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| Titel |
Tracer-based quantification of individual frac discharge in single-well multiple-frac backflow: sensitivity study |
| VerfasserIn |
Julia Ghergut, Horst Behrens, Martin Sauter |
| Konferenz |
EGU General Assembly 2014
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 16 (2014) |
| Datensatznummer |
250093437
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| Publikation (Nr.) |
 EGU/EGU2014-8150.pdf |
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| Zusammenfassung |
| Within the deep-geothermal research project at GroßSchönebeck in the NE German Basin,
targeting volcanic rocks (Lower Rotliegend) and siliciclastics (Upper Rotliegend) in the
Lower Permian by means of a well doublet with several screening intervals between 3815 and
4247m b.s.l., several artificial fractures with different geometric and hydraulic
characteristics were created at each well, aiming to increase reservoir performance [1],
[2].
It could not be told a priori which of the various fracturing treatments was to prove as
most promising in terms of future reservoir productivity. At the intended-production well
(GS-4), one large-area waterfrac was created in the low-permeability volcanic rocks, and two
gel-proppant fractures in selected sandstone layers. Each fracturing treatment was
accompanied by the injection of a water-dissolved tracer slug, followed by a defined volume
of tracer-free (‘chaser’) fluid [3]. Each frac received a different species of a sulfonated
aromatic acid salt, as a conservative water tracer. During subsequent backflow tests (either
gas-based lifting, or production by means of a downhole submersible pump), each frac can
contribute a certain (more or less constant) amount to the measured total discharge (also
depending on whether and when each frac ‘starts’ contributing, and which effective aperture
and area it actually ‘manifests’ during the process). Since these individual-frac
discharge amounts cannot be measured directly, it was endeavoured to indirectly
determine (‘resolve’) them from tracer signals as detectable in the overall backflow
discharge.
Therefore, we need to examine how these tracer signals depend on local discharge values
and on local hydrogeologic parameters (matrix porosity, permeability distribution; frac
transmissivity, thickness, effective area and aperture), and to what extent hydrogeological
uncertainty will impede the inversion of local discharge values.
To this end, a parameter sensitivity study was conducted on a simplified flow and tracer
transport model (using FEOW and assuming Darcian flow within the matrix,
Hagen-Poiseuille flow within the waterfrac, and either D or H-P flow within the gel-proppant
fracs), whose main findings are:
(1) late tracer signals are almost independent on matrix porosity, permeability
distribution, frac area (length), thickness and effective aperture, while being highly sensitive
to local discharge values; ‘late’ means a backflow or production volume at least fivefold the
injected chaser volume;
(2a) early tracer signals (concentration ‘peak’ intervals) may exhibit slight ‘acceleration’
and ‘damping’ with increasing matrix porosity or increasing frac aperture (a ‘paradoxical’
behaviour which is not really surprising for single-well ‘push-then-backflow’ tests, actually
owing to flow-field dispersion[4]), and
(2b) a non-monotonous response to varying frac area, being almost insensitive to frac area
as long as the linear-flow regime prevails against the radial-flow regime (effects of the latter
only becoming visible at very low frac areas);
(2c) the effects of these various factors on early-time tracer signals are not unambiguously
discernible from each other, and this ambiguity would persist even if frac-resolved (in-situ)
discharge metering were feasible.
For each of the three fracs (k=1,2,3), a ‘type-curve’ set Ck(Q,t) (parametrized by
discharge values Q) can be generated. Since every frac received a different tracer,
tracer signals measured within the overall backflow will differ from individual-frac
type-curves by mere dilution (no ‘superposition’). Type-curve dilution by factor
Qk/Qtotal can be compared to measured tracer concentrations in the total discharge,
ck(ti), (i=1, -¦, no. of tracer samplings). From a formal point of view, the unknown
discharge values Qk can be determined as the solution of a linear optimization task
subject to the constraint Q1 + Q2 + Q3 = Qtotal (the latter being a measured
value). It is recommendable to perform ‘optimization’ manually, rather than by
resorting to automated solutions provided by some linear programming software. The
first items to inspect are the late-time height and slope of measured tracer signal
‘tailings’: their height yields a first approximation to dilution factors, and thus a first
estimate for Qk, while late-time consistency of observed tailing slopes can be taken as
indicative of the applicability of model presuppositions. To be noted, dilution factors
associated with individual fracs can vary with time, since a steady-state discharge
pattern might not be reached simultaneously at all fracs. The paper also discusses
some reasons why early-time tracer signals are generally unsuited for frac discharge
inversion.
References:
[1] BlöcherMG, ZimmermannG, MoeckI, BrandtW, HassanzadeganA, MagriF
(2010) 3D numerical modeling of hydrothermal processes during the lifetime of a deep
geothermal reservoir. Geofluids, 10, 406-421.
[2] ZimmermannG, BlöcherG, ReinickeA, BrandtW (2011) Rock specific hydraulic
fracturing and matrix acidizing to enhance a geothermal system – Concepts and field results.
Tectonophysics, 503, 146-154.
[3] http://pangea.stanford.edu/ERE/pdf/IGAstandard/SGW/2007/ghergut.pdf
[4] http://pangea.stanford.edu/ERE/pdf/IGAstandard/SGW/2013/Ghergut3.pdf;
~Ghergut5.pdf
Acknowledgements: Tracer tests at the GroßSchönebeck site were conducted with
financial and operational support from the German Federal Ministry for the Environment,
Nature Conservation and Nuclear Safety (BMU) and from the Helmholtz Research Centre
GeoForschungsZentrum (GFZ) Potsdam. Modelling work was conducted within the ‘gebo’
project (‘Geothermal Energy and High-Performance Drilling’, www.gebo-nds.de),
funded by the Lower-Saxonian government and by Baker Hughes (Celle), Germany. |
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