Chemical flood oil recovery with highly saline reservoir water

In an oil reservoir in which the water contains more than about 9% dissolved salt, oil is produced by injecting an oil-displacing dispersion of at least one surface active alkylaryloxy polyethoxyethane sulfonate in the reservoir water or an equally saline water followed by a mobility controlling dispersion of noncondensible gas in an equally saline water.

BACKGROUND OF THE INVENTION 
The invention relates to a chemical flooding process for producing oil from 
a subterranean reservoir. More particularly, it relates to such a process 
which is best suited for use in an oil reservoir in which the predominant 
aqueous liquid has a dissolved salt content of more than about 9%. 
As indicated in a paper presented at a symposium on improved methods for 
oil recovery in April 1978, SPE Paper No. 7053, two essential criteria 
that must be met for successful recovery of residual oil by chemical 
flooding are (1) very low interfacial tension between the chemical bank 
and the residual oil and between the chemical bank and the drive fluid, 
and (2) small surfactant retention losses to the reservoir rock. The paper 
relates to the phase behavior of such a chemical bank (or microemulsion or 
aqueous surfactant system) as a function of salinity and describes how the 
salinity of the drive water or fluid (or mobility buffer fluid) tends to 
control the amount of the surfactant retention. 
In a collection of papers, "Improved Oil Recovery By Surfactant Flooding," 
D. O. Shah and R. S. Schechter, Academic Press, New York 1977; a paper by 
S. P. Trushenski, "Micellar Flooding: Sulfonate-Polymer Interaction", 
considers the sulfonate-polymer incompatibility in a chemical flood 
process in which the reservoir oil and water are displaced by a micellar 
fluid followed by a polymer-thickened mobility buffer bank. That paper 
indicates that the interaction between the sulfonate and polymer tends to 
increase the sulfonate requirement and the extent of that interaction 
increases with increases in the drive water salinity, regarding drive 
waters thickened by either bipolymers, i.e., xanthan gum polymers, or 
partially hydrated polyacrylamide polymers. 
Although, in some reservoirs, the extent of the polymer-surfactant 
interaction can be reduced by reducing the salinity of a polymer-thickened 
mobility buffer liquid relative to that of an aqueous surfactant system, 
in reservoirs which contain highly saline waters, such a reduction in 
salinity may not be feasible. For example, in U.S. Pat. No. 4,074,755 by 
H. J. Hill, J. Reisberg, F. G. Hellfrich, L. W. Lake and G. A. Pope, it is 
pointed out that, although numerous chemical flooding procedures have been 
suggested for utilizing relatively low salinity surfactant systems and/or 
polymer containing mobility buffers, such procedures have often been 
unsucessful. In such procedures the salinities of the injected fluid are 
often drastically altered by interactions within the reservoir. Among the 
more important physical and chemical mechanisms which operate within 
typical oil bearing reservoirs are the cross flow of fluids between layers 
of different permeability, the dispersive mixing between the fluids being 
displaced and the displacing fluid, the dissolving of minerals when a 
water differing from the formation water contacts the reservoir rock, and 
the cation-exchange reactions between the reservoir rocks and the injected 
water. The patent also mentions that when the reservoir water is highly 
saline, or contains large proportions of multivalent cations, it may be 
impossible, or at least uneconomical, to formulate an active surfactant 
system having an ionic composition (or salinity) equivalent to that of the 
reservoir water. 
In an aqueous surfactant system the capability of forming an effectively 
low interfacial tension against the reservoir oil frequently occurs within 
a range of salinity such that the system forms three equilibrium phases 
when contacted by the oil and brine of the oil-containing reservoir. In 
such a situation a surfactant-rich phase, which may contain a large amount 
of both oil and brine, may be in equilibrium with essentially pure brine 
and essentially pure oil. The salinity at which the system is capable of 
forming the lowest interfacial tension is called the optimum salinity. For 
use in a reservoir in which the brine contains both divalent and 
monovalent cations, the optimum salinity for a surfactant system that 
contains a given surfactant material usually increases with increasing 
surfactant concentration. But, the increasing of the surfactant 
concentration increases the expense of the oil recovery operation. In 
addition, the extent to which the salinity of the surfactant system can be 
increased is limited by the concentration at which that system forms two 
phases (in the absence of the reservoir oil). 
