Note: Descriptions are shown in the official language in which they were submitted.
31~3
METHOD AND COMPOSITIONS FOR
ACIDIZING SUBTERRANEAN FOR~TIONS
This invention relates to acidizing or fracture-acidiz-
ing of subterranean formations to enhance oil and gas produc-
tionA
Oil and gas acc~ulations usually occur in porous and
permeable underground rock fo~nations. In order to produce
oil and gas contained in the formation, a well is drilled
into the formation. The oil and gas, in many instances, are
contained in the pore spaces of the formation and are hydrau-
licall~ connected by means of permeability or interconnecting
channels between the pore spaces. After the well is drilled
into the formation, oil and gas are displaced to the well
bore by means of fluid expansion, natural and artificial
15 fluid displacement, gra~ity drainase, etcThese various
processes may wo:rk together or independently to drive the
hydrocarbons into the well bore through existing flow chan-
nels. In many instances, however, production of the well
may be impaired by insufficient channels leading into the
bore hole. The formation, in many instances, is treated
to increase the ability of the formation rock to conduct
fluid into the well.
Acidizing of a subterranean formation penetrated by a
well bore has been widely employed for increasing the produc-
tion of fluids, e.g., crude oil, natural gas, etc., from the
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formation. The usual technique of acidizing a formation com-
prises introducing a non-oxidizing acid into the well under
sufficient pressure to force the acid out into the formation
where it reacts with acid-soluble components of the formation.
The technique is not limited to fol~ations of high acid solu-
bility such as limestone, dolomite, etc.. The technique is
also applicable to other types of formation such as sandstone
containing streaks or striations of acid soluble components
such as various carbonates.
During the acid treating o~eration, passageways for
fluid flow are created in the formation or existing passage-
ways therein are enlarged, thus stimula ing the production
of fluids from the formation. Acidizing operations wherein
the acid is injected into the formation at a pressure or
rate insufficient to create cracks or frac~ures in the for-
mation is usually referred to as matrix acidizing.
Hydraulic fracturing is also widely used for stimulating
petroleum producing subterranean formations and comprises
the injection of a suitable fracturing fluid down a well
penetra~ing a formation and into the 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 the
flow of fluids through the formation and into the well.
When the pressure of the fracturing fluid is reduced, a
propping agent which is present in the fracturing fluid pxe~
ven~s the complete closure of the fracture. Combination
fracture-acidizing proces~ses are weli known in the art.
Gelled acidic compositions containing crosslinked poly-
mers have been utilized during the acidizing ox fracture-
acidizing of a subterranean formation. These compositions
present a significant problem during the acidizing or frac-
ture-acidizing of a form.ation where the bottom hole tempera~
ture or the environmental temperature at the locus of the
fracture which is to be propagated or enlarged exceed;s about
200~F.
For instance, at such an elevated temperature, these
compositions, when introduced into the formation for the pur
pose of fracturing the formation, undergo gel instability
which is evidenced by precipitation and/or syneresis (bleed-
ing or medium separation). The phenomenon of syneresis occurs
because the space available for solvent molecules, i.e., water
or other medium, in the amorphous network of the gel decreases
as the fraction increases of molecular chains that participate
in forming crystalline regions within the gel. Accordingly,
as the "gel sets", it also "settles" and a supernatant layer
of fluid becomes visible over the gel phase. This material,
when settled, is, in many instances, damaging to the formation.
Other problems occur using gelled acidic compositions
containing crosslinked polymers during the acidizing or frac-
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ture~acidizing of a subterranean formation with an elevated
temperature. In many instances, these polymers undergo de-
generation, e.g., decrease in viscosity, which results in
poor penetration of the gelled acidic composition into the
formation.
The present invention provides a solution for, or at
least mitigates the above discussed problem. The present
invention provides Lmpro~ed methods for acidizing, or frac-
ture-acidizing subterranean formations; and new gelled
acidic compositions for use in said methods.
Thus, in accordance with one broad aspect on the con-
cept of the in~ention, there is provided a method for acid
treating or fracture-acidizing a subterranean formation com-
prising introducing into said formation via a well bore a
gelled acidic composition camprising essentially of water;
an amount of acid which is capable of, and sufficient for,
reacting with a significant amount of acid soluble components
of said formation; a water thickening amount of a water dis-
persible polymer of about 5 to about 60% by weight acryla-
mide and the r~mainder selected from the group consisting
of dialkylaminoethylmethacrylate, trlalkylaminoethylmeth-
acrylate quaternary salts, and acrylamido alkane sulfonic
acid; and maintaining said composition in said formation in
contact therewith for a period of time sufficient for the
acid in said composition to react significantly ~ith 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
çomposition, suitable for fracture-acidizing of a subterra-
nean formatiGn consisting essentially of water; an amount of
acid which is capable of, and sufficient for, reacting with
a significant amount of acid soluble components of said for-
mation; a water thickening amount of a polymer of about 5
to about 60% by weight acrylamide and the remainder selected
from the group consisting of dialkylaminoethylmethacrylate,
trialkylaminoethylmethacrylate quaternary salts, and acryl-
amido alkane sulfonic acid and introducing the composition
into the formation under sufficient pressure to fracture the
formation.
