Eliminate sample heating during common
NMR measurements

Application Note 7

Nuclear Magnetic Resonance (NMR) measurements such as T2 distributions, T1-T2 andT2-diffusion maps are employed to derive porosity, wettability, fluid types and other properties.
These measurements are derived using the Carr-Purcell-Meiboom-Gill (CPMG) NMR pulse
sequence [1]. This pulse sequence employs a train of radio frequency pulses which probe
the exponential decay of the NMR signal. In order to probe the fastest decays, the pulses atthe beginning of the CPMG pulse train must be close together. For slower decays, the pulse
train must be long. The large number of radio frequency pulses will heat the sample throughinductive heating which will alter the NMR decay itself. By smartly spacing the RF pulses, all thedecays can be measured with far fewer pulses thus significantly reducing the heating.
In a typical CPMG sequence, the pulses in the train are

decreasing the density of the pulses as the train gets longer.

linearly spaced. This results in trains with many closely spaced

This leads to a reduction in the total number of pulses andpulses, leading to heating of the sample. Heating is an issue

therefore a reduction in the heating of the sample. We havebecause as the sample’s temperature increases, its NMR signal

implemented these logarithmically spaced CPMG sequences

decreases. This can lead to inaccuracies in the porosity derived

and will show that these sequences still accurately reproduce

from the NMR data. In addition, the petrophysical property

the NMR measurements for all samples, while eliminating

being measured may be temperature dependent and thus

sample heating.

there is no control of an important property. For porosity,
we have explored the degree of inaccuracy and have foundthat heating can lead to changes in retrieved NMR porosity ofKey Findings:

approximately 10% in both bulk fluid and core samples. We• Traditional CPMG sequences can heat samples leading

have also explored the effect of salinity of the sample fluid.
One method to eliminate the unwanted heating of samples
is to reduce the number of RF pulses by logarithmicallyspacing the echoes (Figure 1). This logarithmically spaced
train will maintain the closely spaced pulses at the beginningof the sequence necessary to probe the smallest pores, while

to errors in the porosity of up to 10%.

•	The NMR response often includes a temperaturedependency which will lead to difficulties in interpretation
if the temperature is changed by the measurement itself.

•	Log spaced CPMG eliminates sample heating by reducing
number of pulses by up to 2 orders of magnitude.

• Log spaced CPMG measures the same decay response 		
with far greater accuracy (porosity variation < 1%) andno temperature affects.

Eliminate sample heating during high duty cycle NMR measurements

Figure 1: Linear vs. log spaced CPMG sequences. In the linear sequence the spacing between each 180 degree RF pulse is equal (Tau).
In the logarithmic spaced sequence the spacing between each RF pulse grows exponentially.

Experiments:
All the NMR experiments were carried out using an Oxford

To properly quantify the effects of heating by the CPMG

Instruments GeoSpec 2+/75 rock core analyser [2]. The pulse

sequences in experiments two and three, the volumes

sequence development and data acquisition were performed

retrieved from the T2 distributions needed to be plotted asusing Green Imaging Technologies App Builder software

a function of time. To do this, new pulse sequences were[3]. Three sets of experiments were carried out to explore

developed where the CPMG T2 distribution measurement

reducing heating during NMR measurements by employing

was put inside a loop. Each time through the loop, thea log spaced CPMG sequence. The first set of experiments

CPMG pulse sequence was run several times. This raw NMR

explored the ability of the log spaced pulse sequence todecay data was then averaged and a T2 distribution createdaccurately reproduce pore size distributions. The second set

from this averaged data. The measured volume was thenof experiments were designed to calibrate the change in NMR

retrieved by summing the area under the T2 distribution.

volume observed due to sample heating by the CPMG to theThe volume was then saved to a file along with a date/actual change in temperature of the sample. The final set oftime stamp for each volume measurement. This data wasexperiments explored the effect of heating of different types

then used to plot volume versus time. Changes in retrievedof samples (bulk vs. core and 2% vs. 20% brine saturation)

volume over time were examined as the sample was heated

via CPMG pulse sequences and examined to what degree log

by the CPMG sequence. The same looping pulse sequencespaced sequences can reduce this heating.

was used to retrieve the data of interest with both the log
and linear CPMG pulse sequences.

