Gel and method for reducing steam channeling

A gel-forming composition is provided comprising a PVA based substance selected from the group consisting of a polyvinyl alcohol, a polyvinyl alcohol copolymer, and mixtures thereof, an aldehyde operable for crosslinking with the PVA based substance, and water. The gel-forming composition is useful for reducing steam channeling in subterranean formations. For example, a method is provided for diverting the flow of injected steam in nonproductive steam channels in an oil reservoir during waterflood and steam cycling operations.

TECHNICAL FIELD 
This invention relates to gels, methods of forming gels, and process for 
reducing steam losses in steam channels. A polyvinyl alcohol 
based-aldehyde hydrogel, or gel, is provided which is useful for plugging 
nonproductive steam channels. The gels of this invention are particularly 
valuable because they have improved stability. The gel-forming 
compositions of this invention can be placed into the steam channels and 
are less damaging to the oil-bearing part of the reservoir than many other 
stable plugging agents. 
BACKGROUND OF THE INVENTION 
Steam injection has significantly increased oil production in many 
reservoirs but frequently the efficiency of oil recovery is low because of 
steam channeling. Due to density differences between steam and reservoir 
fluids, steam rises to the top of the nonproductive steam reservoir 
overriding the oil body and penetrates the formation thereby creating a 
nonproductive steam channel. For example, in steam drive floods the 
rejected steam prematurely breaks through the formation and into the 
producing wells in such a manner that the more productive oil-bearing 
parts of the reservoir are short circuited. Steam therefore tends to open 
up channels which did not exist before and many of these channels are 
nonproductive. These nonproductive channels are difficult to plug, 
particularly for long periods of time. Most of the commonly used polymer 
based plugging agents used to plug off water will degrade at steam 
temperatures and lose their plugging capability within a few days. The 
more temperature stable plugging agents usually are too viscous to be 
effective in plugging the usually smaller and more difficult to reach 
nonproductive steam channels. For these reasons, long-term and indepth 
plugging of nonproductive steam channels is considered to be very 
difficult if not impossible to achieve. Consequently, a long-term plugging 
agent stable at steam temperatures would be of great benefit to the oil 
producing industry. 
Presently foaming agents are used to divert steam but because of their 
instability at steam temperatures they tend to break down within a few 
days and are no longer effective for preventing steam loss. As a result, 
foam-forming agents usually must be injected once or twice a week in order 
to plug the steam channels and divert the steam into the more productive 
part of the reservoir. Examples of foam-forming agents and surfactants 
used to divert steam are sulfonates of alpha-olefins, or blends thereof 
such as sodium alkene sulfonate and hydroxy sulfonate, or sodium and amino 
oxyethylene sulfates either with or without admixing with carboxymethyl 
cellulose, or aliphatic sulfonates such as sodium dodecylbenzene 
sulfonate. Carboxymethyl cellulose or "CMC" is thought to encapsulate the 
steam foam and prevents the foaming agent from being activated until the 
CMC thermally degrades. This method was designed to permit the foam to 
penetrate deeper into the formation. Unfortunately the method does not 
appear to be particularly successful and apparently produces no better 
results than the use of the foam agent without CMC. Furthermore, the 
blending of the foaming agent with CMC ostensibly is very costly since 
copious quantities of materials appear to be necessary. Unfortunately most 
of the foaming agents break down at steam temperature and are usually only 
effective for up to about 4 days thereby requiring frequent reinjection of 
foaming agents which is both time consuming and costly. 
Lignosulfonates have been proposed as a plugging agent in U.S. Pat. No. 
4,074,757 but their use, it is believed, has had very limited success and 
relatively very large amounts are believed to be required. Although they 
form a permanent gel at high temperatures, in order to prevent serious 
damage to oil-bearing zones it is believed that it would be necessary to 
isolate the treatment to the nonproductive steam channels. Such isolation 
obviously is very difficult to achieve. These problems are perhaps part of 
the reason why lignosulfonates do not seem to be widely used. 
Thus there is a need in the oil producing industry for a plugging agent 
which is stable at steam temperature, which is mobile enough to be carried 
deeply into the nonproductive steam channels, and which is sufficiently 
compatible with the oil-bearing part of the reservoir that it will not 
seriously damage it. 
A method of reducing the flow of fluid, specifically water, has been 
described in U.S. Pat. No. 3,762,476 wherein a first aqueous polymer 
solution selected from the group consisting of polyacrylamide, a partially 
hydrolyzed polyacrylamide, a polysaccharide, a carboxymethylcellulose, a 
polyvinyl alcohol, and polystyrene sulfonate, is injected into a 
subterranean formation. Thereafter, a complexing ionic solution of 
multivalent cations and retarding anions, and which also comprises 
aluminum citrate, is injected into the subterranean formation. The 
multivalent cations are selected from the group consisting of Fe(II), 
Fe(III), Al(III), Ti(IV), Zn(II), Sn(IV), Ca(II), Mg(II), Cr(III), and the 
retarding anions are selected from the group consisting of acetate, 
nitrilotriacetate, tartrate, citrate, phosphate. Brine is then injected 
followed by a second slug of an aqueous polymer solution which can be the 
same or different from the first aqueous polymer solution. In any event, 
the complexing ionic solution of multivalent cations and retarding anions 
is capable of gelling both the first and second aqueous polymer solution. 
