Method for acidizing siliceous materials contained in high temperature formations

An aqueous fluoride salt solution and a substantially anhydrous acid precursor are introduced through a well into a high temperature subterranean formation containing siliceous materials. The acid precursor, which is a normally liquid halogenated hydrocarbon having one or two carbon atoms, hydrolyzes in situ to generate a hydrohalic acid which combines with the fluoride salt to acidize the siliceous materials.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a method for treating a subterranean formation 
penetrated by a well, and more particularly concerns a method for 
acidizing siliceous materials contained in relatively high temperature 
subterranean formations penetrated by a well. 
2. Description of the Prior Art 
Acidization of wells is a well-known process for increasing or restoring 
the permeability of subterranean formations so as to facilitate the flow 
of formation fluids, such as oil, gas or a geothermal fluid, from the 
formation into the well and also to facilitate the injection of fluids 
through the well into the formation. Acidization involves treating the 
formation with an acid, typically hydrochloric acid, in order to dissolve 
clogging deposits, such as carbonate scale, thereby opening pores and 
other flow channels and increasing the permeability of the formation. 
Hydrofluoric acid or a mixture of hydrofluoric and hydrochloric acids, 
commonly known as "mud acid", is typically employed to dissolve siliceous 
materials. 
Numerous acidization methods have been proposed to cope with varying well 
conditions and special formation problems. However, in recent years the 
increased activity in drilling very deep oil and gas wells and geothermal 
wells has outpaced the development of suitable acidization methods, 
primarily due to the high temperatures of these formations. 
A problem common to all the prior art acidization processes is the 
corrosion of the well equipment, particularly the downhole tubing and 
casing, which is exposed to the acidizing fluid. Because the reactivity of 
an acid is significantly increased at higher temperatures, the corrosion 
of well equipment is especially serious in the acidization of high 
temperature formations. 
Corrosion inhibitors are generally incorporated into the acidizing fluid 
prior to its injection into the well. However, the effectiveness of the 
known corrosion inhibitors decreases at higher temperatures, and the 
expense of the corrosion inhibitors, which is significant even at low 
temperatures, becomes prohibitive at temperatures above about 250.degree. 
C. Another difficulty with the known corrosion inhibitors, especially when 
used in the large quantities required in high temperature wells, is their 
tendency to form insoluble solids upon reaction with formation materials, 
thereby damaging the formation. 
Another problem encountered during the acidization of high temperature 
formations is that the acid is rapidly consumed by the reactive material 
immediately adjacent the borehole before the acid can penetrate any 
significant distance into the formation. Without adequate formation 
penetration, the acidization operation is of little value. In view of 
these problems, the prior art acidization methods are limited, as a 
practical matter, to acidizing formations having temperatures on the order 
of 250.degree. F. and less. Thus, there exists a need for a method for 
acidizing high temperature subterranean formations. 
Accordingly, it is a principal object of this invention to provide a method 
for acidizing high temperature subterranean formations. 
Another object of the invention is to provide an acidization method which 
results in no more than an acceptable rate of corrosion of metal well 
equipment. 
Still another object of the invention is to provide a simple but effective 
method for acidizing siliceous materials contained in subterranean 
formations having temperatures on the order of 250.degree. F. to 
700.degree. F. and higher, which method results in little or no corrosion 
of the well hardware. 
Yet another object of the invention is to provide an acidization method for 
high temperature formations which does not require the use of corrosion 
inhibitors or other expensive chemical additives. 
A further object of this invention is to provide an acidization method in 
which noncorrosive, nonscaling, acid precursors are displaced through a 
well and into a high temperature formation, wherein the acid precursors 
react in situ to generate hydrohalic acids. 
A further object of this invention is to provide a method for acidizing 
those siliceous materials contained in portions of high temperature 
formations which are relatively remote from a borehole. 
Another object of this invention is to provide a method for simultaneously 
acidizing and hydraulically fracturing a high temperature subterranean 
formation. 
Still further objects, advantages and features of the invention will become 
apparent to those skilled in the art from the following description. 
SUMMARY OF THE INVENTION 
Briefly, the invention provides an improved method for acidizing siliceous 
materials contained in subterranean formations having temperatures between 
about 250.degree. F. and about 700.degree. F., wherein (1) a substantially 
anhydrous treating fluid consisting essentially of an acid precursor and 
(2) an aqueous fluoride salt solution are introduced through a well and 
into contact with the formation. The injected fluids are displaced from 
the well into the formation wherein the acid precursor hydrolyzes in situ 
to generate a hydrohalic acid which in turn converts the fluoride salt 
solution into a hydrofluoric acid solution. 
