Hydrocarbon reservoirs with a large column height as well as tight gas rocks require a large range of capillary pressures to describe the saturation of fluids present in these formations. While mercury injection capillary pressure (MICP) can achieve high equivalent capillary pressures, the tests are destructive to the core plugs. Centrifuge techniques have gained in popularity since they are faster than the porous plate technique, but they are limited in the achievable pressure range. Here, we propose the use of fluorinated oils to extend the achievable capillary pressure of the air-brine centrifuge technique by a factor of two. We use Fluorinert FC-70 in an inverted bucket configuration which doubles the radius of rotation and keeps the density contrast comparable to an air-brine system. Furthermore, we show the application to NMR T2 cut-off determination as a function of capillary pressure. Since Fluorinert does not contain any hydrogen, there is no signal overlapping with the brine in the core plugs. Furthermore, in the inverted bucket configuration, the outlet face of the plug is not in contact with a drainage surface so that the Hassler-Brunner boundary condition of Pc = 0 is satisfied. Additionally, the method allows the storage under a liquid Fluorinert phase, which prevents evaporation and significantly extends the available time for NMR measurements at low water saturations.

In this study, digital rock analysis was combined with a variety of experimental core-analysis measurements to investigate the effect of salt saturation and distribution on the porosity and permeability of halite-cemented core samples. Medical and micro-X-ray CT scans of core sections and 2.54-cm (1-in.) diameter plugs indicated that the halite generally occurred in the form of distinct layers. High-resolution micro-X-ray computed tomography (MXCT) images acquired of 0.6-cm diameter plugs revealed that, on the pore scale, halite appeared to be pore filling. Pores were either completely filled with halite or did not contain any halite at all. It was also observed that halite preferentially occurred in the larger pores associated with larger grain sizes. The porosity and permeability results, both measured experimentally on the core plugs and calculated by segmentation of the MXCT images, demonstrated the obstructive effect of halite on storage and flow as well as the decline of both properties with increasing salt saturation. Comparison of calculated and measured values showed that the measured porosity could be up to 6 porosity units (p.u.) higher than the calculated one, while the measured permeability of core plugs after salt removal was an order of magnitude lower than the one obtained by lattice Boltzmann simulation. One possible reason for this discrepancy may be the stratified nature of the samples. While the fully salt-saturated plugs appeared homogeneous in MXCT images, post-flood MRI images revealed that the plug was composed of layers with different MRI intensities, i.e., different amounts of water-filled porosity. Consequently, the petrophysical parameters calculated for the miniplugs may only be representative for a section of the core plug. The results of the MRI-assisted core floods emphasized the importance of considering different scales when interpreting and applying the results of digital rocks analysis.

Over the last two decades, 129Xe-gas NMR has become a well-known method to probe porous materials, as the chemical shift of the gas is highly sensitive towards its surrounding. Thus, this non-destructive technique is suitable to characterize the pore structure and the interconnectivity as well as the dynamical behavior of a fluid inside the pore space. This chapter provides an overview on the application of 129Xe-MR for probing exchange and dynamics in porous media. Therefore, the NMR-related features and hardware requirements are discussed at first before common experimental techniques and their application examples are introduced.

To date, solid-state photo-CIDNP experiments have been performed only using magic angle spinning NMR in a high-field regime, which is not associated with physiologically relevant spin dynamics. Here, we predict that nuclear spin polarization up to 10%, almost 9 orders of magnitude larger than thermal equilibrium polarization, can arise in cyclic photoreactions at the earth field due to a coherent three-spin mixing mechanism in the S-T(-) or S-T(+) manifold. The effect is maximal at a distance of about 30 Å between the two radicals, which nearly coincides with the separation between the donor and secondary acceptor in natural photosynthetic reaction centers. Analytical expressions are given for a simple limiting case. Numerical computations for photosynthetic reaction centers show that many nuclei in the chromophores and their vicinity are likely to become polarized. The theory predicts that only modest hyperfine couplings of a few hundred kilohertz are required to generate polarization of more than 1% for radical-radical distances between 20 and 50 Å, that is, for a large number of radical pairs in electron-transfer proteins.

