Foam How Through an OikWet Porous

Medium: A Laboratory StudyJ.M4. Sanchez, SPE, and RD. HazEett; Mobil R&D Corp.

Summary. A laboratory study of the steady generation and flow of gas and surfactant solution tbmugh an oil-wet porous medium

indicates that foam is formSd and is effective in reducing gas permeability. A comparison of the flow chamcteristics of foam in ofl-wet

and water-wet media showed that at similar surfactant concentrations gas permeability reduction in both systems is approximately equal.

The abpity to form stable foam in situ in ,m oil-wet porous medium appears to result from the alteration of the initially hydrophobic

swface to hydrophilic by two mechanisms surface tension reduction and surfactant adsorption. Nettability alteration of the hydropho-

bic surface is evidenced by a dramatic shift in fiquid relative pernzeabiity when surfactant is present in the aqueous phase. The liquid

relative permeability curve of the oti-wet porous meditim, in the presence of surfact.ant, essenwy.~tched that Of the water-wfi Porous

medium. Such shifts in liquid relative permeabfity have not been observed for foam flow in Wongly water-wet porous media. No

alteration in either liquid Or gas relative permeability occurred with added surfactant when a residual mineral oil was pment in the

oil-wet systeh. Transient measurements, however, indicated the formation of foam in tie water-wet system when residual oil was p&sent.

Introduction

Per3& et al. 1 recentfy presented an excellemt dizmssion on thefundamental behavior of foam flow through water-wet porous me-

dia. When an aqueous foam is injected into water-wet porous me-

dia, the liquid phase and foam pseudopbzse move in the samepathways thatliquid and gas would pass through if the system were

at the same capillary pressure. Furthermore, very little liquid is

tfmsPo@d through the pores, whefe foam flows. Both these state-

ments are supported by the observance of essentially identical fiq-

uid relative pcxmeability curves in both the presence and absence

of an added surfactint. 24

Unchanged liquid relative penneabili~ curves when foam is pres-

ent in porous medii indicate that flow diversion will not occur in

the absence of in increased pressure gradient. Decreased injectivi-

ty of the steadgm phase in the absence of an increased pressure

gradient simply indicates. increased resistance to gas flow through

the same patiways. Hence, flow diversion can occur only for in-

creased values of the capillary pressure (which results in a smaller

liquid. saturation).

Because fmm flows in the pores where gas would normslfy flow,

the foam occupies the larger pores in water-wet systems. Hence,

pxmeability reduction of the gas phase is tremendous became lager

pores are most affected. Furthermore, as Nibid4 pointed out, the

pore network occupied by nonwetdng (gas) phase may have a flow-

ing foam fraction, depending on the foam mobtition pressure

gradient.~ Pressure drop through this flowing iiaction will be

determined largely by the number of Iarnellae and tleir Iifedme rela-

tive to the pore hansit time. 8

The pdm.ary mechanism for foam generation in porous media

hm b- proposed aS capilfzry snap-off Of the gas pha.w. 9-12

Hoh,9 while not explicitly using the term snap-off, fmt ob-

served foam formation by this mechanism. Two typM of snap-off

have been postulated. The first consists of fiquid accwmdation in

pore throats, where the capillary pressure is large emough to allow

liquid flow through a condnuous film from a pore opening into a

pore conshiction. 9-13 As Falls et al. 10 noted, this type of snap-

o.ff occuis only during imbibition in water-wet media. The second

type of smp-off occurs,in pores with L/d ratios greater than r andhas been described for capilkies. 1*115 This type of pore-level in-

stability cccurs only for capillmy pressures 1.s than the enq capl-

lag pressure. Both of the above snap-off de$cziptions may be

reduced to geometrical criteria at a given liquid saturation.

Lmnellae production is also facilitated by stranding during the

initial gas displacement of a surfactant solution from pore

interstices 12 and lamellae division. 17 The stabJIty of foam, once

it is generated, is presumed to be a result of kunellae stabilization

by pore walls. 18 Furthermore, many kmnellae propagation models

rely oh the foundations developed by Bretherton. 19-21 Each of

these mechanisms presumes tie existence of a water-wet state. Eveh

Khatib et al. s~ critical capillary pressure for foain destabtition

C@yrlght i99280clety .1 Peiro!eum Englmm

SPE Reservoir ti@nedng, F.bcuay 1992

relies on tie assumption of a continuous wetting film between the

smafl water-filled pores and the foam Iamellae.