The acid precursor is a normally liquid, halogenated hydrocarbon having a 
generalized formula: 
EQU C.sub.x H.sub.y X.sub.z 
wherein 
x=1 or 2; 
y=0, 1 or 2, but y.ltoreq.x; and 
z=2x-y+2, and 
which is thermally stable under the high temperature and pressure 
conditions to which it is exposed prior to hydrolysis. 
The method of this invention is useful in acidizing those subterranean 
formations in which the prior art acidization methods are rendered 
impractical due to the high formation temperatures. The invention provides 
an acidization method for high temperature formations in which corrosion 
of well equipment is substantially eliminated and the undesirable 
consumption of acid by the formation immediately adjacent the borehole is 
avoided. The method can be employed in high temperature formations having 
a large connate water concentration, such as a formation containing an 
aqueous geothermal fluid, or in high temperature formations having little 
or no connate water. The method has the advantage of being operable with 
conventional well equipment and does not require the use of exotic alloys 
or other materials to avoid corrosion of the well equipment. 
DETAILED DESCRIPTION OF THE INVENTION 
The method of this invention is suitable for acidizing siliceous materials 
contained in relatively high temperature formations and finds particular 
utility in acidizing siliceous materials contained in formations having 
temperatures on the order of 250.degree. F. and higher, especially between 
about 250.degree. F. and about 700.degree. F. 
The aqueous fluoride salt solutions suitable for use in the method of this 
invention are noncorrosive aqueous solutions of water-soluble alkali metal 
and/or ammonium fluoride salts. The fluoride salt must be capable of 
dissociating in situ to provide fluoride ions for the in situ generation 
of hydrofluoric acid. Suitable fluoride salts include the water-soluble 
alkali metal and/or ammonium salts of hydrofluoric acid, fluoroboric acid, 
hexafluorophosphoric acid, difluorophosphoric acid and fluorosulfonic 
acid. Preferred fluoride salts include ammonium fluoride, ammonium 
bifluoride, ammonium fluoroborate, ammonium hexafluorophosphate, ammonium 
difluorophosphate, ammonium fluorosulfonate, cesium fluoride, cesium 
bifluoride, cesium hexafluorophosphate, cesium difluorophosphate and 
cesium fluorosulfonate. These ammonium and cesium salts are preferred 
because the ammonium and cesium fluorosilicate salts formed in situ upon 
acidization of siliceous materials are relatively water-soluble as 
compared to the corresponding sodium, potassium and rubidium salts. Where 
sodium, potassium or rubidium fluoride salts are to be used, suitable 
precautions known in the art must be taken to avoid excessive 
precipitation of the corresponding fluorosilicate salts. For example, an 
overflush fluid, such as water, may be injected to displace these salts 
away from the vicinity of the well before they are precipitated. 
Aqueous solutions of ammonium fluoride are particularly preferred due to 
their low cost and noncorrosivity, and aqueous solutions of ammonium 
fluoroborate are particularly preferred under circumstances in which the 
fluoroborate anion will serve to fuse movable formation fines and/or to 
desensitize clay particles. However, at very high temperatures, depending 
upon the pressure, ammonia gas may evolve from these aqueous solutions, 
which evolution has the effect of causing the solutions to become 
corrosive. In most cases the ammonia evolution can be controlled by 
pressurizing the fluoride salt solution. Where the ammonia evolution can 
not be controlled or where the ammonium ssalts are otherwise deemed 
unsuitable, the use of the corresponding cesium salt is preferred. 
The concentration of the fluoride salt in the aqueous solution may vary 
widely depending, inter alia, upon the desired hydrofluoric acid 
concentration in the acid solution produced in situ and whether the 
injected fluids will be diluted with connate water and/or water otherwise 
injected into the formation. Where no dilution of the fluoride salt 
solution is expected, the fluoride salt concentration is preferably 
sufficient to provide a fluoride ion concentration between about 0.1 and 
about 25 weight percent, more preferably between about 1 and about 10 
weight percent. Conversely where substantial dilution of the injected 
solution is expected a proportionately higher concentration is needed in 
the injected solution to yield the desired fluoride ion concentration in 
the acid solution produced in situ. 
