Prepartially crosslinked gel for retarding fluid flow

58~ PREPARTIALLY CROSS~I~XED GEL FOR RETARDING FLUID FLOW Technical Field This invention relates to gels, methods of forming gels, and uses for gels. A polyvinyl alcohol based-aldehyde hydrogel, or gel, is providPd which is useful for immobilizing large volumes o earth or water. The 10 gel can be used for reducing the permeability of soils and subterranean formations to the flow of fluids, waeers or brines. The gels of this invention are particularly valuable in retarding the flow af fluids, waters or brines in hydrocarbon production from a wellbore, or from solar ponds~ 15 Related Applications ~ he sub;ect m~tter of this application is related to that of Canadian Patent Application Serial ~u~ber 459,031, filed July 71 1984 for "Ge1 for Re~arding Water Flow" and Canadian Patent Application 20 Serial Nu~ber 459,111, filed July 18, 1984 for "GEL AND PROCESS FOR RETARDING FLUID FLOW" by B.B.

Sandiford and R.

Zillmer.

Background of the Invention _. ~ . .. __ _ .

The recovery of hydrocarbons, both liquid and gaseous, from 25 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 grea~ and water disposal costs so high, that hydrocarbon production is not economical. Such wate~ production has in some cases been disposed of 30 in an abandoned or dry well by separating such water from the hydro carbons 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 nonco~mercial. Even i a disposal well 35 i8 close by~ the disposal cost can still be very expensive. ~lerefore it is de9irable to find a way to reduce or shut off the flow o water while permitting hydrocarbon production to continue.

It is well known that the production of large amounts of water ~rom hydrocarbon producing wells is a major expense item in the overall 40 hydrocarbon recovery cost. It i5 not uncommon for an oil well to produce 9~1 i~ .. ..

5~1~ 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 poss;ble. The effluen~ must then be subjected to a 5 costly separation procedure to recovery water-free hydrocarbons~ The foul water separated therefrom also presents a troublesome and expensive disposal problem. Consequently, it i9 desirable to decrease the vol~ne of water produced from hydrocarbon wells. It is, of course, desirable to be ab~e to achîeve this objective and at the same time not materially 10 affe~t the hydro~arbon recovery rate~ However, where the volume of water is ver~ high, e.g., 80 to 99X water, and only 1 - 20% oil, even substantial reduction in hydrocarbon protuction can be tolerated if water production can be substantially reduced.

-One s~ch method ~f reducing the flow of water has been described in 15 ~S. ~atent No, 3,762,476 wherein a first aqueous polymer sGlution ~elec~ed fro~ tke group consisting of polyacrylamide~ a partially -hydroly~ed pol~ y lamide, a polysaccharide, a carboxymethylcellulose, a poly~inyl aIcoho~, and polystyrene sulfonate, is injected into a subterr~ean formation. Thereafter, a complexing ionic solution of 20 multivalent cati~s and retarding anions, and which ~lso comprises aluminu~ cit~ate, i9 injected into the subterranean formation. The ~ultivalent cations are selected from the group consisting o Fe~II), Fe(lII), Al(III), Ti(IV),Zn(II)9 Sn(IV~, Ca(II), ~g(II), Cr(III), and the retardi~g anions are selected fro~ the group consisting of acetate, 25 nitrilotriacetate, tartrate, citrate9 phosphaee. Brine i9 then in~eceed followed by a second slug of an aqueous polymer solution which can be the same or different from ~he first aqueous polymer solution. In any event, the ~o~plexing ionic solut;on of multivalent cations and retarding anions is capab~e 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 proces~Ps which utilize waterflooding. UOS.

Patent No.

~ 4,098,337 discloses a method for forming a hydroxymethylated 35 polyacsylamide gel, in situ, to reduce the permeability of a ~husly treated zone where the waterflood method of oil re~overy i9 employed. In this ca3e ~he gel was for~ed in situ by the injection of an aqueous pvlyacrylamide solution and an aqueo~s formaldehyde ~olution~ In waterflood operations it can be desirable to treDt the water 40 ;njector wells with a polymer gel forming solution to control and/or - -- 3 -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 Df varying permeability.

Although polyacrylamide-based gels can be effective in retarding 5 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 65C. Abo~e about 65C, polyacrylamide-based gel& become very sensitive to hardness of the 10 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 te~per~ture, hardne-ss~ or total dissolved solids content abo~e the afore15 mentioned ranges us~ally are not capable of being successfully treatedwi~h p~l~ac~yla~ide-~ased polymers to retard the flow of ~ater.

P~lyacrylamides usually are formulated with a dichromate which is red~ced from a valence of plus six to plus three in which state the chromic ion then acts as the crosslinking agent for the polymer. There 20 is growing concern that the use of dichromate in formulations for blocking the flow of fluids in subterranean formations might pollute aquifers. Thus there is a growing desire and need to replace formulations ~hich contain dichromates, such ~s some polyacrylamides and lignosulfonates, with formulations which are dichromate free. The gel 25 systems of this invention are dichromate free.

In many hydrocarbon producing wells temperatures of 80C or higher are often encountered. Formation waters frequenEly have hardnesses which -exceed 1000 ppm~ It i9 therefore desirable to develop a gel which can be used to ~etard or block the flow of water in subterranean ~ormations 30 having a temperature of 65C or higher, and a water hardness of 1000 ppm or higher.

In other ilooding operations, rather than water, other fluids can be used. S~me fluids which are used are carbon dioxide nnd steam. Because of the high temperature required in steam flooding or other steam 35 ~timulation methods, many of the ge~s used for blocking water are not suitable or satisfactory for blocking steam. Other steam treating ~ethods such as "Push and Pull" operations, sometimes referred to as ~cyclic steam injection" or "~luff and Puff" operations~ where a production well is steamed for several days and then produced for a month or 40 so result in ~team channels bein~ for~sd which if not blocked will result - 4 ~ 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 5 as carbon dio~ide can also be used in push and pull operations.

