CA1189237A - Water soluble, crosslinkable polymer compositions, their preparation and use - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

The present invention relates to macromolecular compositions crosslinked by a novel bridge having the formula -NRx-CH=N-CO-. This novel bridge is obtained by reacting a macro-molecule, preferably a polymer, having at least two amidocarbonyl groups with a compound having at least two formylamido groups in an aqueous medium under highly acidic conditions. The crosslinking bridge formed by the reaction of this invention is produced in accord with the following formula:

Description

3~

The present invcntion relate to macrolnolecular composi~i.cns crosslinked by a novel bridge having the forrnula ~NTIX-CH~N-CO-. This novel bridge is obtained by recoin a macro molecule, preferably a polymer, having at least t~Jo arnidocarbonyl groups with a compoullc~ havillg at least two formylamido grouts in an aqueous medium under highly acidic conditions. Tlle crosslinkin~ bridge wormed by the reaction ox this invention is produced in accord with the following formula:

R-NRX-C~ H2N-CO R' Ho X~C13=N-CO~ 120 Compositions containing macromolecules havi.ng at least two nmicloc~rbonyl ~,roups an a compound havl.n~, at lest two ~ormylc~ ido groups ,can act as viscos~n~ or gellin~ agents for ~qlleous ails wllerc ken died to aqucous acid, crosslinkill~
occurs. Acids gelled in this manner have important use in the recovery of oil and natural gas from subterranean formations..
The art of well stimulation can employ a technique called frac~ure-ac~iæing Jo enhance the recovery o either oil or gas from subterranean carbonate formations. Carbonate formations include limestone, dolomites, or other reservoir ogle r!~iC~
contain calcarious material. normally, fracture-acidizing involves the injection of an aqueous acid, which rnay or may no contain a ~rop~ant, .into a wellbore at such a rate and pressure 3~7 .

as to exceed the formation stresses whereby causlng rock fa~i~ue and inducing new fractures in the formation.
Fractures are natural or induced cracks or channels ln the formation matrix. Stimulstion by this technique is achieved by allowing the acid to etch the fracture face Since the face is a heterogeneous cotnposition, the acid reaction rates will vary on the exposed surface. After the exerted pressure has been relieved, fracture closure will occur but the fracture face is no longer uniform and in most cases Jill not perfectly align due to the action of the acid. If a proppant is pumped with the acid, the acid will again preferentially act on the formation face.
However, since the formation is heterogeneous, some areas of the face will be nonreactive to the acid. After fracture closure, the etched, non uniform localities of the face again will not align while areas not etched will be "propped"
open by that proppant.
In each case, a more conductive channel is provided to allow the oil or gas to flow to the wellbore aEter the injection pressure is relieved. When an aqueous acid is injected into a wellbore in a fracture-acidi~ing application, it is often advantageous to use a viscosiEying or jelling agent in the 1uid (c~fo U.S. patents 3,415,319; 3,434,971;
3,749,169; 3,236,305 and 3,252,904). Viscous fluids possess several properties ha are favorable to fracture acidizing.
For example, the fluid viscosity is proportionally related to the created fracture volume end racture width, wherefore higher fluid viscosities will generate larger fracture volumes and fracture widths In additlo~, vlscous fluids : - 3 \~

decrease the rate of the acid etching on the formation allowing the acid to penetrate deeply into the fracture and viscous fluids serve as efficient proppant transporting media necessary to place proppant into the etched fracture.
Normally viscous fluids contain homo or copolymer compositions as the viscosifying agents. In particular, copolymer compositions containing amidocarbonyl pendent groups, such as polyacrylamide, are commonly used as viscosifying agents in fracture acidizing fluids. Normally, low polymer concentra-tions, usually less -than 1.0% by weigh-t, are used. To enhance the viscosity of the fluid, it is known from German O~fenlegung-sschrift 2,657,443 that copolymers of acrylamide are readily crosslinked by the addition of aldehydes such as formaldehyde, acetaldehyde or glyoxal. The crosslinkiny reaction normally occurs at ambient temperatures in alkaline solutions or between 40 and 30C in acidic media. However, heating the fluid to initiate crosslinking and the time necessary to complete the crosslinking, sometimes in excess of 45 minutes, make this fluid impractical in fracture acidizing applications.
The present invention relates to a composition cross-linkable in an aqueous acidic medium comprising a compound having at least two formylamido groups and a different compound having at least two amidocarbonyl groups, the formylamido group and the amidocarbonyl group of said compounds being capable of reac-ting in an aqueous acidic medium to form crosslink bridges of the formula -NR -CH=N-CO-, wherein R represents hydrogen, alkyl having 1 to 4 carbon atoms or -CH2OH.
The present invention may also be defined as a cross-linked polymer composition obtained by reactiny in a highly acidic aqueous medium amidocarbonyl groups of component A) with l ., .i .:~;

23~

formylamido groups of component s), component A) comprising a water-soluble homo- or copolymer having amidocarbonyl groups in an amount of more than 10%, calculated on the weight of the polymer and component B) comprising a member selected from the group consisting of (a) a bis-acylamido-compound of the formula I
O O
H-c-N-cnH2n-(ocnH2n)m-N-c-H (I) R R
wherein Rl and R represent hydrogen, alkyl having 1 -to carbon atoms or -CH2OH, n represents a number from 1 to 3 and m represents a number from 0 to 1, and (b) a water-soluble homo-or copolymer having formylamido groups of the formula -N-C /
R9 \ H

wherein R stands for hydrogen or alkyl having 1 to carbon atoms or -CH2OH, the amount of formylamido groups being not less than 1 mol per 500 g o:E the polymer.
In another aspect, the invention compr:Lses a methocl of forming a crosslinked composition having c:rosslin]c briclyes of the formula NR -CH=N-CO in which R represents hydrogen, alkyl having 1 to 4 carbon atoms or -CH2OH comprising reacting a compound having at least two formylamido groups with a compound having at least two amidocarbonyl groups in an aqueous acidic medium.
In a further aspect, the invention comprises in the process for well stimulation by fracture acidizing with an aqueous acidic solution containing a viscosifying amount of a polymer composition, the improvement comprises the aqueous acidic - 4a -solution containing as a viscosifier a crosslinked polymer composition as hereinbefore defined.
The invention also relates to mixtures of ingredients, one comprising macromolecules having at least two amidocarbonyl groups and another comprising a compound having at least two formylamido groups, said mixtures being suitable for reaction in an aqueous medium under highly acidic condi-tions to cross-link with the formation of the -NRX-CH=~-CO-bridge. The invention further relates to methods of crosslinking such mixtures. The invention also relates to the use of the resulting crosslinked polymer compositions as - 4b -I.-`;

viscosifiers for acids that do not have the disadvantagesof the known polymers used for this purpose in fracture acidizing stimulation. I^~ile the new crosslin}<ed polymer compositions of this invention have particular utility as viscosifiers in fracture acidizing stimulation, they may also be used whenever it is desired to increase the viscosity of aqueous acids such as ln the art of metal cleaning.
The compositions according to the present invention when employed in acid fracturing have the advantage that little or no precipitants are formed in ~h2 acid environment as is typical of previously known gelling agents. Precipitants, if formed, tend to plug the underground formations and can severely limit the efficiency of the stimulation. In the present invention, if the crosslinked pol.ymer composition degrades, the resulting polymer residues are solul~le in the aqueous acld. This is because the crosslinking bridge decomposes leav:Ln~-amino functions attached to the polymer haclcbone. These amino functions impart solubillty to the polymer residues.
Crosslinkable mixtures of this invention prei~err~bly comprise:
a) a water-soluble homo- or copolymer having amidocarbonyl groups and by a bis~acylamido-compound of the formula I
O O
H-c-N-cnH2n-~ocnH2n)m-N-c~H I
Rl ' R2 wherein Rl and R2 represent hydrogen, alkyl having 1 to carbon atoms or -CH20H, ~2~7 .. .
represents a number from 1 to 3 end m represents a number from 0 to 1 or a water-soluble homo- or copol~ner having o~nyl~mldo groups of the formula NO
9 .~ Al wheroln stands for hydrogen or alkyl having 1 to carbon atoms or Cll2OH. The cros~llnk~5bl~ mlxtur~s rnay contain ox or more of thy componen ts n together wi if one or more components by ' Component a) oE the crosslinkahLe mixtures this i.nven~ion and component by are resent in a ratio by weight ox from IØ2 Jo 1:20, preferably 1:0.2 to 1:1.
In genera he homo- or copolyln~r of colrlr)on~n~
a) prior to crosslinking may be any macromoleculc, preferably a polymer, which is su~iciently solllble in water end contains in the polymer an amount of amino-carbonyl groups (a least two) suficient to allow k formation of the novel cro~slinklng brid,e, -N~X-C~l~N-CO- D in to m~cromolec~ 3~ Ox C~I
crossllnking agent i* pr~s~nt in thy compositlons llX rivals , or R . Thy ~olu~ility 1ll wator should bo not lets than lO g/l and lie content ox amidoc~rbonyl groups shoulcl be above lo preferably above 15%, calculntcd on the weigtlt of the macromolecul~O
Particularly suitable homo- or copolymers of the component a) prior to ~rosslinklng comprise 50 to 100%
by welght of entities of the formllla II

