CA2006482C

CA2006482C - Method for improving enhanced recovery of oil using surfactant-stabilized foams

Info

Publication number: CA2006482C
Application number: CA 2006482
Authority: CA
Inventors: Laurier Lincoln Schramm; Conrad Ayasse; Karin Mannhardt; Jaromir Novosad
Original assignee: Alberta Oil Sands Technology and Research Authority
Current assignee: Petroleum Recovery Institute
Priority date: 1989-12-20
Filing date: 1989-12-20
Publication date: 1994-06-07
Anticipated expiration: 2009-12-20
Also published as: CA2006482A1

Abstract

"METHOD FOR IMPROVING ENHANCED RECOVERY OF OIL USING
SURFACTANT - STABILIZED FOAMS"
Abstract of the Disclosure A process is provided for enhancing the recovery of oil in a subterranean formation. The process involves injecting a surfactant-containing foam having oil-imbibing and transporting properties. A foam having such properties is selected either by determination of the lamella number or by micro-visualization techniques.

Description

2~6~2 r~
2 The present invention relates to a procass for the

3 enhanced recovery of oil from a ~ubterranean oll-bsaring

4 formation using a surfa~tant-~tabilized foam.

~ 9~ 9E~$~INVEN~LQN
6 In the recovery of oil from a subterranean oil-bearing 7 formatio~ only between about ten to fifty percent o~ the oil in 8 place is recoverable using combined primary and secondary 9 production mode~. As a result, tertiary or enhanced oil recovery processes have been developed. Such proces~es include thermal 11 procesqes exemplary of which are steam flooding and in-~itu 12 combustion, chemical flooding techniques and gassou3 di~placement 13 drives. The gase~ used may include steam, carbon dioxlde or 14 hydrocarbons.
However, saveral problems occur when a g3s pha~e is 16 used as the displaaing medium. First, fingering o~ the ga~ pha~e 17 into the oil will degrade the unifo~m displacement front with 18 co~comitant reduction in oil recovery. This is a result of the 19 adverse mobility ratio between the displacing gas and the oil.
2~ Seoondly, the density differance between the gas and oil phase 21 will cause gravity override wherein the gas will tend to move 22 upwardly, sweeping onl~ th'e upper portion of the oil-bearing 23 zone. Finally, reservoir heterogeneities and zones o~ relatively 24 well swept (i.e. low oil) rock can cause the displacement fluid to channel through the oil-bearing zones. All of these phenomena 26 act to reduce the amount of vil recovered.
27 The use of surfactant-stabilized foams comprises a 28 relatively new technology for circumventing these problems.

2 ~ 8 2 1 The foam, having a viscosity greater than the 2 displacing medlum, will preferentially accumulate in the well-3 swept and/or hlgher permeability zones of the formation. The 4 di~placing m~dium is thus forced to move into the unswept or underswept areas of the formation. It is from these latter areas 6 that the additional oil is recovered. However, when a foam i~
7 used to fill a low oil content area of the reservoir, the oll 8 contained therein i8, for all practical purpose~, lost. This is 9 because the foam functions to divert the displacement fluid from ~uch areas.
11 The selection of suitable foam-forming surfactants 12 which produce foams having the neoessary stability to collapse 13 and viscosity i~ crucial. Such properties as solublllty, surface 14 tension and bulk foam stability must be taken into consideration.
Typical tests for the evaluation of surfactants would include 16 solubility tests in the salinity and temperature environment of 17 the particular reservoir, bulk foam te~t~ to ensure the stability 18 of the foam to collapse, and permeabllity or pressure drop 19 measurements made in packed sand beds or cores containing in~ected foam. U.S. Patents 4,589,276 to Djabbarah and 4,601,336 21 to Dilgra~ et. al. cover some of these tests.
22 Rscent ~tudies have indicated that many foams are 23 destroyed upon being brought into contact with an oil pha3e. It 24 is desirable that the foam not collapse upon contact with the residual oil in the swept portions of the reservoir, nor block 2& unswept portions thereof.
27 The clas~ical description of foam-oil 1nteraction~ has 28 been outlined by S. Ross, J. Phys. Colloid Chem. 54(3) 429-436 29 (1950). Ross sets forth that foam stability ln the presence of -~
o~l can bs described from thermodynamics in ter~ of the `~

` 2~6~2 1 Spreading and Entering Coefficients S and E respectively. ~he~e 2 coefficients are defined as followB:

3 S = Y F ~ Y OF Y O
4 wherein YOF i8 the foamlng solution surface ten~ion;
6 ~OF i the ~oaming solution-oil interfacial ten3ion;
7. and 8 ~O is the surface tension of the oil.

