NO822764L - WATER-SOLUBLE ACID STABLE COPOLYMERIZES, THEIR PREPARATION AND USE. - Google Patents

Description

The present invention.. relates to new water-soluble copolymers which are stable in high concentrations of Bronsted-Lowery acids, and which are particularly

are used as friction reducers for acids, which in turn are used in the method of acid stimulation of oil-natural gas wells. The copolymer according to the invention contains in statistical distribution in its macromolecule at least 5% by weight of units which have a formylamino group A. "And

at least 10% by weight of units containing an aminocarbonyl group B

A well-known stimulation technique for oil and natural gas drilling that is used to increase the degree of transport and the overall yield of underground carbonate formations is the so-called pressure acidizing. Carbonate formation is in this, compound is for example dolomite, limestone or other storage rocks that contain calcareous material. Pressure acidizing is usually carried out so that the aqueous acid, which may also contain so-called support agents, is pumped into the borehole to be stimulated, namely under a pressure that exceeds the formation pressure. This results in new cracks and channels, so-called fractures in the formation, being blown up. In pressure acid techniques, the pressed-in acid also etches the surfaces of the rock fractures and if the acid also contains a support agent, this support agent is transported into the opened fractures. Both the irregular etching of the heterogeneous fracture surface and the embedment of support material mean that the fractures can no longer close completely after pressure relief. In each case, a better continuous channel is thus created which makes it possible for the oil, reps. the soil gas after depressurization to flow to the transfer well.

In the pressure acidizing technique, it is necessary

to pump the acid, the so-called fluid, at high speed into the bore, so that it can build up at the required blast pressure for the rock. Here it turns out that the engraver part of the injection pressure is lost due to frictional forces,

which occurs between what in turbulent flow flows into the fluid and the conduit walls. As this pressure loss, so-called - frictional pressure, increases with the injection speed, with increasing injection speed, an increasingly smaller part of the pressure is transferred as a hydraulic pressure on the formation. By adding smaller concentrations of polymers to the treatment fluid, the flow of the pumped-in fluid is now laminated so that, even at high injection speeds, the frictional pressure is reduced considerably, and thus a significantly larger part of the peak pressure is transferred to the formation stone.

Common friction reducers that are used when carrying out pressure acidification are polysaccharides such as e.g. hydroxyethyl cellulose, xanthan gum (metabolism product of Xanthomonas campestris) hydroxypropylated guar gum (from the seeds of Cyanaposis t etragonol ibus) or synthetic polymers based on polyacrylamide.

When using polysaccharides as friction reducers, problems arise that these compounds are not very stable in highly concentrated acids. Normally, the polysaccharide is hydrolysed and broken down in strong acids, as this friction-reducing effect is lost. The use of synthetic polyacrylamides as friction reducers also has technical drawbacks: In the presence of strong acids, polyacrylamides are rapidly hydrolysed to insoluble degradation products. These fall out of the treatment fluid and, by clogging the fractures, can lead to an obstruction of oil or the gas flow and thereby damage the formation.

The invention now relates to a copolymer that can be used to improve friction reducers for pressure acidizing underground calcareous formations. The polymer in the invention is acid-soluble, and its decomposition products are also both acid-soluble and stable.

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Preferably, the new polymers according to the invention are composed of 5 to 50% by weight of units of formula I

10 to 95% by weight of units of formula II and 0 to 85% by weight of units of formula III

in which R 1 means hydrogen, methyl or hydroxymethyl and R<2>

means hydrogen or methyl, and X means cyan, carboxyl or its alkali or ammonium salts, alkoxycarbonyl with 1 to 6, preferably 1 to 3 carbon atoms, hydroxy alkoxycarbonyl with 1 to 3 carbon atoms, N-methylolamidocarbonyl of the formula HOCh^NH-CO-, whose methylol group e can optionally be etherified with alkanols with 1 to 4 carbon atoms, alkanoylamirio with 1 to U carbon atoms, which can optionally be N-substituted with methylol or alkyl with 1 to U carbon atoms, pyrrolidony-(1), phenyl, pyridinium, the sulfonic acid group, sulfo-alkylamidocarbonyl with 1 to A carbon atoms, the phosphonic acid group, since sulfonic acid and phosphonic acid groups can also be present as alkali or ammonium salts, a residue of formula IV