Aqueous foam generation by snapdf is dependent on the dis-

connection of the nonwetting (gas) phase. For $u*ces ~fig a

umtzct angle (to an aqueouz pba3e) gmter than 70, smp-off cannot

cccur.~ The dependence of snapoff on the water-wet nahme of

the pore .$urface and the premise that foam generations pfimary

mechanism is snap-off leads naturally to the question of what hap-

pem if a reswvoir is mixed-wet or oil-wet.

Reservoir rock nettability stites are inherently ditlicuk to 2-X=-

~. 24 Henm, the precise wetted state of most rwervoti is largely

unknown. The possibility and mdanisms of the formation of

ndxed-wettabfity reservoirs have been described in detail. M Refi-

ance of foam models on the assumption of a strongly water-wet

system and the potenti of foam as an EOR process in a wide va-

riety of reservoirs have prompted tis laboratory investigation of

foam flow through an oil-wet porous mdlum. A careful compti-

son of foam behavior in oil-wet md water-wet systems in the ab-

sence of oil and the effect8 of residual oil on foam formation and

3tabfity aze pre3ented.

Aspects of Foam In 011-Wet Porous Media

Bond and Bernard2$ I%st tested foam in a hydrophobic porom

medium. They observed that, at low smfactant concentmtion, the

bydrophoblc sand was mom. permeable to @s than the watm-wet

sand. At larger surfactant concentrations, this trend was reversed.

No satisfactory interpretation of the mechanism of foam formation

in this hydrophobic porous medium was presented. Kanda and

Schechter18 subsequently treated glass beads with silicone and ob- , ,

serwldmt @itial permeability reduction of the gas was much le8s

than that obtained with noncoated glass beads. They asserted that

soap films bridging pore walls were unstable because of the poor:

nettability.

More recently, Iescure and Claridge27 described macroscopic

C02 flood performance as a function of rock wettabifi~. Their

conclusions pointed out that foam in both ofl-wet and water-wet

media significantly enhanced oil recovery compared with that of

a tYIIic~ VJatir-dtemathg-gm process. Unfortunately, became of

the integmlnahme of the oil recovery cunw presented, fandzmentzl

interpretation of their data i3 diftimdt.

Evidence of foam formation in oif-wet porous media in apparent

cOntiadiction of theory, which rests on a continuous water-wet net-

work, also helped to prompt this investigation. As wilf be sbowu,

foam forms in a hydrophobic >rous medkun as a result of wetta-

bifity alteration of the s~ace to a hydrophilic state.

Experlmenta! Apparatus and Procedures

Fig. 1 is a schematic of the experimental system. Liquid was me-

tered through a 2-Jm3 fker to a tee located upstream of the bead-

pack. Fretiltered (0.2 #m) distilled water was used for all inns.

91

,.. ., t-:. .

. . .. . . .

~R&m=P2121!oE

HPLC PUMP. r.,

,.: ~~) (:cumON

rNUPRO

I flLT3R

ON/wF VANERmJ.4Tm

CRmuAlwCYLINOER

19.1 EXPer@ntal appa~tU$ fol steady two-phase flow measurements in the presence,nd absence of surfactant.

The surfactant chosen for this study was Chaser SD 1000~ (an

alpba+]efi-sulfonate din@.28 prepmitied nitrogen was regda!ed

at 482.6 kpa.and metered tiough a Nupro~ micrometer needle

valve. TM provided simple, accurate control ofnlrogen flow rates

through the beadpack. Nitrogen and aqueous solution were joined

at a tee before entry into the beadpack. Effort was directed at en-

suring that no foam was pregenerated before the two-phase flow

ermy into the pack. However, because of the small D of the infet

tubing, the entering two-phase flow consisted of large slugs of gas

followed by liquid slugs.,

Scala lime glass beads sieved between 100 and 140 mesh were

obtained from AUtech Assocs. For experiments on water-wet me-

dia, uncoated glass beads were used. ,Experiments on oif-wet me-

dia were conducted wih sikumted beads. The beads were sikmated

at AUtech through a condensation reaction with dimetbyl-dicfdoro-

sikme. This treatment effectively rendered the surface of the beads

strongly- oil-wet (hydrophobic). The nature of the surface of the

coated and uncoated beads was checked by placing a drop of pure

di.sdfled water on several layers of beads in a watch glass. The drop

was observed to imbibe readilyinto the uncoated beads and to bead

(not spread) on the sikmated beads. The absolute permeabtity of

each separate pack was 10.9 pm. 2 The packing procedure used is

detailed in Ref. 6. Total PV ranged from 58.5 to 59.4 cIn3.