For obvious reasons, the aqueous fluoride salt solution injected through 
the well must itself be noncorrosive. As used herein, the term 
"noncorrosive" is meant to exclude solutions which cause an unacceptable 
amount of corrosion in the injection equipment. For the purposes of this 
invention, an aqueous fluoride salt solution is noncorrosive if, during 
its passage through the well into the formation, it does not cause an 
unacceptable rate of corrosion of the injection equipment. Preferably, an 
aqueous fluoride salt solution is selected and the injection operation is 
controlled such that the injection of the fluoride salt solution results 
in less corrosion than about 0.05 pounds per square foot. The addition of 
water-soluble buffering agents or corrosion inhibitors to the fluoride 
salt solution is contemplated where necessary to reduce its corrosivity, 
however, care must be taken not to introduce any material which will 
adversely react with or precipitate in the formation. Suitable 
noncorrosive aqueous fluoride salts solutions are known in the art. 
The acid precursor suitable for use in the method of this invention is a 
normally liquid, halogenated hydrocarbon having one or two carbon atoms 
per molecule. More specifically, the acid precursor is a normally liquid 
halogenated hydrocarbon having the generalized formula: 
EQU C.sub.x H.sub.y X.sub.z 
wherein 
x=1 or 2; 
y=0, 1 or 2, but y.ltoreq.x; and 
z=2x-y+2, and 
which is thermally stable under the high temperature and pressure 
conditions to which it is exposed prior to hydrolysis. 
The term "thermally stable" as used herein is meant to exclude compounds 
which spontaneously decompose and/or polymerize under the temperature and 
pressure conditions to be encountered. Halogenated hydrocarbons which 
thermally decompose under the conditions encountered prior to hydrolysis 
are to be avoided since some of the decomposition products, such as 
chlorine, are highly toxic, and other of the decomposition products, such 
as a tar resulting from the pyrolysis of halogenated hydrocarbons having 
three or more carbon atoms per molecule, tend to form plugging deposits 
which are difficult to remove. 
The term "normally liquid" as used herein includes those compounds which 
exist as liquids under the ambient temperature and pressure conditions at 
the well site. In general, a compound which is "normally liquid" for the 
purpose of this invention has a normal melting point less than about 
80.degree. F., preferably less than about 30.degree. F., and has a normal 
boiling point above about 80.degree. F., preferably above about 
120.degree. F. Normally liquid compounds are more easily handled at the 
well site and more easily injected through a well into the subterranean 
formation in the method of this invention. The term "normally liquid" is 
meant to exclude compounds which exist only as a solid or a gas under the 
temperature and pressure conditions to which they will be exposed during 
handling at the well site and introduction into the well. 
The treating fluid injected through a well in the method of this invention 
should consist essentially of the acid precursor or a mixture of acid 
precursors, and not contain any more than a minor amount of other 
materials such as water, hydrocarbons, surfactants or other materials 
which would significantly affect the rate of hydrolysis of the acid 
precursor. The addition of surfactants and/or other hydrocarbon additives 
to the treating fluid retards the rate and degree of hydrolysis of the 
acid precursor and the presence of these compounds at very high 
temperatures often results in the production of pyrolytic products which 
damage the formation. The presence of oxygen-containing compounds, 
including oxygen-containing hydrocarbon solvents, such as alcohols and 
ketones, must be avoided since at high temperatures these compounds are 
often corrosive even in an anhydrous state. 
As discussed above, polymerizable or pyrolyzable compounds must be avoided 
in the acidization method particularly in very high temperature wells. 
Accordingly, unsaturated halogenated hydrocarbons must also be avoided. 
Halogenated hydrocarbons having three or more carbon atoms, and at very 
high temperatures two or more carbon atoms, tend to hydrolyze to form 
polymerizable and/or pyrolyzable side reaction products, such as propylene 
and acetic acid, respectively, and must therefore also be avoided. It is 
also preferable to avoid compounds which are flammable or explodable under 
the temperatures and pressure conditions to which they are exposed during 
handling at the well site. 
The halogenated hydrocarbons having one carbon atom per molecule which are 
suitable for use as the acid precursor in the method of this invention 
include the normally liquid compounds having the general formulas CX.sub.4 
or HCX.sub.3 which are thermally stable under the temperature and pressure 
conditions to be encountered. Suitable compounds of the formula CX.sub.4 
include: tetrachloromethane, fluorotrichloromethane, bromotrichloromethane 
and dibromodichloromethane. Suitable compounds of the formula HCX.sub.3 
include: trichloromethane, tribromomethane, chlorodibromomethane, 
bromodichloromethane, iododibromomethane, chlorodiiodomethane, 
iododichloromethane and fluorochlorobromomethane. 
