Note: Descriptions are shown in the official language in which they were submitted.
$~ 32
The invention relates to the acid treatment of sub-
terranean earth formation/ and more particularly to the acid
treatment of siliceous subterranean format:Lons surrounding oil
wells, gas wells, water injection wells, and similar boreholes.
Acid treatment or acidization is a well known method
of stimulating oil producing and gas producing formations to
increase the production rates of oil and gas therefrom, and to
facilitate the ease with which fluid such as water, brine or
gas can be injected into subterranean ormations surrounding
a well ~ore. Acidization of siliceous formations, e.g., sand-
stone, shale, serpentines, etc., has met with some favorable
results when the formation is treated with hydrogen fluoride.
Various modifications of this hydrogen fluoride acidization
have been disclosed in the prior art. The acid compositions
employed in these prior art methods have mainly consisted of
various mixtures of hydrogen ~luoride and various other mineral
acids such as orthophosphoric acid, fluorophosphoric acid,
sulfuric acid, ~ydrochloric acid, and the like. Although such
mixtures are generally effecti~e, experience has shown that many
formations do not respond to the acid treatment. One difficulty
with the above con~entional acidizing techniques occurs when the
acid solution becomes spent and precipitates solid materials
which plug the pores of the producing formation.
~/ \
In general, calcareous and siliceous formations are
of a heterogeneous nature and contain a number of constituents
such as iron, aluminum, alkali and alkaline earth metals. As
a result, a problem common to all methods of acidizing is the
production of precipitates within the formation interstices
through the action of the acidic-treating reagent or its bypro-
ducts on some precipitate-forming constituent of the formation.
As noted above, acidizing techniques have previously employed
mixtures of phosphoric acid, generally referred to as ortho-
phosphoric acid, with other mineral acids. However, the ortho-
phosphates of polyvalent or heavy metals are all virtually insol-
uable in water. For example, calcium and magnesium compounds
are ~ound in all producing formations and when attacked by
phosphoric acid mixtures form insoluble phosphates. The calcium
and magnesium phosphates are especially difficult to remove and
require expensive procedures to revitalize a producing forma-
tion damaged in this manner. Hence there exists a need for an
acidizing technique which avoids the formation of insoluble
precipitates in siliceous formations as the acid reacts with
the formation and spends, i.e., is neutralized.
Most previously used acidizing solutions have a
relatively low viscosity. In some acidizing treatments, for
example in fracture acidizing or matrix acidizing where it is
desired that the acid penetrate deeply into the formation via
a few channels rather than penetrating the formation in a
uniform manner, it is advantageous to use a relatively viscous
acidizing solution.
Accordingly, a principal object of the invention is
to provide a method for increasing the permeability of silice-
ous subterranean geological formations.
Another object of the invention is to provide a
method for acidizing a siliceous subterranean geological forma-
tion that minimizes damage to the formation caused by plugging
with insoluble precipitates.
Yet another object of the invention is to provide a
method for increasing the permeability of siliceous subterranean ~ !
geological formations containing calcareous and other mineral
constituents.
Still another object of the invention is to provide
a method for acid fracturing a siliceous subterranean geological
formation that minimizes damage to the formation caused by ;
plugging with insoluble precipitates.
A further object of the invention is to provide an
improved acid composition for treating siliceous geological
formations.
A still further object of the invention is to provide
an improved acid composition for treating siliceous geological
formations that does not form insoluble precipitates upon
reaction with polyvalent metal cations in the formation.
Yet another object of the invention is to provide an
improved acid composition for treating siliceous geological
formations containing calcareous and other mineral constituents.
An additional object of the invention is to provide
a viscous acidizing composition.
Other objects and advantages of the invention will be
apparent from the following disclosure.
The drawing is a graph illustrating the relationship
of th~ polymeric P2O5 content of the phosphoric acid ingredient
of the acid composition as a function of the mole ratio of
H2O/P2O5.
-3- ;
This invention provides an acid composition compris-
ing a substantially anhydrous liquid mixture containing about
50 to 99 parts by weight of polyphosphoric acid having about
5 to 9Q percent of the total P2O5 present as polymeric P2O5,
about 1 to 25 parts by weight of hydrofluoric acid, 0 to about
50 parts by weight of a catalyst selected from the group con-
sisting of (a) strong mineral acids selected from the group
consisting of sulfuric, nitric and hydrochloric acids and mix-
tures thereof; (b) unsubstituted aromatic and unsubstituted
aliphatic monocarboxylic, dicarbyoxylic and tricarboxylic
acids having from 1 to about 6 carbon atoms and chlorine sub-
stituted derivatives thereof; and (c) oxidizing agents selected
from the ~roup consisting of organic and inorganic peroxides,
a compound containing the permanganate ion, the hypochlorite
ion, the perchlorate ion or the chlorate ion, and a compound
containing the chromium ion having valence of 61 and wherein
the EI2O/P2O5 mole ratio of the overall acid mixture in less
than 3.4.