Differential pressure was measured with two Rmemount trans-

ducers. One transducer was calibrated witi a mercury manometer

for pressures from 0.69 to 12.4 kpa. When foam was generated

within the porous medium, pressure drops much greater than this

were observed, and a second transducer was calibrated and used

in the pressure range from 13.78 to 68.94 kpa. Pressure taps were

located along the length of the pack spaced 152.4 mm apart. Total

length of the pack was 228.6 mm. AU d~ffemmial .pres.sme data

was recorded on a Soltec, three-point strip-chart recorder. Irjec-

tion pressures were measure@ with a O-to 206.7-kPa gauge. Sur-

face tensions were measured by the maximum-bubble-pressure

method, and values were confirmed by capilfmy rise.

Liquid saturation was measured after steady-state flow was

acbievd by weighing the pack. A Metier bahmx capable of weigh-

ing up to 63(FJ g with a, resolution of 0,1 g was used for all mass

measurements. Because gas conhibuted a negligible amount to the

total mass, the water saturation was dctemdm?d from the follow-

ing relationship

sw=lm, mE/owvp. . . . . . . . . . . . . . . . . . . . . . . . . . . . . ...(1)

Liquid flow rates were obtained for each data point. Once steady

differential pressure ~adients were noted, liquid was coflected at

the outki for a specitied time. Two measurements were made for

92

each @nt9 to ensure that the liquid flow rate remained unchanged

with time. Gas flow rates were measured by colkting g= in an

inverted volumetric flask. The difference between the weight of

the flask (initially fall of H20) before and after collection, corn-

bid with the time of coflecdon, yielded the gas flow rate at at-

mospheric pressure.

AU experiments were conducted under conditions of steady flow

of nitrogen and aqueous solution. Liquid flow rates varied between

1 and 3 cm3 Imin, depending on the degee of Ii@Id desaturation

desired. The porous medium was initially saturated with water or

smfactant solution. For those expwimmfs where surfactant SPIU-

tion was used, 5 PV of surfactznt solution werepamed through the

pbrous medium before beginning gas flow. This effectively satis-

tied reck adsorption, a prerequisite for sNdy of the foam prokss

in the absence of surfactant adsorption dynamics.

A white mineral oil was used for those experiments where ?1

was present. The oil had a density of 0.843 g/cm3 and a viscosity

of about 0.013 Pas at 25C. Ofl was intmducdinto the pack by

fmt saturating the system with water and then displacing it with

mineral oil. When water was no longer produced, the pack was

weighed and the amount of water and oif colfected was noted. The

ol was then dmplaced by the two-phase flow of gas and disdlled

water and produced oif and water again recorded. This effectively

reduced the value of the oil saturation to 12% PV in the oil-wet pack.

Experimental ResuIts and Dlscusslon

Data Analysis. Most of the data were r@ced to reladve pemw

bility to gas and liquid phases. For this, the following equation was

used (where the gas flow rate was cakulated at the mean of the

system pressure):

kti=-qipilti(dpld$ -l. ..: . . . . . . . . . . . . . . . . . . . . . ...(2)

Representing foam tlow in porous medii through the use of rek-

tive permeabtity &ta has many limitations. Darcys law applies

to tie creeping flow of a Newtonian fluid Ommgh a continuous path-

way. For water-wet porous media, experimental evidence indicates

that Darcys law is valid for the aqueous flowing phase. This in-

formation is extremely vahiatde for reservoir simulation. In those

pores where the capillary pressure ~ctates gas flow, the relative

permeability to gas in tie presence of foam-fomdng surfacmnts ii

clearly a function of many variables. W29 Furthermore, theoreti-

cal analysis by Hatziavrzmidis30 demcmstiates that both viscosity

and permeability are altered when dispersions flow through porous

media.

Clearly, for the foamlgas phase, a relative permeabiiV repre.3en-

taticm is inadequate. For illusmating trends and correlating data,

SPE Resmvok En@eexinE, Februarj IW2

13

n O No Surfactant

5a

A ,0.001 WA

E 0.005 W?A0

; l 0.010 WA. @

g :1:~A

mOil Abs9nt

@

azELu

J*0.~ .01: 0F

5

@o

l

E

.0011

0 20 40 60 80 ~WATER SATURATION %

Fig. 2Liquid permeability in two-phase flow through water-

wet m@ia.