Flooding operations using carbon dio~ide and other gases as the drive fluid frequently experience a loss of drive fluid to nonproductive parts of the reservoir because o greater ability of gases to dissipate into such channel ~s opposed eO liquids. Loss of drive gases in flooding 1~ operations and steam in stimulation methods is more difficult to prevent because the flow channels responsible for such losses can be very small i~ diameter or width thereby making it very difficult to fill such chan~els with a blocking agent. Some viscous plugging s~bstances, even though they may have the desired stability at higher te~peratures) are 15 not ~ble-to penetrate and effectively fill narrow channels, particu1arly as ~urh channeLs bece more distan~ from the wellbore.

Thus there is a need for plugging fluids which can be formulated to penetra~e deeply into the formation. The use of this invention addresses this proble~ a~d provides polyvinyl slcohol based gels which can be 20 tailor ~ade to t~e 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. P~tent NoO 2,832,414 discloses such a method wherein an aqueous solution of a water soluble polyvinyl alcohol whirh is capable of 25 formi~g 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 ~etal casing.

30 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 base~. Furthermore, in Patent No. 2~B32,414 the gel i~ used to fill a relatively large but stagnant cavity compared to the volume of a typical channel in a subterranean formation associated 35 with hydrocarbon production from a wellbore.

Studies of the ~acroscopic changes in polyvinyl acetate gels that occur upon removal irom awelling equilibrium with isopr~pyl 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 40 of gels having vsriou3 deBrees of crosslinking and polymer concentration.

8~ ~, -- 5 -The polyvinyl aceeate g21s were formed from precursor polyvinyl alcohol gels that were crosslinked with glutaric dialdehyde ~hich were then converted to acetate gels by polymer homologous acetylation.

~S.

Patent No. 3~2659657 disclose~ a procless for preparing an S aqueous polyvinyl alcohol composition, which re~ains fluid for at lea~t a few seconds after preparation and spontaneously gels thereafter. The gel i9 formed by contacting a gelable fluid aqueou~ polyvinyl alcohol solution ~ith a hexavalent chromium compound and a reductive a~ent to convert Cr(VI) to Cr~III). The compositions are used to produce foams suitable 10 as insulat;ng7 acouseical, and packaging materials. The gel~ are crosslinked ~ith ~hromium, not an aldehyde.

U.S Pateat No. 3,658,745 discloses a hydrogel which is capable of ~ignificant expansion upon cooling in water and reversible ~hrinking upon heati~g which comprises a crosslinked acetalated hydrogel formed by - 15 reactlng a poly~inyl alcohol previously dissolved in watcr and a ~onal-dehyde and-a di~ldehyde. The hydrogel~ are alleged to have sufficient crossli~ki~g to prevent imbibition of macromolecular materials such -~s proteins ~t not the imbibition of ~icromolecular materials such as ~ow molecular weight water solutes. These hydrogels are alleged to be 20 useful for dialytic purification when pure water is added to the macromolecular so-l-ution 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 ~hr-inkage of these gel~ occurs upon slight heating such as from 12 to 25 3TG which indicates that these gels would have little value for blocking water in subterranean formations, especially at temperatures of 37C or higher.

Summary of the Invention By tfie term "aldehyde" as used herein is meant a monoaldehyde, a 30 dialdehyde, a polyaldehyde, and any of the former whether subs~ieu~ed or unsubs-tituted. PreferabLy the aldehyde contains two functional groups such as dialdehyde os a substituted ~onoaldehyde as used herein is meant to include unsaturated carbon-carbon bond as well as substitution of functional groups. Nonlimiting examples of substituted monoaldehyde are 35 acrolein and acrolein dime~hylacetal. Polyaldehydes can be used and may in some ca6es be more desirable, however, polyaldehydes are not as available commercially as dialdehydes and as a con6equence use of polyaldehydes may not be practical.

Non-limiting examples of dialdehyde crosslinking sgents are glyoxal~ 40 ~alonaldehyde~ succinaldehyde~ glutaraldehyde~ adipaldehyde~ -- 6 -terephthaldehyde. Non-limitissg examples of dialdehyde derivatives are glyo~al bisulfite addition compound ~ a~ ~C( oa) S03CH~O~)S03, glyoxal ~rimeric dihydrate, malonaldehyde bisdimethylacetal~ 2,5-dimethoxytetrahydrofuran, 394-dihydro-2-methoxy-2H-pyran, and furfural. Acetals, hemiacetals, cyclic acetals, bisulfite addition cos~pound~, shiffls ba3es or other compounds which generate dialdehydes in water, either alone or in response to ~sn additional agent such as an acid or a condition such as heat are also meant to be ;ncluded in the term 10 "~sldsehyde" as used and claimed herein.

~ on-limiting e~amples of monoaldehyde with a second functional group in addition to the aldehyde group are acrolein and acrolein dimethylacetal.

~ n-lisniting e~a~ples of polyaldehydes are polyacrolein dimethyl15 acetaI, addition ~roducts of acroleils for example, ethylene glycol plus~c~ol~i~ an~ glycer41 plus acrolein.

By-the ter~ "acidic catalyst" or "crosslinking catalyzing substsnce" ~ss used herein is meant a subst~snce which is a proton donor or a substance which in its environment will form or become a proton donor.

20 All acids are operable as an acidic catalyst in the gel ~ystems described herei~, for example, Bronsted acids such a~ mineral and carboxylic acids, ~r-L-~w~-s acids. Non-lisniting examples of a ~ewis acid are zinc chloride, ferrous chloride, stannou~ chloride, aluminum chloride, barium fluoride, and ~tslur trioxide. Some of these chemicals hydrolyse in water ~o 25 produce metal oxides or hydroxides and HCl or HF. ~he rate of hydrolysis of many-Lewis acids is dependent on emperature and the other dissolved compounds in the solution. ~se rate of production of the acidic catalyst~ HCl, from some of the abo~e Lewis acids determines the rate of gel formstion.