The halogenated hydrocarbons having two carbon atoms per molecule which are 
suitable for use as the acid precursor in the method of this invention 
include the normally liquid compounds having the general formulas C.sub.2 
X.sub.6, HC.sub.2 X.sub.5 and H.sub.2 C.sub.2 X.sub.4 which are thermally 
stable under the temperature and pressure conditions to be encountered. 
Suitable compounds of the formula C.sub.2 X.sub.6 include: 
1,2-difluorotetrachloroethane, 1,1,2-trifluorotrichloroethane and 
1,1,2-trifluorotribromoethane. Suitable compounds of the formula HC.sub.2 
X.sub.5 include: pentachloroethane, fluorotetrachloroethane, 
fluorotetrabromoethane, difluorotribromoethane, 
1,2-dichloro-1,1,2-tribromoethane, 1,1-dichloro 1,2,2-tribromoethane, 
dibromotrifluoroethane, dibromotrichloroethane and 
fluorodichlorodibromoethane. Suitable compounds of the formula H.sub.2 
C.sub.2 X.sub.4 include: tetrachloroethane (both the symmetrical and 
unsymmetrical isomers), tetrabromoethane (both the symmetrical and 
unsymmetrical isomers), fluorotrichloroethane, 1-fluoro 
1,1,2-tribrompethane, 1-fluoro 1,2,2-tribromoethane, 
difluorodichloroethane, 1,1-difluoro 1,2-dibromoethane, 1,1-difluoro 
2,2-dibromoethane, chlorotribromoethane, 1,1-dichloro 1,2-dibromoethane, 
1,2-dichloro 1,2-dibromoethane, 1,1-dichloro 2,2-dibromoethane and 
bromotrichloroethane. 
Mixtures of the described acid precursors can also be employed either in 
the form of a solution or an admixture. The use of a plurality of discrete 
slugs of different acid precursors, or mixtures of acid precursors, is 
also contemplated and in some cases is preferred, as is described more 
fully hereinbelow. 
The selection of a particular acid precursor for use in the method of this 
invention will depend, inter alia, upon the hydrohalic acid desired, the 
formation material to be acidized, the temperature and pressure conditions 
to which the acid precursor will be exposed prior to hydrolysis, and the 
availability, cost and handling characteristics of the acid precursor. 
In general, the halogenated hydrocarbons having one carbon atom are 
preferred over the halogenated hydrocarbons having two carbon atoms, 
especially at formation temperatures above about 500.degree. F., because 
various side reaction products of the hydrolysis of the halogenated 
hydrocarbons having two carbon atoms, such as acetic acid, can be 
pyrolyzed to form plugging solid residues at these very high temperatures. 
Of the halogenated hydrocarbons having one carbon atom, the acid 
precursors of the formula CX.sub.4 are preferred, and tetrachloromethane 
(i.e., carbon tetrachloride) is particularly preferred due to its ability 
to hydrolyze readily over the temperature range 250.degree. to 700.degree. 
F., as well as its low cost and availability. 
Hydrochloric acid precursors and acid precursors which reach to form a 
mixture of hydrochloric acid and other hydrohalic acids are preferred for 
use in the method of this invention. The preferred hydrochloric acid 
precursors are tetrachloromethane, trichloromethane, pentachloroethane and 
tetrachloroethane, with tetrachloromethane being particularly preferred. 
Preferred acid precursors which hydrolyze to form a mixture of 
hydrochloric and hydrobromic acids are bromotrichloromethane, 
chlorodibromomethane, bromodichloromethane, trichlorodibromoethane, 
1,1-dichloro 1,2-dibromoethane, 1,2-dichloro 1,2-dibromoethane, and 
1,1-dichloro 2,2-dibromoethane. Preferred acid precursors which hydrolyze 
to form a mixture of hydrochloric and hydrofluoric acids are 
1,1,2-trifluorotrichloroethane, fluorotetrachloroethane and 
fluorotrichloroethane, with 1,1,2-trifluorotrichloroethane being 
particularly preferred. 
In the method of this invention, one or more slugs of a treating fluid 
consisting essentially of the acid precursor is introduced through a well 
and into the subterranean formation to be acidized. During its passage 
through the well, the treating fluid must be in a substantially anhydrous 
state to avoid premature hydrolysis and the resulting corrosion of the 
injection equipment. The term "substantially anhydrous" as used herein is 
meant to include treating fluids having not more than a minor amount of 
water. The amount of water which can be tolerated in the treating fluid 
depends primarily upon the temperature to which the treating fluid is 
heated during its passage through the well. For example, at relatively low 
treating fluid temperatures, such as temperatures on the order of 
250.degree. to 300.degree. F., water concentrations of about 10 weight 
percent may be acceptable because the acid precursor and water are 
immiscible and therefore do not hydrolyze readily. However, at relatively 
high treating fluid temperatures, such as on the order of 500.degree. to 
700.degree. F., water concentrations must be less than about 1 weight 
percent due to the accelerated rate of hydrolysis at these temperatures. 