We have completed an extensive study of 129 Xe longitudinal spin relaxation in the gas phase, involving both intrinsic and extrinsic mechanisms. The dominant intrinsic relaxation is mediated by the formation of persistent Xe2 van der Waals dimers. The dependence of this relaxation on applied magnetic field yields the relative contributions of the spin-rotation and chemical-shift-anisotropy interactions; the former dominates at magnetic fields below a few tesla. This relaxation also shows an inverse quadratic dependence on temperature T; the maximum low-field intrinsic relaxation for pure xenon at room temperature measured here to be 4.6 h, in agreement with previous work increases by 60% for T=100 °C. The dominant extrinsic relaxation is mediated by collisions with the walls of the glass container. Wall relaxation was studied in silicone-coated alkali-metal-free cells, which showed long many hours or more and robust relaxation times, even at the low magnetic fields typical for spin-exchange optical pumping 3m T. The further suppression of wall relaxation for magnetic fields above a few tesla is consistent with the interaction of 129 Xe with paramagnetic spins on or inside the surface coating. At 14.1 T and sufficiently low xenon density, we measured a relaxation time T1 =99 h, with an inferred wall-relaxation time of 174 h. A prototype large storage cell 12 cm diameter was constructed to take advantage of the apparent increase in wall-relaxation time for cells with a smaller surfaceto-volume ratio. The measured relaxation time in this cell at 3 mT and 100 °C was 5.75 h. Such a cell or one even larger could be used to store many liters of hyperpolarized 129 Xe produced by a flow-through polarizer and accumulator for up to three times longer than currently implemented schemes involving freezing xenon in liquid nitrogen.

B. Saam, R. E. Jacob, B. C. Anger, K. R. Minard Physics, University of Utah, Salt Lake City, UT, United States, Pacific Northwest National Laboratory, Richland, WA, United States Introduction Spin-exchange optical pumping (SEOP) [1] is widely employed to generate samples of hyperpolarized He for magnetic resonance studies, most notably MRI studies of the lung [2]. One drawback to SEOP is the time to polarize the gas; existing polarizers typically require 10 h or more to achieve 40-50% polarization in quantities of gas ~1 bar·L. Two recent advances in the physics of SEOP have led to dramatic enhancements in polarization efficiency: the use of spectrally narrowed diode-laser arrays [3] and “hybrid” SEOP, which employs both potassium and rubidium as alkali-metal intermediaries [4]. We have combined these techniques in constructing two polarizers, a prototype system at Utah and a more fully engineered system at PNNL. We report up to 50% He polarization in 0.5 bar·L of gas in valved and refillable glass cells, achieved in 4 h.

We have measured longitudinal nuclear relaxation rates of Xe in Xe-N2 mixtures at densities below 0.5 amagats in a magnetic field of 8.0 T. We find that intrinsic spin relaxation in this regime is principally due to fluctuations in the intramolecular spin-rotation SR and chemical-shift-anisotropy CSA interactions, mediated by the formation of Xe-Xe persistent dimers. Our results are consistent with previous work done in one case at much lower applied fields where the CSA interaction is negligible and in another case at much higher gas densities where transient xenon dimers mediate the interactions. We have verified that a large applied field suppresses the persistent-dimer mechanism, consistent with standard relaxation theory, allowing us to measure room-temperature gas-phase relaxation times T1 for Xe greater than 25 h at 8.0 T. These data also yield a maximum possible low-field T1 for pure xenon gas at room temperature of 5.45±0.2 h. The coupling strengths for the SR and CSA interactions that we extract are in fair agreement with estimates based both on previous experimental work and on ab initio calculations. Our results have potential implications for the production and storage of large quantities of hyperpolarized Xe for use in various applications.