A delayed action catalyst is a substance which is not acidic in and of itself, but which generates an acidic catalyst slowly on insteraction with water at the temperature of isltere~t. For example, the rate of generation of the acid in oil well usage is usually controlled by the reservoir temperature experienced during the in-sit~s gel formstion. In 35 many applicutions 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 t~o cosnponent ~ystem, for example, a two consponent delayed action catalyst can comprlse a first component which 40 will react with a ~econd component~ to form an acidic catalyst. A 53 3~ 7 -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 wi~h the reducing agent to produce sulfuric acidO In another two component delayed action catalyst system 5 the reaction product of the two components can react with water to form the acidic cataly~t.

The acidic catalyst and/or delayed action catalyst must, of course, have some solubility in water. However, in so~me oil field usages the partial solubility of the acidic catalyst in ~he oil product can be 10 advantageous if treatment is to include subterranean zones containing both oil and ~ater~ The fr~ction of the acidic catalyst or delayed action catalyst ~hich dissolutes in oil will, of course, not be available to catalyze the gel formation reaction in such zones of high oil content, co~se~uently such oil-~ater zones will not be blocked by gel formation to 15 the sa~e extent as those zones with li~tle or no oil present.

~ on-limiting-e~æmples of delayed action catalysts are methyl formate, ethyl formate, m~t~yl acetate9 ethyl acetate, glycerol monoacetate or acetIn and glycerol diacetate or diacetin.

Iaboratory tests conducted on core samples have shown that diacetin 20 hydrolysi~ more slowly than methyl formate at all temperatures including the higher temperatures. Therefore, where subterranenan formations having higher temp~ratures 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 ionger time for the 25 gelling forming fluids to penetrate into the pores of such subterranean zones before gelation occurs. Non-limiting examples of delayed act}on catalyst and their acidic catalyst product are:

Delayed_~ction Catalyst Acidic Catalyst Product Meth~l formate Formic acid ~lycerol diacetate Acetic acid Sodium persulfate Sulfuric acid Sodium dodecyl sulfate Sulfuric acid ~ethyl methane sulfonate Methylsulfonic acid Sodiu~ triiodide/sodium Hydroiodic acid bi~ulfate/water Therefore9 delayed action acidic catalysts can be esters which slowly hydrolyze in wates, the rate of hydrolysis being dependent on temperature and initial pH~ Other delayed aceion catalysts are the ana!Logs of esters ~nd acid~ ~uch ~8 sulfones~ xanthates~ xanthic acids, thi~cyarlates, and 40 the like. In some of these examples, hydrolysis produces an acidic ~2'~ -- 8 -catalyst which speeds the c~osslinking reaction and an alcohol which does not affect ~el formation. An example of a delayed action acidic catalyst is methyl formate which is ;nfluenced by the environment ~ith respect eo the rate of formation of acid. For example, the higher the temperaturet 5 ehe faster ~ethyl formate will hydrolyze and generate fo~ic acid~ By the tPrm "Bronsted acid" a~ 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 ~ 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 character;~-tic or the characteristics of the environment in which it is used.

By the term "gel" as used herein is meant a chemically crosslinked three-dimension-al e`lastic network of long-chain molecules with a certain 15 amount of immobilized solvent (diluent~ molecules.

By the term npy~ based substance" or ~PVA" (frequently referred to her~in as the '~irs~ substance") as used herein is meant long-chain molecu1e~ selected ~ro~ the group consisting o polyvinyl alcohols, polyvinyl alcoh~l copolymers, and mixtures thereof.

2n By the term ~PVA-aldehyde gel" as used hesei~ i~ meant a chemically ~rosslinked three-dimensional elastic network of lon~chain molecules ~elected from-the group consisting of a polyvinyl alcohol, a polyvinyl alcohol copolymer, and mixtures thereof~ crosslinked with an aldehyde, ~ad containing a~certain amoune of i~obilized and chemically bound water 25 molecu~e9.

By the term '~VA-glutaraldehyde gels" as used herein i~ meant a chemically three-dimensional elastic network of PVA based substance crossLinked ~ith glutaraldehyde, and containing a certain amount of unmobili~e~ and chemically bound water molecules.

All of the above mentioned acidic catalysts are effective crosslinking catalyzing substances for PVA-glutaraldehyde gel systems.

Non~lim;ting examples of polyvinyl alcohol copolymers are polyvinyl alcohol-co-crotonic acid, pDlyvinyl alcohol-co-acrylic acid, polyvinyl alcohol-co-methacrylic acid, polyvinyl alcohol-co-vinylpyridine, and 35 polyvinyl al~shol-co-vinylacetate, the latter of which i~ frequently present in small amounts in comme~/cial grade polyvinyl alcohols.

It is disclo~ed in-~NMme ~ &,-patent application serial number ~/S~ ~ that PVA based gels can be formed at elevated temperstures by using a high concentration of glutaraldehyde as the 40 cros~linking agent for forming the gel~ and that by u~ing a relat;vely 5~3~ higher concentration of glutaraldehyde that an acidic catalyst i5 not required for crosslinking. This discovery offers a very distinct advantage over oeher PVA-aldehyde gel systems in that it permits the gel-forming composition to be used in subterranean formations having high alkalinity where the alkaline material increases the pH of the gelforming composition to levels which, if not basic, is sufficiently high that gelation will not occur or occurs only after a very long period of time which is too long to be of commercial value for retarding the flow of fluids. It is disclosed that the higher glutaraldehyde concentration 10 somehow produces a slightly acidic condition in the gel-forming co~position.

Unfortunately gels produced with high glutaraldehyde concentration tend to be less stable in the long run than gels having lower glutaraldehyde concentration. We have discovered that the amount of crosslin~ing 15 aj~e~ i.eO, aldehyde and in particular glutaraldehyde, can be reduced by prehea~i~g the gel-forming composition to an elevated temperature for a period ~f time to effect a predetermined amount of partial crosslinking between ehe PVA and the aldehyde. After comple~ing such partial crosslinking the gel-forming mixture can be cooled or allowed to cool to 20 a lower temperature and still a gel will form within a period of time acceptable and useful for gelling in subterranean flow channels.

Howev2~, if the gel-forming composition were not partially crosslinked iQS described above, but ~erely maintained at the lower temperature, then either the gel would not form, or if a gel does form it would take too 25 long to form to be of any practical value in subterranean formations.

