Carbon dioxide foam flooding

A method of conducting an enhanced oil recovery process in a subterranean reservoir wherein, as part of the process, there is injected into the reservoir as a sweep fluid a foam containing carbon dioxide, water and a foaming agent having the formula: ##STR1## wherein R is a straight chain alkyl radical having from 10 to 16 carbon atoms; and M is an alkali metal or ammonium cation, with sodium being preferred. A particularly preferred foaming agent is sodium lauryl sulfoacetate.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a method of enhanced oil recovery utilizing a 
foamed carbon dioxide sweep fluid. More particularly the invention relates 
to such a method utilizing a foam having improved stability. 
2. Description of the Prior Art 
Methods of enhanced oil recovery involving injection of carbon dioxide at 
high pressures into a reservoir via an injection well while withdrawing 
reservoir fluids via a production well are well known in the art. Such a 
process is described by Whorton et al. in U.S. Pat. No. 2,623,596. later 
it was discovered that recovery of petroleum could be increased in such 
processes if the carbon dioxide was used in slug form and driven through 
the reservoir by an aqueous drive fluid such as brine, plain or carbonated 
water. A typical process of this type is described by Holm in U.S. Pat. 
No. 3,065,790. Next, it was discovered that the areal sweep efficiency and 
the overall efficiency of the carbon dioxide-waterflood enhanced oil 
recovery process could be improved by incorporating a foaming agent and 
water into the slug of carbn dioxide to form a foam. U.S. Pat. No. 
3,342,256 to Bernard et al. describes such a process utilizing a wide 
variety of foaming agents including water-soluble salts of esters of 
C.sub.3 -C.sub.6 dicarboxylic acids, such as monosodium dioctyl 
sulfosuccinate and the like. 
In U.S. patent application Ser. No. 621,685, filed Oct. 14, 1975 by Fischer 
et al., now U.S. Pat. No 4,036,764, there is described a method of 
drilling and workover in a high temperature well employing as the drilling 
or workover fluid a foam containing a gas, such as air, carbon dioxide or 
the like and an aqueous solution of a foaming agent having the formula: 
##STR2## 
wherein R is a straight chain alkyl radical having from 10 to 16 carbon 
atoms, and M is an alkali metal or ammonium cation, with sodium being 
preferred. A particularly preferred foaming agent is sodium lauryl 
sulfoacetate. 
It has been the experience that in spite of the wide variety of foaming 
agents known for foaming aqueous base media, the foaming of a mixture of 
carbon dioxide and water for use as a sweep fluid in an enhanced oil 
recovery process presents many problems. The mixture of carbon dioxide and 
water to be foamed constitutes an acidic composition. Many foaming agents 
are not stable in acidic media and tend to decompose. Due to the length of 
time required to carry out an enhanced oil recovery process, it is 
necessary that the foamed carbon dioxide remain in the foamed state for a 
prolonged period of time while passing through a reservoir which often has 
an elevated temperature. Many foaming agents satisfactorily foam carbon 
dioxide initially, but the foam tends to break down after a brief period 
of time, such as a day or two. Other foaming agents are capable of foaming 
carbon dioxide at atmospheric or relatively low pressure but fail to form 
satisfactory foams under the high pressure conditions at which enhanced 
oil recovery processes are often carried out. At low pressures, carbon 
dioxide tends to behave as a gas. At high pressures, carbon dioxide 
becomes a viscous fluid and takes on many of the characteristics of a 
liquid. Some foaming agents must be present in a relatively high 
concentration to foam carbon dioxide. Due to the large volume of injected 
fluids required in the instant process, it is desired to keep the 
concentration of foaming agent in the foam to a minimum. Thus, there is a 
need for an enhanced oil recovery process which employs a carbon dioxide 
foam which is stable at the conditions under which the process is carried 
out. 
Accordingly, a principal object of this invention is to provide a method of 
enhanced oil recovery using an aqueous base carbon dioxide foam as a sweep 
fluid. 
A further object of this invention is to provide such a method conducted in 
a high temperature reservoir. 
A still further object of this invention is to provide such a method 
utilizing a foaming agent which foams carbon dioxide at high pressure. 
Another object of this invention is to provide such a method utilizing a 
carbon dioxide foam which is stable over a long period of time at elevated 
pressure. 
Other objects, advantages and features of this invention will become 
apparent to those skilled in the art from the following discussion and 
appended claims. 
