Acid foam fracturing

Subterranean formations are fractured by means of an acid foam which can contain propping materials.

BACKGROUND OF THE INVENTION 
This invention relates to the treatment of subterranean formations. More 
particularly, this invention relates the use of acid foam for fracturing 
of subterranean formations. 
Various methods have been utilized by the oil industry for increasing the 
oil and gas flow from subterranean formations. One approach is to 
hydraulically fracture such formations with various liquids with or 
without propping agents suspended therein. The hydraulic pressure causes 
fracturing of the rock; the initially formed fractures are then extended 
by the injection of liquids under pressure therein. Thus, fracturing 
throughout these specifications shall mean initiating the fracture, 
propagating it or enlarging it. A propping agent, if one is used, is 
deposited in the fractures to maintain the permeability of the formation 
after liquids are removed. 
An improvement of this approach is to inject an acid solution into the 
formation under sufficient pressure to cause a fracture. The acid 
contained in the solution etches the fracture walls, thereby providing 
highly conductive channels. The degree of stimulation of the flow from the 
subterranean formation is strongly dependent upon the extend of acid 
penetration into the fracture; consequently, various methods have been 
devised to assure deep acid penetration. One such method involves the use 
of emulsions to shield the acid from the formation materials and thereby 
allow unreacted acid to penetrate deeply into the formation. Other methods 
to increase acid penetration involve the use of gelled acids and acids 
containing inhibitors which retard the reaction with the walls of the 
formation. 
The enumerated attempts for increasing the productivity of oil and gas flow 
from subterranean formations suffer from several drawbacks. Thus, 
conventional hydraulic fracturing requires high viscosity liquids 
requiring extremely large pumping equipment to achieve satisfactory 
injection rates. It often necessitates a chemical treatment to improve the 
viscosity, gel strength and fluid loss properties before such liquids can 
be used for fracturing. The removal of the viscous fracturing fluid from 
the resulting fractures presents additional problems. Other difficulties 
are the tendency for the sand or propping agent, often incorporated in the 
fracturing fluid, to plug the entry to the fracture and the failure of a 
viscous fracturing fluid to uniformly deposit the propping agent within 
the fracture. 
The problems associated with the use of emulsions, retarded and gelled 
acids are the size and high horsepower requirement of the pumping 
equipment and the difficulty in removal of the injected liquid. The 
present invention provides an improved acid fracturing process that 
alleviates many of the difficulties associated with prior art techniques. 
Thus, one object of the present invention is to increase the productivity 
of a formation carrying oil, gas or other products. 
Another object of the invention is to provide an improved formation 
fracturing operation. 
A further object of the invention is to provide a method for fracturing 
subterranean formation, which results in the creation of deep and wide 
fractures. 
Another object of the invention is to combine foam and chemical fracturing 
of formations to optimize results. 
A still further object of the invention is to reduce the energy required 
for injection and the size of pumping equipment. 
Still another object of the invention is to reduce the time and effort 
required for removal of the fracturing fluid. 
A still further object of the invention is to provide a fluid which readily 
penetrates the created fractures and the existing cracks in the formation 
and uniformly deposits the propping agent contained therein. 
Still another object of the invention is to provide a fracturing fluid 
which moderates the rate of reaction between the formation minerals and 
the acid contained in the fluid foam until full penetration of the 
fracture is obtained. 
A still further object of the invention is to minimize the damage to 
subterranean formations by liquid fracturing media. 
Still another object of the invention is to decrease the amount of acid 
needed for fracturing of a formation. 
A still further object of the invention is to enable fracturing of 
formations with acid-foam even at elevated temperatures such as 
300.degree. F. (149.degree. C.) to 500.degree. F. (260.degree. C.). 
Other objects and advantages of this invention will be apparent to one 
skilled in the art upon studying this disclosure. 
SUMMARY OF THE INVENTION 
In accordance with one aspect of the invention, the fracturing fluid is an 
acid-foam optionally containing a propping agent. The foam is placed in 
contact with the formation undergoing treatment or to be fractured and 
sufficient pressure is applied to the fluid to fracture the formation. 
In accordance with another aspect of the invention, a fracturing fluid is a 
foam made from a gelled polymer solution containing acid or a gelled acid 
solution either of which may optionally contain a propping agent. This 
foam when placed in contact with the formation to be treated or fractured 
and pressure applied, fractures or enlarges fractures in the formation. 