The invention preferably relates to an acid composi-
tion cornprising a substantially anhydrous liquid mixture con-
taining about 60 to 95 parts by wei~ht of polyphosphoric acid
having about 40 to 86 percent of the total P2O5 present as
polymeric P2O5, about 2 to 8 parts by weight of hydrofluoric
acid, about 2 to ~0 parts by weight of a catalyst selected from
the group consisting of (a) strong mineral acids selected from
the group consisting of sulfuric, nitric and hydrochloric acids
and mixtures thereof; (b) unsubstituted aromatic and unsub-
stituted aliphatic monocarboxylic, dicarboxylic and tricar-
boxylic acids having from 1 to about 6 carbon atoms ancL chlorine
substitu-ted derivatives thereof; and (c) oxidizing agents se-
lected from the group consisting of organic and inorganic
,,; ., I
$~ ~
peroxidesl a compound containing the permanganate ion, the
hypochlorite ion, the perchlorate ion or the chlorate ion,
and a compound containing the chromium ion having valence 0~ !
6, and wherein the ~20/P205 mole ratio of the overall acid
mixture in less than 3.4.
An embodiment of the invention provides a subterranean
geological formation penetrated b~ a well bore which comprises
introducing through said well bore and into the formation
surrounding said well bore a substantially anhydrous liquid
acid composition comprising a mixture of about 50 to 99
parts by weight of polyphosphoric acid, said polyphosphoric
acid containing about 5 to 90 weight percent of the total ~:~
P205 present as polymeric P205; about 1 to 25 parts by
weight of hydrofluoric acid; O to about 50 parts by weight
o a catal~st selected from the group consisting o~ (a)
strong mineral acids selected from the group consisting of
sulfuric, nitric and hydrochloric acids and mixtures thereof;
(b) unsubstituted aromatic and unsubstituted aliphatic
monocarboxylic, dicarboxylic and tricarboxylic acids having
from 1 to about 6 carbon atoms and chlorine substituted
derivatives thereof; and (c) oxidizing agents selected from
the group consisting of organic and inorganic peroxides, a
compound containing permanganate ion, the h~pochlorite ion,
the perchlorate ion or the chlorate ion, and a compound :~
containing the chromium ion having a valence of 6 and
wherein the H20/P205 mole ratio of the overall acid mixture
is less than 3.4.
A further embodiment of the invention provides a
method for increasing the permeability of a siliceous sub-
terranean geological formation penetrated by a well borewhich comprises introducing through said well bore and
into the formation surrounding said well bore a highly
5-
viscous substantially anhydrous liquid acid composition com-
prising a mixture of 60 to 95 parts by weight of polyphos-
phoric acid, said polyphosphoric acid containing about 40 to
86 weight percent of the total P2O5 present as polymeric P2O5;
about 2 to 8 parts by weight of hydrofluoric acid; about 2 to
40 parts by weight of a catalyst selected from the group consis-
ting of (a) strong mineral acids selected from the group consis-
ting of a sulfuric, nitric and hydrochloric acids and mixtures
thereof; (b) unsubskituted aromatic and unsubstituted aliphatic
monocarboxylic, dicarbo~ylic and tricarboxylic acids having
from 1 ~o about ~ carbon atoms and chlorine substituted deriva-
tives thereof; and ~c) oxidizing agents selected rom the group
consisting of organic and inorganic peroxides, a compound con-
taining permanganate ion, the hypochlorite ion; the perchlorate
ion or the chlorate ion, and a compound containing -the chromium
ion having a valence of 6 and wherein the H2O/P2O5 mole ratio
of the overall acid mixture is less than 3.~.
The acid composition can be utilized in both matrix
acidizing and acid fracturing procedures, and also has utility
in many other varied applications such as the drying of gases,
extraction of m~kals from ores, the treatment of metals, the
manufacture of fertilizers and in removing silica deposits
from any surface, e.g., from khe surfaces of steam boilers and
pi.pes.
!
