Note: Descriptions are shown in the official language in which they were submitted.
~L~Z~
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METEIODS FOR ACIDIZING SUBTERRANEAN
FORMATIONS AND GELLED ACID COMYOSITIONS
This invention pertains to improved composi-
tions and method for acid treating or acidizing of
subterranean formations in order to stimulate the
production of formation fluids.
Acid treating or acidizing of porous subter-
ranean formations penetrat~d by a well bore has been
widely employed for increasing the production of fluids
such as, crude oil, natural gas, etc., from said forma-
tions. The usual techni~ue of acidizing a formation
comprises introducing a nonoxidizing acid into the well
under sufficient pressura to force the acid out into
the formation where it xeacts with the acid-soluble
components of the formation. The technique is applied
to formations of high acid solubility, such as lime-
stone, dolomite, etc., and is also applicable to othertypes of formations containing streaks or striations of
acid-soluble componen-ts such as sandstone containing
carbonate striations.
During the acid treatments, passageways in
the formation for fluid ~low are created or enlarged,
thus stimulating the production of fluids from the
32,152~F -1- ~r,
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formation. The ac-tion of the acid on the formation is
o~-ten called etching. There are essentially two well-
-knowrl types of acid treatments used in the field, and
these include matrix acidizing and fracturing acidizing.
S Both types of treatments utilize acid compositions as
pumpable fluids. In matxix acidizing operations, the
acid is injected into the formation at a pressure ox
rake sufficient to force the fluid into the formation
but insufficient to hydraulically fracture the formation.
In fracture acidizing operations, the acid composition,
which is usually in the form of a viscous gel, is used
as a fracturi.ng fluid which is pumped through the well
bore into the formation at a sufficient rate and pressure
to overcome the overburden pressure and thus fracture
the formation.
One of the problems commonly encountered in
acidizing operations ls insufficient penetration of the
formation by the acid. It is desirable that good
penetration is 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. Th~ severity
of the problem increases with well depth and with
increasing temperatures which enhance the reaction of
the acid with the formation.
Poor penetration can also be caused, and/or
aggravated, by fluid loss to the more porous zones of
the formation or "thief zones." Poor penetration can
also be caused and/or aggravated by leak-off at the
fracture faces in combination fracturing-acidizing
operations. Fluid loss or leak-off can frequently
worsen the situation by leaving the tight ~low perme-
ability) zones of the formation unchanged and merely
32,152~F -2-
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openiny up the already high permeability zones. One
solution w~lich has been proposed ~or the above-discussed
problem is to incorporate various thickening or gelling
agents into -the acid solutions. Such agents thicken
the acid solution and increase the viscosity thereof.
- It has been disclosed that polymer-thickened acid
solutions have improved fluid loss properties. For
example, see U.S. Patent Nos. 3,415,319 and 3,434,971.
It has also been reported that the reaction rate of
said polymer~thickened acid solutions with the acid-
soluble portion of the formation is lessened or retarded.
See, for example, U.S. Patent Nos. 3,749,169; 3,236,30S;
3,252,904; 4,055,502; 4,103,742 and 4,191,657. It has
been disclosed that foams, including foamed acids, have
improved fluid loss properties. For example, U.S.
Patent Nos. 3,937,283-and 4,235,734 disclose foamed
acid systems which can be used for foam fracturing.
However, such foams have stability problems.
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 empl~yins acidizing
compositions having thickening or viscosifying agents
incorporated therein, is stability to heat. By
"stability to heat" is meant the retention of the
increased or greater viscosity properties under the
conditions of use. To be satisfactory, such compo-
sitions should be sufficiently stable to resist thermal
32,lS2-F -3-
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clegenerat.ion in the presence of acid ~or a period of
time su~:Eicient to accomplish the intended purpose,
e.y., good penetration and significant etching of the
formation. The degree of stability required in any
particular operation will vary depending upon the type
of formation being -treated, the temperature of the
formation, the well depth, acid concentration, pump
rates, shear rates, etc. For exclmple, acidizing o~ a
kight low permeabi.lity formation will proceed more
slowly (i.e., at lower pump ra-tes and lower pressures)
than with a more permeable formation, other factors
being the same, because a longer time will be required
to obtain a significant amount of etching.
