How to get the most out of your NMR system

Technical NoteIntroduction
There are several ways to optimise the performance of your NMR rock core system. Experts generally talk about this byreferencing the signal to noise ratio (SNR) of the instrument because SNR is directly related to measurement time.
There are generally four ways to increase SNR and thereby decrease measurement time:
1. Match sample size to probe size
2. Optimise parameter selection
3. Employ new technologies (Q-Sense)
4. Use an instrument with higher magnetic field strength
Each provides benefits, but users need to be aware of some issues with each.

Why match sample size to probe size?
Matching probe size to sample size will maximise the filling
factor during NMR measurements. The filling factor is the ratioof sample volume to probe volume and will affect acquisition
times, so this number should be as close to 1 as possible.
The theoretical highlighted results show that if a 38.1 mm
diameter core is measured in a 40 mm probe rather than in a 75
mm probe, the acquisition time is almost 20 times shorter.
Sample Size (mm)OptimalRelative Scan Times (s)Diameter

Length100 mm
probe

75 mm
probe

51 mm
probe

40 mm
probe

28 mm
probe

25.4

25.4

1016.8

181.0

38.1

50.0

108.5

19.3

3.6

1.0-

75.0

60.0

5.6

1.0---

VersusNon-optimal

In addition to better SNR and short Taus, the advanced power dissipation provided by Q-Sense also allows formeasurement of short Tau in combination with long echo trains. If the echo train is not long enough to characterise
the longer T2 components, errors will arise in the resultant distribution as shown in Figure 5.

Correct number of echoes
Not enough echoes

Figure 5. This graph shows an example of correctly acquiring the echoes and not acquiring enough echoes
on a carbonate sample (porosity=13.5%, permeability=2.24 mD).

What can a higher magnetic field do?
One way to increase SNR and reduce acquisition times is to use an NMR instrument with higher field strength (forexample 12 or 20MHz). In rocks, the relationship between SNR and field strength is somewhere between a linear andsquared relationship depending on the sample. So, using a higher field magnet will improve SNR. The main drawback ofusing higher field systems is that the results may not correlate with down-hole measurements taken at lower field.
New technologies, such as the Q-Sense offered by Oxford Instruments, avoid this problem by providing the advantagesof a higher field magnet (improved SNR and shorter Tau) but at the standard 2 MHz field strength. Users therefore gethigh SNR and short Tau, but are still able to correlate measurements with down-hole data, and work at industry standard
field strength.

How to get the most out of your NMR system

What parameters can you optimise and why?
The main parameters that affect SNR are the number of signal averages and the signal filtering employed. Traditionalsignal averaging is performed by repeating the NMR measurement and averaging the results. This increases themeasurement time linearly with the number of signal averages.
The SNR is directly related to the square root of the number of averages. Signal averaging can also be performed within
the NMR measurement, for example by acquiring multiple points on each echo during an NMR T2 measurement.
Averaging these points increases the SNR in the same way as traditional signal averaging does. This type of averaging
does not require additional measurement time but will limit the timings within the NMR pulse sequence (for example,by forcing a longer Tau in a T2 measurement meaning that fewer small pores will be measured).
Digital filtering can increase the SNR by narrowing the filter thus reducing the noise but this will also limit the timing
within the NMR pulse sequence. The filter width has an inverse relationship with SNR (small filter width leads toincreases in SNR).
Optimising the filter width and thenumber of points acquired during themeasurement can drastically improve
SNR and scanning times, as seen in thefollowing table and graph (Figure 1):
These figures show that discussionsof ‘SNR’ or ‘minimum Tau’ alone
are almost meaningless without
corresponding information on filter
values and number of data points perecho.
The main drawback of decreasing thefilter width or increasing the number ofpoints averaged within the sequenceis that these changes increase theminimum sequence timings, forexample forcing a longer Tau for T2measurements meaning that smaller
pores may be ignored. Therefore,
parameter optimisation may not be
possible in some rocks with shorter T2s
without losing information from theshort T2 components (as found in shaleand tight gas reservoirs).

