FRACTURING FLUID

FLUIDE DE FRACTURATION

English Abstract

A subterranean fracturing fluid includes an amphoteric surfactant,
specifically
a betaine surfactant and an alcohol in an aqueous medium. Depending upon the
proportions of the ingredients, the fluid can be foamed.

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. A fracturing fluid comprising an aqueous medium, at least one betaine
surfactant having a saturated or unsaturated alkyl or acyl group and at least
one alcohol
electrolyte having the general formula
R3-OH
wherein R3 is a hydrocarbon aliphatic or aromatic, straight or branched,
saturated or
unsaturated hydrocarbon group with 6-24 carbon atoms.
2. The fracturing fluid of claim 1, wherein the saturated or unsaturated alkyl
or acyl groups contains 14 - 24 carbon atoms.
3. The fracturing fluid of claim 1 or 2, wherein the betaine surfactant has
the
general formula
<IMG>
wherein R is an alkyl group or the group R'NC3H6 - wherein R1 is an alkyl
group, and
R1 and R2 are hydrocarbon aliphatic or aromatic, straight or branched,
saturated or
unsaturated groups.
4. A Fracturing Fluid of claim 3, wherein the groups R1 and R2 contain 1-2
hydrophilic moieties selected from Hydroxy 1 (-OH), ethoxy or propoxy.
5. The fracturing fluid of claim 1 or 2, wherein said alcohol is selected from
the group consisting of benzyl alcohol, decanol, dodecanol or hexadecanol.

6. The fracturing fluid of any of claims 1 to 5, wherein said aqueous medium
is selected from the group consisting of water, an aqueous solution of an
inorganic salt,
aqueous alkaline or acid solution, water and a lower alkanol, glycol or
polyglycol and
mixtures thereof.
7. The fracturing fluid of any of claims 1 to 6, wherein said alkyl group
contains 14-22 carbon atoms.
8. The fracturing fluid of any of claims 1 to 6, wherein said alkyl group
contains 14-24 carbon atoms and 1-2 double bonds.
9. The fracturing fluid of any of claims 1 to 6, wherein said acyl group
contains 14-24 carbon atoms.
10. The fracturing fluid of any of claims 1 to 6, wherein said acyl group
contains 14-24 carbon atoms and 1-2 double bonds.
11. The fracturing fluid of any of claims 1 to 8 wherein said betaine
surfactant
is selected from the group consisting of erucyl dimethyl betaine, docosyl
dimethyl
betaine, octadecyl dimethyl betaine cetyl dimethyl betaine, and tallow
dimenthyl
betaine and myristyl dimethyl betaine.
12. The fracturing fluid of claim 1 or 2, wherein said aqueous medium is a
solution of KC1 in water, said betaine surfactant is octadecyl dimethyl
betaine and said
alcohol is hexadecanol.
13. The fracturing fluid of claim 1 or 2, wherein said aqueous medium is a
solution of KCl in water, said betaine surfactant is octadecyl dimethyl
betaine and said
alcohol is tetradecanol.

14. The fracturing fluid of claim 1 or 2, wherein said aqueous medium is a
solution of KCl in water, said betaine surfactant is octadecyl dimethyl
betaine and said
alcohol is decanol.
15. The fracturing fluid of claim 1 or 2, wherein said aqueous medium is a
solution of KCl in water, said betaine surfactant is octadecyl dimethyl
betaine and said
alcohol is benzyl alcohol.
16. The fracturing fluid of claim 1 or 2, wherein said aqueous medium is a
solution of KCl in water, said betaine surfactant is hexadecyl dimethyl
betaine and said
alcohol is benzyl alcohol.
17. A method of fracturing subterranean formation comprising the step of
injecting the fracturing fluid of claims 1 to 16 into the formation at a
pressure sufficient
to initiate fracturing.