It is known that certain surfactants may exhibit an optimum or adequate 
oil-displacing activity in aqueous liquids which are highly saline. 
Surface active alkylaryloxypolyethoxyethane sulfonate surfactants exhibit 
such a capability and their use in oil recovery processes has been 
previously proposed. For example, U.S. Pat. Nos. 4,018,278 and 4,088,189 
describe (a) using such sulfonates as the predominant surfactants for 
imparting an oil-displacing efficiency within a highly saline aqueous 
surfactant system, and (b) using them in the concentrations indicated by 
capillary displacement tests or interfacial tension measurement tests 
designed for determining the economically efficient proportions to be used 
in aqueous liquids which are the same as or have salinities equivalent to 
those in highly saline reservoirs. U.S. Pat. No. 4,066,124 describes uses 
of such sulfonates as co-surfactants in relatively saline aqueous 
surfactant systems containing mixtures of predominantly water-soluble and 
predominantly oil-soluble petroleum sulfonate surfactants. U.S. Pat. No. 
3,977,471 describes such surfactants as members of a new and improved 
class of surface active agents for use in chemical flood processes in 
reservoirs containing water with salinities of 2% or more. U.S. Pat. No. 
3,827,497 describes waterflood surfactant compositions that contain 
sulfonated oxyalkylated alcohols and indicates that an 
alkylaryloxypolyethoxyethane sulfonate can be included. U.S. Pat. No. 
4,077,471 describes injecting mixtures of said sulfonates with oil soluble 
non-ionic surfactants in highly saline aqueous mixtures ahead of 
polymer-thickened aqueous mobility buffer or drive fluids "if no adverse 
interaction occurs between the polymer and the surfactant." 
U.S. Pat. No. 3,653,440 by J. Reisberg describes a water-flood oil 
production process in which the oil is displaced by injecting an active 
surfactant system followed by drive liquid comprising a mixture of a gas 
and an aqueous liquid, with the gas and liquid being injected so that the 
drive fluid mobility is relatively low and the gas moves ahead of the 
liquid. The patent indicates the desirability of having a surfactant in 
such a liquid to provide a gas-water surface tension at least as low as 
about 30 dynes per centimeter, so that the gas bubbles are relatively 
small and homogeneously distributed throughout the liquid. 
SUMMARY OF THE INVENTION 
This invention relates to an improvement in a process for recovering oil 
from a reservoir containing water which contains more than about 9% 
dissolved salt (or other elecrolyte) by injecting, ahead of a 
mobility-controlling drive fluid, an aqueous surfactant system which 
contains at least one surface active alkylaryloxypolyethoxyethane 
sulfonate dispersed within an aqueous liquid which is the reservoir water 
or has a salinity (or electrolyte content) equivalent to the reservoir 
water. The present improvement comprises a combination of steps. The 
surfactant used is one or a mixture of such sulfonates having a chemical 
composition tailored to suit the physical and chemical properties of the 
reservoir to be treated. The reservoir tailored surfactant comprises at 
least one surface active alkylaryloxypolyethoxyethane sulfonate in which 
the arrangement of the size and structure of the alkyl groups and the 
number of ethoxy groups is such that a dispersion of the 
reservoir-tailored surfactant in the reservoir water or one having an 
equivalent salinity (a) exhibits a significant and substantially optimum 
oil-displacing activity when the surfactant concentration is as low as 
about 0.1% by weight of the aqueous liquid, (b) is at least substantially 
a single-phase system at the reservoir temperature, and (c) is 
sufficiently viscous by itself or when mixed with noncondensible gas to be 
capable of exhibiting within the reservoir formation a mobility at least 
substantially as low as that of the reservoir oil within that formation. 
The aqueous liquid within which the reservoir tailored surfactant is 
dispersed to form the surfactant system to be injected is the liquid 
produced from the reservoir or one that is so similar in salinity as to be 
capable of maintaining a salinity that is substantially unaltered by 
ion-exchange reactions which occur when that liquid is flowed within the 
reservoir. In the surfactant system injected into the reservoir, the 
concentration of the surfactant is a value between about 3 and 10% by 
weight sufficient to cause a significant proportion of the injected 
surfactant system to retain a concentration of at least about 0.1% of the 
surfactant in spite of the dilution that occurs during the displacement of 
the surfactant system within the reservoir. The surfactant system is 
displaced within the reservoir by injecting it immediately ahead of a 
substantially homogeneous dispersion of noncondensible gas in an aqueous 
liquid having a composition the same as or substantially equivalent to 
that of the aqueous liquid used in the surfactant system with said 
dispersion being capable at the reservoir temperature, of exhibiting a 
mobility within the reservoir at least substantially as low as that of the 
injected surfactant system. In a preferred embodiment, the drive fluid, at 
the time it is injected or as soon as it has flowed through a significant 
portion of the reservoir, contains enough surfactant to improve the 
homogeneity of the dispersion of the gas within the liquid. 