The general reactions involved in the attack of the
substantially anhydrous liquid polyphosphoric-based acid compo-
sitions of this invention upon siliceous compounds is expressed
by the Eollowing equation:
H4P207 ~ Si2~ ~ j~~ I+ ~ 4 ~ siP4T+ ~l4P27 + ~2
\si '
_ / ~ _
In the ~irst s~ep, a phosphosilica-te complex is
formed. Under anhydrous condikions the soluble phosphosilicate
compLex reacts with hydrofluoric acid to produce a yas, silicon
ketrafluoride and to regenerate polyphosphoric acid and water.
The overall concept of this invention is that the acid mixture
rapidly dissolves the silica and complexes the other metals.
Polyphosphoric acid mixtures having a mole ratio of water to
phosphorous pentoxide (H20/P205) of between about 2.1 to 3.4,
and particularly between about 2.2 and 2.8, form soluble com-
plexes with most cations. Furthermore, the polyphosphate com-
plexes are stable after neutralization. The in situ formation
of gaseous silicon tetrafluoride provides the additional benefit
of sweeping and carrying undissolved solids through the forma-
tion. Excess polyphosphoric acid is required to remove the
ambient and produced water in order to keep the system in an
anhydrous condition, i.e., maintaining the mole ratio o-f water
to phosphorus pentoxide in the overall acid mixture below 3.4.
The composition of the acid mixture employed in
carrying out this invention will depend upon its ultimate use.
In the treatment of subterranean formations, the composition
of the acid mixture will depend largely upon -the particular
--6--
type of formation to be acidized. In predominately siliceous
geological formatio~s containing sandstone, shale, or other
siliceous rock compositions, acid mixtures are employed which
comprise between abou~ 50 to 99 parts by we.ight of polyphos-
phoric acid having about 5 to 90 weight percent of the total
P2O5 present as polymeric P2O5, about 1 to 25 parts by
weight of hydrofluoric acid, and optionally 0 to about 50 parts
by weight of a catalyst selected from the group consisting of
strong mineral acids, carboxylic acids and oxidizing compounds,
and in which the H2O/P2O5 mole ratio of the overall acid
mixture is between about 2.1 and 3.4.
The preferred acid compositions employed in treat-
ing siliceous formations comprise about 60 to 95 parts by
weight of substantially anhydrous liquid polyphosoric acid
having about ~0 to 86 weight percent of the total P2O5 present
as polymeric P2O5, about 2 to 8 parts by weight of hydrofluoric
acid, and optionall~ about 2 to 40 parts by weight of a catalyst
selected from strong mineral acids, carboxylic acids, and
o~idizing compounds, and in which the H2O/P2O5 mole ratio
is between 2.2 and 2.8.
In mixed formations, i.e., formations containing some
calcareous materials and particularly those containing less
than 15 percent calcareous materials in admixture with siliceous
materials, it is preferred that the substantially anhydrous
liquid acid composition also contain hydrochloric acid. The
compositions employed in these formations preferably contain
about 50 to 99 parts by weight of a substantially anhydrous
liquid polyphosphoric acid having about 5 to 90 weight percent
of the total P2O5 present as polymeric P~O5, and more preferably
about 60 to 95 parts by weight of polyphosphoric acid having
about 40 to 86 weight percent of the total P2O5 pre-
7-
sent as polymaric P2O5; about 1 to 25, and more preferably
about 2 to 8, parts by waight of hydrofluoric acid; and about
1 to 50, and more preferably about 2 to 30, parts by weight
of hydrochloric acid. These compositions exhibit an ~2O/P2O5
mole ratio of less than 3.4, and preferably about 2.2 to 2.8.
The hydrofluoric acid component may be prepared
in situ by adding crystalline ammonium bifluoride to hydro-
chloric acid. The hydrogen chloride reacts with the bifluoride
sal~ to form hydrogen fluoride. The more salt added, the greater
will be the hydrogen fluoride concentration and the lower will
be the hydrogen chloride concentration. Other preparative
methods, including the mixing of hydrofluoric and hydrochloric
acid solutions, can be employed. The use of such mixed acids
is generally preferred.