1?

QOil Absent

ai

00.0

00 Ab=

c .1;

o 4 A,

~

o p*M@m$

~AA%

u!Q ~ 0 &>*

!4 .01, 0 No Surfactant

$m

A 0.001 wt%

l 0.010 W%

aq e q O.loowt%

q.001-1 :

0 20 40 60 60

WATER SATURATION %

Fig. 3-Liquid permeability in two-phase flow through oil-wet

(sl[anated) media.

however, relative permeabtity curve: have a high degree of utility

and are used here. A relative permeabilby representation of the data

cetiy provides sigdkantly more infonnadcm than a simple flOW

resistance factor (which is generally reported without saturation iu-

formation). Undf a goti global model for foam in porous media

exists, relative permeability and apparent viscosity offer the best

hope of analyzing trends in data and estimating magnitudes of ef-

fms when surfactant is present.

Experinmmtaf Resufts. Fig. 2 depicts the relative permeability to

liquid of the water-wet beadpack. These data effectively yielded

a baseline for compariwn to tie experimem conducted in the ini-

tially oil-wet porous meiium. As can be seen, the relative permea-

bility to water remains unchanged when surfackmt is added. These

&ta ccmtinn previously report@ observations. 24

When surfactant is ad&d to the two-phase flow of water and gas

through the initially oil-wet porous medium, the liquid relative per-

meability Shit% d.WMtidy, as ~g. 3 shows. For X WmIM-p!XW

relative permeability of 0.1, the water saturation increases by about

100% when surfactant is present. Likewise, for a water saturation

of 30%, the relative permeability of the aqueous phase decreases

to about one seventiuof its baseline value when surfactant is added.

These obsewations, combined with tie shapes of the curves, indi-

cate that liquid flows through a set of smafler pore pathways when

surfactant Kpresent. This b not previously been reported for foam.

flow in porous media.

No consistent trend was found in the liquid relative permeability

SM with changing smfactit concentration. However, a ?fighUy

mater shift mm be observed in Fis. 3 at surfactant concentrations.

of 0.01 w%.

J.mterpretation of the liquid relative permeability shift in the pres-

ence of foam in the initidfy hydrophobk porous medium is facili,

tated by Fig. 4. Fig. 4 plots (1) the liquid relative permeabfity of

water through the oil-wet porous media in the absence of surfac-

1

s~ Oou~ 00 q g .1: ~ lB~

%~

oa

z. Oil Absent

~#Q

~ .01:

g8

0 No Surfacfant, Oil-VJet

$ l8 . 0.01 vA%, Oil-WetK II No Surfactant, Water-Wet

.001 -r J

o 20 40 60 80

WATER SATURATION /n

Fig. 4Comparisons of liquid permeability in oil-wet media

[o. that in water-wet media.

A

A

.001 !o 20 40 60 E

WATER SATURATION %

Fig. 5Gas permeability in two.ph= flow thr6ugh oil-wet

[silanated) media.

SPE Resemou E@,.ctig, Febmq 1S?2 93

%WAO ~H

q

*.. .mw

No Sutfactantq m

0.001 V/t%

0.005 Wt%

0.010 VA%

A

.001 +

o 20 40 60

WATER SATURATION %

-1

80

Fig. 6Gas permeability in two.phaae flow through water-wet media.

at, (z) tie liquid permeability in the oil-wet porous medii in the

presence of surfactant, and (3) the liquid penneabfity through the

water-wet porous media itl the absence 6f wrfactant. Remarkably,

the Iiauid relative permeabililv to m.dsurfactant solution flow

thmugi the oil-wet ktii ,mbies tbit of gas/water flow throughthe water-wet. beads.

A study of foam in oil-wet media has practical significance otiy

if the mobility of the gas phase is reduced by added surfactsnt. Ffg.

5 depicts the relative permeabfity of the oil-wet beads to the gas

phase in the presence and absence. of surfactmt. Gas permeability

is reduced dramatically at a surfactant concentration of 0.01 W%.

An increase in surf%ctant concentrdion to 0.1 wt% resulted m fur-

ther redwtiqn in gas permeabtity. .Snrfactant concentrations of

0.001 wt% and less had no measurable effect on tie gas relative

Oil Present o

0

q

goo

Oo q.0?7

qO No Surfactant

nq 0.100 Vit%

o

1L .01I

-120 40 60 80

WATER SATURATION %

Fig. 8Llquld permeability In two-phaaa flow througholl-wet(atlanated) media with oil.