. .: ,::, 9 E = Y F t YOF YO :~
wherein ~`
11 r F~ rOF and yO are as de~ined ~La. ~-12 Based on these coefficients, one can predict that three 13 typos of oil-foam interactions could take place. First, (Type 14 Aj an oll will neither spread over nor enter the surface of ~oam lamellae when E and S are le8~ than zero. Seoondly, (Type H) oil 16 wlll enter but not spread over the surface of foam lamellaa when 17 E is greater than zero but S is le~s than zero. Thirdly, ~Type 18 C) oil w~l~ ente~ the ~urfaae of foam la~ellae and then spr2ad l9 over the lamellae surfaces i~ both E and S are greater than zero.
This latter behaviour, typically, will destabilize the foam.
21 However, experimental results have not borne out these 22 predictions. Furthermore, tha theory was developsd 23 assuming that the oil droplets are readily imbibed into the 24 foam lamellaa. Again however, experimental results show that ~-~
~'"r ~ ''"' '~

':~

2~0~2 1 some foams, particularly those of type A ~B do not read~ly 2 lmbibe oil.
3There exl8t8, therefore, a need to distinguish betwe~n 4 foams which are stable to oil but ~lo not signi~icantly imbibe oil, as in type A su~a, foams which are ~table to oil and do 6 imblbe oil as in the second type above and flnally, foam~ that 7 are unstable to oil a~ in the third predicted type.

8~ 5~LIY~Y351~
9In accordancs with the present invention, it has been discovered that foams having the propertie~ of being stable in 11 the presence of an oil phase or additionally being func~ional to 12 imbibe and tran~port the oil phase can be realized.
13Furthermora, it has been found that each foam-forming 14 ~urfa¢tant provlding these propertie3 ¢an be determined by one 15o~ two methods. :
16The first method reliea on the discovery that there 17exi~ts a correlation between foam ~tablllty and oil lmbibing ~.
18proparties and a coefficient referred to herein as the ~amella ~ .
19Number (L). More ~pecif~cally, the lamella numbor L is defined :~
as~
21YOF rO ~
2~ L = ; -~:
2 3 ' ' YOF rp 24 wherein 25~OF is the foaming solution surface tenslon;
26rO i3 the radius of an emulsified drop;
27YOF is the foaming solution-oil interfacial tension;
28and 29rp is the radius of a foam lamella Plateauborderwhere 30it initially contacts the oil. ~he Plateau border ~-11. refers to the part of a foam lamella that has cur~ed 2 ~ 8 2 1 surfaces. Plateau borders occur where a ~oam lamella 2 meet~ either another foam lamella, or a ~urface of 3 another material such as oil or solid.
4 It is to be noted that, becau6e oil imbibition i8 of interest, rO can be equated to one-half the thicknes~ of a foam 6 lamella.
7 Thu3, when L i8 greater than one, emulsification and 8 imbibition of oil into a foam will occur. If L is substantially 9 equal to, or greater than about seven, the imb~bition of large amounts of small droplets will result in foam de~truction. Thu~, 11 a surfactant generating a foam which is stable to collapse in the 12 presence of an oil phase would be one having a lamella number 13 le~s than seven. Additionally a foam having the stabllity and 14 which will imbiba and transport an oil phase would be one having a lamella numbar ranging between one and seven.
16 The second method involves uslng dlrect observatlon of 17 foam behavlour uslng micro-vlsualizatlon apparatus and comparing 18 the observed behaviour with known models 80 as to catagorize said 19 surfactants fun~tional to generate a foam which is stabl~ to ~;:
2~ collapse in the presence of an oil pha~e or which additionally 21 are aapabl~of both imbibing and transporting the oil pha3e.
22 In one broad aspect, the invention provides a method 23 for testing;whethsr a surfactant-containing foam is stable ;~
24 against collapse in the presence of an oil pha~e in a subterranean format:ion, comprising~
26 determini.ng the lamella number, L, of the foam, by:
27 (a) detsrmining the surface tension of the fo~m, YF;
28 (b) cont:acting the foam with an oil phase;

- 6 - ~

','~: ', :