Description

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BACKGROUND OF THE INVENTION
FIELD OF THE INVENTION
This invention relates to a fracturing fluid and to a method of fracturing a
subterranean formation to increase the permeability of the formation.
More specifically, the invention provides a viscoelastic surfactant based
fracturing fluid for fracturing a subterranean formation and transporting
proppant into
thus created fractures.
DISCUSSION OF THE PRIOR ART
Hydraulic fracturing has been used for many years to stimulate the production
of petroleum from subterranean formations. In hydraulic fracturing, a
fracturing fluid
is injected through a wellbore into the formation at a pressure and flow rate
sufficient
to overcome the overburden stress and to initiate a fracture in the formation.
Frequently, a proppant, whose function is to prevent the created fractures
from closing
back down upon itself when the pressure is released, is suspended in the
fracturing
fluid for transport into a fracture. Proppants in use include, for example 20-
40 mesh
size sand and ceramics, but the most common proppant is sand. The proppant
filled
fractures provide permeable channels allowing petroleum to seep through the
fractures
into the wellbore from whence it is pumped to the surface. Accordingly, a
desirable
fracturing fluid should have the following properties: (a) to be compatible
with the
reservoir rock and reservoir fluids, (b) have sufficient viscosity and fluid
structure to
suspend proppants and transport them deep into the formation, (c) be stable
enough to
retain sufficient viscosity and fluid structure throughout proppant placement,
(d)
possess low fluid loss properties and low fluid flow friction pressures, (e)
be easily
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removed from the formation with little residues, (f) be easily made under
field
conditions and (g) be relatively inexpensive. Production of petroleum can be
enhanced
significantly by the use of specialized fracturing fluids, which exhibit high
levels of
rheological performance.
Fracturing fluids in common use include various aqueous gels and hydrocarbon
gels. The gels are formed by introducing cross-linkable polymers or
surfactants into an
aqueous or hydrocarbon fluid, followed by cross-linking of the polymer or
surfactant
molecules. The cross-linking give the fluid high viscoelastic properties that
are
necessary to transport and place proppants into the fractures.
Another widely used fracturing fluid is a foamed, water-based fracturing
fluid.
Such a fluid is described, for example, in U.S. Patent No. 3,980,136, issued
to R.A.
Plummer et al on September 14,1976. Briefly, the foamed fracturing process
involves
generation of foams with a desired quality which are pumped through a wellbore
into
a formation. Typically, far aqueous systems, a polymer has to be hydrated in
water at
the surface before being pumped into the formation. The process of polymer
hydration
is time consuming and often requires bulky equipment at the wellsite. Another
problem common to polymer-based fracturing fluids is that a significant amount
of
polymer residue is left in the formation resulting in negative impact on
formation
permeability.
Viscoelastic surfactants have long been used for well stimulation. A
surfactant
is a type of substance, which contains both hydrophobic and hydrophilic groups
in the
same molecule. The hydrophobic group is usually one of a variety of alkyl
groups and
the hydrophilic group can be ionic, which may be positive (cationic), negative
(anionic)
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or contain both positive and negative moieties (amphoteric), or nonionic -
often
consisting of a neutral polyoxyalkylene group. When dissolved in an aqueous
medium,
surfactants generally form various aggregates called micelles above a critical
micelle
concentration (cmc). At low concentration of surfactant, the micelles usually
are small
and spherical. Under certain conditions and surfactant concentrations,
however, the
spherical micelles grow in size and/or change their shape resulting in the
formation of
long flexible micelles. Above a certain concentration the long flexible
micelles can
become entangled and exhibit strong viscoelastic behavior. Even though this
feature
has been observed in a number of systems containing nonionic and anionic
surfactants,
the effect is more pronounced in cationic surfactants, especially those
containing an
amine or quaternary ammonium group, in the presence of certain organic
counterions
such as, for example salicylate, benzonate and alkyl sulfonate. Viscoelastic
surfactant
fluids have been studied extensively in recent years and have found a wide
variety of
uses in many applications.
U.S. Patent No. 4,061,580, issued to R.W. Jahnke on December 6,1977 discloses
surfactant gelled fracturing and acidizing fluids suitable for well
stimulation. The
gelled fluids are prepared by adding certain amine salts to aqueous acid or
salt
solutions. The amine salts used as thickeners are prepared by merely mixing
one
equivalent of amine per equivalent of acid or, in the case of polybasic acids
such as
sulfuric and phosphoric acids, as little as one-half equivalent of amine per
