Gel and process for preventing loss of circulation, and combination process for enhanced recovery

A rapid setting gel-forming composition is provided comprising a first substance selected from the group consisting of a polyvinyl alcohol, a polyvinyl alcohol copolymer, and mixtures thereof, and aldehyde, and water, which is useful in preventing the loss of circulation fluids in well drilling, completion of workover operations. A combination process is also provided using a slower setting gel-forming composition for retarding the flow of waters or brines in high permeable non-productive channels in combination with a subsequent acidizing step for increasing the permeability of low porous structure in subterranean formations. The combination process is particularly useful in waterflood operations to increase the sweep efficiency of the oil recovery process while improving the flow oil and drive water in the productive parts of the reservoir.

TECHNICAL FIELD 
This invention relates to gels, and processes for forming and using the 
gels. A polyvinyl alcohol based-aldehyde hydrogel, or gel, is provided 
which is useful for immobilizing large volumes of earth or water. One of 
the gels can be used for reducing the loss of drilling, completion or 
workover fluids from a wellbore. Other gels can be used for reducing the 
permeability of subterranean formations to the flow of fluids, waters or 
brines. The various gels of this invention are particularly valuable in 
retarding the flow of fluids, waters or brines in hydrocarbon production 
from a wellbore. 
The subject matter of this application is related to that of commonly 
assigned U.S. Pat. No. 4,498,540, for "Gel for Retarding Water Flow" and 
our Ser. No. 624,111, filed June 25, 1984, now abandoned, for "Gel and 
Process for Retarding Fluid Flow" which are hereby incorporated herein by 
reference. 
BACKGROUND OF THE INVENTION 
The recovery of hydrocarbons, both liquid and gaseous, from subterranean 
zones has frequently resulted in the simultaneous production of large 
quantities of water or brines. In some cases, even though substantial 
flows of hydrocarbons have been shown, water production is so great and 
water disposal costs so high, that hydrocarbon production is not 
economical. Such water production has in some cases been disposed of in an 
abandoned or dry well by separating such water from the hydrocarbons and 
reinjecting the separated water into such wells. Where a disposal well is 
not available nor near the producing well, pipelining the water product 
over a long distance to a disposal site can become so costly that it 
renders the well noncommercial. Even if a disposal well is close by, the 
disposal cost can still be very expensive. Therefore it is desirable to 
find a way to reduce or shut off the flow of water while permitting 
hydrocarbon production to continue. 
It is well known that the production of large amounts of water from 
hydrocarbon producing wells is a major expense item in the overall 
hydrocarbon recovery cost. It is not uncommon for an oil well to produce 
an effluent which is 60-99% by volume water and only 1-40% by volume oil. 
In such situations, the major part of the pumping energy is expended in 
lifting water from the well, a cost which the producer would like to avoid 
if possible. The effluent must then be subjected to a costly separation 
procedure to recovery water-free hydrocarbons. The foul water separated 
therefrom also presents a troublesome and expensive disposal problem. 
Consequently, it is desirable to decrease the volume of water produced 
from hydrocarbon wells. It is, of course, desirable to be able to achieve 
this objective and at the same time not marer-ally affect the hydrocarbon 
recovery rate. However, where the volume of water is very high, e.g., 80 
to 99% water, and only 1-20% oil, even substantial reduction in 
hydrocarbon production can be tolerated if water production can be 
substantially reduced. 
One such method of reducing the flow of 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. 
Water produced from a wellbore can come from the infiltration of naturally 
occuring subterranean water as described above, or the water can come from 
injected water put into the formation in those hydrocarbon recovery 
processes which utilize waterflooding. 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 a aqueous 
formaldehyde solution. 
In waterflood operations it can be desirable to treat the water injector 
wells with a polymer gel forming solution to control and/or redirect the 
water flow profile. Such treatment can prevent channeling of water at the 
injector well and/or control or redirect the water flow through regions of 
varying permeability. 
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. 
In many hydrocarbon producing wells temperatures of 80.degree. C. or higher 
are often encountered. Formation waters frequently have hardnesses which 
exceed 1000 ppm. It is therefore desirable to develop a gel which can be 
used to retard or block the flow of water in subterranean formations 
having a temperature of 65.degree. C. or higher, and a water hardness of 
1000 ppm or higher. 
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, many 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 aa "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. Again because many of the 
existing gels degrade rapidly at steam temperatures, polymers such as 
polyacrylamides are generally not satisfactory. Other fluids such as 
carbon dioxide can also be used in push and pull operations. 
Flooding operations using carbon dioxide 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. 
Thus there is a need for plugging fluids which can be formulated to 
penetrate deeply into the formation. The use of this invention addresses 
this problem and provides polyvinyl alcohol based gels which can be tailor 
made to the particular problem at hand and which can overcome many of the 
shortcomings of prior art plugging agents and gels. 
