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Chapter 10

Surfactants

Surface active agents are included in most aqueous treating fluids to improve the

compatibility of aqueous fluids with the hydrocarbon-containing reservoir. To

achieve a maximal conductivity of hydrocarbons from subterranean formations

after fracture or other stimulation, it is the practice to cause the formation

surfaces to be water-wet.

Alkylamino phosphonic acids and fluorinated alkylamino phosphonic acids

adsorb onto solid surfaces, particularly onto surfaces of carbonate materials in

subterranean hydrocarbon-containing formations, in a very thin layer. The layer

is only one molecule thick and thus significantly thinner than a layer of water

or a water-surfactant mixture on water-wetted surfaces (Penny, 1987a,b;Penny

and Briscoe, 1987).

These compounds so adsorbed resist or substantially reduce the wetting of

the surfaces by water and hydrocarbons and provide high interfacial tensions

between the surfaces and water and hydrocarbons. The hydrocarbons displace

injected water, leaving a lower water saturation and an increased flow of

hydrocarbons through capillaries and flow channels in the formation.

PERFORMANCE STUDIES

The primary purpose of surfactants used in stimulating sandstone reservoirs is

to reduce the surface tension and the contact angle thus to provide control of

the fluid loss. However, many of the surfactants are adsorbed rapidly within

the first few inches of the sandstone formations. In this way, their effectiveness

with respect to deeper penetration is reduced.

Experimental and field studies of various surfactants used in the oilfield have

been described (Paktinat et al., 2007).

Several different surfactants were investigated to determine their adsorption

properties when injected into a laboratory sandpacked column. In addition,

field data were collected from Bradford, Balltown, and Speechley sandstone

formations. The correlation between laboratory and field data was confirmed.

Reservoirs that were treated with microemulsion fluids demonstrate

exceptional water recoveries when compared with conventional surfactant

Hydraulic Fracturing Chemicals and Fluids Technology. http://dx.doi.org/10.1016/B978-0-12-411491-3.00010-8

2013 Elsevier Inc. All rights reserved. 121


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122 Hydraulic Fracturing Chemicals and Fluids Technology

treatments. These investigations are considered to serve as case studies and

can be used to minimize formation damage (Paktinat et al., 2007).

VISCOELASTIC SURFACTANTS

Typical viscoelastic surfactants are N-erucyl-N,N-bis(2-hydroxyethyl)-N-

methyl ammonium chloride and potassium oleate, solutions of which form

gels when mixed with corresponding activators such as sodium salicylate and

potassium chloride (Jones and Tustin, 2007).

A methyl quaternized erucyl amine is useful for aqueous viscoelastic

surfactant-based fracturing fluids in high-temperature and high-permeability

formations (Gadberry et al., 1999).A problem associated with the use of viscoelastic surfactants is that stable

oil-in-water emulsions are often formed between the low-viscosity surfactant

solution, i.e., the broken gel and the reservoir hydrocarbons. As a consequence,

a clean separation of the two phases may be difficult to achieve, complicating

the cleanup of wellbore fluids. Such emulsions are believed to form because

conventional wellbore fluid viscoelastic surfactants have little or no solubility

in organic solvents (Jones and Tustin, 2007).

Cationic Surfactants

A number of cationic surfactants, based on quaternary ammonium and

phosphonium salts, are known to exhibit solubility in water and hydrocarbons

and are frequently as such used as phase-transfer catalysts (Starks et al., 1994).

However, the particular cationic surfactants which form viscoelastic

solutions in aqueous media are poorly soluble in hydrocarbons, and are

characterized by partition coefficients Ko,w for a surfactant in oil and water

close to zero.

The partition coefficient of a substance is the ratio of the concentrations inequilibrium in two non-miscible fluids, such as oil and water.

Ko,w =co

cw

. (10.1)

The partition coefficient can be determined by various analytical techniques

(Sharaf et al., 1986).

For example, cyclic voltammetry has been used for the determination of

the critical micelle concentration of surfactants, self-diffusion coefficient of

micelles, and the partition coefficient (Mandal and Nair, 1991). Also, high-performance liquid chromatography is a suitable technique (Terweij-Groen

et al., 1978).

Typically, the high solubility of a cationic surfactant in hydrocarbon solvents

is promoted by multiple long-chain alkyl groups attached to a head group, as can


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123CHAPTER | 10 Surfactants

be found in hexadecyltributylphosphonium ions and trioctylmethylammonium

ions.

In contrast, cationic surfactants which form viscoelastic solutions generally

have only one long straight hydrocarbon chain per surfactant head moiety (Jonesand Tustin, 2007).

