Long-term physical and structural changes in a fractured-porous system subject to THM
stress (typically, a deep geothermal reservoir under exploitation) essentially
correlate with the specific area (s) of the contact surface between fractures and
rock matrix. This parameter is not properly captured by standard hydraulic or
geophysical tests, nor by flow-path tracings in highly-dispersive flow fields. A
single-well tracing method that increases the sensitivity of tracer signals (BTCs) w.
r. to the surface-area parameter s is described (the dual-tracer push-pull method),
alongside with its application for three crystalline formations in Germany. Fluid
salinity and heat as an artificial tracer could further be used to complement the
information returned by injected solutes. A simplified way of estimating s from
tracer tests is proposed in terms of mid-late BTC approximations.
Determining s from tracer BTCs presupposes reliable knowledge of tracer
physicochemical behaviour under reservoir conditions. A tentative interpretation of
BTC differences between different tracers, beyond the amount accountable for by their
different diffusion coefficients, can first rely on structure-activity considerations
(Behrens 1986), before tracer physico-chemical properties are quantified by
appropriate laboratory experiments. A field push-pull test can substitute the
required laboratory investigations if at least one assuredly reference tracer is
injected alongside with the tracers whose physico-chemical behaviour is less secured.
- An analogous approach had been used by Snodgrass and Kitanidis (1998), Istok et al.
(2001) to get in situ estimates of natural attenuation rates.
In the practice of deep reservoir tracing, there are physical and financial
limitations to test design and duration. Insufficient flushing volumes may render BTC
peak regions unusable for fracture characterization, and insufficient outflow volumes
can make characteristic mid-late BTC slopes difficult to recognize.
A further important measure of reservoir quality is provided by flow-capacity or
matrix freight repartitions, which can equally be derived from tracer tests, with the
advantage of being invariant to flow rates (as long as the hydraulic regime doesnt
change qualitatively) and insensitive to tracer BTC normalization uncertainties
(incurring from poorly-known total injected mass or from poorly-defined analytical
calibration standards). A parametric plot of 0th- and 1st-order truncated time
moments of tracer BTCs can yield a flow-capacity repartition, if derived from a
flow-path tracing, or one by matrix freight, if derived from a push-pull test. In
order to calculate time moments, measured tracer BTCs need to be extrapolated for
large times according to some transport model, however the result is relatively
insensitive to the particular choice of matrix blocks shape and geometry.
Flow-capacity diagrams derived from standard flow-path tracings had been interpreted
by Shook (2003) as a geometric characterization of the reservoir. Flow-capacity and
matrix freight repartitions obtained from several tracings conducted in Germany in
deep sedimentary and crystalline formations before and/or after massive hydraulic
stimulation are compared and explained.
Expert tracer knowledge provided by H. Behrens (Germany) was essential to the design
and the interpretation of all tests.
Financial support from the German Research Foundation (DFG, project Sa-501/16/1-4),
from the Urach-Spa Pilot Geothermal Plant, the Leibniz Geoscience Institute (GGA)
Hannover and the GFZ Potsdam is gratefully acknowledged.