2 SIX)

3~
.

wherein R4 stands for hydrogen or methyl and of O to 50%
by weight of entities of the formula III

-CHIC-. (III) wherein R5 stands for hydrogen, alkyl having 1 to 3 carbon atoms, hydrogen and methyl being preferred, and Y stands for formylamido; ~-suhstituted formylamido where the N-substitutent is methyl or hydroxy methyl, cyano, carboxyl or its alka~, or ammorlium salts; the alkoxycarbonyl group having 1 to 6, preferably 1 to 3 carbon atoms; the hydroxy-alkoxycarbonyl group having 1 to 3 carbon atoms; the N-methylolamidocarbonyl group HOCH2~H-CO-, thy methyl group of which may optionally be etherified with alkanols having 1 to 4 carbon atoms; the allcanoylamino group having 1 to 4 carbon atoms which may optionally be N-substltuted with methylol or alkyl having 1 to 4 carbon atoms; ~yrrolldonyl~ phenylj pyridinium;
the sulfonic acid group; the sulfoalkylamidocarbonyl group having 1 to 4 carbon atoms; the phosphonic acid group, it being possible for sulfonic and phosphonic acid groups to occur as alkali or ammo~i~ salts; for a radlcal of the formula IV
~7 ~PH2~ R~
or for a radical of the formula V
~7 -CONH-CpH2p-N (V3 8~23'7 and the quaterni~ed compounds of (IVY and (V) quaternized with CH3-Cl or dimethyl sulfate, wherein R7 and R8 are the same or different and stand :Eor alkyl having l to 4, preferably l or 2; carbon atoms and p represents a number from l to 4.
Preferred homo- or copolymers of component a) prior to crosslink-ing comprise 60 to 85% by weight of entities oE the formula II and 15 to ~0%
by weight of entities of the formula III.
Furthermore, it is preferable to use as component a) homo- or copolymers in which R5 signifies hydrogen or methyl and Y signifies the carboxyl group; the sulfonic acid group, 3-sulfo-2-methyl-propyl-(2)-amidocarbonyl of the :Eormula, C,~13 an alkanoylamino group having l to 4 carbon atoms which may optionally bc N-substituted with methylol or alkyl having l to carbon atoms; pyrrolidonyl-(l) or a radical of the formulas IV and V given and defined above, wl1ere each acid group can also occur as Na-, I- or NH~-salt.
From among the group of compositions of this invcnt:ion prior to crosslinking containing a bis-acylamido compound o:f the ~Eormu1fl 1: thosc arc prc:Eerred which conta:i.n a comL)ol1nd of the :formula :[ where Rl and R represent hydrogen or -C11201-1, and n represents the number l.
It is possible to use as homo- or copolymer of component b) prior to crosslinking any polymer that is r g;237 sufficiently soluble in water, i.e. not less than 10 g/l which in audition to formylamido groups also has an amount of amidocarbonyl groups su~icient to give stable crosslinked polymers of -the novel chemical structure. Stated differently, it is possible that component a) and component b) each contains the required amounts ox amidocarbonyl and formylamido groups.
Indeed, component a) and component b) may be the same. Self-crosslinkable copolymers ox this special class are described in a concurrently filed Canadian application Serial No. 392,248 entitled "Water-soluble Copolymers" in the names ox Friedrich Engelhardt, Klaus Kuhlein, Ulrich Riegel, Sigmar P. Von Halasz, Jeffrey C. Dawson and Anthony R. Reed.
Component b) must contain a sufficient amount of formylamido groups to form the desired crosslink bridge. A
sufficient amount of formylamido groups should not be less than 1 yram mole of formylamido group (i.e., 71 grams of N-vinyl formamide) per 500 grams of the polymer.
Particularly suitable homo- or copolymers of -the component b) prior to crossllnking comprise 50 -to 100~ by weight of entities of the formula VI
-CH -CH-N O (VI) ' \H
R

wherein R stands for hydrogen, methyl or hydroxymethyl, hydrogen and methyl being preferred, and of 0 to 50% by weight oE
entities of the formula VII

Rll -CH2C- (VII) X

g _ ~8~37 wherein R stands for hydrogen or methyl and X stands for cyano; the carboxyl group or its alkali or ammonium salts; the alkoxycarbonyl group having l to 6, preferably l to 3 carbon atoms; the hydroxy-alkoxycarbonyl group having l to 3 carbon atoms; the amidocarbonyl group, the N-methylolamidocarbonyl group HOC112N11-CO-, the methylol group of which may optionally be etherified with alkanols having l to 4 carbon atoms;
an alkanoylamino group having l to 4 carbon atoms which may optionally be N-substituted with methylol or alkyl having l to 4 carbon atoms;
pyrrolidonyl-(l); phenyl; pyridinium; the sulfonic acid group, a sulfoalkyl-amidocarbonyl group having l to carbon atoms; the phosphonic acid group;
it being possible for sulfonic and phosphonic acid groups to occur as alkali or ammonium salts; for a radical of the formula VIII

o -C~l2~l2--P-R (VIII) whereir1 R and R are the same or different and stand for alkyl having l to 4J preferably l or 2 carbon atoms; for a radical of the formula IX

-COO-Cpl12p-N \ Rl3 ~1:X) wherein R and R have the mean:ings given above and p rep-rese1l~s a tl~1mbe:r from l to 4; or for a radical of the formula X

R L
-CON11-CpH2p-N Rl5 OX) and the quaternized compounds of SIX) and OX) quaternized with ~13Cl or dimethyl sulfate, wherein Rl4 and Rl5 are the same or different and stand for alkyl having l to I, preferably l or 2 carbon atoms and p has the mean-ing given above.
Preferred homo- or copolymers of the component b) prior to cross-linking comprise 60 to 95% by weight of entities of the formula (VI) and ; -~L~89~3~

5 to 40% by weight of entities of the formula ~VII).
Furthermore, it is preferable to use as component b) prior tocrosslinking homo- or copolymers in which X signifies the carboxyl group, the sulfonic acid group, 3-sulfo-2-methyl-propyl-(2)-amidocarbonyl of the formula c,~l3 C~13 an alkanoylamino group having 1 to carbon atoms which may optionally be N-substituted with methylol or alkyl having 1 to carbon atoms, pyrrolidonyl-(l) or a radical oE the formulas IX and X glven and defined above, where each acid group can also occur as Na-, K- or NH~-sal~.
Preferably, copolymers are used as components a) and b) prior to crosslinking having K-values of from 15 to 300 (cf. ~ikentscher "Cellulosechemie" Vol 13, page 5~ (1932).
It stands to reason that the copolymers may contain several differ-ent radicals X and Y respectively. As a rule, the radicals X and Y when present in a single macromolecule have not more than 3, preferably 2, diEEer-ent meanings. Copolymers oE this kind are prepared using several clitforont comonomers carrying a radical X and Y respec-tively, as a file not nlore tlu 3, preEerably 2.
The homo- or copolymer con~positions oE-this invention prior -to crosslink:ing are readily soluble in water to give solutions of` a meclium de-gree of viscosity. Hence these solutions can be handled easily. The com-positions prior to crosslinking retain this property unless they are treated with acids at a pH of 3 or below. Unlike the known polymer-aldehyde combina-tions, the compositions of this invention are storable for an unlimited period of time. Acids are used to induce the novel crosslinking reaction be-tween the amidocarbonyl groups of component a) and the formylamido groups of component b) which results in a drastic rise in viscosity due to the forma-.