2 ~ 8 2 1 (c) determining the radills of a foa~ lamella plateau 2 berder where it initially contacts the oil phases 3 rp;
4 (d) determlning the radl~ of an emulsiflad drop of oil in the foam, rO;
6 (e) determining the inte.rfacial tenslon between the 7 foam and the oil pha~e, ~OF; and a ( f) calculating L from the mathematical model 9 ~F r L =
1 1 ~ OF rp 12 and 13 determining whether the L of the foam i8 le~s than 14 seven, said range being indicative of foams which are ~table lS against collapse in the presence of an oil phase.
16 In another broad aspect, the invention provldes 17 an apparatus for use in selecting a aurfactant-contalning foam 18 which i~ stable again~t collapse in the presence of an oil phase 19 in a subterranean formation, comprising:
a tran~parant cell; ~:
21 a ftrst channel providing for the flow of foam lamella 22 through the cell; and ~.
23 a second channel providing for the flow of oil into the 24 cell, ~aid s~cond channel intersecting and terminating at!said first channel.

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2 Figure 1 is a schematic showing the micro-visualization ,: 1 3 apparatus used in one aspect of the practice of the present 4 invention.
; 5 Figure 2 is an illustrative representation of a Type A
6 foam which upon contact wit:h an oil phase shows little 7 interaction therewith.
. , .
8 Figure 3 is an illustrative representation of a Type B
g foam which upon contact with an oil phase has the capability of imbibing and transporting said oil.
11 Figure 4 is an illustrative representation of a Type C
12 foam which upon contact with an oil phase is destroyed by 13 rupturing of its lamella.
14 Figure 5 is a schematic showing the core flood test apparatus.
16 Figure 6 is a plot of coreflood MRF (defined below) 17 versus micromodel breakage frequency to illustrate the 18 correlation between coreflood MRF (Residual Oil) and micromodèl 19 foam stabilities.
Figure 7 is a plot of coreflood MRF ratio (Residual oil 21 to MRF oil-free) versus micromodel breakage frequency to 22 illustrate the correlation between normalized coreflood, MRF, MRF
23 (Residual Oil/MRF (Oil-free) and micromodel foam stabilities.
24 Figure 8 is a graph of the oil recovered from corefloods hy the foam versus the micromodel lamella number to 26 illustrate the correlation therebetween.
'~

2S)'~ ,2 2 Having reference to the accompanying drawings there is 3 provided in a first aspect a process for enhancing the recovery 4 of oil in a subterranean formation which involves injecting a foam having oil imbibing and transporting properties.
6 The foam exhibiting such properties is selected by 7 either of the two methods described herebelow.
8 Determination of the Lamella Number g There e~ists a correlation between foam stability and oil imbibing properties and a coefficient referred to herein as 11 the Lamella Number (L). More specifically, the lamella number L
12 is defined as:
O
13 L YF rO
14 ~OF rp wherein 16 YF is the foaming solution surface tension;
17 rO is the radius o~ an emulsified drop;
18 rOF is the foaming solution-oil interfacial tension; and 19 rp is the radius of a foam lamella plateau border where it initially contacts the oil.
21 It is to be noted, that because oil imbibition is of 22 interest, rO can be equated to one-half the thickness of a foam 23 lamella.
24 The surface tension can be determined usin~ the standard du Nouy ring technique as is described in standard ~-26 textbooks of colloid chemistry. `~
27 The interfacial tensions between the oil and the foam 28 forming solution may be measured using the spinning drop 29 tensiometer as described by J. L. Cayias, R.S. Schechter, and W.

~0~ 2 ,~

1 H. Wade in Adsorption at Interfaces, ACS Symp. Ser., No. 8, 234-2 247 (1975).
3 The radius of an emulsified drop (rO) was determined using the micro-visualization apparatus described below.
5imilarly, the radius of a foam lamella Plateau border where it 6 initially contacts the oil (rp) was determined using the same 7 apparatus. However, the ratio of the radius rO/rp was measured 8 for a number of surfactant stabilized foams of 95~ quality and 9 found to have a constant value of about 0.15. By ~5% quality is meant 95 volume percent gas and 5 volume percent aqueous 11 solution. Thus a useful approximation is to use a value of about 12 0.15. In this case the micro-visualization apparatus is not 13 needed and only the surface and interfacial ~ensions have to be 14 measured, which can be done using readily available apparatus.
It is to be noted that the lamella number is the ratio 16 of two forces namely a) the capillary suction force exerted by 17 the foam lamellae causing oil to be drawn up into the lamellae 18 where it is pinched off into droplets and b) the resisting force 19 provided by the interfacial tension of the oil which counteracts the capillary suction force.

