Heat and solute tracer tests accompanying fluid injection experiments can provide
additional information on transport properties like fluid velocities and fluid-rock
contact surface areas, which are not reflected by pressure signals. This is
illustrated for long-term (1a) and short-term (1b) moderate-rate hydraulic
stimulations in crystalline formations, as well as for a short-term, high-rate
hydrofracturing (2a) and for the design of a sequence of short-, mid- and long-term
hydraulic tests following hydrofracturing (2b) in sedimentary formations: 

(1a) at the pilot KTB hole (German 'Kontinentale Tiefbohrung' in Bavaria), about
85,000 m3 of cold water (of which the first 22,000 m3 to compensate abstractions by
earlier pumping test) were injected at rates up to 3.3 L/s over 1 year into a
highly-permeable fracture system in 4 km deep crystalline, with the aim of basic
research into injection-induced, coupled THM processes in large-scale fault systems
of suspected extreme heterogeneity. Heat and solute push-pull tests repeated at
similar volume scales before/after injection helped quantify the THM-induced change
of fluid-rock contact-surface per-volume areas. Several post-injection heat and
tracer push-pull tests conducted at different volume scales provide information on
fracture heterogeneity. Late tracer recoveries lower than expected suggest a
large-scale drift away from the pilot hole;
(1b) at the Urach pilot HDR site, a tracer push-pull test conducted at the end of a
short-term stimulation revealed the gradual change in total fracture appertures and
specific surface areas;

(2a) at the pilot geothermal borehole Horstberg in the N-German sedimentary basin,
over 20,000 m3 of cold water were injected at rates up to 15 L/s in 3.8 km depth with
the aim of connecting 2 sandstone horizons by a hydrofrac induced in the separating
clayey sandstone formation, supposed to provide the basis for a innovative,
economical single-well technique of geothermal energy extraction. A multi-tracer slug
was added before the last 1800 m3 injected into the lower horizon, and the tracer
breakthrough curves recorded with the 3600 m3 fluid produced from the upper horizon
(divergent flow field) enabled estimating the transport properties of the established
flow path: fluid residence time (reservoir size), approximate fluid-rock
contact-surface area. After 1.5 year shut-in, tracer BTCs recorded during short-term
production phases from both the upper (flow-path) and the lower (push-pull) horizon
in the still highly-pressurized system provided further information on fluid-rock
contact-surface areas in both horizons and transport property changes;
(2b) for the series of hydraulic tests planned by the GFZ Potsdam in 4 km deep
sandstone and vulcanite formations at GrossSchoenebeck as of 2007, it is shown how
spiking the injected fluid by different tracers at 5 different stages of the test
sequence can aid in characterizing the heterogeneous reservoir at different scales.

Tritiated water and 1,5-naphthalene disulfonate, used as tracers in (1a), (1b) and
(2a), showed similar transport properties, with differences attributable to matrix
diffusion only.

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Germany