U.S. Pat. No. 4,098,337 discloses a method for forming a hydroxymethylated 
polyacrylamide gel, in situ, to reduce the permeability of a thusly 
treated zone where the waterflood method of oil recovery is employed. In 
this case the gel was formed in situ by the injection of an aqueous 
polyacrylamide solution and an aqueous formaldehyde solution. 
Although polyacrylamide-based gels can be effective in retarding water 
production or flow in some subterranean formations, polyacrylamide-based 
gels will not be stable or effective in all formations. In general, 
polyacrylamide-based gels will work satisfactorily in formations having a 
temperature below about 65.degree. C. Above about 65.degree. C., 
polyacrylamide-based gels become very sensitive to hardness of the brines, 
especially where hardness is above about 1000 ppm. The hardness of the 
water becomes a more detrimental factor the higher the temperature, thus 
for very hot regions even low hardness levels can render many gels 
ineffective. Formations which have a higher temperature, hardness, or 
total dissolved solids content above the aforementioned ranges usually are 
not capable of being successfully treated with polyacrylamide-based 
polymers to retard the flow of water. Thus polyacrylamide-based gels are 
not considered useful in preventing steam channeling. 
In other flooding operations, rather than water, other fluids can be used. 
Some fluids which are used are carbon dioxide and steam. Because of the 
high temperature required in steam flooding or other steam stimulation 
methods, most of the gels used for blocking water are not suitable or 
satisfactory for blocking steam. Other steam treating methods such as 
"Push and Pull" operations, sometimes referred to as "cyclic steam 
injection" or "Huff and Puff" operations, where a production well is 
steamed for several days and then produced for a month or so result in 
steam channels being formed which if not blocked will result in an 
inefficient steaming operation due to loss of steam into channels which 
drain into nonproductive parts of the reservoir. Because many of the 
existing gels degrade rapidly at steam temperatures, polymers such as 
polyacrylamides are generally considered unsatisfactory. 
Flooding operations using steam and other gases as the drive fluid 
frequently experience a loss of drive fluid to nonproductive parts of the 
reservoir because of greater ability of gases to dissipate into such 
channel as opposed to liquids. Loss of drive gases in flooding operations 
and steam in stimulation methods is more difficult to prevent because the 
flow channels responsible for such losses can be very small in diameter or 
width thereby making it very difficult to fill such channels with a 
blocking agent. Some viscous plugging substances, even though they may 
have the desired stability at higher temperatures, are not able to 
penetrate and effectively fill narrow channels, particularly as such 
channels become more distant from the wellbore. 
Polyvinyl alcohol gels have been used to protect well casings from 
corrosion. U.S. Pat. No. 2,832,414 discloses such a method wherein an 
aqueous solution of a water soluble polyvinyl alcohol which is capable of 
forming a gel if maintained in a quiescent state, is pumped into the 
annular space between the casing and the wall of the bore hole. After 
allowing the polymer to remain quiescent over a period of time a gel is 
formed. The thusly formed gel prevents the intrusion of formation water 
into the annular space thereby reducing corrosion of the metal casing. 
Apparently, no crosslinking agent is employed and for that reason is not 
believed that this particular gel would be useful for plugging steam 
channels. Furthermore, in Patent No. 2,832,414 the gel is used to fill a 
relatively large but stagnant cavity compared to the substantial flows 
occurring in steam channels. 
Studies of the macroscopic changes in polyvinyl acetate gels that occur 
upon removal from swelling equilibrium with isopropyl alcohol were 
reported in the Journal of Colloid and Interface Science, Vol. 90, No. 1, 
November 1982, pages 34 to 43. These studies were conducted using films of 
gels having various degrees of crosslinking and polymer concentration. The 
polyvinyl acetate gels were formed from precursor polyvinyl alcohol gels 
that were crosslinked with glutaric dialdehyde which were then converted 
to acetate gels by polymer homologous acetylation. 
U.S. Pat. No. 3,265,657 discloses a process for preparing an aqueous 
polyvinyl alcohol composition, which remains fluid for at least a few 
seconds after preparation and spontaneously gels thereafter. The gel is 
formed by contacting a gelable fluid aqueous polyvinyl alcohol solution 
with a hexavalent chromium compound and a reductive agent to convert 
Cr(VI) to Cr(III). The compositions are used to produce foams suitable as 
insulating, acoustical, and packaging materials. The gels are crosslinked 
with chromium, not an aldehyde. 
U.S. Pat. No. 3,658,745 discloses a hydrogel which is capable of 
significant expansion upon cooling in water and reversible shrinking upon 
heating which comprises a crosslinked acetalated hydrogel formed by 
reacting a polyvinyl alcohol previously dissolved in water and a 
monaldehyde and a dialdehyde. The hydrogels are alleged to have sufficient 
crosslinking to prevent imbibition of macromolecular materials such as 
proteins but not the imbibition of micromolecular materials such as low 
molecular weight water solutes. These hydrogels are alleged to be useful 
for dialytic purification when pure water is added to the macromolecular 
solution after each cycle. Apparently these particular hydrogels are able 
to absorb and desorb water and microsolutes with alternate cooling and 
heating cycles. Apparently a major amount of shrinkage of these gels 
occurs upon slight heating such as from 12.degree. to 37.degree. C. which 
indicates that these gels would have little value for preventing steam 
channeling in subterranean formations, especially at temperatures of 
37.degree. C. or higher. 
