www.elsevier.com/locate/jpetscieng

Journal of Petroleum Science and Engineering 39 (2003) 297–308

Gas and oil production from waterflood residual oil: effects of

wettability and oil spreading characteristics

Carlos A. Grattonia,*, Richard A. Daweb,1

aDepartment of Earth Science and Engineering, Imperial College, Prince Consort Road, London SW7 2AZ, UKbChemical Engineering Department, University of West Indies, St. Augustine, Trinidad and Tobago

Received 12 March 2002; received in revised form 4 October 2002

Abstract

In the depressurisation of reservoirs already produced to waterflooded residual oil, solution gas is released when the reservoir

pressure drops to below the bubble point. This gas becomes mobilised when the critical gas saturation has been reached.

Additionally, the oil itself can become mobile from its residual state and can also be produced under suitable physical conditions.

The critical gas saturation, the rate of saturation change, and the gas saturation remaining at the end of the depressurisation process

(unrecoverable gas) are important parameters in determining the overall economic performance when depressurising a reservoir.

In this, and previous work, we are demonstrating that these quantities depend additionally upon other factors which affect

the fluid distribution and the rate of gas generation, particularly the surface and interfacial properties. For instance, earlier visual

experiments in glass micromodels suggested that wettability and oil spreading coefficient could substantially influence both the

value of the critical gas saturation and the growth pattern for the developing gas bubbles, and thus the gas flow. In order to

confirm these observations and to provide quantitative data, further experiments in large sintered packs, with different matrix

wettability and with oils having different spreading coefficients (e.g. oil spreading onto a gas–water interface), have been

carried out and are reported here. These new experiments show that the magnitude of the critical gas saturation for a water-wet

system is about the same irrespective of whether the oil is spreading or non-spreading, but it is much higher than for the oil-wet

case. In addition, oil is also produced but the rate of production is dependent upon the rock wettability and the oil

characteristics. We find that in a water-wet medium, for spreading oils, the physical form of the oil becomes transformed from

being immobile ganglia into mobile oil films, which can then be transported by the gas. For non-spreading oils, oil has to be

pushed out by the gas as discontinuous ganglia so less is oil produced. In contrast, in an oil-wet system, the oil phase already

exists as a continuous film on the surface of the solid so that the generation of gas effectively expands the oil phase, enabling the

oil to be produced in larger quantities even at lower gas saturations.

These new experiments give further evidence that rock wettability has an important influence on the performance of gas

production from residual oil. Additionally, significant amounts of oil may be recovered after waterflooding from the residual

condition, which could have a beneficial impact on the economics of the depressurisation.

D 2003 Elsevier Science B.V. All rights reserved.

Keywords: Waterflooded; Residual oil; Depressurisation; Gas production; Wettability

0920-4105/03/$ - see front matter D 2003 Elsevier Science B.V. All rights reserved.

doi:10.1016/S0920-4105(03)00070-6

* Corresponding author. Fax: +44-207-594-7444.

E-mail addresses: c.grattoni@imperial.ac.uk (C.A. Grattoni), radawe@eng.uwi.tt (R.A. Dawe).1 Fax: +1-868-662-4414.

C.A. Grattoni, R.A. Dawe / Journal of Petroleum Science and Engineering 39 (2003) 297–308298

1. Introduction

After the economic limit for oil production by

waterflooding has been reached, significant quantities

of hydrocarbons can be left in the reservoir. Reser-

voirs with a high solution gas–oil ratio, which have

been waterflooded above the bubble point, are good

candidates for depressurisation. During depressurisa-

tion, solution gas is released from residual oil when

the reservoir pressure drops to below the bubble point.