The temperature of the formation usually has
lS 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 efects. One such effect
is deganeration of the composition, which results in a
decrease in viscosity. Another such effect is increased
rate of reaction of the acid wi-th the formatlon. Thus,
some compositions which would be satisfactory in a low
temperature formation such as in the ~ugoton field in
the Anadarko basin might not be satisfactory in forma-
tions encountered in deeper wells as in some West Texas
fields. Another problem which is sometimes encountered
when using thickened compositions in treating formations
involves flow-back or removal of the treating composi-tion
after the operation is completed.
In ordinary acidizing operations using unthick-
ened acids there is usually no problem in removing the
32,152-F -4-
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spent acid because it is essentially wat~r. However,
some residues from the sperlt thickened or viscous acid
solutions are difficult to remove from the pores of the
formation of the fracture after the operation is complete.
Sometimes the polymeric gelling agent precipitates from
the spent acid and forms a clogging residue 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.
During such acid treatments, the treatment
acid often dissolves iron scale in pipes and iron-
containing minerals in the formation. The dissolved
iron normally remains in solution until the acid is
spent. Upon spending, ferric hydroxide begins to
precipitate and plug the formation. Complete precipi-
tation of ferric hydroxide is reached at a pH of about
3.2. Ferrous hydroxide, being more soluble, does not
begin to precipitate un-til a pH of approximately 7.7
and is not generally a problem.
The deleterious effects of ferric hydroxide
in wells was recognized by Grebe in U.S. Patent
No. 2,175,081 as early as 1937 where a strong acid
containing sulfurous acid is disclosed as a means of
countering the precipitation pro~lem. Numerous other
procedures have been proposed for avoiding the ferric
hydroxide problem. For example, u.S. Patent No.
2,175,095 suggests including withi~ the acidizing fluid
a material such as lactic acid, ammonium acetate,
glycine, glycolic acid, citric acid, or the like, which
is capable of preventing the precipitation of iron or
aluminum hydrates at normal precipitation pH values.
U.S. Patent No. 2,335,689 suggests adding an iron
32,152-F - -5-
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sequestering agent, such as a polyhydric phenol, wi-thin
the injec-ted acids. U.S. Patent No. 3,142,335 suggests
the use of a seques-teri~lg agent containing a mixture of
ingredients that function as a pH buffer, such as
citric acid or a citrate salt mixed with acetic or
formic acids or thelx salts. U.S. Patent No. 3,150,081
suggests using mixtures of hydroxyacetic and citric
acids, which mixtures are alleged to be cheapar and
more efective than the use of either acid alone.
The most common se~uestering agents in
commercial practice are citric acid, ethylenediamine-
tetraacetic acid (EDTA), nitrilotriacetic acid (NTA),
acetic acid, lactic acid, and citric acid/acetic acid
mixtures. Data on these sequestering agents is found
in the paper by Smith et al., Paper No. SPE 2358,
Society of Petroleum Engineers of AIME (presented
November 7-8, 1968).
The problem with most iron sequestering
agents is that they are not particularly effe~tive at
temperatures beyond about 125 to 150F (51.7 to
65.6C). Only NTA, citric acid and EDTA hava shown any
effectiveness at higher temperatures. Of these, EDTA
is the only practical sequesteriny agent because citric
acid tends to form insoluble citrates during the course
of the well treatmentO Such citrates can block the
well pro~uction almost as effectively as the ferric
hydroxide.
The presence of ferric ions in the treatment
acid solution is known to cause other serious problems
30 as well. For example, U.S. Patent No. 4,096,914 teaches
that ferric ion reacts with asphaltenic oil to form
32,152-~ l6-
1220~30
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insoluble iron asphaltene compounds. These compounds
are insoluble pxecipitates which likewis~ plug the
formation channels and inhibit production of the
des.ired fluid. The pa-tent teaches that the problem can
be solved by incorpoxating salicylic acid into the
treatment acid.
Ferric ion corrosion can also be a problem.
