Rock core NMR - 12MHz versus 2MHz:Why go to higher field?

Technical Paper

Rock core NMR - 12MHz versus 2MHz: Why go to higher field?
Nuclear Magnetic Resonance (NMR) laboratory based core analysis has been performed for over 20 years, and for much of that time
most of the measurements have been carried out at 2MHz. Laboratory based NMR rock core analysis was first devised to calibrate
well logs which were collected using down hole NMR tools that operated at 2MHz, so it made sense to use 2MHz instruments in thelaboratory. In recent years, as laboratory based NMR has expanded its tool box of measurements beyond well log measurements, andas more unconventional rocks are scanned, the tendency to use 2MHz is changing.
The sensitivity of NMR instruments increases with operating frequency, such that sensitivity at 12MHz is more than 10 times that at2MHz, making measurements up to one hundred times faster. While the minimum echo spacing, and thus the smallest detectablepore size, are very similar for 12MHz and 2MHz instruments, the greater sensitivity of a 12MHz instrument means that adequate signalcan be acquired from small pores in less time, with fewer scans
So, should users with 2MHz NMR benchtop systems upgrade to higher field instruments such as the 12MHz GeoSpec+? What are
the advantages? This technical note will lay out the advantages of a 12MHz GeoSpec versus a 2MHz GeoSpec with the aim ofanswering these questions.

MethodThe University of Oklahoma’s Mewbourne School ofPetroleum and Geological Engineering has completed astudy comparing the 2MHz and 12MHz NMR responses,and much of this technical note is based on their findings.
To properly compare results, one must ensure the same
samples are compared and that no alteration occurs tothe sample between measurements. Table 1 overleaf
shows the samples used in this comparison.1
The samples were scanned using a GeoSpec 2MHz
rock core analyser and a GeoSpec 12MHz rock Ccore
analyser, both owned by the University of Oklahoma.
continued overleaf

Rock core NMR - 12MHz versus 2MHz: Why go to higher field?

Samples

Porosity %Permeability md

Berea Sandstone

19157

Tennessee Sandstone

40.003

Lyons Sandstone

100.006

Fontainebleau Sandstone

7.2

25.5Kansas Limestone

188.5

Fused Glass Beads

37

35,800

Aluminum Oxide

4520,100

Eagle Ford Shale

6-

Utica Shale

5-

Woodford Shale

8-

Method continued
All samples were cleaned using Soxhlet extraction at100C, then dried in a vacuum oven for 12 hours, andfinally saturated with dodecane at 7000psi for 48 hours.1

Table 1 – Sample selection with porosity and permeability.1

Signal to Noise (SNR) Comparison
If the same measurement time (number ofscans) is used, the higher frequency of the12MHz GeoSpec results in about 15 times
more NMR signal being captured, meaning
the SNR will be substantially higher. This is
illustrated in Figure 1. As one would expect,high permeabiity samples have a higher SNR
than low permeability samples.1

Figure 1 – SNR vs measurement time. All data were collected with a TE of300µs. The circled data are from the 12MHz system while the lower data points
are from the 2MHz.1

Technical Paper

T2 Spectra Shift
When comparing T2 spectra measured at
2MHz with those measured at 12MHz,
the 12MHz spectra can be seen to shift
left and become better defined. Theleftward shift can be explained by the factthat the higher frequency of the 12MHzGeoSpec causes faster relaxation ofhydrogen protons at the pore walls, sothe observed T2s are shorter and the T2spectra shift left.
Figure 3 shows the T2 spectra from thesame sample measured at 2MHz and12MHz and illustrates the spectra shift. In
the Kansas limestone, the 12MHz spectra
is bimodal, which can be attributed tothe higher SNR of the 12MHz GeoSpec,
resulting in better visibility of the smaller
pores spaces.

Figure 3 – T2 spectra of two samples measured at 2MHz and 12MHz at Tau=75us.1

2D Correlation Maps
One of the key areas of interest in laboratory based NMR core analysis measurements in recent years has been 2D correlation
maps, which measure two parameters and correlate them in a 2D map. The two most popular are the T1-T2 map and the T2Diffusion map. Both these maps are signal intensive and if one can measure at better SNR one gets better defined maps andgets them more quickly.
As seen above with the T2 spectra shift, the 12MHz GeoSpec provides better resolution of the T2 (and T1) spectra, whichmeans the correlation between the two in a T1-T2 map is better defined, or put another way, the data populations will bebetter separated and better defined on the maps. While 2D maps are have been produced successfully at 2MHz, the 12MHz
GeoSpec instrument with Q-Sense probe technology offers exponentially better SNR and thus can measure these signal intense
measurements more quickly and with better resolution. 2D map measurements not only require 4 times as much data as a 1D
measurement, but also require the data to spread over a larger area due to the 2 dimensional nature of the measurement.

Conclusions

Oxford Instruments Magnetic Resonance

It has been determined in independent tests by Oklahoma

For more information: magres@oxinst.com

University’s Mewbourne School of Petroleum and Geologicalwww.oxinst.com/geospec

Engineering, that a 12MHz GeoSpec NMR rock core analyser
can obtain an SNR that is an average of more than 11 times
higher than the SNR obtainable on the 2MHz GeoSpec, inhigh porosity samples. In low porosity samples such as shales,
using the same number of scans in each instrument, the SNR isan average of almost
9 times higher in the12MHz instrument.
The 12MHzUKTubney Woods, Abingdon, Oxfordshire, OX13 5QX, UK
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USA300 Baker Avenue, Suite 150, Concord, MA, 01742, USA
Tel: +1 978 369 9933 Fax: +1 978 369 8287
China

GeoSpec has distinct

Floor 1, Building 60, No.461, Hongcao Road, Shanghai,advantages over200233, China

the 2MHz system in

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terms of signal tonoise ratio. The vastly
better SNR means
measurements areFor more information: info@greenimaging.com

acquired more quickly,

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and allow for better resolution in T2 spectra, more accurate

Canadaporosity measurements with tight rock cores, better defined 2D
correlation maps, and use of a wider range of signal filters.
1

Comparison of 2MHz and 12MHz NMR Responses inConventional and Unconventional Reservoir Rocks. B. Min, C.H.
Sondergeld, C.S. Rai, Society of Petrophysicists and Well Log
Analysts

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