SUMMARY OF THE INVENTION 
By the term "aldehyde" as used herein is meant a monoaldehyde, a 
dialdehyde, a polyaldehyde, and any of the former whether substituted or 
unsubstituted. Preferably the aldehyde contains two functional groups such 
as dialdehyde or a substituted monoaldehyde as used herein is meant to 
include unsaturated carbon-carbon bond as well as substitution of 
functional groups. Nonlimiting examples of substituted monoaldehyde are 
acrolein and acrolein dimethylacetal. Polyaldehydes can be used and may in 
some cases be more desirable, however, polyaldehydes are not as available 
commercially as dialdehydes and as a consequence use of polyaldehydes may 
not be practical. 
Non-limiting examples of dialdehyde crosslinking agents are glyoxal, 
malonaldehyde, succinaldehyde, glutaraldehyde, adipaldehyde, 
terephthaldehyde. Non-limiting examples of dialdehyde derivatives are 
glyoxal bisulfite addition compound 
EQU Na.sub.2 HC(OH)SO.sub.3 CH(OH)SO.sub.3, 
glyoxal trimeric dihydrate, malonaldehyde bisdimethylacetal, 
2,5-dimethoxytetrahydrofuran, 3,4-dihydro-2-methoxy-2H-pyran, and 
furfural. Acetals, hemiacetals, cyclic acetals, bisulfite addition 
compounds, shiff's bases or other compounds which generate dialdehydes in 
water, either alone or in response to an additional agent such as an acid 
or a condition such as heat are also meant to be included in the term 
"aldehyde" as used and claimed herein. 
Non-limiting examples of monoaldehyde with a second functional group in 
addition to the aldehyde group are acrolein and acrolein dimethylacetal. 
Non-limiting examples of polyaldehydes are polyacrolein dimethylacetal, 
addition products of acrolein for example, ethylene glycol plus acrolein, 
and glycerol plus acrolein. 
By the term "acidic catalyst" or "crosslinking catalyzing substance" as 
used herein is meant a substance which is a proton donor or a substance 
which in its environment will form or become a proton donor. All acids are 
operable as an acidic catalyst in the gel systems described herein, for 
example, Bronsted acids such as mineral and carboxylic acids, or Lewis 
acids. Non-limiting examples of a Lewis acid are zinc chloride, ferrous 
chloride, stannous chloride, aluminum chloride, barium fluoride, and 
sulfur trioxide. Some of these chemicals hydrolyse in water to produce 
metal oxides or hydroxides and HCl or HF. The rate of hydrolysis of many 
Lewis acids is dependent on temperature and the other dissolved compounds 
in the solution. The rate of production of the acidic catalyst, HCl, from 
some of the above Lewis acids determines the rate of gel formation. 
A delayed action catalyst is a substance which is not acidic in and of 
itself, but which generates an acidic catalyst slowly on interaction with 
water at the temperature of interest. For example, the rate of generation 
of the acid in oil well usage is usually controlled by the reservoir 
temperature experienced during the in-situ gel formation. In many 
applications the rate of acidic catalyst generation or release can be 
controlled by the gel-forming fluid formulation to range from a few 
minutes to a few days or more. 
The acid catalyst can be a two component system, for example, a two 
component delayed action catalyst can comprise a first component which 
will react with a second component, to form an acidic catalyst. A 
non-limiting example of such a two component delayed action catalyst is 
sodium persulfate and a reducing agent. In such a delayed catalyst system 
the sodium persulfate reacts with the reducing agent to produce sulfuric 
acid. In another two component delayed action catalyst system the reaction 
product of the two components can react with water to form the acidic 
catalyst. 
The acidic catalyst and/or delayed action catalyst must, of course, have 
some solubility in water. However, in some oil field usages the partial 
solubility of the acidic catalyst in the oil product can be advantageous 
if treatment is to include subterranean zones containing both oil and 
water. The fraction of the acidic catalyst or delayed action catalyst 
which dissolutes in oil will, of course, not be available to catalyze the 
gel formation reaction in such zones of high oil content; consequently 
such oil-water zones will not be blocked by gel formation to the same 
extent as those zones with little or no oil present. 
Non-limiting examples of delayed action catalysts are methyl formate, ethyl 
formate, methyl acetate, ethyl acetate, glycerol monoacetate or acetin and 
glycerol diacetate or diacetin. 
Laboratory tests conducted on core samples have shown that diacetin 
hydrolysis more slowly than methyl formate at all temperatures including 
the higher temperatures. Therefore, where subterranean formations having 
higher temperatures are encountered, diactin or acetin because of their 
slower rate of hydrolysis are used to provide a longer time for 
crosslinking reactions to occur and hence provide a longer time for the 
gelling forming fluids to penetrate into the pores of such subterranean 
zones before gelation occurs. Non-limiting examples of delayed action 
catalyst and their acidic catalyst product are: 
______________________________________ 
Delayed Action Catalyst 
Acidic Catalyst Product 
______________________________________ 
Methyl formate Formic acid 
Glycerol diacetate Acetic acid 
Sodium persulfate Sulfuric acid 
Sodium dodecyl sulfate 
Sulfuric acid 
Methyl methane sulfonate 
Methylsulfonic acid 
Sodium triiodide/sodium 
Hydroiodic acid 
bisulfate/water 
______________________________________ 
Therefore, delayed action acidic catalysts can be esters which slowly 
hydrolyze in water, the rate of hydrolysis being dependent on temperature 
and initial pH. Other delayed action catalysts are the analogs of esters 
and acids such as sulfones, xanthates, xanthic acids, thiocyanates, and 
the like. In some of these examples, hydrolysis produces an acidic 
catalyst which speeds the crosslinking reaction and an alcohol which does 
not affect gel formation. An example of a delayed action acidic catalyst 
is methyl formate which is influenced by the environment with respect to 
the rate of formation of acid. For example, the higher the temperature, 
the faster methyl formate will hydrolyze and generate formic acid. 