94

14

ma

g . ~&mOil Presant

g 00

$

: .1:

I.#00

u.!>

i=

~ ,

O No Surfactant

E D 0.100 Wt%

.01-1

0 20 40 60. 8

WATER SATURATION

Fig. 7Gas permeability in two-phase flow through oil-w(sllanated) media with oil present.

Gas permeability reducdon by added surfactant is essentially iden-

tical in both water-wet and the ioitially oil-wet porous media. These

remarkable rasuks may be seen by a comparison of Figs. 5 and

6. Fig. 6 depicts the gas relative permeability in the water-wet me-

dia in ~e presence of various smfactant concentrations. A surfac-

tant concentration of O.001 WI% is sesn to have little effect on the

gas permeability. However, comparison of the. gas pe~eability iu

the water-wet beads at a surfactant concentration of 0.01 W% (Fig.

6) with gas pennmbifity in the oil-wet beads (Fig. 5) at the sane

.swfactant concemation in~catis similar cume shape and perme-

ability reduction. Previous compakon of foam flow in hydropho-

bic and hydrophilic porous media have shown large differences Is;

bowever, the &ta were taken undec unsteady conditions..

Figs. 5 and 6 demonstrate that foam can form in-sire in a

hydrophobic porous medium and signiiicmtly lower gas permea-

. Sutfactant Injaction

60-

50-

40-

30- Water-Wat Pack

20-

Oil-Wet Pack10-

0

0 10 20 30 40 50 ~

TIME (minutes)

~9. 94mpafls0n of transient rasponse of oil-wet VS. &Nat pack when sutiactant ia inJected with oil present.

SPE Reservou E@r.eeF.mz, February 1992

w.. . . . . . . .,

btity iri tie absence of oil. Experiments were performed to deter.

mine if foam waz effective ti m oil-wet pomuz medium in the pus.

ence of interstitial oil. Figs. 7 and 8 present the rexdts:

Foam did not form in the .&wet porous medium when the imer-stbial oil was present with SD 1000 surfactant., Fig. 7 depicts a

comparison of the gas permeability in the absence and presencf

of swfactant. The experimental details maybe found in the nmte-

rials W-up section. Permeab* was calculated with Eq. 2. Water

saturation was calculated by modifying the total PV by subtracting

the, volume of interztitizl oil This modifd PV was used io J?q.

.1. No permeability reduction of gas was noted in Fig. 7. Hence,

foam either did not form or collapsed rapidly.

Relative water permoabtity remaimd essentially identical to that

in the absence of surfactant when the mineral oil was present for

the oil-wet porous medium. Fig. 8 illustrates this result. This be-

havior is in marked contrast with that of two-phase flow in the ab-

sence of oil (Fig. 3). Indeed, essentially no change in the flow

behavior was noted with added surfactant m the presence of min-

erzf oil, indicating that water remained a nonwetting phzse.

A complete set of experiments on foam flow through water-wet

porous meiia in tie prezence of oil waz not performed. Fig. 9,

however, depicts a comparison of the transient injection-pressure

bebavior.when gas and surfactant solution wqe introduced iom both

water-wet and oil-wet beadpacks. Interstitial mineral oif saturation

for the oil-wet pack was 12%, and residual oil saturation for the

water-wet pack was 15%. From the transient re~omse, f~ clearly

formed in the watw-wet pack and resulted in a rise in injection pressure w a steady value.

Discussion.- From the experimental reiidts, it is clear that foamgenerates and reduces gas mobility in an initialfy oil-wet porous

medium. If snap-off, which presumes a hydrophilic condition, is

the dominant mechanism of foam generation, how are Owe data

reconciled?

Ms seeming paradox is resolved by concluding that the aaifi-

tion of suifactant alters the wettabifi~ of the @drophobic porousmsdium to hydrophilic. fhis conclusion is suppoti by the datain Fig, 4. The liquid relative pernwbifity cume of the ol-wet bead-

@ck bi the presence of surfactant shifted so @at it essentially match-

es that of the water-wet md]a with or without surfactmt.

Mccaffery$l observed liquid flow in smaller pores at similar liq-

uid saturations when surfactant is present in the initially oil-wet

porous medium. This clearly indicates a change ffom hydrophobic

tO hy&opbilic SU1faCe characteristics.

