NMR and Core Analysis

Technical Datasheet

Introduction
Most people involved in core analysis know that NMR (Nuclear Magnetic Resonance) has been part of the available suite of welllogging measurements since the mid-90s, and that it is also used for routine laboratory core analysis – in part for calibration of NMR
well logs. But how many people really know how NMR works, and the full extent of what it can do for the core analyst? Most core
analysts know that NMR can determine porosity and pore size distributions easily and quickly, but what about fluid mobility such as
Bound Volume Irreducible (BVI), Free Fluid Index (FFI), Clay Bound Water (CBW), and effective porosity? Furthermore, what aboutpermeability, capillary pressure, and oil/water or gas/water contents? NMR can measure all these parameters quickly and accurately,
and current software makes it possible to do so without extensive knowledge of NMR.
This note is an attempt to introduce NMR and these capabilities to the non-NMR expert petrophysicist and core analyst.

Technical background
Essentially, NMR signals are generated from liquids (oil or brine)
when the sample is placed in a magnetic field and then excitedwith a brief pulse of radio frequency (RF) energy. Immediately
after the pulse, an NMR signal appears, which then dies awaywith a characteristic relaxation time or decay rate known as
T2. The amplitude of the signal immediately after the pulse isan indication of the total amount of fluid present, while theT2 of the signal gives valuable information about the physicalenvironment of the liquids.
In pores filled with a single fluid, there are two main
components to the NMR signal – one coming from the fluid
far from the pore walls, and the other from fluid close to thepore walls. Fluids far from the pore walls give NMR signals
similar to those from bulk fluids, with relatively long relaxation
times, while fluids close to the pore walls undergo a process ofadsorption and desorption with the pore walls which has theeffect of dramatically reducing their NMR relaxation times.

NMR and Core Analysis

In large pores, the dominant effect is from the bulk fluids, so larger pores tend to have longer NMR relaxation times. In smaller pores,
the surface-to-volume ratio is much higher, so the fluids close to the pore walls tend to dominate the NMR signal, and smaller poresshow overall shorter NMR relaxation times. This process is illustrated in the figures below.

Signals from fluids insmaller pores are modified
by surface interactions

Fluids in large
pores havelong T2 decay
times, likebulk fluids

Of course, in a real measurement it is not possible to take NMR
measurements from individual pores. The whole core must be
measured at once, so the resulting NMR signal is a composite of
all the NMR signals from the different pore sizes in the core.

++Technical Datasheet

The next step is to use a mathematical procedure known as an inversion to process the composite NMR signal and separate
it out into its component parts. In theory, there is one T2 component for each different pore size in the core. But in practice,
the analysis is usually restricted to a maximum of about 256 individual T2 components, which is more than adequate for mostpractical purposes. The result of the inversion process is a T2 distribution showing the relative population of the individual T2
decay times that make up the composite NMR signal from the core. Because long T2s come from large pores, and short T2s fromsmall pores, this T2 distribution is in effect a model of the pore size distribution in the core.

Inversion

Beyond the T2/pore size distribution – porosity, BVI,FFI, CBW and permeability
Once we have the basic pore size distribution from the NMR
data, a number of useful petrophysical parameters can bequickly and easily inferred. The integral of all the T2s (area underthe curve) gives us the total porosity when compared to thesignal from a known reference. If we centrifuge the core andthen repeat the NMR measurement, the integral of the second
data set is the irreducible fluid (Bound Volume Irreducible, orBVI), while the difference between the two is the Free Fluid
Index (FFI).

NMR and Core Analysis

It is very important to note that the area under the saturated T2 distribution curve (which gives us a measure of total porosity) isof course influenced by the shape of the curve. If the T2 curve does not represent all the T2s present in the sample, then signal
from some pores will be missing and porosity will be under-reported. This situation can occur, for example, if the instrument
being used is not capable of detecting short T2 signals from small pores, in which case porosity due to small pores will not beaccounted for. Technically, this means that the instrument being used must be able to perform sustained measurements using
short Time-to-Echo (TE) values. This effect is illustrated in the following figure, which shows three T2 distributions obtained on
the same core sample with TE=100µs (green), 200µs (red) and 600µs (blue).
It is clear that the green (TE=100 µs)
and the red (TE=200 µs) distributions
show more detail at the shorter T2values (smaller pores) than does theblue distribution (TE=600 µs). The green
distribution also records a much higher
total integral signal, corresponding toa higher porosity value. The results are
summarised in the following table:Sample

1-4R

TE (µs)AcquisitionTime (min)Number ofScans

Signal toNoise Ratio

NMR Porosity(ml)100280220.18

4.248

200

5.5

224

203.06

2.175

600

21.5

864

200.16

1.151

It is standard practice nowadays to assume that all T2s of 2.5 ms or less come from clay boundwater (CBW), so these can be integrated separately to report CBW, and thence EffectivePorosity.