SUMMARY OF THE INVENTION 
Briefly, this invention involves a method of conducting an enhanced oil 
recovery process in a subterranean reservoir wherein there is injeced into 
the reservoir via an injection well as a sweep or displacement fluid a 
mixture of carbon dioxide at high pressure, water, and, as a foaming 
agent, an alkali metal or ammonium salt of an alkyl sulfoacetate wherein 
the alkyl radical is straight chained and has from 10 to 16 carbon atoms. 
The sweep fluid can be followed by a drive fluid to push the sweep fluid 
through the reservoir towards a production well through which reservoir 
fluids are produced. 
DETAILED DESCRIPTION OF THE INVENTION 
In enhanced oil recovery processes, a sweep or displacement fluid is 
injected into a hydrocarbon-bearing resevoir via an injection well to 
displace reservoir hydrocarbons through the reservoir towards a production 
well through which they are produced. The sweep fluid may constitute the 
entire body of enhanced oil recovery fluid or a slug of sweep fluid can be 
followed by a drive fluid, such as thickened or unthickened plain or 
carbonated water or brine or a gas such as air, carbon dioxide, combustion 
gases or hydrocarbon gases. In the instant invention, the sweep fluid is a 
foam containing carbon dioxide, water and an alkali metal or ammonium salt 
of an alkyl sulfoacetate as a foaming agent. The foam has good stability 
under pressures of the magnitude commonly encountered in subterranean 
reservoirs. The stability is even greater at elevated pressures than it is 
at lower pressures. 
The foam may be formed in any one of a number of ways. The foam may be 
generated at the surface by combining the ingredients in a suitable mixing 
device, such as a foam generator. The foam is then injected into the 
reservoir. More commonly the foam is generated in situ by injecting via 
the injection well the ingredients of the foam either separately or 
simultaneously. Alternatively, two or more of the components can be mixed 
together at the surface prior to introduction into the well. In these 
instances, the foam forms during its passage down the well and in the 
reservoir in the vicinity of the well. When the components are injected 
separately, the foaming agent and water should be introduced before or at 
the same time as the carbon dioxide. 
It must be noted that the enhanced oil recovery processes described above 
in which carbon dioxide is used as a sweep fluid employ the carbon dioxide 
at pressures in excess of about 700 p.s.i. It is well known that the 
solubility characteristics of carbon dioxide have a distinct effect on oil 
recoveries when the carbon dioxide is at pressures in excess of about 700 
p.s.i. At these high pressures, carbon dioxide exists as a dense fluid or 
liquid, rather than as a gas, even though the critical temperature of 
carbon dioxide is about 89.degree. F. That is to say, carbon dioxide has 
not been liquefied at temperatures above 89.degree. F., regardless of the 
pressures applied. However, below 89.degree. F., carbon dioxide exists 
either as a gas, a dense fluid or a liquid, depending upon pressures 
applied. Inasmuch as the typical pressures employed in enhanced oil 
recovery processes when carbon dioxide is used are in excess of 700 
p.s.i., and the temperatures are below about 200.degree. F., the carbon 
dioxide exists as a dense fluid, rather than as a gas, and, in most 
typical situations where the reservoir temperature is below 89.degree. the 
carbon dioxide exists as a liquid. A dense fluid is more like a liquid 
than a gas, as evidenced by considering solubility factors. For instance, 
a foaming agent will readily dissolve in carbon dioxide when the carbon 
dioxide exists as a dense fluid, whereas it will not dissolve in the 
carbon dioxide when the carbon dioxide exists in the gaseous state. 
The amount of the carbon dioxide injected into the reservoir will of course 
vary for different reservoirs, and will be dependent upon total reservoir 
pore volume, hydrocarbon pore volume, and other unique reservoir 
characteristics. However, throughout this description and claims the term 
"effective pore volume" is to be taken to mean that portion of the 
reservoir which is expected to be contacted by the carbon dioxide 
injected. "Effective pore volume" is determined by conventional laboratory 
and field techniques, well known to petroleum engineers. 