3L

wherein R and R are the same or different and mean alkyl of 1 to 4, preferably 1 to 2 carbon atoms, a residue of formula V

wherein R 3 and R k have the above meaning, and p means a number from 1 to A, or a residue of formula VI

in which R 5 and R 6 are the same or different and mean alkyl with 1 to 4-, preferably 1 or 2 carbon atoms, and p means a number from 1 to 4-, as well as those of the formulas V and VI corresponding, for example, by means of dimethyl sulfate or methyl chloride quaternized groups.

The copolymers according to the invention are statistical, i.e. the distribution of the units I to III is statistical in the macromolecules. • Of course, the copolymer according to the invention can contain several different units of formula I and III,

e.g. different units of formula III which separate eeg from each other with different meanings of the residues X or R 2 . Usually, in a single macromolecule X has no more than three, preferably no more than two different meanings. The copolymer of this type is produced by using different comonomers with the copolymerization, usually no more than three, preferably no more than two, which differ from each other by different meanings of the residue X and/or R . Generally, the copolymer according to the invention has K-values (cf. Fikentscher, "Cellulosechemie", volume 13, page 58 (1932)) between 25 and 250, which corresponds to molecular weights between 20,000 and 15 x 10^. Copolymers according to the invention which have K values between 50 and 200 corresponding to molecular weights between 100,000 and 4 x 106 are preferred as friction reducers.

Preferred copolymers according to the invention consist of

5 to 30 wt% of units of formula I,

50 to 80% by weight of units of formula II, and

1 to 60% by weight of units of formula III.

Furthermore, such copolymers are preferred in which R 2 means hydrogen or methyl or those in which X means carboxyl, sulfonic acid groups, 3-sulfo-2-methyl-propyl-(2)-amidocarbonyl with the formula

alkanoylamino with 1 to 4 carbon atoms, which can optionally be •N-substituted with methylol or alkyl with 1 to 4 carbon atoms, pyrrolidonyl-(1 ) or a residue of the above-mentioned and defined formulas V and VI, as each acidic group can also exist as sodium-potassium or ammonium salts.

Particularly preferred for use as friction reducers when stimulating natural gas or petroleum wells are such copolymers according to the invention in which the units of formula I were introduced by copolymerization with the co-use of N-vinylformamide, and the units of formula III with the co-use of N-vinyl-N-methyl -acetamide, N-vinylpyrrolidilone or 2-acrylamido-2-methyl-propanesulfonic acid or mixtures thereof.

The copolymers according to the invention are easily soluble in water or aqueous acids, and can therefore be easily handled. When used in a pressurized acid process, the polymers can be added to the acid in the form of aqueous 0.5 to 6% by weight

in solutions (polymer solutions with a concentration above 6% by weight are too viscous, and therefore difficult to handle) in the form of hydrocarbon dispersions containing emulsifiers and 20 to 50% by weight of the polymer, or in the form of oil in water or water in oil emulsions containing emulators and 20 to 50% by weight of the polymer. Preferably, the copolymer according to the invention is used in the form of a water in oil emulsion with the highest possible polymer content, normally 20 to 75% by weight. The oil, which is suitable for the production of these water in oil emulsions, is for example branched or unbranched paraffin hydrocarbon fractions with a boiling range from 150 to 250°C. Depending on the monomer composition of

resulting properties, the copolymers according to the invention act in concentrations from 0.06 to 0.12% by weight, at temperatures from normal room temperature up to temperatures above 80°C as highly effective friction reducers, as the duration of action lasts for a longer time, for example more than 10 days . The very entry of the hydrolytic of building the side chain of the copolymer remains a breakdown product with acid-soluble and does not separate from the solution. Preferably, the copolymer according to the invention is combined with acids which are liquid under normal conditions, and especially with inorganic and strong lower organic acids. Examples of acids which can be used together with the copolymer according to the invention are hydrofluoric acid, chlorohydrogen acid, formic acid, acetic acid, monochloroacetic acid, dichloroacetic acid or trichloroacetic acid. The most common use is hydrochloric acid, which is used in concentrations from 3 to 28% by weight, depending on the required treatment conditions and the characteristics of the formation site.