The present invention combines a particular way of selecting a sulfonate 
surfactant for use in highly saline water, a particular way of 
proportioning a so-selected surfactant within a particular type of such 
water, and a particular way of displacing the resulting surfactant system 
within a reservoir formation. It thus provides at least a substantial 
elimination of the problems due to an oil-displacement inefficiency 
resulting from the dilution of the surfactant system by diffusive mixing 
within the reservoir, a lack of or loss of mobility control due to an 
absence of polymeric thickening agent, or a loss of surfactant efficiency 
due to a polymer-surfactant incompatability within the reservoir. 
As used herein, the term alkylaryloxypolyethoxyethane sulfonate refers to a 
sulfonate of a sulfonic acid of the formula 
EQU (alkylaryl)--O--(CH.sub.2 CH.sub.2 O).sub.n --CH.sub.2 CH.sub.2 SO.sub.3 H 
where n is a number of from about 1 to 6 and the alkylaryl group is a 
phenyl group to which at least one saturated or unsaturated alkyl group is 
attached, with the total number of carbon atoms within said alkyl groups 
being from about 6 to 12.

DESCRIPTION OF THE INVENTION 
The present invention is at least in part premised on a discovery that 
alkylaryloxypolyethoxyethane sulfonate surfactants can be selected and 
utilized so that they provide a particularly beneficial combination of 
interfacial tension-lowering, mobility-decreasing and foam-forming 
properties when they are dispersed within relatively highly saline aqueous 
liquids at temperatures common in oil bearing reservoirs. When used as 
presently specified, these sulfonates can eliminate or at least reduce any 
problems due to a diffusive mixing of fluids or a surfactant-polymer 
incompatibility, in addition to any problems due to high salinity of the 
reservoir water. 
In properly proportioned dispersions of such surfactants in highly saline 
aqueous liquids, the structure and molecular weight of the alkyl groups 
and number of the ethoxy groups which are contained in one or more of such 
surfactants can be adjusted so that a high oil-displacing activity is 
provided by a selected moderately low surfactant concentration. And, 
unobviously, such an activity can remain high throughout dilutions of the 
surfactant system, for example, to a concentration as low as 0.1% by 
weight. As known, such dilutions of the surfactant systems are 
substantially unavoidable when a bank of surfactant-rich aqueous liquid is 
being driven by aqueous liquid through a reservoir that contains a 
surfactant-free aqueous liquid. 
The present surfactant systems are capable of exhibiting a relatively low 
mobility within an oil reservoir without the necessity of using polymeric 
water thickeners. The foam forming properties of the surfactants selected 
in accordance with the present procedures are such that, at an 
economically low concentration within highly saline aqueous liquids, they 
are capable of forming substantially homogeneous dispersions of 
noncondensible gases, which dispersions have mobilities low enough to 
adapt them for use as mobility-controlling drive liquids for displacing 
the surfactant systems and oil within the reservoir. 
Laboratory Tests 
FIG. 1 shows the effects of the concentration of sodium and calcium ions on 
dispersions of surfactants in aqueous liquids. The surfactants tested were 
(a) a petroleum sulfonate surfactant comprising a sulfonated hydrocarbon 
fraction having an average equivalent weight of 430 and containing about 1 
sulfonate group per molecule which is available from Witco Chemical 
Company, and (b) Triton X-200 surfactant, comprising a polyethoxylated and 
sulfonated octylphenol in which each molecule contains an average of two 
ethoxy groups and one sulfonate group which "caps" the chain of ethoxy 
groups with an ethanesulfonate group, available from Rohm and Haas 
Chemical Company. The oil-displacing activities were measured by means of 
the microscopic screening technique described in SPE Paper No. 3798 
presented at the Improved Oil Recovery Symposium in April 1972. In that 
procedure one observes the degree of deformation, breakup and stringer or 
filament formation of a microscopic drop of an oil in a stream of flowing 
surfactant system. 