21 Micro-visualization Determination 22 Having particular reference to Figure 1, the micro-23 visualization apparatus of the present invention utilizes a pair 24 of glass plates 2. A flow pattern 3 is etched into one of the two glass plates 2a. A key feature of the flow pattern is that 26 advancing foam lamellae and oil could be brought into contact in 27 a con~rollable fashion. The etched pattern represented a model 28 of a small part of the micro-structure in a porous medium. The : 1 typical pore areas ranged from 380-3000 X 55-65 ~ m. The plates 2 2 were placed together in a holder (not shown) adapted for observation in a conventional microscope (again, not shown). A
. , .
4 first inlet part 3 was provided for the introduction of foam therein. A second inlet port 4 communicating with a series of 6 divided inlet ports 5 was provided for introduction of the oil between the plates 2 and its subsequent contact with the foam.
The novel features of the flow network are (1) that 9 separate injection and control of the flow of foam lamellae and of the oil are possible, and (2) that channels are provided such 11 that flowing foam lamellae can be observed under two very 12 important conditions as follows. First, the flow behaviour of 13 foam lamellae can be observed in pores and throats in the absence 14 of oil (upper pathways in Figure 1). Secondly, the behaviour of the foam lamellae can be observed during their initial and 16 subsequent encounters with oil in khe region of the cell where 17 the oil channels intersect the foam channels.
18 The microvisualization apparatus permits assessment of 19 two properties: (1) the ability to imbibe and transport oil, which can be observed directly, and (2) the stability to breakage 21 in the presence of the oil, which can be measured in the 22 apparatus. The combination of these is assessed versus known 23 models.

-The experimental results given in Table I herebelow 26 show the interaction of various foams with David crude oil. The 27 crude oil used was a well head sample produced from the 28 Lloydminster sand, in the David field having a density of 0.9259 :
r)J)6~2 1 g/mL and viscosity of 207 mPa.s, both at 23.0C.

O O
3 Surfactant Brine ~F Yo ~OF E S L fb Foam 4 (0.5% mass) Solution mN/N mN/m mN/m mN/m mN/m Type

5 Concentration mass% s-l

6 Fluorad FC-751 0.0 19.3 29.3 6.6 -3.4 -16.6 0.4 0.00 A

7 Fluorad FC 751 2.1 1~.0 29.37.0 -3.3 -17.3 0.4 0.00 A

8 Dow XS84321.05 0.0 35.0 29.34.5 10.2 1.2 1.1 0.02 B
g Dow XS84321.05 2.1 32.2 29.31.2 4.1 1.7 3.9 0.02 B
10 Stepanflo 60 0.0 29.2 29.3 2.5 2.4 -2.6 1.7 0.02 B
11 Varion CAS 0.0 36.0 29.3 0.8 7.5 6.0 7.1 0.03 C
12 Atlas CD-413 o.o 35.0 29.3 0.4 6.1 5.3 13.8 0.03 C
13 Atlas CD-413 2.1 30.7 29.3 0.2 1.6 1.2 23.4 0.05 C ;;

14 Example II ~-The crude oil was a well head sample produced from the 16 Judy Creek field, Beaverhill Lake pool having a density of 0.8296 17 g/mL and viscosity of 4.6 mPa.s, both at 23.0+ 0~5C. Values 18 obtained for the physical properties and fb are given in!Table II ~`
19 herebelow.
Examples I and II show that for a range of oils ancl 21 foams there is a correlation between the micro-visual method 22 (combination f fb and Foam Type columns) and the first method (L
23 column). Thus either method yields the same needed information.

2 Surfactant Brine Foam Oil E S L fb Foam ~ 0.5% mass Conc. Surface Surface Initial Type 4 Tension Tension IFT
mass~ mN/m mN/m mN/m mN/m mN/m s~
.
6 Fluorad FC 751 0.0 19.3 24.3 4.7 -0.3 -9.7 0.60 0.002 A
7 2.1 19.0 24.3 5.2 -0.1 -10.50.53 0.000 A
8 Mixture + 2.1 30.6 24.3 0.667.0 5.6 6.7 0.020 B
Na Dodecyl ~0 Sulfate 0.0 38.3 24.3 5.119.1 8.9 l.1 0.013 B
11 Dow XS84321.05 0.0 35.0 24.32.3 13.0 8.4 2.2 0.018 B
12 2.1 32.2 24.3 0.628.5 7.3 7.6 0.041 C
13 Varion CAS o,o 36.0 24.3 0.5712.3ll.1 9.2 0.042 C
14 2.1 35.7 24.3 0.5011.910.9 10.4 0.039 C
15 Atlas CD-413 0.0 35.0 24.3 0.5111.210.2 10.0 0.039 C
16 2.1 30.7 24.3 0.416.~ 6.0 10.9 0.037 C
. _ .
17 + Mixture of 0.49% VarLon CAS plus 0.01% Fluorad FC-751 surfactants.