The gas starts to flow only after the critical gas

saturation has been reached. The critical gas saturation

is thus an important parameter in determining the

economic viability of depressurising a reservoir after

it has been waterflooded. If the value of the critical

gas saturation is high, some considerable time elapses

before any gas is produced, so that its discounted

economic value is reduced, whereas a low value of

critical gas saturation produces an early return on the

investment (Dumore, 1970; Moulu and Longeron,

1989; Beecroft et al., 1999). The rate of gas produc-

tion is controlled by the relative permeabilities,

mainly the gas values. This affects the ultimate gas

saturation, gas remaining at the end of the depressur-

isation, and ultimately the economic viability of the

process (Ligthelm et al., 1997; Naylor et al., 2000).

The evolution of gas saturation in three-phase

systems has been studied under a wide range of

conditions, particularly the critical gas saturation

(Moulu and Longeron, 1989; Kortekaas and van

Poelgeest, 1991; Li and Yortsos, 1991; Kashchiev

and Firoozabadi, 1993). It is worth noting that exter-

nal gas injection processes are intrinsically different

from internal gas generation and bubble growth into

gas clusters. When gas is released from waterflooding

residual oil, the gas saturation has to reach a critical

value before it can start to move (Kortekaas and van

Poelgeest, 1991; Firoozabadi et al., 1992; Li and

Yortsos, 1993; Egermann and Vizika, 2000). This

critical gas saturation is highly dependent on the rate

of pressure depletion (Scherpenisse et al., 1994).

Earlier studies of the physical processes of nucleation,

growth and mobilisation of the gas released from

solution showed that the gas and oil mobility are

controlled by the oil–water interfacial tension if the

oil has a positive spreading coefficient and the matrix

is water-wet (Hawes et al., 1996). This scenario is

consistent with the results from high-pressure core

experiments (Kortekaas and van Poelgeest, 1991;

Braithwaite and Schulte, 1992; Scherpenisse et al.,

1994). In previous work, using glass micromodels,

Hawes et al. (1996, 1997) indicated that both rock

wettability and the oil spreading coefficient could

substantially influence the value of the critical gas

saturation and the growth pattern of the developing

gas bubbles. Naylor et al. (2000) performed exper-

imental studies in aged cores and suggested that

wettability might play an important role in hydro-

carbon recovery. Further experiments for oil-spread-

ing oils in water-wet beadpacks (Grattoni et al., 2001)

showed that the gas relative permeability is much

smaller, up to an order of magnitude, than those

obtained from displacement experiments or from

calculations using traditional correlations. These low

values of relative permeabilities are consistent with

published data on depressurisation of cores at reser-

voir conditions (Naylor et al., 2000; Drummond et al.,

2001). If this occurs within the reservoir, then low gas

flow rates will occur at the early stages of depressur-

isation.

The fluid distribution and flow within a porous

media is controlled by the fluid–fluid interactions

(interfacial tensions) and fluid–solid interactions

(wettability). The combination of all these interactions

will determine the topology and morphology of the

fluids within the porous media. The spreading coef-

ficient of a fluid, So, is the imbalance between the

interfacial tensions (forces) acting along a single line

(contact line between fluid phases). For the gas–oil–

water system, the oil spreading coefficient on a

water–gas interface can be defined as (Rowlinson

and Widom, 1982):

So ¼ cwg � cow � cog ð1Þ

where cwg is the water–gas surface tension, cow is

the oil–water interfacial tension and cog is the oil–

gas surface tension. The spreading coefficient is

really the difference between the work of adhesion

and cohesion. An oil positive spreading coefficient

indicates that oil will form a spreading film on the

gas–water interface. If the oil has a negative coef-

ficient (non-spreading), the fluids will form a non-

zero contact angle with the other two phases along

the contact line. In three-phase systems, only two

contact angles can be independent (Blunt, 2001).

C.A. Grattoni, R.A. Dawe / Journal of Petroleum Science and Engineering 39 (2003) 297–308 299

Some examples of fluid distributions for different

wettability conditions, spreading coefficients and

flow mechanisms can be found elsewhere (Grattoni

et al., 1997).