equivalent
of acid may be used resulting in the formation of an acidic salt. The aqueous
acid or salt
solution can be gelled by the addition of the above-described salts. For
example,15
by weight of HCl can be gelled by the addition of a small amount, usual 3-10%
by
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weight and typically about 5 % by weight of an amine or amine salt as
described above.
For fracturing fluids, aqueous solutions containing some inorganic salts can
be gelled
by the addition of 3-10% by weight, preferably about 5% by weight, of an amine
salt
described above.
U.S. Patent No. 4,163,727, issued to C.G. Inks on August 7, 1979 discloses an
acidizing-gel composition which consists essentially of, for example, about
15% by
weight of HCI, about 20% by weight of a suitable nonionic gel-forming
surfactant
containing oxyethylene and oxypropylene units, a corrosion inhibitor to the
extent
needed, and the balance water.
U.S. Patents Nos. 5,551,516, issued to W.D. Norman et al on September 3,1996
and 5,964,295, issued to J.E. Brown et al on October 12,1999 disclose a
fracturing fluid
composition comprising a quaternary ammonium salt, erucyl bis (2-hydroxyethyl)
methyl ammonium chloride, an organic salt such as sodium salicylate, inorganic
salts
such as ammonium chloride and potassium chloride and water. The patents state
that
the fluid has good viscoelastic properties and is easily formulated and
handled.
Furthermore, no or very little residue is left in a formation after the
completion of the
fracturing process. It is worth noting, however, that cationic surfactants
such as amine
and quaternary ammonium salts usually degrade very slowly, both aerobically
and
anaerobically, and moreover are highly toxic to marine organisms. The
combination
of low biodegradability and high toxicity is a fundamental criterion for a
product
injurious to the environment. In addition, cationic surfactants tend to render
the
formation, especially sandstone formations, oil-wet by adsorbing on the
surface of clays
and sands. The alteration of the formation wettability often reduces the
relative
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permeability of petroleum leading to high water/ petroleum ratio and low
production
rates. The strong adsorption of cationic surfactant on the clay and sands may
also
adversely affect fluid viscosity.
GENERAL DESCRIPTION OF THE INVENTION
Thus, there is a general demand for surfactants, which are less harmful to
both
the environment and to subterranean formations, but which have the same
excellent
ability as above-mentioned cationic surfactants to form viscoelastic
surfactant based
fracturing fluids. An object of the present invention is to meet this demand.
Another object of the present invention is to overcome the disadvantages
inherent to existing fracturing fluids by providing a fracturing fluid having
relatively
good foaming capability and foam stability in a wide range of temperatures.
According to one aspect, the invention relates to a fracturing fluid
comprising
an aqueous medium, at least one betaine surfactant having a saturated or
unsaturated
alkyl or acyl group containing 14-24 carbon atoms and an alcohol having the
general
formula
R3-OH
wherein Rs is a hydrocarbon group with 6-24 carbon atoms.
According to a second aspect, the invention relates to a method of fracturing
a
subterranean formation comprising the step of injecting a fracturing fluid
into the
formation at a pressure sufficient to initiate fracturing, said fluid
including an aqueous
medium, at least one betaine surfactant having a saturated or unsaturated
alkyl or acyl
group containing 14-24 carbon atoms and at least one alcohol having the
general
formula
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R3-OH
wherein R3 is a hydrocarbon with 6-24 carbon atoms.
DESCRIPTION OF THE PREFERRED EMBODIMENT
As described above, the basic composition of the invention includes a betaine
surfactant and an alcohol which are readily degradable. In addition, since
betaine
surfactants have strong foaming capability, the present composition also gives
an
excellent foaming capability and foam stability within a given temperature
range,
without employing additional foaming surfactants. The present fluid may also
contain
a gas, for example, N2 or C02, and thereby be in the form of foams or
energized fluids.
In other words, the present fluid may also be utilized as a foamed water-based
fracturing fluid in the presence of gas.
The betaine surfactant has a saturated or unsaturated alkyl or acyl group with
14-24 carbon atoms. The generally molecular structure of the betaine
surfactant is
represented by the general formula
R1
R-N+-CHzC00
R2
where R is the alkyl group or the group R'NHCsH6, in which R' is an acyl
group.
R1 and RZ are hydrocarbon aliphatic or aromatic, straight or branched,
saturated
or unsaturated groups and may also contain one or two hydrophilic moieties,
such as
hydroxyl(- OH), or ethoxy or propoxy moieties. The alcohol having the general
structure
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R3- OH
where Rs is a hydrocarbon group with 6-24 carbon atoms. The hydrocarbon group
Rs
