A comparison of magnetic resonance imaging methods for fluid content imaging in porous media

Quantitative measurements are important for imaging fluid content in porous media. Conventional MRI methods suffer from contrast because of relaxation times in porous media, resulting in measurements of apparent fluid content, not the true fluid content. We compare four magnetic resonance imaging methods for fluid content imaging in several water‐saturated reservoir core plugs: frequency‐encoded spin echo, single point ramped imaging with T₁ enhancement, hybrid spin echo single point imaging (SE‐SPI), and T₂ mapping SE‐SPI. 1‐D profiles obtained with each method were compared in terms of image quality, image sensitivity, and quantification of water content. The image quality of short T₂ lifetime samples suffered from blurring in hybrid SE‐SPI images. Image sensitivity was the highest in the profiles obtained with frequency‐encoded spin echo. The quantification of frequency‐encoded spin echo, T₂ mapping SE‐SPI, and hybrid SE‐SPI suffered in core plugs with a significant population of short T₂ components because of T₂ attenuation. Overall, single point ramped imaging with T₁ enhancement was found to be the most general method for fluid content imaging. Copyright © 2013 John Wiley & Sons, Ltd.     

CO2 dynamic adsorption/desorption on zeolite 5A studied by 13C magnetic resonance imaging

We present the first 13C magnetic resonance imaging study of CO2 transient adsorption/desorption processes in a zeolite 5A column. CO2 transient concentration profiles were measured with a centric scan spin-echo single point imaging technique. The adsorption wave profiles were determined under flow conditions, with the results analyzed by the Bohart-Adams model. The model adequately accounts for the spatial and the temporal behavior of CO2 in the column. CO2 adsorption rate constants were calculated from the fit. Desorption profiles were acquired by blowing a helium stream through a zeolite 5A column saturated with CO2. An asymmetry between the adsorption and desorption profiles is readily apparent. A linear relationship between the CO2 condensed phase concentration and square root of time was observed.     

Some Isoperimetric Inequalities in Electrochemistry and Hele Shaw Flows

Isoperimetric inequalities are applied to a moving-boundaryproblem for doubly-connected domains. This problem occurs forexample in electrochemistry, in which case the domains in questionare the electrolyte of an electrolytic cell. The two electrodessurrounding the electrolyte are assumed to grow or dissolve,at different rates in general, by electrochemical reaction.We obtain optimal estimates showing, for example, that the leastchange in volume of each electrode always occurs in sphericalsymmetry.     

Measurement of rock-core capillary pressure curves using a single-speed centrifuge and one-dimensional magnetic-resonance imaging

Capillary pressure curves are widely used in materials, soil, and environmental sciences, and especially in the petroleum industry. The traditional (Hassler-Brunner) interpretation of centrifugal capillary pressure data is based on several assumptions. These assumptions are known to lead to significant errors in the measurement of capillary pressure curves. In this work, we propose a new "single-shot" method to measure the capillary pressure curve of a long sedimentary rock core using a single-speed centrifuge experiment and magnetic-resonance imaging to directly determine the water saturation distribution along the length of the sample. Since only a single moderate centrifuge speed is employed, the effect of gravity can be ignored and the outlet boundary condition of the core plug was maintained. The capillary pressure curve obtained by the single-shot method is remarkably consistent with results determined with conventional mercury-intrusion methods. The proposed method is much faster and more precise than traditional centrifuge methods.     

Monitoring oil displacement processes with k‐t accelerated spin echo SPI

Magnetic resonance imaging (MRI) is a robust tool to monitor oil displacement processes in porous media. Conventional MRI measurement times can be lengthy, which hinders monitoring time‐dependent displacements. Knowledge of the oil and water microscopic distribution is important because their pore scale behavior reflects the oil trapping mechanisms. The oil and water pore scale distribution is reflected in the magnetic resonance T₂ signal lifetime distribution. In this work, a pure phase‐encoding MRI technique, spin echo SPI (SE‐SPI), was employed to monitor oil displacement during water flooding and polymer flooding. A k‐t acceleration method, with low‐rank matrix completion, was employed to improve the temporal resolution of the SE‐SPI MRI measurements. Comparison to conventional SE‐SPI T₂ mapping measurements revealed that the k‐t accelerated measurement was more sensitive and provided higher‐quality results. It was demonstrated that the k‐t acceleration decreased the average measurement time from 66.7 to 20.3 min in this work. A perfluorinated oil, containing no ¹H, and H₂O brine were employed to distinguish oil and water phases in model flooding experiments. High‐quality 1D water saturation profiles were acquired from the k‐t accelerated SE‐SPI measurements. Spatially and temporally resolved T₂ distributions were extracted from the profile data. The shift in the ¹H T₂ distribution of water in the pore space to longer lifetimes during water flooding and polymer flooding is consistent with increased water content in the pore space. Copyright © 2015 John Wiley & Sons, Ltd.     