Many experiments have been performed on the

effect of wetting and spreading for gas injection

displacement processes (Grader and O’meara, 1988;

Oak, 1990, 1991; Vizika and Lombard, 1996; Dicarlo

et al., 2000), but data on the internal gas production or

depressurisation process are more scarce. In view of

this, an experimental study has been undertaken at

Imperial College using large sintered packs with

different matrix wettability, and for oils with different

spreading coefficients (e.g. oil spreading onto a gas–

water interface). This paper reports the results on the

influence of wettability and oil spreading character-

istics on critical gas saturation, ultimate gas saturation

and oil recovery for gas production from waterflooded

residual oil.

Table 1

Fluid interfacial tensions, spreading coefficients and densities at

35 jC

Spreading system,

n-pentane +

heavy paraffin +

distilled water

Non-spreading system,

n-pentane + heavy

paraffin + distilled

water + n-butyl alcohol

cgw (mN/m) 55.6 30.1

cow (mN/m) 29.3 8.5

cgo (mN/m) 23.6 22.8

So (mN/m) + 2.7 � 1.2

qo (kg/m3) 777 779

qw (kg/m3) 1003 993

qg (kg/m3) 2 2

2. Experimental approach

The experiments to observe and quantify the oil–

water–gas dynamic behaviour were performed in

large rectangular, core-scale, sintered packs of 13

cm width, 30 cm length and 0.4 cm depth, containing

more than 200,000 randomly packed but sintered

glass beads of 0.85–1.23 mm in diameter. Although

these homogeneous test sections are a simplistic

representation of the pore space, we believe that they

accurately represent the main flow characteristics of

reservoir rocks and have the merit of being transparent

so that any movement of the fluids can be observed.

During the sintering process, the test sections were

heated to a temperature of 730 jC, so making the

surfaces of the models strongly water-wet. Strongly

oil-wet conditions could be achieved by treating the

surfaces with a commercially available water-repel-

lent, 2% dimethyl dichlorosilane in 1-1-1 trichloro-

ethane (Repelcote from Merck). The wettability was

preserved by first contacting the test section with the

wetting fluid.

Before starting any experiment, the sintered pack

was first filled with CO2 and, for the water-wet

condition, flooded with degassed water until it was

fully saturated. It was then flooded with oil to irredu-

cible water saturation and re-flooded with degassed

water to produce residual oil saturation. For the oil-

wet condition, the test section was first filled with

CO2 then flooded with several pore volumes of oil,

and re-flooded with degassed water to the residual oil

saturation.

As in our previous experiments, the oleic phase

was modelled as a simple hydrocarbon mixture

consisting of n-pentane dissolved in heavy paraffin

(nonvolatile oil) in a volumetric ratio 40:60 (Hawes

et al., 1996; Grattoni et al., 2001). For gas gener-

ation, we have adopted the procedure of heating the

mixture to produce gas from the oil phase. The

physics involved and effects we are wishing to

observe are identical to those in pressure reduction

(actual reservoir behaviour), but the experimental

procedures are simpler than performing experiments

by pressure reduction where the gas would be

released from solution by reducing the pressure

below the bubble point. The three-phase mixture that

was created (oil –gas–water) had a positive oil

spreading coefficient. Non-spreading oil conditions

were obtained by adding a small amount of n-butyl

alcohol to the mixture. Table 1 gives the properties

of the fluids. The oil–water phase was pre-equili-

brated for a week before use.

A diagram of the experimental arrangement is

shown in Fig. 1. The test section was mounted

vertically in a temperature controlled water bath

and the outlet was connected to a condensing sec-

tion, which was kept at low temperature (i18 jC)by circulating water through an outer jacket. During

the experiment, the temperature of the water bath

was slowly increased from 30 up to 46 jC at a

Fig. 1. Diagram of the experimental setup.

C.A. Grattoni, R.A. Dawe / Journal of Petroleum Science and Engineering 39 (2003) 297–308300

constant rate (about 1 jC/day) to release the gas

from solution. The fluids that were expelled from the

sintered pack were collected (condensing section) in

a series of test tubes, where the condensed pentane

became mixed with the nonvolatile component

(heavy paraffin) that was produced in the effluent.

