 |
| Titel |
Single-well tracer test sensitivity w. r. to hydrofrac and matrix parameters (case study for the Horstberg site in the N-German Sedimentary Basin) |
| VerfasserIn |
I. Ghergut, H. Behrens, E. Holzbecher, R. Jung, M. Sauter, T. Tischner |
| Konferenz |
EGU General Assembly 2012
|
| Medientyp |
Artikel
|
| Sprache |
Englisch
|
| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 14 (2012) |
| Datensatznummer |
250064032
|
|
|
|
|
|
| Zusammenfassung |
| At the geothermal pilot site Horstberg in the N-German Sedimentary Basin, a complex field
experiment program was conducted (2003–2007) by the Federal Institute for Geosciences and
Natural Resources (BGR) together with the Leibniz Institute for Applied Geosciences
(GGA), aimed at evaluating the performance of innovative technologies for heat extraction,
for direct use, from a single geothermal well[1],[2]. The envisaged single-well operation
schemes comprised inter-layer circulation through a large-area hydrofrac (whose successful
creation could thus be demonstrated), and single-screen ‘huff-puff’ in suitable (stimulated)
layers, seated in sandstone-claystone formations in 3–4Â km depth, with temperatures
exceeding 160Â Ë C.
Relying on Horstberg tracer-test data, we analyze heat and solute tracer transport in three
characteristic hydraulic settings:
(A) single-screen, multi-layer push-pull, with spiking and sampling at lower
well-screen in low-permeability sandstone layer (‘Detfurth’), from which
hydrofrac propagation (through several adjacent layers) was initiated;
(B) single-screen, single-layer push-pull, with spiking and sampling at upper
well-screen within a more permeable sandstone layer (‘Solling’);
(C) inter-layer vertical push through above-mentioned hydrofrac, with spiking at
well-screen of A, and sampling at well-screen of B.
Owing to drill-hole deviation, the hydraulically-induced frac will, in its vertical propagation,
reach the upper sandstone layer in a certain horizontal distance X from the upper well-screen,
whose value turns out to be the major controlling parameter for the system’s thermal lifetime
under operation scheme C (values of X below ~8Â m leading to premature thermal
breakthrough, with the minimum-target rate of fluid turnover; however, the injection pressure
required for maintaining the target outflow rate will also increase with X, which
renders scheme C uneconomical, or technically-infeasible, when X exceeds ~15Â m).
Tracer signals in C are, as well, sensitive w. r. to X, but the effects of increasing X,
upon tracer signals, are largely indistinguishable from those of increasing Solling
porosity.
Further numerical simulations of heat and solute tracer transport in above-named test
settings reveal significant disparities between parameter sensitivities attainable in the same
kind of test (A, B) conducted at different layers, as well as between solute concentration and
temperature signal sensitivities w. r. to transport parameters in one and the same test (C).
Why? – Test A features fracture flow, and dual-continuum transport, whereas test B features
single-continuum flow and transport (within the host rock, with negligible losses to the
hydrofrac). Flow is rapid in test A (being fracture-dominated), but slow in test B
(being confined to the host rock). In test C, fluid first flows through the hydrofrac
mainly, next it ‘must’ flow through the upper sandstone; heat transport is dominated
by matrix diffusion across the hydrofrac (along which it thus experiences strong
retardation), whereas solute transport is dominated by matrix micro-fissure and
intra-particle diffusion within the upper sandstone (where it experiences strong
retardation).
We examine the implications of these findings upon the inversion of transport-effective
hydrofrac parameters from measured tracer signals, and upon the tracer-based predictability
of the system’s thermal lifetime under different operation schemes.
[1]http://www.geothermal-energy.org/pdf/IGAstandard/SGW/2005/jung.pdf
 Â
[2]http://www.geothermal-energy.org/pdf/IGAstandard/WGC/2010/2272.pdf  Â
 Â
 Â
Acknowledgement: This study is funded by MWK Niedersachsen (Lower-Saxony’s
Science and Culture Ministry) and by Baker Hughes (Celle) within task units ‘G6’ and ‘G7’
of the Collaborative Research Project ‘gebo’ (Geothermal Energy and High-Performance
Drilling). |
|
|
|
|
|
|