Polyphosphoric acid is a generic term used to deine
the phosphoric acids having less water of constitution than
orthophosphoric acid. Orthophosphoric acid contains one atom
of phosphorus per molecule and has a theoretical mole ratio
of water to phosphorus pentoxide of 3.0 or greater. Polyphos-
phoric acids have two or more atoms of phosphorus in a chainor ring structure in alternating sequence with oxygen, and a
theoretical mole ratio of water to phosphorous pentoxide of
less than 3. Polyphosphoric acid has two general forms, the
acyclic and cyclic. The latter is commonly referred to as
metaphosphoric acid. In the acyclic form, which is derived by
limited molecular dehydration of orthophosphoric acid, the
individual chains of phosphorus and ox~gen atoms have terminal
ends and a theoretical mole ratio of water to phosphorus pen-
toxide of between 2 and 3. In metaphosphoric acid, which is
derived from the acyclic form by continued molecular dehydra-
tion, the chain i6 endless, forming rin~ structures. Meta-
phosphoric acids have theoretical mole ratios o~ water to
phosphorus pentoxide of 2 or less. However, in some cases it
,.., I
, -8-
is preferred that the concentration or dehydration of the ortho-
phosphoric acid is stopped before the meta species begins to
form. The reason is that the acylic form o~ polyphosphoric
acid is a much better complexing agent for alumlnum and transi-
tion metals like iron, cobalt, nickel, copper, zinc, etc.
Therefore, in geological ~ormations which contain substantial
amounts of compounds of the aforementioned metals, a polyphos-
phoric acid-based acidizing mixture with little or no meta
polyphosphoric acid present would be most effectlve. Thus,
the preferred acid compositons exhibit H20/P205 mole ratios
above about 2.
The substantially anhydrous polyphosphoric acid
component of the acid mixture of this invention may be pre-
pared from either furnace acid or wet process acid. The
various components are introduced into a suitable vessel with
agitation or stirring preferably in a closed vessel or system.
Open vessels are provided with a cooling means to avoid fum-
ing ~apors which are generated by the exothermal mixing of
-the acid components. The composition of this invention can
be obtained by any suitable method depending on the source
materia]s used. For example, a dilute wet-process phosphoric
acid is processed to polyphosphoric acid by the addition of
dilute, concentrated, or fuming sulfuric acid followed by con-
centration of the mixture through any suitable step, such as
evaporation of water or by the addition of anhydrous phos-
phorus pentoxide and anhydrous hydrofluoric acid. When a
polyphosphoric acid having an ~20/P205 mole ratio of less than
2.6 is used, it is preferred to add concentrated (98% strength)
sulfuric acid to avoid dilution of the P205 content. On the
other hand fuming sulfuric acid, sulfur trioxide, and/or hydro-
fluoric acid can be added to polyphosphorîc acid to obtain the
proper percent of the other acids in the mixture. It is noted
from the drawing that poly acid begins to form in the eguili-
brated acid at a mole ratio of water to P2O5 about 3.6, i.e.,
an acid containing about 95 weight percent orthophosphoric acid
and still containing about 5 weight percent uncombined water.
Although this composition has some free water, the acid is
herein referred to as a substantially anh~drous acid since it
is anhydrous in a sense that it has reached its maximum concen-
tration of orthophosphoric acid and further concentration
increases the poly acid content.
The total P2O5 content of the substantially anhydrous
polyphosphoric based acid mixture is determined by diluting a
representative sample with water, adding perchloric and nitric
acids and boiling the mixture to convert all forms of phosphoric
acid to orthophosphoric acid. Samples are then passed over a
cationexchange resin to replace the metal cations with hydrogen
as these cations will interfere with subsequent analysis. The ;~
ion exchanged sample is thereafter titrated with a strong base
through two break-points, the first of which corresponds to
the neutralization of the strong acids present, hydrochloric,
nitric, etc., and the most strongly ionized hydrogen of the
orthophosphoric acid. The second break-point in the titra- ;~
tion curve occurs at a pH of about 9.S to 10 and corresponds
to neutralization of the second less strongly ionized hydrogen
o~ the orthophosphoric acid. The difference in titer between
these break-points corresponds to the total phosphate present
which is reported as total P2O5.
The water content of the acid existing as water of
constitution and water of dilution is determined by placing a
weighted portion of the acid in a crucible with zinc oxide in
excess of that needed to react with the acid. The crucible is
then weighed, dried at 100 C. for one hour and placed in an
--10~
5~
oven at 500 C. for an hour. The loss in wei~ht corresponds
to the total water present in the acid mixture.