tion of -NR -C~l=N-C0-linkages forming a three dimensional novel polymer net-work according to this invention. This is why these compositions are used to increase the viscosity of acids. Depending upon the amount of the com-position added, the viscosity may be varied within a wide range. The lower limit of this range is the viscosity of the pure acid free from copolymers.
If, on the other hand, a sufficient amount of a composition of this inven-tion is added, gels of the novel cross linked polymer composition may be ob-tained which do not flow spontaneously but keep any shape given to them. Be-tween these extremes any viscosity may be obtained by varying the composi-tion contents of the acid.
In fracture-acidi~ing applications, the concentration of the acrylamido homo or copolymer (component a) usually ranges from 0.24% to 0.72% by weight on total fracturing or treating fluid depending of the polymer properties, well treating conditions or reservoir characteristics.
The crosslinking agent (component b) will have a concentration range from 0.05% to 1%, preferably 0.25% to 0.6%, based on total weight of the fractur-ing or treating Eluid. The acrylamido homo or copolymers are introduced into the aqueous phase as a fine solid powder, a hydrocarbon dispersion con-taining 20% to 50% by weight of component a, or as an oil in water or water in oil emulsion normally containing 20% to 50% by weight of component a.
The preferred system is an emulsion containing the highest possible percent-age of component a to produce a stable emulsion, normally 35% to 75% by weight. Suitable oils useful in such water in oil emulsions include normal or branched paraffinic hydrocarbons having a boiling point of 150C. to 250C.
The crosslinking agents (component b) are normally introduced as aqueous solutions containing up to 50% by weight of crosslinking agent, a fine solid powder, a hydrocarbon, dispersion containing 20% to 50% by weight of crosslinking agent or water in oil or oil in water emulsions containing 20% to 50% by weight of crosslinking agent. The preferred state would be as I ,/ 's, ~8~Z37 an aqueous solution containing 50% by weight active crosslinking agent.
When the preferred concentration of acrylamido homo or copolymer, 0.2~% to 0.72%, and preferred concentration of the crosslinking compositions of this invention, 0.25% to 0.5%, are placed together in water only a medium viscosity is obtained. A Fann 35 viscometer at a shear rate of 511 sec 1 will indicate a viscosity between 20 and 60 centipoise. The composi-tions will retain this viscosity until they are treated wikh a Bronsted-Lowery acid. The acids are used to induce the crosslinking reaction between the acrylamido homo or copolymer and the formylamido compositions leading to the new crosslinked polymers of this invention by the formation of linkages according to the following chemical structure -NRX-CII=N-CO- producing a drastic rise in viscosity usually exceeding 300 cps at a shear rate of 511 sec 1 on a Fann 35 viscometer.
The normal method oE preparing the novel acid gel according to this invention is to add the preferred concentration of acrylamido homo or copolymer to an acidic solution allowing the polymer to hydrate for usually 3 min. to ~5 min. Generally, any acid or aqueous acid solution may be thickened accordlng to the present invention. In fracture-acidizing applica-tions, acid strengths normal range from l to 28% by weight of either hydrochloric, acetic or formic acid. The preferred strength is 3% to 15%
depending on the well treating conditions and reservoir characteristics.
After 90% of the viscosity from the hydrating polymer has been obtained the acidic solution is treated with the preferred concentration of crosslinking agent, 0.25% to 0.5% by weight, a substantial increase in viscosity occurs according to the formation of the new crosslinked polymer. It stands to rea-son that any other way of combining the components of this invention with the acrylamido homo or copolymer and the acid likewise yields acid gels or highly viscous acid preparations.
The viscosity of the acid thickened with the compositions of this invention is maintained at room temperature for long periods of time, e.g., `' 3~

exceeding 3 months. At elevated temperatures, e.g., above 50C., preferably above 80C., the viscosity diminishes and the highly viscous, gelled acids turn to liquids of low viscosity.
This change is due to a hydrolytical degradation of the novel three dimensional crosslinked polymer -to linear polymer chains. The time necessary for the change from the gel to the liquid state depends, to a cer-tain extent, on the composition of the polymer molecule and hence, may be varied within certain limits, but not exceeding 12 hours, by selection of appropriate monomer compounds and monomer ratios.
Generally, any acid or aqueous acid solution may be thickened according to the present invention by Eorming the novel crosslinked three dimensional polymer. Acids being solid at normal temperatures, e.g., aromatic sulfonic acids, have to be used as aqueous solutions. Preferred acids are those which are normally liquid and9 with respect to economical use in the field of oil recovery, are strong and inexpensive. I-lence in the first place, inorganic and strong lower organic acids are taken into con-sideration .
Examples of acids which may be thickened according to this ;nvcn-tion are hydrochloric acid, sulEuric acid, ni-tric acid, l~erchloric acid, phosplloric acid, formic acid, acetic acid, monocllloroacetic ceil dicllLoro-acetic acicl ~uld trichloroacetic acid.
Preferred acids Eor oil recoveriTIg pur~)oscs are hydrocllloric acld, Eormic acid auld acetic acid The homo or copolymers of the component a) prior to crosslinking are produced by (co-)polymerizing 50 to 100%, preferably 60 to 85% by weight of acrylamide or methacrylamide and 0 to 50%, preferably 15 to ~0% by weight of a comonomer of the formula IIIa ,R5 CH2=C-Y (IIIa) wherein R and Y have the meanings given above.

- l -Z3~7 1onomers having the ormul~ (IIla) and suitable for copolymeriza~ion with acrylamide or methacrylamide include the following:

R 5~ C11~
- -CHO

5¦-CH3 -I ~3 3 _ _ . __ . . . _ .. __ _ _ _ _ _ Y -COOC~3 -CO~C2H5 COOC2~5 -COOC' E1 -COOC~

~5 CH3 c~3 . __ ., _ . . . _ Y -C~C6~13 2 4 -COOC2H~OH 3 6V~1 1~CC3H6VrI

R5 c~3 -H ~~3 _ I, ................ .~ . .
y CO~H.C~20~ -~O~IC~20~ -CONHCF~20C~I3 -CO~'~C~OC~13 R5 ~~ ~3 ~~
,. . . - . - .
Y ~COI~CH20C2H5 -CONHC~20C~H5 ~COl~Hc~2 _ I__ __I
Y ~HCOC~3 -~COC2~5 ~COC3~7 7 WHO

~5 ..~_ %37 R5 1 -I ¦ Of Y -N-COC~3 -~COC2~1$ -N-COC3~7 -~-CHO -N-CHO
l l l ~2 C~2 C~2~I C~13 CH2 -OH

. .
~,5 _~ -El _~
______ _ _ _ Y COC E~3 -X'-CC~ 3H~; - N C OC 2E~5 CE~3 CE~3 C2H5 ~5 -El -CE~3 __ __ __ _ ___ _ Y a 3 -N? SO3 R5 a H `;`El OH
_ J _ ____ 3 -S03~) 'P3~2~ P~3~2 ~,5 -H _}~