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 channels or 
fractures on a permanent bases. Furthermore, in U.S. Pat. No. 2,832,414 
the gel is used to fill a relatively large but stagnant cavity compared to 
the volume of a typical channel in a subterranean formation associated 
with hydrocarbon production from a wellbore. 
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 blocking water 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 acida 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 subterranenan 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 various PVA based substances 
crosslinked with glutaraldehyde and containing a certain amount of 
immobilized and chemically bound water molecules. 
By the term "water" as used herein is meant to include any source of water, 
including brine, sea water, brackish water, formation water, fresh water 
and pure water. Furthermore the brine can be saturated and at an elevated 
temperature. 
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. 
A problem which sometimes occurs in the oil field is the loss of 
circulation of special fluids such as drilling, completion and workover 
fluids into the subterranean formation. Loss of circulation fluids into 
the formation can cause damage to the drill bit caused by overheating and 
large decrease in drilling rate either of which can cause large increases 
in the cost of drilling, collapse of the formation at the wellbore which 
can damage the wellbore beyond repair, or in-depth plugging of the 
formation which can damage the reservoir to such an extent that the 
reservoir may have to be abandoned. 
In order to stop or retard the loss of circulation fluids into the 
reservoir it is desirable to plug the flow passages responsible for such 
losses very quickly. Cements and silicates are frequently used, however, 
because of the flow properties of cement and silicates completely 
effective plugging is not always achieved. The large particles in cement 
prevent it from penetrating much beyond 8 centimeters (8 cm) into the low 
flow rate channels. Whereas in high flow rate channels the cement often 
does not stop the loss of circulation fluids probably because the cement 
did not set, which could be because of dilution due to formation water 
infusion or merely because the fast flow rate prevented setting. Cement 
plugs near the wellbore are frequently short circuited by the circulation 
fluid shortly after the resumption of drilling, completion, or workover 
operation. Thus, there is a need for a system that will plug both low flow 
and high flow rate channels adjacent a wellbore and not allow circulation 
fluids to pass. 
The PVA-aldehyde gel systems of this invention can penetrate the formation 
for distances much greater than 8 cm whether the formation be a sand-like 
or carbonate-type matrix, and also stick to the matrix after gelation. The 
gel times of PVA-aldehyde gel systems can be varied from a couple of 
minutes to days. However, in most cases drilling, completion or workover 
operations is very costly. Consequently time delays are avoided wherever 
possible. Loss of circulation fluid problems therefore need to be 
corrected rapidly. Fast setting plugging agents for use close to the 
wellbore are a long sought solution to the problem. 
In serious lost circulation cases often 20 cubic meters or 120 barrels or 
more of circulation fluid can be lost in 10 minutes. 
In our invention, there is provided rapid setting PVA-aldehyde gels 
formulated so that they are gelled within a period of time from about 1 
second to no greater than 12 minutes, and preferably from immediately or 
almost immediately to about 10 minutes, i.e. from about 1 second to about 
10 minutes, after formulation. These gels can be partially formulated at 
the surface but completely formulated in the wellbore preferably at or 
near the point of lost circulation. Our gel-forming compositions are 
gelled in the presence of an acidic catalyst, which in combination with 
the amount of aldehyde, causes a rapid setting of the gel to occur. In one 
embodiment the acidic catalyst is added to the gel-forming mixture either 
in the wellbore or preferably at or near the point of lost circulation. In 
another embodiment the aldehyde is the last component of the gel-forming 
composition to be added to the mixture and it is added at the wellbore 
near the point of lost circulation. The use of such rapid setting 
PVA-aldehyde gel systems offers additional advantages of ease of wellbore 
clean-up as opposed to cements, and greater elevated temperature stability 
over other gel systems such as polyacrylamide based gels which in general 
are not stable at temperatures of 65.degree. C. or higher. Our 
PVA-aldehyde gel-forming compositions also have the advantage that they 
can be formulated using formation brine rather than merely fresh water 
which is substantially pure water, that is H.sub.2 O. This compatibility 
is an important advantage in locations where fresh water is not readily 
available. 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. 
Accordingly, there is provided a process for reducing the loss of 
circulation fluids into flow passages of a subterranean formation during a 
well drilling, completion or workover operations, the circulation fluids 
being selected from the group consisting of drilling fluids, completion 
fluids and workover fluids, the process comprising stopping the injection 
of the circulation fluid into the wellbore; introducing into the flow 
passages, an effective amount of a gel-forming composition comprising (i) 
an aqueous solution comprising a first substance selected from the group 
consisting of polyvinyl alcohols, polyvinyl alcohol copolymers, and 
mixtures thereof, (ii) an 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, 
and (iii) an amount of a crosslinking catalyzing substance, which in 
combination with the amount of the aldehyde is operable for effecting 
gelation, at the temperature of the subterranean formation, of the 
gel-forming composition in a period of time no greater than about 12 
minutes after the gel-forming composition is formulated or introduced into 
the subterranean formation; and allowing the gel-forming composition to 
flow into the flow passages and to form a gel therein within such period 
of time thereby reducing the loss of circulation fluid upon resuming well 
drilling, completion or workover operation. In one embodiment the total 
aldehyde content of the gel-forming composition is from about 0.03 to 
about 10% of the weight of the gel-forming composition or thusly formed 
gel. In a preferred embodiment the amounts of a crosslinking catalyzing 
substance and aldehyde are operable for effecting gelation, at the 
temperature of the subterranean formation, of the gel-forming composition 
in a period of time from about zero, or one second, to about 10 minutes 
after being formulated or introduced into the subterranean formation. 