For the purposes of this invention, a treating fluid is "substantially 
anhydrous" when it contains less than the amount of water required to 
cause a significant amount of hydrolysis during passage through the well, 
which significant amount results in an unacceptable rate of corrosion of 
the injection equipment. Best result are obtained when the treating fluid 
is introduced into the well as an ahydrous liquid. 
The introduction of the treating fluid into the subterranean formation can 
be accomplished by a variety of well-known fluid injection methods, 
provided that the acid precursor is not prematurely mixed with water. In 
the method of this invention the substantially anhydrous treating fluid 
and the fluoride salt solution are injected through a well into the 
subterranean formation. Both the substantially anhydrous treating fluid 
and the fluoride salt solution may be introduced through an injection 
tubing, preferably in the form of a plurality of small, discrete 
alternating slugs. Because the acid precursors are relatively insoluble in 
water there will be little mixing and/or hydrolysis during the passage 
through the injection tubing at these temperatures. Alternatively, a slug 
of the substantially anhydrous treating fluid may be injected through a 
water-free injection tubing and the aqueous fluoride salt solution may be 
injected through the well annulus between the injection tubing and the 
walls of the borehole. In either case, the injected fluids are preferably 
displaced from the well into the formation by an inert displacement fluid 
which is injected through the injection tubing and/or well annulus to 
over-displace the mixed acid precursor and aqueous solution into the 
formation. This latter-described procedure is preferred because it 
provides for mixing of the acid precursor and aqueous solution in the 
borehole prior to entry into the formation, and yet, because the 
hydrolysis rate is relatively slow, the hydraulic acid is not produced to 
any significant extent until the reaction mixture has been displaced well 
into the formation. Excess water may be injected prior to the displacement 
fluid to remove any acid precursor remaining in the injection tubing. 
The displacement fluid can be any inert fluid, such as nitrogen or an 
aqueous or oleaginous fluid is noncorrosive and nonplug-forming under the 
conditions encountered in the injection well. Preferred aqueous 
displacement fluids contain ammonium chloride, ammonium iodide, ammonium 
bromide salts or the like which serve to stabilize any water-swellable 
clays in the formation. Preferred oleaginous displacement fluids are the 
solvent refined paraffinic lubricating oil base stocks, known as neutral 
oils and bright stocks, such as are used conventionally in the manufacture 
of lubricating oils for industrial turbines and other machines operating 
at high temperatures. 
In a preferred embodiment of the method of this invention a high 
temperature subterranean formation is hydraulically fractured as the acid 
precursor is hydrolyzing in situ. The technique of fracture-acidizing is 
well known and therefore need not be described more fully herein except 
for the following novel features. In a preferred method of fracture 
acidizing, the substantially anhydrous treating fluid is introduced under 
pressure through the injection tubing and into the subterranean formation 
while an aqueous fluoride salt solution which preferably also contains a 
water-soluble, viscosity-increasing agent is simultaneously injected under 
pressure through the well annulus and, subsequently, through the injection 
tubing to displace the acid precursor into the subterranean formation as 
the formation is being hydraulically fractured. The exposed fracture 
surfaces provide a hot, clean surface for reaction with the in situ 
produced acid. Suitable viscosity increasing agents include the thermally 
stable, water-soluble polymers normally used in hydraulic fracturing, such 
as polyacrylamides and polyvinylpyrrolidones. As is conventional, propping 
agents can be injected to hold the newly formed fractures open after the 
pressure is reduced. 
The factors to be considered in selecting the quantity of acid precursor 
and/or water to be injected in the method of this invention are 
essentially the same as in a conventional acidization operation. By way of 
example, an acid treatment which would conventionally call for the use of 
about 100 gallons of an acid solution containing 12 weight percent 
hydrochloric acid and 3 weight percent hydrofluoric acid per foot of 
perforated interval, requires the injection of about 12.2 gallons of 
tetrachloromethane and, in a relatively water-free formation, about 100 
gallons of a 5.5 weight percent aqueous ammonium fluoride solution. In a 
water-containing formation, the volume of the fluoride salt solution to be 
injected may be reduced and a higher concentration of the fluoride salt 
employed to from the desired concentration of hydrofluoric acid upon 
dilution with the connate water. The design of a particular acidization 
operation using the method of this invention will therefore become obvious 
to those skilled in the art from these well known factors when taken in 
view of this disclosure. 
