Process for secondary recovery

Hydrocarbons are recovered from a subterranean hydrocarbon-bearing formation penetrated by an injection well and a production well by displacing hydrocarbons toward the production well using a drive fluid such as water thickened with polyacrylamide or partially hydrolyzed polyacrylamide or the sodium, potassium or ammonium salt thereof and a minor amount of polyacrylamide or partially hydrolyzed polyacrylamide or the sodium, potassium or ammonium salt thereof alkoxylated with an alkylene oxide. Optionally, the drive fluid can be saturated with carbon dioxide and/or natural gas at the injection pressure. An aqueous fracturing fluid containing a small amount of alkoxylated polyacrylamide or partially hydrolyzed polyacrylamide is also described.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a process for recovering hydrocarbons from a 
subterranean hydrocarbon-bearing formation penetrated by an injection well 
and a production well wherein a drive fluid such as water thickened with 
(I) polyacrylamide or partially hydrolyzed polyacrylamide or the sodium, 
potassium or ammonium salt thereof and (II) a minor amount of 
polyacrylamide or partially hydrolyzed polyacrylamide or the sodium, 
potassium ammonium salt thereof alkoxylated with ethylene oxide or a 
mixture of ethylene oxide and propylene oxide is utilized to displace 
hydrocarbons in the formation toward a production well. In another aspect 
this invention relates to a method of fracturing a formation using an 
aqueous fluid containing a small amount of alkoxylated polyacrylamide or 
partially hydrolyzed polyacrylamide. 
2. Prior Art 
The production of petroleum products is usually accomplished by drilling 
into a hydrocarbon-bearing formation and utilizing one of the well-known 
recovery methods for the recovery of hydrocarbons. However, it is 
recognized that these primary recovery techniques may recover only a minor 
portion of the petroleum products present in the formation particularly 
when applied to reservoirs of viscous crudes. Even the use of improved 
recovery practices involving heating, miscible flooding, water flooding 
and steam processing may still leave up to 70-80 percent of the original 
hydrocarbons in place. 
Thus, many large reserves of petroleum fluids from which only small 
recoveries have been realized by present commercial recovery methods, are 
yet to reach a potential recovery approaching their estimated 
oil-in-place. 
Water flooding is one of the more widely practiced secondary recovery 
methods. A successful water flood may result in recovery of 30-50 percent 
of the original hydrocarbons left in place. However, generally the 
application of water flooding to many crudes results in much lower 
recoveries. 
The newer development in recovery methods for heavy crudes is the use of 
steam injection which has been applied in several modifications, including 
the "push-pull" technique and through-put methods, and has resulted in 
significant recoveries in some areas. Crude recovery of this process is 
enhanced through the beneficial effects of the drastic viscosity reduction 
that accompanies an increase in temperature. This reduction in viscosity 
facilitates the production of hydrocarbons since it improves their 
mobility, i.e., it increases their ability to flow. 
However, the application of these secondary recovery techniques to depleted 
formations may leave major quantities of oil-in-place, since the crude is 
tightly bound to the sand particles of the formation; that is, the 
sorptive capacity of the sand for the crude is great. In addition, 
interfacial tension between the immiscible phases results in entrapping 
crude in the pores, thereby reducing recovery. Another disadvantage is the 
tendency of the aqueous drive fluid to finger, since its viscosity is 
considerably less than that of the crude, thereby reducing the efficiency 
of the processes. Another disadvantage is the tendency of the aqueous 
drive fluid to remove additional gas by diffusion from the in-place oil, 
thus further reducing the already lowered formation oil volume and 
increasing the viscosity of the oil. 
There is a definite need in the art for a water flooding process in which 
the disadvantages discussed above are largely eliminated or avoided. 
SUMMARY OF THE INVENTION 
This invention relates to a process for recovering hydrocarbons from a 
subterranean hydrocarbon-bearing formation penetrated by an injection well 
and a production well which comprises: 
(A) injecting into the formation via an injection well a drive fluid 
comprising water having dissolved therein about 0.05 to about 5 weight 
percent of polyacrylamide or partially hydrolyzed polyacrylamide or the 
sodium, potassium or ammonium salt thereof and a minor amount of 
polyacrylamide or partially hydrolyzed polyacrylamide or the sodium, 
potassium or ammonium salt thereof alkoxylated with a material selected 
from the group consisting of (a) ethylene oxide or (b) a mixture of 
ethylene oxide and propylene oxide wherein the weight percent of ethylene 
oxide in the said mixture is from about 60 to about 95, 
(B) forcing the said fluid through the formation and 
(C) recovering hydrocarbons through the production well. 
An additional embodiment of this invention relates to the driving fluid 
compositions utilized in step (A) which may be saturated with carbon 
dioxide and/or natural gas, if desired. 
