Understanding wettability in unconventional
rocks and how it can change over time

Technical PaperIntroduction
In a recent technical paper [1], we described an NMR wettability index (NWI) measurement which we had recently implemented inour software. This measurement was based on the fact that NMR response and hence T2 distributions [2,3] change as a function ofwettability in core plug samples. In order to derive this NWI based on T2 distributions, the NMR T2 spectra of 100% brine saturated,
100% oil saturated, bulk oil and bulk brine are needed. These spectra are then mixed to give a predicted T2 spectrum which iscompared (via a least squares fit) to a T2 spectrum recorded from a sample partially saturated with both water and oil and whosewettability is to be determined. In the previous technical paper, we presented a new method which coupled this T2 based NWI tospatially resolved T2 NMR measurements to monitor changes in wettability and saturation along rock core plugs.

This technical paper describes further work we havechallenging in low permeability unconventional samples. In

completed expanding the utility of the NWI measurements

addition, the Amott or USBM method will alter saturation andon two fronts. Firstly, we have adapted the existing NWI

wettability conditions in a core sample rendering it uselessmeasurement to unconventional samples in order tofor further aging studies without cleaning, drying and re-provide robust wettability measurements for tight rocks.

saturation.

Unconventional samples prove challenging for wettability

For example, to measure the wettability change in a core

analysis because of their tight pore structure and their dual
pore networks, one associated with organics and the otherbeing the intergranular network. These two networks may
have different wetting conditions complicating wettability
analysis. Secondly, we have explored the ability of the NWI
measurement to track wettability changes in core samples

sample daily using the Amott or USBM methods for 10 days
would require 10 identical core plugs, one tested each day.
As will be shown in this technical paper, the NWI method
allows the wettability to be tracked as a function of time on
the same core plug.

as a function of time. The restoration of wettability inreservoir rock core samples to its original state is highly

Key Findings:

critical in preliminary core preparation for special core analysis•	The NMR wettability index (NWI) is a new method toexperiments [4,5]. Tracking these changes in wettability
can be difficult using Amott or USBM methods. Thesemeasurements involve forced imbibition which can beaccurately determine wettability of unconventional rockcore samples.

•	NWI can be successfully used to non-destructively monitorchanges in wettability in real time.

Understanding wettability in unconventional
rocks and how it can change over time

Experiment:Two sets of twin unconventional rock core plugs from theThe goal of this experiment was to observe wettability

same South Texas well, but different units, were used in thischanges in the unconventional samples under study. Towettability study. The physical properties of these samples areachieve changes in wettability, the decane saturated samples

summarized in Table 1. For each set of twins (Twins 1: 7-P2/7-

were submerged in brine while the brine saturated samples

P3A and Twins 2: 2-P2/2-P1A), one of the twins was brine

were submerged in decane. Spontaneous imbibition of eachsaturated (4% NaCl in water) while the other one was decane

fluid/sample combination was monitored over time and T2

saturated. See Table 1 to reference which twin was saturated

distributions were acquired periodically during the imbibition

by brine and which was saturated by decane. T2 NMR spectra

process. Figure 1 shows water droplets forming on thewere acquired on each sample in these initial states, using

surface of one of the 100% brine saturated shale samples

a GeoSpec 2+/75 NMR instrument [6] and Green Imaging

after imbibition of decane.

Technologies software [7]. Table 2 summarizes the NMR
parameters employed. T2 spectra for bulk brine and decane
were also recorded.

SampleLength(cm)Diameter Rock Type(cm)Original
State2-P2

5.21

2.58

chalk-marl

100%
Brine
Saturated

Porosity (%)
Helium, MICP

4.3, 4.1
2-P1A

4.59

2.6

chalk-marl

100%
DecaneSaturated

7-P2

4.93

2.61

marl

100%
Brine
Saturated

marl

100%
DecaneSaturated

12.5, 127-P3A

4.97

2.55

Table 1 – Sample Information.

LECO TOC(wt%)

XRD
Mineralogy (wt%)

0.39

Quartz: 3.8Feldspar: 2.3
Carbonate: 85
Total clay: 6.6
Pyrite: 1
Marcasite: 0.7

4.38

Quartz: 21.1
Feldspar: 3.9
Carbonate: 46.5
Total clay: 23.4
Pyrite: 4.6
Marcasite: 0.6

Technical Paper

SampleRecycledelay (ms)Signal toNoise Ratio

Tau(µs)Number ofEchoes

NMR Porosity(p.u.) orVolume (ml)Time(min)

2-P2

750

200

50

5000

4.7 p.u.

8.5

2-P1A

750

200

50

5000

4.3 p.u.

11.75

7-P2

3000

200

50

20000

13.7 p.u.

1.07-P3A

750

20050

5000

12.8 p.u.

1.5Bulk Decane

18750

16450

1250004.69 ml

9.0

Bulk Brine

22500

23650150000

6.38 ml

12.0

Table 2 – CPMG Parameters.