In accordance with still another aspect of the invention, the other 
ingredients of the foam partly shield the acid present therein from 
reacting with the side of the formation, thus allowing preponderance of 
acid to penetrate deeply into the formation, so that bulk of the reaction 
takes place in the optimum location upon collapsing the foam. 
In accordance with a further aspect of the invention, nitrogen and other 
ingredients of foam absorb the heat of reaction between acid and the 
formation, thus limiting the reaction rate. 
In accordance with a further aspect of the invention said foam will break 
upon reduction in pressure by expansion of the enclosed gases resulting in 
acid being available for etching the formation and for depositing the 
suspended propping agent (if any). Said pressure reduction causes the flow 
to reverse directions and liquids originally present as foam, as well as 
reaction products, flow back into the well bore where they can be 
effectively removed to leave a relatively dry fractured zone which is 
propped open and acid etched to provide minimum resistance to flow of the 
gas and/or oil to be produced. 
In accordance with a still further object of the invention incorporating of 
a gelling agent into the foam enables the foam to remain thermally stable 
even when exposed to elevated temperatures such as 300.degree. F. 
(149.degree. C.) to 500.degree. F. (260.degree. C.). 
Other aspects of this invention will become apparent to one skilled in the 
art upon studying this disclosure and the appended claims. 
DETAILED DESCRIPTION OF THE INVENTION 
A foam containing an acid of the type that readily reacts with the 
formation is used to fracture subterranean formations. The fracturing is 
accomplished by means of pressure under which it is injected and chemical 
reaction of the acid with the formation. Since only a small portion of the 
acid contained in the foam is at the foam's surface, the majority of the 
acid does not react with the formation immediately upon injection. 
Furthermore, most of the heat generated by the reaction is absorbed by the 
foam, thus reducing the temperature and limiting the reaction rate of the 
acid with minerals of the formation. As the result, substantially all the 
acid is carried deep into the fracture remaining available for reaction 
until the foam is broken. 
The foam used is a dispersion of a gas in an acid solution containing a 
foaming agent. The volumetric gas content, or foam quality, can range from 
about 50 to about 95 percent at the temperature and pressure of injection, 
but the preferable range is between 65 and 85 percent by volume. Although 
any gas can be employed, it is preferable to use an inert easily available 
gas, such as nitrogen. Examples of other gases that can be utilized with 
the invention include: carbon dioxide, air, hydrocarbon gases, argon, 
helium, krypton and xenon. 
The liquid may be water, light oil, or other carrier medium but it must be 
compatible with the acid and surfactant used. In the preferred embodiment, 
water is chosen because of its availability and compatibility with many 
commonly employed acids and surfactants. 
Any foaming agent compatible with other ingredients of the foam and capable 
of producing foam in an acidic environment can be used in the invention. 
Many of the foaming agents disclosed in U.S. Pat. Nos. 3,269,468; 
3,313,362; 3,136,361 and 3,572,440 meet both criteria; however, foaming 
agents which are preferred are: 
1. Igepal DM 970, a trademark for a sulfonated fatty acid produced by 
General Aniline and Film Corporation, 
2. Orvus K, a trademark for a sulfonated fatty acid produced by Proctor and 
Gamble Co., 
3. Cor 180, a trademark for a material produced by Chemical Oil Recovery 
Co., and 
4. GAFAC, a trademark for a material produced by General Aniline and Film 
Corp. 
The amount of the foaming agent depends on the type of the agent used, the 
other ingredients in the foam and the required foam stability at the 
injection conditions. Generally the more surfactant is added the more 
stable is the foam. In the preferred embodiment 0.5 to 3 weight percent of 
surfactant is added to the liquid. Alternatively, instead of adding more 
surfactant, to form a foam of high stability, the solution can be gelled 
by a suitable gelling agent prior to incorporation of the surfactant in 
accordance with a method described in U.S. Pat. No. 3,727,688 for example. 
Especially useful for increasing thermal stability is the addition of 
sodium carboxymethylcellulose polymer, polyacrylamides or polysaccharides; 
a foam gelled with sodium carboxymethylcellulose remains stable even at 
elevated temperatures such as 300.degree. F. (149.degree.) to 500.degree. 
F. (260.degree. C.). The concentration of the gelling agent in the 
solution from which foam is made, can be from about 250 to about 15,000 
ppm. In most applications, a concentration between about 350 and about 
10,000 ppm is preferred. 