To determine the amount of orthophosphoric acid
present, various analytical techniques can be employed. Regard-
less of the analytical method employed, prior thereto, the acid
sample is prepared by dilution with water, and then acidifying
it with concentrated sulfuric or nitric acid, followed by fur-
ther dilution. Care should be taken to avoid elevated tempera-
-tures and the sample preparation should be done in an ice bath
to avoid hydrolysis of the polyphosphoric acid. The resultant
solution is passed over a strong acid, cation exchange resin,
e.g., ~mberlite IR-120~I*, to remove the metallic cation impuri-
ties which inter~ere with subsequent analysis. Immediately
after passage over the resin, the acid should be neutralized to
a p~I of about 3.5 to about 6.0 to reduce the tendenc~ of poly
phosphoric acid to hydrolyze. The acid is thereafter titrated
to the break-point, falling at a p~I bet~een about 9.5 and 10,
corresponding to the neutralization of the second ionized
hydrogen of the orthophosphoric acid. Thereafter an excess
of a silver nitrate solution is added to precipitate silver
orthophosphate and release the third, very weakly ionized hy-
drogen ion of the orthophosphoric acid. The resultant solution
is then titrated to determine the amount of hydrogen ion re-
leased in the silver precipitation, and this titer value corres-
ponds to the amount of orthophosphoric acid present in the
sample which is reported on a P2O5 basis.
The amoun~ of phosphorus pentoxide existing in the
form of polyphosphoric acid can be determined by the difference
between the total P2O5 present and that existing as orthophos-
phoric acid. When, however, the polyphosphoric acid is pre-
*Trademark
--11--
sent in low concentrations, constituting 5 percent or less ofthe total P2O5 content, it is preferred to analyze for the poly-
phosphoric acid directly, by an ion exchange chromatoyraphy
method such as described by Peters and Rieman in Analytica
Chimica Octa, 1~, page 131 and by Weiner in Journal American
Oil Chemist Soc_et~, 34, page 124.
Catalytic agents which can be used to catalyze the
above described ~eneral reaction are strong mineral acids,
organic carboxylic acids, oxidizing compounds or mixtures
thereof. These catalysts can be employed in concentration
ranges of 0 to about 50 parts by weight and preferably in
the range of about 2 to 40 parts by weight.
Strong mineral acids such as swlfuric, nitric,
perchloric and hydrochloric acids or mixtures thereof can be
used. One drawbac~ with using sulfuric acid as a catalyst in
acid mixtures for treating hydrocarbon formations is the pos-
sibility of sludge formation due -to sulfuric acid attack on
formation hydrocarbons. However, for the other utilities men- ;
tioned above, sulfuric acid is preferred for, in addition to
the catalytic effect, it aids in dehydration and depresses
the freezing point of polyphosphoric acid to yield a final
product having a freezing point of less than about 30 F.,
thereby insuring that the mixture is liquid at ambient tempera-
tures. Furthermore, sulfuric acid has an additional and sur-
prising effect on the viscosity of the phosphoric acid for it
reduces -the acid viscosity by 50 to 75 percent at concentrations
of about 20 to 40 weight percent based on 100 percent strength
sulfuric acid, thereby allowing the use of a polyphosphoric
acid with a lower mole ratio of water to phosphorus pentoxide.
Suitable or~anic carboxylic acids useful as catalysts
in the above-described reaction are those that form water 901u-
ble or acid-soluble salts of alkali metals and alkaline earth
- -12-
~.~
metals. Aromatic and aliphatic monocarboxylic, dicarboxylic
and tricarboxylic acids having from 1 to about 6 carbon atoms
can be used. The carboxylic acids can be saturated or unsatu-
rated and substituted or unsubstituted. ~he most common sub-
stituent is the chloride ion. For example, benzoic, formic,
acetic, chloroacetic, peracetic, trichloroacetic, citric,
oxalic and maleic acids or mixtures thereof are operable.
Oxidizing agents which can be employed include organic
and inorganic peroxides, a compound containing the permanganate
ion, the hypochlorite ion, the perchlorate ion or the chlorate
ion, or a compound containing ~he chromium ion having a valence
of 6. Specific compounds which may be used are, for example,
benzoyl peroxide, hydroyen peroxide, potassium permanganate,
potassium perchlorate, chromic acid, potassium chromate or
mixtures thereof.
Other additives such as acid inhibitors are not
normally required. For example, at temperatures below 160 F.
acid inhibitors are not necess~ry. However, if additives are
employed, they should be compatible with the acid mixture.
Suitable inhibitors above this temperature may include inor-
ganic arsenic compounds and acetylenic alcohols, thiophenols,
heterocyclic nitrogen compounds, substituted thioureas, rosin
amine derivatives, quakernary ammonium compounds and similar
organic agents.
The novel acid mixtures of this invention can be
prepared by mixing the components in suitable vessels. It is
preferred that these vessels are vented and provided with cool-
ing means to avoid fuming vapors since the mixing reaction is
exothermic. Generally, the order of addition involves first
adding the substantially anhydrous liquid phosphoric acid to
the mixing vessel and the other components can be added in any
order except that the hydrofluoric acid is introduced last.
