NMR Core Analysis for Unconventional Reservoirs

Application Note 1

Oil and gas production is highly dependent on understanding key properties of reservoirrock, such as porosity, permeability and wettability. Geoscientists have developed a variety ofapproaches to measure these properties, including log and core analysis techniques.
Advantages of NMR

MethodLow field nuclear magnetic resonance, NMR, is routinely used

To visualise the fluids in small pores, the measurement parameterin the oil exploration industry to examine the T2 relaxation

known as TE (Time to Echo) must be as short as possible. The

time distribution of fluids in core plugs. These distributions canOxford Instruments GeoSpec rock core analyser acquires

be interpreted to give information on pore size distributions,data with TE as short as 60 µs, the best commercially available

permeability, porosity, free fluid index and bound water volumes.

performance in the industry. This class-leading performance is a

Instruments for routine measurements of relaxation distributions
generally operate at 2MHz in order to reduce artefacts causedby ferromagnetic material in the samples and to matchdirect result of our newly developed Q-Sense enhanced signal
sample probe. This has substantially decreased acquisition times
while simultaneously increasing signal to noise ratio.

the operation of the NMR logging tools. Laboratory-based

In a typical laboratory NMR core analysis procedure, a sample

instruments are used to help calibrate downhole logging tools

is scanned when fully saturated with one fluid (like brine orand for more accurate and advanced measurements on cores.

oil). This gives the T2 distribution which represents the pore

Higher field instruments are also available for use inapplications where greater sensitivity is required – for example,in imaging studies.

size distribution within the core sample. The shorter the echo
time provided by the NMR instrument, the shorter T2 thatcan be resolved, enabling smaller pore sizes to be detected.
This is especially important in unconventional reservoirs, morespecifically, shale oil, gas and heavy oil reservoirs.
In unconventional reservoirs, the very small pores are often thedominant contribution to the total porosity of the core samples,
which is a key parameter in determining the estimated outputof any reservoir. Before the short echo times provided by theGeoSpec instrument, measurements were being taken using
longer echo times resulting in an undervalued total porosity andcorresponding undervalued reservoir potential.

NMR Core Analysis for Unconventional Reservoirs

Results
To illustrate the performance of the GeoSpec, data are compared on three different shale samples. Table 1 shows the results
obtained at different echo times. All scans were run using the following parameters: Recycle delay: 500 ms, Gain 54 dB, T2 Max:
100 ms. Echo times used were 100 µs, 200 µs and 600 µs.

Sample

1-1R

1-4R

1-11R

TE (µs)(=2xTau)

Acquisition
Time (min)Number ofScans

Signal toNoise Ratio

NMR Volume
(ml)

100

1.5

64

203.72

4.512

200

7272

201.741.983

600

321312200.71

0.939

100

280

220.18

4.248

200

5.5

224

203.06

2.175

600

21.5

864

200.16

1.151

100

5192

200.48

2.309

200

17640

203.93

1.271

600

46

1840

200.81

0.754

The results show that shorter TE times produce fastermeasurements and detect more signal in these samples. Thismeans that a significant amount of signal comes from short
T2 relaxation times and thus smaller pores. This is further
confirmed in the adjacent figures.
Using the Green Imaging Technologies (GIT) Systems software,
the adjacent figures show the T2 pore size distribution curves
derived from the acquired T2 NMR data at different echo times
for the three samples. This gives an indication of pore size
distribution in the samples.
In the figures, the green line represents T2 pore size
distribution acquired at 100 µs echo spacing, the red line
represents T2 pore size distribution acquired at 200 µs echo
spacing and the blue represents T2 pore size distribution
acquired at 600 µs echo spacing.

Application Note 1

It is evident from the variation in the curves for thethree samples that more signal is acquired at shorter echo
times. In all three graphs, the green line is representing asubstantially greater variance of volumes and is a bettermeasure of the actual pore size distribution in the coresample. It will also provide a more accurate porosity
input for estimations of reservoir potential.
Figure 1: Sample 1–1R

Figure 2: Sample 1–4RFigure 3: Sample 1–11RConclusion
It is clear that shorter echo time measurements are highly beneficial when calculating total
porosity and pore size distribution, as the difference in detected NMR volume is greater by afactor of two or more for shorter echo times.
The shorter echo and acquisition times provided by the GeoSpec range of core analysers
and enhanced by the GIT Systems software, provides one of the quickest and easiest ways to
get an accurate pore size distribution and total porosity measurement of core samples. Moreaccurate and timely data inputs make for a more comprehensive and timely reservoir model.

The Advantages of the Q-Sense Technology

Improved Signal to Noise for faster analysis: Q-Sense offers
a 2-fold improvement in Signal to Noise Ratio over existing rockcore analysers. This major improvement in sensitivity allows areduction in experimental time by a factor of four.
Improved sensitivity for measurements on very low

For more information: magres@oxinst.com
www.nmr.oxinst.com
UKporosity rocks: With its significant improvement in sensitivity,

Tubney Woods, Abingdon, Oxfordshire, OX13 5QX, UK

Q-Sense provides scope to measure very low porosity samples

Tel: +44 (0) 1865 393 200 Fax: +44 (0) 1865 393 333(2% or less) within a reasonable time frame when the penalty forexperiments on very low porosity rocks used to be measurement
times in excess of one day.
Shorter echo times to characterise tightly bound rocks: TheUSA
300 Baker Avenue, Suite 150, Concord, MA, 01742, USA
Tel: +1 978 369 9933 Fax: +1 978 369 8287

fast recovery of the Q-Sense system allows the T2 measurement

Chinaof samples with echo times down to 60μs. This is particularly

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

important for characterising fluids in very small pores, such as clay

200233, China

bound water and gas shale where T2 values are typically of theTel: +86 21 6073 2925 Fax: +86 21 6360 8535

order of milliseconds or less.
Low susceptibility to probe detuning for improved ease ofuse: NMR probes fitted with Q-sense technology are now much

Green Imaging Technologies

less susceptible to detuning which means there is often no need

For more information: info@greenimaging.com

to retune the probe over a wide range of rocks.

www.greenimaging.com

External power dissipation for reduced thermal effects:

CanadaQ-Sense technology is fitted with active energy dissipation so the RF

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

power deposited from the transmitter does not affect the sample

Canadatemperature during T2 measurements, especially when samples areToll Free: +1 888 944 8462

measured over a large number of closely-spaced echoes.
Improved power handling capability for better 180º pulses:
Q-Sense provides an improvement in 180º pulses. This results in T2
distributions with less artefacts and therefore fewer false readings.
Built-In Calibration feature for a better accuracy over a wide
range of conductive rocks: Q-Sense probes have a built in signal
injection coil that can be used to calibrate the system sensitivity toensure better accuracy in porosity measurements on samples withhigh saline content and magnetically susceptible rocks.

3374

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