FIG. 1 shows the results of the microscopic screening tests of the 
relationship between sodium and calcium ion concentrations on the oil 
displacement capability (and/or interfacial tension lowering activity) of 
dispersions of the above-described surfactants in solutions of the 
indicated molar amounts of sodium chloride and calcium chloride in 
distilled water with respect to a typical reservoir crude oil at a 
temperature of 75.degree. C. The data points denoted by squares refer to 
inactive (below optimum) surfactant systems, those marked by triangles 
relate to coacervate systems whch exhibit two clear liquid phases, and 
those marked by circles designate systems having effective oil displacing, 
low interfacial tensions against the oil. The vertically-hatched region in 
the lower left corner of FIG. 1 shows the solubility limits of the 
Brighton 430 petroleum sulfonate. It is believed to be clear from the data 
shown that the systems containing the Triton X-200 surfactant are much 
more effective than those containing the petroleum sulfonate surfactants. 
FIG. 2 shows the result of oil displacement tests of two surfactant systems 
in Berea cores having an effective permeability to water of about 300 md. 
The cores were 2 inches in diameter and 10 inches long and were maintained 
at a temperature of 75.degree. C. In each test the core was initially 
preflooded with the saline water to be used as a preflood followed by a 
crude oil which, at that temperature (with a 20% isooctane content), had a 
viscosity of 2.9 centipoise. The saline waters which were tested were 
synthetic reservoir water (designated as SDS water) containing 
approximately 11% sodium chloride, 1% magnesium chloride, 0.6% calcium 
chloride and minor amounts of barium and strontium, and distilled 
water-dilutions of the SDS water. 
For Test A the oil-containing core was preflooded with the SDS water to a 
residual oil saturation to water and then was flooded with an aqueous 
surfactant system consisting of 5.4% by weight Triton X-200 surfactant (on 
an active basis) in SDS water; which system has a viscosity of 1.3 
centipoise. A dispersion of Triton X-200 in the SDS water forms a milky 
suspension of an insoluble material which tends to cause face plugging of 
such a Berea core. The suspended material appears to consist of a 
non-ionic component, the presence of which was confirmed by nuclear 
magnetic resonance analysis. The surfactant system which was tested was 
subjected to centrifugation after which the supernatent layer was utilized 
in the test. 
As indicated by curve A, of FIG. 2, an oil saturation of less than 8% of 
the pore volume of the core was attained by injecting 1 pore volume of the 
surfactant system. 
In Test B the core was preflooded with half strength SDS water (with the 
diluent being distilled water) then flooded with a surfactant system 
consisting of 4.9% Triton X-200 surfactant (active basis) in 1/2 strength 
SDS water; which system had a viscosity of 1.05 centipoise. As shown by 
curve b, this provided very little displacement. 
The Triton X-200 surfactant has a combination of structure and size of 
alkyl groups and number of ethoxy groups adapting it to be at near optimum 
oil-displacing activity in 100% SDS water. An otherwise similar 
surfactant, Triton X-202, which contains an average of one ethoxy group 
per molecule was indicated (by the microscopic screening test of such 
surfactant in distilled water solutions of sodium chloride) to have an 
optimum oil-displacing activity at a salt concentration of only 1.0 to 1.5 
molar (rather than the 2 to 3 molar optimum region of Triton X-200. Such a 
difference in salinity requirement is indicative of how the number of 
ethoxy grups can be varied to provide an alkylaryloxypolyethoxyethane 
sulfonate surfactant for use in an aqueous liquid having a given salt 
content. 
FIGS. 3A and 3B show the relationship between surfactant concentration and 
optimum electrolyte concentration for an alkylaryloxypolyethoxyethane 
sulfonate surfactant system of the present invention, as compared to that 
of a petroleum sulfonate surfactant system of the prior art. Dispersions 
of the indicated percentages by weight of the surfactants in aqueous 
solutions of varying salt concentration (prepared by distilled water 
dilutions of the SDS water) were screened for their oil-displacing 
activity against a reservoir crude (by the microscope method). The 
rectangular bars appearing under the various surfactant concentrations 
indicate the regions of effective oil-displacing activity and the solid 
(or shaded) portions of those bars indicate the regions of highest or 
optimum activity. The tested petroleum surfactant (which was a Petronate 
10-80 sulfonate surfactant, comprising a sulfonated petroleum fraction 
having an equivalent weight of 420) exhibited the high response to 
concentration of surfactant that is typical of petroleum sulfonate 
surfactants. For example, the Petronate surfactant activity was optimum 
with salt concentrations equivalent to from about 15 to 18% SDS water, 
when the surfactant concentration was 5%, but, when the surfactant 
concentration was reduced to 1%, the optimum activity was limited to SDS 
water concentrations of only from about 5 to 6%. 