The amounts of pentane, paraffin and water pro-

duced were determined experimentally. First, the n-

pentane component was determined by evaporating it

from the mixture at 80 jC, then the nonvolatile oil

component was extracted by petroleum ether to leave

only water in the test tube. Once the volume of each

of the fluids produced had been measured, the

individual volumes were corrected to allow for the

thermal expansion of the fluids contained within the

pack and the change in pore volume. The average

saturation of the individual phases remaining in the

test section at any time was then calculated from

these data.

3. Experimental results

3.1. Water-wet media with spreading oil

The accumulated volumes of the individual com-

ponents produced in the water-wet pack and a spread-

ing oil are shown in Fig. 2 as a function of the gas

saturation, Sg. No gas or oil was produced during the

initial stages, when the gas saturation was lower than

the critical value. This is consistent with the behaviour

observed during earlier micromodel studies (Hawes et

al., 1996). Pore scale observations have shown that

the oil phase was disconnected at connate water

conditions, and the positive spreading coefficient

ensured that any gas released from solution formed

as bubbles within the oil ganglia. These gas bubbles

were initially disconnected and immobile. As the

volume of gas increased, the bubbles grew displacing

the oil and thinning the oil film around it. In this case,

Fig. 2. Cumulative effluent production for water-wet media and spreading oil. (A) Gas production; (B) oil and water production.

C.A. Grattoni, R.A. Dawe / Journal of Petroleum Science and Engineering 39 (2003) 297–308 301

the controlling parameter for gas mobility is the oil–

water interfacial tension, because as the oil is spread-

ing, the gas–oil interface always precedes the oil–gas

interface, Fig. 3. During the initial stage, only water

was produced, until the gas saturation reached 9%.

Then a sufficient number of bubbles had become

connected to allow gas to flow through the matrix

and out of the sintered pack (we define this as the

critical saturation). It was observed that although the

gas started to flow when the saturation reached 9%,

Fig. 3. Oil, gas and water distribution for a water-wet

the critical saturation, both the gas flow rate and gas

saturation continued to increase as more gas was

released from solution, until the gas saturation reached

an ultimate value of 30%. It was also noticed that

some of the nonvolatile oil was produced as soon as

gas flowed out of the core, and at the end of the

experiment, oil accounted for about 32% of the total

liquid collected. Thus, the nucleation and expansion

of the gas bubbles had mobilised oil ganglia that were

initially trapped within the matrix by interfacial

media. (A) Spreading oil; (B) non-spreading oil.

C.A. Grattoni, R.A. Dawe / Journal of Petroleum Science and Engineering 39 (2003) 297–308302

forces. As the oil is spreading, it becomes redistrib-

uted as films around the gas bubbles, and when these

bubbles become connected, the oil films also become

connected and redistributed within the pore space. It

had been previously observed that the gas filaments

forming the flow channels were not always continu-

ous; they followed a cycle of filaments being formed

and subsequently broken inducing intermittent gas

flow (Grattoni et al., 2001). The disconnected gas

ganglia moved upwards due to its own buoyancy. The

oil was transported upwards, ‘piggy-backing’ on the

gas bubbles in the form of films and head meniscus,

whenever the gas moved as individual bubbles or

when gas filaments become subdivided (Hawes et al.,

1996). As a result of this behaviour, the oil can be

transported upwards and produced along with the gas

(Fig. 4). Oil production by this mechanism was

confirmed during this experiment.

Initially, the water flow rate increased as the gas

was released from solution, but after the critical gas

saturation has been attained, the gas become better

connected and the water flow rate decreased. The flow

of water out of the test section virtually ceased after

the gas saturation had reached about 26%, as shown in

Fig. 2B. At this point, the maximum gas saturation

and gas relative permeability had been achieved.

3.2. Water-wet media with non-spreading oil

With a water-wet matrix and non-spreading oil, the

production of oil, water and gas is in general similar to

the spreading oil (Fig. 5). Once again only water was

Fig. 4. Gas filament rupture and oil transport by

produced, as it is the wetting phase, until the gas

saturation reached a value of 11%. The fact that the

critical gas saturation was similar in magnitude to the

value for the spreading oil case suggests that the

controlling parameters for gas mobility are similar.