When the above described method for acidizing or frac-
ture acidizing of a subterranean formation is employed, no
difficulty i5 encountered in handling and pumping the mate-
rials required to make the composition, or the composition
itself. Surprisingly, the acidizing or fracture-acidizing
composition does not exhibit the phenomenon of syneresis at
temperatures over 200F and excellent penetration of the
formation by the acid is achieved.
The group of copolymers which are useful in the practice
of the invention are from about 5 to about 60% by weight
acrylamide and the remainder selected from the group of mono-
mers having the structural formula:
0 R +
(l) 3 1I C 0 _ CH2CH2 - I - R X
CH2 R
where R is selected from the group consisting of methyl or
ethyl; and X is Cl, Br, I, CH30S0~,
0 / R
(2) CH3 - C - C - 0 CH2 - CH2 - N
H2 R
R is ethyl or methyl, and
0 H R
1~ 1 13
(3) R - C - C - N - C - CH S0 0 - M
CH R2
Rl
wherein R, Rl, R2, R3 are independently selected from the
group consisting of hydrogen and alkyl having from l to 5
carbon atoms and M is selected from the group consisting of
hydrogen, sodium, potassium and ammonium.
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The preferred copoi~mer of the present invention is a
copolymer of 20% by weisht acrylamide with the remainder
from formula (l) wherein: each R is methyl, X is CH30S03,
and said polymer has a weight average molecular weight of
about 7,000,000.
Generally, the amount of acidic gelling polymer useful
in the camposition of the invention can vary widely depend-
ing upon the viscosity of the composition of the invention
desired in the formation to be treated and the bottom hole
temperature of the formation. In general, the amount of
thickening or gelling agent used will be in the range of
from about 0.10 to about 20 percent weight. There is, how-
ever, really no limit on the amount of gelling agent which
can be used as long as the gelled acidic composition can be
pumped in accordance with the method of the invention.
Generally, the acidic polymer composition will contain
an inhibitor to prevent or greatly reduce the corrosive
attack of the ac:id on metal. Any of a wide variety of com-
pounds known in the art and employed for this purpose can
be used. The amount of the inhibitor utilized is not highly
critical and may be varied widely. Usually this amount is
defined as a small but effective amount, e.g., from about
0.10 percent to about 2.0 percent by weight or more of the
acidic polymer solution.
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When it is desirable to contact the subterranean forma-
tion with a foam containing the composition of the present
invention, any foaming agent compatible with the composition
of the invention and capable of producing foam in an acidic
environment may be used. Many of the foaming agents are
disclosed in U. S. Patent No. 4,044,083.
Various methods are known in the art for preparing
monomers utilized in the composition of the invention. For
instance, see U. S. Patent No. 3,573,263 for the preparation
of the monomers of formulae (1) and (2) and U. S. Patent No.
3,507,707 for the preparation of monomer of formula (3).
The weight average molecular weight of the polymers
useful in the practice of the invention range from about
5/000~000 to about 20,000,000. The most preferred copolymer
weight average molecular weight ranges from about 5,000,000
to about 15,000,000.
As used herein and in the claims, weight average molec-
ular weight is defined by the following formula:
~_Ci
~0 weight average molecular welght = C
where Ci is the concentration of molecular species i, Mi is
the molecular weight of the species i, and the summation is
of all species i.
Acids useful in the ~ractice of the invention includes
any acid which is effec-tive in increasing the flow of fluids,
e.g., hYdrocarbons, through the formation and into the well.
Thus, under proper conditions of use~ examples o~ such acids
can include inorganic acids such as hydrochloric acid, hydro-
fluoric acid, and phosphoric acid; Cl - C4 organic acids such
as formic acid, acetic acid, propionic acid, butyric acid
and mixtures thereof and combinations of inorganic and or-
ganic acids. The concentration or strength of the acid can
vary depending upon the type of acid, the type of formation
being t-eated, and the above stated compatibility require-
ments, and the results desired in the particular treating
operation. Generally speaking, the concentration can vary
from 0.4 to about 60 weight`percent, depending upon the type
of acid, with concentrations within the range of lO 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 pro~ide an amount of hydro-
chloric acid within the range of from abou~ 0.4 to about 35,
more preferably at least about 10 percent, weight percent
based on the total weight of the gelled acidic composition.
Amounts within the range of about lO to about 30 weight
percent will frequently be practical amounts of use. The
acids used in the practice of the invention can contain
~ 1~a~
any of the known corrosion inhibitor~, de-emulsifying agent~
sequestering agents, surfactants, friction reducers, etc.,
known in the art. The preferred acids for carrying out the
invention are hydrochloric acid, acetic acid, formic acid
and mixtures thereof.