Solid evidence for the alteration of tie wettabili~ of low-energy

solids (hydmphoblc and oil-wet solids fall into this class) when in-

terracial tension (IFT) is decressed by added surfactaot was first

observed by Bemdt and Zimmm. 32 They measured the comm au-

gle of water on polyethylene as a fonction of surface tension. Vari-ous values of surface tensiou were obtained by the addition of fivediffwent mimic surfacmms. The contact angle of tk~ watedairlsolid

system decreased from a value of >90 at a surface tension of 72

film to 0 at a surface tension of about 30 tilm. Indeed, the

aqueous phase was able to spresd completely on the surface of the

polyethylene at surface tensions below 30 mN/m. Siar results

were found for aqueous solutions on polytetrafluoroethylenq how-

ever, a lower surface-tension value was necessary before complete

spreading occurred..

Various authors 1$33. have suggested that taildown surfactamt

adsorption on a hydrophobic surface may result in wettabtity sd-

teration. While &is may occur to some extent, it is not necessay

for the alteration of a hydrophobic sorface to hydrophilic. Fox and

Zisinan34 demonstrated that contact angle changes on Iow+mwgy

solids can be a function solely of the surface tension of the fiquids,

They measured the contact angles of tie homologous series of n-

alkanes (pentsne to hexa.d&ne) on polytetrafluoroethykme. Con-

tsct angles ranged from 46? at a surface tension of 27.6 mN/m (hex-

adecane) to full spreading at a surface tension of 16.0 hN/m

(peutane). Much care was taken m the experiments to ensore that

no stmy surface-active material w prosem.

Adsorption onto the water-wet and oil-wet beads was measured

at a surfactant concentration of 0.1 W%. fn c0ntr23t to Le.3cure

SPE Reservoir Er@meti& February 1992

TABLE l-SURFACE TENSION OF SD 1000 AS AFUNCTION OF CONCENTRATION

Concentration(~ 0/,)

0.0010.0050.0100.1000.500

Surface Tension

(mN/m)

e2.s

59.7

547

S9.8

37.9

and CIaridges27 assertion that anionic sorfactant adsorption was

grester OD the water-wet media, we find greater adsorption on.the

oil-wet media. Adsorption on the water-wet beads at pH =7 was

not measurablewith a stmic bomb mntaining 11 g of bmxis, wheress

adsorption on the oif-wet besds was 5.4X 104 g/g. Adsorption

was messured by the bol?+sobuiondepletion method. A static tube

containing 35 mL of mrfactznt solution at 0.1 wt% concentration

was used. ?he difference between the adsorption onto oif-wet and

water-wet beads supprits authors who have suggested wettabiliiy

aftcratiou resulting from surfacomt adsorption. Clearly, smfactsnt

adsorpdcm was signillcant on the oil-wet beads.

The works of Fox and Zwman34 and Bemett and Zisman32 sug-

gest that contact-angJe reduction and ultimate spreading on

hydrophobic low-energy surfaces was caused solely by reducing

tie surface tension of water.34 This claim is supported by the

Young-Dupre equation

17,$y=c=+o=v cosoc . . . . . . . .. . . . . . . . .. . . . . . . . . . . . ...(3)

Fox and Zism2n34 point out that if the solidhmpor and solidiliq-

uid fFfs play a miiior role, a decrease in tie surface ton.$ion at

the liquidlvapor interface must cause a ccmcomitamt increase in tie

cosine of the contzct angle. Hence, the wettsbility is altered from

hydrophobic to hydrophilic. A critical. surface tension was

defmed~ below which a solution @. spread on a hydrophobic

surface.

Alteration of the initiaffy hydrophobic porous medium to

hydrophilic by the two mechanisms described provides a consis-

td reconciliation of the experimental results in the absence of oil

reported here with current foam-generation theory. At the lower

surface tensions with added SD 1000 (see TabIe 1), the silanated

besdpack becomes watm-wet, allowing snap-off to occur. Further-

more, tie altered hydrophobic surface $ more likely to be able to

support fifms that ze generated, Models for foam in water-wet me-

dia may accurately reflect the behavior of foam in an initially oil-

wet porous medium because of nettability akemticm to a hydrophilic

(water-wet) state.

Surface alteration from hydrophobic to hydmpti]c is suFPorted

further by the similarity in gas relative permeability reduction ob-

served by comparison of Figs. 5 .ad 6. ffthe hydrophobic surface

has been aftered to hydrophilic, then the two porous media become

essentially identical The experiments were operated with tie same

gas and liquid flow rate ratios at each Iiqoid saturation vafue.Thus,

similar gas permeabfli~ reduction would be expected.

