Spin echo SPI methods for quantitative analysis of fluids in porous media

Abstract

Fluid density imaging is highly desirable in a wide variety of porous media measurements. The SPRITE class of MRI methods has proven to be robust and general in their ability to generate density images in porous media, however the short encoding times required, with correspondingly high magnetic field gradient strengths and filter widths, and low flip angle RF pulses, yield sub-optimal S/N images, especially at low static field strength. This paper explores two implementations of pure phase encode spin echo 1D imaging, with application to a proposed new petroleum reservoir core analysis measurement. In the first implementation of the pulse sequence, we modify the spin echo single point imaging (SE-SPI) technique to acquire the k-space origin data point, with a near zero evolution time, from the free induction decay (FID) following a 90 degrees excitation pulse. Subsequent k-space data points are acquired by separately phase encoding individual echoes in a multi-echo acquisition. T(2) attenuation of the echo train yields an image convolution which causes blurring. The T(2) blur effect is moderate for porous media with T(2) lifetime distributions longer than 5 ms. As a robust, high S/N, and fast 1D imaging method, this method will be highly complementary to SPRITE techniques for the quantitative analysis of fluid content in porous media. In the second implementation of the SE-SPI pulse sequence, modification of the basic measurement permits fast determination of spatially resolved T(2) distributions in porous media through separately phase encoding each echo in a multi-echo CPMG pulse train. An individual T(2) weighted image may be acquired from each echo. The echo time (TE) of each T(2) weighted image may be reduced to 500 micros or less. These profiles can be fit to extract a T(2) distribution from each pixel employing a variety of standard inverse Laplace transform methods. Fluid content 1D images are produced as an essential by product of determining the spatially resolved T(2) distribution. These 1D images do not suffer from a T(2) related blurring. The above SE-SPI measurements are combined to generate 1D images of the local saturation and T(2) distribution as a function of saturation, upon centrifugation of petroleum reservoir core samples. The logarithm mean T(2) is observed to shift linearly with water saturation. This new reservoir core analysis measurement may provide a valuable calibration of the Coates equation for irreducible water saturation, which has been widely implemented in NMR well logging measurements.

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

“…Spin echo single point imaging (SE-SPI) MRI measurements generate a series of T 2 -weighted images and, thereby, a spatially resolved T 2 distribution. [5,6] SE-SPI is, however, time consuming because only one k-space point is acquired for each RF excitation, and there is a 5T 1 relaxation delay for magnetization recovery between each gradient step. [5][6][7] In previous work, displacing phase injection was suspended to avoid fluid content change during lengthy MRI measurements.…”

“…[5,6] SE-SPI is, however, time consuming because only one k-space point is acquired for each RF excitation, and there is a 5T 1 relaxation delay for magnetization recovery between each gradient step. [5][6][7] In previous work, displacing phase injection was suspended to avoid fluid content change during lengthy MRI measurements. [8,9] However, the oil and water pore scale distribution may change when fluid injection is suspended.…”

Direct measurement of magnetic field gradient waveforms

“…5 is crucial for advanced MRI pulse sequences that switch gradients rapidly, for example, fast spin echo SPI measurements (35).…”

A Nonparametric Approach for Determining NMR Relaxation Distributions

“…The first is the determination of the amounts of the observed fluids. This information is used to determine fluid content, which may in turn be used to determine porosity and fluid saturations (Chen et al 1993;Kulkarni and Watson 1997;Grunewald and Knight 2012;Li et al 2009). It also plays an essential role in determining permeability distributions (Uh and Watson 2011).…”