Note: Descriptions are shown in the official language in which they were submitted.
21027S9
APSULATED ACID COMPOSITION AND PROCESS FOR PREPARING AND USING SAME
92-369
Background of the Invention
The invention relates to the field of well stimulation and,
more particularly, to an encapsulated acid composition for use in
the acid treatment of wells, especially hydrocarbon producing
wells, and a process for preparing such a composition.
Well stimulation refers generally to the treatment of wells
in order to improve production from such wells. Numerous methods
and techniques are known for stimulating such wells. One field
of well stimulation is known as acid treatment. This technique
generally involves pumping an acid solution into a formation at a
rate which stresses the formation beyond its structural strength
and thereby fractures the formation. The acid solution then
flows into such fractures, where the acid "etches" the walls of
the fracture. After pressure is returned to normal, the fracture
closes but the walls no longer line up due to the acid etching.
Thus, new channels are formed from the formation into the well
bore and production is enhanced.
Acids typically used in such processes include hydrofluoric,
hydrochloric, formic, acetic and mono, di and trichloroacetic
acids in aqueous solution having acid concentration typically
ranging between about 3 to 28%. It has been found, however, that
the increased formation temperatures combined with the relatively
strong acid concentrations results in a large and unacceptable
corrosion rate on the tubing strings of the well.
2102769
92-369
One known method for preventing this corrosion involves the
use of corrosion inhibitors which can control the corrosion rate
for tubing fabricated from low carbon steel, duplex steel and low
chromium steel up to maximum temperatures of about 300~F (150~C).
In wells which produce H2S and CO2, however, high chromium tubing
is typically used. Further, corrosion inhibitors do not protect
such high chromium tubing from corrosion rates which are so high
that acid treating such wells involves a significant risk of
losing the well.
It is therefore the principal object of the present
invention to provide a composition for acid treating a formation
without exposing tubing in the formation to increased corrosion
from acid.
It is a further object of the invention to provide a process
for preparing such a composition.
It is a still further object of the invention to provide a
process for using such a composition in an acid treatment
process.
Other objects and advantages will appear hereinbelow.
SUMMARY OF THE INVENTION
The invention relates to a composition comprising an aqueous
solution of acid encapsulated in a polymeric material which is
chemically resistant to the acid in the aqueous solution of acid
~ 1 n 2 7 6 9
and which polymeric material is substantially impermeable to
acid and water.
More particularly, according to one aspect of the
invention there is provided an encapsulated acid composition,
comprising an aqueous solution of acid encapsulated in a
polymeric material which is chemically resistant to said acid
in said aqueous solution of acid, which polymeric material is
substantially impermeable to water and acid, and which
polymeric material is a heat-sensitive cross-linked material
which ruptures when exposed to elevated temperatures.
According to another aspect of the invention there is
provided a process for preparing an encapsulated acid
composition, comprising the steps of: forming an emulsion
having a continuous phase of oil and a dispersed phase of an
aqueous solution of acid; mixing said emulsion with a polymer
which is soluble in said oil and insoluble in said aqueous
solution of acid, said polymer being chemically resistant to
said acid in said aqueous solution of acid, said polymer being
substantially impervious to said acid and to water, and said
polymer, when cross-linked, being heat-sensitive and subject to
rupture when exposed to elevated temperatures; and mixing said
emulsion and said polymer with a cross-linking agent so as to
encapsulate droplets of dispersed phase of said aqueous
solution of acid in a cross-linked polymer encapsulant by
deposition of the cross-linked polymer on the droplets of the
dispersed phase.
B-
21 ~27 69
- According to yet another aspect of the invention there is
provided a process for treating a subterranean formation,
comprising the steps of: mixing an encapsulated acid
composition, comprising an aqueous solution of acid
encapsulated in a polymeric material which is chemically
resistant to said acid in said aqueous solution of acid, which
polymeric material is substantially impermeable to water and
acid, and which polymeric material is a heat-sensitive cross-
linked material which ruptures when exposed to elevated
temperatures, with a carrier fluid; and injecting said fluid
into said formation, whereby elevated temperatures of said
formation cause said encapsulated acid composition to rupture,
thereby contacting said aqueous solution of acid with said
formation.
According to yet another aspect of the invention there is
provided a process for neutralizing a basic solution,
comprising the steps of: mixing said basic solution with an
encapsulated acid composition, comprising an aqueous solution
of acid encapsulated in a polymeric material which is
chemically resistant to said acid in said aqueous solution of
acid, which polymeric material is substantially impermeable to
water and acid, and which polymeric material is a heat-
sensitive cross-linked material which ruptures when exposed to
elevated temperatures; heating said mixture so as to cause said
encapsulated acid composition to rupture, thereby mixing said
4a
.
B~-
~11 027 69 ~
aqueous acid solution with said basic solution so as to
neutralize said basic solution.