-e~OhV~-C~I2SC\;~ -CN~C~2C~2CH~;3~) R5 _~

. Y -CO~EIC2~-S03~) -~o~HCH-C~32~$03~) ~3 .. ..
_ Y -coy H2c~2~H~c ~2 S03 O C~-C~2~H2 5~3 ~3 P~5 . EI
.-Y C O SO
, 3 ~:~13 3123~7 R~ H3 -CII
..... _ , , go c2lI4so3~ 1 C~13 c~3 _ _ _ _ _ A _ , . ' . . . _ -Cot 2c~2-N 3 coo-c~2c~l2-l~ 3 Y ~COO-c8~cH2 2 5 1 ~COOa CE~2C~-N 3 7 .--~ _______--_ . CH~C~2-N 4 9 -COO-C~I~CE~2-N 4 9 ~R5~ -I O
_ 3 y -coo~cE~2c~ 2c~l2~N 3 C~3~C~2t~2_N 3 Y O ~C2E~i -COC~o~c~ 3 ~:~32~5 ~3 ~C~3 . a .,, _ Y -coy 2c~ N 3 CON~-C~1~2C~N~

5 -El O
_ ___ _ _ j 3 Y -CO~;R~CE~2C~2-N~c . 2C~12 My C l _ ~CON~I-CE12C~ N ' 32 ~2 No C

Y2 H:~ No i Oh'H-CEI2C~2~2~N 3 _ Y -CO~R-CR2C~2CH2-~ 3 -CONH-C WCH 2_N~ 2 y I -CONH-C13-cH2~J 3 CONH-C-CR 11 ~H3 ~N~ l coo 2c~N 2 5 H2-~ 3 7 Since N-vinyl amides are no stable under acidic conditions reaction mixtures containing acidic substances mus t be neutralized prior to the polymerizn~lon such as with the above men~oned basic comonvmers.

m e homo or copolymers of the col-nponent b) p-ior to cross-lin~;in~ are prod-lced by (co-)pol~ncri~ing 50 to 100/~, preferably 60 to 95~/~ by weight of a monomer having the formula VIa:
,~,0 Cl~=CII-N-C (via) wherein Pi has the meaning given above and O T O 50%, preferably 5 to 40% by weight ox a comono~er of he formula VIIa:
pll I (VIIa~
CH2 = C-X

herein Rll and Ye have the neanings given above.

.onomers having the formula (VIa) and suitable for the production of homopolymers of component b or copolymerization with monomers of the fo~ula vIra to forTn copolyners of component b are N-vinyl-formamide, N-vinyl N-methyl-formrlmide ancl N-vinyl-~-hydroxymethyl-folmamide. Monomers having the fonnula VIIa include acrylrlmide/al~e monomers illustrrltecl above for formula IIIa.
Crosslinling of component a may also be achieved by employing compounds of formula I as a crosslinking agent.
Specific compounds of the formula I include the following species:

_ _ 3~7 Rl ' R2 n . . . _ c~3 C~13 1 0 ~H5 G2H5 1 0 C~H5 ~2H5 C4Hg C~Hg 1 o C4~29 CH3 1 0 C3H7 C2~I5 1 0 CH3 C~23 2 0 Ci~3 C~13 2 C2H5 C2~5 2 0 C3~17 : C3}27 2 0 C~13 C''3 3 }I H 3 C3H7 C3~7 3 o Preferred compounds ox ormula I ore:
O
,. ..
H-C-NH-CH2-NII-C-H ~Methylene-bis-for~amide) Fmd o o H-C-NIl-CH2-0-C}l2-N~I-CH (Bis-(N-fol~yl-aminomethyl) ether).

The compounds of formula I can easily be produced according to British Patent 1,410,722.
If copol~ners having several different radios Y and X
in components a and b respectively are desired, several different comonomers of the fonnula IIIa and VIIa respectively are used in the copoly~erization, as a rule 3 or preferably 2.
The polymerization to procluce coMponents a or b may be performed according to any known polymerizing process. Ion one of the monomers is a vinyl-form~mide monomer, R pal range from 6 to 12, preferably 7 to 9 shou].d be employed.
To adjust the pll value, allcaline reacting salts of alkali metals, e.g., alkalicarbonates, alkalihydrop,encarl~onates, alkaliborates, di- or trialkaliphospha~es , alkali-hydroxides, ammonia or organic amines of the formula NR316 are used, wherein R16 is hydrogell, alkyl hazing 1 to 4 carbon atoms or hydroxyethyl whereby at l.east one of the radicals Rl6 is differënt prom hydrogen. Preferred bases fol: adjus~in~-the pi value are the alkali compounds mentioned above, especially sodium hydroxide, potassi.um hydroxide, sodium carbonate and hydrogen carbon~lte, potassium carbonaLe and hydrogen carbonate and sodium- alld potassium bora~es. ~.nother preferred base is NH3.
The polymerization reaction can be initiated by energetic electromagnetic or corpuscular radiation or by all substances which form radicals. Accordingly, possible polymerization initiators are organic per-compounds such as, for example, benzoyl peroxide, alkyl hydroperoxides, such as, for example, butyl hydroperoxide, cuTnene hydroperoxide, p-mentllane hydro-peroxide, dialkyl peroxides, such as di-tert.-butyl peroxide, or inorganic per-compounds such as, for example, potassium, sodium or am~onium persulfate and hydrogen peroxide, and azo ,~( compounds such as, for example, azobisisobutyronitrile~2,2' azobis(2-amidinopropane)hydrochloride or azobisisobutyramide.
It is advantageous to employ the organic or i.norganic per-compounds in combination with reducing agents. Examples of suitable reducing agellts are sodium ~yrosul~hite, ~odiu~
bisul~hite or condensation products of formaldehyde with sulphoxylates. The polymerization reaction can be carried out particularly advantageously using ~lannich adducts of sulphinic acids, aldehydes and amino compounds, such as are described in German Patent 1,301,566.
It is know furthermore to add to thy pol~nerization batches small amounts of moderators, which harmonize the course of the reaction by flattening the reaction rat~/time di~t,ram, improve the reproduciblll~ of the reaction anal hence foal to uniform products with extremely little variation in q~lali.ty.
Examples of suitable moderators of this type are ni~rilo-tris-propionylamide or hydrohalides of monoalkylamines) d:ialkyl-amines or.~ri~lkylamines, such as, for example, dibu~yl~ i.ne hydrochloride. In m.anufacturin~ the copolymers ox the i.nvention, such compounds can also be present with advantage.
urthermore, scatted regulators can be added to the polymerization batches, that Is Jo say compounds itch influenee the molecular weight of the polymers formed. Usable known regulators are, for examp~ e, alcohols such as methan~1, ethanol, propanol, isopropanol, n-butanol, sec.-butanol and amyl alcohol, alkylmercaptans 6uch as dodecylmercaptan and tert.-dodecylmercap~an, lsooctyl ~hioglycola~e, and Rome halogen compounds, ah as carbon tetrachloride, chloroform and methylene chloride.
o2~2 3~7 As usual, the polymerization is carried out in an atmosphere of protective was, preferably nitrogen.
The reaction may be perfonned in solution, in emulsion or under the conditions of precipitative polymerization at a temperature of from 20 to 1~0C., preferably from 40 to 100C.
If water is used as a solvcnt for the reaction, the polymerization is run ln sollltion and a viscous, aqueotls solution of the (co-)polymerizates is obtained. The product can be isolated either by distilling off the waxer from the solution or by mixing the aqueous solution with organic solvcllts miscible with water, whereby the (co-)polymer precipitates ànd can be separated from the liquid phase, e.g., by filtration.
It is preferred, however, to use the aqueous solution of the (co-~pol,~ner obtained directly, optiona11y after having adjusted a certain desired concentration.
If (co-)polymerizing is performed in an organic solvent, as for instarlce in a lower alkanol, prefera~)ly, e.g., in Bert.
butanol, the reaction runs under the con~:i.tions oE rrecini~ativ~
polymerization. In this case, the (co-)polymer Honed pre cipitates from the starting solution as a solid com~ouncl during the course of the reaction. it eon be easily isolated in the usual nlanner, e.g,., by iiltration under suctiorl an-l drying the jilter residue. 0~ course, it is also possible, and sometimes referred, -to distill off the organic solvent.
The following working examples demonstrate the use of compositions according to the invention or the production of hi~h~viscous acid compositions, especially acld gels.
further illustrative~worklng examples of the preparation of the polymers of component b, as well as pol~ners of component a here Y is formylamido, can be found in the aforesaid application ox Engelhardt et al entitled "later Soluble Copolymers".
ale abbreviations used in the examplcs and in the included tables have the following meanings:
AM: acrylamide vinyl-N-methylacetamide A~IPS: 2-acryla~ido-2-methylpropane sulionic acid where the exponent 1 signifies khe a~onium salt, 2 signifies the salt with dimethyl- ~-hydroxyl-ethylamine, AS; acrylic acid BIAS: methacrylic acid N~A: N-methylol-acrylamide VSSNa: sodi~n salt of vinylsulfonic acid era: N me~hylol-vinylaceta~ide VA: vinylacetamide VF. vinylformamide N~F: N-methylol-vinylforma~ide VPA: vinylphosphonic acid I: ammoniu~eroxidisulfate B: a combination of a oniumperoxidisulfate + dib~l~yl-ammoniuDI hydrochloridet COOH

C: azo-isobutylronitrile VIP: vinyl~yrrolidone.