In one embodiment of our rapid setting gel-forming composition, the amount 
of the second substance or aldehyde is from about 0.03 to 10 percent of 
the weight of the gel-forming composition or gel, and the pH of the 
gel-forming composition is no greater than about 5.5. In one embodiment 
the second substance or aldehyde is from about 0.03 to about 5 percent of 
the weight of the gel-forming composition or gel. In another embodiment 
the second substance or aldehyde is from about 0.03 to about 4 percent of 
the weight of the gel-forming composition or gel. In still another 
embodiment the second substance or aldehyde is from about 2 to about 3 
percent of the weight of the gel-forming composition or gel. Preferably 
the pH of the gel-forming composition is no greater than about 5 or about 
4. In one embodiment the pH of the gel-forming composition is from about 2 
to about 5. In another embodiment of the amount of the second substance or 
aldehyde is no more than about 2 percent of the stoichiometric amount 
required to react with all of the crosslinkable sites of the first 
substance. In another embodiment the aldehyde is glutaraldehyde. In yet 
another embodiment the gel-forming composition is at least about 64 weight 
percent water, i.e. H.sub.2 O. In still other embodiments the gel-forming 
composition is at least 85%, and preferably at least about 90 or 91% by 
weight brine. In another embodiment the amount of the PVA based substance 
or first substance is from about 0.1 to about 5% of the weight of the 
gel-forming composition or gel. Preferably the first substance or PVA is 
from about 0.1 to about 5% of the weight of the gel-forming composition or 
gel. In still another embodiment the first substance has an average 
molecular weight of at least about 30,000, preferably at least about 
100,000. 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. Preferably the first substance is polyvinyl alcohol. In a 
preferred embodiment the gel-forming composition is from about 2 to about 
3% by weight polyvinyl alcohol and from about 1 to about 2% by weight 
glutaraldehyde. This composition is useful in treating a well or wellbore 
having a subterranean temperature at least as high as 80.degree. C. 
In a further embodiment, prior to introducing the rapid setting gel-forming 
composition into the wellbore, a short-term plugging agent is introduced 
into the wellbore and into the flow passages to temporarily plug the flow 
passages until a gel is formed in the flow passages from the gel-forming 
composition. Examples of short-term plugging agents are diatomaceous 
earth, ground up nut shells, wax beads, and mixtures thereof. In another 
further embodiment, cement is introduced into the wellbore and from the 
wellbore into the subterranean formation after introducing the rapid 
setting gel-forming composition into the wellbore. In another further 
embodiment, a silicate is introduced into the wellbore and from the 
wellbore into the subterranean formation after introducing the rapid 
setting gel-forming composition into the wellbore. Preferably a silicate 
followed by a cement is introduced into the wellbore after introducing the 
gel-forming composition into the wellbore. 
The above processes are especially useful for reducing the loss of 
circulation fluids in wells having a severe loss circulation problem. 
Where the loss circulation occurs in fractures having extremely high 
permeability, it is desirable to precede the introduction of the 
gel-forming composition into the wellbore with a temporary plugging agent 
such as diatomaceous earth, ground up nut shells, wax beads or other 
substances to initially reduce the permeability in such severe fractures 
so that the gel-forming composition will have an opportunity to set up and 
form a gel in such fractures. In general, the gel-forming compositions 
used for reducing the loss of drilling fluid will be relatively quick 
setting. This is achieved, for example, by having a relatively high 
glutaraldehyde concentration in the gel-forming composition with a low pH. 
The PVA-aldehyde gels as described are stable at high temperatures for 
long periods of time and offer a definite advantage over many other 
polymer based gels which are not effective in wells or formations having a 
high temperature. By having the gel set up in a period of time no greater 
than about 12 minutes after it is formulated or comes in contact with the 
formation, this process offer a definite advantage over cements which will 
not penetrate as deeply into the formation but require a longer time to 
set up. In some wells having severe fractures the use of cement by itself 
is ineffective because the cement is lost before it has a chance to set 
up. In such situations the loss of circulation fluid is only partially 
corrected. However, in our invention described above, the gel-forming 
composition sets up rapidly and the loss of circulation fluid is greatly 
reduced. 
There is also provided a gel, which is especially useful for reducing the 
loss of circulation fluids but is also useful for other purposes which 
require a rapid setting gel, formed by reacting, in the presence of an 
effective amount of an acidic catalyst, the components of a gel-forming 
composition comprising 
i. 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, and 
iii. water, wherein H.sub.2 O provides at least about 62 or 64 percent of 
the weight of the gel, and wherein the amount of the acidic catalyst in 
combination with the amount of the second substance is operable for 
effecting gelation, at a predetermined temperature, of the gel-forming 
composition in a period of time no greater than about 12 minutes after the 
gel-forming composition is formed. In one embodiment the total aldehyde 
content of the gel-forming composition is from about 0.03 to about 10 
percent of the weight of the gel-forming composition. Preferably the 
amount of the acidic catalyst in combination with the amount of second 
substance or aldehyde is operable for effecting gelation, at a 
predetermined temperature, of the gel-forming composition in a period of 
time from about zero or near zero to about 10 minutes after the 
gel-forming composition is formed. 
In one embodiment the amount of second substance or aldehyde is from about 
0.03 to about 4% of the weight of the gel-forming composition. In another 
embodiment the amount of the second substance or aldehyde is at least 
about 2% of the stoichiometric amount required to react with all the 
crosslinkable sites of the PVA-based substance or first substance. In a 
preferred embodiment the aldehyde is glutaraldehyde. In yet another 
embodiment the amount of the acidic catalyst is sufficient to maintain the 
pH of the gel-forming composition at a value no greater than about 5.5, or 
about 5, or about 4. In a further embodiment the pH of the gel-forming 
composition is at least about 2. In yet another embodiment the amount of 
the PVA-based substance or first substance is from about 0.1 to about 5% 
of the gel-forming composition or gel. In still another embodiment the 
first substance is from about 1.5 to about 5% of the gel-forming 
composition or gel. In a preferred embodiment the first substance is from 
about 2 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 at least about 100,000. In a further 
embodiment the average molecular weight of the first substance is from 
about 100,000 to about 1,000,000. Preferably the average molecular weight 
of the first substance is about 125,000. In yet another embodiment the 
first substance is polyvinyl alcohol. In yet another embodiment the water 
of the gel-forming composition is provided by a brine, and the brine is at 
least about 85, or 90, or 91 percent of the weight of the gel-forming 
composition or gel. 
Another problem which frequently occurs in enhanced oil recovery operations 
such as flooding or stimulating is the loss of injected fluids into 
nonproductive and usually high permeability parts of the reservoir while 
the productive parts of the reservoir remain largely inaccessible because 
of their usually low permeability. This invention also provides a 
combination process in which the high permeability channels which are 
usually nonproductive are treated to retard the flow of fluids, especially 
water or brines, while the low permeability zones which are usually 
oil-bearing are acidized to increase the permeability thereof to the flow 
of oil or drive or stimulating fluids. Our particular combination of steps 
provides a relatively easy and effective way to treat reservoirs 
experiencing both inefficient loss of fluids to nonproductive areas while 
at the same time improving the recovery from the productive oil-bearing 
areas of the reservoir. 
Accordingly our process provides a method to penetrate the nonproductive 
high permeability channels for relatively large distances from the 
wellbore and effectively block the flow of water or brines therein so that 
better use of the drive or stimulating fluids can be achieved. This is 
then closely coordinated with an acidizing process to improve flows in 
desired areas such that as a result of our combination process the 
efficiency and profitability of the enhanced oil recovery operation is 
improved. Accordingly, there is also provided a process for retarding the 
flow of water in high permeability channels in a subterranean formation 
and increasing the permeability of low permeability oil-bearing porous 
structure in the subterranean formation comprising introducing into the 
subterranean formation a predetermined amount of a gel-forming composition 
which when gelled in the high permeability channels is operable for 
retarding the flow of water therein, the gel-forming composition 
comprising (i) an aqueous solution comprising 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 aceral crosslinkages, wherein the total aldehyde content of 
the gel-forming composition is from about 0.1 to about 5 percent of the 
weight of the gel-forming composition, sufficient to form a gel with said 
aqueous solution, when in the presence of an effective amount of a 
crosslinking catalyzing substance; and allowing the gel-forming 
composition to form a gel, in the presence of an effective amount of the 
crosslinking catalyzing substance, in the high permeability channels which 
is effective for retarding the flow of water therein; after introducing 
the gel-forming composition into the high permeability channels, 
introducing into the subterranean formation a predetermined amount of an 
acidizing substance which is operable for penetrating the low permeability 
porous structure and dissolving flow inhibiting deposits therein; and 
allowing the acidizing substance to dissolve the flow inhibiting deposits 
thereby increasing the permeability of the low permeability porous 
structure. In one embodiment of our combination process the acidizing 
substance is introduced into the subterranean formation within a period of 
time from about zero to about one day after the gel is formed in the high 
permeability channels. In another embodiment the acidizing substance is 
introduced into the subterranean formation after the gel is formed in the 
high permeability channels. In a further embodiment the acidizing 
substance is introduced into the subterranean formation within a period of 
time from about zero to about 10 hours after the gel is formed in the high 
permeability channels. In another embodiment after introducing into the 
subterranean formation the predetermined amount of the gel-forming 
composition, an effective amount of a crosslinking catalyzing substance is 
introduced into the subterranean formation which is operable to cause the 
gel-forming composition in the high permeability channels to gel therein. 