By the term "Bronsted acid" as used herein is meant a chemical which can 
act as a source of protons. By the term "Lewis acid" as used herein is 
meant a chemical that can accept an electron pair from a base. By the term 
"delayed action acid" as used herein is meant any acidic catalyst which 
makes available or generates donor proton over a period of time or after 
an initial period of time either as a consequence of its characteristic or 
the characteristics of the environment in which it is used. 
By the term "gel" as used herein is meant a chemically crosslinked 
three-dimensional elastic network of long-chain molecules with a certain 
amount of immobilized solvent (diluent) molecules. 
By the term "PVA based substance" or "PVA" (frequently referred to herein 
as the "first substance") as used herein is meant long-chain molecules 
selected from the group consisting of polyvinyl alcohols, polyvinyl 
alcohol copolymers, and mixtures thereof. 
By the term "PVA-aldehyde gel" as used herein is meant a chemically 
crosslinked three-dimensional elastic network of longchain molecules 
selected from the group consisting of a polyvinyl alcohol, a polyvinyl 
alcohol copolymer, and mixtures thereof, crosslinked with an aldehyde, and 
containing a certain amount of immobilized and chemically bound water 
molecules. 
By the term "PVA-glutaraldehyde gels" as used herein is meant a chemically 
three-dimensional elastic network of PVA based substance crosslinked with 
glutaraldehyde, and containing a certain amount of unmobilized and 
chemically bound water molecules. 
By the term "water" as used herein, unless otherwise specified, is meant to 
include any source of water, including brine, sea water, brackish water, 
formation water, fresh water and pure water which is H.sub.2 O. 
Furthermore if the water is a brine, the brine can be saturated at an 
elevated temperature. By the term "aqueous" as used herein, unless 
otherwise specified, is meant to include aqueous solutions comprising such 
water. Thus, for example, an aqueous solution of the first substance is to 
be understood to include the first substance dissolved in brine or fresh 
water. 
All of the above mentioned acidic catalysts are effective crosslinking 
catalyzing substances for PVA-aldehyde and PVA-glutaraldehyde gel systems. 
Non-limiting examples of polyvinyl alcohol copolymers are polyvinyl 
alcohol-co-crotonic acid, polyvinyl alcohol-co-acrylic acid, polyvinyl 
alcohol-co-methacrylic acid, polyvinyl alcohol-co-vinylpyridine, and 
polyvinyl alcohol-co-vinylacetate, the latter of which is frequently 
present in small amounts in commercial grade polyvinyl alcohols. 
This invention is concerned with placing a gel-forming composition in the 
nonproductive steam channels of an oil reservoir which is effective for 
retarding the flow of steam in such channels. In particular, the 
gel-forming composition comprises PVA based substance, an aldehyde or 
aldehyde generating substance operable for crosslinking with the PVA based 
substance under acidic conditions, and water. The water can be supplied by 
a brine, and the water or brine can be acidic. It is convenient to 
dissolve the PVA based substance in formation brine and form an aqueous 
premixture which is acidic. Then just before injection into the 
subterranean formation, the aldehyde is added to the premixture thereby 
producing the gel-forming mixture. In an alternative embodiment the fresh 
softened water used to produce the injected steam can be used to formulate 
the gel-forming composition. The PVA-aldehyde-water gels used by this 
process have much superior stability at steam temperature than most foams, 
are easier to place in steam channels than most viscous substances such as 
tars which are stable at steam temperature, and are less damaging to the 
oil-bearing part of the reservoir than lignosulfonates. 
Accordingly, in accordance with the principles of this invention, in a 
hydrocarbon recovery operation conducted in a subterranean formation in 
which a loss of steam channels has occurred, there is provided a process 
for reducing steam loss to nonproductive steam channels comprising 
introducing an effective amount of a gel-forming composition into the 
subterranean formation and into the nonproductive steam channels, the 
gel-forming composition being operable when gelled in the nonproductive 
steam channels for retarding the flow of steam therein, the gel-forming 
composition comprising 
i. an aqueous solution comprising an effective amount of a first substance 
selected from the group consisting of polyvinyl alcohols, polyvinyl 
alcohol copolymers, and mixtures thereof, and 
ii. an effective amount of a second substance selected from the group 
consisting of aldehydes, aldehyde generating substances, acetals, acetal 
generating substances, and mixtures thereof capable of crosslinking with 
the first substance through the formation of acetal crosslinkages, which 
is operable for effecting, under acidic conditions, crosslinking of the 
first substance and the second substance or aldehyde and for forming a gel 
in the nonproductive steam channels; and allowing the gel-forming 
composition, under acidic conditions, to form a gel in the nonproductive 
steam channels which is effective for retarding the flow of steam in the 
nonproductive steam channels, thereby reducing steam loss to nonproductive 
steam channels upon resumption of a hydrocarbon recovery operation. 