Application Note 7

Experiment 1: Pore size distribution correlation
To test the ability of the log spaced CPMG sequence toFigure 2 shows the comparison of the T2 distribution

accurately reproduce pore size distributions, three glass filled

retrieved for the linear CPMG sequence (Figure 2 – bluesamples (2.5 cm diameter x 1 cm high) were stacked into thetrace) to that retrieved from the log spaced CPMG (Figuremagnet. Each sample contained approximately 2.5 ml of 2%2 – red trace). Table 1 shows the NMR parameters

KCl brine. Each sample was doped with different amounts ofemployed for both the linear and log spaced CPMG

copper sulfate which is paramagnetic and causes the principal

sequences employed. As mentioned in the experimental

T2 component for each sample’s distribution to be shifted.

section, this T2-distribution was retrieved from three

The higher the copper sulfate concentration, the lower thesamples with three different peak T2 values stacked on top

peak T2 value. The three samples employed had peak T2of each other in the magnet. Figure 2 shows that there

values at approximately 0.5 ms, 10 ms and 100 ms. Whenis excellent agreement between the distributions derived

the three samples were stacked together in the magnet, theyfrom the linear CPMG sequence as compared to the log

produce one combined T2 distribution with three distinct

CPMG sequence. The total volume of the three samples

peaks. This distribution was then probed with both the linear

derived from the log and linear spaced sequences differs

and log CPMG sequences.

by less than 1%.

Figure 2: Comparison of T2 distributions retrieved from the linear spaced CPMG (blue trace) and log spaced CPMG (red trace) pulse sequences
retrieved from three samples with three different peak T2 values stacked on top of each other in the magnet.

Eliminate sample heating during high duty cycle NMR measurements

Log Spaced CPMG

Linear Spaced CPMG

Tau (µs)

Log spaced between 100

Number of Echoes

512

2500

Max T2 (ms)100100Recycle Delay (s)

0.5

0.5

Number of Scans

1616

P90 (µs)

11.22

11.22

P180 (µs)

22.42

22.42

NMR Resonance (MHz)

2.457

2.457

1380

100

Table 1: NMR parameters for comparison of performance of log vs linear spaced CPMG

Experiment 2: Calibration of NMR volumeto temperature variationsThe second set of experiments set out to quantify the effect ofCPMG heating by tracking the volumes measured as a functionof time. As mentioned earlier, as a sample heats there will be adecrease in the volume measured from the NMR data. The blue
dots in the upper panel of Figure 3 show the associated change
in NMR volume as a bulk 2% KCl sample is probed as a functionof time. The volume clearly drops from approximately 17.3 ml to16.7 ml during the experiment. It should be noted that the scatterin the blue dots is associated with instability in the NMR data andis not truly caused by variations in temperature. The NMR volumedata can also be plotted as a percentage change (Figure 3 – lowerpanel – blue dots). Also shown in the upper panel of Figure 3
is the sample temperature as a function of time. This data wasrecorded simultaneously with the NMR volume using an infrared
thermometer and shows that the sample increases in temperaturefrom approximately 27.50 C to 34.75 C. As expected, there wasclearly an inverse correlation between the temperature of thesample and the observed NMR signal or volume. Specifically, theNMR signal is tied to temperature via a Boltzmann distribution.
Therefore, the observed sample temperatures can be employed topredict the change in observed NMR volume.

Application Note 7This was completed for the temperature
data recorded and the predicted percent
change in NMR volume is plotted as thered dots in the lower panel of Figure 3.
As can be seen there is good agreement
between the measured change in NMR
volume and predicted change in NMR
volume. Any difference between thepredicted and measured change in NMR
volume can be attributed to the fact that
the IR thermometer measures the surface
temperature of the brine whereas theNMR measurement probes the entire bulk
volume. The surface temperature could
be cooler than the average temperature of
the bulk sample as it is cooled by the air
above the sample.

Figure 3: NMR volume of a bulk 2% KCl sample (blue dots – upper panel) measured as a function of time by a linear CPMG sequence.
The temperature of the bulk sample (red dots – upper panel) was also logged simultaneously to the NMR measurement with an infraredthermometer. The lower panel shows the NMR volume plotted as a percent change (blue dots). The red dots in the lower panel are the percent
change in NMR volume as predicted via the Boltzmann distribution using the temperature of the bulk sample.

Eliminate sample heating during high duty cycle NMR measurements

Experiment 3: Log versus linear CPMG heating changes
Once we were confident that the observed changes in NMR volume accurately reflected heating of the samples by the CPMG
sequence, we began to study sample heating with both the log and linear CPMG sequences on various sample types. Table 2summarizes the NMR parameters employed for the log and linear CPMG pulse sequences employed.

Figure 4: Plots of percent change in NMR volume measured with both the linear spaced CPMG pulse sequence (blue trace) and the log
spaced pulse sequence (red trace). The log spaced CPMG sequence clearly reduces heating for both bulk samples (upper two panels) andsaturated sandstone samples (lower two panels).

Figure 4 compares the percent change in NMR volumeIt should be noted that there remains a small change in themeasured with the log and linear CPMG pulse sequences for

NMR volume measured by the log CPMG sequence. Thisthe samples tested (Bulk 2% KCl, Bulk 20% KCl, sandstone

indicates that despite having fewer echoes then the linear

saturated with 2% KCl, sandstone saturated with 20% KCl).