Two moles o~ ferric ion reacts with base metal to form
three moles of ferrous ion. Almost any oxidizing
source readily converts the ferrous ions to fexric
ions, and a vicious circle results. Additives used to
control problems associated with ferric lons in treat-
ment of wells have been called "iron stabilizexs" or
"iron control agents" by practitioners in the field.
Another problem associated with the presence
of ferric ions in a polymer thickened acid composition
involves the precipitation of the polymer. That is, a
precipitate forms which is believed to contain iron in
a chemically combined form (i.e., such a metal ion
crosslinker) with the polymer. The precipitate is
usually in the form of a gummy, insoluble mass that is
very difficult, if not impossible, to resolubilize
under the conditions of use for acid treatment fluids.
In view of the deficiencies of the prior art,
it would be highly desirable to provide a gelable acid
composition for use in acidic well treatments and the
method of using the sama, which composition is viscous,
stable and generally free of insoluble precipitates
during use
32,152-F ~7-
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ln one aspect, the present invéntion is a gellableacid composition which compri.ses (A) an aqueous acid thickened
with a functlonally effective amount of a water-soluble
polymer comprising (1) from greater than 0 to 50 weight
percent of a nonionic water-soluble ethylenically unsatu-
rated monomer, (2) from 50 to less than lO0 weight percent
of a water-soluble ethylenically unsaturated monomer con-
taining an anionic moiety and (3) from 1 to 1000 ppm of a
polyvinyl crosslinking monomer, wherein said polymer is
crosslinked in amounts sufficient to provide increased
viscosity to the composition during acid treatment of
porous subterranean formations, which amount of polymer
is sufficient to cause gelation and to form a gelled acid
composition and sufficiently stable to degradation by heat
of porous subterranean formationsi and (B) at least one
reducing agent in an amount sufficient to prevent or sub-
stantially prevent the foramtion of an insoluble residue
as the gelled acid reacts with the acid-soluble components
of said formation in the presence of dissolved ferric ion.
In another aspect, the present invention is a
method for acid treating a porous subterranean formation
susceptible to attack by an acid which acid is penetrated
by a wellbore, which method comprises injecting into said
formation via said wellbore a gellable or gelled acidic
composition comprising (A) an aqueous acid i.n an amount
capable of, and sufficient for, reacting with a significant
amo~mt of the acid-soluble components of said formation;
and a water-thickening amount of a water-soluble polymer
comprising (1) from greater than 0 to 50 weight percent
of a nonionic water-soluble ethylenically unsaturated
monomer, (2) from 50 to less than 100 weight percent of
a water-soluble ethylenically unsaturated monomer containing
an anionic moiety and (3) from 1 to 1000 ppm of a polyvinyl
crosslinking monomer; and (B) at least one reduciilg agent
32,152-F -8-
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"`` ~L22~930
g
in an amount sufEicien~ to prevent or substantlally prevent
the ~ormation of an insoluble residue as the gelled acid
reacts with the acid-soluble components of said formation
in t`he presence of dissolved ferric ion, said polymer 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 said composition having sufficient
stability to degradation by the heat oE said formation
to permit good penetration o said composition into said
formation; and maintaining said composition in said formation
in contact therewith for a period of time sufficient usually
for the acid in said composition to react significantly
with the acid~soluble components of said formation and
stimulate the production of fluids therefrom.
The process of this invention yields a means
of providing increased production of oil or gas. The
acid composition can be used in fracture acidizing. The
polymeric thickener provides an improved means for
reducing the rate at which fluid is lost into the
subterranean formation. The acid is also used for
matri~ acidizing where the polymeric thickener provides
a slow rate of reaction of acid within the subterranean
formation to increase penetration of said acid into the
formation. In either case, the reducing agent prevents
problems associated with insoluble residues caused by
ferric ions.
Ethylenically unsaturated, water-soluble
monomers suitable for use in this invention are those
which are sufficiently water-soluble when dissolved in
32,152-F -9-
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--10 -
water and wh.ich :reaclily undergo addit.ional polymeriza-
tion to .~orm polymers which are at. least inheren-t:ly
water-dispexsible and preferably water-soluble. By
"inherently water-dispersible" is meant thak -the
polymer, when contacted with an a~ueous medium, will
disperse therein without the aid of surfactan-t to ~orm
a colloid dispersion o polymer in the aqueous medium.
