45861-G9
Studying Flow Dispersion through Porous Media by Novel Molecular Contrast NMR Imaging
The objective of the proposed research was the development and application of a new magnetic resonance imaging (MRI) methodology to directly visualize macroscopic flow dispersion through porous media. Our method development utilizes the principle of Dynamic Nuclear Polarization (DNP) to hyperpolarize, i.e. greatly enhance, the MRI signal of the molecule of our choice. The conventional MRI approach to visualize flow is to use (1) paramagnetic tracer molecules or (2) pulsed field gradient based methods to characterize steady state flow dynamics. Our method focuses on capturing flow dynamics as they develop with time and over space, similar to tracer methods, except that no foreign tracer molecules are added to the flowing matter to visualize flow, but the signal of the flowing molecules themselves are utilized. Authentic flow contrast is achieved through high MRI signal amplification of the inflowing molecules through DNP hyperpolarization (high magnetic spin alignment), which physical principle relies on the effective transfer of higher (up to 3 orders of magnitude) polarized electron spins of stable radicals to nuclear spins that provide the MRI signal. DNP relies on the ability to highly saturate the electron’s spin resonance (ESR) with frequencies in the 10-300 GHz range, which by itself is a technological challenge. My group developed state of the art DNP instruments and methods, particularly successfully in the 10 GHz ESR frequency range. We have focused our effort on hyperpolarizing water and (crude) oil because our goal was to provide an approach to study molecule-specific flow dispersion images of water and hydrocarbons through reservoir rock porous media of varying permeability, porosity and heterogeneity. Such imaging capability will help understand how the spatial and temporal development of flow correlates with the microscopic pore-level characteristics.
Following are key scientific achievements that were made possible through the ACS PRF support, as acknowledged in our publications.
A. We developed an approach to covalently tether the polarizing agent, i.e. the stable nitroxide radical, to agarose-based gel beads, where water that is flowing over an spin labeled agarose column becomes continuously hyperpolarized, but is perfectly radical-free upon leaving the column. This represents a key development for producing long-lived hyperpolarized water in continuous supply.
B. We developed a fully portable prototype magnetic resonance instrument that is capable of greatly enhancing (up to 130 fold) the proton NMR signal of pure water instantaneously and at room temperature. We demonstrated its portability and utility through a research visit to the C. We have demonstrated the use of hyperpolarizaed water as a contrast-agent free MRI contrast agent for flow and perfusion imaging. We first demonstrated that the continuous polarization of radical-free, flowing water allows the distinctive and direct visualization of flow vortices and mechanical flow dispersion through a molecular sieve bead packing. We furthermore varied the bead diameter of a packed bead column from 1 mm down to 100 μm and demonstrated that the flow dispersion pattern distinctively changes. Water easily channels through the column packed with large beads, while significant backflow was observed for flow through the column packed with small beads. Research supported through this grant had visible impact on our scientific community and my own as well as my student’s career. We have demonstrated for the first time that water with highly amplified proton NMR signal can be used as an authentic contrast agent for imaging macroscopic flow dispersion of water. Our imaging method provides a unique tool to capture transiently developing flow patterns, as opposed to steady state flow dynamics. The importance of our method is based on the fact that the transiently developing flow dispersion characteristics is difficult to study without the use of external tracers, while it represents a key parameter in describing the function of flow columns in chemical engineering, flow imbibition of petroleum reservoir rocks as well as physiological blood flow through the cardiovascular system. Hyper-polarization represents an ideal contrast mechanism to highlight the ubiquitous and specific function of water in physiology, biology and materials because the physiological, chemical and macroscopic function of water is not altered by the degree of magnetization. This development on visualizing flow through packed bead columns led my student and I to pursue the development of hyperpolarized water as a physiological MRI contrast agent. Our effort in making our research relevant to petroleum-related studies led us to discussions with my colleagues at the Schlumberger Doll research center on the hyperpolarization of hydrocarbons, especially crude oil. The goal behind our collaboration is to develop DNP amplification at low magnetic fields that corresponds to the same magnetic fields of NMR well logging, and most importantly, to possibly utilize free radicals inherent to crude oil as DNP polarization agent.