~923~7 Example 1 a) To 100 g of a 1% by weight aqueous solution of a copolymer of 80%
by weight of acrylamide, 15% by weight of AMYS and 5% by weight of vinyl-pyrrolidone (K = 201) 5 g of N-methylene-bis-formamide are added. The clear, slightly viscous solution thus obtained is stable and storable for an un-limited period of time. Upon mixing this solution with 100 ml of concen-trated hydrochloric acid the viscosity rises rapidly and within 30 minutes a gel is formed. At ordinary temperatures of 20 to 25C. the acid gel does practically not change its property over a period of more than 15 days. At 80 to 90C., however, the gel degrades within 20 minutes to give a slightly yellowish brown, clear liquid of low viscosity.
The copolymer used in this Example can be produced as follows:
b) 600 ml of deionised water are Eirst introduced into a 2 1 polymer-isation vessel equipped with a stirrer, thermometer, gas inlet tube, dropp-ing funnel and heating bath, and the following monomers are then introduced, while stirring the mixture and passing a slight stream oE nitrogen through it:
80 g oE acrylamide, 15 g of AMPS and 5 g of vinylpyrrolidone.
ThereaEter 0.5 g oE azodiisobutyronitrile is aclcled ancl the tempera-ture of the reaction mixture is brought to 50C. by means oE a heating bath.
One ml of a 10% solution of benzoyl peroxide in acetone is then adcled to cause the polymerisation to commence, with a rise in temperature and in viscosity, a maximum temperature oE 58C. being reached.
AEter completion of the reaction, the mixture is stirred or a further hour at 80C. The highly viscous polymer mass is now diluted to 1%
strength by adding water, while stirring.
c) Gels of similar properties are obtained if the copolymer used above is replaced by a copolymer of 55% by weight of acrylamide, ~0% by ~9~37 weight of AMPS and 5% by weight of vinyl-methylacetamide (K = 165) or with a copolymer of 60% by weight of acrylamide, 35% by weight of AMPS and 5% of vinyl-methyl-acetamide (K = 179).
Example 2 To a 3.5% by weight aqueous solution of a copolymer of 70% by weight of acrylamide, 10% by weight of AMPS, 15% by weight of vinylformamide and 5% by weight of vinylpyrrolidone (K-value = 152) was admixed, with stirr-ing, 1% by weight of N-methylene-bis-formamide. The clear solution obtained was storable without change for an unlimited period of time. Upon adding thereto the same volume of concentrated hydrochloric acid there was obtained, within a period of 20 minutes an acid gel which does not alter its state within a 10 day's period at 20 to 25C.
At 80 to 90C., however, the gel "fuses" within 20 minutes to give a clear liquid of low viscosity.
Acid gels of similar behavior may be obtained if the copolymer used in this Example is replaced by the copolymers given in the following table:
AM AMPS net k-value . . .

20 VIP 39.~6 2l5 20 VIP 30.3 206 20 VIP 32.6 208 All amounts given in this table are parts by weight.

Example 3 To 100 ml of a 1% by weight aqueous solution of a copolymer of 65% by weight of acrylamide and 35% by weight of sodium acrylate (K = 203) 100 ml of concentrated hydrochloric acid are added. The strongly acidic mix-ture obtained represents a slightly viscous liquid which can be stored em-changed for an unlimited period of time. Upon adding to it 0.5% by weight f of methylene-bis-formamide, a gel s-table in shape is formed wi-thin a period of 10 minutes. At 20 to 25C. the acid gel remains unchanged for at least 12 days, while at 80 to 90 C. a clear liquid of low viscosity is formed within 30 minutes. A similar acid gel may be produced if the methylene-bis-formamide used above is replaced by the same amount of a compound having the formula ~C-NH-CH2-0-CH -Nll-C~

The aqueous polymer solution used can be obtained by adding, under vigorous stirring, 2.6 ml of a copolymer emulsion produced as described be-low, into 97.5 ml of water.
The copolymer emulsion is produced as Eollows:
1.85 g of sorbitan monostearate are dissolved in 30 g of a technical isomeric hydrocarbon solvent having a boiling range of from 200 to 2~0C.
The solution is introduced into a 2-liter reaction vessel fitted with a mechanical stirrer, thermometer and gas inlet for nitrogen. A solu--tion of monomers is produced by dissolving 33.9 g of acrylamide and 18.2 g of acrylic acid in 50 ml of deionized water and the pll-value of the mixture is adjusted to 8.5 by adding a 20% aqueous solution of sodium hyclroxkle.
Then the aqueous monomeric solution is slowly added to -the orgallic sorbitan monostearate solution while vigorously stirring and the air -in tlle react:io vessel :is replaced by nitrogen.
0,07 g of 2,2'-Azobis-iso-butyronitrile, dissolved in ace-tone, are added to the emulsion of the monomers and then the reaction vessel is gen-tly heated to 60 C. while stirring. The polymerising reaction was finished with-in 2.5 hours resulting in a stable 39% by weight strength emulsion of the copolymer having a K-value of 203.

1% by weight aqueous solution of a copolymer of 90% acrylamide and 10% of dimethylaminoethyl-methacrylate (used for polymerising as the acetate) '. ' containing 5% by weight of N-methylene-bis-formamide was mixed, at a volume ratio of 1 19 with concentrated hydrochloric acid which results in an acid gel within 10 minutes. At 20 to 30C., the gel obtained remains unchanged for more than 8 days but collapses to give a clear liquid of low viscosity at 80 to 90C.
Replacement of the N-methylene-bis-formamide by the same amount of a compound having the formula C - N - CH - N - O

leads to a similar result.
The aqueous polymer solution used can be obtained by adding, under vigorous stirring, 4.8 g of a copolymer emulsion produced as described below, into 95.2 ml of water and subsequent addition of 5 g of N-methylene-bis-formamide.
The copolymer emulsion is produced as follows:
185.0 g oE a mi.xture of 84% strength saturated aliphatic hydrocarbons and 16% strength naphthenic hydrocarbons (boiling point of the mixture: 192 to 254C.), 188.3 g of a chloroalkane having a chlorine content of 66.5 and a density of 1.575 kg/m3 and 32 g of sorbitan monooleate are mixed in a vessel equipped with a stirrer, a thermometer and a nitrogen inlet and outlet.
After having well intermixed the constituents, a solution of 180 g of acrylamide and 20 g of dimethylaminoethyl-methacrylate-acetate in 387.5 parts of water is added and the aqueous phase is emulsified in the organic phase. Nitrogen is allowed to pass through the mixture for 30 minutes, which is then heated within 15 minutes to a temperature of 60C. At this temperature a solution of 0.212 parts of 2,2'-azo-bis-isobutyronitrile in a slight amount of acetone is added. After having heated the mixture to 60 C.
for 3 hours the polymerization is complete.