In one embodiment the gel-forming composition is introduced into the 
subterranean formation simultaneously with steam. In yet another 
embodiment the introduction of steam into the subterranean formation is 
discontinued during the introduction of the gel-forming composition 
therein. 
In further embodiments of the above described processes comprise the step 
of introducing a foam-forming composition into the subterranean formation, 
and forming a foam in the nonproductive steam channels, prior to 
introducing the gel-forming composition into the subterranean formation. A 
still further embodiment comprises mixing the gel-forming composition with 
a part of the foam in the nonproductive steam channels and forming a gel 
from the gel-forming composition in the interstices of that part of the 
foam in the nonproductive steam channels. In this embodiment the foam 
forming agent or composition is admixed with the gel-forming composition 
and thusly formed admixture is injected into the steam channels 
simultaneously with steam. The steam causes the foaming agent to generate 
the foam which as it penetrates into the steam channels forms the cells of 
the foam. Since the gel-forming composition is admixed with the foaming 
agent the cells of the form are reenforced by the gel after the gel has 
formed. Thus the gel is formed in the interstices of the thusly formed 
foam after the foam is in the steam channels. The foam is more stable by 
the fact that the foam cells are supported from within and without by the 
gel. Whereas foams formed by the agent and steam alone tend to collapse 
when the steam condenses. Furthermore the cellular structure imposed on 
the gel as a consequence of the foaming agent being admixed with the gel 
improves the stability of the gel by inhibiting infusion of the material 
into the gel or migration of material out of the gel as by syneresis. In 
this embodiment the steam eventually becomes part of the water of the 
gel-forming composition or gel. By being able to force the admixture of 
foaming agent and gel-forming composition with steam deep into the steam 
channels very stable plugging of the steam channels can be achieved 
without the need for frequent subsequent treatment with additional 
foam-forming agent. In one embodiment the foam is not formed until after 
its injection into the subterranean formation in admixture with the 
gel-forming composition. In any event if some collapse of the foam does 
occur due to condensation of the steam injected with the foam-forming 
composition and the gel-forming composition, the collapse will not be 
complete and foam cells still remain which contain the gel-forming 
composition. Further if any condensation of the media, i.e. steam, used to 
form the foam does occur, the condensate since it is water merely becomes 
part of the gel-forming composition and not a separate gas phase as a 
noncondensable gas would be. Thus this invention uses a media for 
expanding the foaming agent, i.e. steam, which if it condenses becomes 
part of the gel-forming composition. In these various embodiments it is to 
be understood that gelation of the gel-forming composition in the 
admixture occurs later in time than the creation of the foam. Thus in one 
embodiment the foam expanding gas is such that upon its condensation it 
becomes part of the gel-forming composition and thense part of the gel. 
Foams containing gels in their cell structure are superior foams than 
foams supported by just a gas phase, and gels which are reenforced by 
foams are superior to the gels which are not so reenforced. Thus in this 
invention the gel has a synergistic effect on the foam and the foam has a 
synergistic effect on the gel and thus there is provided a more stable 
composition for plugging steam channels. 
In another embodiment, wherein the nonproductive steam channels are at a 
substantially higher temperature than the temperature of the oil-bearing 
part of the subterranean formation, the gel-forming composition is capable 
of gelling within a predetermined amount of time if maintained at or near 
the temperature of the steam channels, but is not capable of completely 
gelling within a predetermined period of time when maintained at the 
temperature of the oil-bearing part of the subterranean formation. This 
embodiment is particularly useful where the temperature of the 
nonproductive steam channels is about 22.degree. C. or more higher than 
the temperature of the oil-bearing part of the subterranean formation. The 
greater the differences in temperature between the steam channels and the 
oil-bearing part of the reservoir the greater the advantage of this 
embodiment. 
In accordance with the principles of this invention there is also provided 
a gel formed from a gel-forming composition comprising 
i. an aqueous solution of an effective amount of a first substance selected 
from the group consisting of polyvinyl alcohols, polyvinyl alcohol 
copolymers, and mixtures thereof, 
ii. an effective amount of a second substance selected from the group 
consisting of aldehydes, aldehyde generating substances, acetals, acetal 
generating substances, and mixtures thereof capable of crosslinking with 
the first substance through the formation of acetal crosslinkages, said 
second substance being operable for crosslinking with the first substance 
under a predetermined acidic condition when the gel-forming composition is 
maintained for a predetermined period of time at an elevated temperature, 
the gel-forming composition not being capable of completely gelling within 
said period of time when maintained at a cooler temperature which is 
substantially lower than the predetermined elevated temperature, and 
a foaming agent operable for being foamed, in admixture with said 
gel-forming composition, with steam, the gel-forming composition being 
formed by heating or maintaining the gel-forming composition and foaming 
agent admixture under the predetermined acidic condition for the 
predetermined amount of time at the predetermined elevated temperature. In 
a further embodiment the predetermined elevated temperature is at least 
about 125.degree. C., and the cooler temperature is about 22.degree. C. 
lower or more than the predetermined elevated temperature. In another 
embodiment the second substance or aldehyde is glutaraldehyde. In one 
embodiment the predetermined period of time is no greater than about 5 
days. In another embodiment the predetermined period of time is from about 
1 hour to about 4 days. In a preferred embodiment the predetermined period 
of time is from about 2 hours to about 3 days, and in an especially 
preferred embodiment from about 3 or 4 hours to about 1 or 2 days. By way 
of example, if the predetermined period of time for gelation at the 
elevated temperature is 20 hours, then the gel-forming composition is 
formulated so that it will form a substantially complete gel when 
maintained at the predetermined temperature for 20 hours; whereas, if the 
gel is maintained at a substantially lower temperature than the 
predetermined elevated temperature then a substantially complete gel is 
not formed in 20 hours. The difference in temperature between the steam 
channels and the oil-bearing strata permit the gel-forming composition to 
be removed or flushed from lower temperature zones after the gel-forming 
composition has gelled in the higher temperature zones. 