-13-
The substantially anhydrous li~uid phosphoric acid
based mixtures of this invention can be used in both matrix
acidizing and fracture acidizing. In matrix acidizing, -the
method of this invention is carried out by injecting the acid
solution to be used into the producing formation surrounding
the well bore. The injec-tion pressure is kept below that
necessary to fracture the formation so that penetration of the
acid into the formation matrix occurs. The injection rate
selected should be generally sufficient to keep the pressure
below that necessary to fracture the formation. The acid mix-
ture of the invention has a high solubility for siliceous for-
mations resulting in products which are either solubilized or
chelated in the form of low viscosity solutions. After the
acid mi~ture has remained in contact with the exposed formation
surfaces for a time sufficient to react therewith and to enlarge
the formation passages, the low viscosity reaction effluent is
flushed from the formation. Generally a spacer fluid such as
a low-boiling, low aromatic-containing aliphatic hydrocarbon,
e.g., diesel oil, jet fuel, etc., is injected followed by the
injection of an afterflush fluid such as filtered crude oil,
low calcium containing water, etc. Injection of the afterflush
fluid displaces the spacer fluid and the low viscosity reaction
effluent and is continued until the desired quantity is intro-
duced. The well may be returned to production as soon after
the afterflush has been injected as is practicable.
The acid mixtures of this invention exhibit high vis-
cosities under most reservoir conditions and are particularly
useful in fracture acidizing; which treatment, due to much
lower fluid loss, pxomotes the formation of larger fractures
and greater penetration than do the conventional fracturing
techniques. The P2O5 content of the acid mixture has the
greatest influence on viscosity. The viscosity generally
-la-
~3~b P~
ranges from about 500 to about 2,000 centipoises. Additional
benefits derived from the high viscosity charactexistics of
the acid mixtures of this invention are that gelling agents
need not be added to the acidizing mixture, and the use of
diverting agents in the acidizing operation may be avoided.
~onventional fracture acidizing equip~ent may be used in this
operation. ~s mentioned above, because of the high viscosity
characteristics, the acid mixtures of this invention can func-
tion as both the fracturing fluid and the acidizing reagent.
Conventional propping agents can be used. In some instances,
it is desirable to employ a graded sand of uniform spherical
granular configuration such as a 20-40 mesh silica sand. This
sand is retained within the fractured crevices after the acid
mixture has been flushed therefrom and functions as a propping
agent to retain the formation in a fractured condition.
Through this use of sand, a temporary propping condition is
attained inasmuch as the subsequent introduction of the acid
mixture, capable of reacting with the siliceous componen-ts of
the formation, will react with the siliceous propping agents
resulting in their substantial disintegration. EIowever, even
functioning in a temporary capacity, the siliceous propping
agents serve a useful purpose in retaining the formation in a
fractured condition and thus facilitate the deeper penetration
of subsequently injected acid mixtures into the fractured
formation.
The invention is further illustrated by the following
examples which are illustrative of specific modes of practicing
the invention and are not intended as limiting the scope of the
invention as defined by the appended claims.
Example 1
The acid composition is prepared by adding 6,3~0
gallons of polyphosphoric acid (76~ P2O5, 50% polymeric P2O5)
~15-
to a stainless steel mixing tank equipped with a stirrer and
a circulating water jacket. With slow stirring, 470 gallons
of hydrofluoric acid (70% concentration) are added. The mix-
ture is cooled to remove the heat of reaction. The acid mix-
ture contains 94.1 weight percent polyphosphorlc acid contain-
ing 50 weight percent of the total P2O5 present as polymeric
acid and 6.9 weight percent of the hydrogen fluoride (70%
aqueous solution), and in which the H2O/P2O5 mole ratio in the
overall acid mixture is 2.7. The specific gravity of the acid
mixture is 1.88 at 79 F. Twenty-four tons (4 six-ton batches)
of the acid composition were prepared, shipped to the well site
and stored for about one week.
Example 2
A waste water disposal well designed for the disposal
of 30,000 barrels per day of water is drilled. When completed,
the well will only -take about 1,~00 barrels per day of water
a-t a 200 pound per square inch injection pressure measured at
the surface. The well is treated with a conventional acid
treatment utilizing a hydrochloric-hydrofluoric acid mixture.
The volume rate of injection only increased to 2,160 barrels
per day.