A dispersion is obtained which does not clot, is safe against sedi-mentation, has a viscosity of 710 m Pa sec. at 29.3 sec. 1 and contains a ,"~ - 28 -polymer having a value of 201. Polymer contents: 20.8%.
Example _ a) 1.85 g of sorbitan monostearate are dissolved in 28 g of Isopar* M, a technical isomeric hydrocarbon solvent having a boiling range of from 200 to 240 C., sold by Exxon Corporation. The solution is introduced into a 2-liter reaction vessel fitted with a mechanical stirrer, thermo~leter and gas inlet for nitrogen. A solution of monomers is produced by dissolving 33.9 g of acrylamide and 2,4 g of acrylic acid in 40 ml of deionized water and the pH-value of the mixture is adjusted to 8.5 by adding a 20% aqueous solution of sodiumhydroxide.
Then the aqueous monomeric solution is slowly added to the organic sorbitan monostearate solution while vigorously stirring and the air in the reaction vessel is replaced by nitrogen.
0.07 g of 2,2'-azobis-iso-butyronitrile, dissolved in acetone, are added to the emulsion of the monomers and then the reaction vesscl is gently heated to 60 C. while stirring. ale polymerising reaction was finished wi-th-in 2.5 hours resulting in a stable emulsion oE the copolymer.
by 67 g of Isopar M are introduced into a l-liter polymerization Elask and heated to a temperature of 60C., with weak stirring, 0.27 g of sorbitan monolaurate, 1.3 g of sorbitan monostearate, 0.17 g of sorl)itall monooleate and 4.3 g oE polyoxyethylene sorbitan monostcarate bring SLlCCeS-sively added and dissolved. With further weak s-tirring a s-tream oE nitroge is passed into the solution and the temperature is adjus-tecl-to 60 C.
94 ml of water are given into a separate vessel and 2.9 g oE AMPS, 10.0 g of VIMA, 10.0 g of vinyl ~yrrolidone and l0.~ g of N-vinyl formamide are added and dissolved while stirring. This solution is adjusted to a pll-value of 8 to 10 by the dropwise addition of 10% strength aqueous solution oE sodium hydroxide, 0.1 g of ammonium persulfate being subsequently added.
This monomeric solution is then emulsified in the organic phase via a dropp-ing funnel in the reaction flask, with rapid stirring. The polymerization * Trade Mark - 29 -23~

reaction starts after approx. 30 minutes, which is recognizable by a rise in the temperature. In the course of 15 minutes the reaction temperature rises to 80 - 90C. The polymerizing reaction having faded out, the solution is heated for another two hours at 80C. A stable emulsion having a polymeric content of 30 percent by weight is obtained. The molecular weight of the polymer is 1.5 . 106.
The two emulsions obtained under items a) and b) of this Example are intermixed and thoroughly homogenised by vigorously stirring the mixture for 5 Minutes. The copolymer composition thus obtained has a copolymer con-tent of about 27% and represents a highly active, valuable thickening agent for aqueous acids.
Example 6 a) 185.0 parts of a mixture of 8~% strength saturated aliphatic hydro-carbons and 16% strength naphthenic hydrocarbons boiling point of the mix-ture: 192 to 25~C.), 188.3 parts of a chloroalkane having a chlorine con-tent of 66.5 and a density of 1,575 kg/m3 and 32 parts of sorbitan mono-oleate are mixed in a vessel equipped with a stirrer, a thermometer and a nitrogen inlet and outlet.
After having well intermixed the constituents, a solution of 212.5 parts of acrylamide in 387.5 parts of water is added and the aqueous phase is emulsified in the organic phase. Nitrogen is allowed to pass through the mixture for 30 minutes, which is then heated within 15 minutes to a temperature of 60 C. At this temperature a solution of 0.212 parts of 2~2'-azo-bis-isobutyronitrile in a slight amount of acetone is added. After having heated the mixture to 60C. for 3 hours the polymerization is com-plete.
A dispersion is obtained which does not clot, is safe against sedi-mentation, has a viscosity of 710 m Pa sec. at 29.3 sec. 1 and contains a polymer having a K value of 201.
b) 150 g of Exsol* D, a deodorized kerosene boiling between 190 to * rrade Mark _ 30 -Phi 23~7 240 O sold by Esso Chemie of Germany, are introduced into a l-liter poly-merization flask and hea-ted to a temperature of 60C., with weak stirring, 1.3 g of sorbitan monolaurate~ 6.5 g of sorbitan monostearate, 0.8 g of sorbitan monooleate and 22 g of polyoxyethlene sorbitan monostearata being successively added and dissolved. With further weak stirring a stream of nitrogen is passed into the solution and the temperature is adjusted to 60 C.
150 ml of water are given into a separate vessel and 30 g of AMPS, 45 g of vinyl pyrrolidone and 55 g of N-vinyl formamide are added and dissolved while stirring. This solution is adjusted to a pH value of 8 to 10 by the dropwise addition of 10% strength aqueous solution of sodium hydroxide, 0.3 g of atmmonium persulfate being subsequently added. rhis monomeric solution is then emulsified in the orgculic phase via a dropping funnel in the reaction flask, with rapid stirring. The polymerization reaction starts after approx.
30 minutes, which is recognizable by a rise in the temperature. In the course oE 15 minutes the reaction temperature rises to 80 - 90 C. The poly-merizing reaction having Eaded out, the solution is heated for another two hours at 80C. A staple emulsion having a polymeric content of 30% by weight is obtained. The molecular weight of the polymer is 95,000.
The two emulsions obtained under items a) and b) of this example are intermixed and thoroughly homogenised by vigorously stirring the mixture for 5 minutes. The copolymer composition thus obtained has a copolymer con-tent of about 27% and represents a highly active, valuable thickening agent for aqueous acids.

A gas well in West Texas is selected to fracture-acidize. The well has production interval from a depth of 9~650 to 9,740 feet. At this interval, the bottom hole static temperature is 170F while the formation permeability averaged 0.1 md. The well productivity prior to treatment is

4 MCF/day and 5 barrels of condensate by natural flow.
The fracture-acidizing fluid is prepared by blending 40,000 3~7 gallons of a 15% hydrochloric acid solution containing 80 gallons, 0.2% by weight, of a common corrosion inhibitor with 1,600 pounds (40 lb per 1000 gallons) of a fine powdered (lOO-120 mesh) copolymer. The copolymer con-tains 55% by weight acrylamide, 40% by weight oE AMPS and 5% by weight of N-methyl-N-vinyl acetamide (K = 165). In addition, 80 gallons, 0.2% by weight, of a nonionic fluorosurfactant is also blended into the treating fluid. After one hour of storage in two frac tanks, a low viscosity fluid (I 35 cps at 511 sec. 1 on a Fann 35) is obtained. Upon injection of the fluid into the well bore, the aqueous crosslinking solution containing 50%
by weight of active polymer from a composition of 80% N-vinyl formamide, 10%
acrylic acid and 10% N-vinylpyrrolidone is added at a rate of 4 gallons per 1000 gallons of treating fluid. The injection rate is 12 barrels per minute at a surface treating pressure of 6000 psi. After approximately 85 minutes, the well is shut-in for 6 hours to allow the acid to react. After this time, the well head pressure is relieved and the well is placed back into a flow-ing status. The well productivity is tested and found to be significantly improved.
In its broadest aspects, highly viscous acid preparations or acid gels of the present invention may contain from 70 to 99.8% by weight of one or more of the aforementioned acids and 0.2 to 30% by weight of the cross-linked polymer composition having characteristic -NR -CH=N-CO- bridges. It is generally preferred that substantially all (at least 50% and preferably at least 70%) crosslink bridges have the aforesaid formula. Other known crosslink bridges may also appear in the final product depending upon the presence of other moieties in components a or b) and/or the addition of known crosslinking agents reactive with such moieties.
As stated earlier, the amidocarbonyl groups of component a) are carried on macromolecules. This is because the macromolecule seems to stablize the resulting bridge. Where the molecule to which the amido-carbonyl groups are attached is of insufficient size, no or only a small , ~8~Z37 number of desired bridges are stabilized, and the bridges appear to be de-graded to formic acid, ammonium ion, amine, carboxylic acid and possibly other degradation products by hydrolytic action.

..

Claims (28)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A composition crosslinkable in an aqueous acidic medium comprising a compound having at least two formylamido groups and a different compound having at least two amidocarbonyl groups, the formylamido group and the amidocarbonyl group of said compounds being capable of reacting in an aqueous acidic medium to form crosslink bridges of the formula -NRx-CH=N-CO-, wherein Rx represents hydrogen, alkyl having 1 to 4 carbon atoms or -CH2OH.