CPMG sequence, the log sequence still heats the sample

For each of the samples, there is clearly more change in theto a certain degree. This heating however amounts to only

volume measured for the linear CPMG sequence as comparedapproximately 1% change in the NMR volume measured.

to the log sequence. For example, for the upper right panel ofFigure 4 the percent change in the measured NMR volume isapproximately 7% for the linear CPMG sequence (blue dots).
Conversely a percent change of only approximately 1% for thelog CPMG sequence (red dots) is observed.

The linearly spaced CPMG data presented in Figure 4 also
allows interesting conclusions to be drawn on the difference insample heating between bulk vs core samples and the effect ofsalinity on sample heating. Firstly, if we compare the linearly
spaced CPMG data (blue dots) in the upper two panels of theApplication Note 7figure, we see that the higher salinity bulk sample heats more than the lower salinity sample (7% reduction in NMR volumefor higher salinity sample vs. 4% reduction in NMR volume for lower salinity sample). This is expected because the increased
salt concentration of 20% KCl brine lowers the specific heat capacity of the water allowing it to reach a higher temperature
as compared to the 2% KCl brine. The same comparison of salinity for the core samples (Figure 4 – lower panels – blue dots)shows that again when saturated with the higher salinity brine the core sample heats more than when it saturated with thelower salinity brine. (8% reduction in NMR volume for higher salinity sample vs. 7% reduction in NMR volume for lower salinitysample). The effect however is not as pronounced as with the bulk samples.

Bulk KCl Samples(2% and 20%)

KCl Saturated(2% and 20%) Sandstone Sample

CPMG

Log

LinearLog

LinearTau (µs)

Log spaced between 50→ 179550

Log spaced between 50→ 1795

50

# of Echoes

5125000

5125000

Max T2 (ms)

100

100

100100Recycle Delay(s)

0.75

0.75

0.750.75# of Scans

1616160160# of Meas.

1000

1000

100100

Table 2: NMR parameters for study of heating by CPMG sequences in pore size distribution measurements.

Eliminate sample heating during high duty cycle
NMR measurements

Conclusion:

Oxford Instruments Magnetic ResonanceThis paper presents results showing that the linearly spaced RF

For more information: magres@oxinst.com

pulses in CPMG sequences (employed in T2 distributions andT1-T2 maps) can deposit excess energy into samples leadingwww.oxinst.com/geospec

to unwanted heating. This heating can lead to inaccuracies

UKin the measurements retrieved from the NMR data. ThisTubney Woods, Abingdon, Oxfordshire, OX13 5QX, UK

work has shown that depending on the type of sample (bulkTel: +44 (0) 1865 393 200 Fax: +44 (0) 1865 393 333

vs sandstone core), the pulse parameters employed and thesalinity of the brine, heating can lead to a discrepancy ofup to 10% in the pore volume or porosity retrieved from T2

USA
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A new pulse sequence was presented which eliminates theChina

heating of samples by the many pulses in a linearly spaced

Floor 1, Building 60, No.461, Hongcao Road,

CPMG pulse sequence. This new pulse sequence employs

Shanghai, 200233, China

logarithmically spaced pulses. This new sequence maintains

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the closely spaced pulses at the beginning of the sequencenecessary to probe the smallest pores while decreasing thedensity of the pulses as the train gets longer. This leads

Green Imaging Technologies

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the total number of pulses necessary to accurately probe

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the exponential decay of the NMR signal and therefore
a reduction in the heating of the sample. This work has
shown that this new sequence can accurately reproduce T2

Canada520 Brookside Drive, Suite B, Fredericton, NB, E3A 8V2,

distributions and retrieve correct pore volumes from these

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in this modified CPMG sequence lead to no unwanted heating

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of the sample.
References
1.	Meiboom, S. and Gill, D., “Modified Spin‐Echo Method for Measuring
Nuclear Relaxation Times”, Review of Scientific Instruments (1958),
29, 688-691
2.	Geo-Spec 2-75 User Manual, Version 1.8, Oxford Instruments.
3.	GIT App Builder User Manual, Version 1.1 Green Imaging Technologies.
For further information:
M. Dick, D. Green, D. Veselinovic , “Reducing Sample Heating During NMRMeasurements”, Society of Core Analysts Annual Symposium, Pau France, 2019
M. Dick, D. Green, D. Veselinovic, “Accurate Pore Size Measurement Via NMR
On Unconventionals”, Unconventional Resources Technology Conference,
Colorado USA, 2019

5161

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© Oxford Instruments plc, 2019. All rights reserved. Ref: MR/211/0619