The treatment according to this in~ention consists of
first stopping the injection of waste water, backwashing the
disposal well, and then sampling several times by gas lifting
the first, middle and last fluids produced. The well is back-
washed again until the water becomes clean. Five hundred gal-
lons of the substantially anhydrous liquid phosphoric acid
based mixture described in Example 1 are injected at a maximum
surface tubing pressure of 350 psi and a maximum of 180 psi on
the casing. The next 2,000 gallons are injectsd a~ ]/2-barrel
per minute and at 50 psi at the wellhead and 300 psi on the
casing. A last slug of 1,100 gallons i 5 started at the rate of
1/2-barrel per minute at vacuum and is reduced to 1/4-barrel
per minute after the first 300 gallons are injected. After the
acidizing treatment, the acid i5 displaced to total depth with
300 barrels of filtered lease water. First injection rates are
measured after injecting about 30 barrels of filtered lease
water at a wellhead pressure of 340 psi and at a rate of 13-1/2
barrels per minute. During the next injection of about 300
barrels of filtered lease water, the injection rate through
the casing increases to 14 barrels per minute and the injec-
tion pressure gradually decreases to 18Q psi at the wellhead.
The injection of lease water was never interrupted and is
continued at l/2-barrel per minute while the well was switched
to the ~ield injection system. The first rate recorded was
18,000 barrels per day, increasing rapidly to 22,000 barrels
per day at 115 psi surface pressure.
Example 3
This example illustrates the use of the method of
th~ invention in fracture acidizing a subterranean ~il pro-
ducing formation. A production well is completed in a rela-
tively shallow reservoir haviny a temperature of 130~ F. and
with a total productive interval of 15 ~eet per~orated with
two holes per foot at depths of 2,903 and 2,908 feet and
2,918 to ~,92~ feet. The initial fracture is formed by the
following treating fluid injections:
(1) A spearhead injection of 3,000 gallons of
the substantially anhydrous liquid phosphoric acid
based mixture prepared in accordance with the pro-
cedure described in Example 1 is pumped into the
formation under fracturing pxessure. The initial
portion of the acid fracturing fluid is injected
at a pressure of 2,000 p.s.i.g. After fracturing
the injection pressure dropped to 600 p.s.i.g. and
the well is placed on ~acuum. ~ ~-
(2) An additional injection of 5,000 gallons
of 40 gravity lease crude containing about 4 pounds~ -~
per gallon of small, solid particles of sand sus-
pended therein is injected at a pressure of 2,000
p.s.i.g., the injection pressure dropped to 600
p.s.i.g. and the well placed on vacuum.
(3) 3,000 gallons of another slug of the
substantially anhydrous liquid phosphoric acid
based mixture fracturing fluid is injected at
an initial pressure of 2,000 p.s.i.g., the in-
jection pressure dropped 600 p.s.i.g.
A second stage fracture is formed by repeating the
above fluid in~ections. Fluid injection rates average 16 to 18
barrels per minute at well head injection pressures of between
1,800 and 2,200 p.s.i.g. A third stage fracture is formed by
repeating these injections at flow rates and pressures compara-
ble to those encountered in the second stage fracturing. The
acid fracturing fluid is flushed from the tubing into the forma-
tion, and the well is returned to service in conventional manner.The production rate of oil is observed and the increase is found
to be about four fold over the 29 barrels per day production
rate prior to fracturing. This indicates that the fracturing
operation is successful.
Example 4
Another acid composition is prepared by adding 69.50
pounds of polyphosphoric acid (76% P2O5, 50% polymeric P2O5)
to a stainless ~teel mixing tank equipped with a stirrer and
a circulating water jacket. With slow stirring, 6.85 pounds
hydrofluoric acid (70% by weight aqueous solution) and 23.65
pounds sulfuric acid (98% by weight aqueous solution) are
added to the mixing tank. The mixture is cooled by circula-
-18-
ting water during the mixing operation to remove the heat of
reaction. The specific gravity of the resulting acid composi-
tion is 1.785 at 79~ F. A total of 3,600 gallons of this acid
composition are prepared and shipped to a Louisiana well site.
E~ample 5
A Louisiana waste water disposal well is designed
to have a 30,000 barrels per day (B/D) capacity for waste
water. When completed, the well will take only about 1,400
harrels per day of water at an injection pressure measured at
the surface of 200 pounds per square inch gauge (p.s.i.g.).
