Composition for acidizing subterranean formations

Gelled acidic compositions suitable for either matrix-acidizing or fracture-acidizing of subterranean formations, and methods of using said compositions, are provided. Said compositions comprise water, a water-dispersible polymer of acrylamide, an acid, a water-soluble compound of a polyvalent metal wherein the metal can be reduced to a lower polyvalent valence state and cause gelation of the water containing said polymer and said acid, and a reducing agent capable of reducing said metal and causing said gelation.

This invention relates to acid treating or acidizing of subterranean 
formations. 
Acid treating or acidizing of porous subterranean formations penetrated by 
a well bore has been widely employed for increasing the production of 
fluids, e.g., crude oil, natural gas, etc., from said formations. The 
usual technique of acidizing a formation comprises introducing a 
non-oxidizing acid into the well under sufficient pressure to force the 
acid out into the formation where it reacts with the acid-soluble 
components of the formation. The technique is not limited to formations of 
high acid solubility such as limestone, dolomite, etc. The technique is 
also applicable to other types of formations such as a sandstone 
containing streaks or striations of acid-soluble components such as the 
various carbonates. 
During the acid treating operation, passageways for fluid flow are created 
in the formation, or existing passageways therein are enlarged, thus 
stimulating the production of fluids from the formation. This action of 
the acid on the formation is often called etching. Acid treating or 
acidizing operations wherein the acid is injected into the formation at a 
pressure or rate insufficient to create cracks or fractures in the 
formation is usually referred to as matrix-acidizing. 
Hydraulic fracturing is also commonly employed to increase the production 
of fluids from subterranean formations. Hydraulic fracturing comprises the 
injection of a suitable fracturing fluid down a well penetrating a 
formation and into said formation under sufficient pressure to overcome 
the pressure exerted by the overburden. This results in creating a crack 
or fracture in the formation to provide a passageway which facilitates 
flow of fluids through the formation and into the well. Combination 
fracture-acidizing processes are well known in the art. 
Thus, it is within the scope of the present invention to inject the gelled 
acidic compositions of the invention into the formation under insufficient 
pressure to cause fracturing of the formation and carry out a matrix 
acidizing operation, or inject said gelled acidic composition at 
sufficient rates and pressure to cause fracturing and carry out a 
combination fracture-acidizing operation. 
One of the problems commonly encountered in acidizing operations is 
insufficient penetration of the formation by the acid. It is desirable 
that good penetration be obtained in order to realize maximum benefits 
from the operation. Too often the acid is essentially completely spent in 
the area immediately adjacent and surrounding the well bore. The severity 
of the problem increases as the well temperature increases because acid 
reactivity with the formation increases with increasing temperatures, as 
in deeper wells. 
Poor penetration can also be caused, and/or aggravated, by fluid loss to 
the more porous zones of the formation where low permeability is not a 
problem. Poor penetration can also be caused, and/or aggravated, by 
leak-off at the fracture faces in combination fracturing-acidizing 
operations. Either said fluid loss or said leak-off can frequently worsen 
the situation by leaving the tight (low permeability) zones of the 
formation unchanged and merely opening up the already high permeability 
zones. 
One solution which has been proposed for the above discussed problem is to 
incorporate various polymeric thickening or viscosifying agents into the 
acid solution. Said agents serve to thicken the acid solution and thus 
increase the viscosity thereof. It has been reported that so thickened 
acid solutions have reduced fluid loss properties. For example, see U.S. 
Pat. No. 3,415,319 issued in the name of B. L. Gibson; and U.S. Pat. No. 
3,434,971 issued in the name of B. L. Atkins. It has also been reported 
that the reaction rate of said so-thickened acid solutions with the 
acid-soluble portions of the formation is lessened or retarded. See, for 
example, U.S. Pat. No. 3,749,169 issued in the name of J. F. Tate; U.S. 
Pat. No. 3,236,305 issued in the name of C. F. Parks; and U.S. Pat. No. 
3,252,904 issued in the name of N. F. Carpenter. 
Higher viscosities are also advantageous in combination 
fracturing-acidizing operations in that the more viscous acidic solutions 
produce wider and longer fractures. More viscous acid solutions are also 
more effective in carrying propping agents into the formation when 
propping agents are used. 