Results:1) T2 Distributions:Figure 2 shows the T2 spectra recorded for the first
30-40 days of imbibition for the 2-P1A/2-P2 twin
shale samples. The left-hand panel of Figure 3
shows several distributions recorded as twin 2-P1A(originally 100% decane saturated) imbibed brine.
The right-hand panel of Figure 3 shows several
distributions recorded as twin 2-P2 (originally 100%brine saturated) imbibed decane. At first glance, theT2 distributions recorded during imbibition seem to bechanging more rapidly for sample 2-P1A as comparedto sample 2-P2. This was an early indication thatthese twins are water wet, because the 100% decane
saturated sample (2-P1A) is readily imbibing brine
while the 100% brine saturated sample (2-P2) is not
imbibing decane as quickly.
Figure 1: Water droplets seen on the surface of thesample after it was submerged in decane.

Understanding wettability in unconventional
rocks and how it can change over time

Figure 2: The T2 spectra recorded for the 2-P1A/2-P2 twin shale samples. The left-hand panel shows several distributions recorded as twin
2-P1A (originally 100% decane saturated) imbibed brine. The right-hand panel of shows several distributions recorded as twin 2-P2 (originally100% brine saturated) imbibed decane. Also shown are the 100% brine saturated, 100% decane saturated, bulk brine and bulk decane T2distributions. This data was employed to derive the wettability and saturation as a function of imbibition time.

A similar analysis has also been carried out on twins 7-P3A/7-P2and Figure 3 shows the T2 distributions recorded during the first30-40 days of the imbibition process for each twin. The lefthand panel of Figure 3 shows the distributions recorded as twin
7-P3A (originally 100% decane saturated) imbibed brine. The
right-hand panel of Figure 3 shows several distributions recorded
as twin 7-P2 (originally 100% brine saturated) imbibed decane.
As with the 2P set of twin shale samples, at first glance the T2
distributions recorded during imbibition gives an indication as
to whether these shales are water wet or oil wet. Sample 7-P2
seem to be imbibing decane more rapidly than sample 7-P3A
was imbibing brine indicating that the sample is likely oil wet.

2) Wettability Analysis:
The T2 distributions recorded during the imbibition process
for both twins were processed through the wettability fitting
routine described in our previous technical paper [1]. Figure 4
shows the results of the wettability analysis for twins 2-P1A and2-P2. The left-hand panel displays the wettability as a functionof imbibition time for both samples, while the right-hand panel
shows the water saturation as a function of imbibition timefor both samples. Looking at the wettability change for twin
2-P1A (Figure 4 – left panel - blue trace), it is observed that therock started oil wet but changed to water wet within the first
24 hours of brine imbibition. The NMR wettability index thencontinued to increase for the next three days until it stabilizedaround 0.90 at about day five.

Technical Paper

Figure 3: The T2 spectra recorded for the 7-P3A/7-P2 twin shale samples. The left-hand panel shows several distributions recorded as twin
7-P3A (originally 100% decane saturated) imbibed brine. The right-hand panel of shows several distributions recorded as twin 7-P2 (originally100% brine saturated) imbibed decane. Also shown are the 100% brine saturated, 100% decane saturated, bulk brine and bulk decane T2
distributions. This data was employed to derive the wettability and saturation as a function of imbibition time.

During the first five days of the experiment, the water saturation

result the saturations predicted by the wettability model during

for twin 2-P1A (Figure 4 – right panel – blue trace) did not appear

this time are not accurate. Since the wettability stabilized at

as stable. It started around 30% during day one and dropped

day five, the water saturation has shown a slow increase fromto approximately 20% by day five. Clearly it is not physicallyapproximately 0.2 to 0.3 and the saturations measured by D2O

possible for a decane saturated rock which is imbibing brine

agree well with the predicted results.

to have its water saturation go down. Instead the instability
in the data during the first the first five days reflects active
changes ongoing in the rock during this period. These changes
lead to instability in the predictions. This was confirmed byindependent D2O measurements on another 2P twin sample.
The D2O measurements allowed the water saturation to bemeasured directly via NMR and are plotted as black dots in Figure
4. The D2O data confirmed the assumption that the sample isundergoing active changes during the first five days and as a

The wettability and saturation levels for sample 2-P2, decane
imbibition into brine, did not change as rapidly as for sample
2-P1A. While there were no measurements during the first fivedays of decane imbibition, neither the wettability (Figure 4 – leftpanel – red trace) nor the water saturation (Figure 4 – right panel– red trace) changed significantly between days five and thirtyfive. The wettability only slowly decreased from 0.96 to 0.83
during this time while the water saturation decreased from 97%
to 92% during the same time.

Understanding wettability in unconventional
rocks and how it can change over time

Figure 4: The results of the wettability analysis for twins 2-P1A and 2-P2 are shown. The left-hand panel shows the wettability as afunction of imbibition time for both samples, while the right-hand panel shows the water saturation as a function of imbibition time forboth samples. The black dots the water saturations as determined in a separate NMR measurement on a different 2P sample 100% D2Osaturated imbibing decane.