In yet another embodiment the crosslinking catalyzing substance and the 
acidizing substance have the same composition. 
In a further embodiment of our combination process the amount of second 
substance or aldehyde is from about 0.005 or about 0.01 to about 4% or 
about 5% of the weight of the gel-forming composition. In another 
embodiment the amount of second substance or aldehyde is at least 0.7% of 
the stoichiometric amount required to react with all of the crosslinkable 
sites of the first substance. In a preferred embodiment the aldehyde is 
glutaraldehyde. In yet another embodiment the PVA-based substance or first 
substance is from about 0.1 to about 5% of the weight of the gel-forming 
mixture or gel. In still another embodiment the first substance is from 
about 1.5 to about 5% of the gel-forming composition or gel. In another 
embodiment the first substance has an average molecular weight of at least 
30,000, and preferably at least 100,000. 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 the 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 yet another embodiment, 
the first substance is polyvinyl alcohol. In another embodiment the 
gel-forming composition is at least about 64% by weight fresh water or 
H.sub.2 O. In another embodiment the water of the gel-forming composition 
is provided by a brine, and the brine is at least about 90 or 91 percent 
of the weight of the gel-forming composition or gel. In one embodiment the 
first substance or polyvinyl alcohol is from about 2 to about 3 of the 
weight of the gel-forming composition or gel. In a preferred embodiment 
the gel-forming composition or gel is about 2.5% by weight polyvinyl 
alcohol and about 0.1% by weight glutaraldehyde. These gel-forming 
compositions are particularly useful in subterranean formations having a 
formation temperature at least as high as about 65.degree. C. 
In another embodiment of our combined process, the aldehyde is 
glutaraldehyde and the amount of glutaraldehyde is operable for promoting 
crosslinking of the first substance and glutaraldehyde under weakly acidic 
conditions and a separately provided acidic catalyst is not required. In a 
further embodiment, other than glutaraldehyde and acidic products produced 
in the gel-forming composition from the glutaraldehyde, the gel-forming 
composition is substantially free of effective amounts of crosslinking 
catalyzing substances which are operable for promoting substantial acidic 
catalysis of a crosslinking reaction between the first substance and 
glutaraldehyde; and wherein the gel is formed in the subterranean 
formation without the necessity of contacting the gel-forming composition 
with any additional effective amounts of a crosslinking catalyzing 
substance. This particular gel-forming composition and method of forming 
is more fully described in our copending Ser. No. 624,111 filed on June 
25, 1984. 
In a further embodiment of our combined process further comprises, after 
forming the gel in the high permeability channels and dissolving the flow 
inhibiting deposits in the low permeability porous structure, recovering 
oil from the subterranean formation. In a further embodiment oil is 
recovered by water flooding. 
In a preferred embodiment of our combination process the crosslinking 
catalyzing substance is a delayed action catalyst such as an ester which 
slowly hydrolyzes as it moves away from the wellbore into the formation. 
The ester is selected so that it will slowly form a weak organic acid as 
it penetrates into the formation. In this embodiment the weak organic acid 
also reacts with the flow inhibiting deposits in the low permeability 
channels. The combination of selecting the crosslinking catalyzing 
substance which allows the gel-forming composition to slowly gel thereby 
enabling in-depth plugging of the high permeability channels, and which 
also allows in-depth acidizing of the low permeability channels, is a 
particularly useful combination. This embodiment has the advantage over 
many prior art processes in that only two compositions are involved; 
namely, the gel-forming composition and the crosslinking catalyzing 
substance which also serves as the acidizing substance. These two 
substances can be initially premixed, or simultaneously injected, or 
injected in alternate slugs, into the formation. If separately but 
simultaneously injected, after the high permeability zones are plugged, 
the injection of the gel-forming composition is terminated while injection 
of the acidizing substance is continued. The advantage of not having 
several different formulations to pump into the well or to premix at 
various times greatly facilitates the use of this method for enhanced oil 
recovery. Having only two compositions for injection also reduces the 
chances for operating error. The method is particularly valuable in remote 
locations where providing several compositions and several storage tanks 
would be difficult. For example, jungle locations or other remote 
locations are very difficult to operate in because the general lack of 
utilities renders complex processes susceptible to prolonged down times 
for relatively minor break downs. Thus there is a need for a relatively 
simple method of enhancing oil recovery. 