SPRITE MRI with Prepared Magnetization and Centrick-Space Sampling

A technique for imaging materials with short transverse relaxation times and prepared longitudinal magnetization is proposed. The technique is single-point ramped imaging withT₁-enhancement (SPRITE) MRI with centrick-space sampling. The effects of transient state behavior on image resolution and signal/noise are estimated. Centric sampling in the basic SPRITE sequence gives increased signal-to-noise and permits a quantitative determination of the MR parameters associated with longitudinal spin preparation. Spin-lock and inversion recovery preparation experiments are presented.     

Single point measurements of magnetic field gradient waveform

Pulsed magnetic field gradients are fundamental to spatial encoding and diffusion weighting in magnetic resonance. The ideal pulsed magnetic field gradient should have negligible rise and fall times, however, there are physical limits to how fast the magnetic field gradient may change with time. Finite gradient switching times, and transient, secondary, induced magnetic field gradients (eddy currents) alter the ideal gradient waveform and may introduce a variety of undesirable image artifacts. We have developed a new method to measure the complete magnetic field gradient waveform. The measurement employs a heavily doped test sample with short MR relaxation times (T(1), T(2), and T(2)(*)<100 micros) and a series of closely spaced broadband radiofrequency excitations, combined with single point data acquisition. This technique, a measure of evolving signal phase, directly determines the magnetic field gradient waveform experienced by the test sample. The measurement is sensitive to low level transient magnetic fields produced by eddy currents and other short and long time constant non-ideal gradient waveform behaviors. Data analysis is particularly facile permitting a very ready experimental check of gradient performance.     

Water content profiles with a 1D centric SPRITE acquisition

The purpose of this work is to develop a rapid MRI method amenable to profiling with minimal or no T(1) relaxation weighting. The behavior of a signal during a centric SPRITE acquisition is analyzed. It is shown that the technique can be made immune to a broad range of T(1) changes. In a properly executed measurement, only T(2)* and proton density parameters define the image intensity. A T(2)* mapping technique can be easily applied, separating T(2)* and proton density contributions to the image. A drying soil sample with low initial water content is experimentally studied as a demonstration of the technique. A characteristic baseline artifact is easily removed from the profiles by a simple operation.     

Two-dimensional T2 distribution mapping in porous solids with phase encode MRI

Two pure phase encode MRI sequences, CPMG-prepared SPRITE and spin-echo SPI with compressed sensing, for two-dimensional (2-D) T2 distribution mapping have been presented. The sequences are 2-D extensions of their 1-D predecessors previously described and are intended for studying processes in porous solids and other samples with short relaxation times whenever 2-D T2 maps are preferable to simple 1-D profiling. The sequences were tested on model samples and natural water-saturated rocks, in a low field MRI instrument. 2-D spin-echo SPI and CPMG-SPRITE demonstrate a similar performance, enabling measurement of T2 down to 1-2 ms. Both experiments are time consuming (up to 2-2.5 h sample dependent). As such, they can be recommended mostly for measurement during steady state conditions or when studying relatively slow dynamic processes (e.g. enhanced oil recovery, cement paste hydration, curing rubber, infiltration of paramagnetic ions).     

Magnetic Resonance Imaging of Gases: A Single-Point Ramped Imaging with T1 Enhancement (SPRITE) Study

A pure phase-encoding MRI technique, single-point ramped imaging withT₁enhancement, SPRITE, is introduced for the purpose of gas phase imaging. The technique utilizes broadband RF pulses and stepped phase encode gradients to produce images, substantially free of artifacts, which are sensitive to the gasT₁andT^*:₂relaxation times. Images may be acquired from gas phase species with transverse relaxation times substantially less than 1 ms. Methane gas images,¹H, were acquired in a phantom study. Sulfur hexafluoride,¹⁹F, images were acquired from a gas-filled porous coral sample. High porosity regions of the coral are observed in both the MRI image and an X-ray image. Sensitivity and resolution effects due to signal modulation during the time-efficient acquisition are discussed. A method to increase the image sensitivity is discussed, and the predicted improvement is shown through 1D images of the methane gas phantom.