The gelled acidic compositions of the in~ention are
aqueous compositions. They normally contain a significant
amount of water. The amount of water can vary depending
upon the concentrations of the other components in the com-
position, particularly the concentra~ion of the acid. For
example, when an organic acid such as acetic acid is used
in a 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 concentration of
about 35 weight percent. Thus no precise overall range of
water content can be set forth. Based on the above-stated
overall ranges for tne concentratîons of the other compo-
nents, the water content of the compositions can be in the
range of from about 30 to 99, frequently about 60 to 90,
weight percent. However, amounts of water outside the
ranges can be used.
Propping agents which can be used in the practice
of the invention include any of the known in the ar~,
e.g., sand grains, walnut shell fragments, sintered
bauxite, tempered glass beads, nylon pellets or any
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mixture of two or more thereof and similar materials. 5uch
agents can be used in concentrations of about 0.1 to 10
pounds per U. S. gallon of fracturing fluid~ In general,
propping agents with particle sizes of 6 mesh to about 400
mesh, more preferably 8 to lO0, and most preferably 12 to
~0 mesh are employed.
Any suitable method can be employed for preparing the
gelled acidic composition of the invention. Thus, any
suitable mi~ing technique or order of addition of the com-
ponents o~ the composition to each other can be employed
and will provide a composition having sufficient stability
to degeneration by the heat of the formation (to which the
composition is to be used) and to provide good penetration
of the composition into, and significant etching of, said
formation. It is ordinarily preferred, however, to first
disperse the polymer in a non-hydrating compound such as
an alcohol or an oil before contacting the polymer with
water or acid. Thus, it is within the scope of the inven-
tion to moisten or slurry the polymer with a small amount
of a low molecular weight alcohol, e.g., Cl to C3 alcohols,
or a hydrocarbon such as diesel oil or mineral oil as a dis-
persion aid to disperse the polymer in the water or acid
solution. The gelled acidic composition of the invention
may be held at ambien~ temperatures for several days prior
to its introduction into the formation.
The gelled acidic composition of the invention can be
prepared on the surface and a suitable t~nk equipped with
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suitable mixing means. The composition is then pumped down
the well and into the formation employing conventional equip-
ment for pumping acidic compositions. It is, however, within
the scope o the invention to prepare the compositions while
they are being pumped down the well. Thus, a dispersion of
the polymer and water can be prepared in a tank adjacent to
the well head and then, a few feet down stream from the tank,
a connection can be provided for introducing the acid into
the polymer dispersion.
The composition is next introduced into the subterr~-
nean formation. The acid is introduced into the subterra-
nean formation whereby the calcareous rock in the formation
is dissolved thereby increasing the permeability, and per-
mitting better flow of fluids through the formation. T~e
pumping rate and pressures utili~ed will obviously depend
upon the characte:ristics of the formation and whether frac-
turing of the formation is desired. After the gelled acid
composition has been injected in this manner, the well will
normally be shut in and allowed to stand for a period rang-
ing from se~eral hours to a day or more. If there is pressure
on the well, pressure is then released and the spent acid com-
position containing salts formed by the reaction of the acid
composition, is permitted to Elow back into the well bore and
is pumped or flowed to the surface. The well may thereafter
be placed on production or used for other purposes.
The following example will serve to illustrate the
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invention but should not be considered as unduly limiting
the invention.
EXAMPLE I
The affect of temperature on the viscosity of various
polymers in a 20 percent hydrochlcric acid was determined
by first homogeneously mixing the components with water and
heating the resulting fluid from 100F to 250F in a Fann
Model 50 Viscometer while taking the 50 rpm dial reading,
using a number l spring and sleeve at a temperature indi-
cated on Table I. This reading is reported in Table I as
the shear stress exerted by the fluid in lb/ft2. All
samples contained l percent by weight of the polymer.
Sample A contained a copolymer made up of 20 percent
by weight acrylamide and 80 percent by weight of trimethyl-
aminoethyl methacrylate quaternary salt. Samples B and C
contained a copolymer containing 60 percent by weight
acrylamide and 40 percent by weight 2-acrylamido-2-methyl-
propane sulfonic acid. Samples D, E and F contained 60
percent by weight acrylamide and 40 percent by weight 2-
acxylamido-2-methylpropane sulfonic acid. The weight
average molecular weight of samples A, B, C, D, E, and F
was greater than 5,000,000 but less than lO,000,000. In
addition, samples D, E and F were crosslinked using form-
aldehyde. The results of these tests are shown in Table I.
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Unable to recognize this page.
The results of the Table show that the composition of
the invention, Samples A, B, and C, were very stable at high
temperatures including 250F while samples D, E, and F, which
were crosslinked using formaldehyde, displayed syneresis a~
over 200F.
While the invention has been described in terms of cer-
tain embodiments and illustrated by examples, the skilled
artisan will readily appreciate that various modifications,
changes, substitutions and omissions may be made without
departing from the spirit thereof. Accordingly, it is
intended that the scope of the present invention be limited
solely by the scope of the following claims.
What is claimed is:
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