This low capacity of the well for injected water is believed
due to the presence, in the gravel pack opposite the formation
into which the water is injected and in the formation itself,
of plugging material made up principally of finely divided
solids from the clay-containing aqueous drilling mud with
which the well has been drilled. About three months after
being drilled the well is acidized according to established
field practice in an attempt to remove such drilling mud solids
and clean up the well. The acidizing treatment involves se~uen-
tially in~ecting through the gravel pack and into the formation:(a) a 500 gallon batch of inhibited 15 percent by weight hydro-
chloric acid; (b) a 1,000 gallon batch of inhibited aqueous
solution containing 12 percent by weight hydrochloric acid and ~ -
3 percent by weight hydrofluoric acid; and (c) a second 500
gallon batch of the solution described in (a) above. Immedi-
ately following the acidi~ing treatment, waste water can be
injected into the well at a rate of 3,000 B/D using 100 p.s.i.g.
injection pressure. However, this improved injection rate
lasts only a few hours and then drops to 2,160 B/D at an injec-
tion pressure of 200 p.s.i.g. Five months after this treatment
the injection rate is an unsatisfactorily low 4,000 :B/:D at
250 p.s.i.g.
-19-
The well is then given an acidizing treatment accor-
ding to this invention. The well is backwashed by producing
it for several hours while sampling the first, middle and last
fluids produced. At first considerable amounts of oil, bac-
teria and debris are produced along with the water. By the
end of this first backwashing stage, the aqueous fluid pro-
duced is clear. The well is shut in overn:ight. The next day
a second backwashing stage is carried out. At first, consid-
erable amounts of oil, bacteria and debris appear again along
with the water, but the produced fluids clear up within a few
hours. There is injected into the formation through a 2 micron
filter 180 barrels of fresh water containing l,000 pounds sodium
hydroxide and 900 pounds potassium chloride followed by a flush
of 20 barrels o~ fresh water containing lO0 pounds oE potassium
chloride. The purpose of this step is to place sodium hydroxide
in the gra~el pack and surrounding formation. This sodium hydro-
xide reacts exothermically with the subsequently injected acid-
izing solution to raise the temperature of the acidizing solu-
tion to enable it to more readily dissolve the plugging material
in the vicinity of the well. The well is shut in for 40 hours.
The well tubing is displaced by injecting therein 20 barrels of
filtered lease brine. There is injected into the well a 500
gallon batch of the acidizing solution described in Example 4
at a rate of 4 barrels per minute ~B/M) at a maximum surfacP
tubing pressure of 350 p.s.i.g. and a maximum casing pressure
of 180 p.s.i.g. Next, a 2,000 gallon batch of the acidizing
solution of Example 4 is in~ected at a rate of 0.5 B/M at a
maximum surface tubing pressure of 50 p.s.i.g. and a maximum
casing pressure of 30-0 p.s.i.g~ Finally~ a 1,100 gal:Lon batch
of the acidizing solution of Example ~ is injected. This batch
is star-ted at a rate of 0.5 B/M at vacuum and trimmed down to
0.25 B/M a~ter the first 300 gallon portion has been injected.
-20-
~he acidizing solution is displaced out of the tubing and
out into the formation by injecting 300 barrels of lease brine.
The first injection rate, measured after about 30 barrels of
this batch of filtered lease brine has been injected, is
13.5 B/M at a surface pressure of 340 p.s.i~g. Another 300
gallon batch of filtered lease brine is injected. The injec-
tion rate increases to 14 B/M and the injection pressure
gradually decreases to 180 p.s.i.g. Injection of filtered
lease brine is continued at the rate of 0.5 B/M while the
well is switched to the field injection system, i.e., waste
water which has been produced along with oil from other wells
in the field and which is to be reinjected into the formation.
The first injection rate recorded is 18,000 B/D increas.ing
rapidly to 22,000 B/D at 115 p.s.i.g. surface pressure.
The well is used daily for injecting waste water
produced from other wells in the field. The amount of waste
water injected is periodically monitored over a period of
more than two years as follows:
:
Volume Waste Water Injection
Test Injected Pressure
Number (B/D) (p.s.i.
1 13,060 225
2 11,200 200
3 17,680 250 ~ :~
4 12,710 1~0 ~-
11,550 ~oo :
6 15,280 290
7 18,510 150
8 19,370 260
9 22,590 350
19,210 360
11 20,690 ~25
12 21,550 490
13 23,610 375
14 34,590 400
:,
Various embodiments and modifications of this inven-
tion have been described in the foregoing description and
examples, and further modifications will be apparent to those
skilled in the art. Such modifications are included within
the scope of this invention as defined by the following claims.
Having now described the invention, we claim:
