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| Titel |
Microscale simulations of NMR relaxation behaviour in the presence of fluid flow in porous media |
| VerfasserIn |
O. Mohnke, N. Klitzsch, C. Clauser |
| Konferenz |
EGU General Assembly 2009
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 11 (2009) |
| Datensatznummer |
250029962
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| Zusammenfassung |
| Structure and state of soils have considerable influence on their flow and transport properties
in particular for the vadose zone. In petrophysical applications of nuclear magnetic
resonance (NMR), the measured relaxation signals originate from the fluid filled pore
space. Hence, in (partially) saturated rocks or sediments the water content directly
corresponds to the initial amplitude of the recorded NMR relaxation signals. The
rate of relaxation (longitudinal / transversal relaxation time T1/T2) is sensitive to
the pore size and physiochemical properties of the rock-fluid interface (surface
relaxivity), as well as the concentration of paramagnetic ions in the fluid phases (bulk
relaxivity).
Joint numerical simulations of the NMR relaxation behaviour (Bloch equations) and fluid
flow (Navier-Stokes) on a pore scale dimension have been implemented in a finite element
model using Comsol Multiphysics. Solving the differential equations for general cases allows
to simulate NMR responses for arbitrary pore geometries and heterogeneous distributions of
surface properties. The simulations can cover every possible NMR relaxation regime, e.g.
diffusion or surface limited as well as intermediates, depending on the model’s properties.
The implementation of the KST approach for NMR surface properties allows to link the
(magnetization) sink terms in the differential equations with spatially distributed
concentrations of paramagnetic ions in the fluid phase and at the fluid-rock interfaces. A
strong inter-pore coupling due to connectivity of pores with different NMR properties (size,
surface properties) can significantly shift the relaxation time distribution that is
not accounted for in the determination of pore size distribution from NMR decay
times.
These simulations are verified by corresponding NMR and SIP laboratory experiments on
fully and partially saturated reference samples with accurately defined pore spaces
determined by computer tomography (see Wiens et al, MPRG7). Based on these
investigations and pore scale simulations of frequency dependent behaviour of the complex
resistivity (Spectral Induced Polarization, SIP; see also Volkmann et al, MPRG7) we aim at
an interpretation scheme combining NMR and SIP to assess structure, state and thus flow
properties of partially saturated soils. |
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