The copolymers according to the invention are produced by copolymerization of 5 to 50% by weight of an optional N-substituted vinylformamide of formula Ia

10 to 95% by weight of acrylamide and 0 to 85% by weight of an ethylenic unsaturated monomer of formula Illa

1 2

wherein R , R , and X have the above meanings.

Preferably, 5 to 30% by weight of the vinylformamide of formula Ia, 50 to 80% by weight of acrylamide and 1 to 60% by weight of the monomer of formula IIa are copolymerized. A particularly preferred composition is 10 to 20% by weight of N-vinylformamide, 50 to 70% by weight of acrylamide and 10 to 40% by weight of a monomer of formula IIIa.

If the copolymer is to be produced which has several different residues X and/or R 2 in component III, then several different, usually three or preferably two different comonomers of formula Illa are used for the copolymerization.

Copolymerization can be carried out according to all known polymerization methods in the range from pH 4 to 12, preferably 6 to 9.

For setting the pH value, alkaline reacting salts of alkali metals are suitably used, e.g. alkali carbonates, alkali hydrogen carbonates, alkali borates,

di- or tri-alkali phosphates, alkali hydroxides, ammonia

7 7

or organic amines of the formula NR^, where R means hydrogen, alkyl with 1 to K carbon atoms or hydroxyethyl, and wherein at least one of the residues R 7 is different from hydrogen. Preferred bases for adjusting the pH value of the above-mentioned alkali compounds, in particular sodium hydroxide, potassium hydroxide, sodium carbonate, sodium hydrogen carbonate, potassium carbonate, potassium hydrogen carbonate, and sodium or potassium borates. A further preferred base is NH 2 .

The polymerization reaction can be initiated by high-energy electromagnetic or corpuscular radiation, or by substances that form radicals. Consequently, as polymerisation ion initiators, suitable organic perforin parts such as e.g. benzoyl peroxide, alkyl hydroperoxide, such as butyl hydroperoxide, cumene hydroperoxide, p-menthane hydroperoxide, dialkyl peroxides such as e.g. di-tert.-butyl peroxide or inorganic per-f o rbinds, such as e.g. potassium or ammonium persulfate or hydrogen peroxide, further azo compounds such as e.g. azo-bis-isobutyronitrile, 2,2'-azo-bis-(2-amidinopropane) hydrochloride or azo-bis-isobutyramide. It is advantageous to use the organic or inorganic surfactants in combination with reducing substances. Examples of suitable reducing substances are sodium pyrosulphite, sodium hydrogen sulphite or condensation products of formaldehyde with sulphoxylates. Particularly advantageously, the polymerization can be carried out using Mannich adducts of sulfinic acid aldehydes and amino compounds, such as are described for example in German patents 1,301,566.

It is further known to the polymerization mixture

to add smaller amounts of so-called moderators that harmonize the course of the reaction by flattening the reaction rate-time diagram. They thus lead to an improvement in the reproducibility of the reaction, which makes it possible to produce uniform products with very small quality deviations. Examples of suitable moderators of this type are nitrilo-tris-propionylamide, and hydrohalides of monoalkylamines, dialkylamines or trialkylamines, such as e.g. dibutylamine hydrochloride. Such moderators can also be advantageously used in the preparation of the copolymers according to the invention.

