Study on the Thermal Stability of Viscoelastic Surfactant-Based Fluids

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Industrial & Engineering Chemistry Research

Study on Thermal Stability of Viscoelastic Surfactant based fluids bearing lamellar structures Atrayee Baruah, Akhilendra K. Pathak, and Keka Ojha* Department of Petroleum Engineering, Indian School of Mines, Dhanbad, Jharkhand 826004, India S Supporting Information ABSTRACT: A detailed study has been done on the phase behavior and rheologic al properties of Viscoelastic Surfactant (VES) bas ed fracturing fluids bearing lamellar liquid crystal structures that are developed from two surfactants (S DS and NaOA ) and three c o-surfactants (propan-2-ol, iso-amyl alcohol and 2-ethyl hexanol) in t he presence of clove oil as the organic phase and water as the aqueous phase. Lamellar liquid c rystals prepared from NaOA/2-ethyl hex anol/clove oil/water system demonstrated the best viscoelastic properties among all the developed fluids as it exhibited resistance to high temperature and shear conditions. Addition of alkali and nano-particles enhanced t he viscoelastic properties which were observed from static and dynamic rheological tests.

1. INTRODUCTION Surfactants form a unique class of chemical compounds which have found important practical applications in almost every area of chemistry as they consist of a hydrophilic he ad-group and hydrophobic tail that can make possible molecular aggregate in water solutions above a Critical Micelle Concent ration (CMC).

1-3

Surfactants are amphiphilic molecules which reduc e the interfacial

tension (IFT) between water and oil phases. When a co-surfactant (non-ionic molecule) is added, it plays the function of dec reasing the repulsive forces between the charged hydrophilic parts of the 4

surfactant molecule. As a c onsequence, co-surfactants change the CMC, the size of the micelles, 5

and the phas e behavior in the surfactant solutions. Surfactant solutions resulting in the formation of spherical micelles (isotropic ) have low viscosity, whereas, the formation of anisot ropic micelles (e. g., 6

rod-shaped/ non-spherical) results in a distinct rise in the viscosity of the solution. With the increase in surfactant concentration, inter-micellar forc es become progressively more important as they may cause either a change in the critical packing parameter leading to a further shape transition or a disorder/order transition to the liquid crystalline state. These self -assembled structures are easily tunable by changing the concentration of surfactant that leads to the formation of discontinuous cubic phase, lamellar phase, hexagonal phase, discontinuous cubic phase, and reverse hexagonal phase that contribute to viscoelastic behavior.

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These self-assemblies are, however, considered as an

ordered network of giant micelles that have found application in detergents and related industries, pharmacy, lubricants, oil industries, etc. The liquid crystalline phas e, are considered an intermediate state of matter bet ween liquids and solids. In the c rystalline state, the movement of molecules is practically restricted, while in liquids, these exhibit structures with short-range order where molecules can readily flow.

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However, molecules in liquid crystals achieve some freedom of relative movement

while they still retain some kind of long-range order.

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ACS Paragon Plus Environment


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Viscoelastic surfactant (VES) based fluid systems have been used for various oilfield operations, including gravel -pack completions, frac packs, acidizing and lost circulations. These are the preferred gelling and viscosity generating agents for fluids used in the production zone because of their nondamaging effects on the res ervoir.

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However, VES fluids can also be used as an alternative to

conventional fracturing fluids as thes e are operationally simple to use, leave no residue t o damage the formation and facilitate a much faster clean up.

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These fluids constituting of micelles are in

dynamic equilibrium with their monomers as they can break and recombine rapidly unlike the polymer based fluids.

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In the petroleum industry, shallow oil fields ha ve already been exhausted, and consequently the natural res ourc es are now being extracted from subterranean formations or reservoirs.

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The

development of fracturing fluids has obt ained great attention for high temperature conditions prevailing in greater depths for increased oil production. P olymer-based fracturing fluids have been utilized for reservoir temperature conditions as high as 220°C; however surfactant based fluids with its various advantages over polymer based fluids could be developed for high temperature conditions. In our previous work, we performed a comparative study on the development of fracturing fluid synthesized from mixed (Cocamidopropyl Betaine, CAPB+ Sodium Dodecyl Sulfate, SDS) surfactant/iso-amyl alcohol/pine oil/ water and single surfactant (S DS)/iso-amyl alcohol/pine oil/wat er system. Worm-like micelles constituting the fluid developed from the mixed surfactant system at 30% C/S (percentage of co-surfactant and surfactant mixture in the solution) exhibited satisfactory viscosity at 100 s

-1

shear rate under a temperature of 60°C.

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Subsequently, we have worked on t he

development of fracturing fluid having lamellar structures from S DS/iso-amyl alcohol/pine oil/wat er and SDS/tert-amyl alcohol/pine oil/water system. The paper report ed the development of fracturing fluid that could be utilized up to 70°C temperature only.