In wells which have a serious nonproductive steam channeling problem, 
injecting a foam-forming composition prior to injecting the gel-forming 
composition can improve the effectiveness of the steam channel plugging 
operation. Non-limiting examples of foam forming compositions or 
surfactants are sulfonates of alpha-olefins, hydroxy sulfonates, aliphatic 
sulfonates and mixtures thereof. The foaming composition by itself or in 
admixture with the gel-forming composition can be carried into the steam 
channels with steam. This can also be followed by injecting the 
gel-forming composition into the steam channels. In one embodiment the 
gel-forming composition is carried into the nonproductive steam channels 
with steam. It is to be understood however that injection of the 
foam-forming composition by itself is only temporary and the principal 
improvement resides in the injection of the gel-forming composition by 
itself or in admixture with the foaming agent and steam. The gel-forming 
composition can penetrate the foam and set up and back up the foam. In one 
embodiment the foam is used to incorporate the acid and second substance 
or aldehyde and the injected PVA based substance must penetrate into the 
foam-acid-aldehyde mixture before it can begin to gel. In another 
embodiment, either the acid catalyst or the second substance or aldehyde 
is injected along with the foam and this is followed by injection of the 
remaining parts of the gel-forming composition which until the first 
substance comes into contact with the foam containing the second substance 
and acidic catalyst will not gel. In one embodiment, the acid-aldehyde 
combination injected with the foam is glutaric acid and glutaraldehyde. 
In still another gel or process embodiments, the amount of second substance 
or aldehyde is from about 0.01 to about 4 percent of the weight of the 
gel-forming composition or gel. In still another embodiments, the amount 
of the second substance or aldehyde is at least about 0.7% of the 
stoichiometric amount required to react with all of the crosslinkable 
sites of the first substance. In another embodiments the second substance 
or aldehyde is glutaraldehyde. In one embodiment the amount of PVA based 
substance is from about 0.5 to about 5% of the weight of a gel-forming 
composition or gel. In a preferred embodiment the first substance is from 
about 1 to about 4% of the weight of the gel-forming composition or gel. 
Preferably the first substance is from about 1.5 to about 3% of the weight 
of the gel-forming composition or gel. In another embodiment the first 
substance has an average molecular weight of at least about 30,000. 
Preferably the average molecular weight of the first substance or 
polyvinyl alcohol is from about 100,000 to about 1,000,000. Higher 
molecular weights can be used; however, the higher molecular weight the 
higher the viscosity of an aqueous solution of the first substance or 
polyvinyl alcohol. Average molecular weights for the first substance over 
5,000,000 will probably form too viscous a solution to be useful. In one 
embodiment the average molecular weight of the first substance is about 
125,000. In a preferred embodiment the PVA based substance or first 
substance is polyvinyl alcohol. In still another embodiment the 
gel-forming composition is at least about 64 weight percent water, i.e. 
H.sub.2 O. In yet another embodiment the water is provided by a brine and 
the brine is at least about 91% of the weight of the gel-forming 
composition. The brine can be saturated with dissolved salt and be hot or 
near its boiling point. Some hot saturated brines near their boiling 
points can contain as much as about 30% by weight dissolved salt or as 
little as about 70% by weight H.sub.2 O. Exact amounts of dissolved salt 
will vary depending on the various species of salts involved and the 
extent of any supersaturation. For example a gel-forming composition which 
is 91% by weight brine, wherein the brine is saturated and has a H.sub.2 O 
content of 70% by weight, will be about 64% by weight H.sub.2 O. In one 
embodiment, the first substance is polyvinyl alcohol. In another 
embodiment the first substance or polyvinyl alcohol is from about 2.5 to 
about 3 percent of the weight of the gel-forming composition or gel, and 
the amount of the second substance or aldehyde, preferably glutaraldehyde, 
is from about 0.01 to about 1 percent of the weight of the gel-forming 
composition or gel, and the remainder of the gel-forming composition is a 
brine having pH less than 7. In another embodiment the pH of the 
gel-forming composition is from about 2 to about 6.9, and preferably from 
about 3 to about 6. In yet other embodiments the gel-forming composition 
also comprises a separately added acidic catalyst. 
In one embodiment the total aldehyde content, i.e. mono-, di- and 
polyaldehyde, of the gel-forming composition is from about 0.01 to about 
4% of the weight of the gel-forming composition or thusly formed gel. 
This process is useful not only for steamfloods but for stimulation 
processes such as "push and pull" operations in which steam is injected 
for about 9 days more or less followed by pumping of oil for about 6 
months more or less. Accordingly, in one embodiment the hydrocarbon 
recovery operation is a steam push and pull operation. In another 
embodiment the hydrocarbon recovery operation is a steamflooding 
operation. These processes are particularly useful where the subterranean 
formation has an average formation temperature of at least about 
125.degree. C. 