In a highly preferred embodiment of the method of this invention, a 
preflush fluid is injected prior to the treating fluid and fluoride salt 
solution. The preflush fluid consists essentially of an acid precursor 
which hydrolyzes to form hydrochloric acid, hydrobromic acid, hydroiodic 
acid or a mixture thereof. Suitable preflush fluids include 
tetrachloromethane, trichloromethane, pentachloroethane, 
tetrachloroethane, bromotrichloromethane, chlorodibromomethane, 
bromodichloromethane, trichlorodibromoethane, dichlorodibromoethane, and 
mixtures thereof. The preflush fluid is allowed to hydrolyze in situ and 
react with the formation before the treating fluid and fluoride salt 
solution are injected. The function of the acid produced by hydrolysis of 
the preflush fluid is (1) to provide a low pH environment relatively free 
of cations which would otherwise form insoluble fluoride or fluosilicate 
salts, such as calcium fluoride and sodium fluosilicate, respectively, 
with the subsequently produced hydrofluoric acid, and (2) to consume 
carbonates and other highly reactive, nonsiliceous materials in the 
formation thereby conserving the later-introduced hydrofluoric acid for 
reaction with the siliceous materials to be acidized. 
After the preflush acid has solubilized the nonsiliceous materials, the 
solubilized materials should be removed from the portion of the formation 
which is to be acidized by the subsequently injected fluids. The removal 
may be accomplished by injecting a displacement fluid to overdisplace the 
solubilized materials deep into the formation, or, alternatively, the 
solubilized materials may be produced through the well. 
Whenever the formation to be acidized by a method fo this invention 
contains water-swellable clays, care must be taken to avoid swelling these 
clays, which swelling could result in severely reducing the permeability 
of the formation. Various methods are known for avoiding clay swelling. In 
particular any aqueous solutions injected before or during the acidization 
operation should contain an agent, such as ammonium chloride, to prevent 
clay swelling. 
An inert gas, such as nitrogen, can be added to the fluids injected in the 
method of this invention to aid in the mixing of the acid precursor and 
water in the formation. Between about 50 and about 5,000 standard cubic 
feet of the inert gas per barrel of the injected fluid is preferred, and 
good results are obtained when about 1,000 standard cubic feet of nitrogen 
per barrel of fluid is employed. 
After displacement of the injected fluids into the formation, the well is 
shut in for a preselected time to allow the acid precursor to hydrolyze 
and the in situ-produced acid to be consumed in the desired acidization of 
formation materials. The degree of hydrolysis achieved in situ is 
determined by the length of time the well is shut in. When less than 
complete hydrolysis is achieved, precautions must be taken to handle the 
unreacted acid precursor and any noxious intermediate reaction products. 
After the preselected period, the borehole is preferably flushed with a 
conventional well cleaning fluid, such as water, and the well effluent is 
contacted in a pit or other contacting device with a dilute ammonium 
hydroxide solution for a short time prior to returning the well to its 
normal injection or production operation. 
The degree of hydrolysis achieved in a preselected period of time will 
depend, inter alia, upon the particular acid precursor, and the 
temperature and pressure conditions in the formation. The rate of 
hydrolysis generally increases with increases in temperature and/or 
pressure. In formations having temperatures between about 250.degree. F. 
and about 350.degree. F., at least about 50 percent hydrolysis is desired, 
preferably at least about 80 percent hydrolysis. In these formations, less 
than complete hydrolysis will normally be employed, as a practical matter, 
due to the long shut-in period required for complete hydrolysis. The 
quantity of acid precursor injected must, of course, be larger at these 
lower degrees of hydrolysis in order to provide the same yield of 
hydrohalic acid. In higher temperature formations, substantially complete 
hydrolysis can be achieved within a relatively short time period, such as 
less than 48 hours, and is therefore preferred. The time required for any 
desired degree of hydrolysis can be determined in the laboratory by a 
simple test which is described below in the determination of the time 
required for complete hydrolysis of tetrachloromethane. 