Although any acid, or a mixture of acids, which is capable of reacting with 
the minerals of the formation and which is compatible with the other 
ingredients may be used, the preferred acids include mineral acids such as 
hydrochloric, hydrofluoric, phosphoric, sulfuric, sulfamic, and nitric 
acids. Other acids suitable for the use with this invention can be 
selected from those disclosed in U.S. Pat. No. 3,572,440. It should be 
emphasized that the selection of an acid depends on its ability to react 
with the formation. The concentration of the acid in the liquid component 
depends on the particular acid or acids used, the desired pH and the 
desired foam stability. Generally 5 to 50 weight percent by weight acid 
solution provides a satisfactory foam for most applications. 
Optionally, a propping agent may be added to the foam at any stage of 
operation before the foam is injected into the formation. The propping 
agent is carried into fractures in the formation and becomes wedged there 
upon collapse of the foam, hence, it keeps open passages through the 
formation thus allowing increased flow of oil and gas therefrom. Many 
particulate materials may be employed as propping agents, it is preferred, 
however, in most applications to use sand. Especially useful is smooth 
round grain sand whose size is between 10 and 80 mesh. The amount of sand 
that can be carried in the foam varies with the type of acid foam used, 
but commonly 0.2 to 3 pounds of sand per gallon of acid foam is used. 
In operation, the acid, liquid and surfactant can be mixed together in any 
desired order. It is preferred, however, to add an acid to water and then 
add a surfactant. Optionally where thermal stability of foam is desired, 
sodium carboxymethylcellulose polymer can be added in such concentration 
as to gell the liquid prior to addition of the surfactant. Agitation, 
helpful in making the solution homogenous, can be provided by means of a 
static mixer in addition to pump and pipe mixing. The solution is aerated 
with the gas to form a foam of desired quality. A propping agent, if it is 
used, can be added either to the solution, or it can be incorporated into 
foam as the foam is being pumped into the formation. 
The foam is then injected into the formation at a pressure required for 
fracturing which may be anywhere from 500 to 20,000 pounds per square inch 
and even more. Since acid is uniformly distributed throughout the foam, 
only the acid near or at the surface of the foam is available for reaction 
with the wall of the formation. The products of the reaction usually 
include a gas and water, the gas being a component of the original acid or 
a resultant gas, such as carbon dioxide, resulting from reaction with the 
formation material. The reaction is usually exothermic; consequently, the 
reaction is reduced further by absorbtion of heat by the foam and by the 
rapid flow of the fluid foam past the reaction site. As the foam 
penetrates into the fracture, it uses up only a minor amount of acid 
allowing the remaining part of the acid to penetrate deeply into the 
fracture. 
The propping agent, if one is used, is transported in the foam into the 
cracks produced by fracturing and becomes wedged there. The foam is stable 
only for a certain period of time, which is dependent on the type, the 
amount of a surfactant agent and whether a gelling agent such as sodium 
carboxymethyl-cellulose was used. It is stable only under high pressure; 
consequently, it can be collapsed either by a lapse of time or by a sudden 
release of pressure. Upon collapsing of foam, the propping agent remains 
in the formation holding open the fracture and enabling oil and gas to 
pass therethrough. The acid reacts with the formation producing channels 
therein. 
In the light of this disclosure many modifications, changes and 
substitutions will be apparent to those skilled in the art. It is intended 
that those modifications, changes and substitutions which fall within the 
spirit or scope of the invention be considered as a part thereof.

The following examples are included for illustrative purposes and are not 
intended to limit the scope of the invention: 
EXAMPLE I 
A solution contaning 121/2 percent by weight of HCl in water was prepared 
and placed in eleven 100 ml samples. To each sample, varying amounts of a 
surfactant were introduced. 
One sample was prepared using only tap water and a surfactant. 
Within 24 hours all samples were converted into foam by aeration with 
nitrogen. The foam was placed in Ross-Miles apparatus and its heights were 
measured. The measurements were repeated after five minutes. The results 
appear in Table I. 
