Rapid non-destructive Capillary Pressure
Measurements with NMR

Application Note 2

Capillary pressure is a critical parameter for the oil exploration and production industry, and is acommon measurement performed in core analysis laboratories. However, traditional techniques suchas porous plate, centrifugation and mercury injection are laborious, time consuming and expensive.
The method described here was first reported by Dr. Derrick
Green of Green Imaging Technologies at a meeting of the Societyof Core Analysts in 2007. Called GIT-CAPTM, it centrifuges coreplug samples and then directly measures the water saturationdistribution using a one-dimensional NMR profile (a simplified
version of Magnetic Resonance Imaging, or MRI). This technique
is as accurate as porous plate and is 5 times faster than traditional
centrifuge capillary pressure measurements.

MethodThe patented GIT-CAP capillary pressure measurement techniqueuses capillary pressure theory combined with NMR-determinedsaturation profiles, allowing for the accurate development ofcapillary pressure curves. With this technique, the core plug is
first fully saturated with brine, and is then centrifuged to createa distribution of fluid in the plug which is in turn dependent
on capillary pressure. The fluid distribution is quantified using
NMR. The capillary pressure at each position along the plug athydrodynamic equilibrium is derived from equation (1) and thesaturation at the corresponding positions is measured using
NMR (illustrated in Figure 1).
[1]

Where Pc(r) is the capillary pressure at the given radial position,
Dr is the density difference of the two fluids, w is the rotational
speed and r2 is the distance to the outlet face of the plug. Thisequation assumes that the outlet boundary condition of 100%water saturation (or zero capillary pressure) is maintained. Thismeans in any centrifuged plug, there will be a distribution ofsaturations from 100% at the outlet to water saturation at theinlet (governed by the rotational speed).

Figure 1: GIT-CAP capillary pressure technique measuring
30 - 40 data points
Saturation profiles acquired after centrifugation at different
speeds can be plotted on the same curve expanding the range
and resolution of the capillary pressure curve. This technique
was proven to work on a wide range of samples (Reference 1).
Accurate saturation profiles are ensured by using advanced
SPRITE-based MRI techniques (Reference 2).

Rapid non-destructive Capillary Pressure Measurements with NMR

Results
Typical results are shown in the following figures for two sandstone samples. The associated saturation profiles
are included to illustrate the amount of fluid movement within the samples at each centrifuge speed.
Figure 3a:
Saturation profiles
for Figure 3b

Figure 2a:
Saturation profiles
for Figure 2b

Figure 2b: GIT-CAP capillary pressure curve for sandstone
sample with a permeability of 5.74% and porosity of 20.4 mD
As can be seen in these figures, thisGIT-CAP Advantages

technique only requires two different
centrifuge speeds to determine the fullcapillary pressure curve. The obvious
advantage is that the result is obtained• Uses representative fluids
Mercury
Injection

in a fraction of the time required

Traditional
Centrifuge• Accuracy/more data (GIT-CAP provides 30-40 data points per
centrifuge speed)• Simpler centrifuge set-up (no costly upgrades required)technique to the traditional capillarypressure measurement methods:• Direct measurement• Speed (GIT-CAP takes days, centrifuge takes weeks)

techniques but it is also an accurate,compares the GIT-CAP measurement• Non-destructive• Simpler sample handling proceduresfor other traditional measurement
direct measurement. The following table

Figure 3b: GIT-CAP capillary pressure curve for sandstone
sample with a permeability of 13.7% and porosity of 9.7 mD

PorousPlate• Speed (GIT-CAP takes days, porous plate takes weeks or months)• More data points

Application Note 2With the inherent time and accuracy advantage, the GIT-CAP NMR measurement can help a laboratory increase their sample
throughput by at least three times per centrifuge. The following table illustrates how to maximise sample throughput using thismeasurement technique:

1 Centrifuge

2 Centrifuges

Traditional

GIT-CAP

Traditional

GIT-CAP

Samples per centrifuge

44

4

4

Test duration in days
(including sample preparation)

10

3

10

3

Days per year

250

250

250250Pc Tests/year

100

330

200660HardwareConclusion

The advantages of the GIT-CAP measurement are bolstered byCapillary pressure measurements using the GIT-CAP methodthe GeoSpec hardware, which has substantially decreased

are substantially faster than those made by traditional

acquisition times, as well as an increased signal to noise ratio.

techniques, particularly for low permeability samples with

Decreased acquisition times make the GIT-CAP measurement

longer equilibrium times. They also provide significantly moreeven faster while the increased signal to noise ratio anddata points, allowing more accurate prediction of additional

short echo times ensure the data collected is more accurate.

parameters such as pore throat distributions, wettability, andMeasurements are controlled by the GIT-CAP software andrelative permeability.

can be made easily by core laboratory technicians without indepth knowledge of NMR techniques.

References
Reference 1: Comparison study of capillary pressure curves obtained using traditional
centrifuge and magnetic resonance imaging techniques. D.P. Green, J.R. Dick, J. Gardner, B. J.
Balcom, B. Zhou. International Symposium of the Society of Core Analysts, Calgary, Alberta,
Canada, (September, 2007)
Reference 2: Spatially resolved measurement of rock core porosity. F. Marica, Q. Chen, A.
Hamilton, C. Hall, T. Al, B.J. Balcom. Journal of Magnetic Resonance 178, 136 - 141. (2006)
Additional references are provided at www.greenimaging.com/publications

GeoSpec – The new approach to core analysis using NMR

Until recently, a high level of expertise and knowledge of theprinciples and practice of NMR has been needed to perform
NMR core analysis. Consequently, NMR core analysis has been
confined to a relatively small group of Special Core Analysis

For more information: magres@oxinst.com
www.nmr.oxinst.com

Laboratories. A partnership between Oxford Instruments andUKGreen Imaging Technologies brings optimised hardware andTubney Woods, Abingdon, Oxfordshire, OX13 5QX, UK

software to improve performance and enable any core analysis

Tel: +44 (0) 1865 393 200 Fax: +44 (0) 1865 393 333

laboratory to generate the petrophysics values that the industry
needs without the requirement that every dataset be analysed

USAby an NMR expert. Now, any core analysis laboratory can add

300 Baker Avenue, Suite 150, Concord, MA, 01742, USA

NMR to the techniques they can offer, allowing:

Tel: +1 978 369 9933 Fax: +1 978 369 8287

• Well Log Calibrationand giving fast measurement of:• Total Porosity• Pore Size Distribution• Bulk Volume Irreducible (BVI)• Free Fluid Index (FFI)• Clay Bound Water (CBW)• Effective Porosity• Permeability• Capillary Pressure• Gas/Water Content• Oil/Water Content• Shale Analysis• Water/BitumenContent

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