Anionic Surfactants

A few anionic surfactants exhibit a high solubility in hydrocarbons but low

solubility in aqueous solutions. A well-known example is sodium bis (2-

ethylhexyl) sulfosuccinate (Manoj et al., 1996). This compound does not form

viscoelastic solutions in aqueous media. So, the addition of a salt causes

precipitation. Thermodynamic studies suggest that the micellization processis endothermic in nature so that it is mainly an entropy governed process.

The solubility of a surfactant in hydrocarbon tends to increase as the size

of the side chain decreases. It is believed that this occurs because smaller

side chains cause less disruption to the formation of inverse micelles by the

surfactant in the hydrocarbon, such inverse micelles promoting solubility in the

hydrocarbon (Jones and Tustin, 2007).

By altering the degree and type of branching from the principal straight

chain, the surfactant can be tailored to be more or less soluble in a particular

hydrocarbon. Preferably the side chain is bonded to the -carbon atom. Bylocating the side chain close to the charged head group promotes the most

favorable combinations of viscoelastic and solute properties. The synthesis of

a -branched fatty acid is shown schematically in Figure10.1.

Preparation 10.1. Synthesis of 2-methyl methyl oleate (Jones and Tustin,

2007): Sodium hydride is washed with heptane and then suspended in

tetrahydrofuran. 1,3-Dimethyl-3,4,5,6-tetrahydro-2(1H)-pyrimidinone is then

added and the mixture is stirred in a nitrogen atmosphere. Then methyl oleate

O

OCH3

O

OCH3

CH3

O

OH

CH3

OH-, H+

CH3CuLi

FIGURE 10.1 Synthesis of-branched fatty acids (Jones and Tustin, 2007).


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O

OCH3

O

OCH3

CH3

O

CH3

O-

CH3 J

NaH

OH-

FIGURE 10.2 Synthesis of 2-methyl methyl oleate (Jones and Tustin, 2007).

is dropwise added over a period of 2h and the resulting mixture is heated to

reflux for 12 h and then cooled to 0 C.

Further, methyl iodide is added dropwise and the reaction mixture is again

heated to reflux for a further 2h. Afterwards the reaction mixture is cooled

to 0 C and quenched with water, concentrated in vacuo and purified by

column chromatography to give the end product 2-methyl methyl oleate as a

yellow oil.

The synthesis of 2-methyl methyl oleate and the subsequent hydrolysis of

the ester is shown in Figure10.2. A rigid gel is formed when a 10% solution of

potassium 2-methyl oleate is mixed with an equal volume of a brine containing

16% KCl.

Contacting this gel with a representative hydrocarbon, such as heptane,

results in a dramatic loss of the viscosity and the formation of two low-viscosity

clear solutions:

1. An upper oil phase.

2. A lower aqueous phase.

The formation of an emulsion was not observed. Thin-layer chromatography

and infrared spectroscopy showed the presence of the branched oleate in both

phases.

The gel is apparently broken by a combination of micellar rearrangement and

dissolution of the branched oleate in the oil phase. Consequently the breaking

rate of the branched oleate is faster than that of the equivalent linear oleate. This

is demonstrated in Figure10.3which is a graph of gel strength against time

at room temperature for an unbranched potassium oleate gel and a branchedpotassium 2-methyl oleate gel.

Both gels were prepared from 10% solutions of the respective oleate mixed

with equal volumes of a brine containing 16% KCl. Each gel was then contacted

with an equal volume of heptane.


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1

1.5

2

2.5

3

3.5

4

0 2 4 6 8 10 12 14

Gelstrengthcode

Break time/[h]

Potassium oleate2-Methyl potassium oleate

FIGURE 10.3 Gel strength versus time (Jones and Tustin, 2007).

TABLE 10.1 Gel Strength Codings (Jones and Tustin, 2010)

Number Description

1 Original viscosity

2 Weak flowing gel

3 Tonguing gel

4 Deformable nonflowing gel

The gel strength is a semiquantitative measure of the flowability of the

surfactant-based gel relative to the flowability of the precursor fluid before

addition of the surfactant. There are four gel strength codings. These codings

are summarized in Table10.1.

Using infrared spectroscopy, the value ofKow for the potassium 2-methyl

oleate of the broken branched gel was measured as 0.11. In contrast the value

ofKow for the potassium oleate of the broken unbranched gel was measured as

effectively zero.The rapid breakdown of the branched oleate surfactant gels, with little or no

subsequent emulsion, leads to the expectation that these gels will be particularly

suitable for use as wellbore fluids, such as fluids for hydraulic fracturing of oil-

bearing zones. Excellent cleanup of the fluids and reduced impairment of zone


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matrix permeability can also be expected because emulsion formation can be

avoided (Jones and Tustin, 2007).