Exemplary nonionic monomers suitably employed
in the practice of this invention are those ethylenically
unsaturated monomers that are sufficiently water-soluble
to form at least a 5 weight percent solution when
disæolved in water and readily undergo addition polymeri-
zatioIl to form polymers that are water-soluble. Examples
of such nonionic monomers include ethylenically unsatu-
rated amides such as acrylamide, methacrylamide andfumaramide; their water soluble N-substituted nonionic
derivatives such as the N-methylol derivatives of
acrylamide and methacrylamide as well as the N-methyl
and N,N-dimethyl derivatives of acrylamide and methacryl-
amide; hydroxyalkyl esters of unsaturated carboxylicacids such as hydroxyethyl acrylate and hydro~ypropyl
acrylate; and the like. Of the foregoing nonionic
monomers, the ethylenically unsaturated amides are
pre~erred, with acrylamide being especially prPferred.
Examples of suitable water-soluble, ethylen-
ical~y ur.saturated monomexs containing anionic moieties
include the sulfonate-containing monomers including,
for example, N-sulfoalkyl, a,~--ethylenically unsaturated
amides such as 2~acrylamido-2-methylpropane sulfonic
acid, 2-acrylamido propane sulfonic acid, 2-acrylamido
ethane sul'fonic acid and the alkali metal salts thereo
such as the sodium and potassium salts thereof as well
32,152-F . -10-
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a~ other such monome.rs listed iII U.S. Paten-t No.
3,692,673; sulfoalkyl esters of unsaturated carboxylic
acids such as 2-sulfoethyl methacrylate and other such
sulfoalkyl esters as listed in U.S. Patent No.
4,075,134, as well as the alkali.metal salts thereof;
sulfoarylalkenes such as vinylbenzyl sulfonic acid and
the various salts o vinylbenzyl sulfonate, p-styrene
sulfonic ac.id and the salts thereof; sulfoalkenes such
as vinyl sulfonic acid and salts thereo~; and the like.
Of the foregoing sulfonate monomers, the sulfoalkyl
derivatives of acrylamide and methacrylamide are
preferred with those of acrylamide being especially
preferred, particula.rly 2~acrylamido-2-methylpropane
sulfonic acid (AMPS), 2-acrylamido-2-propane sulfonic
acid and the salts thereof. In the most preferred
embodiments, the sulfo group is in the form of an
alkali metal sulfonate salt such as sodium sulfonate.
The polyvinyl crosslinking monomers are
copolymerized with the aforementloned ethylenically
unsaturated water-soluble monomers including, for
example, divinyl benzene, divinyl esters of polycar-
boxylic acid, diallyl esters of polycarboxylic acids,
diallyl maleate, diallyl fumarate, divinyl adipate,
glyceryl trimethylacrylate, diallyl succinate, divinyl
ether, the divinyl esters of ethylene glycol or diethy-
le~e glycol diacrylate, polyethylene glycol diacrylates
or methacrylates, and the like. Of the aforementioned
polyvinyl crosslinking monomers, the most preferred is
methylene bisacrylamide.
Polymers useful herein are anionic polymers.
Polymers are prepared by using techniques known in the
art for preparing water-soluble polymers. That is,
32,152-F -11-
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polymer~ are pr~pared by copolymerizing the afore~len-
tioned mono~ers using techniques known in the art for
prepariny water-soluble polymer~. For example, polym-
exiæatio~ can be carried out in an aqueous medium in
the presence of a small but effective amount of a
water-soluble oxygen~containing catalyst at a tempera-
ture of from 80F to 190F (26.7 to 87.8C). Tha
resulting polymer is recovered from the aqueous tnedium,
as by drum drying, and can be subsequently ground to
the desired particle size. The particle size should be
fine enough to facilitate the dispersion of the polymer
in water. Polymers are also prepared using pol~meriza-
kion techni~ues described in U.S. Patent No. 3,284,393
and U.S. Patent RE 28,474. Alternatively, the polymers
can be prepared by reacting various known alkylating
reagents with suitable copolymers to form anionic sites
on the pol~mer.