Example III

19 Using the same light crude oil as in Example II, the invention was tested in low pressure ambient temperature 21 corefloods.
22 Method. The porous medium used was Berea sandstone cut 23 into 2.5 x 2.5 x 20 cm blocks that had been wrapped in fiberglass 24 tape and cast in epoxy resin. These blocks had pore volumes of about 30 mL and absolute air permeabilities of about 630 to 1040 26 md. Although the Berea cores were selected to have similar 27 properties there is some unavoidable variation. Th~ cores were 28 flooded using the coreflooding apparatus illustrated in Figure 5.

:
- - ~0~ 2 1 Foam was pregenerated by passing gas and surfactant solution 2 throu~h a 7 micrometre in-line filter. Oil and aqueous phase 3 productions were measured by separating them in a glass buret and 4 drawing off the aqueous phase to a separate container. The kests were conducted at constant imposed rates of gas and liquid 6 injection. For each ~xperiment a fresh epoxy-coated sandstone 7 core was prepared and saturated with brine by imbibition.
8 Subsequently, the core was flooded as follows: -

9 l. Brine was injected to saturate the core and measure the absolute permeability to brine.
11 2. Oil was injected into the core, displacing brine, until 12 the residual water saturation was attained. The first 13 few pore volumes were injected from the top down with 14 the core in a vertical orientation and at a very low rate (2 mL/hr), subsequently 6-8 pore volumes (PV) were 16 injected with the core horizontal, at a high rate (72 17 mL/hr).
18 3. The core was mounted in the apparatus and brineflooded l9 at a rate of 2-18 mL/hr (linear superficial velocity =
0.3-3 m/day), until the (unchanging) residual oil 21 saturation was obtained, usually after 6-8 PV of brine.
22 4. Gas and brine were injected simultaneously at pre-23 determined rates in order to measure the pressure drop 24 base lines.
5. Surfactant solution was injected for surfactant pre-26 equilibration (to satisfy the adsorption requirement of 27 the core and to determine any oil recovery due to 28 surfactant alone); 6-7 PV were injected in 24-48 hours 29 at a :rate of 2-18 mL/hr.

- ~` 2~ 2 1 6. Foam was pregenerated in the in-line filter and 2 injected into the core at an initial pressure smaller 3 than the expected pseudosteady-state injection 4 pressure.
To assure the repeatability of the tests, foam flooding 6 was carried out in a series where rate and foam quality were 7 changed in such a way that the pressure drop was always 8 increasing. Each surfactant was tested using total volumetric g rates of about l9 mL/hr (3 m/day) and foam qualities of about g6%. Sufficient time was allowed to attain the pseudosteady-11 state, an average duration was 2 weeks of continuous operation.
12 From the data in Example III, ~oams predicted to yield 13 types A, B and C behaviour were selected for coreflood testing.
14 The efficiency of the selected foams with re~ard to their capacity to improve volumetric sweep efficiency in porous media 16 was evaluated based on mobility reduction factor (MRF): a ratio 17 of the pseudo-steady state pressure drops across a core with foam 18 and with only gas and brine flowing at rates equivalent to those 19 in the foam. Figure 6 shows the MRF's measured for foam in the presence of residual oil versus micromodel breakage frequencies 21 in the presence of oil. Since each foam did not behave 22 identically in the oil-free cores, that is the oil-free core 23 MRF's were not all the same, the ratio of residual oil NRF to 24 oil-free core MRF for each foam is plotted in Figure 7. These results establish the correlation between micromodel and 26 coreflood foam stabilities to oil.

~0~ 2 1 Example IV
2 The final example illustrates the use of the invention 3 in a secondary flood application using the surfactant found to 4 yield type B behaviour in Example II and used in a tertiary foam flood in Example III. In a sepa:rate experiment a Berea core was 6 brine and oil saturated as in Example III, but instead of 7 flooding with a sequence of brine (waterflooding), then 8 gas/brine, surfactant solution and foam, in this case it was 9 flooded directly with foam. Thus the foam was injected as a secondary recovery process. The results are shown in Table III
11 herebelow.