2. A crosslinkable, water-soluble composition according to claim 1 comprising:
A. a water-soluble homo- or copolymer having amidocarbonyl groups in an amount of more than 10%, calculated on the weight of the polymer and B. a member selected from the group consisting of (a) a bis-acylamido-compound of the formula I

(I) wherein R1 and R2 represent hydrogen, alkyl having 1 to 4 carbon atoms or -CH2OH.
n represents a number from 1 to 3 and m represents a number 0 or 1, and (b) a water soluble homo- or copolymer having formylamido groups of the formula wherein R9 stands for hydrogen or alkyl having 1 to 4 carbon atoms or -CH2OH, the amount of formylamido groups being not less than 1 mol per 500 g of the polymer.

3. A crosslinkable, water-soluble composition according to claim 2 wherein component A comprises 50 to 100% by weight of entities of the formula II

(II) wherein R4 stands for hydrogen or methyl and of 0 to 50% by weight of entities of the formula III

(III) wherein R5 stands for hydrogen, alkyl having 1 to 3 carbon atoms, and Y stands for formylamido; N-substituted formylamido where the N-substituent is methyl or hydroxy methyl; cyano; carboxyl; the alkali or ammonium salts of carboxyl; the alkoxycarbonyl group having 1 to 6 carbon atoms; the hydroxy-alkoxycarbonyl group having 1 to 3 carbon atoms; the N-methylolamido-carbonyl group HOCH2NH-CO-; the N-methylolamidocarbonyl group wherein the methylol group is etherified with alkanols having 1 to 4 carbon atoms; the mono- or dialkylamino group having altogether 1 to 8 carbon atoms in the alkyl radicals; the alkanoylamino group having 1 to 4 carbon atoms; the alkanoylamino group having 1 to 4 carbon atoms which is N-substituted with methylol or alkyl having 1 to 4 carbon atoms; pyrrolidonyl-(1); phenyl;
pyridinium; the sulfonic acid group; the sulfoalkylamidocarbonyl group hav-ing 1 to 4 carbon atoms; the phosphonic acid group; the alkali or ammonium salts of said sulfonic acids and phosphonic acid groups; a radical of the formula IV

(IV);

a radical of the formula V

(V) wherein R7 and R8 are the same or different and stand for alkyl having 1 to 4 carbon atoms and p represents a number from 1 to 4; or quaternary salts of formulas IV and V.

4. A crosslinkable, water-soluble composition according to claim 2 or claim 3 wherein component B) comprises 50 to 100% by weight of entities of the formula VI

(VI) wherein R9 stands for hydrogen, methyl or hydroxy methyl and of 0 to 50% by weight of entities of the formula VII

(VII) wherein R11 stands for hydrogen or methyl and X stands for cyano; the carboxyl group; the alkali or ammonium salts of carboxyl; the alkoxy carbonyl group having 1 to 6 carbon atoms; the hydroxy-alkoxycarbonyl group having 1 to 3 carbon atoms; the N-methylolamidocarbonyl group HOCH2NH-CO; the N-methylolamidocarbonyl group wherein the methylol group is etherified with alkanols having 1 to 4 carbon atoms; the alkanoyl-amino group having 1 to 4 carbon atoms; the alkanoylamino group having 1 to 4 carbon atoms which is N-substituted with methylol or alkyl having 1 to 4 carbon atoms; pyrrolidonyl-(1); phenyl; pyridinium; the sulfonic acid group;
a sulfoalkylamidocarbonyl group having 1 to 4 carbon atoms; the phosphonic acid group; the alkali or ammonium salts of said sulfonic acids and phos-phonic acid groups; a radical of the formula VIII

(VIII) wherein R12 and R13 are the same or different and stand for alkyl having 1 to 4; a radical of the formula IX

(IX) wherein p represents a number from 1 to 4; a radical of the formula X

(X) wherein R14 and R15 are the same or different and stand for alkyl having 1 to 4 carbon atoms; or quaternary salts of formulas IX and X.

5. A crosslinkable, water-soluble composition according to claim 2 wherein components A) and B) are present in a ratio by weight of from 1 :
0.2 to 1 : 20.

6. A crosslinkable, water-soluble composition according to claim 1 comprising a water-soluble homo- or copolymer having amidocarbonyl groups and a bis-acylamido- compound of the formula I

(I) wherein R1 and R2 are hydrogen, alkyl having 1 to 4 carbon atoms or CH2OH.

7. A crosslinkable, water-soluble composition according to claim 6 where the compound of formula I is methylene-bis-formamide or bis-(N-formyl-aminomethyl)ether.

8. A crosslinkable, water-soluble composition according to claim 1 comprising a water-soluble homo- or copolymer having amidocarbonyl groups and a water-soluble homo- or copolymer having formylamido groups of the formula wherein R9 is hydrogen or alkyl having 1 to 5 carbon atoms.

9. A crossslinkable, water-soluble composition according to claim 1 wherein the polymer having formylamido groups is a homopolymer of vinyl formamide.

10. A polymer composition according to claim 1 together with an aqueous acidic medium wherein crosslink bridges having the formula -NRx-CH=N-CO-wherein Rx represents hydrogen, alkyl having 1 to 4 carbon atoms or -CH2OH
exist between said compounds.

11. A crosslinked composition according to claim 10, wherein crosslink bridges of the formula -NRx-CH=N-CO- are formed by the reaction in an aqueous acidic medium of at least two formylamido groups with at least two amidocar-bonyl groups.

12. A crosslinked polymer composition obtained by reacting in a highly acidic aqueous medium amidocarbonyl groups of component A) with formylamido groups of component B), component A) comprising a water-soluble homo- or copolymer having amidocarbonyl groups in an amount of more than 10%, calcu-lated on the weight of the polymer and component B) comprising a member selected from the group consisting of (a) a bis-acylamido-compound of the formula I

(I) wherein R1 and R2 represent hydrogen, alkyl having 1 to 4 carbon atoms or -CH2OH, n represents a number from 1 to 3 and m represents a number from 0 to 1, and (b) a water-soluble homo- or copolymer having formylamido groups of the formula wherein R9 stands for hydrogen or alkyl having 1 to 4 carbon atoms or -CH2OH, the amount of formylamido groups being not less than 1 mol per 500 g of the polymer.

13. A crosslinked polymer composition according to claim 12 wherein component A) comprises 50 to 100% by weight of entities of the formula II

(II) wherein R4 stands for hydrogen or methyl and of 0 to 50% by weight of entities of the formula III

(III) wherein R5 stands for hydrogen, alkyl having 1 to 3 carbon atoms and Y stands for formylamido; N-substituted formylamido where the N-substituent is methyl or hydroxymethyl; cyano; carboxyl; or the alkali or ammonium salts of carboxyl; the alkoxycarbonyl group having 1 to 6 carbon atoms; the hydroxy-alkoxycarbonyl group having 1 to 3 carbon atoms; the N-methylolamido-carbonyl group HOCH2NH-CO-; the N-methylolamidocarbonyl group wherein the methylol group is etherified with alkanols having 1 to 4 carbon atoms; the mono- or dialkylamino group having altogether 1 to 8 carbon atoms in the alkyl radicals; the alkanoylamino group having 1 to 4 carbon atoms; the alkanoylamino group having 1 to 4 carbon atoms which is N-substituted with methylol or alkyl having 1 to 4 carbon atoms; pyrrolidonyl-(1); phenyl;
pyridinium; the sulfonic acid group; the sulfoalkylamidocarbonyl group hav-ing 1 to 4 carbon atoms; the phosphonic acid group; the alkali or ammonium salts of said sulfonic acids and phosphonic acid groups; a radical of the formula IV

(IV);

a radical of the formula V

(V) wherein R7 and R8 are the same or different and stand for alkyl having 1 to 4 carbon atoms and p represents a number from 1 to 4; or quaternary salts of formulas IV and V.