FIG. 3B shows the results of such tests of systems containing Triton X-200 
surfactant systems in the concentrations specified by the present 
invention. The difference is dramatic. From surfactant concentrations as 
low as 0.1% by weight to as high as 5.0% by weight, the range of salt 
concentrations that provided optimum oil displacing activity were the same 
for each concentration of the surfactant--and were equivalent to the high 
salinity of from 90 to 100% of the SDS water. 
Additional comparative oil displacement experiments were conducted in three 
Berea sandstone cores having diameters of 2" and lengths of 10". In each 
test the core was preflooded with SDS water, flooded with a typical crude 
oil then water flooded with SDS water to a residual oil saturation of 
about 39% plus or minus 1% of the pore volume of the core. In each test 
the surfactant system was used in the form of a slug (amounting to 25% of 
the core pore volume) which contained the Triton X-200 surfactant in SDS 
water in the concentrations indicated below, and was injected immediately 
ahead of a drive fluid of the type indicated below (with the core 
maintained at a temperature of 75.degree. C.). 
In the first test the surfactant system contained 7% by weight of the 
surfactant and was displaced through the core with SDS water. This 
resulted in a residual oil saturation of 27% of the core pore volume. 
In the second test the surfactant concentration was 7% and the surfactant 
system was displaced through the core with SDS water containing 1,000 
parts per million of xanflood biopolymer and having a viscosity of 10.4 
centipoises. This provided a residual oil saturation of 21.1% of the core 
pore volume. 
In the third experiment a concentration of the surfactant was only 5% and 
the surfactant system was displaced through the core by alternate 
injections of slugs having volumes of 10% of the core pore volume, of an 
aqueous solution of 0.2% of the same surfactant in SDS water and nitrogen 
gas. This reduced the residual oil saturation to only 13% of the core pore 
volume. 
In the first, in which no mobility control was provided, only a negligible 
amount of additional oil was recovered. In the third test, despite the 
lower chemical concentration of only 5% rather than 7%, the use of the 
drive fluid comprising the alternating slugs of liquid and nitrogen gas 
provided a significantly greater oil recovery than that obtained in the 
second test, in which the drive fluid was a conventional type polymer 
thickened aqueous liquid. The third test simulated a situation in which a 
highly saline reservoir was waterflooded with the reservoir brine, and 
then chemically flooded in accordance with the present process with the 
reservoir brine being used as the aqueous phase of the surfactant system. 
Since, as indicated by the present test, such a proedure is relatively 
insensitive to dilutions of the surfactant system, it substantially 
completely eliminated problems due to surfactant polymer interaction, 
cation exchanges within the reservoir, or polymer-surfactant 
incompatibilities. 
SUITABLE CHEMICALS AND TECHNIQUES 
The oil-bearing reservoirs to which the present invention is applied are 
preferably those containing a significant saturation such as from about 
20-50% of an oil having a viscosity of from about 1 to 10 centipoise at 
the reservoir temperature. Such reservoirs can be substantially any having 
permeabilities suitable for oil production by means of a waterflood 
process. 
In general, the alkylaryloxypolyethoxyethane sulfonate surfactants suitable 
for use in the present process are those which contain a total of from 
about 6 to 12 (preferably 8 to 9) carbon atoms in saturated or unsaturated 
alkyl groups attached to a phenyl group which is attached by an 
ether-oxygen atom to a chain of from about 1 to 6 (preferably 2 to 4) 
ethoxy groups which chain is terminated by an ethanesulfonate group. 
Numerous individual members of such compounds and procedures by which they 
can be manufactured are mentioned in "Surface Active Agents" by A.M. 
