Solid state NMR imaging of irreducible water in reservoir cores for spatially resolved pore surface relaxation estimation

A review of non-invasive imaging methods and applications in contaminant hydrogeology research

Contaminant hydrogeological processes occurring in porous media are typically not amenable to direct observation. As a result, indirect measurements (e.g., contaminant breakthrough at a fixed location) are often used to infer processes occurring at different scales, locations, or times. To overcome this limitation, non-invasive imaging methods are increasingly being used in contaminant hydrogeology research. Four of the most common methods, and the subjects of this review, are optical imaging using UV or visible light, dual-energy gamma radiation, X-ray microtomography, and magnetic resonance imaging (MRI). Non-invasive imaging techniques have provided valuable insights into a variety of complex systems and processes, including porous media characterization, multiphase fluid distribution, fluid flow, solute transport and mixing, colloidal transport and deposition, and reactions. In this paper we review the theory underlying these methods, applications of these methods to contaminant hydrogeology research, and methods' advantages and disadvantages. As expected, there is no perfect method or tool for non-invasive imaging. However, optical methods generally present the least expensive and easiest options for imaging fluid distribution, solute and fluid flow, colloid transport, and reactions in artificial two-dimensional (2D) porous media. Gamma radiation methods present the best opportunity for characterization of fluid distributions in 2D at the Darcy scale. X-ray methods present the highest resolution and flexibility for three-dimensional (3D) natural porous media characterization, and 3D characterization of fluid distributions in natural porous media. And MRI presents the best option for 3D characterization of fluid distribution, fluid flow, colloid transport, and reaction in artificial porous media. Obvious deficiencies ripe for method development are the ability to image transient processes such as fluid flow and colloid transport in natural porous media in three dimensions, the ability to image many reactions of environmental interest in artificial and natural porous media, and the ability to image selected processes over a range of scales in artificial and natural porous media.

Continuous wave MRI of heterogeneous materials

A prototype continuous wave MRI system operating at 7T has been used successfully to study a variety of heterogeneous materials exhibiting T2 relaxation values ranging from 10 micros to 50 ms. Two-dimensional images of a poly(methly methacrylate) (PMMA) resolution phantom (T2=38 micros) exhibited a spatial resolution of approximately 1mm at a magnetic field gradient strength of 200 mT/m. The technique was used to study the hydration, drying, and subsequent water penetration properties of cement samples made from ordinary Portland cement, and revealed inhomogeneities arising from the cure conditions. Sandstone samples from an oil reservoir in the North Sea were also studied; structure within these materials, arising from the sedimentary bed layering in the reservoir, was found to have an effect on their water transport properties. A section from a confectionery bar (T2* approximately 50-60 ms) was also imaged, and its internal structure could be clearly discerned.

Single-point magnetic resonance imaging study of water adsorption in pellets of zeolite 4A

The water uptake process in commercial type particles of zeolite 4A has been studied using a single-point MRI method. True proton density, T1, T2, and T*2 relaxation times were obtained with submillimetric resolution, overcoming the restrictions of short T*2 signals. The molecular mobility in nonequilibrium conditions has been characterized by relaxation time mapping. A clear reduction of the water sorption rate was observed by comparing MRI profiles of a loosely packed bed and gravimetric measurements of spread particles from the same sieved zeolite batch.

Relaxation time mapping of short T*2 nuclei with single-point imaging (SPI) methods

New techniques for quantitative mapping of T1, T2, and T*2 are proposed, based on the single-point imaging (SPI) method, for materials with short nuclear magnetic resonance relaxation times which cannot be imaged with traditional methods. Relaxation times extracted from two-dimensional images of uniform doped agarose phantoms (T*2 approximately 60-210 microseconds) as well as hardened mortar (T*2 approximately 220 microseconds) and polymers (T*2 approximately 20-100 microseconds), using these techniques, agreed with bulk measurements. The method was then applied to a partially dried cylindrical concrete sample (T*2 approximately 115 microseconds).

Concrete thawing studied by single-point ramped imaging

A series of two-dimensional images of proton distribution in a hardened concrete sample has been obtained during the thawing process (from -50 degrees C up to 11 degrees C). The SPRITE sequence is optimal for this study given the characteristic short relaxation times of water in this porous media (T2* < 200 micros and T1 < 3.6 ms). The relaxation parameters of the sample were determined in order to optimize the time efficiency of the sequence, permitting a 4-scan 64 x 64 acquisition in under 3 min. The image acquisition is fast on the time scale of the temperature evolution of the specimen. The frozen water distribution is quantified through a position based study of the image contrast. A multiple point acquisition method is presented and the signal sensitivity improvement is discussed.