In still further embodiments, the water used to form the gel has a hardness 
of at least about 1000 ppm. In further embodiments the water has a 
hardness of at least about 3000 ppm, or 6000 ppm, or higher. In other 
further embodiments of the above described gels, the water used to form 
the gel has a total dissolved solids content of at least about 30,000 ppm. 
In a still further embodiment such water has a total dissolved solids 
content of at least about 80,000 ppm or more and can be a saturated brine. 
In the embodiments of this invention the aldehydes crosslink with the 
polyvinyl alcohol or polyvinyl alcohol copolymer through formation of 
acetals. It has been found that gels formed in this way are adaptable to 
the hardness of the water from which they are formed or exposed. These 
gels are also more stable at high temperatures than polyacrylamide based 
gels or gels made from biopolymers or polyvinyl alcohols gelled by other 
crosslinking agents such as borate. 
Because of the adaptability and compatibility of these gels to water 
hardness or total dissolved solids content, these gels can be prepared 
using formation water, brackish water, sea water, brine or usually any 
other available source of water conveniently at hand as well as fresh 
water, i.e. H.sub.2 O. Because the largest ingredient used to formulate 
the above described gels is principally water, substantial economic 
advantage is provided by this invention which permits gels to be formed 
with the cheapest source of available water. However, the advantages of 
this invention are not limited merely to economic advantages because these 
gels also provide substantial technical advantages over other gels. For 
example, in many of their uses these gels are subjected to the infusion of 
severely contaminated water into the gelling mass prior to reaching its 
gelation point. Where such contaminated water infusion occurs in many 
other gelling fluids the gelation thereof is destroyed or so severely 
harmed that such other gels, if in fact they do gel, would be rendered 
ineffective for their intended use. 
Due to their stability at elevated temperatures, the above described gels 
can also be formed and used in formations having an average formation or 
in-situ temperature of about 125.degree. C. or higher, and in some 
embodiments where the average formation or in-situ temperature is 
200.degree. C. or higher. 
The above described methods of forming a gel in situ in subterranean 
formations be be practices using all of the gels provided by this 
invention. 
The gels of this invention have improved resistance to heat and are stable 
in hard water. These characteristics make these gels particularly useful 
for reducing steam channeling.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
It is preferable to conduct flow tests on core samples of crushed rock from 
the reservoir to be sure that the gel times are not materially different 
from that of neutral rock. The first example demonstrates a method for 
determining gel times. 
Example No. 1 
Since normally fresh soft water is used to produce injected steam, this 
same water is preferably used to produce the gel-forming composition. 
Polyvinyl alcohol having an average molecular weight of about 125,000 is 
added to softened fresh water to produce a 10 percent concentration and 
the mixture is heated to 95.degree. C. for 45 minutes to completely 
dissolve the polymer. A high pressure core holder is packed with crushed 
reservoir rock to form a 60 centimeter (60 cm) long, 5 cm diameter test 
core sample. The core sample is then saturated with softened water and 
heated to 230.degree. C. in preparation for a flow test. Steam, at 
230.degree. C. is injected into the core sample at a rate, based on a 
water equivalence, of 300 cm per day and the pressure drop across the core 
sample measured. The gel-forming composition is prepared by mixing 24 
parts of the thusly prepared 10 percent polymer solution with one part by 
weight of a 50 percent glutaraldehyde solution (commercial grade). The 
gel-forming composition is injected at a rate of 60 cm per day 
simultaneously with 230.degree. C. steam or steam and a forming agent at a 
rate, based on a water equivalence, of 180 cm per day into the core 
sample. The gel-forming composition is nominally designed for a gel time 
of 3 hours at 230.degree. C. The variation of gel-time from nominal is an 
indication of the influence of the reservoir rock on reaction rates. 
Example No. 2 
Preferably after determining the optimum gel-forming composition in crushed 
reservoir rock for a given steam channel temperature and a desired gel 
time, treatment of an injection well experiencing severe steam channeling 
can be conducted with the PVA-aldehyde gel systems of this invention. For 
example, an injection well having a 15 meter reservoir interval with the 
top 1.5 meters taking about 90 percent of the injected steam and the top 3 
meters taking about 100 percent of the injected steam, and having an 
average interval permeability of about 500 millidarcies (500 md) and 
porosity of about 25 percent, is receiving about 130 cubic meters per day 
(130 CMPD), based on a water equivalence, of 230.degree. C. injected steam 
at a surface pressure of about 50 kilograms per square centimeter gauge 
(50 kpscg). 
An aqueous gel-forming composition, formulated as in Example No. 1, is 
heated to 95.degree. C. in an in-line heater and stored in an insulated 
tank for about 2 hours. The gel-forming composition is then fed at a rate 
of 30 CMPD to an eductor which is simultaneously receiving 100 CMPD of 
steam or steam and a foaming agent, based on water equivalence. The 
gel-forming composition is conveyed into the reservoir by the injected 
steam for a 12 hour period, and thereafter displaced into the formation 
with steam at 100 CMPD, based on water equivalence, until fully displaced 
from the wellbore into the reservoir. The well is then shut in for about 
24 hours and thereafter steam injection then resumed. It is expected that 
the top 300 cm of the interval will receive less than 50 percent of the 
injected steam after treatment. 