Some nonvolatile oil was produced as soon as the gas

started to flow. When the gas saturation reached about

26%, water production almost stopped. The gas sat-

uration increased to an ultimate value of 31%. At the

end of the experiment, the oil produced accounted for

less than 12% of the total liquid collected, which is

noticeably less than for the spreading oil, even though

the non-spreading oil residual saturation was higher.

This is probably due to the configuration of the phases

as the oil does not surround the gas and is less tightly

bound to the gas bubbles.

At the pore level, as the oil is non-spreading the

gas expansion can either displace water or oil (push-

ing a slug of oil through a pore throat). In this context,

either the oil–water or the gas–water interface can

control the movement, depending on the values of the

interfacial tensions and the pore throat size at which

each interface is located (Fig. 3B). Visual observa-

tions in micromodels have shown that for the system

used here the gas displaces both oil and water. The gas

forms flow channels that are being formed and sub-

sequently broken, and the upper part of the gas bubble

moves upwards due to its own buoyancy. When this

happens, small oil ganglia are transported upward by

the gas bubble (Grattoni et al., 1997). Thus, small

slugs of oil can be transported through physical

interaction even when the oil is discontinuous within

the gas bubble (from Grattoni et al., 2001).

Fig. 5. Cumulative effluent production for water-wet media and non-spreading oil. (A) Gas production; (B) oil and water production.

C.A. Grattoni, R.A. Dawe / Journal of Petroleum Science and Engineering 39 (2003) 297–308 303

the porous media. As not all the gas menisci are in

contact with oil, this mechanism is less efficient in

producing oil than the spreading case.

3.3. Oil-wet media with spreading oil

This oil-wet case produced very different results

to either of the water-wet cases. Oil and water were

produced almost as soon as gas was released from

solution (Fig. 6). The critical gas saturation at which

the gas phase became mobile (i3%) was also very

much lower than in the water-wet case. These results

are in keeping with the behaviour observed in our

earlier micromodel observations (Hawes et al., 1996)

Fig. 6. Cumulative effluent production for oil-wet media and sprea

and consistent with results from experiments on

aged cores carried out under reservoir conditions

(Naylor et al., 2000; Drummond et al., 2001). A

lower critical gas saturation is due to two main

reasons.� the gas–oil interface controls the gas movements,

as the oil is spreading (separating the gas from the

water) and the gas evolves from the oil.� the residual oil is already in the form of continuous

films, because of the oil-wet condition, thus

formation of the gas phase effectively produces

an expansion of the hydrocarbon (increasing its

saturation), and the oil can immediately become

mobile without changing its physical form.

ding oil. (A) Gas production; (B) oil and water production.

Fig. 7. Oil, gas and water distribution for an oil-wet media and

spreading oil. The oil and water form continuous networks.

C.A. Grattoni, R.A. Dawe / Journal of Petroleum Science and Engineering 39 (2003) 297–308304

At the end of the experiment, the gas saturation had

increased to 33%, and both oil and water production

continued to increase. Although the oil–water produc-

tion seem to have reached a plateau, the ultimate gas

saturation seems to be much higher, indicating that a

higher liquid production could have been achieved if

the experiment was not stopped. It may be noted that in

this case, the nonvolatile oil accounted for about 40%

of the total liquid produced and the water saturation

continued to decrease even when the gas saturation

reached a value of 33%. This may be due to fluid

configuration, and the fact that both oil and water form

simultaneously continuous networks, oil through

smaller pore throats and water through medium and

large pore throats (Fig. 7). The fluid configuration in

oil-wet media is very similar for both spreading and

non-spreading oil, and therefore their behaviour is

expected to be similar.

4. Analysis of results

4.1. Relative permeabilities

After the critical gas saturation has been reached, a

ratio of relative permeabilities for gas–water, gas–oil

and oil–water can be calculated from the flow rates

and the difference in hydrostatic pressures as, for

example, for a water-wet media:

Krg

Krw

¼Qglg

Qwlw

1þ DqghDPwf max

� ��1

ð2Þ

where Q is the flow rate out of the porous medium, lis the viscosity, Dq is the water–gas density differ-

ence, g is the gravitational constant, h is the height of

the pack, DPwf max is the maximum differential

pressure for water flow, and the subscripts g and w

refer to gas and water, respectively. Similar equations

can be written for the other relative permeability

ratios.

For the oil-wet condition, both oil and water, which

are continuous, can control the hydrostatic pressure.

At the pore level, a gas interface may need to displace

water or oil in order to expand. At a larger scale, the

saturation of oil and water represents the hydrostatic

forces acting on the gas phase. Thus, we can define

the density difference for the oil-wet condition as:

Dq =DqgoSo +DqgwSw. However, in our experiments,

the density differences are similar, but because they

are influencing the pressure ratio, which has a minor

influence in the permeability ratio, any density effect

is negligible.

The calculated relative permeability ratios as a

function of the gas saturation are presented in Fig.

8. It can be seen that for the water-wet conditions,

both the spreading and non-spreading oils have sim-

ilar trends but their values are noticeably different.

The main reason for the lower values of Kro/Krw for

the non-spreading oil (i5 times smaller) is due to the

oil discontinuity, which in turn blocks some pores and

is produced by the gas using the less efficient push–

pull mechanism. This, in turn, induces higher ratios of

the gas–water and gas–oil relative permeabilities (the

liquid permeabilities are smaller instead of the gas

permeability being higher). The oil–water permeabil-

ity ratio increases with gas saturation because the

water mobility is larger at lower gas saturation and

the oil permeability should increase as more gas is

generated.

The oil-wet case, on the other hand, has a com-

pletely different behavioural scenario. The Kro/Krw

ratio decreases as the gas saturation increases, mainly

due to the decrease in oil mobility, which is larger at

Fig. 8. Relative permeability ratios as a function of gas saturation. ( ) Water-wet media and spreading oil; (x) water-wet media and non-

spreading oil; (5) oil-wet media and spreading oil.

C.A. Grattoni, R.A. Dawe / Journal of Petroleum Science and Engineering 39 (2003) 297–308 305

lower gas saturation and decreases as the oil is tightly

bound to the solid and fills the smaller pore throats.

The ratios of the gas–water and gas–oil relative

permeability have reversed tendencies compared to

the water-wet case. This is possibly because the

interactions between gas and wetting fluid are similar.

4.2. Oil recovery

To compare the amount of oil production for the

different experimental cases, an oil recovery factor,

RFo, has been defined as:

RFo ¼Soi � So

Soið3Þ

where Soi is the residual oil saturation after water-

flooding and So is the oil saturation remaining at a

given gas saturation after depressurisation has begun.

As the oil production is associated with the gas

evolution, the oil recovery factor was analysed as a

function of the cumulative gas production (Fig. 9A).

The oil recovery curves start at different positions as

the critical gas saturation is different for each case.

Due to the oil continuity, the oil-wet media with

spreading oil have a higher recovery at the beginning,

but the recovery becomes lower than the water-wet

case at higher gas saturations as the oil, being the

wetting fluid, is strongly bounded to the solid. For

the water-wet media, the recovery factor is always

higher for the spreading oil, probably due to the

different fluid distributions (degree of reconnection)

Fig. 9. Oil recovery factor as a function of cumulative gas production (A) and gas saturation above the critical value (B). ( ) Water-wet media

and spreading oil; (x) water-wet media and non-spreading oil; (5) oil-wet media and spreading oil.

C.A. Grattoni, R.A. Dawe / Journal of Petroleum Science and Engineering 39 (2003) 297–308306

and mechanisms of oil transport. A larger gas–oil

interfacial area can enhance the mass transfer of

volatile component between phases and thus the gas

production, but this interfacial area also influences

the oil transport.

In order to normalize the results, the oil recovery

factor is plotted as a function of the increase of

saturation above the critical gas saturation (Fig. 9B).

The behaviour follows the same trends observed in

Fig. 9A, but all the curves are much closer. This

confirms that the oil recovery is linked to the fluid

configuration and displacement mechanism. At early

stages, more oil is produced under oil-wet conditions

as residual oil is already in the form of continuous

films. A similar argument applies to the water pro-

duction under water-wet conditions. At later stages,

the production of the non-wetting liquid is improved,

thus increasing the oil recovery for the water-wet case.

The majority of the non-spreading oil remains trapped

due to the fluid distribution and a discontinuous oil

configuration.

5. Conclusions

The results of these experiments confirm the qual-

itative behaviour previously observed at the pore scale

in micromodels and provide quantitative data at the

core-scale. Our earlier visual studies with glass micro-

models had suggested that the rock wettability could

affect the nucleation of gas bubbles within a porous

medium, and would strongly influence the growth

pattern and mobilisation of the gas bubbles. The

results presented in this paper have shown that these

effects are reflected in the magnitude of the critical

gas saturation, the relative permeabilities and the oil

recovery.

� The magnitude of the critical gas saturation was

about the same for a water-wet system irrespective

of whether the oil was spreading or non-spreading,

but it was much lower in the oil-wet case. The

maximum or ultimate gas saturation has similar

values for both the water-wet cases and a larger

value for the oil-wet system.� The gas saturation reached at the end of the

experiments has a small range of variation (30–

33%). These results suggest that there may be little

influence of wettability and oil spreading character-

istics on the ultimate gas saturation.� The relative permeability ratios also reflect the

effect of wettability and oil spreading character-

istics. The oil–water ratio is smaller for the non-

spreading oil than that for the spreading oil under

water-wet conditions, reflecting the negative effect

that the discontinuous oil have on the flow. The

gas-wetting fluid relative permeability has a similar

trend for both wettabilities.

C.A. Grattoni, R.A. Dawe / Journal of Petroleum S

� The behaviour of the oil phase and the rate of

recovery depend upon the rock wettability, and on

the oil spreading characteristics. Although the oil is

transported by the gas and produced in all the

experiments, the results suggest that this may be

achieved by different mechanisms. In water-wet

systems, for spreading oils, the physical form of

the oil becomes transformed from being immobile

ganglia into films in which the oil is mobile and

can be transported by the gas phase. For a non-

spreading case, less oil is produced. Under this

condition, the oil remains discontinuous, but is re-

mobilised as small droplets or slugs that are

transported by push–pull by the gas. In contrast,

in the oil-wet system, the oil already exists as

continuous films on the surface of the solid, and

formation of the gas phase effectively expands the

hydrocarbon phase. As a result, oil becomes

mobile and is produced as soon as gas is released

from solution.

The experiments have confirmed that rock wett-

ability and oil spreading characteristics influence the

critical gas saturation, the recovery rates and the

ultimate gas saturation when gas is released from

residual oil after waterflooding, such as during the

depressurisation of waterflooded reservoirs. Clearly,

care is needed in the choice of values of the gas

critical saturation and relative permeabilities for

simulating reservoir performance, as well as estimat-

ing the feasibility or economic performance of the

gas and oil production from reservoir waterflooded

residual oil.

Nomenclature

g gravitational constant, m/s2

h height of the pack, m

K relative permeability

P pressure, Pa

Q flow rate, m3/s

RFo oil recovery factor

S saturation

So oil spreading coefficient, N/m

Greek symbols

c surface or interfacial tension, N/m

l viscosity, Pa s

q density, kg/m3

Subscripts

g gas

i initial at residual oil saturation

o oil

r relative

w water

cience and Engineering 39 (2003) 297–308 307

Acknowledgements

The authors wish to thank Dr. X.D. Jing and Dr.

R.W. Zimmerman for encouragement, R.I. Hawes for

valuable discussions, and EPSRC for financial sup-

port.

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