WO2002018745A2

WO2002018745A2 - Fracturing fluid

Info

Publication number: WO2002018745A2
Application number: PCT/CA2001/001227
Authority: WO
Inventors: Kewei Zhang
Original assignee: Trican Well Service Ltd.
Priority date: 2000-09-01
Filing date: 2001-08-31
Publication date: 2002-03-07
Also published as: AU2001287435A1; WO2002018745A3; CA2318297A1

Abstract

A subterranean fracturing fluid includes a surfactant, specifically a cationic surfactant and an organic salt in an aqueous medium. The fluid can be foamed for fracturing applications without employing an additional foaming surfactant.

Description

BACKGROUND OF THE INVENTION FIELD OF THE INVENTION

This invention relates to a fracturing fluid and to a method of using the fluid to

fracture a subterranean formation to increase the permeability of the formation.

More specifically, the invention provides a viscoelastic surfactant-based

fracturing fluid and foam, and the use of the fluid or oam to fracture a subterranean

formation and transport proppant into thus created fractures.

BACKGROUND OF THE INVENTION

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, the function of which is to prevent the created fracture from

closing 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, the most common proppant being 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, good fracturing fluid should

have the following properties: (a) compatability with the reservoir rock and reservoir

fluids, (b) sufficient viscosity and fluid structure to suspend proppants and transport

them deep into the formation, (c) enough stability to retain sufficient viscosity and fluid structure throughout proppant placement, (d) low fluid loss properties and low fluid

flow friction pressures, (e) easily removable from the formation with little residues, (f)

easily made under field conditions and (g) relatively inexpensive. Production of

petroleum can be enhanced significantly by the use of specialized fracturing fluids,

which exhibit high levels of Theological performance.

Fracturing fluids in common use include various aqueous 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.

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, i.e. positive (cationic), negative (anionic) 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 if the surfactant concentration is

above a critical micelle concentration (cmc). At low concentration of surfactant, the

micelles usually are small and spherical. Under certain conditions and surfactant

concentrations, 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.

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 HC1 can be gelled by the addition of a small amount, usual 3-10% by

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, approximately 15%

by weight of HCl, approximately 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, very little if any residue is left in a formation after the completion of the

fracturing process.

Another widely used fracturing fluid is a foamed, water-based fracturing fluid.

Foams are defined as dispersions of gas in a liquid. Typically, foamed fluids contain

sufficient amount of surfactants in an aqueous liquid. The composition of the foamed

fluid is such that the quality of the foam at the bottom of the well is in the range of from

about 53% to 90%. The pressure at which the foamed fluid is pumped into the well is

high enough to initiate and extend a fracture in the hydrocarbon-bearing formation.

Foamed fracturing fluids, which contain a significant quantity of gas, have several

advantages over conventional fracturing fluids. Firstly the gas aids in cleanup after the completion of the hydraulic fracturing process. The gas "energizes" the fluid, and when

the pressure is decreased, such as by producing fluids from the well, the gas expands

and drives much of the liquid component of the fracturing fluid out of the formation.

Secondly, foams have a lower liquid content compared to conventional fracturing fluids

and therefore are less likely to cause formation damage. As aqueous fluids invade a

formation, they can sometimes cause formation damage by interacting undesirably with

the formation or by interfering with gas and/ or oil flow into the wellbore causing a

decrease in the relative permeability.

A water based foamed fracturing 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 described in the patent involves generation of foams with a desired

quality and stability which are pumped through a wellbore into a formation.

US Patent No. 5,258,137 issued to Bonekamp et al on November 2, 1993 discloses

a viscoelastic surfactant-based foamed fracturing-fluid. The fluid includes a cationic

surfactant, namely an alkyltrimethyl ammonium chloride, wherein the alkyl group is

a long chain alkyl having 12-22 carbon atoms' which may be saturated or contain one

or more double bonds, with an organic salt such as sodium salicylate to associate with

the cationic surfactant to form a viscoelastic surfactant in an aqueous fluid, and a

foaming surfactant in order to have sufficient foam quality. The viscoelastic surfactant-

based foamed fluids are said to have high foam quality and stability and therefore good leak-off properties.

As disclosed, for example, in US Patent No. 5,575,335 issued to King on

November 19, 1996 and in US Patent No. 5,711,376 issued to Sydansk on January 27,

1998, it has become a common practice to incorporate various polymers into the foamed

fracturing fluids in order to increase the viscosity of the fluid and the foam stability to

transport a high concentration of proppant and provide more favourable leakoff

properties. Natural polymers such as guar gums and their derivatives are commonly

used. Typically a polymer has to be hydrated above ground before being pumped into

the formation. The process of polymer hydration is time consuming and often requires

bulky equipment at the well site. Moreover, the viscosity of such compositions can

change substantially with variations in temperature, and the compositions are shear

degradable, can have short shelf lives, and can leave a significant amount of polymer

residue is in the formation having negative impact on formation permeability.