The polymeric material is heat sensitive and is preferably
formulated so as to provide a desired temperature at which the
polymeric material ruptures so as to release the aqueous
solution of acid. Such a desired temperature may be, for
example, about 200~C. Such a composition can be mixed with any
carrier fluid such as water or methanol and pumped into a
formation. The acid solution remains encapsulated, and
therefore cannot damage the tubing, until the desired
temperature is reached, the polymeric material ruptures, and
the acid solution is released into the formation.
According to the invention, such a composition is prepared
in a process comprising the steps of: forming an emulsion
having a continuous phase of oil and a dispersed phase of an
aqueous solution of acid; mixing said emulsion with a polymer
which is soluble in said oil and insoluble in said aqueous
solution of acid, said polymer being chemically resistant to
said acid in said aqueous solution of acid, and said polymer
being substantially impervious to said acid and to water; and
mixing said emulsion and said polymer with a cross-linking
agent so as to encapsulate droplets of said dispersed phase of
said aqueous solution of acid in a cross linked polymer
encapsulant.
4b
B~
2 1~276g
-
92-369
DETAILED DESCRIPTION
The invention relates to a composition for use in the acid
treatment of subterranean formations, especially for stimulating
the production of hydrocarbon wells drilled into such formations.
Acid treatment generally induces new fractures into the formation
and "etches" the new fractures so as to form new flow areas from
the formation into the well bore, thus enhancing production.
The present invention is drawn to a composition comprising
an encapsulated acid which prevents the acid from damaging tubing
and other well equipment while being pumped into the formation,
and which composition then releases the acid into the formation
as desired.
According to the invention, such a composition comprises an
aqueous solution of acid encapsulated in a polymeric material.
The polymeric material is preferably chemically resistant to the
encapsulated acid so as to prevent the acid from breaking through
the polymeric material. The polymeric material is also
preferably impervious to water and acid so as to prevent water
from diluting the aqueous acid solution and to prevent the acid
form escaping through the polymeric material.
The acid to be encapsulated should be selected based upon
the composition of the formation to be treated. The acid should
also be oil insoluble for reasons which will become apparent
during the following discussion of the process for preparing the
composition. Suitable acids include mineral acids, preferably
21 027 69 ~
92-369
hydrofluoric acid, hydrochloric acid, and mixtures thereof. The
concentration of acid in the aqueous solution may preferably be
based upon the desired amount of "etching" and the susceptibility
of the formation to the acid. The aqueous solution of acid may
preferably have an acid concentration of up to about 15% by
weight.
The polymeric material which encapsulates the acid is
preferably a cross-linked polymeric material such as, for
example, cured epoxy resin, vinyl ester, and polyester. Such
cross-linked polymeric material may preferably be cross-linked
with a cross-linking agent which may preferably be aliphatic
amides, aromatic amides, and mixtures thereof. More preferably,
the cross-linking agent could suitably be diethylenetriamine,
triethylenetriamine, Melamine, formaldehyde butylamine, and
mixtures thereof. Styrene and divinylbenzene are preferred
cross-linking agents for vinyl ester and polyester polymeric
material.
According to the invention, the cross-linked polymer
material is heat sensitive, that is, it ruptures at a particular
temperature so as to release the aqueous acid solution. Thus,
the cross-linked polymer is formulated so as to provide an
appropriate rupture temperature, that is, the cross-linked
polymer will preferably have a rupture temperature higher than
the temperature in the tubing, and lower than the formation
temperature. By rupture temperature is meant the temperature at
Trade Mark
f~
,~
21027~9
92-369
which the polymeric material decomposes or disintegrates or is
otherwise broken so as to release the aqueous acid solution. The
rupture temperature may, for example, suitably by about 200~C.
The composition may preferably have a particle size in the
range of about 0.5 to 3000 ~m, more preferably between about 1 to
100 ~m, and more preferably still between about 2 to 30 ~m.
Such a composition may suitably be used in any acid
treatment of a well where it is desirable to prevent contact
between the acid and the tubing and pumping equipment. Such a
composition also serves to preserve the acid solution and prevent
its dilution or breaking down before contact with the formation.
The composition may preferably be used in an acid treatment
by mixing the composition with a carrier fluid such as, for
example, water, methanol, or mixtures thereof, and pumping the
mixture into the formation to be treated. The acid is
encapsulated and therefor does not contact the tubing or pumping
equipment, thereby avoiding the aforesaid corrosion. Further,
the formation temperature heats the mixture to the point where
the polymeric material ruptures to free the acid solution. After
the fluid including composition has been pumped into the
formation for the desired time, the fluid and composition is
preferably pumped out of the formation and tubing, thus leaving
the formation enhanced or stimulated, and capable of improved
production.