Accordingly~ there is provided herein an improved process for retarding fl~id flow in the channels of a subterranean formation. In particularS in a subterranean formation having channels which have a high permeability to the flow of fluids, a process is provided or retarding fluid 3~ flow in the channels comprising providing a gel-forming composition which when substantially completely gelled in the high permeability channels is operable for retarding fluid flow therein, said gel-forming composition comprising (i~? an aqueous solution comprising a first substance selected from the group consisting of polyvinyl alcohol, a polyvinyl alcohol 35 copolymer, and mixtures thereof, and (ii.) an effective amount of an aldehyde sufficient to form a gel with said aqueous solution when said gel-forming composition is acidic; heating at least a part of the gel-forming composition to a preheat temperature which is higher than the average in situ temperature oE the subterranean formation; maintaining the gel-forming composition at the preheat temperature for a period oE -- 10 -t;me sufficient to effect a predetermined amount of partial crosslinking of the gel-forming composition thereby producing a partially crosslinked composition; introducing the partially ~rosslinked composition into the 5 high permeability channels; and allowing the Ipartially crosslinked composition to form a gel which is at least substantially completely gelled in the high permesbility channels thereby retarding fluid flo~7 in the channels.

In another embodiment the predetermined amount of partial cross0 linking is sufficient to csuse the partially crosslinked composition to substantially completely gel in the high permeability channels in a predetermined length of time. In one embodiment the predetermined length ~f time is no greater than about 5 days. In another emkodiment the p~edetermined length of time is at lesst about 1/2 a day.

I~ a further embodiment of the above process includes the steps 15 comprising heating a second part of the gel-forming composition thusly providea to a ~erond preheat temperature which is higher than the average ~n-situ temperature of the subterranean formation; maintaining the second part of the gel-forming composition at the second tempersture for a second period of time sufficient to effect a second predete~mined amount 20 of partial crosslinking of the gel-foruling composition which is greater - than the predetermined amount of partial crosslinking previously effected in the first part of the gel-forming composition introduced into the high permeability hannels, thereby producing a higher partially crosslinked composition; introducing the higher partially crosslinked composition 25 into the high permeability channels in back of or after the first partially crosslinked compositon; and allowing the higher partially crosslinked composition to form a gel which i~ at least substantially completely gelled in the higher permeability channels in back of or after the partially crosslinked composition first allowed to form a gel 30 th~rein, thereby further retarding fluid flow in such higb permeability channels. In a further embodiment, the second predetermined amount of partial crosslinking is sufficient to cause the higher partially crosslinked composition to substantially completely gel in the high permeability channels within a Recond predetermined length of time which 35 is lesa than the first predetermined length of time. In yet a further embodiment the first prede~ermined length of time is no greater than about 5 days, and the second predetermined length of time is no greater than about 3 days. In still another embodiment, the first predetermined length of time in which the first part of the gel compos;tion is gelled 40 is at least about 2 days, and the second predetermined length of time in 589~ which the second part nf the gel-f~rmin~ ~om~n.~i tinn forms a gel is at least about 1/2 a day.

In a further embodiment, the second preheat temperature to which the second part of the gel-forming composition is heated is higher than the 5 first preheat temperature to which the first part of th~ gel-forming composition is heated. In yet another embodiment, the second period of time for which the second part of the gel-forming composition is maintained at the second preheat temperature is greater than the period of time for which the Eirst part of the gel-forming composition is main10 tained at the first preheae temperature.

In another embodiment~ the PVA based substance or first 3ubstance ispoly~inyl alcohol and the aldehyde is glutaraldehyde. In a still further embodiment the concentration of glutaraldehyde in the gel-forming composition i8 less than about 4% and in yet another embodiment less than lS about 007~ In another embodiment the ~olecular weight of the PVA based s~b~tan~e is ~t l-e~t about 30,000 and in another embodiment at least about lQO,OOO.

I~ still anot~er-embodiment the preheat temperature is at least about 65C.- In another embodiment the preheat temperature i8 at least about 90C, and in yet another embodiment the preheat temperature is at least about 125~C.

In another embodiment of the above pro ess, the thusly provided gel-forming composition will not $orm a substantially completely gel within about a 5 day period of time in the channels without first beîng heated to an eievated tempe~ature higher than the average in situ 25 temperature of the subterranean formation for a period of time sufficient to effect the predetermined amount of partial crosslinking of the gel~forming romposition. In yet another embodiment, the gel formed.in the channels has a smaller degree of shrinkage after gelation than other gels formed in the channels from other gel-forming compositions, wherein the other gel-forming compositions have the same fonmulation as the gel-forming composition provided by this invention except that the other gel-forming compositions have a higher concentration of the aldehyde component than the gel-forming composition provided in thi~ invention and, further, wherein the higher conc~ntrations of aldehyde in such other gel-fonming compositions are sufficient to form ~ubstantial1y completely gelled gels therefro~ within a period of time no greater than about 5 days after being introduced into the ehannels ~ithout being preheated to, and maintained for a period of time at~ a temperaeure higher than the average in situ tempera~ure of the subterranean formation. In other 5~ - 12 ~ words, the gcls of this cmbodimcnt will not form 2 gcl within 5 days of injection into the channels unless the gel is first preheated for a period of time æufficient to form a predetermined amount of crosslinking, whereas similar gels of the same composition, except for the fact that they have a higher aldehyde component coneentration, will gel without preheating within a 5 day period of time after injection. These higher aldehyde component gels have a higher degree of shrinkage than the gels of this particular embodiment of this invention described herein.