Furthermore, so-called regulators can be added to the polymerization mixture, since such compounds affect the molecular weight of the polymers produced. Usable known regulators are e.g. alcohols, such as methanol, ethanol, propanol, isopropanol, n-butanol, sec-butanol and amyl alcohols, alkyl mercaptans, such as dod ecyl mercaptan and tert.-do-decyl mercaptan, isoo ethyl thi o glycol and some halogen compounds such as e.g. carbon tetrachloride, chloroform and methylene chloride.

Usually, the polymerization is carried out in a protective gas atmosphere, preferably under nitrogen.

The reaction can be carried out in solution, in emulsion, or under the condition of the precipitation polymerization after temperatures from 20 to 120°C, preferably from 4-0 to 100°C. When water is used as a solvent for the polymerization reaction, it proceeds in solution, and an aqueous viscous solution of the polymer is obtained. The reaction product can be isolated, either by distilling off the water from the solution, or by mixing the aqueous solution with an organic solvent that is completely miscible with water, where: however, the copolymer is insoluble. When such organic solvents are added to the aqueous polymeric solution, the polymer formed resp. copolymer out, and can be separated from the liquid phase, e.g. by filtering. Preferably, however, the formed one is used

r

aqueous solution of the polymer or copolymer directly for further use, possibly after setting a certain desired concentration.

When the copolymerization is carried out in one organic solvent, such as e.g. in a lower alkanol, e.g.

in tert.-but anol, then it proceeds under the conditions of the precipitation polymerization. In this case, the polymer formed resp. copolymer in the course of the reaction in solid form, and can be easily isolated in the usual way, such as e.g. by suction and subsequent drying. Of course, it is also possible and in many cases preferred to distill the solvent from the react ion mixture.

Monomers that introduce units into the copolymer

of formula I is N-vinylformamide, N-vinyl-N-methylformamide,

and N-vinyl-N-hydroxymethylformamide, the latter two being preferred. When acrylamide is co-used in the polymerization, units of formula II are introduced into the copolymer. Units of formula III are introduced into the copolymer according to the invention by co-using one or more, preferably two monomers of formula

where R p and X have the following meanings:

(D As N-vinylamides are not stable under acidic conditions, the mixture containing acidic substances must be neutralized before polymerization, for example with the above-mentioned inorganic or organic bases or basic comonomers.

Examples of preferred monomers into which units of formula III are introduced into the copolymers according to the invention are:

N-vinylformamide,

N -vinyl-N- metyl -form amide, _

N-vinyl acetamide,

N-vinyl-N-methyl-acetamide,

N-vinylpyrrolidone,

acrylic acid, alkali or ammonium salts of acrylic acid or its alkali or ammonium salts

or

The above-mentioned copolymers according to the invention are, as far as is known, new, with the exception of two in which X

means a sulfo group, or a sulfoalkyl-amidocarbonyl group, or a salt of these groups (cf. US patent no. 4.04.8.077).

The above-mentioned copolymers according to the invention can also be used for thickening, resp. for gel formation of acid solutions, e.g. by the process with pressure acidification, as described in the concurrently filed application entitled "Water-soluble cross-linkable polymer compositions, its preparation and use", as well as in the priority US application entitled "Watersoluble Cross-linkable Polymer Compositions Containing a Metal Compound, Their Preparation and Use", by Jeffrey C. Dawson, Robert R. Mc. Daniels and Lawrence Sedillo.

The following examples illustrate the production of the copolymer according to the invention.

The abbreviations used in the examples and tables have the following meanings:

Example 1

In a 2-liter flask, equipped with a water bath, stirrer, reflux condenser, dropping funnel and gas inlet, add 4-00 ml of deionized water and 7.7 ml of a 25% aqueous ammonia solution. While introducing a weak stream of nitrogen, 9.3 g of acrylamido-methylpropanesulfonic acid and, as soon as a clear solution is present, 60 g of acrylamide, 18.6 g of N-vinylpyrrolidone and 14.7 g of N-vinylformamide are added. The solution's pH value is 6.5. The mixture is heated to 50°C and the polymerization is triggered by the addition of 5 ml of a 20% aqueous solution of ammonium peroxydisulfate. After an induction period of approx. After 10 minutes, the reaction starts, the temperature rises to 65°C, and the mixture becomes viscous. The mixture is then heated to 80°C, held at this temperature for 2 hours. After cooling to room temperature, a highly viscous solution of the copolymer is obtained.