Figure 5 shows the results of the wettability analysis for twins

The wettability and saturation levels changed more rapidly for

7-P3A and 7-P2. These samples were tested for over ninety

sample 7-P2 as compared to sample 7-P3A. Unfortunately,days, considerably longer than the 2P samples. The left-handthere were no measurements during the first five days of decane

panel displays the wettability as a function of imbibition timeimbibition. However, after the fifth day, the wettability (Figurefor both samples, while the right-hand panel shows the water

5 – left panel – red trace) and the water saturation (Figure 5 –saturation as a function of imbibition time for both samples.

right panel – red trace) changed significantly. During the firstThe wettability change for twin 7-P3A (Figure 5 – left panel -forty days, the NWI decreased steadily from ~0.05 to -0.30 whileblue trace) shows that the NWI started around -0.7 and slowly

the water saturation remained relatively constant (~35%). After

increased to around -0.4 by day thirty of the 100% decane

day forty, both the wettability and saturation seemed to havesaturated sample imbibing brine. During this same time period,stabilized.

the water saturation for twin 7-P3A (Figure 5 – right panel –blue trace) slowly increased from approximately 15% to around
30% by day thirty. Since day thirty both the wettability andsaturations were relatively constant.

Technical Paper

Figure 5: The results of the wettability analysis for twins 7-P3A and 7-P2 are shown. The left-hand panel shows the wettability as afunction of imbibition time for both samples, while the right-hand panel shows the water saturation as a function of imbibition time forboth samples.

Both the 7P and 2P samples display a dynamic wettability
index that slowly asymptotes towards equilibrium as imbibition
proceeds. Despite significantly lower porosity than sample
7-P3A (see Table 1), sample 2-P1A, a chalk, imbibes water
more successfully than sample 7-P3A, an organic-rich marl;correspondingly, the NWI for the chalk sample is very high, > 0.8,
indicating strongly water wet. The majority of brine imbibition in
the chalk occurred very quickly, within 24 hours, and the chalkdid not readily imbibe decane, remaining highly water wet interms of NWI. Meanwhile, sample 7-P2 imbibed decane only
slightly more rapidly than sample 7-P3A imbibed brine, indicatingthat the sample is slightly oil wet. leaning towards mixed wet;correspondingly, the NWI for these sample is within the boundsof -0.4 to 0.2 for the duration of the experiment.

Understanding wettability in unconventional
rocks and how it can change over time

Conclusions:

Oxford Instruments Magnetic Resonance

The NWI has been successfully tested on unconventional

For more information: magres@oxinst.com

samples for quantifying changes in sample wettability over

www.oxinst.com/geospec

time. This is an important result for two reasons. Firstly, itUK

establishes a new method for determining the wettabilityof unconventional samples. Determining wettability of
unconventional samples can be difficult due to their tightTubney Woods, Abingdon, Oxfordshire, OX13 5QX, UK
Tel: +44 (0) 1865 393 200 Fax: +44 (0) 1865 393 333

pore structure and their dual pore networks, one associatedUSAwith organics and the other being the intergranular network.

300 Baker Avenue, Suite 150, Concord, MA, 01742, USA

Secondly, it has been shown that NWI can be employed to
non-destructively monitor changes in wettability in real time

Tel: +1 978 369 9933 Fax: +1 978 369 8287

(i.e. during a flooding experiment or an aging procedure).

China

This is potentially a powerful new tool in Special Core Analysis

Floor 1, Building 60, No.461, Hongcao Road, Shanghai,(SCAL) because unlike Amott or USBM wettability methods,

200233, Chinaa single core sample can be employed to monitor wettability

Tel: +86 21 6073 2925 Fax: +86 21 6360 8535

change as a function of time. This should lead to greater
efficiency in determining how to restore core samples to their
original wettabilities in preparation for SCAL experiments.

Green Imaging Technologies
For more information: info@greenimaging.com

References

www.greenimaging.com

1.	Technical Paper, Determination of rock core wettability by NMR, publishedby Green Imaging Technologies and Oxford Instruments in 2018.

Canada2.	Looyestijn, W.J. and Hofman, J.P., “Wettability Index Determination by
Nuclear Magnetic Resonance”, SPE 93624, presented at the MEOS, Bahrain,
March 2005. Published in SPEREE April 2006, pp 146 – 153.

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

3.	Looyestijn, W., Zhang, X., and Hebing, A., “How can NMR assess thewettability of a chalk reservoir”, Society of Core Analysts, Vienna, Austria,
27 August-1 September 2017.

Toll Free: +1 888 944 8462

4.	Spiral, E., James, L.A. “Application of an optimisation method for therestoration of core samples for SCAL Experiments”, Snowmass, Colorado,
USA, 21 August-26 August 2016.
5.	Spiral, E., James, L.A. “Application of an optimisation method for therestoration of core samples for SCAL Experiments - Part II”, Vienna, Austria,
27 August-1 September 2017.
6. Geo-Spec 2-75 User Manual, Version 1.8, Oxford Instruments.

5170

7.	GIT Systems and LithoMetrix User Manual, Revision 1.9, Green Imaging
Technologies.

CanadaTel: +1 506 458 9992 Fax: +1 506 458 9615
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