DETAILED DESCRIPTION OF THE INVENTION 
Prior to practicing the process of this invention it is sometimes desirable 
to open up a communication path forcing a liquid such as water, oil or any 
other suitable hydrocarbon fraction into the formation at pressures of 
from about 300 to about 3000 psig which are sufficient to rupture the 
formation and to open up channels therein. By use of this method it is 
possible to position the fracture at any desired vertical location with 
respect to the bottom of the oil-filled zone. It is not essential that the 
fracture planes be horizontally oriented, although it is, of course, 
preferable that they be so oriented. After the fracture has been 
established, and without diminishing the fracture pressure, a propping 
agent may be injected into the fraction in order to prevent healing of the 
fracture which would destroy its usefulness for fluid flow communication 
purposes. Gravel, metal shot, glass beads, sand, etc. and mixtures thereof 
are generally employed as propping agents. When sand is utilized as the 
propping agent particles having a Tyler mesh size of from about 8 to about 
40 are preferred (i.e., from about 0.016 to about 0.093 inches). 
Generally, the number average molecular weight of the polyacrylamide or 
partially hydrolyzed polyacrylamide or salts thereof utilized in this 
invention and of the alkoxylated polyacrylamide or partially hydrolyzed 
polyacrylamide or salts thereof will range from about 10,000 to about 
2,000,000 or more. Polyacrylamide, partially hydrolyzed polyacrylamide or 
salts thereof which are manufactured and sold by a number of chemical 
manufacturers, are prepared by the usual vinyl compound polymerization 
methods. 
The partially hydrolyzed polyacrylamides which are useful in preparing the 
ethoxylated, partially hydrolyzed polyacrylamides employed in the drive 
fluid of this invention consist of about 12 to about 67 mole percent of 
recurring units of the formulas: 
##STR1## 
where M represents hydrogen, sodium, potassium or ammonium and about 33 to 
88 mole percent of recurring units of the formula: 
##STR2## 
The preparation of such partially hydrolyzed polyacrylamides is well known 
in the art and is described in detail in U.S. Pat. Nos. 3,039,529; 
3,002,960; 3,804,173, etc. 
The alkoxylated polymers employed in the process of this invention comprise 
(1) polyacrylamide or (2) partially hydrolyzed polyacrylamide or the 
sodium, potassium or ammonium salt thereof alkoxylated with, i.e., reacted 
with, from about 2 to about 100 percent by weight of ethylene oxide or 
with a mixture of ethylene oxide and propylene oxide wherein the weight 
percent of ethylene oxide in the said mixture is about 60 to about 95. In 
another embodiment, alkoxylated polymers useful in the secondary recovery 
process of this invention are prepared by reacting polyacrylamide or 
partially hydrolyzed polyacrylamide with 2,3-epoxy-1-propanol (i.e., 
glycidol). 
The alkoxylation of the acrylamide polymers, i.e., the reaction of these 
polymers with an alkylene oxide, can be conveniently conducted using 
methods well known in the art. For example, an aqueous solution comprising 
about 10 to about 30 weight percent or more of the acrylamide polymer in 
water along with about 0.5 weight percent or more of potassium hydroxide 
or sodium hydroxide is charged to an autoclave and the autoclave and 
contents heated to a temperature of about 125.degree. to about 200.degree. 
C. after which the required weight of ethylene oxide or a mixture of 
ethylene oxide and propylene oxide is pressured with nitrogen into the 
autoclave over a period of 1 to about 3 hours or more following which the 
autoclave is allowed to cool to room temperature and then vented. The 
reaction product remaining after being stripped to remove volatile 
materials yields the water-soluble, alkoxylated polymer. 
A number of other methods are set out in the art for conducting such 
alkoxylation reactions including those described in U.S. Pat. Nos. 
2,213,477; 2,233,381; 2,131,142; 2,808,397; 3,879,475; 2,174,761; 
2,425,845; 3,062,747; 3,380,765 and German Offen. No. 2,021,066 of Nov. 
11, 1971 (CA 76 86780b (1972)). 
The following example which illustrates the preparation of the alkoxylated 
acrylamide polymers is to be considered not limitative.

EXAMPLE I 
A total of 450 cc of water, 5 g. of powdered potassium hydroxide and 65 g. 
of polyacrylamide (number average molecular weight of about 250,000) are 
added to an autoclave which is then heated to a temperature of 125.degree. 
C. Ethylene oxide in the amount of 40 g is added to the autoclave under 
nitrogen pressure over a 1.05 hour period during which time the 
temperature of the autoclave is maintained at 130.degree. C. Next, the 
autoclave and contents are allowed to cool to room temperature after which 
the autoclave is vented. The reaction mixture is then stripped of 
volatiles using a nitrogen purge. The resulting water-soluble product is 
polyacrylamide alkoxylated with about 37 weight percent of ethylene oxide. 