Filter(kHz)

Numberof Points

Relative
Tau

Relative
SNR

Relative
Scan Time (s)

100

1

1.0

1.0

88.4

250

64

3.4

2.4

15.3

100

32

3.4

4.0

5.5

50

64

14.2

9.4

1.0

100 KHz, 1 Pts
100 KHz, 32 Pts
100 KHz, 64 Pts

Figure 1. Graph showing the noise produced by differing filters and number of pointsacquired. For example, the 100 kHz filter with only 1 point acquired produces the most noise.

If the Tau is too long, the porosity measurement is incorrect and pore information from the smaller pores islost, as shown in the figure below (Figure 2).

Tau = 50u
Tau = 100us
Tau = 300us

Figure 2. This graph shows how the T2 distribution (pore size) and measured porosity can change when differing Taus areused on shale.

Increasing the number of points acquired may cause problems when measuring rock samples with high magnetic
susceptibility (for example high iron content). High magnetic susceptibility causes the echo peaks in a T2 sequence
to have steeper sides. In a T2 measurement on such samples, if too many points are acquired, the calculated
porosity will be too low because signal is being measured on the sides of an echo peak that is no longer flat.
True Value

AverageEchoTechnical Note

Figure 3. This graph shows the effects of using single
point versus multi-point acquisition on a sandstone(porosity=14.7%, permeability=83 mD).

1 Pt per echo
70 Pts per echo

12Optimise Parameters
Match ProbeNo Optimisation

10Throughput

8

In these cases, what is being measured is a combination of poresize and magnetic susceptibility (i.e. a combination of T2 andT2*). Figure 3 shows an example of a sample that exhibits bothof these issues. The distribution with one point per echo has
the correct porosity and has the correct detail around the 10msec point.
The following graphic illustrates the benefits of using optimisation
techniques; in this example increasing sample throughput by12 times!

64200Time

1Figure 4. Optimisation on 1.5” mid permeability sandstonecore. Probe size optimised from 53 mm to 40 mm diameterand employing a Tau of 250 µsec versus 50 µsec (whichallows for increased filtering).

What can new technologies offer?
Recent advances in NMR technology have significantly improved SNR and shortened Tau by using signal enhancement
techniques. Oxford Instruments has developed the Q-Sense technology that improves SNR and sensitivity, and provides
shorter echo times. With the new Q-Sense technology, users can realise performance advantages on the industrystandard 2 MHz system that would previously only have been achievable on higher field instruments.
These new technologies mean you can:
• Measure difficult samples quickly and accurately.
• Measure rocks with low porosity faster due to improved SNR.
• Measure rocks with very small pore sizes more accurately due to shorter Tau measurement times.
• Measure difficult carbonates with very small and very large pore sizes better due to short Tau times and the ability
to handle high power and long duty cycle RF pulses (i.e. long echo trains).
• Increase the stability of the instrument due to the low/non-existentprobe detuning effects for almost all rocks.

How to get the most out of your NMR system

Recommendations for consideration
Optimising parameters is the simplest and most economical
method of increasing the throughput in the laboratory but
it is also the easiest way to create errors in the results. Users
should not begin the parameter optimisation process unlessthey fully understand the impact on their data. Software
products such as GIT Systems integrate many of theparameter optimisation methods discussed herein and help
ensure that testing results are accurate for users.
For a relatively modest expense, a user can optimise probe
size for sample size by having more than one probe for each
NMR instrument. That way the probes can be interchanged
depending on the core being tested.
Although new NMR technologies and higher field systems
are a higher capital expense, they do provide opportunities
to significantly increase laboratory throughput and toemploy newer testing methodologies. This brochure wasprepared to provide a sampling of techniques that can help
enhance NMR rock core testing.
Our NMR Rock Core Team is available to discuss what
may be the best way to enhance the performance ofyour existing NMR system or what new technologies mayenhance the testing you can currently perform.

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