Wettd+fity Aa-ation may not occur in tie presence of oil in 23

oil-wet porous medium. f& is clearly demonstrated in Figs. 7znd

8. G= and liquid relative pemmabilily are essentially unstTected

by the presence of surfactsnt when the mineral DI. is present. Most

remarkable are reported contact angle cWnpaIi.$Ons Of fie

water/air/solid and the water/oil/solid interface on low-energy

solids. 35 f..ucassen-Reynder@ confimmd the wettabi!i(y akeratibn

observed by Bemett and ZIsmanQ for waterhidhydrophobic-solid

systems when surface tension is lowered. However, no change in

the contact angle was observed with decrwm-ing IFTS for the

water/oilhydrophobic-solid system. Proportional adsorption at the

oilhlid and water/oif interfaces was proposed as a possible expla-

mtion for the differences io the two types of systems. Possibly,

surfactant partitioning into the oil allows smfactznt adsorption at

the oillsolid interface. Sii adsorption at a vaporlsolid interface

may be absent. A glance at the Ymmg-Dupre equation indicates

that the ratio of the diffqence between the solid/oif and solidlwater

95

IFTs to the waterloif ti must remain unchanged for a constagt

, contact angle. Tbis supports the propordonakdsorption assertion.

Dsta kxficadng no change in waterloilkdid contact angle with lower

surface tension suggest that the Jack of any observable reduction

in gss permea.btity @lg. 7) is a result of the pefnkment presence

of an oti-wet surface.

Transient pressure re.spnse during gadsurfactant injection in-

dicates tht foam forms in the water-wet beadpack when ofl is pres-

ent, as demonwated in Fig. 9. Because residuaf oif occupies large

pore. centers, smp-off in smaJJer pores may sdJJ occur. No con-

crete explanation for the difference in foam befwior in thepres-

ence of oiJ is currently avaif able, however, and more work is needed

to dehmeate these mechanisms.Foam forms in aninitialJy oiJ-wet porous medium in the absence

of oiJ because of an akeration of the surface from hydrophobic to

hydrophilic. This wettabiJity alteration redts horn surface tension

reduction and smfactant adsorption at the soJid/liquid interface. It

is noted that the nettability alteration is a generaJ phenomenon as-

swiated with reducing the smface temion m gasibtieihydrophoblc-

soJid systems.

Conclusions

1. Gas permeabdity is significantly reduced compared with thatin the absence of suriwtant under condkions of steady two-phase

flow of gas and smfactant solution through a sikmated beadpack,

This indicates that ftiam forms in situ in sn M.iaJJy oikwet porous

medium h the presence of surfactsnt.

2. FoaRI formation in the initialfy oil-wet porous medium is a

rmdt of wettabiJily aheration of tie hydrophobic solid to hydrophil-

ic. This reconcikzs the data with foam-genemdon tbebry, W of which

rests on the assumption of a water-wet network.

3. WettabjJity alteration is evidenced by a substantial shifl in the

Jiquid-phase relative penneabihly of the initially oiJ-wet media when

Wrfactmt is present.

4. Liquid relative peim~btity curves iJJ the presence of surfac-

tam for the initiaJJY c&wet porous medium essential, match tiose

for watir-wet media both in the absence and presence of swfactant.

5. The observed wettaboily change resuk.$ from suface tension

reduction and surfactant adsorption and is a general phenomenon

associated with the gasilxinelhydrophobic-solid system.

6. Gas permeability reduction for the same surfactamt concent-

ration is e.ssmtidJy identicaJ for both water-wet and kdtia!ly oiJ-

wet media. Tbi.Y further sumorts the a?.serdon that the initiaJJyhydrophobic surface has be_e; aJtered to hydropbific. -

Nomenclature

A = flow area, IIIM2

d = pore diameter, mm

k = absolute permeabflty, fim2

kn = relative permeability to Flnid i .L,= pore length, mm

~E = mS Of pack IIn&l steady two-phase flcrz, kgMs=m=Of pack tldJy ssturated with water, kg

dp/dz = differential pressure gradient, kpahm

qi = lOhlllletriC flow rate of Fluid i, Cm3jh, Sw = water saturation

Vp = pore volume, m3

dc = contact angle (measured ti Jiquid phase), degr~s

fq = viscosity of Phase i, Pa. spi = density of Phsse i, g/m3

oLv = :yface tension of Jiquidlvapor interface, ti/m

OsL = ~@Xfmid temlon of solid@quid interface, IUN/m

nsv = mtwfacid tension of solidhapor interface, &Jm

Acknowledgments

We thank G.A. Bsrtos for his help in conducting the caretld meas-

urement of afl dats presented. JU addition, we thsnl B. G.. Hurd

for his caretid reading of tie manuscript and for his many helpfd

suggestions. We also thank the management of MobJ R&D Corp.for permission to publish this paper.

96

Referqicee

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,,

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24

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Quantities of Surfa&nt o. NitmgewWater Relative Permeabiiities,3>

paper SPE 15446 presented at the 19g6 SPE AMuat Technical Con-

femme and Exiibitim, New Orleans, Ott 5-8.

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5-s.

SPE Reservoir E@neexinz, February 1992

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J. l%ys. Chem. (1963) 67, 969.

S1 Metric *13VWSI033 FactorsCp x 1.0* E-03 = Pas

dyneslcm x 1 .O* E+OO = mN/m

F ~F32)/l.S= ..=

in. x 2.54* E+(M = Cm

md X 9.S69 233 E04 = ymqpsi X 6.S94 757 E+OO = kpa

.Wwerslo fmt.a, Is mad, sPER31

C&?lal SPE m4nuwdPt mcabnd far ti,w Oti, 9, 1989. Revlti rmnu,crlPt recatidFeb. 4 t WI. Pwr -P& for PubllmMn Aug. 20, 1690, Ps@, [SPE iw fimlPresenIti m the 1989 SPE Am.al Twbnlti Cmfwa.m & 3xhlblUon hsld In San An!*10. cu. e-n,

SPE Reservoir Engin-, Februmy 1S%2

DnMb *diss rsassrches msthods esfd

mechanisms of heavy.oil recovery by

steam pmcsssss at Mobil R&D Corp. in

Dallss. He holds a BS degfss hum Texss

.+ +, A&M U. and MS and PhD degress from

.-.. %+

$%3=

the U. of Texas, ali in chemissl engineer-

..... v ing. (Photo unavailable.) Randy Doyle

.. . ;,. ffazlstf Is the technical leader of a

roc!dfluld interactions project within the,.Improved Recovety Group at Mobil R&D

Corp. w[th special interest in inteffaciai

phenomena. He holds qS, MS, and PhD degress in chemicalengineering from the U. of Texas.

?7

,0-30 PsMPRESSUEGALU

HPLCPIJMP

3 WAYVALVE

vPREsATummt=%

6R#uAlED

FGURE 1: Experimental Apparatus for Steady, Two-Phase Flow &Measurements in-Preserce and Absence of Surfactant. *

I

.0010

0 No SumtA 0.001 wt%m 0.005 w%

o

l 0.015 Wt% l:QA -

GIAbsenld

20 40 60 80

oil Absml00

o NoswfmatEf@@oA O.001WI%l 0.010 Wf%q o o O.loowl%

q

o 20 4-o 60 60

k? o No surfamnt, oil-wetl l O.cl ?fR6,oil-wet

El NOSdaCat, water-wet

.Oolk--~ .

n 211 40 80WATER SATURATKN % WA7ER 8ATlN?A~% WAmR SATURATION%

Fgure 2 Liquid ~ity in tw-pt.ase flow Film 3: Lquidthmqh water-wet mafia

~ifl~- Fgure 4 Comparison of fii permeability inthrouoh oil-wat @m#sd) mdm Oil-l@ media to that m wa$w-wwt.

?J

-001 ~o 20 40 60 80

WATER 8ATWfATfON %

OJu(!3 1 csBAbI#

-m &--T-r420 40 60 80

WAIER 8AlURATfON %

O(-J

.01-i 10 20 40 60 80

WATER SATURATION

19687

0

i-,.

0

q 0 No Surfactant1 q 0,100 M% I

o.01! 1 1 1 4

0 20 40 60 80

WATERSATURATION%

F~ure 8: Liquidpermeabilityin two-phaseflow thoughoil-wet (silanated) media- oil present.

70-SurfactantIn@cWon

60-at Time Zero

sa; 50-U

$40-WK

~ 30- Water-WetFackl

iii20~

10FOi>WetPack

o 10 20 30 40 50 60TIME(minutes)

Figure9: Comparisonof transient responseof oil-wet vs water-wetpack when surfactantis injected - oil present.