A series of tests are performed to determine the time required for complete 
hydrolysis of tetrachloromethane at about 350.degree. F. Approximately 1 
gram of tetrachloromethane, 15 grams of deionized water and a calcium 
carbonate chip weighing about 2.6 grams are placed in a glass test tube 
and the test tube is sealed. The sealed tube is placed inside a 
cylindrical autoclave having an internal dimension slightly larger than 
the external dimensions of the sealed tube. The autoclave is pressurized 
with nitrogen to about 1,200 p.s.i.g. The contents of the autoclave are 
heated rapidly, e.g., at a rate of 50.degree. F. per minute, up to 
350.degree. F. and then held at this temperature for varying preselected 
periods of time. At this temperature, the pressure inside the sealed tube 
is approximately 1000 p.s.i.g. At the end of the preselected time period, 
the contents of the autoclave are rapidly cooled to room temperature by 
circulating nitrogen through the autoclave. The glass tube is broken to 
recover the unreacted calcium carbonate chip for weighing. Calculating 
from the equation which represents the overall hydrolysis-acidization 
reaction: 
EQU CCl.sub.4 +2CaCO.sub.3 .fwdarw.2CaCl.sub.2 +3CO.sub.2 
the weight loss expected for 100 percent hydrolysis of the 1 gram of 
tetrachloromethane is about 1.3 grams. The test in which the autoclave 
contents are maintained at 350.degree. F. for 44 hours indicates complete 
hydrolysis of the tetrachloromethane. Similar series of tests are 
conducted for 400.degree. F. and 500.degree. F., and the time required for 
complete hydrolysis under these conditions is found to be about four hours 
and one hour, respectively. 
If the rate of hydrolysis of a selected acid precursor is too rapid for a 
particular acidizing treatment, a retarder may be incorporated into the 
treating fluid to retard the hydrolysis reaction. The retarder should be 
nonpolymerizable and nonpyrolyzable under the high temperature and 
pressure conditions in the formation, and should be nonreactive with the 
formation constituents and the in situ-produced acid. Suitable retarders 
include the solvent refined, paraffinic lubricating oil base stocks, known 
as neutral oils and bright stocks. Preferred retarders are the highly 
paraffinic "white oils" which are acid refined from these base stocks. 
Exemplary retarders and their properties are as follows: 
______________________________________ 
Gravity Nominal Boiling Point 
Viscosity 
Retarder .degree. API 
Range (.degree. F.) 
(SSU) 
______________________________________ 
90 Neutral Oil 
30.8 640- 790 90 @ 100.degree. F. 
300 Neutral Oil 
27.7 710- 980 300 @ 100.degree. F. 
175 Bright Stock 
24.3 800- plus 175 @ 210.degree. F. 
______________________________________ 
These base stocks typically are between 70 and 90 percent saturated 
hydrocarbons with the balance being aromatic hydrocarbons. White oils are 
even more highly paraffinic. 
Normally only very high temperature formations, such as geothermal 
formations having temperatures between about 500.degree. F. and about 
700.degree. F. will require the use of a retarder. However, use of a 
retarder in acidizing other subterranean formations having temperatures 
above about 400.degree. F. is contemplated. When a retarder is required, 
the treating fluid injected through the well into the formation in the 
method of this invention will consist essentially of a mixture of the 
retarder and the acid precursor. Exemplary treating fluids are mixtures 
consisting of from about 50 to 95 weight percent acid precursor with the 
balance being the retarder. The amount of retarder required for a 
particular acidization treatment is easily determined by repeating the 
aforementioned test for determining the time required for the desired 
degree of hydrolysis with differing amounts of retarder. For example, the 
time required for complete hydrolysis of a treating fluid consisting of 1 
gram of a neutral oil marketed by Union Oil Company of California under 
the name Union 300 Neutral Oil and 1 gram of tetrachloromethane when mixed 
with 15 milliliters of a 3 weight percent NaCl solution was determined by 
this test to be between 16 and 20 hours at 400.degree. F. as compared to 
less than 6 hours for tetrachloromethane in a 3 weight perent NaCl 
solution without the retarder. 
The rate of hydrolysis of the acid precursor can also be retarded by 
increasing the salt concentration of the aqueous fluid with which it 
reacts in situ to generate the hydrohalic acid. For example, at 
400.degree. F. the complete hydrolysis of tetrachloromethane requires 
about six hours in a 3 weight percent NaCl solution as compared to only 
about four hours in fresh water. The total salt content of the aqueous 
fluoride salt solution, i.e., the fluoride salt plus other salts, may be 
adjusted between about 2 and about 30 weight percent to control the rate 
of hydrolysis of the acid precursor. Preferably ammonium chloride, 
ammonium bromide, ammonium iodide or mixtures thereof are used, as 
required, to retard the rate of hydrolysis of the acid precursor. When the 
well is shut in to allow hydrolysis of the acid precursor a spacer fluid 
containing a retarder, such as a 30 weight percent solution of ammonium 
chloride, is preferably positioned in the well adjacent to the formation 
to be acidized in order to substantially prohibit hydrolysis of any acid 
precursor remaining in contact with the well. 