In another embodiment of the above process the gel-forming 10 composition will not form a substantially complete gel within about a 5 day period of time in 20 ml vial samples maintained at about 25C without first being heated Lo an eleva~ed temperature of about 65C or higher for a period of time sufficient to effect the predetermined amount of partial crosslinking required to cause substantially complete gelation within the 15 predet~rmined length of time. In a further embodiment, the gel-forming com~o~i-ti~n is such that when the partially crosslinked compssition in the 20 ml vial sEmple i8 cooled or allowed to cool, to a temperature of ab-out 25DC or lower immediately after the preheat period, a gel will form within a length of time no greater than about 5 days, the gel being at 20 least substantially completely gelledO In a further embodiment the gel thu~ly formed in the 20 ml viai sample has a smaller degree of shrinkage after gelation after other gels formed in 20 ml vial samples from other gel-for~ing compositions having the same formulation as the gel-forming composition provided in this embodiment except that the other gel-forming 25 compositions have higher concentrations of the aldehyde than the gel-forming composition provided in this embodiment, and still further wherein the higher concentrations of aldehyde in other gel-forming compositions are sufficient to form substantially completely gelled gels therefro~ within a length of time no ~reater than about 5 days after being fonmulated in the 20 ml vial samples without being preheated to, and maintained for a period of time at, a temperature of about 65C or h;gher~ In yet another embodiment the subterranean formation is an oil-bearing formation, and further comprising after allowing the 35 partially cro6slinked composition to Eorm a gel in the high permeability channels~ subjecting the oil-bearing formation to a waterflood operation thereby producing oil~ Most of the state-of-the-art gels, such as polyacrylamide based gels, used to block water in oil-bearing formations either have v,ery little 40 ~tability at elevated temperatures or in the presence of hard brines.

~L2~5~ - 13 ~ PVA-aldehyde-water gel systems have much better stability at elevated temperatures, or in hard brines, or both. However, it has been found that some of the PVA-aldehyde-water gels will shrink at some point in time, sometimes after 40 days or longer, when held at elevated temperatures.

It has been found that very stable PVA based glutaraldehyde-w~ter gels can be made from gel forming compositions having a low concentration of glutaraldehyde. For ~xample, very stable gels have been formed from such gel-forming compositions where the glutaraldehyde concentration was about 0.5% by weight. Such low glutaraldehyde gels remain stable for longer periods of time at a temperature of 90C. The difficulty with such low glutaraldehyde gel-forming compositions is that they take a very lon~ time to gel or in some cases fail to produce a compleeely gelled geI. This disadvantage has been overcome in this inventioa by preheating 15 the gel-forming compo~i~;ons at an elevated temperature for a period of ti~e su~ficient to ~a~se a predetermined amount of partial crosslinking of the gel-fonmi~g co~position. After achieving the predetermined amount of partial crosslinking, the gel-forming composition is then injected into the reservoiL where gelation is allowed to proceed to completion.

20 The gel-forming composition becomes substantially completely gelled withi~ a predetermined length of time ~hich depends on surh factors as ~he extent of the partial cro~slinking of the composition beore injection into the subterranean formation, the average in ~itu temperature of the formation, the pH of the gelling mixture when prese~t in the flow cha~nels, and the chemistry of the brine.

By determining the subterranean conditions that the particular gelorming composition of interest will encounter, the proper amount of partial crosslinking required to substantially completely gel withih a desired length of time after being injected into the formation can be 30 determined~ The length of time for gelation to occur is a factor in determining how far in-depth the gelling composition ~ill penetrate into the formation and block the flow of fluids. Therefore, this invention i9 useful for many formations regardless of the ormation temperature because it allows a lower amount of aldehyde ~o be used, thereby pro35 ducing a more stable gel. For example, in high temperature formations amore stable gel can be formed by using a Lower glutaraldehyde gel-for~ing composition which has been first partially crosslinked before it's injection into the subterranean formation. In low temperat~re formation, ~imilar improved gels cfln be formed by providing a greater amo~nt of partial crosslinking before injection.

5~3~ - 14 In one embodiment, the gelling comp~sitinn is partislly crosslinked to such an extent that upon injection it will penetrate into the subterranean formation for about 4 days before completely gellingO This is followed with a second slug of gelling composition which has been 5 partially crosslinked to a greater extent so that it will penetrate into the formation for about 3 days before completely gelling. Thus the second slug can be followed by a third slug of gelling composition which has been par~ially crosslinked to a still gr-eater extent so that it will penetrate into the formation about 2 days before completely gelling. The 10 third slug will form in the channels in back o or after the second slug.

The process can be repeated with each succeeding slug of gelling composition being partially crosslinked to a greater extent than the former slug thereby decreasing the time for the composition to compietely gel. The process permits the gel to be formed at greater distances from 1~ the ~llbore and o~er greater lengths in the high permeability flow ~han~els without the need for continually changing the formulation of the gel-forming composieion while at the same time allowing the more stable low-aldehyde or low glutaraldehyde component gels to be placed in such channels regardless of the in situ formation temperature.

In one embodiment, the gelling composition is heated a short time to temperatures as high as about 125C or about 150C or higher thereby enabLing the desired amount partial crosslinking to be accomplished relatively rapidly.

In yet another embodiment the gel-forming composition is preheated at 25 a temperature of at least about T = (Tf + 60C) / 1.3, and preferably to a temperature of at least about Tp = (Tf + 50C) / 1.2 where~T i8 the tempesature~ in C~ to which the gel-forming composition 30 is preheated and maintained for the predetermined period of time to effect the predetermined amount of partial crosslinking, and Tf is the - average subterranean formation temperature in ~C.

In still another embodiment, wherein the subterranean formation has a reservoir brine having a pH higher than 7, the process ~urther comprises 35 recovering a predetermined amount of the reservoir brine and adjusting the p~ thereof to a value from about 5 to less than 7, and using the adjusted brine as at least a major part of the water of the gel~forming composition.

There is also provided a gel formed from a gel-forming composition 4 compri~ing a PVA based substance, an aldehyde operable Eor crosslinking - 15 with the PVA based substance when the gel-forming composition is acidic, and water~ by heating the gel-forming omposit;on to a predetermined elevated temperature for a predetermined period of time sufficient to effect a predetermined amount of partial crosslinking of the gel-forming 5 composition such that ~hereafter the gel-forming ~omposition, ~hen allowed to cool to a predetermined lower tempe~ature, will form a gel at the predetermined lower temperature within a length of time from about 1¦2 day to no greater th n about 5 days9 and wherein the gel-forming composition will not form a gel ~ithin about 5 days at the predetermined 10 lower temperature unless the gel-forming composition is preheated to the predetermined elevated temperature for a length of time at least as long as the predete~mined period of time.