In the secondary recovery process of this invention, generally the aqueous 
drive fluid will contain from about 0.05 to about 5 weight percent or more 
of the polyacrylamide or partially hydrolyzed polyacrylamide or the 
sodium, potassium or ammonium salt thereof and from about 0.01 to about 
2.0 weight percent of the alkoxylated polyacrylamide or partially 
hydrolyzed polyacrylamide or salts thereof. Optionally,, the aqueous drive 
fluid may be saturated with carbon dioxide and/or natural gas at the 
injection pressure which generally will be from about 300 to about 3000 
psig or more. 
If desired, the aqueous drive fluid having dissolved therein the 
above-described polymeric thickening agent may be made alkaline by 
addition of an alkaline agent. The advantageous results achieved with the 
aqueous alkaline medium used in the process of this invention are believed 
to be derived from the wettability improving characteristics of the 
alkaline agent. 
Useful alkaline agents include compounds selected from the group consisting 
of alkali metal hydroxides, alkaline earth metal hydroxides, and the basic 
salts of the alkali metal or alkaline earth metals which are capable of 
hydrolyzing in an aqueous medium to give an alkaline solution. The 
concentration of the alkaline agent employed in the drive fluid is 
generally from about 0.005 to about 0.3 weight percent. Also, alkaline 
materials such as sodium hypochlorite are highly effective as alkaline 
agents. Examples of these especially useful alkaline agents include sodium 
hydroxide, potassium hydroxide, lithium hydroxide, ammonium hydroxide, 
sodium hypochlorite, potassium hypochlorite, sodium carbonate and 
potassium carbonate. 
A wide variety of surfactants such as linear alkylaryl sulfonates, alkyl 
polyethoxylated sulfates, etc. may also be included as a part of the 
aqueous drive fluid composition. Generally about 0.001 to about 1.0 or 
more weight percent of the surfactant will be included in the drive fluid. 
This invention is best understood by reference to the following example 
which is offered only as an illustrative embodiment of this invention and 
is not intended to be limitative. 
EXAMPLE II 
In a field in which the primary production has already been exhausted, an 
injection well is completed in the hydrocarbon-bearing information and 
perforations are formed between the interval of 3970-3985 feet. A 
production well is drilled approximately 410 feet distance from the 
injection well, and perforations are similarly made in the same 
hydrocarbon-bearing formation at 3975-3990 feet. 
The hydrocarbon-bearing formation in both the injection well and the 
production well is hydraulically fractured using conventional techniques, 
and a gravel-sand mixture is injected into the fracture to hold it open 
and prevent healing of the fracture. 
In the next step water saturated with carbon dioxide at 1200 psig at a 
temperature of 70.degree. F. to which there has been added about 0.22 
weight percent sodium hydroxide and containing dissolved therein 0.32 
weight percent of a polyacrylamide having a number average molecular 
weight of about 135,000 and 0.12 weight percent of the sodium salt of 
partially hydrolyzed polyacrylamide having about 16 mole percent of the 
carbonamide groups originally present in the polyacrylamide hydrolyzed to 
carboxyl groups and having a number average molecular weight of 300,000 
alkoxylated with about 13 weight percent of ethylene oxide in the manner 
previously described in Example I above is injected via the injection well 
into the formation at a pressure of about 1200 psig and at the rate of 
0.95 barrel per minute. Injection of the driving fluid is continued at the 
rate of about 1 barrel per minute and at the end of 63 days the rate of 
production of oil is substantially greater than with water injection 
alone. 
Another embodiment of this invention relates to a method of fracturing a 
fluid-bearing formation such as a hydrocarbon-bearing formation whereby 
artificial channels or fractures of high fluid conductivity within the 
formation can be formed. The pressures employed in the fracturing process 
may range from 500 to 10,000 psi or more (measured at the surface). 
The method of this invention for fracturing a fluid-bearing formation 
penetrated by a well comprises injecting a viscous fracturing fluid down 
the well and into contact with the said fluid-bearing formation at a 
pressure and volume flow rate sufficient to fracture the said formation 
and wherein the viscous fracturing fluid comprises an aqueous solution of 
a water-soluble, high molecular weight, partially hydrolyzed 
polyacrylamide alkoxylated with an alkylene oxide and, optionally, a small 
amount of amorphous colloidal silica. 