Schwartz and J.W. Perry, Vol. 1 and 2, Interscience Publishers, N.Y., 
1949, and U.S. Pat. Nos. 3,827,497; 3,977,471; 4,066,124; 4,018,278 and 
4,088,189. Particularly suitable compounds comprise 
octylphenoxypolyethoxyethane sulfonates having oil-displacing and 
foam-forming properties at least substantially equivalent to those of the 
Triton X-200 sulfonate surfactant. 
In the present process, in effect, both the chemical composition and 
concentration of both the surfactant and the aqueous liquid of the 
injected aqueous surfactant system are correlated with the physical and 
chemical properties of the rocks and oil and water of the reservoir. The 
chemical composition of the reservoir tailored surfactant and the aqueous 
liquid in which it is dispersed are such that the resulting surfactant 
system exhibits the above-described properties of oil-displacing activity, 
phase behavior, mobility, and avoidance of deleterious changes in salinity 
due to ion exchange within the reservoir, with the surfactant 
concentration being as low as about 0.1% by weight. In addition, at the 
time the surfactant system is injected, the surfactant concentration 
should be less than about 10% but more than about 0.1% by enough so that, 
within a significant portion of the surfactant system being displaced 
through the reservoir, the concentration will remain about 0.1% and the 
oil-displacing activity will remain high. In general, this is ensured by 
injecting the surfactant system at a sulfonate concentration of from about 
3 to 10% by weight, where the volume of the injected system is from about 
10 to 20% of the pore volume of the portion of the reservoir through which 
the injected fluid will be displaced. 
Where the mobility of the aqueous surfactant system prepared in accordance 
with the present process is undesirably high relative to that of the 
reservoir oil, the effective mobility of that system can readily be 
reduced by dispersing a noncondensible gas within at least a portion of 
the surfactant system. This utilizes applicant's discovery of the 
relatively low mobility foam-forming capabilities of the presently 
specified surfactant systems, and can be done in numerous ways. For 
example, (a) the injection of a relatively small proportion such as about 
0.1 pore volume of the swept zone of the reservoir of the surfactant 
system can be followed by an injection of a portion of noncondensible gas 
having a substantially equal volume at the pressure and temperature of the 
reservoir with such alternations being repeated until a suitable total 
volume of surfactant system, such as from about 10-20% of the pore volume 
of the swept zone of the reservoir has been injected, (b) the 
noncondensible gas can be injected simultaneously with some or all of the 
selected amount of surfactant system, (c) the proportion of noncondensible 
gas which is dispersed in liquid to form the drive fluid injected 
immediately behind the surfactant system can be made high enough to cause 
a relatively prompt migration of gas into the surfactant system, or the 
like. 
Aqueous liquids suitable for use in the present process can comprise 
substantially any aqueous liquid having a salinity which is the same as or 
is equivalent to that of the water present in a subterranean oil reservoir 
in which the predominant liquid phase is an aqueous solution containing at 
least about 9% by weight total dissolved salt. As used herein "salinity" 
refers to the total organic and inorganic electrolyte content of the 
aqueous liquid, including the small proportions of the surfactant sulfonic 
acid salts which are dissolved and ionized within the aqueous liquid phase 
of the surfactant system. Suitable aqueous liquids include the brines 
contained in high salinity oil reservoirs, and/or surfactant systems 
formed from such brine. Such aqueous liquids, preferably contain a ratio 
(in parts by weight) of about 10 parts monovalent cation to one part 
polyvalent cation. A preferred aqueous liquid is typified by the SDS water 
described above. 
The aqueous surfactant systems and the dispersions of noncondensible gas in 
aqueous saline liquid which are used in accordance with the present 
process can advantageously contain additives such as corrosion inhibitors, 
stabilizers, bactericides, and the like, such as those conventionally used 
in chemical flood oil recovery processes, as long as such additives are 
compatible with the surfactant systems and gas dispersions of the present 
invention. 
The noncondensible gas utilized in the present process can be substantially 
any which remain relatively inert and is neither significantly condensed 
or dissolved at the temperature, pressure and fluid content conditions 
created during the application of the process to an oil-containing 
reservoir. Examples of such gases include nitrogen, air, flue gas, 
combustion gas, and the like. Particularly suitable procedures for forming 
and/or injecting mobility-controlling liquids containing such gases are 
described in U.S. Pat. No. 3,653,440 and such procedures are incorporated 
herein by cross-reference.