Whether these inventions are used for preventing loss of circulation fluids 
as first described herein, or for a combination process as subsequently 
described herein, in all of these embodiments there is the additional 
advantages of being able to use the formation brine as a source of water 
for the injected mixtures, and the stability of the gels at elevated 
temperatures. In still further embodiments of the above described gels, 
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 various crosslinkable aldehydes 
and glutaraldehyde 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 
these inventions 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 in-situ or 
formation temperature of about 80.degree. C. or higher, and in some 
embodiments where the average in-situ or formation temperature is 
125.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. 
With regard to our combination process for enhanced oil recovery, the 
principles of this invention can be used where the subterranean 
water-conveying zone or flow channel, or nonproductive or high 
permeability part of a reservoir is under the subterranean 
hydrocarbon-producing formation; or where the subterranean water-conveying 
zone surrounds the subterranean hydrocarbon-producing formation; or where 
at least part of the subterranean water-conveying zone coincides with at 
least part of the subterranean hydrocarbon-producing formation. 
In one embodiment of this invention directed to a water flood operations, 
it frequently is desirable to treat the water injector wells with a 
polymer gel-forming solution to control the water flow profile. In this 
embodiment such treatment prevents channeling of water at the injector 
well and/or controls and/or redirects water flow through regions of 
varying permeability. Since in this embodiment the polymer is injected as 
a relatively low viscosity aqueous phase it penetrates preferentially the 
region of highest permeability to water. Accordingly, after formation of 
the gel in high permeability regions, such regions are converted to low 
permeability to further retard water flow thereby causing, upon further 
water injection, a water sweep of previously inaccessible areas in the 
formation which usually have relatively low permeability. By extending the 
water flow to such previously inaccessible regions, more hydrocarbons can 
be recovered than would be recovered in the absence of such polymer 
treatment. 
The gels of these inventions have improved resistance to heat and are 
stable in hard water. These characteristics make these gels particularly 
useful for many oil field applications such as water mobility control. 
These gels can be advantageously used in other harsh environments such as 
solar pond construction where they can be used to consolidate loose soil 
and to retard or stop the leakage of brine through the pond floor, or to 
prevent convective flow from lower intervals containing hotter water into 
upper intervals containing cooler water. For oil field application, no 
other gels are known which exhibit the stability and durability of the 
gels of this invention especially in high temperature reservoirs. 
Accordingly, one objective of this invention is to provide a means of 
controlling water movement in oil wells and subterranean formations 
especially in formations having temperatures 80.degree. C. or higher, or 
where the waters involved are saline or hard. 
Still another object of this invention is to provide a gel which can be 
formulated using hard water and water containing a high level of dissolved 
solids such as sea water and formation water encountered in deep off-shore 
hydrocarbon fields. 
Another object of this invention is to provide a gel which is quick setting 
so that it can be used to stop the loss of circulating fluids such as 
drilling, completion and workover fluids. 
Another object of this invention is to provide a gel which is stable at 
high temperatures and in particular more stable than other gels at such 
high temperatures. 
DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In one embodiment the gel-forming mixture is first formed at or near the 
point of lost circulation in the wellbore by injecting one of the 
components of the gel-forming mixture separately into the formation 
adjacent or near the point of lost circulation. For example, the acidic 
catalyst by itself or mixed with the aldehyde, or the aldehyde by itself 
can be injected into the formation through a separate tubing run down the 
wellbore adjacent to, or near, the point of lost circulation. The aqueous 
solution of PVA based substance can be introduced directly into the 
wellbore or in another separate tubing the outlet of which is adjacent to 
the point of loss circulation. The two separate streams meet and mix for 
the first time adjacent to, or near, the point of lost circulation. In the 
fast setting gel-forming composition of this invention isolation of any 
one of the three components, i.e., aldehyde, acidic catalyst, or PVA based 
substance, from the other two components will prevent the initiation of 
the crosslinking and hence gelation reaction.

EXAMPLE NO. 1 
This example demonstrates how to determine the relationship between the 
aldehyde concentration and the pH of the gel-forming composition and the 
gel time thereof in a particular reservoir rock associated with a well 
experiencing a loss of circulation fluid. Reservoir injection water (RIW) 
or reservoir brine is preferably used to produce the gel-forming 
composition. Polyvinyl alcohol having an average molecular weight of about 
125,000 is added to RIW to produce a 3% concentration and the mixture 
heated to 95.degree. C. for 45 minutes to completely dissolve the polymer. 
The pH of the mixture is adjusted to 5.0 by the addition of 12% HCl 
solution. 
A high pressure core holder is packed with crushed reservoir rock to form a 
60 centimeters (60 cm) long, 5 cm diameter test core sample. The test core 
sample is saturated with RIW and heated to 90.degree. C. in preparation 
for a flow test. RIW, at 90.degree. C., is injected into the core sample 
at a rate of 30 cm per day and the pressure drop across the core sample 
measured. Mineral oil, at 90.degree. C., having a viscosity of 10 
centipoise (10 cp) at 25.degree. C., is then pumped through the core 
sample at 30 cm per day until no more RIW is displaced therefrom. 
