 |
| Titel |
Short-term fluid, heat, and solute transport in deep ‘georeservoirs' likely to become ‘EGS': some challenges to ICDP hydrogeologists who might like using artificial tracers |
| VerfasserIn |
Julia Ghergut, Horst Behrens, Ernst Huenges, Peter Rose, Martin Sauter |
| Konferenz |
EGU General Assembly 2014
|
| Medientyp |
Artikel
|
| Sprache |
Englisch
|
| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 16 (2014) |
| Datensatznummer |
250097160
|
| Publikation (Nr.) |
 EGU/EGU2014-12710.pdf |
|
|
|
|
|
| Zusammenfassung |
| During Fall 2013, the Integrated Continental Scientific Drilling Programme (ICDP) set out to
define a new Science Plan that shall replace its past-decade version (Harms et al., eds., 2005)
for the decade to come. Geoscientists worldwide were welcomed to suggest new imaging and
exploration methods, new sites to drill, new challenges to be addressed with a view at new
‘societal needs’ (Harms and Wiersberg 2013).
Save for two outstanding exceptions at the Mutnovsky volcano in Russia and the KTB
site in Germany, the use of artificial tracers, especially within forced-gradient tests, has not
been on the agenda of most ICDP projects so far (other than for purposes of monitoring
microbial contamination in conjunction with drilling activities); deep-reservoir exploration
and characterization efforts were restrained to non–fluid-invasive techniques on the one hand,
and to sites featuring some unique earth-historical traits, on the other hand. Surely, this was
not for lack of interest in quantifying fluid transport in the deep subsurface in general, but
mainly due to operational, technical, and financial constraints (lack of resources / lack of
opportunity for significant fluid turnover within the target, deep-seated georeservoirs, and fear
of persistent, large-scale georeservoir contamination by non-pristine fluids). – This is likely to
change during the forthcoming decade(s), owing to worldwide increased interest in some
‘georesource’ or ‘georeservoir’ play types (Moeck 2013) that have not been in
the ICDP focus so far, including non-volcanogenic geothermal, and allowing for
man-made design and intervention into how those ‘georesources’ or ‘georeservoirs’ shall
work for us. Among the latter, petrothermal systems (Jung 2013, Huenges and
Jung 2004) acquire growing recognition as a promising (and maybe unique) option
for baseload energy supply in vast areas of the Northern hemisphere, at very low
emissions and (in the long run) moderate costs. With petrothermal coming into play,
forced-gradient turnover of significant fluid amounts shall be on the agenda, and with it also
a more comprehensive use of artificial tracers, for far more ambitious tasks than
just the monitoring of drilling-related contamination. Since any ICDP project is
likely to start (and mostly also end) with not more than one deep well, single-well
forced-gradient test experience gained within the non-ICDP areas of hydrocarbon and
geothermal exploration and engineering seem predestined for a specific knowledge
transfer.
Single-well ‘push-then-pull’ (SW) tracer methods appear as attractive for a number of
reasons: less uncertainty of design and dimensioning, and lower tracer quantities required,
than for inter-well (IW) tests; stronger tracer signals, enabling easier and cheaper metering,
and shorter metering duration required, reaching higher tracer mass recovery, than in IW
tests; last not least: no need for a second well! However, SW tracer signal inversion faces a
major issue: the ‘push-then-pull’ design weakens the correlation between tracer residence
time distribution (RTD) and fluid transport parameters, inducing insensitivity or ambiguity of
tracer signal inversion against some of those georeservoir parameters that are supposed to be
the target of tracer tests par excellence: pore velocity, transport-effective porosity,
fracture aperture and spacing or density (where applicable), fluid/solid or fluid/fluid
phase interface density. Hydraulic and geophysical methods cannot measure the
transport-effective values of such parameters, because the signals they detect correlate neither
with fluid motion, nor with material fluxes through (fluid-rock, or fluid-fluid) phase
interfaces. Typically, pressure signals obtained in hydraulic tests do not enable to
distinguish between geological formations with equal permeability, but different
transport-effective porosity, nor between formations with equal permeability, equal
transport-effective porosity, but different fluid-rock interface area. The ability to
measure this latter parameter is crucial to lifetime predictability for geothermal, gas
storage, waste disposal, or hydrocarbon reservoirs (especially in advanced depletion
stages).
From IW tracer signals, fluid RTD can be derived, whose statistical moments relate to
major hydrogeological properties of target georeservoirs. By contrast, SW tests can be used to
quantify processes other than advection-dispersion: typically, the exchange of some extensive
quantity (mass, energy) between fluid and solid/fluid phases by matrix diffusion or
sorption/partitioning, whose rate or amount depends on the phase saturation and/or interface
area density. Flow-field reversal during the ‘pull’ stage of a SW test is supposed to largely
compensate the effects of flow path heterogeneity, and to enhance the effects of tracer
exchange processes at phase interfaces. However, it always destroys the equivalence between
fluid RT and reservoir size; transport-effective porosities, closely relating to fluid RT, can
only be measured reliably by means of conservative-tracer IW tests. The SW inability to
determine porosity also (indirectly) impedes the SW-based ability to determine those
complementary, non-advective parameters: increased sensitivity comes along with increased
ambiguity.
Reactive tracers can aid overcoming some of the limitations to parameter determinability
associated with the SW design. Specific use of reactive tracers has first been made by Tomich
et al. (1973), for a SW-based measurement of residual-oil saturation in (depleted) oil
reservoirs; later on by Robinson (1985), for tracking temperature fronts in laboratory-scale
geothermal-reservoir IW-test ‘analogues’; more recently by Schaffer et al. (2013),
for tracking brine-(sc)CO2 interfaces during CO2 injection within CCS research
projects.
The paper discusses some lessons derived from a twelve-year experience using artificial
tracers in various IW and SW field tests in Germany, aimed at deep-georeservoir
characterization and/or short- to mid-term process monitoring during reservoir
operation:
– if those tests have been successful to a certain extent, it was primarily owing to
ascertainedly conservative tracer transport behavior;
– if those tests have been of limited success, it was because of lack of reactive
tracer species with well-defined, and reasonable properties (reasonable means:
sensitive to ‘something’, but not to ‘everything’ that may ‘happen’ within the target
georeservoir).
If the artificial-tracer–based quantification of deep-georeservoir hydrogeology and of
induced (short- to mid-term) transport processes therein is to become a task for some future
ICDP projects, they will need to effectively address this dilemma. Further, if EGS, and
especially the petrothermal type shall be on the agenda, then SW tests will be ‘unavoidable’.
Finally, if the most is to be made out of a SW test, then tailored reactive tracer pairs (Tomich
et al. 1973, Ghergut et al. 2013) are a must: not just reactive, not just retarded, but:
conservative alongside with reactive, and with contrasting retardation behavior between
product and reactant.
Selected references:
HarmsU, KoeberlC, ZobackM, eds (2005) Continental Scientific Drilling: A Decade
of Progress, and Challenges for the Future. Springer, 366pp.
HarmsU, WiersbergT (2013) Conference on ICDP’s New Science Plan. Scientific
Drilling, 15: 77.
HuengesE, JungR (2004) Technologies for the Utilisation of Enhanced Geothermal
Systems (www.bgr.de/
veransta/renewables_2004/presentations_DGP/Block1Introduction_pdf/2_Huenges_Jung.pdf)
JungR (2013) EGS – Goodbye or Back to the Future. Chapter 5, dx.doi.org/10.5772/56458
(www.intechopen.com/
books/effective-and-sustainable-hydraulic-fracturing)
MoeckI (2013) Classification of geothermal plays according to geological habitats. IGA Academy Report
0101-2013 (www.geothermal-energy.org/iga_service_gmbh/projects/ifc_project/workshop_izmir.html)
RobinsonBA (1985) Non-reactive and chemically reactive tracers: Theory and
applications. PhD Thesis, MIT, Cambridge, MA, 551pp.
RoseP, MellaM, KastelerC, JohnsonS (2004) The estimation of reservoir pore volume
from tracer data. Procs 29th Workshop on Geothermal Reservoir Engineering, Stanford
University, SGP-TR-175 (pangea.stanford.edu/ERE/pdf/IGAstandard/SGW/2004/Rose.pdf)
SchafferM, MaierF, LichaT, SauterM (2013) A new generation of tracers for the
characterization of interfacial areas during supercritical carbon dioxide injections into deep
saline aquifers: kinetic interface-sensitive (KIS) tracers. Intl J Greenhouse Gas Control, 14:
200-208.
TomichJF, DaltonRLJr, DeansHA, ShallenbergerLK (1973) Single-Well
Tracer Method to Measure Residual Oil Saturation. J Petrol Technol Transact, 255:
211-218.
GhergutI, BehrensH, SauterM (2013) Can Peclet numbers depend on tracer species?
going beyond SW test insensitivity to advection or equilibrium exchange. Procs 38th
Workshop on Geothermal Reservoir Engineering, Stanford University, SGP-TR-198
(pangea.stanford.edu/ERE/pdf/IGAstandard/SGW/2013/Ghergut5.pdf)
Acknowledgements: The Göttingen group gratefully acknowledges financial support
(to developing tracer SW methods for petrothermal-reservoir testing) from Baker
Hughes (Celle) and the Lower-Saxonian Government within the interdisciplinary
research project ‘gebo’ (‘Geothermal Energy and High-Performance Drilling’). |
|
|
|
|
|
|