can be aliphatic or aromatic, straight or branched, saturated or unsaturated.
The
combination of the specific betaine surfactants and alcohols in an aqueous
medium
gives good viscoelastic properties within a given temperature range. The
carbon
numbers of the hydrocarbon groups R will determine the useful temperature
range for
a particular fluid so that high carbon numbers usually give products suitable
for high
temperatures.
The group R can suitably be tetradecyl, hexadecyl, octadecyl, oleyl, rape seed
alkyl and tallow alkyl, erucyl, docosyl or the corresponding acyl group.
The preferred betaine surfactant is octadecyl dimethyl betaine. Alternative
betaines may be employed either alone or in combination, including erucyl
dimethyl
betaine, docosyl dimethyl betaine, cetyl, dimethyl betaine, tallow dimethyl
betaine, and
myristyl dimethyl betaine. The preferred alcohol is benzyl alcohol.
Alternatives
include decanol, dodecanol and hexadecanol.
By "aqueous medium" is meant that at least 50% by weight, preferably at least
90% by weight, of the water-based liquid system consists of water. Within the
term are
plain water and aqueous solutions of inorganic salts and aqueous alkaline or
acidic
solution. Other exemplary aqueous liquids include mixtures of water and water-
miscible liquids such as lower alkanols, e.g., methanol, ethanol or propanol,
glycols and
polyglycols. Also included are emulsions of immiscible liquids in the aqueous
liquids,
aqueous slurries of solid particulates such as sands, ceramics, or other
minerals and a
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number of conventional components such as clay stabilizers, antifreeze agents
and
bactericides. All of the additives, as well as the betaine surfactants,
alcohols and water,
are employed in amounts that do not deleteriously affect the viscoelastic
properties of
the fluid.
The present invention is described below in greater detail by means of the
following examples.
EXAMPLES
The foaming properties of the compositions according to the present invention
were tested by a simple method involving the measuring the viscosity of the
gel.
Example 1
1.5 g active substance of octadecyl dimethyl betaine (in the following called
C18
-betaine) was first dissolved in 200 ml of 5 wt% KCl aqueous solution. The
resulting
surfactant solution was mixed with 0.8g active substance of hexadecanol at
55°C. A clear
gel with high elasticity was formed. The viscosity of the gel was measured
using a
Brookfield viscometer (Model LVT, Spindle 1 at 12 rpm) at 55°C. The
results are listed
in Table I.
Example 2
1.5 g active substance of C18 -betaine was first dissolved in 200 ml of 5 wt%
KCl
aqueous solution. The resulting surfactant solution was mixed with 0.6g active
substance of tetradecano( at 40°C. A clear gel with high elasticity was
formed. The
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viscosity of the gel was measured using a Brookfield viscometer (Model LVT,
Spindle
1 at 12 rpm) at 40°C. The results are listed in Table I.
Example 3
1.5 g active substance of C18 -betaine was first dissolved in 200 ml of 5 wt%
KCl
aqueous solution. The resulting surfactant solution was mixed with 0.2g active
substance of decanol at 30°C. A clear gel with high elasticity was
formed. The viscosity
of the gel was measured using a Brookfield viscometer (Model LVT, Spindle 1 at
12
rpm) at 30°C. The results are shown in Table I
Example 4
1.0 g active substance of C18 -betaine was first dissolved in 200 ml of 5 wt%
KCl
aqueous solution. The resulting surfactant solution was mixed with 0.5g active
substance of benzyl alcohol at 22°C. A clear gel with high elasticity
was formed. The
viscosity of the gel was measured using a Brookfield viscometer (Model LVT,
Spindle
1 at 12 rpm) at 22°C. The results are shown in Table I.
Example 5
1.5 g active substance of C16 -betaine was first dissolved in 200 ml of 5 wt%
KCl
aqueous solution. The resulting surfactant solution was mixed with 0.6g active
substance of benzyl alcohol at 22 oC. A clear gel with high elasticity was
formed. 22 °C.
The viscosity of the gel was measured using a Brookfield viscometer (Model
LVT,
Spindle 1 at 12 rpm) at 22 °C. The results are shown in Table I.
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Table I
Com ositions V_iscosit of els
1. C18-Betaine 0.75% 340 cp
Hexadecanol 0.4%
KCl 5.0
2. C18-Betaine 0.75 % 470 cp
Tetradecanol 0.3
KCl 5.0%
3. C18-Betaine 0.75% 750cp
Decanol 0.1
KCl 5.0
4. C18-Betaine 0.5 % 660 cp
Benzyl alcohol0.25%
KCl 5.0
5. C16-Betaine 0.75% 220 cp
Benzyl alcohol0.3
KCl 5.0
From the results of testing set out in Table I it is evident that combinations
of a
betaine surfactant and an alcohol in the aqueous medium form clear gels with
good
viscoelastic properties. These gels can be used for hydraulic fracturing
applications.
For applications requiring higher viscosity, higher surfactant loading is
generally
required. The present fluid may also contain a gas, for example, N2 or C02,
and
thereby be in the form of foams or energized fluids. In other words, the
present fluid
may also be utilized as a foamed water-based fracturing fluid in the presence
of gas
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