U. M. Scheven

Quantitative nuclear magnetic resonance measurements of preasymptotic dispersion in flow through porous media

We use pulsed field gradient nuclear magnetic resonance to probe molecular displacements in preasymptotic Stokes flow through a pack of beads with bead diameter d=100±20μm, through a Bentheimer sandstone, and a Portland carbonate rock core, for a common range of flow velocities v and interrogation times Δ. For flow through the bead pack the length scale of the pore is well defined, as are the Peclet number Pe∊[20–80] and the Reynolds number Re<0.1. Probability distributions of molecular displacements P(ζ) are determined. The mean displacement ⟨ζ⟩, the variance σ2≡⟨(ζ−⟨ζ⟩)2⟩ and the skewness γ3≡⟨(ζ−⟨ζ⟩)3⟩ of P(ζ) are determined by a self-consistent cumulant analysis designed to minimize the systematic errors to which any cumulant analysis of non-Gaussian distributions is susceptible. Systematic errors in σ and γ arising from surface relaxation effects and flow displacements through the internal fields of rocks are quantified.

Small Angle Neutron Scattering (SANS and V-SANS) Study of Asphaltene Aggregates in Crude Oil

We report small angle neutron scattering (SANS) experiments on two crude oils. Analysis of the high-Q SANS region has probed the asphaltene aggregates in the nanometer length scale. We find that the radius of gyration decreases with increasing temperature. We show that SANS measurements on crude oils give similar aggregate sizes to those found from SANS measurements of asphaltenes redispersed in deuterated toluene. The combined use of SANS and V-SANS on crude oil samples has allowed the determination of the radius of gyration of large scale asphaltene aggregates of approximately 0.45 microm. This has been achieved by the fitting of Beaucage functions over two size regimes. Analysis of the fitted Beaucage functions at very low-Q has shown that the large scale aggregates are not simply made by aggregation of all the smaller nanoaggregates. Instead, they are two different aggregates coexisting.

Quantifying transport within a porous medium over a hierarchy of length scales

Magnetic resonance techniques are used to probe transport within a porous medium over length scales of microns to centimeters. In particular, the apparent discrepancy between estimates of dispersion within porous media determined by pulsed field gradient magnetic resonance techniques and a conventional elution analysis is addressed. The model porous medium considered is a packed bed of height and internal diameter 22.5 and 16.8mm, respectively, packed with highly porous cross-linked dextran particles approximately 50μm in diameter. Experiments were performed for Peclet numbers in the range 1<Pe<31. First, a nonspatially resolved displacement encoding Alternating Pulsed Field Gradient Stimulated Echo Nuclear Magnetic Resonance (APGSTE NMR) measurement was used to yield estimates of bed porosity (0.898±0.004), mobile phase volume fraction (0.29±0.02), intraparticle diffusion coefficient [(2.8±0.2)×10−10m2s−1], and characteristic time, Te, for exchange between the intra- and interparticle pore space (∼300ms)...

Quasi‐asymptotic Dispersion

The experimental characterization of voidspaces in porous media generally includes measurements of volume averaged scalar properties such as porosity, dispersivity, or the hydrodynamic radius rh = V/S, where V and S are the volume and surface area of the pore space respectively. Displacement encoding NMR experiments have made significant contributions to this characterization. It is clear, however, that NMR derived dispersivities in packed beds—the one random porous system for which there exist canonical but incompatible theoretical predictions with few or no adjustable parameters—can be affected by the same experimental complications which have substantially contributed to the puzzling scatter in published dispersion results based on elution experiments. Notable among these are macroscopic flow heterogeneities near walls, and inhomogeneous flow injection. Using the first three cumulants we delineate a transition from a pre‐asymptotic to a quasi‐asymptotic dispersion regime and determine the true dispersi...