NMR Profiling in 1D

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NMR Profiling in 1-D

• Rice University
• Michael Rauschhuber
• George Hirasaki
• March 26, 2008

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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.
  

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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

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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.

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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.

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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.

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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
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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
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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
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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
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Porosity Profiles
fRARE 0.200 fGrav. 0.208 fCPMG 0.208
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Porosity Profiles
fRARE 0.242 fGrav. 0.245 fCPMG 0.243
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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.

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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
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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.
  

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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
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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
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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.
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Diffusion Profile - Water
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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.

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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.

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