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WAG-CO2 process :
pore- and core-scale experiments
© IFP
M. Robin, V. Sygouni, J-P Duquerroix
S. Bekri, S. Gautier, O. Vizika and E. Fernandez Righi
Introduction
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© IFP
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Among the parameters affecting the flow processes
(Vizika et al, 1994, Avraam and Payatakes, 1999), the
pore surface wettability is very important.
It depends both on the mineralogy and on the
physicochemical properties of the used fluid system
(Hirasaki 1991, Anderson 1986, Drummond and
Israelashvilli, 2002).
It strongly affects the flow mechanisms and therefore
the oil recovery (Cuiec, 1990, Jadhunandan and
Morrow 1995, Schramm and Manhardt 1996, Bradford
and Leij 1995, Odlenburg et al 2001).
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IEA Collaborative Project on EOR - 30th Annual Workshop and Symposium - 21-23 September 2009, Canberra, Australia
Introduction
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© IFP
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Both enhanced oil recovery and CO2 storage in
reservoirs turned the Water Alternating Gas (WAG)
injections to an interesting process (Christensen et al,
1998, Olsen et al, 1992, Akervoll et al, 2000).
In this paper we try to investigate the Water Alternating
CO2 step by step starting from micromodel and core
experiments aiming to a future modeling of the
hysteretic relative permeability curves.
Thus, gas injection experiments have been performed
both in transparent micromodels and in a composite
core. The visualization experiments in micromodels
have been conducted under reservoir conditions
using dead oil and reconstituted brine.
IEA Collaborative Project on EOR - 30th Annual Workshop and Symposium - 21-23 September 2009, Canberra, Australia
Spreading coefficient (water wet system)
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The fluid distribution in the pore network depends on the value of
the spreading coefficient S:
S

wg


ow


og
S > 0 : oil can easily flow
within the porous medium
© IFP
S < 0 : oil can be trapped
within the pore space
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(Vizika et al. 1994)
IEA Collaborative Project on EOR - 30th Annual Workshop and Symposium - 21-23 September 2009, Canberra, Australia
Experimental set-up (micromodels)
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© IFP
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The confining cell containing
the micromodel is designed
to be able to work up to 200
bars; it is equipped with an
integrated heating device
allowing experiments up to
60°C.
Two sapphire windows, one
located at the top and the
other at the bottom of the
confining cell, allow lighting
and visualization.
IEA Collaborative Project on EOR - 30th Annual Workshop and Symposium - 21-23 September 2009, Canberra, Australia
Glass micromodels
The micromodel consist of two glass plates; one
supports the etching representing the “porous”
network and the other the entrance and the exit of the
fluids, the average size of the circular grains (circles)
is 0.3 mm.
© IFP
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IEA Collaborative Project on EOR - 30th Annual Workshop and Symposium - 21-23 September 2009, Canberra, Australia
Experimental conditions
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Three wettability conditions were reproduced : clean
state (WW), restored state (aged in oil for two
months) and silanated state (OW).
Two different initial saturations were investigated
prior to gas injection : Swi and Sor.
The fluids which were used were reconstituted brine,
stock tank oil (dead oil) and CO2.
Visualization experiments have been performed
under 2 reservoir conditions:
© IFP
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60 bars and 55°C / CO2 under gaseous conditions (standard
conditions)
90 bars and 55°C (SC CO2)
IEA Collaborative Project on EOR - 30th Annual Workshop and Symposium - 21-23 September 2009, Canberra, Australia
Clean state (WW) : CO2 injection at Swi
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© IFP
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Oil is brown, brine is white and CO2 is grey.
Brine is either present under the form of a film.
surrounding the grains or under the form of pendular
rings. This is a typical situation of a water-wet system.
Oil phase remains a continuous one and oil spreads on
the brine. The spreading coefficient is positive.
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IEA Collaborative Project on EOR - 30th Annual Workshop and Symposium - 21-23 September 2009, Canberra, Australia
Clean state (WW) : CO2 injection at Sor
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© IFP
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Both contact angles and fluid distribution show that this
porous medium is water-wet.
Oil spreads on brine film and is found in between the
brine and the gas phase. On the third picture (red circles),
some gas blobs are observed in the middle of the pore
bodies, surrounded by oil.
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IEA Collaborative Project on EOR - 30th Annual Workshop and Symposium - 21-23 September 2009, Canberra, Australia
Restored state : CO2 injection at Swi
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© IFP
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This is a typical situation of a water-wet system.
These results are fairly similar to the ones obtained
for the clean state. Aging has not significantly altered
the wettability of the micromodel.
It can clearly be seen that the oil phase spreads on
the brine; it can be deduced that the spreading
coefficient is positive.
IEA Collaborative Project on EOR - 30th Annual Workshop and Symposium - 21-23 September 2009, Canberra, Australia
Restored state : CO2 injection at Sor
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© IFP
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Oil spreads on the brine film and is found in between
the brine and the gas phase. Sometimes, it forms
bubbles in the center of some pores.
On the second picture (red circles), some gas blobs
are observed in the middle of the pore throats,
surrounded by oil.
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IEA Collaborative Project on EOR - 30th Annual Workshop and Symposium - 21-23 September 2009, Canberra, Australia
Capillary pressure curves
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© IFP
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In order to compare the micromodel wettability results
with the wettability of a real core, the capillary
pressure curves were measured by centrifuge for both
cleaned and restored plugs.
These results are fairly similar to the ones obtained for
the micromodels (cleaned and aged).
Aging has not significantly altered the wettability
neither of the micromodel, nor of the plugs. The
wettability after aging could be characterized as
intermediate.
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IEA Collaborative Project on EOR - 30th Annual Workshop and Symposium - 21-23 September 2009, Canberra, Australia
Silanated state (OW) : CO2 injection at Swi
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© IFP
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It can be seen that the medium is preferentially oil-wet.
There is no contact between the brine and the glass.
The CO2 has flown through the oil phase. Oil is the
only phase which is in contact with the pore walls. It
seems that CO2 is always separated from the pore
walls by an oil film (red circles).
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IEA Collaborative Project on EOR - 30th Annual Workshop and Symposium - 21-23 September 2009, Canberra, Australia
Silanated state (OW) : CO2 injection at Sor
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© IFP
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It can be seen on these pictures that the medium is
oil-wet. Dark films of oil covers the grains and it is
also under the form of pendular rings.
Brine is included within the oil phase (red circles).
There is no contact between the brine phase and
the CO2. They are always separated by an oil film.
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IEA Collaborative Project on EOR - 30th Annual Workshop and Symposium - 21-23 September 2009, Canberra, Australia
Silanated state (OW)
brine and CO2 (90 bars 55°C : SC CO2)
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© IFP
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Left picture represents a silanated (OW) porous medium,
saturated with brine, after CO2 injection. It can be noticed
that the pore walls wettability is intermediate.
On many occurrences (red circles) the observed contact
angles tend to show that the system is even slightly
preferentially CO2-wet.
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IEA Collaborative Project on EOR - 30th Annual Workshop and Symposium - 21-23 September 2009, Canberra, Australia
Silanated state (OW)
Three phase (90 bars 55°C : SC CO2)
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© IFP
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Silanated (OW) porous medium, saturated with brine, oil, and
supercritical CO2. It can be seen on these pictures that the
medium is oil-wet.
Dark films of oil covers the grains and it is also under the
form of pendular rings. Brine is included within the oil phase
(red circles). There is no contact between the brine phase
and the CO2, they are always separated by an oil film.
IEA Collaborative Project on EOR - 30th Annual Workshop and Symposium - 21-23 September 2009, Canberra, Australia
Remarks after micromodel experiments
In water wet micro models:
 the spreading coefficient is positive
© IFP
In oil wet models:
 when CO2, brine and oil coexist, the CO2 is a nonwetting phase whereas the oil is the wetting phase.
 when only two phases, CO2 and brine, are present,
the CO2 may be the wetting phase.
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IEA Collaborative Project on EOR - 30th Annual Workshop and Symposium - 21-23 September 2009, Canberra, Australia
© IFP
Experiments in a composite core
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Plug
Length
(cm)
Diameter
(cm)
Porosity
(%)
Permeability
(mD)
Swi
1
5.98
4.9
16.2
2.7
0.26
2
5.98
4.9
15.9
1.6
0.28
3
5.98
4.9
16.4
2.6
0.28
4
5.98
4.9
15.7
1.8
0.29
IEA Collaborative Project on EOR - 30th Annual Workshop and Symposium - 21-23 September 2009, Canberra, Australia
Experimental conditions
Flow experiments performed in a composite under ambient
conditions.
 Experiments :
1.
2.
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Gas injection at Swi
Brine injection at Swi
Fluids :
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Gas : N2
Oil : dodecane
Brine
Dodecane
Brine
Density (g/cm3)
0.749
1.1
Viscosity (cp)
1.37
1.08
IFT (mN/m)
35.5
© IFP
The two phase flow relative permeabilities were calculated
with history matching using Pumaflow.