In carrying out the process of this invention, about 0.01 to 0.2 effective 
pore volume of foam containing about 88 to 98.9 volume percent carbon 
dioxide measured at reservoir conditions of temperature and pressure, 
about 1 to 10 volume percent water and about 0.1 to 2 volume percent 
foaming agent, is injected through an injection well and into the 
reservoir. Depending upon the reservoir temperature and the pressure 
utilized, the carbon dioxide will exist either as a liquid or as a dense 
fluid. The carbon dioxide is preferably injected into the reservoir as a 
liquid and under sufficient pressure to maintain it in the reservoir as 
either a dense fluid or a liquid. The subsequently injected drive medium 
is injected in an amount sufficient to drive the carbon dioxide through 
the reservoir from the injection to a production well. Injection of the 
driving fluid is continued until the liquids produced in the production 
well have a high water/oil ratio, at which time injection of drive fluid 
is terminated. Subsequent to the termination of driving fluid injection, 
the reservoir is depressured to allow the dissolved gases to come out of 
solution and thereby to form additional foam to drive additional oil 
towards the production well. 
In another embodiment of this process, the foaming agent may be injected 
into the reservoir prior to injection of the carbon dioxide slug. For 
instance, the foaming agent may be incorporated into a liquid such as 
water, mineral oil, or those hydrocarbons which are normally liquid at 
pressures in excess of about 700 p.s.i., such as LPG, propane, etc. In 
this instance, since the water or hydrocarbon acts as a carrier vehicle 
for the foaming agent, it is necessary that the foaming agent be soluble 
in the vehicle in which it is to be incorporated. Where the foaming agent 
is incorporated in a liquefied light hydrocarbon and is introduced into 
the reservoir ahead of the carbon dioxide slug, the presence of cracks and 
fissures, etc., wherein very high pressure gradients exist, will cause the 
liquefied light hydrocarbon to flash to a gas thereby generating foam at 
these areas of very high permeability. When the foaming agent is 
incorporated in a water vehicle, the water may contain carbon dioxide or 
other dissolved gases in amounts sufficient to cause them to come out of 
solution and generate foam when an area of low pressure is encountered. 
When the foaming agent is introduced into the reservoir prior to injection 
of the carbon dioxide slug, it is readily apparent that the subsequently 
injected carbon dioxide will not channel through highly permeable strata 
inasmuch as the foam will retard such channeling. Not only does formation 
of foam at the highly permeable strata, streaks or fissures retard loss of 
carbon dioxide from the zone of interest, but any foam increases its 
viscosity, thereby making the carbon dioxide a much more efficient 
displacing fluid. While the foaming agent-containing solution may have 
incorporated in it dissolved gas which will come out of solution, it is 
also possible to inject an aqueous or non-aqueous foaming agent-containing 
solution and follow it with gases such as carbon dioxide, methane, etc. 
When this mode is utilized, foam will be formed within the immediate 
vicinity of the injection well and will preferentially be driven into the 
more highly permeable strata rather than the less permeable strata, the 
solution taking the path of least resistance. The carbon dioxide then 
injected will more readily penetrate the less permeable strata because of 
the plugging effect created by the foam in the highly permeable strata. 
The foaming agent employed in the process of this invention, which is 
capable of producing a stable form under reservoir conditions when 
intimately contacted with water and carbon dioxide, is an alkali metal or 
ammonium salt of a carboalkoxy methane sulfonic acid, werein the alkyl 
radical is straight chained and has from 10 to 16 carbon atoms, and is 
characterized by the following generalized formula: 
##STR3## 
wherein R is the alkyl radical; and M is the alkali metal or ammonium 
cation, with the sodium cation being preferred. Specifically, the alkyl 
radical can be n-decyl, n-undecyl, n-dodecyl, n-tridecyl, n-tetradecyl, 
n-pentadecyl, or n-hexadecyl. A particularly preferred foaming agent is 
sodium lauryl sulfoacetate, which has the formula described above wherein 
R is a n-dodecyl radical and M is a sodium cation, and which is marketed 
by Stepan Chemical Company under the trademark Lathanol LAL 70 which is 70 
percent active. 
The water used in the foam is preferably fresh water or brine containing 
less than about 3 percent by weight salt content. If brine containing a 
higher concentration of salts is employed, the foam is difficult to form 
and the foam that does form has decreased stability. 
In one embodiment of this invention, an aqueous foaming agent solution is 
prepared by admixing the foaming agent in a carrier such as water, such 
that the solution contains about 0.20 to 10 weight percent of the foaming 
agent, preferably about 0.5 to 2 weight percent, and more preferably about 
1 weight percent of the foaming agent.