Another problem encountered in acidizing operations, particularly when 
employing acidizing compositions having thickening or viscosifying agents 
incorporated therein, is stability to heat. By stability to heat, it is 
meant the retention of the increased or greater viscosity properties under 
the conditions of use. Such compositions to be satisfactory should be 
sufficiently stable to resist degeneration by the heat of the formation 
for a period of time sufficient to accomplish the intended purpose, e.g., 
good penetration and significant etching of the formation. The degree of 
stability required in any particular operation will vary with such 
operating variables as the type of formation being treated, the 
temperature of the formation, the well depth (time to pump the acidic 
composition down the well and into the formation), the acid concentration 
in the composition, etc. For example, acidizing of a tight low 
permeability formation will proceed more slowly than a more permeable 
formation, other factors being the same, because a longer time will be 
required to obtain a significant amount of etching and the composition 
must remain in place and effective for a longer period of time. Also, more 
time will be required to pump the acidic composition into place in the 
formation. 
The temperature of the formation usually has a pronounced effect on the 
stability of the acidizing compositions and, generally speaking, is one of 
the most important operating variables when considering stability. 
Increased formation temperatures usually have at least two undesirable 
effects. One such effect is degeneration of the composition, e.g., 
decrease in viscosity. Another such effect is increased rate of reaction 
of the acid with the formation. Thus, some compositions which would be 
satisfactory in a low temperature formation such as in the Hugoton field 
in the Anadarko basin might not be satisfactory in formations encountered 
in deeper wells as in some West Texas fields. 
In ordinary acidizing operations using unthickened acids there is usually 
no problem in removing the spent acid because it is essentially water. 
However, a problem which is sometimes encountered when using thickened 
compositions in treating formations is the ease of removal of the treating 
composition after the operation is completed. Some thickened or highly 
viscous solutions are difficult to remove from the pores of the formation 
or the fracture after the operation is complete. Sometimes a clogging 
residue can be left in the pores of the formation, or in the fracture. 
This can inhibit the production of fluids from the formation and can 
require costly cleanup operations. It would be desirable to have gelled 
acidic compositions which break down to a lesser viscosity within a short 
time after the operation is completed. 
The present invention provides a solution for, or at least mitigates, the 
above discussed problems. The present invention provides improved methods 
for acidizing, or fracture-acidizing, subterranean formations; and new 
gelled acidic compositions for use in said methods. 
Thus, in accordance with one broad aspect of the concept of the invention, 
there is provided a method for acid treating a porous subterranean 
formation susceptible of attack by an acid and penetrated by a well bore, 
which method comprises: injecting into said formation via said well bore a 
gelled acidic composition comprising water; an amount of a 
water-dispersible polymer of acrylamide which is sufficient to thicken 
said water; an amount of a water-soluble compound of a polyvalent metal 
wherein the metal present is capable of being reduced to a lower 
polyvalent valence state and which is sufficient to cause gelation of an 
aqueous dispersion of the components of said composition when the valence 
of at least a portion of said metal is reduced to said lower valence 
state; an amount of a water-soluble reducing agent which is effective to 
reduce at least a portion of said metal to said lower valence state and 
cause said gelation; an amount of a non-oxidizing acid which is capable of 
reacting with a significant amount of the acid-soluble components of said 
formation; said polymer, said polyvalent metal compound, said reducing 
agent, and said acid, in the amounts used, being sufficiently compatible 
with each other in an aqueous dispersion thereof to permit said gelatin 
and thus form a said composition having sufficient stability to 
degeneration by the heat of said formation to permit good penetration of 
said composition into said formation; and maintaining said composition in 
said formation in contact therewith for a period of time sufficient for 
the acid in said composition to significantly react with the acid-soluble 
components of said formation and stimulate the production of fluids 
therefrom. 
Further, in accordance with another broad aspect of the concept of the 
invention there is provided a gelled acidic composition, suitable for 
matrix acidizing or fracture-acidizing of a subterranean formation, 
comprising: water; a water-thickening amount of a water-dispersible 
polymer of acrylamide; an amount of a water-soluble compound of a 
polyvalent metal wherein the metal present is capable of being reduced to 
a lower polyvalent valence state and which is sufficient to cause gelation 
of an aqueous dispersion of the components of said composition when the 
valence of at least a portion of said metal is reduced to said lower 
valence state; an amount of a water-soluble reducing agent which is 
effective to reduce at least a portion of said metal to said lower valence 
state and cause said gelation; and an amount of a non-oxidizing acid which 
is capable of reacting with a significant amount of the acid-soluble 
components of said formation; said polymer, said polyvalent metal 
compound, said reducing agent, and said acid, in the amounts used, being 
sufficiently compatible with each other in an aqueous dispersion thereof 
to permit said gelation and thus form a said composition having sufficient 
stability to degeneration by the heat of said formation to permit good 
penetration of said composition into said formation and the maintenance of 
said composition in said formation in contact therewith for a period of 
time sufficient for the acid in said composition to significantly react 
with the acid-soluble components of said formation and stimulate the 
production of fluids therefrom. 