14. A crosslinked polymer composition according to claim 12 or claim 13 wherein component B) comprises 50 to 100% by weight of entities of the formula VI

(VI) wherein R9 stands for hydrogen, methyl or hydroxy methyl and of 0 to 50% by weight of entities of the formula VII

(VII) wherein R11 stands for hydrogen or methyl and X stands for cyano; the carboxyl group; the alkali or ammonium salts of carboxyl; the alkoxycarbonyl group having 1 to 6 carbon atoms; the hydroxy-alkoxycarbonyl group having 1 to 3 carbon atoms; the N-methylolamidocarbonyl group HOCH2NH-CO-; the N-methylolamidocarbonyl group wherein the methylol group is etherified with alkanols having 1 to 4 carbon atoms; the alkanoyl-amino group having 1 to 4 carbon atoms; the alkanoylamino group having 1 to 4 carbon atoms which is N-substituted with methylol or alkyl having 1 to carbon atoms; pyrrolidonyl-(1); phenyl; pyridinium; the sulfonic acid group;
a sulfoalkylamidocarbonyl group having 1 to 4 carbon atoms; the phosphonic acid group; the alkali or ammonium salts of said sulfonic acids and phos-phonic acid groups; a radical of the formula VIII

(VIII) wherein R12 and R13 are the same or different and stand for alkyl having 1 to 4 carbon atoms; a radical of the formula IX

(IX) wherein p represents a number from 1 to 4; a radical of the formula X

(X) wherein R14 and R15 are the same or different and stand for alkyl having 1 to 4 carbon atoms quaternary salts ox formula IX and X.

15. A crosslinked, polymer composition according to claim 12 wherein components A) and B) are present in a ratio by weight of from 1:0.2 to 1:20.

16. A crosslinked, polymer composition according to claim 12 wherein component A) is a water-soluble homo- or copolymer having amidocarbonyl groups and component B) is a bis-acylamide-compound of the formula I

(I) wherein R1 and R2 are hydrogen, alkyl having 1 to 4 carbon atoms or CH2OH.

17. A crosslinked, polymer composition according to claim 16 wherein component B) is methylene-bis-formamide or bis-(N-formyl-aminomethyl)ether.

18. A crosslinked polymer composition according to claim 12 wherein component A) is a water-soluble homo- or copolymer having amidocarbonyl groups and component B) is a water-soluble homo- or copolymer having formyl-amide groups of the formula wherein R9 is hydrogen or alkyl having 1 to 4 carbon atoms or -CH2OH.

19. A crosslinked, polymer composition according to claim 12 wherein component B) is a homopolymer of vinyl formamide.

20. A highly viscous aqueous acid composition comprising an aqueous solution containing 70 to 99.8% by weight of an acid and 0.2 to 30% by weight of a crosslinked polymer composition according to claim 10 or 12.

21. A method of forming a crosslinked composition having crosslink bridges of the formula -NRx-CH=N-CO in which Rx represents hydrogen, alkyl having 1 to 4 carbon atoms or -CH2OH comprising reacting a compound having at least two formylamido groups with a compound having at least two amidocar-bonyl groups in an aqueous acidic medium.

22. A method of forming a crosslinked composition in which Rx repre-sents hydrogen, alkyl having 1 to 4 carbon atoms or -CH2OH which comprises reacting in a highly acidic aqueous medium amidocarbonyl groups of component A) with formylamido groups of component B), component A) comprising a water-soluble homo- or copolymer having amidocarbonyl groups in an amount of more than 10%, calculated on the weight of the polymer and component B) comprising a member selected from the group consisting of (a) a bis-acylamido-compound of the formula I

(I) wherein R1 and R2 represent hydrogen, alkyl having 1 to 4 carbon atoms or -CH2OH, n represents a number from 1 to 3 and m represents a number from 0 to 1 and (b) a water-soluble homo- or copolymer having formylamino groups of the formula wherein R9 stands for hydrogen or alkyl having 1 to 4 carbon atoms or -CH2OH the amount of formylamido groups being not less than 1 mol per 500 g of the polymer.

23. A method according to claim 21 wherein component A) comprises 50 to 100% by weight of entities of the formula II

(II) wherein R4 stands for hydrogen or methyl and of 0 to 50% by weight of entities of the formula III

(III) wherein R5 stands for hydrogen, alkyl having 1 to 3 carbon atoms, and Y stands for formylamido; N-substituted formylamido where the N-substituent is methyl or hydroxy methyl; cyano; carboxyl; or the alkali or ammonium salts of carboxyl; the alkoxycarbonyl group having 1 to 6 carbon atoms; the hydroxy-alkoxycarbonyl group having 1 to 3 carbon atoms; the N-methylolamido-carbonyl group HOCH2NH-CO; the N-methylolamidocarbonyl group wherein the methylol group is etherified with alkanols having 1 to 4 carbon atoms; the mono- or dialkylamino groups having altogether 1 to 8 carbon atoms; the alkanoylamino group having 1 to 4 carbon atoms, the alkanoylamio group hav-ing 1 to 4 carbon atoms which is N-substituted with methylol or alkyl having 1 to 4 carbon atoms, pyrrolidonyl-(1); phenyl; pyridinium; the sulfonic acids group; the sulfoalkylamidocarbonyl group having 1 to 4 carbon atoms; the phosphonic acid group; the alkyl or ammonium salts of said sulfonic acids and phosphonic acid groups, a radical of the formula IV

(IV);

a radical of the formula V

(V) wherein R7 and R8 are the same or different and stand for alkyl having 1 to 4 carbon atoms and p represents a number from 1 to 4; or quaternary salts of formulas IV and V.

24. A method according to claim 21 wherein component B) comprises 50 to 100% by weight of entities of the formula VI

(VI) wherein R9 stands for hydrogen, methyl or hydroxyl methyl and of 0 to 50% by weight of entities of the formula VII

(VII) wherein R11 stands for hydrogen or methyl and X stands for cyano; the carboxyl group; the alkali or ammonium salts of carboxyl; the alkoxy carbonyl group having 1 to 6 carbon atoms; the hydroxy-alkoxycarbonyl group having 1 to 3 carbon atoms, the N-methylolamidocarbonyl group HOCH2NH-CO-; the N-methylolamidocarbonyl group wherein the methylol group is etherified with alkanols having 1 to 4 carbon atoms; the alkanoyl amino group having 1 to 4 carbon atoms; the alkanoylamino group having 1 to 4 carbon atoms which is N-substituted with methylol or alkyl having 1 to 4 carbon atoms; pyrrolidonyl-(1); phenyl; pyridinium; the sulfonic acid group;
a sulfoalkylamidocarbonyl group having 1 to 4 carbon atoms; the phosphonic acid group; the alkali or ammonium salts of said sulfonic acids and phos-phonic acid groups; a radical of the formula VIII

(VIII) wherein R12 and R13 are the same or different and stand for alkyl having 1 to 4 carbon atoms; a radical of the formula IX

(IX) wherein p represents a number from 1 to 4; a radical of the formula X

X) wherein R14 and R15 are the same or different and stand for alkyl having 1 to 4 carbon atoms; or quaternary salts of formulas IX and X.

25. A method according to claim 21 wherein component A) is a water-soluble homo- or copolymer having amidocarbonyl groups and component B) is a bis-acylamide-compound of the formula I

(I) wherein R1 and R2 are hydrogen, alkyl having 1 to 4 carbon atoms or CH2OH.

26. A method according to claim 21 wherein component A) is a water-soluble homo- or copolymer having amidocarbonyl groups and component B) is a water-soluble homo- or copolymer having formylamide groups of the formula wherein R9 is hydrogen or alkyl having 1 to 4 carbon atoms or -CH2OH.

27. In the method of increasing the viscosity of aqueous acidic solu-tions wherein a viscosifier is added to an aqueous acidic solution the im-provement comprises using as the viscosifier a crosslinked polymer composi-tion according to claim 10 or 12.

28. In the process for well stimulation by fracture acidizing with all aqueous acidic solution containing a viscosifying amount of a polymer composi-tion, the improvement comprises the aqueous acidic solution containing as n viscosifier a crosslinked polymer composition according to claim 10 or 12.