210271~9
92-369
It should be noted that the composition of the present
invention may also suitably be used to neutralize a basic
solution. In such a use, the composition may be mixed with the
solution, suitably in an amount sufficient to provide a
neutralizing amount of acid. The mixture can then be heated to a
temperature exceeding the rupture temperature of the composition
so as to release the acid solution, allowing same to mix with the
basic solution and thereby neutralize the basic solution.
According to the invention, the encapsulated acid composi-
tion is prepared as follows.
First, an emulsion is formed having a continuous phase of
oil and a dispersed phase of an aqueous acid solution. The emul-
sion may preferably be formed having a ratio of oil continuous
phase to aqueous acid solution dispersed phase of between about
95:5 to 70:30.
The continuous oil phase may be any oil solution which forms
a stable emulsion with the aqueous acid solution and which does
not interfere with the subsequently described cross-linking step.
A suitable oil may be, for example, any isomer of xylene.
The dispersed aqueous acid solution phase is an aqueous
solution of the appropriate acid. The type and concentration of
the acid are as set forth above.
A surfactant may be used, if desired or necessary, in order
to provide a stable emulsion. Such a surfactant may be selected
from known surfactants.
2102~9
92-369
The emulsion is preferably formed so as to provide a droplet
size of the dispersed aqueous acid solution of between about 0.1
to 100 ~m. Such a droplet size facilitates the overall desired
particle size of the end product.
The emulsion is then mixed with a number of materials, as
follows, so as to form encapsulating material in the continuous
phase of the emulsion, surrounding the dispersed aqueous acid
solution droplets.
The emulsion is initially mixed with a polymer material. To
facilitate the formation of encapsulated droplets of aqueous acid
solution, the polymer is preferably oil soluble, particularly
soluble in the oil used as the continuous phase. Further, the
polymer material is preferably insoluble in the aqueous acid
solution. Since the polymer material is to form the encapsulant
of the aqueous acid solution, the polymer material is preferably
chemically resistant to the acid of the aqueous acid solution,
and is substantially impervious to acid and to water. Suitable
polymer and cross-linked polymer material has been set forth
above. The polymer material is preferably used in amounts
sufficient to provide a ratio of polymer to acid in the emulsion
of between about 0.1:1 to 4:1. More preferably, the weight ratio
is between about 0.2:1 to 2:1.
Suitable cross-linking agents have been set forth above.
The cross-linking agent is preferably added in amounts, by weight
of the polymer, of between about 3 to 20%.
- 2 ~ ~27 69
92-369
The cross-linking between the polymer and the cross-linking
agent may be expedited or facilitated through the use of a
promoter selected for such purpose. Suitable promoters include
trimethylaminomethylphenol and dimethylaminomethylphenol, here-
inafter referred to as DMP-30 and DMP-10, respectively. The
promoter is preferably added in amounts, by weight of the poly-
mer, of between about 2 to 10%.
The solution is then allowed to settle. Spheres which
contain the aforedescribed encapsulated aqueous acid composition
may then be recovered.
The preparation of the composition of the present invention
is further illustrated by the following examples.
EXAMPLE 1
This example demonstrates the preparation of an aqueous acid
solution in oil emulsion.
3 g of surfactant (Pluronic L122) were mixed and dissolved
with 80 g of xylene. 20 g of a 10% (wt.) hydrochloric acid
aqueous solution was added to the xylene mixture and stirred to
form a water in oil emulsion, wherein the water contains
hydrochloric acid.
EXAMPLE 2
This example demonstrates the encapsulation of the aqueous
acid solution of the emulsion.
Trade Mark
A
~,
2102769
92-369
8 g of epoxy resin were mixed with 0.78 g of diethylene-
triamine (cross-linking agent) and 0.48 DMP-30 (reaction
promoter) and dissolved in 20 g of xylene. This mixture was
added to the emulsion of Example 1 under stirring and let react
to form an encapsulating coating around the emulsion dispersed
droplets to form microcapsules. After the reaction finished, the
product collected consisted of spherical particles having a core
of hydrochloric acid solution encapsulated by the epoxy resin.
EXAMPLE 3
This example demonstrates the temperature resistance of the
encapsulated aqueous acidic solution.
A sample of dry powder (spherical particles) was analyzed by
differential scanning calorimetry to determine the rupture tem-
perature of the capsules. The heat was increased 25~C/min from
40 to 250~C. The capsules exhibited a rupture temperature of
200~C.
Thus disclosed is a novel composition which allows acid
treatment of hydrocarbon producing formations without subjecting
well tubing and pumping equipment to harmful corrosive effects of
the acid, as well as a process for preparing and a process for
using such a composition.
This invention may be embodied in other forms or carried out
in other ways without departing from the spirit or essential
- 2102769
92-369
characteristics thereof. The present embodiments are therefore
to be considered as in all respects to be illustrative and not
restrictive, the scope of the invention being indicated by the
appended claims, and all changes which come within the meaning
and range of equivalency are intended to be embraced therein.