Mclecular weights of the polymers of this
i~vention can vary. Molecular weights typically range
20 from 100,000 to 25 million. Most preerably, molecular
weights range from 1 million to 5 million.
The polymers of this invention comprise
sufficient monomer containing anionic moieties in order
that the composition of this invention does not yield
an insoluble residue under acid treatment of subterranean
formations. The polymers also comprise sufficient
nonionic monomer in order that the polymer will provide
sufficient thickening ability to the composition. The
amount of polyvinyl crosslinking monomer which is
employed will depend upon the type of crossllnker which
is employed and the molecular weight of the linear
polymer.
32,152~F -12-
93(1
-13-
The pol.ymers of this i.nventlon are ~ypicall~
from greater than 0 to 50, preferably from greater than
0 to 35, most preferably from 20 to 30, weight. percent
nonionic ethylenically unsaturated monomer, which is pre-
ferably acrylamide; from 50 to less than 100, preferablyfrom 65 to less than 100, most preferably from 70 to 80,
weight percent ethylenically unsaturated monomer comprising
anionic moieties, which is prefereably 2-acrylamido-2-methyl
propane sulfonic acid or a salt thereof; and from 1 to 1000
ppm, preferably from 50 to 100 ppm, most preferably from
75 to 100 ppm polyvinyl crosslinking monomer which is
preferably methylene bisacrylamide. It is undersood that
the amount of ethylenically unsaturated monomer containing
anionic moieties within the polymer can vary depending
on the anionic behavior of the monomer. It is also
understood that the nonionic monomer within the polymer
can undergo a small amount of hydrolysis.
The composition of this invention comprises
from 0.4 to 35, preferably from 3 to 28, weight percent
acid, from 0.1 to 5, preferably from 0.5 to 2, weight
percent polymer, and from 60 to 99.5, preferably from
70 to 96.5, weight percent water based on the total
weight of the composition. To this is added the
reducing agent. In addition, it is desirable to add
corrosion inhibitors, sequestering agents, demulsifying
agents, surfactants, friction reduction reducers, etc.,
which are commonly added to such types of compositions.
In addition, -the acid compositions can be foamed by
including a foaming agent and a gas, such a nitrogen.
Small amounts of polymer will usually produce
liquid mobile gells which can be readily pumped. Large
amounts of polymer will usually produce thicker, more
1;226)~3(~
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viscous, somewhat elastic gels~ Gels having a viscosity
"too thick to measure" by conventional methods can
still be used in the pxactice of the invention. Thus,
- there is really no fixed upper limit on the amount o
polymer which can be used so long as the gelled acidic
composition can be pumped in accordance with the methods
of the invention.
The "ixon control agents" useful herein are
reducing agents which are known classes of compounds
having many members. Any member(s) of these known
classes of compounds can be used herein so lony as the
chosen member(s) is compatible with the gelled acidic
composition, i.e., the ¢hosen member(s) is solu~le or
dispersible in the acidic composition and does not
preven-t formation of the gelled acidic composition or
cause premature breaking of the gel. Examples of
suitable reducing agents include reducing organic acids
(e.g., ascorbic acid, erythorbic acid, and the like)
and soluble salts thereof (e.g., sodium erythorbate,
potassium ~rythorbate, ammonium ascorbate, and the
like), hydrazine, iodide salts (e.g;, sodium iodide and
the like) and other s~ch compounds. Of these, the
organic reducing acids are preferred, and ascorbic
acid, erythorbic acid and/or the soluble salts thereof
are most preferred. The reducing agent is included in
the gelled acidic composition in an effective amount,
i.e., the amount sufficient to prevent or substantially
prevent the formation of an insoluble residue when the
gelled acidic composition is contacted, for example,
with calcium carbonate in the presence of dissolved
iron.
32,152-F ~14-
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Iron control ayents are typically used in
concentrations oE ~rom 0.01 to 0.36 g/100 ml o~
thickened acid composition, preferably from 0.06 to
0.24 g/100 ml of thickened acid composition.
Typically, iron control agents are employed in a
concentration of 0.06 g/100 ml of thickened acid
composition for every 1000 ppm ferrlc ion present and
such concentrations are very effective in combating
residue formation.