Example 2

In a 2 liter flask, equipped with a water bath, piping, reflux condenser, dropping funnel and gas introduction tube, 500 ml of tert.-butanol are placed, and 20 g of AMPS are suspended in it while stirring.

2.2 1 of ammonia gas are then introduced, and 65 g of acrylamide and 15 g of N-vinylformamide are added. While introducing a weak stream of nitrogen, the mixture is heated to 50°C and 1 g of azo-iso-butyronitrile is added. After an induction period of a few minutes, polymerization reduction starts. The temperature thereby increases to 81°C, and the polymer precipitates out. The reaction mixture is kept for a further 2 hours at 80°C,

as a viscous suspension occurs. The polymer can be isolated by suction, dried at 50°C in a vacuum. Meanwhile, it is also possible to isolate the polymer by distilling off the solvents under reduced pressure. The resulting polymer is a white, light powder, which is easily soluble in water. The K-value according to Fikentscher is 14-8.

According to the above-mentioned embodiments, the copolymers of the following tables 1 and 2 can also be produced:

Example 67

In a 500 ml flask equipped with a water bath, stirrer, tilabek condenser, dropping funnel and gas inlet, a solution of 0.23 g sorbitan monolaurate, 1.09 sorbitan mono-stearate, 0.14 g sor;bitan- monooleate and 3.61 g of polyoxyethylene sorbitan monostearate in 56.4 g of a straight paraffin hydrocarbon fraction ("Norpar" 13 from the company Exxon). During the introduction of nitrogen, the solution is heated to 60°C.

In a separate vessel is prepared by dissolving

33.6 g acrylamide, 5.4 g AMPS, 6.6 g N-methyl-N-vinylacetamide, 6.6 g N-vinylformamide and 7.8 g vinyl pyrrolidone in a mixture of 78.8 g deionized water , 2.6 ml 10 wt$-5.g sodium hydroxide solution a monomer solution. This solution's pH

value amounts to 7.4-. The monomer solution is mixed with ..23 mg of 2, 2 '-azo-bis-(2-amidinopropane)-hydrochloride, and is then vigorously added dropwise while stirring gradually into the organic emulsifier solution, a milky, white emulsion being obtained. As soon as the temperature of the mixture has again been increased to 60°C, the stirring speed is reduced to approx. 60 revolutions per minute. After an induction time of 10 minutes, a temperature rise begins. At approx. At 80°C, the emulsion becomes transparent, and the stirring speed is increased. The temperature rises further to a maximum of 97°C. After the mixture has cooled again to 80°C, it is stirred again for 2 hours at this temperature. You get a stable emulsion that contains 30% by weight of active polymer.

Example 68

In a 1 liter polymerization vessel. heat 67 g

of a deodorized kero sinf raks ion with a boiling range from 190

to 24-0°C ("Exsol D", commercial products from Esso Chemie, Germany) to one temperature of 60°C. Under gentle stirring, 0.27 g of sorbitan monolaurate, 1.3 g of sorbitan monostearate, 0.17 g of sorbitan monooleate and 4-.3 g of polyoxyethylene sorbitan monostearate are dissolved in this order. Under further weak re-

stirring and introducing a stream of nitrogen, is then heated

the solution to 60°C. In another vessel, 6.4 g of AMPS, 4-4-, 8 g of acrylamide, 9.2 g of vinylpyrrolidone and 10.6 g of N-vinylformamide are dissolved in 94 ml of water while stirring. The solution thus formed is adjusted by dropwise addition of 10 wt.g. of aqueous

■ sodium hydroxide solution at a pH value from 8 to 10, is mixed with 0.1 g of ammonium persulphate. The thus formed aqueous monomer solution is dripped and emulsified with rapid stirring through a dropping funnel in the organic emulsifier solution. The polymerization reaction starts after approx. 30 minutes while increasing the temperature. During the 15

minutes, the temperature increases to 80 to 90°C. * After completion of the polymerization, the solution is still stirred for two hours at 80°C. A stable emulsion is obtained with a polymer content of 30% by weight. The molecular weight of the polymer is 95,000.

Example 69

1.85 g of sorbitan mono stearate is dissolved in a technically isomeric hydrocarbon mixture with a boiling range from 200 to 24-0°C ("Isopar" M from the company Exxon Corp.). This solution is found in

a two liter reaction flask equipped with stirrer, thermometer, gas inlet tube.

In a separate vessel, a monomer solution is then prepared by dissolving 25 g of acrylamide, 3.7 g of acrylic acid and 9.2 g of N-vinyl-formamide in 4-0 ml of deionized water, and the solution's pH value is set at addition of an aqueous 20% by weight sodium hydroxide solution of 8.5. With vigorous stirring, the aqueous monomer solution is introduced slowly into the organic sorbitan monostearate solution, and then the air in the reaction vessel is displaced with nitrogen.

After adding 0.07 g of 2,2<1->azo-bis-isobutyronitrile dissolved in acetone, the emulsion is slowly heated with stirring to 60°C. After 2 1/2 hours, the polymerization is complete and a stable emulsion of the copolymer is obtained.

Example 70

67 g of a technically isomeric hydrocarbon fraction

with a boiling range from 200 to 24-0°C ("Isopar" M from Exxon Corp.)

is placed in a 1 liter polymer isation vessel and heated to 60°C. With stirring, 0.27 g of sorbitan monolaurate, 1.3 g of sorbitan monostearate, 0.17 g of sorbitan monooleate and 4.3 g of polyoxyethylene sorbitan monostearate are dissolved in order.

In a separate vessel, 12.8 g of VIMA, 39.8 g of acrylamide, 12.0 g of vinylpyrrolidone and 6.6 g of N-methylformamide are dissolved in 94 ml of water while stirring. The pH value of this solution is adjusted by dropwise addition of 2% by weight aqueous sodium hydroxide solution of 8 to 10, and then 0.1 g of ammonium persulphate is added. This monomer solution is then dripped under vigorous stirring through a dropping funnel into the organic emulsifier solution, and emulsified therein. The polymerization reaction starts after approx. 30 minutes, with the reaction temperature rising from 80 to 90°C within 15 minutes. After the end of the reaction, the solution is stirred for a further 2 hours at 80°C. A stable emulsion is obtained with a polymer content of 30% by weight. The molecular weight of the polymer is 95,000.

According to manufacturing examples 78 to 70, the products in the following table III can also be produced which can also be used as valuable friction reducers for aqueous acids.

All quantities in Table III are parts by weight.

Example 71

In order to show the free ion-reduced end property of the copolymers according to the invention, flow tests were carried out with 7.5% hydrochloric acid, once without and once with an addition of the copolymers according to the invention produced according to embodiment 1.

To carry out the experiment, a pipe loop was used in which the fluid is re-pumped through a number of pipes of known length and known diameter with a specific flow rate. Thereby, the hydromechanical pressure is measured at both ends of a given defined pipe section. Since one knows the flow rate and the pressure difference, and the relevant pipe section, it is thus possible to graphically show the frictional pressure of different fluids, depending on the flow rate of the fluid in the pipe.

The first experiment was carried out with 208 1 7.5 wt$-ig hydrochloric acid, which. contained 760 ml of a common hand-filed corrosion inhibitor, using pump rates from 38 to 57 l/min using a tube with an inner diameter of 0.925 cm, 19 to 170 l/min using a tube with an inner diameter of 1.25 cm, oe 19 to 4-54- l/min, using a tube of 2.66 cm inner diameter. After this series of experiments, 224 g of the copolymer produced in Example 1 was added to the 7.5 wt% hydrochloric acid corresponding to a copolymer concentration of 0.1 wt% acid. With this polymer-containing acid, the flow-through sample was repeated. The measurement results obtained in both cases on the stirrer sections with 0.925 cm, 1.25 cm and 2.66 cm inner diameter are shown in fig. 1, 2 or 3.

Example 72

In order to show the acid solubility of the copolymers according to the invention, they were compared with commercially available friction reducers and subjected to the following tests:

Three 225 ml vessels were filled each time

200 ml 15% by weight hydrochloric acid. To the acid in vessel 1 was then added 2.0 g of the polymerizate according to the invention, prepared according to example 2, in vessel 2 was added 2.9 g of a commercially available free ion reducer emulsion "LFR-.1" and in In vessel three, 3.2 g of a commercially available friction reducer emulsion "LFR-2" was added. The commercial products "LFR-1 M and "EFR-2" are copolymers of acrylamide and acrylic acid with molecular weights from 500,000 to 2,000,000. The polymer content of the emulsion "LFR-1" and

"LFR-2" is unknown, however, it is to be assumed that there are differences between the two products with regard to the amount of active copolymer, comonomer ratio of the polymer and the type of emulsifier. After this time, the solution in vessel 1 containing the copolymer according to the invention was, as before, completely clear, the copolymer became completely soluble. In vessels 2 and 3, on the other hand, white precipitation plugs had formed each time. The separated solids were sucked off and washed out with water, and then with methanol. The filter residue was then dried in a vacuum at 4-0°C overnight. The quantities obtained are listed in the following table IV.

Example 73

A natural gas well in West Texas is selected for pressurized acidification. Borin.gen has a production interval between a depth of 1100 to 1120 metres. The sole temperature in this interval amounts to 43°C, the permeability of the formation averages 0.15 md (millidarcy). The tube width was 6.0 cm,

the spontaneous productivity of the drilling amounted before the treatment to approx. 168 cm gas/day.

The pressurized acidizing fluid used is produced by first mixing 3785 liters of 28% by weight hydrochloric acid with 35960 1 of fresh water, and then adding 79.5 1 (0.2% by weight) of the acid according to example 1 3 to the diluted acid formed prepared polymer emulsion. The thus formed fluid is forced into the bore at a speed of 1589.1 l/min under a pressure of 310.5 bar. After 5 minutes, the connection is finished, the borehole is closed to give the acid the opportunity to react with the rock.

After 60 minutes, the pressure is relieved at the top of the borehole, and transport is restored. A measurement of the degree of improvement shows a significant increase in drilling productivity.

Claims (11)

1. Method for stimulating natural gas and natural oil wells by pressure acidizing with an acid solution characterized in that the acid contains 0.06 to 0.12% by weight of a water or aqueous acid-soluble copolymer which in its macromolecule contains at least 5% by weight of units which has a formylamino group of formula where R 1 means hydrogen, methyl or hydroxymethyl, and at least 10% by weight of units having an aminocarbonyl group of the formula 2. Process according to claim 1, characterized in that the copolymer is composed in a statistical distribution of 5 to 50% by weight of units of formula I 10 to 95 wt$ of units of formula II 0 to 8 <*> 5 wt$ of units of formula III 1 2 in which- R means hydrogen, methyl or hydroxymethyl and R means hydrogen or methyl and X means cyan, carboxyl, or these alkali or ammonium salts, alkoxycarbonyl with 1 to 6 carbon atoms, hydroxyalkyloxycarbonyl with 1 to 6 carbon atoms, N-methylolamidocarbonyl with the formula HOCR^NH-CO-, whose methylol group may optionally be etherified with alkanols with 1 to 4 carbon atoms, alkanoylamino with t to 4 carbon atoms, which may optionally be N-substituted with methylol or alkyl with 1 to 4 carbon atoms, pyrrolidonyl-(1), phenyl, pyridinium, the sulfonic acid group, sulfoalkylamido-carbonyl with 1 to 4 carbon atoms, phosphonic acid groups, the sulfonic acid and the phosphonic acid groups can also be present as alkaline or ammonium salts, a residue with formula IV wherein R and R are the same or different and mean alkyl of 1 to 4 carbon atoms, a residue of formula V wherein R 3 and R L have the above meaning and p means a number from 1 to 4-, or a residue of formula VI 5 6 in which R and R are the same or different and mean alkyl with 1 to 4 carbon atoms, and p means a number from 1 to 4-, as well as quaternized groups corresponding to formulas V and VI. 3. Process according to claims 1 and 2, characterized in that the copolymer is composed of 5 to 30% by weight of units of formula I 5 to 80% by weight of units of formula II 1 to 60% by weight of units of formula III 4-. Method according to claims 1 to 3, characterized in that a copolymer is used in which X means carboxyl, the sulfonic acid group, 3-sulfo-2-methyl-propyl-(2)-amidocarbonyl with the formula alkanoylamino with 1 to 4 carbon atoms which can optionally be N-substituted with methylol or alkyl with 1 to 4 carbon atoms, pyrrolidonyl-(1) or a residue of the above-mentioned and defined formulas V and VI, each acid group can also be present as sodium -, potassium or ammonium salts. 5. Method according to claims 1 to 4, characterized in that a copolymer is used with a K value between 25 and 250, corresponding to a molecular weight from 20,000 to 15 - 10 . 6. Water-soluble copolymer composed in statistical distribution of "5 to 50% by weight" of units of formula I 1.0 to 95 wt% of units of formula II „ 0 to 85% by weight of units of formula III in which R i means hydrogen, methyl or hydroxymethyl - and R 2 means hydrogen or methyl and X means cyan, carboxyl or these alkali or er^ammonium salts, alkoxycarbonyl with 1 to 6 carbon atoms, hydroxyalkyloxycarbonyl with 1 to 3 carbon atoms, N-methylolamidocarbonyl of formula H0CH2 NH-C0-, whose methylol group can optionally be etherified with alkanols with 1 to 4 carbon atoms, alkanoylamino with 1 to 4 carbon atoms, which can optionally be N-substituted with methylol or alkyl with 1 to 4 carbon atoms, pyrrolidonyl -(1 ), phenyl, pyridinium, the phosphonic acid group or their alkali or ammonium salts, a residue of formula IV 3 / • wherein R and R are the same or different and mean alkyl of 1 to 4 carbon atoms, a residue of formula V wherein R and R have the above meanings, and p means a number from 1 to 4-, or a residue of formula VI in which R^ and R are the same or different, and means alkyl with 1 to 4- carbon atoms, and p means a number from 1 to 4-, as well as the quaternized groups corresponding to the formulas V and VI. 7. Water-soluble ■ ■ ■ copolymer according to claim 6, characterized in that it is composed of 5 to 4-0 weight,? of units of formula I 50 to 80% by weight of units of formula II 1 to 60% by weight of units of formula III. 8. Water-soluble copolymer according to claims 6 and 7, characterized in that X means carboxyl, alkanoylamino with 1 to 4 carbon atoms which may optionally be N-substituted with methylol or alkyl with 1 to 4 carbon atoms, pyrrolidonyl-(1), or a residue with the above-mentioned and defined formulas V or VI, the acyl group can also be present as sodium, potassium or ammonium salts. . 9. Water-soluble copolymer according to claims 6 to 8, characterized in that it has a K-value between 25 and 250, corresponding to a molecular weight of 20,000 to 15. 10 <6> . 10. Aqueous copolymer solution, contains 0.5 to 6% by weight of a copolymer according to claims 6 to 9. 11. Water-in-oil emulsion which as aqueous phase has a polymer solution of a copolymer according to claims 6 to 9.