Moreover, the fluid is formation benign in the sense that very little, if any residue is left

in the formation upon completion of the fracturing process. However, the fluid

disclosed in Bonekamp (supra) requires the introduction of an additional foaming

surfactant into the viscoelastic surfactant base get in order to have sufficient foam

quality and stability.

SUMMARY OF THE INVENTION

An object of the present invention is to overcome the disadvantages inherent to

existing polymer-based fracturing fluids by providing a surfactant-based fracturing fluid, which leaves no significant residue in a subterranean formation upon completion

of the fracturing process.

Another object of the present invention is to overcome the disadvantages

inherent to existing fracturing fluids by providing a surfactant-based fracturing fluid

having relatively good foaming capability and foam stability over a wide range of

temperatures without employing additional foaming surfactants.

According to one aspect, the invention relates to a fracturing fluid comprising

an aqueous medium, at least one cationic surfactant and at least one organic salt having

the general formula.

wherein Ri is an organic anion and M is a cation.

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 cationic surfactant and at least one organic salt having the general

formula

wherein Ri is an organic anion, and M is a cation. The composition may also be formed

into a gel of desired viscosity for a particular application by selecting an appropriate combination of cationic surfactant and an organic salt.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The cationic surfactant has a saturated or unsaturated alkyl group containing 14-

22 carbon atoms. The general formula of the cationic surfactant is

CH₃

I

R-N-CHsX I

CHs where R is an hydrophobic group and X is an anionic group.- The group R can be

aliphatic, straight or branched, saturated or unsaturated containing 14-22 carbon atoms.

The group R can suitably be tetradecyl, hexadecyl, octadecyl, oleyl and tallow.

The composition of the invention includes an aqueous medium, at least one cationic

surfactant and an organic salt. The fluid may also contain a gas, for example N₂ or C0₂

and thus be in the form of a foam. The composition also has excellent foaming

capability and foam stability within wide temperature ranges without employing

additional foaming surfactants.

The organic salt has the general formula

where Ri is an organic anion selected from the group consisting of 3-hydroxy-2-

naphthalenecarboxylate, cumene sulphonate, salicylate and toluene sulphonate, and M is a cation preferably a monovalent cation such as sodium.

Apart from the cationic surfactants and organic electrolyte salts, the aqueous-

medium may contain a number of conventional components such as clay stabilizers,

anti-freeze agents and bactericides. In addition, a foam of desired stability for a

particular practical application can be created by selecting a proper combination of

cationic surfactant and organic salt

The preferred cationic surfactant is a tallow trimethyl ammonium salt or halide.

Other cationic surfactants which may be employed either alone or in combination

include octadecyl trimethyl ammonium salts or halides, cetyl trimethyl ammonium salts

or halides, tetradecyl trimethyl ammonium salts or halides and mixtures thereof.

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. The combination of

the specific cationic surfactants and the organic salts provides an excellent foaming

capability and foaming stability within a wide temperature range. The carbon numbers

of the hydrophobic groups R will usually determine the useful temperature range for

the mixture, high carbon numbers usually yielding products suitable for high

temperatures.

The present invention is described below in greater detail by means of the

following examples. EXAMPLES

Foaming properties of the compositions according to the present invention were

tested by a simple method involving the measuring foam volume and of foam half-life.

Example 1

0.5 g of octadecyl trimethyl ammonium chloride (hereinafter referred to as

OTAC) with the structure

(hereinafter referred to as OTAC) and 0.56 g of sodium toluene sulphonate,

CHsCβHiSOsNa (hereinafter referred to as STS) were mixed with 200ml of tap water.

A clear viscoelastic gel was formed. The resulting gel was poured into a 1 litre Waring

blender jar, and mixed at maximum blender speed for 30 seconds. The resulting foam

was poured into a 1000 ml graduated cylinder, and a timer was started. The foam

volume and foam half li e were measured, (the foam volume is the maximum volume

occupied by the foam, and can be used to calculate foam quality, and the foam half -life

is the time required for 100 ml of solution to accumulate on the bottom of the graduated

cylinder). The results are listed in Table I.

Example 2

0.5 g of OTAC and 0.4 g of sodium salicylate (hereinafter referred to as SS) were

mixed in 200 ml of tap water. A clear viscoelastic gel was formed. The gel was tested

in the same manner as described in Example 1. The results are listed in Table I. Example 3

0.5 g of tallow trimethyl ammonium chloride (hereinafter called TTAC) and 0.17

g of the sodium salt of 2-hydroxy-3-naphthoic acid, CιoH₆ (OH) COONa (hereinafter

referred to as Na-BON) in aqueous NaOH solution were mixed in 200 ml of tap water.