Technical Datasheet

Finally, using either the Coates or the Schlumberger model,permeability can be estimated from the relaxation data. All ofthese calculations are carried out and reported automaticallyby the LithoMetrix software incorporated in every GeoSpec
instrument – and the NMR measurement is non-destructiveand much faster than conventional methods of obtaining thesame parameters.

There’s more – Capillary Pressure, Pore Throat
Distribution, Wettability and Fluid Typing
By adding an accessory called Pulsed Field Gradients (pfg)
to the NMR instrument, it becomes possible to carryout measurements dependent on the diffusion, flow, ordistribution of the fluids within the core sample. As with thestandard measurements discussed above, integrated GIT
Systems software controls the NMR instrument to deliverpfg-based measurements without expert knowledge of theNMR theory behind the measurements.

The 1-D profile records fluid saturation at between 30 and
40 points along the length of the core
The core is then spun in a standard centrifuge, which has theeffect of shifting the fluid distribution towards the outer end of
the core, and another 1-D profile with 30 – 40 points is obtained.

Capillary Pressure
One of the more exciting recent developments in the useof NMR for core analysis is the ability to measure Capillary
Pressure. Measurements that took weeks or months by
special centrifuge or porous plate can now be done by NMR
in hours or days – and with many times more data points.
The patented GIT-CAP technique is available in Green
Imaging Technologies software offerings, which are available
exclusively on Oxford Instruments’ GeoSpec instrument range.

Saturation profiles before and after the first centifugation

GIT-CAP uses a combination of conventional centrifuge anda technique called 1-D Profiling, which is a simple version ofThis process is repeated at one more centrifuge spin speed,

MRI (Magnetic Resonance Imaging) that records NMR signals

then the data is analysed. Capillary pressure is calculated fromin one dimension only – along the axis of the core.

knowledge of the centrifuge spin speeds and the changes in theHere’s how it works. After saturating a core, a 1-D profile
measurement is acquired via the NMR instrument. The result
is usually a more-or-less uniform distribution of fluid along
the length of the core, as seen in the figure top right.

saturation profile.

NMR and Core Analysis

Several points are important to notein this process:1. Only two centrifuge speeds are required,compared to eight in a traditional centrifuge
Pc measurement, so four times more
samples can be measured in a given time.
2. Each saturation profile yields 30 – 40 data
points along the core sample instead ofjust one per spin speed in the traditional
method. So typically 10 times more data
points are collected in the NMR method.
3. The NMR method uses a standard
centrifuge without stroboscope or fluid
collection measurement, so the centrifuge
Primary drainage, imbibition, and secondary drainage capillary pressure curve

itself is likely to be less expensive.

Pore Throat Distributionand Wettability
Having obtained a Pc curve, pore throat
distributions can now be calculated using
the Washburn equations. The imbibitionand secondary drainage curves generated
by the NMR Pc measurement also allow
USBM wettability to be predicted. Bothof these calculations are integrated into
the software, and follow as a naturalconsequence of the NMR Pc data. A
further added advantage is that the NMR
measurement is non-destructive and canuse reservoir fluids and reservoir wettability.

Pore throat distribution obtained from Pc data

Technical Datasheet

Fluid Typing
It was mentioned earlier that GeoSpec instruments fitted with
Pulsed Field Gradients (pfg) are able to carry out measurements
dependent on the diffusion, flow, or distribution of the fluids within
the core sample. One application of this is for fluid typing.
It is possible that different fluids, such as heavy oils and partially boundwater, may exhibit similar T2 values, which makes it difficult to identify
which fluid is which using a standard T2 distribution. However, even if
they have similar T2 values they are unlikely also to have similar diffusion
characteristics, so a measurement that collects both T2 and diffusion data
should in principle be able to separate the fluid types. GIT Systems software,
together with a pfg accessory on a GeoSpec2+ NMR instrument, enables suchmeasurements to be done. The result is a two-dimensional data map illustrating
the relative proportions of the different fluid types in the sample.

Conclusions
NMR is now a well established core analysis
tool, capable of making a wide range of coremeasurements, from pore size distributions tocapillary pressure, on a single instrument. Thepartnership between Oxford Instruments andGreen Imaging Technologies allows an integrated
hardware and software solution to be offered
to core analysts so that this wide range ofapplications can be covered by technicians with nospecialised knowledge of NMR technology.

Diffusion T2 map

GeoSpec – The new approach to core analysis using NMR

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