Anionic Brominated Surfactants

In the case of ionic surfactants, an excess of counterions reduces the repulsive

interaction between the charged head groups. The surfactant molecules at both

ends of such cylindrical aggregates bear an excess of energy in comparison to

the molecules in the inner of the cylindrical part. This excess energy is addressed

as end cap energy and it is the driving force for the linear growth of cylindrical

micelles. Subsequently, the micelles entangle with each other, which results in

a viscoelastic behavior (Lee et al., 2010).

A problem associated with the use of such surfactants is the potentialformation of stable oil in water emulsions during a cleanup operation. This

behavior arises due to the limited solubility in hydrocarbon of conventional

viscoelastic surfactants.

However, it has been demonstrated that a conventional surfactant, such as

potassium oleate, can be brominated and in this way the properties are improved

(Lee et al., 2010). The bromination of oleic acid is done in a hexane solution to

which a solution of HBr in acetic acid is added. In this way 9-bromo stearate is

obtained.

The partition coefficient and the gel break time for various concentrationsof 9-bromo stearate and potassium oleate in 8% KCl solution are shown in

Table 10.2. As can be seen from Table 10.2, the bromination causes a significant

change in the partition coefficient, but does not change the gel break time.

Furthermore, the bromination of the hydrocarbon chain keeps the

viscoelasticity of the surfactant. Thus, the essential property for oilfield

applications is maintained. When the shear stress is removed in the formation,

the solution reverts to its viscous state. However, the bromination reduces the

zero shear viscosity from 528Pas to 180Pas. In contrast, the shear viscosity

at 100s1 is reduced from 0.48 Pa s to 0.16Pa s by the bromination at aconcentration of 5% in 8% KCl.

TABLE 10.2 Partition Coefficient and Gel Break Time (Lee et al., 2010)

Concentration (%) Partition (%) Gel Break Time (h)

BST OLE BST OLE

1 17.1 0 12 14

5 13.28 0 82 84

10 13.49 0 94 98

BST 9-Bromo stearate and OLE Oleate.


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REFERENCES

Gadberry, J.F., Hoey, M.D., Franklin, R., Del Carmen Vale, G., Mozayeni, F., 1999. Surfactants

for hydraulic fracturing compositions. US Patent 5 979 555, assigned to Akzo Nobel NV, 9November 1999.Jones, T.G.J., Tustin, G.J., 2007. Surfactant comprising alkali metal salt of 2-methyl oleic acid

or 2-ethyl oleic acid. US Patent 7 196041, assigned to Schlumberger Technology Corp.,Ridgefield, CT, 27 March 2007. .

Jones, T.G.J., Tustin, G.J., 2010. Process of hydraulic fracturing using a viscoelastic wellbore fluid.US Patent 7 655 604, assigned to Schlumberger Technology Corp., Ridgefield, CT, 2 February2010. .

Lee, L., Salimon, J., Yarmo, M.A., Misran, M., 2010. Viscoelastic properties of anionic brominatedsurfactants. Sains Malays. 39 (5), 753760.

Mandal, A.B., Nair, B.U., 1991. Cyclic voltammetric technique for the determination of thecritical micelle concentration of surfactants, self-diffusion coefficient of micelles, and

partition coefficient of an electrochemical probe. J. Phys. Chem. 95 (22), 90089013.http://dx.doi.org/10.1021/j100175a106.Manoj, K.M., Jayakumar, R., Rakshit, S.K., 1996. Physicochemical studies on reverse micelles of

sodium bis(2-ethylhexyl) sulfosuccinate at low water content. Langmuir 12 (17), 40684072.http://dx.doi.org/10.1021/la950279a.

Paktinat, J., Pinkhouse, J.A., Williams, C., Clark, G.A., Penny, G.S., 2007. Field case studies:damage preventions through leakoff control of fracturing fluids in marginal/low-pressure gasreservoirs. SPE Prod. Oper. 22 (3), 357367.

Penny, G.S., 1987a. Method of increasing hydrocarbon production from subterranean formations.US Patent 4 702 849, 27 October 1987.

Penny, G.S., 1987b. Method of increasing hydrocarbon productions from subterranean formations.EP Patent 234 910, 2 September 1987.

Penny, G.S., Briscoe, J.E., 1987. Method of increasing hydrocarbon production by remedial welltreatment. CA Patent 1 216 416, 13 January 1987.Sharaf, M.A., Illman, D.L., Kowalski, B.R., 1986. Chemometrics. Wiley, New York.Starks, C.M., Liotta, C.L., Halpern, M., 1994. Phase-transfer catalysis: fundamentals, applications,

and industrial perspectives. Chapman & Hall, New York.Terweij-Groen, C.P., Heemstra, S., Kraak, J.C., 1978. Distribution mechanism of ionizable

substances in dynamic anion-exchange systems using cationic surfactants in high-performanceliquid chromatography. J. Chromatogr. A 161, 6982. http://dx.doi.org/10.1016/S0021-9673(01)85213-4.

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