Additional RIW, at 90.degree. C., is then pumped through the core sample 
at 30 cm per day until no more mineral oil is displaced therefrom and the 
pressure drop measured. 
The gel-forming composition is prepared by mixing 24 parts of the thusly 
prepared 3% polymer solution with one part by weight of a 50% aqueous 
glutaraldehyde solution (commercial grade) in a mixing tee located at the 
inlet of the high pressure core holders. The gel-forming composition is 
thereafter injected into the core sample immediately after its formation. 
The injection rate is 30 cm per day until the gel point is reached which 
is indicated by a rapid increase in pressure. The actual gel time is 
compared to the nominal gel time in neutral rock. The variation of 
gel-time from nominal is an indication of the influence of the reservoir 
rock on crosslinking reaction rates. 
EXAMPLE NO. 2 
Preferably after determining the effective aldehyde concentration and pH of 
the gel-forming composition in core samples of crushed reservoir rock, for 
example as described in Example No. 1, treatment of a well experiencing a 
loss of circulation fluid can be conducted with a gel-forming composition 
which is effective for stopping the loss of circulation fluid. For 
example, in a well having a temperature of about 90.degree. C. and 
experiencing a loss of circulation fluid to a 30 cm interval at the 2700 
meter (2700 m) depth, the end of the drill pipe is set adjacent the 30 cm 
interval at the 2700 m depth. Separate tubing is also set opposite the 30 
cm interval. 
A 3% polyvinyl alcohol RIW mixture having its pH adjusted to 5.0 is 
injected into the wellbore at a rate of about 20 cubic meters per hour (20 
CMPH) until 160 cubic meters is injected. Simultaneously with the 
injection of the polymer mixture, a 50% aqueous glutaraldehyde solution 
(commercial grade) is injected into the separate tubing at a rate which 
corresponds to the effective glutaraldehyde concentrate determined in a 
core sample of crushed reservoir rock, or alternatively at a predetermined 
rate which corresponds to a predetermined ratio of glutaraldehyde to 
polyvinyl alcohol. For example the glutaraldehyde can be injected at a 
rate of about 0.83 CMPH. 
If after injection of 80 cubic meters of the gel-forming composition the 
pressure has not increased substantially, then to the remaining 80 cubic 
meters of polymer mixture, is added about 0.04 cubic meters of a bridging 
agent. Nonlimiting examples of bridging agents are diatomaceous earth, wax 
beads, crushed walnut shells, and other plugging agents. 
At any point in the plugging operation, when the injection pressure 
increases rapidly indicating that plugging of the 30 cm interval has been 
completed, the injection of gel-forming aqueous solutions into the 
formation is stopped and the gel-forming mixture is displaced into the 
formation with no more than about 0.5 cubic meters of brine of over 
displacement. 
The gel-forming composition is formulated so that it will gel within 5 
minutes after entering the lost circulation interval. The first part of 
the formed gel is pushed away from the wellbore in-depth into the 
interval. This process is repeated with subsequently formed gel segments 
until sufficient gel is formed in the entire interval and the injection 
pressures for the aqueous mixtures increase rapidly. Even though the 
mixture is designed to gel rapidly, several hours of injection will 
probably be required in order to inject all of the mixtures necessary to 
completely plug the interval. In most cases, about 80 to 160 cubic meters 
of gel-forming composition is required. After plugging the lost 
circulation interval, the drilling operation can be resumed. 
EXAMPLE NO. 3 
This example demonstrates how to determine the relationship between the 
gel-forming composition and the pH and gel-time thereof in a particular 
reservoir rock associated with a well experiencing water channeling in a 
water flood operation. Reservoir injection water (RIW) or reservoir brine 
is preferably used to produce the gel-forming composition. Polyvinyl 
alcohol having an average molecular weight of about 125,000 is added to 
RIW to produce a 2.5% concentration and the mixture heated to 95.degree. 
C. for 45 minutes to completely dissolve the polymer. 
A reservoir test core sample 60 centimeters (60 cm) long, and 0.8 cm in 
diameter is wrapped with Teflon.TM. tape and saturated with RIW. A 0.4 cm 
hole is drilled in the core along its axis thereby producing an annular 
core sample of reservoir rock. A porous distribution disk is sealed to one 
end of the annular core sample with Teflon.TM. tape. The central 
cylindrical cavity of the annular core sample is then packed with crushed 
reservoir rock and packed annular core sample with attached distribution 
disk is inserted into a tightly fitted heat shrinkable Teflon.TM. tube and 
the tube sealed. The packed central column represents a zone of high 
permeability and the annular core a zone of low permeability. The core 
unit is then mounted in an overburden cell, saturated by RIW and heated to 
90.degree. C. in preparation for a flow test. RIW, at 90.degree. C., is 
injected into the core sample at a rate of 30 cm per day and the pressure 
drop across the core sample measured. Mineral oil, at 90.degree. C., 
having a viscosity of 10 centipoise (10 cp) at 25.degree. C., is then 
pumped through the core sample at 30 cm per day until no more RIW is 
displaced therefrom. Additional RIW, at 90.degree. C., is then pumped 
through the core sample at 30 cm per day until no more mineral oil is 
displaced therefrom and the pressure drop measured. 