The novel viscous fracturing fluid comprises an aqueous base, which is 
preferably water, containing from about 0.01 to about 1.0 weight percent 
of a water-soluble, partially hydrolyzed polyacrylamide alkoxylated in the 
manner previously described above with about 2 to 100 weight percent or 
more of ethylene oxide or a mixture of ethylene oxide and propylene oxide 
wherein in the said mixture the weight percent of ethylene oxide is about 
65 to about 95. If desired, the fracturing fluid may include suspended 
therein about 0.2 to about 4.8 pounds per gallon of a propping agent such 
as sand grains having a Tyler mesh size of about 8 to about 40. Further, 
the fracturing fluid may contain suspended therein about 0.05 to about 0.5 
weight percent of amorphous colloidal silica having a particle size range 
of from about 7 to 15 millimicrons such as that sold by the Cabot Corp. of 
Boston, Mass. under the tradename CAB-O-SIL.RTM.. Surfactants such as 
ethoxylated nonyl phenol, etc. can also be added to the fracturing fluid 
to increase the viscosity. 
The number average molecular weight of the partially hydrolyzed 
polyacrylamides useful in preparing the alkoxylated polyacrylamides 
employed in the fracturing fluids of this invention will range from about 
500,000 to about 2,000,000 or more. The partially hydrolyzed 
polyacrylamides utilized in forming the alkoxylated materials will have 
from about 12 to 67 mole percent of the original carboxamide groups 
hydrolyzed to carboxyl groups. 
The fracturing method of this invention is illustrated by the following 
example which is to be considered not limitative. 
EXAMPLE III 
An oil bearing formation at 3440-3505 ft. composed primarily of limestone 
having good porosity (about 23%) contains a large quantity of oil (about 
62% of the pore space) and adequate reservoir pressure (about 650 psi) but 
has a very low permeability (less than 1.1 millidarcies) and the 
productivity is about 4 bbls. oil per day. 
The formation is fractured in 4 stages using conventional techniques for 
proportioning the fracture fluid into each set of perforations. The 
fracture fluid is water having 0.2 weight percent of partially hydrolyzed 
polyacrylamide having about 16 mole percent of the originally present 
carboxamide groups hydrolyzed to carboxyl groups and having a number 
average molecular weight of about 655,000 which provides a viscosity 
sufficient to permit a sand concentration of 3 lb./gal. 
The formation is fractured using this fluid and sand (100,000 lbs. total). 
The sand size is: 
______________________________________ 
First 80,000 lb. 20-40 mesh 
Remaining 20,000 lb. 10-12 mesh 
______________________________________ 
The well is placed on production and produces at a rate substantially above 
that experienced before the fracturing step was completed. 
A substantial reduction in the friction loss in the fracturing process of 
this invention can be achieved when the proppants, such as sand grains, 
employed in the viscous aqueous fracturing fluid are coated with, for 
example, a thin film of polyacrylamide or partially hydrolyzed 
polyacrylamide in which about 12 to about 67 mole percent of the original 
carboxamide groups have been hydrolyzed to carboxyl groups, propoxylated 
with about 20 to about 100 weight percent of propylene oxide or a mixture 
of propylene oxide and ethylene oxide where the weight percent of 
propylene oxide in the mixture ranges from about 65 to about 95. The 
propoxylated polyacrylamides which are water insoluble can be prepared by 
methods well known in the art by reacting, for example, a propylene glycol 
of suitable molecular weight with the polyacrylamide in the presence of 
sodium hydroxide at a temperature of about 100.degree. C. The number 
average molecular weight of the polyacrylamide or partially hydrolyzed 
polyacrylamide suitable for use in making the propoxylated materials will 
vary from about 200,000 to about 2,000,000 or more. 
The thickness of the propoxylated polymer coating on the proppant needed 
will range from about 5 microns up to a preferred thickness about 0.001 
inch to about 0.005 inch although a thickness more or less than set out in 
this range can be utilized, if desired. 
In preparing polymer-coated, sand grains a solution of from 0.01 to about 
1.5 percent or more by weight of the propoxylated polymer is first 
prepared by dissolving the polymer in ethyl alcohol, acetone or any other 
suitable solvent, at temperatures about 50.degree. to about 100.degree. C. 
Sand grains having a Tyler mesh size of about 8 to 40 are then added with 
stirring to the thus-prepared polymer solution after which the 
polymer-coated grains are separated from the solution and dried. In a 
specific illustration of the preparation of such polymer-coated sand 
grains a solution of 0.03 weight percent is formed by adding partially 
hydrolyzed polyacrylamide having about 16 mole percent of the carboxamide 
groups hydrolyzed to carboxyl groups alkoxylated with 32 weight percent of 
propylene oxide (number average molecular weight 650,000) with stirring to 
1,000 gallons of ethyl alcohol at 70.degree. C. A total of 2,000 lbs. of 
clean sand having a Tyler mesh size of 20 is added with vigorous mixing to 
the alcohol solution maintained at about 105.degree. C. Mixing is 
continued for about 25 minutes after which the coated sand grains are 
recovered by filtration and then dried by tumbling in hot air at a 
temperature of about 160.degree. F.