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IEA Collaborative Project on EOR - 30th Annual Workshop and Symposium - 21-23 September 2009, Canberra, Australia
Gas injection at Swi
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© IFP
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The gas injection at the desired Swi was performed
under constant pressure drop (0.5, 1.8 and 3.8 bars),
and a backpressure of 5 bars was applied.
The oil and gas relative permeabilities (Kro and Krg),
were calculated by history matching using the IFP's
reservoir code "Pumaflow" (adapted to laboratory
conditions).
Experimental and simulated results are in good
agreement.
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IEA Collaborative Project on EOR - 30th Annual Workshop and Symposium - 21-23 September 2009, Canberra, Australia
Gas injection at Swi
Saturations
Initial
Final
Oil
0.73
0.27
Brine
0.27
0.27
Gas
0.00
0.46
Average saturations in the composite
before and after gas injection.
1
Relative permeabilities
calculated with Pumaflow
Kro, Krg
0,1
0
0,2
0,4
© IFP
0,8
Krg
Kro
0,01
0,001
0,0001
0,00001
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0,6
0,000001
Sg
IEA Collaborative Project on EOR - 30th Annual Workshop and Symposium - 21-23 September 2009, Canberra, Australia
Gas injection at Swi
© IFP
Experimental and simulated oil
and gas production
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IEA Collaborative Project on EOR - 30th Annual Workshop and Symposium - 21-23 September 2009, Canberra, Australia
© IFP
Gas injection at Swi
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Experimental and simulated gas saturation vs. time
IEA Collaborative Project on EOR - 30th Annual Workshop and Symposium - 21-23 September 2009, Canberra, Australia
Brine injection at Swi
Saturations
Initial
Final
Oil
0.73
0.16
Brine
0.27
0.84
Average saturations in the composite
before and after brine injection.
1,2
Relative permeabilities
calculated with Pumaflow
© IFP
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Kro, Kr w
Brine relative permeability is
very low and rapidly increases
after the breakthrough.
1
0,8
Krw
Kro
0,6
0,4
0,2
0
0
0,2
0,4
0,6
Sw
IEA Collaborative Project on EOR - 30th Annual Workshop and Symposium - 21-23 September 2009, Canberra,
Australia
0,8
1
Brine injection at Swi
Experimental and simulated oil
production and pressure drop
© IFP
The displacement seems to be
frontal due to the late
breakthrough after which the oil
production is almost stabilized.
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IEA Collaborative Project on EOR - 30th Annual Workshop and Symposium - 21-23 September 2009, Canberra, Australia
Brine injection at Swi
© IFP
Experimental and
simulated water
saturation vs. time
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IEA Collaborative Project on EOR - 30th Annual Workshop and Symposium - 21-23 September 2009, Canberra, Australia
Remarks after core flow experiments
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The N2 injection is less efficient than the brine
injection at Swi, where the displacement was frontal,
and where, after the breakthrough, the oil recovery
was almost stabilized.
© IFP
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During the N2 injection at Swi, fingering and early
breakthrough occurred
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IEA Collaborative Project on EOR - 30th Annual Workshop and Symposium - 21-23 September 2009, Canberra, Australia
Conclusions
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Visualization experiments have been conducted in micromodels of
different wettabilities. The aging process in dead oil did not strongly
alter the wettability in both micromodels and cores. From the
micromodel experiments it is shown that the fluid distribution varies
with the surface wettability and the experimental conditions. The
temperature and pressure conditions under which an experiment is
performed determine the CO2 behavior which does not always play
the role of a non-wetting phase.
© IFP
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The aim of this study is to investigate the flow mechanisms during
CO2 / N2 injection in porous media, for varying thermodynamic
conditions and wettability, and the relative permeability curves
hysteresis that occurs during WAG experiments.
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IEA Collaborative Project on EOR - 30th Annual Workshop and Symposium - 21-23 September 2009, Canberra, Australia
Conclusions
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© IFP
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Flow experiments have been also performed in a composite core,
consisting of four plugs, in order to estimate the two-phase
relative permeabilities which will be used in the future to model a
WAG experiment.
The experiments showed that the N2 injection at Swi, where
fingering and early breakthrough occurred, is less efficient than
the brine injection at Swi, where the displacement was frontal,
and where, after the breakthrough, the oil recovery was almost
stabilized.
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IEA Collaborative Project on EOR - 30th Annual Workshop and Symposium - 21-23 September 2009, Canberra, Australia