In a further embodiment the predetermined elevated temperature is at least as high as about 65C and the predetermined lower temperature i~ no 15 greater than aboue 25Co In a further embodiment the predetermined ele~ated *emperat~r~ -is at ieast as high as about 80C and the predeter~ined lower temperature is no greater than about 50C. In a still fnrther embodi~ent ~he predetermined elevated temperature i8 at least as hi&h-as-about lO0-C and the predetermined lower temperature is no greater 20 than about 75C. In yet anuther embodiment the predetermined elevated temperature is at least as high as about 125C and the predetermined lower temperature is no greater than about 95~C~ In still another embodiment the predetermined lo~er temperature is no greater than about TL = 1 r3 T~ - 60C, ~ and p-eferably no greater than about TL = 1.2 TE ~ 50 C ~here TE and-TL are ~he predetermined elevated temperature and the pred2termined lower te~perature expressed in C respectively.

In another embodiment the gel is formed from a gel-forMing 30 composition having at least about 64 percent of itæ weight as water.

In another embodiment the ~ater of the gel-forming composition is provided by a brine~ a~d the brine is at }east about 91 percent of the weight of th~ gel-forming composition. In a further embodiment the brine is a reservoir brine. In a ætill further embodiment the brine i~ a 3S re3ervoir brine with a pH higher than 7 which has been adjusted to a pH from about 6 to les6 than 7.

In one embodiment the gel i3 produ~ed from a gel-forming composition wherein the aldehyde is from about 0.01 to less than about 4% of the weight of the gel-Eorming compo6ition. In another embodi~e~t the aldehyde is glutaraldehyde; and in a still further embodiment the amount ~451~ of glutaraldehyde i5 from 0.03 to less than about 4% of the weight o~ the gel-forming composition.

In another embodiment the gel is produced from a gel-forming composition wherein the PVA based substance i8 from about 1.5 to about 5% 5 of the weight of the gel-forming composition. In a further embodiment the PVA based substance is polyvinyl alcohol.

In yet another embodiment the gel is produced from a gel~for~ing composition which further comprises an acidic catalyst. In an alternative embodiment the gel is produced from a gel-forming composition 10 wherein, othe~ than the aldehyde and acidic substances produced by the conversion of a part of the aldehyde in the gel-forming co~position, the gel-forming composition is substantially free of effective amounts of other acidic catalyst effective for promoting crosslinking of the PVA based s~bstance a~d the aldehyde; and in a further embodiment the 15 aldehyde is about 0.15 to less than about 4% of the weight of the gel formir4~ COmpOsitI~ I~ seill furthe~-embodiments of the ~bove descriged gels~ the water used-to for~ the g~l has a hardness of at least about 1000 ppm. In further embodim~ts the water has a hardness of at least about 3000 ppmy 2 or 60~0 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 embodi~ent such water has a total dissolved solids content of at least about 80,000 ppm.

In the embodi~ents of this invention the aldehyde crosslinks with the polyvinyl alcohol or polyvinyl alcohol copolymer through formation of acetals. It has been found that gels formed in ~hîs way are adsptable to the hardness of the water from which they are formed or exposed. These gels are also ~ore stable at high temperatures than polyacrylamide based 3~ gel~ Dr gel~ made from biopolymers or polyvinyl alcohols gelled by other cros61inking agents such as borate.

Because of the adaptability and compatibility of these gels to water hardne~s-or total dissolved solid6 content, these gels can be prepared using formation water~ brackish water, sea wa~er or usually any other 35 available source of water conveniently at hand. Because the largest ingredient used to formulate the above described gels is principally water, substantial economic advantage is provided Sy this inven~ion ~hich permits gel~ to be formed with the cheape3t source of available water~ Elowever, the advantages of ehis invention are not limited merely to economic advantsgea because these gels also provide ~ubstarltial technical - 17 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. ~here such contaminated water inu3ion occurs in many other gelling fluid~ the 5 gelation thereof is destroyed or so severely harmed that surh other gels, if in fact theg do gel9 would be rendered ineffective for their intended ~se.

Due to their stability at elevsted temperatures~ the above described gels can also be formed and used in formations having an average in-situ 10 temperature of about 80C or higher, and in some embodiments ~here the average in-situ temperature is 125~C or higher.

The above described methods of forming a gel in situ in subterranean formation~ be be practices using all of the gels provided by this inventio~.

The principles of this invention can be used where the subterranean ~ater-conveying ~one is under the subterranean hydrocarbon-producing z-one, or ~here the subterranean water-conveying zone surrounds the suhterra~ean-hyarocarbon-producing zone; or where at least part of the subterranean water-conveying zone coincides with at least part of the 20 subterra~ean hydrocarbon-producing zone.

In one embodiment of this invention directed to a water flood operstions, it f~equently is desirable to treat the water injector wells ~ith a poIymer gel-forming solution to control the water flow profile.

In this ~mbodiment such treatment prevents channeling of water at the 25 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 for~ation of the gel in high permeability regions, such regions are 30 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 pe~meability. By extending the water flow to such previously inaccessible regions, more hydrocarbons can be recovered than ~ould be 35 recovered in the absence of such polymer treatment.

The gels of this invention 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 ~ater mobility control.

These gels ran be advantageously used in other harsh environments such as ~olar pond con~truction where they can be used to consolidate loose soil - 18 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 applications the gels of this invention provide excellent stability and durability.

Accordingly, one objective of this invention is to provide a means of controlling water ~ovement in oil wells and subterranean formations especially in formaeions having temperatures 80C or higber, or where the waters ;nvolved are saline or hard.

Another object of this invention is to provide a ~eans to thicken a 10 gel water with an inexpensive polymer for other oil field developmental uses such as fracture fluids and fluids for secondary and tertiary oil reco~ery~ It is another object of tlli8 invention to provid~ a gel which can be formulated using ~ard water and water containing a high level of dissolved solids such as sea water ~nd formation water encountered in 15 deep off-shore hydrocarbon fields.

Another object Qf this invention i5 to provide a gel which is stable at high temperature~ and in pareicular more stable than other gels at ~uch high temperatures.

