Application
Note 3.
Routine Core Analysis
Free fluid, bound fluid
and T2cutoff
Knowing the value of T2cutoff enables the amount of mobile and bound fluids to
be calculated from log data. The T2cutoff is the size boundary between small pores
containing bound fluid and larger pores where the fluid is free. While various
rock types have standard T2cutoff values, the geometry of the pores and the rock’s
mineralogy may shift the T2 spectrum, so laboratory core analysis experiments are
used to determine the value of T2cutoff. This application note demonstrates how the
total porosity, free fluids and bound fluids are used to calculate T2cutoff.
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Concept
called the bound volume irreducible (BVI). The quantity
of unbound or mobile fluid in large pores is called the
free fluid index (FFI). NMR measurements of a core are
used to determine BVI and FFI, which are then used
to calculate T2cutoff. The T2cutoff can be used to find the
amount of free and bound fluids in a well log.
NMR detects a decaying signal from excited fluid
molecules in a rock sample. The decay time constant
is proportional to the size of the pores in which the
fluids are contained (see Application Notes 1 and 2).
Very small pores bind the fluids tightly with capillary
forces; the amount of fluid bound in these pores is
Sample preparation
To determine a T2cutoff in a laboratory, two NMR
measurements of a core sample are made – the first
when it is saturated with brine and the second when it
has been desaturated. When a core is saturated, fluids
occupy the entire pore space but when desaturated,
water remains bound only in the small pore spaces
between the grains, and in clays.
FluidC
Grains
Clay bound water
In Figure 1, a Berea sandstone core has been
saturated, then desaturated by centrifugation. More
details of these preparation methods can be found
in the American Petroleum Institute’s Recommended
Practices for Core Analysis.
apillary bound water
Figure 1: T2 distributions of saturated
(blue) and bound (red) cores plotted as
incremental porosity.
NMR measurements
Both brine-saturated and desaturated rock cores can
be analyzed in a benchtop NMR instrument. Following
analysis of the saturated sample, the raw data is
processed to give a T2 incremental porosity spectrum
(as described in Application Note 2). The core is
then desaturated to remove all mobile water and
reanalyzed. The T2 distributions of the saturated (blue)
and bound (red) are shown in Figure 1. Note that both
traces have signals from the bound water, but only
the saturated sample has signals from the free fluid.
FFI and BVI determination
To determine BVI using NMR, the total volume of
water in the desaturated sample is first measured
by integrating the incremental porosity spectrum
to produce a cumulative porosity spectrum (Figure
2, red line). BVI is then the fraction of water bound
in pores below a certain size. The saturated sample
is also integrated to determine the total porosity of
the core (blue line). As shown in Equation 1, FFI is the
difference between the total porosity and the BVI.
In this example using a sandstone sample (Figure 2),
total porosity is 18.8 p.u., BVI is 3.4 p.u. and FFI is 15.4
p.u. (1 p.u. = 1 porosity unit = 1 per cent of the bulk
volume).
Equation 1
FFI = Total porosity – BVI
Saturate Core
BVI
Total Porosity
FFI
Saturated T2
Distribution
Centrifuge Core
Bound T2
Distribution
BVI and FFI
Determination
Figure 2: A cumulative porosity spectrum is produced by integrating an incremental porosity spectrum.
Total porosity, FFI, and BVI are determined from the cumulative porosity spectrum.
T2cutoff determination
Figure 3: A cumulative
porosity plot, showing the
calculation of T2cutoff as
40 ms.
BVI
FFI
black line indicates that the saturated sample
has a cumulative porosity of 3.4 p.u. at 40 ms.
T2cutoff therefore equals 40 ms for this sample.
Total Porosity
The T2cutoff is defined as the value of T2 where
the cumulative porosity of the saturated
sample is equivalent to the BVI. For the
example in Figure 3, BVI = 3.4 p.u., the dotted
T2cutoff
How T2cutoff is used
Free fluid is in larger pores where T2 is greater
than the T2cutoff, therefore the T2cutoff is used by
well log analysts to determine the quantity of
free fluid in a reservoir. Figure 4 shows the use
of T2cutoff to differentiate between bound and
free fluids in a saturated sample.
It is impossible to measure the BVI of a
reservoir in situ, but it can be calculated from
the T2 spectra of a well log using the T2cutoff
determined from NMR analysis of a core sample
in a laboratory.
Figure 4: An incremental
porosity plot divided into
bound and free fluids with
T2cutoff.
T2cutoff
Summary
The T2cutoff is a very important measurement
for the interpretation of well log data. Using
NMR in a laboratory, the T2cutoff is calculated by
measuring the total porosity and the bound
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and free fluids in the pores of core samples. The
NMR experiment simply requires measuring the
same core sample when saturated with brine
and desaturated.
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