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| Titel |
Single-well and inter-well tracer test design for characterizing the Heletz site (Israel) with a view at CCS |
| VerfasserIn |
Julia Ghergut, Jac Bensabat, Horst Behrens, Tobias Licha, Friedrich Maier, Mario Schaffer, Martin Sauter |
| Konferenz |
EGU General Assembly 2011
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 13 (2011) |
| Datensatznummer |
250054115
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| Zusammenfassung |
| The Heletz site in Israel was chosen for conducting a CO2 transport experiment within the
MUSTANG project, whose aim is to demonstrate and validate leading-edge techniques for
CCS site characterization, process monitoring and risk assessment. Currently available
knowledge on physical, hydrogeological and hydrogeochemical features of the Heletz site
can be found at http://www.co2mustang.eu/Heletz.aspx
The Heletz major experiment will be preceded and accompanied by a sequence of
single-well and inter-well tracer tests, aimed at characterizing
the transport properties of the storage formation (cf. Behrens et al., 2010), and
CO2 – brine – rock interface processes (cf. Licha et al., 2009).
Generally, inter-well tracer tests are used to determine fluid residence time distributions
(RTD); ’statistical’ moments of RTDs provide important information about the
reservoir:
the zeroth-order RTD moment can tell something about reservoir boundaries;
the first-order RTD moment, or mean residence time (MRT) represents a measure
of reservoir size (the reservoir volume that can be used for fluid storage, under a
certain injection regime)
higher-order RTD moments provide information about reservoir heterogeneity;
traditionally, the second-order moment is associated with flow-path dispersion
(from hydrodynamic up to reservoir scale); from RTD analysis also a
flow-storage repartition (FSR) can be derived, which is sometimes interpreted as
representing reservoir shape (Shook, 2003), with certain limitations when matrix
diffusion or kinetic exchange processes become important (Behrens et al., 2010).
Complementarily, single-well tracer push-pull tests are used to quantify processes other
than advection-dispersion: typically, the exchange of some extensive quantity (mass, energy)
between mobile and immobile fluid regions by processes like sorption and/or matrix
diffusion, whose magnitude depends on the density (area per volume) of involved fluid/rock
interfaces. Flow-field reversal during the ’pull’ phase is supposed to largely compensate the
effects of flow-path heterogeneity (excepting the hydrodynamic level), and to enhance the
effects of tracer exchange processes at fluid-rock interfaces, thus enabling to quantify
interface densities from measured tracer return signals; but this depends on whether
the fluid volumes and flow/shut-in durations used in the push-pull test match the
system’s homogeneity scale, the intrinsic diffusivity of rock matrix and its interface
density. The sensitivity of tracer signals w. r. to interface densities depends upon
the type of process that dominates at the space-time scale of the test, which can
be:
fast-equilibrium sorption,
kinetic exchange (sorption-desorption), or matrix diffusion with high diffusivity
(typical for heat exchange in fractured hard rock aquifers),
matrix diffusion with low diffusivity (typical for most solutes in most rocks, and
for heat exchange in unconsolidated, highly-porous aquifers).
At the Heletz site, four tracer tests will be conducted:
prior to CO2 injection: dual-tracer single-well push-pull test (monopole
divergent followed by convergent flow field), using several tracers with
contrasting sorption and diffusion properties, aimed at characterizing fluid-rock
interfaces and estimating fluid-rock interface densities (also serving as an aid
in tracer species selection, dimensioning and instrumentation for all subsequent
tests);
prior to CO2 injection: brine-phase dual-tracer inter-well circulation test
(forced-gradient, divergent-convergent dipole flow field), using two tracers
with contrasting sorption or diffusion properties, aimed at estimating storage
reservoir size, determining brine RTD and FSR, characterizing reservoir-scale
heterogeneity
prior to main CO2 transport experiment, but including small-sized CO2 slugs:
dual-tracer, single-well multiple-push of alternating brine/CO2 slugs, followed
by prolonged push stage, using single-phase tracers as well as phase-partitioning
tracers, aimed at dynamic characterization of CO2-brine-rock interfaces (Licha
et al., 2009)
during the main CO2 transport experiment: dual-tracer,
inter-well injection-extraction test (forced-gradient, divergent-convergent dipole
flow field), using single-phase tracers as well as phase-partitioning tracers, aimed
at quantifying the storage capacity, characterizing brine displacement processes,
and determining RTD and FSR under two-phase flow conditions.
The poster will explain the design and dimensioning of the first, second and fourth tracer
test.
Unlike the brine-phase spiking conducted at the Ketzin site in Germany (www.co2sink.org),
where only passive sampling was possible (yielding so-called ’resident’ values of tracer
concentration, inconsistent with the reservoir-scale transport equations), the Heletz
experiment offers the advantage of fluid extraction at well-defined rates, rendering measured
values of tracer concentrations (actually, tracer fluxes) consistent with the transport equations
from which parameter inversion is endeavoured. Forced-gradient extraction of fluid is not
meant to be representative of how a CCS site would be operated in reality, but it ensures the
meaningfulness of measured experiment quantities. The MUSTANG experiment would
not be worthwhile being conducted just to see ’when’ CO2 will arrive in a certain
distance; its aim is not to mimic CCS, but to quantify transport processes, which is
not possible without well-defined fluid and solute fluxes. Moreover, we want the
fluxes measured at one borehole to reflect reservoir-scale fluid motion, and not just
borehole-scale flow gradients induced by the particular device used to collect fluid
samples.
References:
Licha T, Schaffer M, Sauter M (2009) ’Smart tracers’ for dynamic
characterization of CO2-brine-rock interfaces. MUSTANG Report WP4,
http://www.co2mustang.eu/MustangDeliverables.aspx
Shook GÂ M (2003) A Simple, Fast Method of Estimating Fractured Reservoir
Geometry from Tracer Tests. Geothermal Resources Council Transactions, 27,
407-411
Behrens H, Ghergut I, Sauter M (2010) Tracer properties, and tracer test results (3):
modification to Shook’s flow-storage diagram. Stanford Geotherm Prog Tech Rep,
SGP-TR-188
Acknowledgements:
The Göttingen authors are grateful to Jac Bensabat who is the ’secret architect’ behind the
Heletz experiment planning complexity, in a way which is not sufficiently reflected by merely
naming him in the list of co-authors.
Field and laboratory work for implementing the tracer methods at the Heletz site are
funded by the EU Seventh Framework Programme FP7Â /Â 2007-2013, within the MUSTANG
project (grant agreement no. 227286). |
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