Brie~ Descril~tion of the Drawings 2~ Fig. 1 is a diagram-of a gel quality scale for grading gelation of gel-forming composition as referred to in Fig 2.

Fig. 2 is a graph of a gel quality for a polyvinyl alcoholglutaraldehyde-brine system which demonstrates the effect of preheating the gel-forming composition during gelationO Descri tion of the Preferred Embodiment An oil well having an average in-situ temperaeure of about 50C or higher, and al90 having an undesirable amount of water production, is treated by injecting a polyvinyl alcohol-glutaraldehyde-water mixture into the~wellbore and from the wellbore into the reservoir. The mixture 30 contains about 2.5~ polyvinyl alcohol having an average molecular weight of 125,000 or higher, about 0O5% glutaraldehyde~ and the remainder a brine from the oil re~ervois having 8 totsl dissolved solids content of about 50,000 and a hardness of about 5 ,000 ppm. The Tnixture is preheated for 4 hours at 90~C and then injected into an oil reservoir.

35 Crosslinking iR completed and a gel is formed in situ in a period of time ranging from about 2 to about 3 days depending upon, in part, the alkalinity of the reservoir.

The following examples demonstrate how some of the gels of this invention can be made and how some formulations will not gel upon cooling because of insufficient partial crosslinking during 40 preheating at the elevated temperature.

~2~5~ - 19 ~xample NoO 1 Seven ge1-forming compositions were formulated and preheated for various times according to the principles of this invention. All seven gel-forming compositions were made from a 2.5,~ by weight polyYinyl 5 alcohol solution dissolved in synthetic brine A having a pH of about 7Ø Various amounts of glutaraldehyde were then added to the filtered aamples. The samples were then preheated to, and maintained at, 93C for various predetermined periods of time, and immediataly thereafter allow to cool to room temperature. The gels were then evaluated at 10 Yarious specified periods of time after preheating as shown in Table 1.

The concentration of glutaraldehyde in the gel-forming composition, the duration of preheating o~ the gel-forming romposition, and the resulting gel quality for the 7 tests, are given in Table 1. Gel q~uality was graded according to the scale as shown in Fig. 1~ The ~ diagrams in 15 Figo l represent the appearance of the gel-forming composition as it appea~d in a 20 ml vial afte~ ~he Yial ~as turned upside down. The dark compos~ ~ 0 n pa~t-represents the~4~ and the white-part an air space in the vial~ Gel quaLity ratings-are sho~n under the lower end of each vial in Fig. 1.

The gel quality r2ting corresponds to the following gelation states "0" 20 no gel, "2" viscous liquid, ~'4" flowing gel, "6" soft gel, "8" semi firm gel, and "10" firm gel. Sample Nos. 3 and 4 of Table I demonstrate the principles of this-Lnvention.

As shown by Sample No. 4, with 4 hours of preheating, a gel was formed after about 4 days; where~s as shown by Sample No. 3 with only 2 hours of preheating no gel was formed. The data 25 sho~s that the gel time can be altered by maintaining the gel at the preheat temperature long enough to achieve a predetermined amount of partia-l crosslinking thereby enabling a full gel to be formed at a lower temperature. It is to be understood that the desired amount of crosslinking can be achieved by various combinations of elevated 30 temperatur2s and preheating times. For example, 4 hours of preheating at 90C will produce a certain amount of partial crosslinking for a gi~en gel-forming composition. This same amount of erosslinking ca~ be achieved at lo~er preheat temperatures with longer preheat times or at higher preheat temperatures with shorter preheat times.

35 Exa~ple No._2 Fig~ 2 is a graph of gel quality as function of time for n gelforming composition consisting of ~ynthetic brine A having 2.57 polyvinyl alcohol and 2.0% o glutaraldehyde Synthetic brine A was prepared by adding the following amount~ of salts to deionized water and adjusting 40 the volume to one liter:

~2~5~}~ - 20 NaC1 lS.0 gr.

NaHC03 0.297 gr.

SrC12 S ~ 0 0.072 gr.

~gC1 6~0 0.788 gr.

CaC12 2 ~ 0 1.800 gr~ KCl 0.186 gr.

BaCl~ 2 ~ 0 0.072 gr.

Brine A was filtered through a 0.22 micron filter bef~re using for the various tests described herein.

10 The gel of CurYe A was produced by preheating the above-described gel-forming composition for 4 hours at 93C and thereafeer allowing it to cool to room temperature and gel. A compleee gel was formed about 48 hours after preheating. This i~ to be compared to Curve B, which lies a-long the gel-time a~is of the graph3 ~hich represents the same gel15 f~ing co~position but without preheating. Curve A data points areshown- by small circles and Curve B data points by small x's. hs shown in the graph, withou~ preheating no gel was formed even after about 1~4 hours a~d the c3mposition was given a rating of zero. As shown in Fig.

1, a geI quality of ~ero means that no apparent gelation occurred.

The polyvinyl alcohol used in formulating the gel-forming composition in E~ample ~os~ 2`and 3 was a ~ommercial grade having an average molecular weight of about 126,000.

Example ~o. 3 This example demonstrates how to determine the relationship between ~5 the aldehyde concentration in the gel-forming composition~ the preheat temperature, and the period of time the gel-forming c~mposieion is maintained at the preheat temperature, for a particular reservoir rock.

~eservoir injection wateI ~RIW) or reservoir brine is preferably used to produce ~he gel-forming composition. Polyvinyl alcohol having an sverage 3~ molecular weight of about 125,000 is added to RIW to produce a 2.5% con centration and ~he mixture heated to 95C for ~5 minute~ to completely di~solve 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 35 test core sample is`saturated with RIW and heated to 50C in preparation for ~ flow test. RI~, at 50C 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 50C, having a viscOsiey of 10 centipoise (L0 cp) at 25C, i8 then pumped through the core sample at 30 cm per day until no 40 more ~IW i8 displaced therefrom. Additional RIW, at 50C~ i9 then pumped 5~3~ - 21 through the core ~ample at 30 cm per day until no more mineral oil is displaced therefrom and the pressure drop is measured.