The invention is further described by the following examples which are 
illustrative of prior art methods and specific modes of practicing the 
invention and are not intended as limiting the scope of the invention as 
defined by the appended claims. 
EXAMPLES 1-10 
To illustrate the persistence of foam in oil-bearing reservoirs, laboratory 
tests are made to determine the stability at elevated pressure of foam 
produced from carbon dioxide, water and various foaming agents in the 
presence of oil. In a Jurguson windowed high pressure gauge having a 
volume of 90 milliliters (mls.), there are mixed 21 mls. synthetic tap 
water containing 300 parts per million (p.p.m.) sodium chloride, 170 
p.p.m. calcium chloride and 30 p.p.m. magnesium chloride, 1.3 mls. Kansas 
crude oil and 2,000 p.p.m. by volume foaming agent. The air space in the 
gauge is purged with carbon dioxide. Additional carbon dioxide is 
introduced to raise the pressure of the system to various values. The cell 
is then shaken vigorously by hand for 15 seconds to form a foam. The 
specific volume of the foam and the length of time the foam persists 
before breaking completely, i.e., the foam stability, are observed. The 
results of these tests are shown in the Table. These results show that 
while both foaming agents tested form a satisfactory foam under these 
conditions, the stability of the foams made using the foaming agent of 
this invention, Examples 1-5, is far greater than the stability of the 
foams made using an alkyl aryl polyether alcohol, a well known foaming 
agent, Examples 6-10. Also, the stability of the foam made using the 
foaming agent of this invention appears to increase dramatically with 
increasing pressure whereas that of the foam made using the well known 
foaming agent decreases with increasing pressure. 
TABLE 
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FORMATION AND STABILITY OF CARBON DIOXIDE 
FOAM UNDER PRESSURE 
Specific 
Foaming Volume of 
Foam 
Pressure Agent Foam Stability 
Example 
(p.s.i.g.) 
(2,000 p.p.m.) 
(mls./gram) 
(seconds) 
______________________________________ 
1 925 Lathanol LAL 70* 
5.7 92 
2 1,135 " 3.4 308 
3 1,285 " 1.9 1,267 
4 1,630 " 1.4 1,800 
5 2,015 " 1.3 2,100 
6 900 Triton X-100** 
6.1 20 
7 1,125 " 3.2 26 
8 1,325 " 1.7 40 
9 1,590 " 1.4 19 
10 1,975 " 1.3 4 
______________________________________ 
*Lathanol LAL 70 foaming agent is sodium lauryl sulfoacetate, 70 percent 
active, in flake form marketed by Stepan Chemical Company. 
**Triton X-100 foaming agent is an alkyl aryl polyether alcohol, 
octylphenoxy polyethoxy ethanol marketed by Rohm and Hass Co. 
EXAMPLE 11 
As a specific example of one of the embodiments of the process of this 
invention, a foam flooding process is carried out in a five-spot well 
pattern in which the central well is an injection well. The five-spot 
pattern is in an oiil-containing reservoir having a temperature of 
135.degree. F. and a pressure of 2,000 p.s.i.g. An aqueous foaming agent 
solution is prepared at the surface by mixing together 99 parts by weight 
fresh water and 1 part by weight Lathanol LAL 70, sodium lauryl 
sulfoacetate, foaming agent. Since the foaming agent is 70 percent active, 
the aqueous foaming agent solution contains 0.7 percent by weight active 
foaming agent. The aqueous foaming agent solution is injected into the 
reservoir via the injection well at the rate of 100 barrels per day until 
a volume equal to 1 percent of the effective pore volume of the reservoir 
has been injected. Next, 150 barrels per day of carbon dioxide is injected 
until a volume equal to 10 percent of the effective pore volume of the 
reservoir has been injected. The injected carbon dioxide is a liquid at 
the surface but becomes a critical fluid in the reservoir. As the carbon 
dioxide is injected, a substantial back pressure is produced which 
indicates the in situ generation of a foam bank in the reservoir. The 
injection of carbon dioxide is followed by water as a drive fluid to drive 
the oil and injected fluids toward the four production wells. Injection of 
the drive fluid and production of oil via the production wells are 
continued until the produced water/oil ratio reaches an uneconomical 
level. 
Various embodiments and modifications of this invention have been described 
in the foregoing specification, and further modifications will become 
apparent to those skilled in the art. Such modifications are included 
within the scope of this invention as defined by the following claims.