Example No. 3 
A producing well, 60 meters from a steam injection well in a steam-flood 
operation, is producing steam and hot water equivalent to 80 CMPD of water 
and 10 CMPD of oil. A production survey shows that the top 3 meters of a 
15 meter interval is experiencing steam breakthrough. The temperature of 
the steam at the production point in the producing well is 125.degree. C. 
whereas the original reservoir temperature was 50.degree. C. The steam 
injection raises the average fluid temperature near the wellbore to 
90.degree. C. A decision is made to treat the production well by the 
method of this invention. Accordingly, 160 cubic meters of gel-forming 
composition having a concentration of 2.5% polyvinyl alcohol having an 
average molecular weight of about 125,000 and 0.1% glutaraldehyde, and 
having a pH of 5.5 is prepared for injection into the production well. 
Prior to injection of the gel-forming composition, the producing well is 
shut down and 60 cubic meters of cold produced water is injected into the 
production well at a rate equal to 160 CMPD. Most of the cold water enters 
the steam channels in the top 3 meters of the interval but about 25% of 
the cold water enters the bottom 12 meters thereof. After the cold water 
cools down the bottom 12 meters of the interval, steam is then injected 
into the producing well and because of the higher permeability of the 
steam channels, the steam channels reach a higher temperature than the 
less permeable oil bearing part of the formation. The interval is now 
ready for injection of the gel-forming composition or admixture of 
gel-forming composition and foam-forming composition. 
The gel-forming composition is designed to gel in about 30 hours at 
120.degree. C. but to not gel within seven days at a temperature no 
greater than 80.degree. C. The gel-forming composition with or without a 
foaming agent is injected over a 24-hour period, and then the well shut in 
for 24 hours. During this period only gel-forming composition in the 
higher temperature steam channels gels. The well is put back on production 
and the ungelled composition in the lower 12 meters of the interval is 
produced or purged from the production well. It is expected that after 
treatment water production is reduced to 20 CMPD and oil production is 
increased to 20 CMPD. 
Example No. 4 
A producing well is on its third cycle of steam injection and the results 
of the second cycle shows a drop in efficiency of oil production as 
compared to the first cycle. Other wells in the field show similar 
results. Evaluation of the formation based on geologic and core data 
indicates a high permeability channel near the bottom of the production 
interval. A decision is made to plug the high permeability channel with 
the method of this invention. Core sample data shows that a gel-forming 
composition having a concentration of 2.5% of polyvinyl alcohol with an 
average molecular weight of 125,000 and 0.5% glutaraldehyde forms with 
softened water a gel in 3 hours at a temperature of 230.degree. C. This 
composition is approximated by preparing a polymer mixture having a 
concentration of 10% polyvinyl alcohol with an average molecular weight of 
125,000 and injecting the mixture at a rate of 24 CMPD in a stream of 
injected steam which can also contain a foaming agent which is injected at 
a rate equal to 100 CMPD of water. A 50% glutaraldehyde aqueous solution 
is injected at the wellhead at a rate of one CMPD. The polymer mixture and 
aqueous glutaraldehyde solution are simultaneously injected into the 
wellhead for a period of six hours, starting one day after beginning the 
third steam injection cycle. The total steam cycle is continued for 9 days 
and during that time the gel-forming composition and the gel produced 
therefrom are placed in-depth in the steam channels. After about 2.5 days 
into the third cycle the thusly formed gel causes steam to be diverted 
into parts of the oil-bearing structure that had not been swept with 
steam. Production rates one week after stopping steam injection show an 
increase in oil and a decrease in water production in the third cycle. 
Production data expected one week after stopping steam injection in each 
of the first three steam cycles is as follows: 
______________________________________ 
Cycle Oil (CMPD) Water (CMPD) 
______________________________________ 
First 60 40 
Second 30 80 
Third 50 50 
______________________________________ 
The various steam rates mentioned herein are to be understood to be 
expressed as their equivalent water rate whether expressed as a velocity 
or volumetric rate. 
In alternative embodiments the second substance in above examples is 
admixed with the foaming agent and the second substance-foaming agent 
admixture is injected with steam into the formation as the gel-forming 
composition (without the second substance) is simultaneously injected. 
Separate conduits can be provided so that mixing of the two streams occurs 
(i.e. the first stream containing the first substance and acidic catalyst 
and the second stream containing the second substance, foam-forming 
composition and steam) at the wellbore proximate to the steam channels. 
Unless otherwise specified herein, all percents are weight percents. 
The gels, the methods of forming the gels, and the processes for preventing 
steam channeling have some degree of flexibility. It is permissible to use 
the formation brine of the subterranean zone as the water part of the 
gel-forming composition since the gel will form even with hard water. 
Other variations of formulations, methods and processes will be apparent 
from this invention to those skilled in the art. 
The foregoing disclosure and description of the present invention is 
illustrative and explanatory thereof and various changes in gel formation 
procedures and gel composition may be made within the scope of the 
appending claims without departing from the spirit of the invention. For 
example, many gel formulations can be produced and variations in forming 
such gels in situ in steam channels will be apparent to one skilled in the 
art from this invention. For example, any number of sequential injection 
steps of the gel-forming compositions can be made. Furthermore, the 
necessary concentrations, amounts and sequence of injection of the 
gel-forming compositions can be tailored to suit the particular well or 
subterranean formation being treated.