A clear viscoelastic gel formed. The gel was tested in the same manner as in Example

1. The results are listed in Table I.

Example 4

0.5 g of TTAC and 0.18 g of the sodium salt of cumene sulphonate (hereinafter

called SCS) were mixed in 200 ml of tap water. A clear viscoelastic gel formed. The gel

was tested in the same manner as in Example 1 and the results are listed in Table I

Table I

Example 5

3.0 g of OTAC was first dissolved in 300 ml of tap water. The resulting solution

was mixed with 2.2g of STS. A clear gel was formed. The viscosity of the gel was

measured using a Brookfield viscometer (Model LVT, Spindle 2) at room temperature.

The results are listed in Table II

Example 6

3.0 g of TTAC was dissolved in 300 ml of tap water. The resulting solution was

mixed with 2.2 g of STS. A clear gel formed. The gel was tested in the same manner as

in Example 5. The results are shown in Table II.

Example 7

3.0 g of OTAC was dissolved in 300 ml of tap water. The resulting solution was mixed with 0.72 g active substance of SS). A clear gel formed. The gel was tested in the

same manner as in Example 5. The results are shown in Table II.

Example 8

4.5 g of TTAC and 1.26 g of of Na-BON in aqueous NaOH solution were mixed

in 300 ml of tap water. A clear gel was formed. The gel was tested in the same manner

as in Example 5. The results are listed in Table II.

Table II

From the results, it is evident that combinations of the cationic surfactant and an

organic salt in an aqueous medium form clear gels with good rheological properties.

These gels can be used for hydraulic fracturing applications. Furthermore, the gels can

also be used as ioamed fracturing fluids having high foam quality and stability without

employing additional foaming surfactant. For applications requiring higher viscosity,

higher surfactant loading is generally required.

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

cationic surfactant of the general formula

CHs I

I

CH₃

wherein R is a hydrophobic group and X is an anionic group and at least one organic

salt of the general formula

wherein R, is an organic anion and M is a cation.

2. The fracturing fluid of claim 1, wherein the hydrophobic group R

is a saturated or unsaturated alkyl group containing 14 -22 carbon atoms.

3. The fracturing fluid of claim 1, wherein said organic anion Ri is selected from the group consisting of 3-hydroxy-2-naphthalenecarboxylate; cumene sulphonate; salicylate and toluene sulphonate.

4. The fracturing fluid of claim 1, wherein said cation M is a monovalent cation.

5. The fracturing fluid of claim 2 wherein said hydrophobic group R is selected from the group consisting of tetradecyl, hexadecyl; octadecyl; oleyl; and tallow.

6. The fracturing fluid of claim 2 or 5, wherein said anionic group X is a monovalent group.

7. The fracturing fluid of any of claims 1 to 6, wherein said cationic surfactant is a tallow trimethyl ammonium halide.

8. The fracturing fluid of any of claims 1 to 6, wherein said cationic surfactant is a tallow trimethyl ammonium salt.

9. The fracturing fluid of any of claims 1 to 6, wherein said cationic surfactant is selected from the group consisting of octadecyl trimethyl ammonium, cetyl trimethyl ammonium and tetradecyl trimethyl ammonium salts and mixtures thereof.

10. The fracturing fluid of any of claims 1 to 6, wherein said cationic surfactant is selected from the group consisting of octadecyl trimethyl ammonium, cetyl trimethyl ammonium and tetradecyl trimethyl ammonium halides.

11. The fracturing fluid of claim 1, wherein said aqueous medium is water, said cationic surfactant is octadecyl trimethyl ammonium chloride and said organic electrolyte is sodium toluene sulphonate.

12. The fracturing fluid of claim 1, wherein said aqueous medium is water, said cationic surfactant is octadecyl trimethyl ammonium chloride and said organic electrolyte is sodium salicylate.

13. The fracturing fluid according to claim 1, wherein said aqueous medium is a solution of NaOH in water, said cationic surfactant is tallow trimethyl ammonium chloride and said organic electrolyte is the sodium salt of 2-hydroxy-3-naphtoic acid.

14. The fracturing fluid according to claim 1, wherein said aqueous medium is water, said cationic surfactant is trimethyl ammonium chloride and said organic electrolyte is the sodium salt of cumene sulphonate.

15. The fracturing fluid according to claim 1, wherein said aqueous medium is water, said cationic surfactant is tallow trimethyl ammonium chloride and said organic electrolyte is sodium toluene sulfonate.

16. A fracturing fluid according any one of claims 1 to 15 including a gas.

17. A fracturing fluid according to claim 16, wherein said gas is N₂ or CO2.

18. A method of fracturing subterranean formation comprising the step of

injecting the fracturing fluid of any of claims 1 to 17 into the formation at a

pressure sufficient to initiate fracturing.