NMR log calibration from laboratory core measurements

Application Note 3

The fundamental basis of Nuclear Magnetic Resonance (NMR) measurements on fluid-bearing
rocks is that the decay or relaxation time of the NMR signals (T2) is directly related to the pore size.
The NMR signal detected from a fluid-bearing rock therefore contains T2 components from every
different pore size in the measured volume. Using a mathematical process known as inversion, thesecomponents can be extracted from the total NMR signal to form a T2 spectrum or T2 distribution,
which is effectively a pore size distribution. From this distribution, various petrophysical parameterssuch as porosity, permeability, and free and bound fluid ratios can be measured or inferred.
However, as might be expected, things are not quite so simple inpractice. The T2 distributions calculated from NMR signals depend on
many other parameters besides simply the sizes of the fluid-containing
pores. Other factors that influence the shape of the distributions
include: the nature of the fluid (in particular the viscosity); the spacing
between the data points (echoes) in the NMR signal; the logging
speed; the wettability of the rock; and the hydrogen index of thefluid(s) being observed. The variability of these factors in the naturalenvironment can lead to errors in interpreting the logging informationcollected during the exploration process.
In the laboratory, most of these additional factors can be eliminatedor at least controlled by making measurements on fully saturated coresamples and on ones that are at irreducible water saturation. These
measurements help to distinguish between fluid and pore size effects.
In the logs, these effects have a significant influence on the recorded
data making interpretation difficult. Calibration of logs refers tocomparing data from the logs with data from well characterised coreplugs taken from the same well, in an attempt to make interpretationof the log data easier and more accurate. The value of this process isrecognised by logging experts:“...careful calibration of petrophysical interpretation
models with core data is essential for obtaining
accurate permeability and bound fluid answers
from NMR logs, as compared to using default
parameters in the petrophysical model.”Schlumberger leaflet on NMR core analysis

NMR log calibration from laboratory core measurements

What factors can be calibrated?
The two NMR log measurements most often calibrated using core plugs are the T2 cut-off dividing the bound and free fluid, andpermeability modelling coefficients.

T2 cut-off
The so-called “T2 cut-off” in a T2 distribution is the T2 value that divides the small pores that are unlikely to be producible fromthe larger pores that are likely to contain free fluid. The integral of the distribution above the T2 cut-off is a measure of the freefluid (mobile fluid) in the rock, and is clearly influenced by the position of the T2 cut-off point, as shown in Figure 1. The portionof the curve below the cut-off is known as bound fluid and is made up of the clay bound fluid and the capillary bound fluid.
An accurate determination of the T2 cut-off point is essential
for an accurate determination of recoverable reserves(mobile fluid). T2 cut-off can be easily determined in theT2 cut-off

laboratory by using two NMR measurements; one on acleaned and re-saturated plug, the other on the same plug
after it has been spun in a centrifuge to irreducible water
saturation. T2 distributions are plotted for both data sets,Free Fluid
Bound Fluidalong with the cumulative values of the distributions. TheT2 cut-off is taken to be the point at which the cumulativevalue of the saturated distribution (yellow horizontal arrow inFigure 2) equals the final cumulative value of the irreducible

Figure 1 - T2 distribution showing integral for free fluid

distribution (vertical yellow arrow in Figure 2). The dataplotting and calculation for this measurement are carried out
automatically by the LithoMetrix software supplied as standard
with every GeoSpec NMR core analyser.
This measurement should be done on core plugs taken fromvarious zones in the well, with the resulting T2 cut-off values
applied retrospectively to the log data from each depth. This
will result in a more accurate prediction of Bound Volume
Index (BVI) and Free Fluid Index (FFI) than those obtained byapplying a single default
value of the T2 cut-offT2 cut-off

Figure 2 - T2 distributions showing T2 cut-off point

to the entire log.