Table I 
______________________________________ 
Results of Foaminess Tests on Surfactant Solutions: 
Ross-Miles Apparatus; 100 ml Solutions 
______________________________________ 
Amount and type of a 
Foam Collar Height, cm 
Surfactant added to acid solution 
0 Minutes Five Minutes 
______________________________________ 
0.5% Igepal DM 970 6.0 4.5 
1.5% " 6.0 5.0 
3.0% " 5.8 4.5 
0.5% Orvus K 3.0 2.7 
1.5% " 5.0 4.0 
3.0% " 6.0 5.0 
0.5% Cor 180 2.5 2.2 
1.5% " 3.5 2.7 
3.0% " 3.0 2.5 
3.0% GAFAC LO 529 5.0 3.0 
Amount and type of a 
Surfactant added to water 
3.0% Igepal DM 970 6.2 5.2 
______________________________________ 
The results indicate that stable foams can be obtained using acidic 
liquids. The foam stability is dependent on both the type and the amount 
of the surfactant used. With a proper choice of these variables (1.5% 
Igepal DM 970 and 3% Orvus K in Example I) foam in the acidic environment 
can be as stable as the one in the neutral environment. 
EXAMPLE II 
A 12.5 percent by weight HCl solution was prepared. To the solution, Orvus 
K liquid surfactant was added until the concentration of Orvus K liquid 
reached 3 percent by weight. The solution was subdivided into two samples. 
The first sample was aerated with nitrogen until foam was formed. A chip 
of carbonate salt was immersed in each of the samples. Before the run was 
concluded, the chip in the foam accidently dislodged and dropped into the 
unfoamed liquid. The chips were removed and their weights were compared to 
the original weights. The chip in the foam lost 28 percent of its original 
weight, whereas the chip immersed in the unfoamed liquid lost 39 percent 
of its original weight. 
The result indicates that the foam shields some of the acid preventing its 
reaction with the carbonate rock. The difference in the loss of weight may 
have been even larger had the chip in the foam not fallen into the 
unfoamed liquid. 
EXAMPLE III 
The solution containing Cor 180 surfactant was placed in a glass lined 
pressure vessel equipped with a 3,000 psi pressure gauge and a 1,300 psi 
rupture disc. The vessel was sealed and then heated to 325.degree. F. 
(163.degree. C.) in a laboratory oven. The temperature of the test 
solution was monitored continuously with an iron-constantan thermocouple 
sealed into the vessel. The resistance factor was determined at intervals 
shown in Table II in accordance with the following procedure. 
A 1-inch I.D. schedule 40 steel pipe 1-foot long was threaded at the ends 
and mounted in a vertical position. The tube was equipped with stainless 
steel sand retaining screens, filled with Ottawa Sand and capped with 
steel reducing couplings having needle valves. The tube was filled with 
water, and nitrogen from a 100 psi source introduced into the top. Water 
was then displaced out the bottom by nitrogen flow and the pressure drop 
across the sand pack during water displacement by the gas flood was 
measured with a capacitance-type transducer (Dynasciences Corporation, KP 
15 and Model D 25 translator) and recorded on a strip chart recorder 
(Sargent model SR). 
Surfactant solutions (normally three pore volumes) were introduced into the 
top of the tube with a hypodermic syringe, displacing water out the 
bottom. The surfactant solution was pushed through the sand pack with 
nitrogen causing foam to be generated in situ, and the pressure drop 
across the sand pack was measured in the same manner as described above 
for the gas-water flood. The ratio of the maximum differential pressure 
observed during the foam flood divided by the maximum pressure during the 
gas-water flood provided a qualitative measure of foam quality. 
This product was repeated under the same conditions except the Cor 180 was 
added to solution gelled by sodium carboxymethylcellulose (CMC). The 
results of the runs appear in Table II. 
Table II 
______________________________________ 
Thermal Stability of 
Surfactant Solutions 
RESISTANCE FACTOR* 
Hours at 3% Solutions 3% Cor 180 added to 
325.degree. F 
of Cor 180 Gelled (CMC) Solutions 
______________________________________ 
4.8 4.9 7.7 
4.9 9.3 ** 
7.8 -- 6.5 
8.8 -- 7.7 
9.0 1.2 5.7 
______________________________________ 
*The resistance factor for a surfactant solution is defined as the ratio 
of the maximum pressure drop observed across the sand pack when the pore 
space was filled with the surfactant foam to the maximum pressure drop 
when the pack was filled with water and gas and was being displaced by 
nitrogen. 
**Remained gelled. 
This example indicates that gelled CMC polymer increases the thermal 
stability of the foamed solution.