The gel-forming composition is prepared by mixing 99 part~ of the thusly prepared 2.5% polymer solution with 1 part by weight of a 50% S aqueous glutaraldehyde solution (commercial grade). The thusly prepared gel-forming composition i9 then heated to a preheat temperature of 90C for 2 hours and immediately thereafter injected into the core sample at a rate of 30 cm per day until the gel point is reached ~hich is indicated by a sapid increase in pressure~ The actual gel time is compared to the nominal gel time in neutral ~ock. The variation of gel-time from naminal i~ an indication of the influence of the reservoir rock on crosslinking reaceion ratesO Exa~ple NoO 4 -To determiue the effect of preheat temperature on the amount of 15 partial crosslinking of the polyvinyl alcohol and the glutaraldehyde, the test-~-escribed i~ E~-ample No. 3 is repeated except the preheat temperature i8 ircreased from 90 to 100C.

Example No. 5 To determine the effect of the length of time of preheating on the amount of partial crosslinking of the polyvinyl alcohol and the glutaraldehyde, the test described in Example No7 3 is repeated except the duration of preheating the gel-forming composition is increased from 2 to 4 hours~ E~ample No. 6 Preferably after determining the optimum preheat temperature and duratioa to achieve a predetermined amount of partial crosslînking of the gel-forming composition in test core samples of crushed reservoir rock, for e~ample as describe~ in Example No. 3, treatment of an injection well experiencing ~ater channeling or loss can be conducted with the desired gel-forming c position. For example, an injertion ~ell having a 30 meters (30 m) reservoir interval with the bottom 1.5 m thereof taking abou~ 50% of the RIW, the bottom 3 M thereof taking about 75% of the RIW9 and the bottom 6 m thereof taking about 100% of the RIW, and having an average interval permeability of about 200 millidarcies (200 md), porosity of about 20%, and reservoir temperature of 50C, is receiving about 1~0 cubic meters per day (160 CMPD) of RlW at a surface pressure of abou~ 20 kilograms per square centimeter gauge t20 kac~g). A 2.5% polyvinyl alcohol aqueoua solution, formulated as in Example No. l, is beated to 95C in an in-line heater and stored in an insulated tank for 40 at lea~t about 45 minutea to completely dissolve the polymer~ The gel ~;~J~ forming composition is formulated from the 2.5% aqueous polymer solutionand a commercial grade 50% glutaraldehyde aqueous solution as in Example ~o~ 3, heated in an in-line heater to a specîfic temperatur~, and injected into the interval according to the following program, in the 5 following sequential order:

(a) heat 80 cubic meters of the gel-forming composition to 85C and inject at a rate of 160 CMPD9 (b) heat 80 cubic meters of the gel-forming composition to 90C and inject at a rate of 160 CMPD, ~ (c) heat 80 cubic meters of the gel-form;ng composition to 95C and inject at a rate of 160 CMPD9 [d~ heat 80 cubic meters of the gel-forming composition to 100C and in3ect at a rate of 160 CMPD, and (e) displace the gel-forming composition into the interval with RIW 15 at a rate oF 160 CMPD wi~hout more than one cubic meter of overdisplace ~e~t, i.e., e~ceeding the volume of ~he wellbore by no more than one cubic-meterr Duri~g steps (~) through (e), the injection pressure i8 not allowed to incr~ase aboYe the fracture pressure of the reservoir~ If it appears 20 that the racture-pressure will be reached at an injection rate of 160 CMPD, -then the in~ectioh rate is reduced so that the injection pressure will not reach the fracture pressure The well is then shut in for about 2 days, and thereafter waterflooding is resumed.

it is expected that the injection surface pressure will increase to about 50 kpscg during treatment, to about 80 kpscg after treatment, that the bottom 1.5 ~ of the interval will take about 10% of the RIW, the bottom 3 m about 20%, the bottom 6 m about 25%, the bottom 12 m about 50~, wit~ 100% of the RIW entering the entire interval.

Unless otherwise specified herein9 all percents are weight percents.

The gels, the ~eehods of forming the gels, and the processes for retarding the flow of fluids have some degree of flexibility. For example~ if the environment in which the gel~ are to be used has a 35 relatively high temperature, gel time can be slowed by using a smaller amount of aldehyde or glutaraldehyde and/or acidic catalyst. Similarly, if the environmental temperature is relatively low9 ~elation can be speeded by the use of larger amounts of aldehyde or glutaraldehyde and or acidic catalyst~ Gel times can also be varied by the amount of partial 40 cro8slinking produced by the preheating step. Furthermore~ it is - 23 ~ permlssible to usP 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 invene;on to those ~killed in ~he art.

~he foregoing disclosure and description of the present invention i5 illustrative and explanatory thereof and various changes in gel formation procedures and gel composition as well as ehe use~ and applications of such gels to form them in situ in subterranean formations and Lo retard or block fluids in subterranean formations may be made within ~he scope 10 Of the appending claims without departing from the spirit of the invention. For example, many gel formulations can be produced and ~any met~ods of forming such gels in situ in subterranean for~ations will be apparent to one skilled i~ the art from this inveneion. For example, any number of sequential injection steps of the gel-forming compositions can 15 be made. Furthermore~ the necessary concentrations, amounts and sequence of i~iection of ~he ~el-forming compositions can be tailored to suit the parti~ular well o~ subterranean formation being treated.

~0 - 2~ TABLE 1 S~mpl~ .

No. 1 2 3 4 5 6 7 ~ GA) 5 5 2 2 Preheating Time (hrs) 2 4 2 4 2 46.5 Ti~e After Preh~ti~g ~hrs) Gel Quality Rating ~See Fig~ 1) 3.75 19 - 0 - 0 0 0 4.1 10 10 0 ~ 0 0 0 5.5 10 10 6 6 0 0 0 6.5 10 10 0 6. 0 ~ 0 22.5 10 10 0 0 0 0 2 29.5 10 10 0 6 ~ 0 2 47~5 10 10 0 10 0 n 2 73 10 10 0 10 ~ ~ 2 360 41% 43~ 96% O.V.O.V. 10O.V. 2 1010 OOV. means original volume of the gel