NMR Profiling in 1D - PowerPoint PPT Presentation

Loading...

PPT – NMR Profiling in 1D PowerPoint presentation | free to view - id: 9bb9c-OTYyM



Loading


The Adobe Flash plugin is needed to view this content

Get the plugin now

View by Category
About This Presentation
Title:

NMR Profiling in 1D

Description:

Spatial resolution will allow for the determination of sample properties such as ... the determination of spatial distributions of properties such as pore size, ... – PowerPoint PPT presentation

Number of Views:57
Avg rating:3.0/5.0
Slides: 23
Provided by: michaelra
Category:
Tags: nmr | profiling

less

Write a Comment
User Comments (0)
Transcript and Presenter's Notes

Title: NMR Profiling in 1D


1
NMR Profiling in 1-D
  • Rice University
  • Michael Rauschhuber
  • George Hirasaki
  • March 26, 2008

2
NMR Profiling Motivation and Goals
  • Standard NMR techniques look at the sample as a
    whole or only at a thin slice.
  • By relying on MRI techniques, spatial resolved
    T2 distributions or apparent diffusion
    coefficients can be obtained.
  • Spatial resolution will allow for the
    determination of sample properties such as
    porosity or saturation as a function of position.

3
Frequency-Encoding Gradients
  • Standard MRI technique to detect spatial
    information from a desired sample.
  • In a uniform magnetic field,
  • In the presence of a linear magnetic field
    gradient gx,
  • Precession frequency is linearly related to
    spatial position

4
T2 Profiling
  • A CPMG-style imagining sequence, known as Rapid
    Acquisition with Relaxation Enhancement (RARE),
    can be used to generate a collection of profiles
    with increasing T2 relaxation.
  • Spatial distribution of T2 can be achieved by
    analyzing the decay of the acquired profiles.

5
Image Construction
  • Acquisition of the full echo is necessary for
    image construction.
  • Application of a FFT reconstruction is performed
    in order to generate the 1-D profile.
  • Finally, frequency can be converted to distance
    using the linear relationship between frequency
    and sample position.

6
T2-weighting with Multiple Echoes
Sample Brine g1 g2 0.800 G cm-1, d1 1.30
msec, t 3.00 msec, DW 40.0 msec, SI 64
Profiles in the figure to the left are presented
in a geometric progression
  • Every echo from the RARE sequence corresponds to
    a profile. FFT reconstruction is applied to each
    echo individually
  • T2 distributions are generated by applying a
    multi-exponential fit to the attenuating
    profiles.

7
T2 Profiles
Sample Brine (1 wt NaCl), g1 g2 0.800 G
cm-1, d1 1.30 msec, t 3.00 msec, DW 40.0
msec, SI 64
8
Yates Core Sample (D)
Sample Yates D, g1 g2 0.800 G cm-1, d1
1.30 msec, t 3.00 msec, DW 80.0 msec, SI
32
9
Texas Cream Limestone (TCL 30)
Sample TCL 30, g1 g2 0.800 G cm-1, d1
1.30 msec, t 3.00 msec, DW 80.0 msec, SI
32
10
Porosity Profiles
  • Porosity profiles can be constructed by
    comparing the RARE profiles of the brine
    saturated core sample and a bulk brine sample.
  • A profile of intrinsic magnetization, M0, was
    generated by extrapolating the first few profiles
    for both the rock and brine sample.
  • The brine sample is a mixture of H2O and D2O such
    that it had a similar Hydrogen content and bulk
    volume as the core. This allows for the sample
    to occupy the same region within the probe while
    maintaining the same gain for core and bulk water
    measurements.
  • Porosity was calculated as function of height
    using

where A cross-sectional area vH20 volume
fraction of H2O
11
Porosity Profiles
fRARE 0.200 fGrav. 0.208 fCPMG 0.208
12
Porosity Profiles
fRARE 0.242 fGrav. 0.245 fCPMG 0.243
13
Saturation Profiles
  • Saturation profiles can be determined for samples
    exhibiting non-overlapping T2 peaks for water and
    oil by using the equation below.
  • Experiments were performed with a sandpack
    initially flooded Mars Yellow crude (So 0.92).
    A saturation gradient was imparted unto the
    system by performing a water flood that removed
    about a quarter of the crude.
  • T2 maps were acquired in 4 cm segments and then
    were stacked together creating a composite image
    for the entire 1 ft. sand column.

14
Saturation Profiles
So, mass 0.71 So, avg 0.68
White T2 log mean Pink min. value between oil
and water peaks.
Red Sat. via Mass Balance Green Average of
Sat. Profile
15
Diffusion Profiling in 1-D
  • By incorporating diffusion gradient pulses, NMR
    imaging techniques become sensitive to molecular
    diffusion.
  • A quantitative map of diffusion coefficients
    could be extracted from a set a DW experiments,
    in which each experiments has a different degree
    of diffusion sensitivity.
  • The most notable applications would extend to
    systems were the saturating fluids with different
    viscosities exhibit overlapping T2 distributions.

16
Pulsed Field Gradient Stimulated Echo Imaging
  • The stimulated echo will have half the amplitude
    of its direct echo conterpart, but will preserve
    more T2 information.
  • Attenuation due to diffusion is increased by
    varying the diffusion gradients (gD).

Readout Gradient
Prephasing Gradient
17
Diffusion Profiles
  • The Stimulated Echo Diffusion Imaging sequence is
    repeated at different diffusion gradient (gD)
    strengths.
  • Loss of echo amplitude due to diffusion is
    mirrored in the attenuation of the profiles.

t 50 msec D 100 msec d 2 msec gD 0 to 32
G cm-1 b 1.3 msec W 154 msec gf 0.80 G cm-1
18
Attenuation Due to Diffusion
  • The presence of imaging gradients in this
    diffusion sequence can impact the diffusion
    sensitivity for a given experiment . Therefore,
    the attenuation equation for the standard PFG
    stimulated echo (shown below) should not be used.
    Overestimation of the diffusion coefficient can
    result if the effect of the imaging pulses are
    neglected.
  • Diffusion due to the imaging gradients as well as
    the diffusion and imaging gradient cross-terms
    must be taken into account in order accurate
    determination of the diffusion coefficient
    (equation shown below).

Neeman, Freyer, Sillerud (1990).
Pulsed-gradient spin echo diffusion studies in
NMR imagiing Effects of the imaging gradient on
the determination of diffusion coefficients.
Journal of Magnetic Resonance, 90, 303-312.
19
Diffusion Profile - Water
20
Diffusion Profile Squalane (20 cP, synthetic
oil)
  • t 40 msec, D 60 msec, d 12 msec, gD 0 to
    32 G cm-1
  • 1.3 msec, W 102 msec, gf 0.80 G cm-1
  • Required 30 min. wait time between measurements
    to prevent a large valley forming in the center
    of the attenuation profiles.

21
Conclusions
  • NMR profiling is a valuable tool enabling the
    determination of spatial distributions of
    properties such as pore size, porosity, and
    fluid saturation.
  • Diffusion coefficients can be calculated as a
    function of height by comparing several images
    which have undergone various amounts of diffusion
    attenuation.
  • Possibility of combining diffusion and T2
    profiling will allow for an imaging analog of
    diffusion editing.

22
Questions?
Acknowledgements
We would like to acknowledge the Consortium for
Processes in Porous Media and the Department of
Energy for their financial support.
About PowerShow.com