As noted above, the gelled acidic compositions of the invention must be 
suitable for matrix acidizing or fracture-acidizing of subterranean 
formations. In order to satisfy this requirement, the polymer, the 
polyvalent metal compound, the reducing agent, and the acid, in the 
amounts used, must be sufficiently compatible with each other, in an 
aqueous dispersion thereof, to permit the gelation of said dispersion and 
thus form a said composition having sufficient stability to degeneration 
by the heat of the formation to permit good penetration of said 
composition into the formation. Furthermore, once said penetration has 
been attained, the said stability must be sufficient to permit the 
maintaining of said composition in contact with the formation for a period 
of time sufficient for the acid in the composition to significantly react 
with the acid-soluble components of the formation and stimulate the 
production of fluids therefrom, e.g., by creating new passageways or 
enlarging existing passageways through said formation. 
Herein and in the claims, unless otherwise specified, the term "good 
penetration" means penetration of live or effective acid into the 
formation a sufficient distance to result in stimulating the production of 
fluids therefrom, e.g., by the creation of sufficient new passageways, or 
sufficient enlargement of existing passageways, through said formation to 
significantly increase the production of fluids from the formation. This 
can vary for different formations, well spacings, and what it is desired 
to accomplish in a given acidizing treatment. Those skilled in the art 
will usually know what will be "good penetration" for a given formation 
and a given type of treatment. However, generally speaking, for guidance 
purposes in the practice of the invention and not by way of limitation of 
the invention, "good penetration" will usually be considered to be a 
distance of a few feet, e.g., up to 5 or more, in a small volume matrix 
acidizing operation, and several hundred feet, e.g., up to 500 or more, in 
a large volume fracture-acidizing operation. 
Herein and in the claims, unless otherwise specified, the term "polymer" is 
employed generically to include both homopolymers and copolymers; and the 
term "water-dispersible polymers" is employed generically to include those 
polymers which are truly water-soluble and those polymers which are 
dispersible in water or other aqueous medium to form stable colloidal 
suspensions which can be gelled as described herein. Also, the term 
"aqueous dispersion" is employed generically to include both true 
solutions and stable colloidal suspensions of the components of the 
compositions of the invention which can be gelled as described herein. 
Any suitable polymer of acrylamide meeting the above stated compatibility 
requirements can be used in the practice of the invention. Thus, under 
proper conditions of use, such polymers can include various 
polyacrylamides and related polymers which are water-dispersible and which 
can be used in an aqueous medium, with the gelling agents described 
herein, to give an aqueous gel. These can include the various 
substantially linear homopolymers and copolymers of acrylamide and 
methacrylamide. By substantially linear it is meant that the polymers are 
substantially free of crosslinking between the polymer chains. Said 
polymers can have up to about 75, preferably up to about 45, percent of 
the carboxamide groups hydrolyzed to carboxyl groups. One presently 
preferred group of polymers includes those wherein from about 20 to about 
25 percent of the carboxamide groups are hydrolyzed. As used herein and in 
the claims, unless otherwise specified, the term "hydrolyzed" includes 
modified polymers wherein the carboxyl groups are in the acid form and 
also such polymers wherein the carboxyl groups are in the salt form, 
provided said salts are water-dispersible. Such salts include the ammonium 
salts, the alkali metal salts, and others which are water-dispersible. 
Hydrolysis can be carried out in any suitable fashion, for example, by 
heating an aqueous solution of the polymer with a suitable amount of 
sodium hydroxide. 
Substantially linear polyacrylamides can be prepared by methods known in 
the art. For example, the polymerization can be carried out in aqueous 
medium, in the presence of a small but effective amount of a water-soluble 
oxygen-containing catalyst, e.g., a thiosulfate or bisulfate of potassium 
or sodium or an organic hydroperoxide, at a temperature between about 
30.degree. and 80.degree. C. The resulting polymer is recovered from the 
aqueous medium, as by drum drying, and can be subsequently ground to the 
desired particle size. The particle size should be fine enough to 
facilitate dispersion of the polymer in water. A presently preferred 
particle size is such that about 90 weight percent will pass through a 
number 10 mesh sieve, and not more than about 10 weight percent will be 
retained on a 200 mesh sieve (U.S. Bureau of Standards Sieve Series). 
Under proper conditions of use, examples of copolymers which can be used in 
the practice of the invention can include the water-dispersible copolymers 
resulting from the polymerization of a major proportion of acrylamide or 
methacrylamide and a minor proportion of an ethylenically unsaturated 
monomer copolymerizable therewith. It is desirable that sufficient 
acrylamide or methacrylamide be present in the monomers mixture to impart 
to the copolymer the above-described water-dispersible properties, for 
example, from about 60 to 99 percent acrylamide and from about 1 to 40 
percent other ethylenically unsaturated monomers, preferably from about 75 
to about 95 percent acrylamide and from about 5 to about 25 percent other 
ethylenically unsaturated monomer. Such other monomers include acrylic 
acid, methacrylic acid, vinylsulfonic acid, vinylbenzylsulfonic acid, 
vinylbenzenesulfonic acid, vinyl acetate, acrylonitrile, methyl 
acrylonitrile, vinyl alkyl ether, vinyl chloride, maleic anhydride, vinyl 
substituted cationic quaternary ammonium compounds, and the like. Various 
methods are known in the art for preparing said copolymers. For example, 
see U.S. Pat. Nos. 2,625,529; 2,740,522; 2,729,557; 2,831,841; and 
2,909,508. Said copolymers can also be used in the hydrolyzed form, as 
discussed above for the homopolymers. 
Crosslinked polyacrylamides and crosslinked polymethacrylamides, at various 
stages of hydrolysis as described above, and meeting the above-stated 
compatibility requirements, can also be used in the practice of the 
invention. In general, said crosslinked polyacrylamides can be prepared by 
the methods described above, but including in the monomeric mixture a 
suitable amount of a suitable crosslinking agent. Examples of crosslinking 
agents can include methylenebisacrylamide, divinylbenzene, vinyl ether, 
divinyl ether, and the like. Said crosslinking agents can be used in small 
amounts, e.g., up to about 1 percent by weight of the monomeric mixture. 
Such crosslinking is to be distinguished from any crosslinking which 
occurs when solutions of polymers and the other components of the gelled 
acidic compositions of the invention are gelled as described herein. 
All the polymers useful in the practice of the invention are characterized 
by high molecular weight. The molecular weight is not critical so long as 
the polymer has the above-described water-dispersible properties. It is 
preferred that the polymer have a molecular weight of at least 500,000, 
more preferably at least about 2,000,000. The upper limit of molecular 
weight is unimportant so long as the polymer is water-dispersible, and the 
gelled acidic composition therefrom can be pumped. Thus, it is within the 
scope of the invention to use polymers having molecular weights as high as 
20,000,000 or higher, and meeting said conditions. 
The amount of the above-described polymers used in preparing the gelled 
acidic compositions of the invention can vary widely depending upon the 
particular polymer used, the purity of said polymer, and properties 
desired in said compositions. In general, the amount of polymer used will 
be a water-thickening amount, i.e., at least an amount which will 
significantly thicken the water to which it is added. For example, amounts 
in the order of 25 to 100 parts per million by weight (0.0025 to 0.01 
weight percent) have been found to significantly thicken water. Distilled 
water containing 25 ppm of a polymer of acrylamide having a molecular 
weight of about 10.times.10.sup.6 had a viscosity increase of about 41 
percent. At 50 ppm the viscosity increase was about 106 percent. At 100 
ppm the viscosity increase was about 347 percent. As another example, 
distilled water containing 25 ppm of a polymer of acrylamide having a 
molecular weight of about 3.5.times.10.sup.6 had a viscosity increase of 
about 23 percent. At 50 ppm the viscosity increase was about 82 percent. 
At 100 ppm the viscosity increase was about 241 percent. Generally 
speaking, amounts of the above-described polymers in the range of from 
0.01 to 4, preferably from 0.1 to 1.5, more preferably 0.1 to 0.5, weight 
percent, based on the total weight of the composition, can be used in 
preparing gelled acidic compositions for use in the practice of the 
invention. However, amounts outside said ranges can be used. In general, 
with the proper amounts of polyvalent metal and reducing agent, the amount 
of polymer used will determine the consistency of the gel obtained. Small 
amounts of polymer will usually produce liquid mobile gels which can be 
readily pumped. Large amounts of polymer will usually produce thicker, 
more viscous, somewhat elastic gels. 
Metal compounds which can be used in the practice of the invention are 
water-soluble compounds of polyvalent metals wherein the metal is present 
in a valence state which is capable of being reduced to a lower polyvalent 
valence state, and which will meet the above-stated compatibility 
requirements. Thus, under proper conditions of use, examples of such 
compounds can include potassium permanganate, sodium permanganate, 
ammonium chromate, ammonium dichromate, the alkali metal chromates, the 
alkali metal dichromates, and chromium trioxide. Sodium dichromate and 
potassium dichromate, because of low cost and ready availability, are the 
presently preferred metal-containing compounds. The hexavalent chromium in 
said chromium compounds is reduced in situ to trivalent chromium by 
suitable reducing agents, as discussed hereinafter. In the permanganate 
compounds the manganese is reduced from +7 valence to +4 valence as in 
MnO.sub.2. 
The amount of said metal-containing compounds used will be a small but 
finite amount which is effective or sufficient to cause gelation of an 
aqueous dispersion of the starting components of the compositions of the 
invention when the metal in the polyvalent metal compound is reduced to a 
lower polyvalent valence state. The lower limit of the concentration of 
the starting metal-containing compound will depend upon several factors 
including the particular type of polymer used, the concentration of the 
polymer, and the type of gel product desired. For similar reasons, the 
upper limit on the concentration of the starting metal-containing compound 
also cannot always be precisely defined. However, it should be noted that 
excessive amounts of the starting metal compound, for example +6 chromium, 
which can lead to excessive amounts of +3 chromium when there is 
sufficient reducing agent present to reduce the excess +6 chromium, can 
adversely affect the stability of the gelled compositions. It is believed 
this can provide one valuable method for controlling stability or life 
span so as to obtain gelled acidic compositions which will break down with 
time and/or temperature to permit ready well clean-up subsequent to an 
acidizing fracturing-acidizing operation. As a general guide, the amount 
of the starting polyvalent metal-containing compound used in preparing the 
gelled acidic compositions of the invention will be in the range of from 
0.05 to 30, preferably 0.5 to 20, weight percent of the amount of the 
polymer used. However, in some situations it may be desirable to use 
amounts of the starting polyvalent metal-containing compound which are 
outside the above ranges. Such use is within the scope of the invention. 
Those skilled in the art can determine the amount of starting polyvalent 
metal-containing compound to be used by suitable experiments carried out 
in the light of this disclosure. 
Suitable water-soluble reducing agents which can be used in the practice of 
the invention are those meeting the above-stated compatibility 
requirements. Under proper conditions of use this can include 
sulfur-containing compounds such as sodium sulfite, potassium sulfite, 
sodium hydrosulfite, potassium hydrosulfite, sodium metabisulfite, 
potassium metabisulfite, sodium bisulfite, potassium bisulfite, sodium 
sulfide, potassium sulfide, sodium thiosulfate, potassium thiosulfate, 
ferrous sulfate, thioacetamide, hydrogen sulfide, and others; and 
nonsulfur-containing compounds such as hydroquinone, ferrous chloride, 
p-hydrazinobenzoic acid, hydrazine phosphite, hydrazine dichloride, and 
others. Some of the above reducing agents act more quickly than others. 
For example, sodium bisulfite has been found to cause extremely rapid 
gelation with the higher concentrations of polymer. 
One presently preferred group of reducing agents are the water-soluble 
organic compounds containing from 1 to about 10 carbon atoms per molecule 
and which release hydrogen sulfide upon hydrolysis. These compounds 
contain the group .dbd.C.dbd.S and include organic amides, xanthate salts, 
trithiocarbonate salts, and dithiocarbamate salts. Some examples are: 
thioacetamide, thiourea, thioformamide, thiopropionamide, sodium ethyl 
xanthate, N,N-diethyl sodium dithiocarbamate, sodium 
butyltrithiocarbonate, and the like. Mixtures of said reducing agents can 
also be used. 
The amount of reducing agent to be used in preparing the gelled acidic 
compositions of the invention will be a small but finite amount which is 
effective or sufficient to reduce at least a portion of the higher valence 
metal in the starting polyvalent metal-containing compound to a lower 
polyvalent valence state. Thus, the amount of reducing agent to be used 
depends, to some extent at least, upon the amount of the starting 
polyvalent metal-containing compound which is used. In many instances, it 
will be preferred to use an excess of reducing agent to compensate for 
dissolved oxygen in the water, exposure to air during preparation of the 
gels, and possible contact with other oxidizing substances such as might 
be encountered in field operations. As a general guide, the amount of 
reducing agent used will generally be within the range of from 0.1 to at 
least 150, preferably at least about 200, weight percent of the 
stoichiometric amount required to reduce the metal in the starting 
polyvalent metal compound to said lower polyvalent valence state, e.g., +6 
Cr to +3 Cr. In most instances, it will be preferred to use at least a 
stoichiometric amount. However, in some instances, it may be desirable to 
use amounts of reducing agent outside said ranges. The use of such amounts 
is within the scope of the invention. Those skilled in the art can 
determine the amount of reducing agent to be used by suitable simple 
experiments carried out in the light of this disclosure. 
Acids useful in the practice of the invention include any non-oxidizing 
acid meeting the above-stated compatibility requirements and which is 
effective in increasing the flow of fluids, e.g. hydrocarbons, through the 
formation and into the well. Thus, under proper conditions of use, 
examples of such acids can include inorganic acids such as hydrochloric 
acid and sulfuric acid; C.sub.1 -C.sub.3 organic acids such as formic 
acid, acetic acid, propionic acid, and mixtures thereof, and combinations 
of inorganic and organic acids. The concentration or strength of the acid 
can vary depending upon the type of acid, the type of formation being 
treated, the above-stated compatibility requirements, and the results 
desired in the particular treating operation. The concentration can vary 
from about 1 to about 60 weight percent, with concentrations within the 
range of 5 to 50 weight percent usually preferred, based upon the total 
weight of the gelled acidic composition. When an inorganic acid such as 
hydrochloric acid is used it is presently preferred to use an amount which 
is sufficient to provide an amount of HCl within the range of from 1 to 
12, more preferably up to about 10, weight percent based on the total 
weight of the composition. The acids used in the practice of the invention 
can contain any of the known corrosion inhibitors, deemulsifying agents, 
sequestering agents, surfactants, friction reducers, etc., known in the 
art, and which meet the above-stated compatibility requirements. 
The gelled acidic compositions of the invention are aqueous compositions. 
They normally contain a significant amount of water. The amount of said 
water can vary widely depending upon the concentrations of the other 
components in the compositions, particularly the concentration of the 
acid. For example, when an organic acid such as acetic acid is used in the 
maximum concentration of 60 weight percent the amount of water present in 
the composition clearly will be less than when an inorganic acid such as 
HCl is used in the preferred maximum concentration of about 10 weight 
percent. Thus, while no precise overall range of water content can be set 
forth, based on the above-stated overall ranges for the concentrations of 
said other components the water content of said compositions can be in the 
range of from about 5 to about 99, frequently about 50 to about 95, weight 
percent. However, amounts of water outside said ranges can be used. 
Propping agents can be included in the gelled acidic compositions of the 
invention if desired. Propping agents which can be used include any of 
those known in the art, e.g., sand grains, walnut shell fragments, 
tempered glass beads, aluminum pellets, and similar materials, so long as 
they meet the above-stated compatibility requirements. Generally speaking, 
it is desirable to use propping agents having particle sizes in the range 
of 8 to 40 mesh (U.S. Sieve Series). However, particle sizes outside this 
range can be employed. When propping agents are used they should be made 
of materials which are not severely attacked by the acid used during the 
time they are exposed to said acid. 
Any suitable method can be employed for preparing the gelled acidic 
compositions of the invention. Thus, any suitable mixing technique or 
order of addition of the components of said composition to each other can 
be employed which will provide a said composition having sufficient 
stability to degeneration by the heat of the formation (in which the 
composition is to be used) to permit good penetration of the composition 
into, and significant etching of, said formation. However, it is 
ordinarily preferred to first dissolve or disperse the polymer in water 
before contacting the polymer with acid. Thus, it is preferred to avoid 
contacting the dry polymer with aqueous acid. Some suitable mixing orders, 
with the components named in order of mixing, include: 
water--polymer--polyvalent metal compound--reducing agent--acid; 
water--polymer--acid--polyvalent metal compound--reducing agent; and 
water--polymer--polyvalent metal compound--acid--reducing agent; and the 
like. It is within the scope of the invention to moisten or slurry the 
polymer with a small amount, e.g., about 1 to about 6 weight percent based 
on the weight of the polymer, of a low molecular weight alcohol, e.g., 
C.sub.1 to C.sub. 3 alcohols, as a dispersion aid, prior to dispersing the 
polymer in water. Contact of the polyvalent metal compound and reducing 
agent in the absence of the dispersed polymer should be avoided. Since the 
acid may sometimes have a degrading effect on the polymer, it is preferred 
that the acid not be in contact with the polymer, even in aqueous 
solution, unduly long in the absence of the gelling agents. Similarly, it 
is preferred that there be no undue delay between completing the 
preparation of the gelled acidic composition and its introduction into 
contact with the formation. 
As used herein and in the claims, unless otherwise specified, the stated 
values for "degree of hydrolysis" or "percent hydrolyzed," and like terms, 
refer to initial values prior to use or test of the polymer. Unless 
otherwise stated, said values are obtained by the following analytical 
procedure. Place 200 ml of distilled water in a beaker provided with a 
magnetic stirrer. Weigh a 0.1 gram polymer sample accurately to .+-.0.1 
mg. Start the stirrer and quantitatively transfer the weighed sample into 
the water vortex. Stir at a rapid rate overnight. Using a pH meter and 1:1 
HCl, adjust the pH of the sample solution to less than 3.0. Stir the 
solution for 30 minutes. Adjust the pH of the solution to exactly 3.3 by 
dropwise addition of 0.1 N NaOH. Then slowly titrate with standard 0.1 
NaOH from pH 3.3 to pH 7.0. 
##EQU1## 
where: V=ml of base used in titration; N=normality of base; W=grams of 
polymer sample; and 0.072= milliequivalent weight of acrylic acid. 
The gelled acidic compositions of the invention can be prepared on the 
surface in a suitable tank equipped with suitable mixing means, and then 
pumped down the well and into the formation employing conventional 
equipment for pumping acidic compositions. However, it is within the scope 
of the invention to prepare said compositions while they are being pumped 
down the well. This technique is sometimes referred to as "on the fly." 
For example, a solution of the polymer in water can be prepared in a tank 
adjacent the well head. Pumping of this solution through a conduit to the 
well head can then be started. Then, a few feet downstream from the tank a 
suitable connection can be provided for introducing the polyvalent metal 
compound into said conduit, either dry through a mixing hopper, or 
preferably as an aqueous solution. Then, a few feet farther downstream the 
reducing agent can be similarly introduced, preferably as an aqueous 
solution. The acid can then be introduced into said conduit a few feet 
downstream from the reducing agent. As will be understood by those skilled 
in the art, the rate of introduction of said components into said conduit 
will depend upon the pumping rate of the polymer solution through said 
conduit. Any of the above-mentioned orders of addition can be employed in 
said "on the fly" technique. Mixing orifices can be provided in said 
conduit, if desired. 
It is within the scope of the invention to precede the injection of the 
gelled acidic composition into the well and out into the formation with a 
preflush of a suitable cooling fluid, e.g., water. Such fluids serve to 
cool the well tubing and formation and extend the useful operating 
temperature range of said compositions. The volume of said cooling fluid 
so injected can be any suitable volume sufficient to significantly 
decrease the temperature of the formation being treated, and can vary 
depending upon the characteristics of the formation. For example, amounts 
up to 20,000 gallons, or more, can be used to obtain a temperature 
decrease in the order of 100.degree. to 250.degree. F.

The following examples will serve to further illustrate the invention, but 
should not be considered as unduly limiting on the invention. 
EXAMPLE I 
A 15 g quantity of an acrylamide polymer (Dow Pusher 700) having a 
molecular weight of about 5,500,000 and a degree of hydrolysis of about 
23.5% was blended into 500 ml of tap water with the aid of a high speed 
mixer (Hamilton Beach malt mixer) for one minute. After standing at room 
temperature for 5 days, a 100 ml portion of this 3 weight percent polymer 
solution was transferred to a pint jar. To this was added 2.5 ml of a 10 
weight percent solution of sodium dichromate dihydrate with stirring 
followed by the addition of 0.3 g thioacetamide. About 1 minute layer, 100 
ml glacial acetic acid was added and the mixture was stirred for 1 minute 
more. All the reagents and acid blended quite well. 
A portion of this composition (50% acetic acid, 1.5% polymer, 1250 ppm 
Na.sub.2 Cr.sub.2 O.sub.7.2H.sub.2 O, 1500 ppm thioacetamide, by weight) 
was transferred to a capillary viscometer (Kimax No. 500) and the 
viscometer was placed in a water bath at about 85.degree. F. The 
temperature of the bath was then increased at a rate sufficient to reach 
200.degree. F. in about 1 hour. The efflux time of the composition was 
measured at intervals and recorded along with the time and temperature. 
For purposes of comparison, another similar composition was prepared 
similarly, but omitting the dichromate and thioacetamide crosslinking 
agents. The essential conditions and results of these rests are shown in 
Table I below. 
TABLE I 
______________________________________ 
Gelled Non-Gelled 
Bath Efflux Bath Efflux 
Temp. Time Temp. Time 
Time in Bath (min) 
(.degree.F.) 
(sec) (.degree.F.) 
(sec) 
______________________________________ 
0 83 59.2 81 71.0 
5 88 63.7 87 42.0 
10 98 63.5 101 35.0 
15 110 69.0 115 31.0 
20 123 82.2 127 27.4 
25 139 92.5 139 22.5 
30 149 85 151 21.0 
35 161 77.8 162 20.8 
40 172 68.0 171 18.1 
45 179 61.6 180 16.4 
50 185 38.7 185 15.1 
55 191 36.8 191 14.3 
60 194 27.3 195 12.6 
65 198 26.0 201 12.0 
70 201 40.9 -- -- 
______________________________________ 
The data in Table I show that, with increasing time and increasing 
temperature, the non-gelled acid system exhibited a relatively low 
viscosity (short efflux time) which continuously decreased throughout the 
test period. In constrast, the gelled acid system of the invention 
exhibited a significantly greater viscosity (longer efflux time) 
throughout the test period. Moreover, the viscosity was seen to increase 
up to about 139.degree. F. at which point the material appeared to gel. 
The composition also appeared to re-gel at 201.degree. F. as evidenced by 
an increase in viscosity. 
It should be noted that both compositions were more viscous than water even 
at the completion of the test. For comparison, pure water would exhibit an 
efflux time of one second or less with the viscometer used above. 
Based on the above data, it is concluded that the gelled acidic composition 
comprising a solution of a substantially linear polyacrylamide having a 
degree of hydrolysis of about 23.5% and having incorporated therein sodium 
dichromate dihydrate, thioacetamide reducing agent, and acetic acid is a 
preferred composition in accordance with the invention. From the above 
viscosity data it is concluded that because of its greater viscosity the 
gelled composition of the invention would be markedly superior to the 
ungelled composition, particularly in fracture-acidizing operations. From 
said viscosity data, it is further concluded that the components of the 
gelled composition have sufficient compatibility with each other to permit 
good penetration (as defined above) into the formation, and permit 
maintaining of the composition in contact with the formation for a period 
of time usually sufficient for the acid to significantly react with the 
acid-soluble components of the formation. Thus, it is further concluded 
that suitable compositions in accordance with the invention could be 
advantageously for acidizing operations in wells having a depth of up to 
at least 10,000 feet, and at formation temperatures of up to at least 
200.degree. F. The use of a preflush cooling fluid injected down the well 
and into the formation prior to the injection of the gelled acidic 
composition would extend said ranges of operation. As will be understood 
by those skilled in the art, the actual attainable ranges of effective 
acidizing operation will depend upon the viscosity of the gelled 
composition, the formation temperature, the composition of the formation, 
the rate of injection of the gelled acidic composition, the acid 
concentration in said gelled acidic composition, etc. 
While certain embodiments of the invention have been described for 
illustrative purposes, the invention is not limited thereto. Various other 
modifications or embodiments of the invention will be apparent to those 
skilled in the art in view of this disclosure. Such modifications or 
embodiments are within the spirit and scope of the disclosure.