tracertest KTB-pilot hole

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

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

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

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

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

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)

(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