13 Foam Initial Residual Oil Saturations Total Oil 14Injection Oil After After Recovery Mode Saturation Brine Foam 16 (~ PV) (~ PV) (% PV) (% OOIP) 17Brine only 63 28 n.a. 56 18Tertiary Foam 63 28 23 63 19Secondary Foam 63 n.a. 20 68 .~

n.a. : not applicable.
21 % PV : percent of pore volume 22 % OOIP : percent of original oil in place 23 Examples III and IV show that having selected the 24 stable oil-imbibing and transporting type of foam using the methods of the present invention more efficient enhanced and 26 secondary oil recovery processes are achieved. This is shown for ~ -":' ' f~J3~4~12 1 enhanced recovery by the optimal incremental oil recovery (i.e., 2 above and beyond that from brine flooding and surfactant solution 3 flooding) of the Type B foam compared with the poorer recoveries 4 for the Types ~ and C foams. The Type B foam matches both of our selection method criteria (micro-visual criteria of behaviour and 6 fb:~ and the correlation: L=6.7 is between 1 and 10). Example IV
7 shows that the Type B foam is not only optimal for enhanced oil 8 recovery but in a secondary oil process the total oil recovery is g even better. The extra oil recoveries are due to the oil-imbibing and transporting property of the Type B foams selected 11 from by the present methods.

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A process for enhanced oil recovery from a subterranean oil-bearing formation wherein a foam is utilized to reduce and control the mobility of a subsequently injected gaseous displacement fluid, comprising:

selecting a surfactant-containing foam which is functional both to imbibe and transport an oil phase in the formation to thereby enhance the recovery of oil; and injecting the selected foam into the formation, either as preformed foam or by alternate injections of surfactant solution and gas.

2. In a secondary process for the recovery of oil from a subterranean oil-bearing formation wherein a drive fluid is utilized to flood the formation the improvement which comprises:

selecting a surfactant-containing foam which is functional to both imbibe and transport oil; and injecting the selected foam into the formation as the drive fluid, either as a preformed foam or by alternate injections of surfactant solution and gas.

3. A method for testing whether a surfactant-containing foam is stable against collapse in the presence of an oil phase in a subterranean formation, comprising:

determining the lamella number, L, of the foam, by:
(a) determining the surface tension of the foam, ?°F;
(b) contacting the foam with an oil phase;
(c) determining the radius of a foam lamella plateau border where it initially contacts the oil phases r?;
(d) determining the radius of an emulsified drop of oil in the foam, r?;

(e) determining the interfacial tension between the foam and the oil phase, ?OF; and (f) calculating L from the mathematical model and determining whether the L of the foam is lass than seven, said range being indicative of foams which are stable against collapse in the presence of an oil phase.

4. The method as set forth in claim 3, for testing whether a surfactant-containing foam is functional to both imbibe and transport an oil phase in a subterranean formation, which further comprises:
determining whether the L of the foam lies inside or outside the range of one to seven, said range being indicative of foams which are functional to both imbibe and transport an oil phase.

5. A method for testing whether a surfactant-containing foam is stable against collapse in the presence of an oil phase in subterranean formation, comprising:
determining the lamella number, L, of the foam, by:
(a) determining the surface tension of the foam, ?°F;
(b) contacting the foam with an oil phase;
(c) determining the interfacial tension between the foam and the oil phase, ?OF; and (d) calculating L from the mathematical model and determining whether the L of the foam is less than seven, said range being indicative of foams which are stable against collapse in the presence of an oil phase.

6. The method as set forth in claim 5, for testing whether a surfactant-containing foam is functional to both imbibe and transport an oil phase in a subterranean formation, which further comprises:
determining whether the L of the foam lies inside or outside the range of one to seven, said range being indicative of foams which are functional to both imbibe and transport an oil phase.

7. An apparatus for use in selecting a surfactant-containing foam which is stable against collapse in the presence of an oil phase in a subterranean formation, comprising:
a transparent cell;
a first channel providing for the flow of foam lamella through the cell; and a second channel providing for the flow of oil into the cell, said second channel intersecting and terminating at said first channel.

8. The apparatus as set forth in claim 7, wherein said first channel extends through the cell between spaced apart foam inlet and foam outlet ports, said second channel extends into the cell from an oil inlet port, and said second channel is divided to intersect said first channel at a plurality of locations.

9. The apparatus as set forth in claim 8, wherein the cell is formed by a pair of glass plates, said first and second channels being etched in the surface of at least one plate.