Primary advantages realized in the method of this invention result from the 
fact that the acid precursors and aqueous fluoride salt solutions employed 
are noncorrosive under the high temperature conditions encountered prior 
to hydrolysis in the formation. To demonstrate the noncorrosivity of the 
anhydrous acid precursors, a series of tests are performed to determine 
the rate of corrosion of tetrachloromethane at a variety of high 
temperatures. Anhydrous tetrachloromethane and a weighed corrosion test 
specimen of N-80 steel are placed in a glass tube which is then sealed. 
The sealed tube is placed in an autoclave and is heated to a preselected 
high temperature for a selected period of time, after which it is cooled 
to 100.degree. F. for the balance of 22 hours. The test specimen is then 
removed from the glass tube and weighed. The weight loss is converted to 
pounds per square foot of surface area. A weight loss of about 0.050 
pounds per square foot is considered the maximum acceptable rate. Results 
of this series of tests are as follows: 
______________________________________ 
Temperature Time Corrosion Rate 
(.degree. F.) 
(Hours) (pounds/sq. foot) 
______________________________________ 
400 6 0.002 
600 6 0.005 
650 6 0.015 
650 6 0.021 
700 4 0.012 
700 6 0.029 
______________________________________ 
This data indicates that metal surfaces exposed to anhydrous 
tetrachloromethane prior to hydrolysis in the method of this invention 
will not be corroded to any signficant extent. Accordingly, the method of 
this invention is suitable for the acidization of subterranean formations 
having temperatures much higher than the 250.degree. F. practical maximum 
temperature of the prior art acidization methods.

The invention is further illustrated by the following example which is 
illustrative of a specific mode of practicing the invention and is not 
intended as limiting the scope of the invention as defined by the appended 
claims. 
EXAMPLE 
A subterranean formation contains a geothermal fluid at a temperature of 
about 450.degree. F. A steam-producing zone of the formation is penetrated 
by a production well at a depth of about 5,500 feet, and is 
fracture-acidized in accordance with the method of this invention. 
First, a 20-barrel slug of a 5 weight percent ammonium chloride solution 
followed by an 80-barrel slug of fresh water are injected through the well 
into the formation in order to stabilize any clay contained thereon. A 
volume of the formation around the well is then preflushed to remove 
nonsiliceous acid-soluble materials by alternately injecting through the 
well and into the formation ten 3.5-barrel slugs of tetrachloromethane and 
ten 35-barrel slugs of fresh water. The injected fluids mix in a mixing 
zone of the well to form a preflush mixture which is then displaced into 
the formation by the later-injected fluids. A 20-barrel slug of fresh 
water is injected to displace the preflush mixture away from the well and 
a 20-barrel slug of a 15 weight percent ammonium chloride solution is 
positioned in the well adjacent the producing zone to protect the well 
from the hydrochloric acid produced upon hydrolysis of the 
tetrachloromethane. 
The preflush tetrachloromethane hydrolyzes in situ to generate hydrochloric 
acid which solubilizes nonsiliceous acid-soluble materials in the 
formation. The later-injected fluids will displace these solubilized 
materials away from the volume of the formation surrounding the well. 
After an appropriate period of time to allow about 80 percent hydrolysis of 
the preflush acid precursor, such as about one hour or less, fifty 
6.5-barrel slugs of tetrachloromethane and fifty 50-barrel slugs of an 
aqueous solution containing 5.5 weight percent of ammonium fluoride and 
1,000 ppm of polyacrylamide polymer marketed by The Dow Chemical Company 
under the trademark Pusher.RTM. 1000 are injected through the well into 
the formation at a rate sufficient to hydraulically fracture the 
steam-producing zone of the formation. The injected fluids mix in the 
mixing zone of the well to form a reaction mixture which is then displaced 
into the formation. A 20-barrel slug of fresh water is injected to 
displace the last of the reaction mixture away from the well, and a 
20-barrel slug of a 30 weight percent solution of ammonium chloride is 
positioned in the well adjacent the formation to protect the well. 
The well is shut in for about two hours to allow substantially complete 
hydrolysis of the acid precursor and substantially complete reaction of 
the hydrofluoric acid produced in situ. Thereafter, the well is brought 
back on production. 
While particular embodiments of the invention have been described, it will 
be understood, of course, that the invention is not limited thereto since 
many obvious modifications can be made, and it is intended to include 
within this invention any such modifications as will fall within the scope 
of the appended claims.