Acids useful in the practice of this
invention include, for example, inorganic acids such as
hydrochloric acid, phosphoric acid, nitric acid,
hydrofluoric acid; and a mixture of hydrochloric acid
and hydrofluoric acid; C1-C4 organic acids such as
formic acid, acidic a~id, propionic acid, bu-tyric acid
and mixtures thereof and combinations of inorga~ic and
organic acids. The nonoxidizing acids are preferred.
The concentration or strength of the acid can vary
depending upon the type of acid, the type of formation
being treated and the results desired in the particular
treating operation. Most preferably, the acid used in
the practice of this invention is an inorganic acid
such as hydrochloric acid.
The gelled acidic composition of the present
invention can be prepared on the surface in a suitable
tank equipped with a suitable means for mixing and then
pumped down into the well and into the formation employing
conventional equipment for pumping acidic compositions.
Most preferably, the pol~mer is mixed with the acid at
temperatures of from 60~F to 90F (15.6 to 32.2C).
Viscosity development is quite rapid and complete
viscosity development occurs in 60 to 90 minutes.
32,152-F ~15-
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-16-
It is within the scope of this invention to precede the
injection of the gelled acid composition wi-th a pad
fluid or preflush such as gelled water~ or a~ueous
potassium chloride, aqueous ammonium chloride, etc.
Such fluids serve to cool the well tubing and formation
and extend the useful operating temperature range of
said compositions. The volume of the pad 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 char-
acteristics of the formation. Typically, the compo-
sitions of this invention are employed at temperatures
up to 300F (148.9C), depending upon conditions of
employment and the amount of polymer which is used.
The gelled acid compositions of the invention
can be prepared on the surface in a suitable tank
equipped with suitable mixing means, and then pumped
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 tech-
nique 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 weli head can be
staxted. Then, downstream from the tank, a suitable
connection can be provided for introducing-the acid
into said conduit~ A foamed acid can be generated by
subse~uently introducing a gas such as nitrogen into
the flowing stream of acid composition. As will be
understood by those skilled in th~ art, the rate of
introduction of said components into said conduit will
depend upon the pumping rate of the polymer solution
32,15a-F -16-
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through said conduit. Any ot the above-mentioned
orders of addition can be employed in said "on the fly"
techni~le. Mixing devices can be provided in said
conduit, if desired.
S For fracture acidizing treatments, the composi-
tions of this invention are injected into a bore hole
at a sufficient rate and pressure to initiate and
pxopagate a crack or fracture in the formation. Sand,
bauxite, or other proppant material can be included in
the treating fluid to prevent the fracture from closing.
In a matrix acidizing operation, the treatment fluid is
injected into the borehole at a rate and pressure
sufficient to force the acid out into the formation but
insufficient to cause fracturing of the formation.
The following examples will serve to further
illustrate the invention but should not be considered
as limiting the scope thereof. Unless otherwise noted,
all parts and percentages are by weight.
Example
The present composition is prepared as follows.
A volume of concentrated hydrochloric acid is mixed
with suffici~nt water to yield an acid concentration of
28 percent. To this solution is added enough ferric
~ chloride to yield a ferric ion concentration of 3000 ppm.
To this solution is added at a r~tio of 1.2 g/100 ml of
acid solution, a random copolymer containing 70 percent
by weight of a sodium salt of 2-acrylamido-2-methyl-
propane sul~onic acid (AMPS) and 30 percent acrylamide
monomer crosslinked with 100 ppm of methylene bisacryl-
amide and having a molecular weight greater than 3million. To this solution is also added 0.6 g/1000 ml
32,152-F -17-
0~3~
~18-
o acid solution of a corrosion inhibitor. ~o this
yelled solution is added sodium erythorhate at a ratio
of 2 g/100 ml of said solution. Marble chips are added
to the gelled solution and the mixture is heated to
150F (65.6C). After bubbling of the composi-tion is
complete, the solution is examined for precipitate.
The polymer composition yields no residue formation.
Conversely, a similar formulation not containing sodium
erythorbate but treated in a similar fashion yields
substantial residue formation.
32,152-F -18-