Application Note 3

The following example illustrates how reserve

SamplePorosity (%)

Permeability(mD)

FFI at33msec (%)

Perm. Est.
(mD)

Measured
FFI(%)

Cut-off(ms)standard cut-off value is used without laboratory

1

5.71

0.003

0.46

0.000068

1.34

10.00

calibration. The samples used in this study were

2

7.59

0.003

1.04

0.000080

2.78

7.08

3

4.59

0.001

0.24

0.000014

0.47

14.13

In this particular example, laboratory measurements

4

7.36

0.003

1.460.00170

3.13

7.94

correct the standard cut-off estimate of free fluid

512.01

0.28

2.71

0.045

7.88

1.79

index by an average of 260%. This means that

6

10.03

0.22

1.51

0.004

2.42

10.66

7

10.2

0.13

2.00

0.032

4.42

7.77

8

8.8

0.15

1.17

0.031

5.08

2.66

estimates can be seriously under-reported if afrom a tight sandstone play.

this reserve could produce 2.6 times more oilthan would have been estimated if the laboratory
calibration had not been completed.

Permeability
Permeability can be calculated from the T2 distribution datausing one of two commonly accepted mathematical models:the free-fluid or Coates model (Equation 1) can be applied informations containing water and/or hydrocarbons, while theaverage-T2 or Schlumberger model (Equation 2) can be applied
to pore systems containing only water and gas. In either case,measurements on core samples are necessary to refine thesemodels by determining the correct values of the coefficients,
and produce a model customised for local use.

Figure 3 – Free Fluid Index using standard and measured cut-off valuesak= φc

FFI
BVI() ( )bEquation 1 - The Coatesmodel for permeability

2k = aT2 log meanφEquation 2 - The Schlumberger
model for permeability

Customisation of these models is done by inserting values ofthe log mean T2, FFI and BVI obtained from core plug NMR
measurements, and then varying the coefficients until thebest linear fit is obtained against permeability measurements
determined by other techniques. LithoMetrix (includedwith every GeoSpec) provides tools to perform this on
single samples while LithoMetrix has advanced regressiontools allowing optimum T2 cut-offs and coefficients tobe determined for a set of samples. Figure 4 shows NMR
permeability versus actual permeability using the optimized
Figure 4 –NMR permeability versus actualvalues for a large set of samples.

4GeoSpec – The new approach to core analysis using NMR

Necessary apparatus

Oxford Instruments Magnetic ResonanceTools needed to perform NMR log calibrations as described

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

in this note include: a GeoSpec NMR Core Analyser with
LithoMetrix software; a centrifuge capable of removing
all mobile (free) fluid from the core plug; and equipment
to measure permeability. Although not required to perform
log calibrations, pulsed field gradients on the GeoSpec
instrument and optional GIT Systems software can allow
users to perform additional advanced measurements tofurther enhance interpretation of NMR logs.

UKTubney Woods, Abingdon, Oxfordshire, OX13 5QX, UK
Tel: +44 (0) 1865 393 200 Fax: +44 (0) 1865 393 333
USA300 Baker Avenue, Suite 150, Concord, MA, 01742, USA
Tel: +1 978 369 9933 Fax: +1 978 369 8287
China

Recommendations for consideration

Floor 1, Building 60, No. 461 HongCao Road, Shanghai,

Expanding our supply of oil and gas requires exploration. With200233,PRC, China

increasing focus on unconventional reserves, it is imperative

Tel: +86 21 61273858 Fax: +86 21 6360 8535

that we do not rely on previously acceptable “standards”
without question. Example after example shows that using
standard cut-offs significantly misrepresents reserve estimates.

Green Imaging Technologies

Easy-to-use GeoSpec NMR systems allow rapid, consistent,

For more information: info@greenimaging.com

reliable laboratory measurements to support work completed

www.greenimaging.com

at the well site; providing reliable results for the reservoir

Canadaengineers and petrophysicists.

520 Brookside Drive, Suite B, Fredericton, NB, E3A 8V2,
CanadaToll Free: +1 888 944 8462
Tel: +1 506 458 9992 Fax: +1 506 458 9615

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