The gel-forming composition is prepared by mixing 99 parts of the thusly 
prepared 2.5% polymer solution with one part by weight of a 50% aqueous 
glutaraldehyde solution (commercial grade) and the pH adjusted to 4.0. by 
the addition of 12% of HCl solution. Preferably the amount of 
glutaraldehyde and the pH of the composition is designed to gel in about 3 
hours. 
The thusly formed gel-forming composition, at 90.degree. C., is then 
injected into the packed core sample at a rate of 150 cm per day until the 
gel point is reached which is indicated by a rapid increase in pressure 
drop across the core sample. At this point the packed column of crushed 
reservoir rock has been plugged while the annular core sample has not. 
RIW, at 90.degree. C., is injected into the core sample at a rate of 30 cm 
per day and the pressure drop measured. The ratio of the pressure drops 
across the core sample before treatment with the gel-forming composition 
and after treatment and gelation is an indication of the effectiveness of 
the plugging procedure. Accordingly, the higher such ratios are more 
effective in the plugging operation. 
The low permeability of the annular core sample is now increased by 
injecting a 3% HCl aqueous solution into the core sample at a steady flow 
rate until a significant decrease in pressure drop occurs. RIW, at 
90.degree. C., is then injected at a rate of 30 cm per day and the 
pressure drop again measured. The effectiveness of the acidizing step is 
indicated by the reduction in pressure drop across the sample. 
EXAMPLE NO. 4 
Preferably after determining the effective gel-forming composition and pH 
thereof in core samples as described in Example No. 3, treatment of a 
reservoir experiencing water channeling in waterflooding can be conducted 
with a gel-forming composition which is effective for reducing water flow 
in high permeability channels. The reservoir has a temperature of 
90.degree. C., an average permeability of 200 millidarcies (200 md) and a 
porosity of 20%, and before treatment is experiencing a RIW injection rate 
of 32 cubic meters per day (32 CMPD) at an injection surface pressure of 
70 kilograms per square centimeter gauge (70 kscmg) or 1000 psig. Before 
treatment, approximately 50% of the RIW is entering the bottom 25 cm of a 
470 cm interval, 75% of the bottom 50 cm of the interval, and about 100% 
of the bottom 100 cm of the interval. 
A 2.5% polyvinyl alcohol aqueous solution, formulated as in Example No. 3, 
is heated to 95.degree. C. in an in-line heater and stored in an insulated 
tank for at least 45 minutes to completely dissolve the polymer. The 
gel-forming composition is prepared as in Example No. 3 by mixing 99 parts 
of the polymer solution with one part by weight of a 50% aqueous 
glutaraldehyde solution and the pH adjusted to 4.0. About 100 cubic meters 
of the gel-forming mixture is injected into the injection well at a steady 
rate over a period of three days. The gel-forming composition is then 
displaced into the reservoir preferably with no more than about one cubic 
meter of RIW over displacement. The well is then shut in for about two 
days which is then followed by acidizing. Acidizing is accomplished by 
injecting 7.6 cubic meters or 2000 gallons of 3% HCl solution. The 
acidizing solution is followed by resumption of the water-flood operation. 
It is expected that the combined process of retarding water flow in the 
high permeability channels with the gel-forming composition and increasing 
the permeability of the low permeable channels with the subsequent 
acidizing step will reduce the injection surface pressure to about 35 
kscmg, increase the RIW injection rate to about 160 cubic meters per day, 
and provide an improved injection profile in the 470 cm interval such that 
about 10% of the RIW enter the bottom 25 cm of the interval, 20% the 
bottom 50 cm, 25% the bottom 100 cm, 50% the bottom 200 cm, and 100% the 
total interval. 
Unless otherwise specified herein, all percents are weight percents. 
The gels, the methods of forming the gels, and the processes for preventing 
loss circulation and retarding the flow of fluids have some degree of 
flexibility. For example, if the environment in which the gels are to be 
used has a relatively high temperature, gel time can be slowed by using a 
smaller amount of acidic catalyst and aldehyde. Similarly, if the 
environmental temperature is relatively low, gelation can be speeded by 
the use of larger amounts of acidic catalyst and aldehyde. 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 or saturated brine. 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 as well as the uses and applications of 
such gels to form them in situ in subterranean formations and to retard or 
block fluids in subterranean formations 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 many methods of 
forming such gels in situ in subterranean deposits will be apparent to one 
skilled in the art from this invention. For example, the necessary 
concentrations, amounts and sequence of injection of the gel forming 
fluids can be tailored to suit the particular well or subterranean 
formation being treated.