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In this present study, we endeavored to develop fracturing fluids for temperature conditions in the range of 100-120°C. Six quaternary systems were used for the development of VES fluids by using two surfactants (Sodium oleate, NaOA and Sodium Lauryl Sulfate, SDS ), three co -surfactants (propan-2-ol, iso-amyl alcohol and 2-ethyl hexanol), clove oil and distilled water. Clove oil was selected as the organic phase since it provided a higher boiling point (~251°C) than pine oil (~195°C) that could help in the development of fracturing fluid for high temperature conditions. Pseudo-ternary phase diagrams were plotted separately to determine the gel region for the development of fracturing fluid. Static and dynamic rheological t ests were carried out to determine the viscoelastic properties of the prepared gels. The effect of adding alkali (sodium hydroxide, NaOH) and nano -particle (silicon oxide, SiO2) on the lamellar liquid crystal was explored. The lamellar liquid crystals prepared from NaOA/2-ethyl hexanol/clove oil/wat er system, presented superior thermal stability than the rest of t he fluids. Miscibility test and static particle settling test were carried out for the fluid that provided superior rheological outcome.

2. EXP ERIMENTAL SECTION 2.1. Materials. Sodium Lauryl Sulfat e (SDS, >85%), 3-methyl butane-1-ol (iso-amyl alcohol, >98%), sodium hydroxide (NaOH, >98%), 2-ethyl hex anol (> 99%), propan-2-ol (>99% ) were all


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received from Loba Chemie P vt. Ltd. S odium oleate (NaOA, >82% ) and Clove oil was procured from Sigma Aldrich. Silicon dioxide (SiO 2) nano-particle of 15 nm, size was obt ained from S RL P vt. Ltd. Distilled wat er was used for sample preparation. 2.2. Pseudo-ternary pha se diagram and Critical Micelle Concentration (CMC). Pseudo-ternary phase diagrams were plotted for six quaternary systems developed from two surfactants (NaOA and SDS), three co-surfactants (propan-2-ol, iso-amyl alcohol and 2-ethyl hexanol), clove oil and distilled water. These diagrams helped in the determination of t he range of composition that forms monophasic region. The co-surfactant to surfactant (C/S) ratio was fix ed at 1: 2 (or 0.5). The phase diagram was constructed using a water titration method fix ed at 3 0 ± 1°C. The ratio of oil to C/S was varied from 1: 9 to 9:1 by weight and 1 ml of water was added to the mixture consisting of surfactant, cosurfactant and oil at their specific weight ratios which was stirred in a magnetic stirrer at a moderate speed. The mixture systems were equilibrated for 30 minutes to visually differentiate the various phases and were classified according to Winsor’s classification (WI, WII, WIII and W IV). The addition of wat er was continued until a Winsor’s type II fluid system was obt ained. The diagrams were constructed by plotting the weight percent of C/S, oil and wat er. The CMC of the t wo surfactants in the presence of thr ee different co-s urfactants were determined by using the i nterfacial tension (IFT) measurement met hod. The int erfacial tension of the different concentrated surfactant solutions was measured by a programmable tensiometer (K russ GmbH, Germany, model: K20 Easy Dyne) under atmospheric pressure and temperature of 30 ± 1 °C by the Du Nouy ring method. The platinum ring of the tensiometer was cleaned with acetone and flame dried before each measurement and the standard deviation did not exceed ±0.1 mN/m. 2.3. Rheological measurements. The static and dynamic rheological properties were measured on a Bohlin Gemini Rheometer (M/s Malvern Instruments Ltd., U.K) using a parallel plate geometry having a diameter of 25 mm. The distanc e bet ween the plates was maintained at 500 microns. The -1

variation of viscosity with shear rate (1 -1000 s ) at a specific test temperature was carried out. In t he dynamic rheological test, the amplitude sweep test was carried out to determine the linear viscoelastic region of the prepared samples. Later a frequency sweep test (0.01 -10 rad/s) was performed at a constant stress in the linear viscoelastic zone of the samples at 40°C. 2.4. Miscibility test. Miscibility tests of the prepared fluids were carried out in the presence of diesel oil and water at definite mixing proportions with gel to water/diesel oil ratio fixed at 1:3, 1:2, 1:1, 2:1 and 3:1 under 80, 90, 100, 110 and 120°C test temperature. The mixing of the fluids was done in a mechanical stirrer. The viscosities of the mixed solutions were determined in a Cannon-Fensky Viscometer by using different orifice size depending on the viscosities of the samples. The viscomet er determined kinematic viscosities which were converted to absolute viscosities by measuring t he densities of the fluids. 2.5. Static Particle Settling Test. Static particle settling experiments was performed at different temperatures (100, 110, 120°C) in a 250 ml measuring glass cylinder having diameter at least 25 times the diameter of the particles. This was done to ensure that there is no effect of confining walls on the settling velocity of the particles.

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The test was carried out at 4 ppg sand concentrations



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having 20/40 mesh size.

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To determine the average settling velocity of the particles , the following

steps were followed: a) Particles of predetermined size and concentration were mixed homogeneously with the VES fluid by means of a mechanical stirrer. b) The mixture was then kept in static condition at a specific test temperat ure. c) Rate of change of height of the clear solution was determined visually at 3hrs interval up to 24hrs. This rate is determi ned as the settling velocity. d) A verage settling velocity at 24 hrs is reported as the settling rate of the particles. e) To ensure reproducibility, at least three measurements we re performed under each test temperature for the reported average settling velocity of the particles. 3. RES ULTS AND DISCUSSION 3.1. Pseudo-ternary pha se behavior studies of quaternary system s. The pseudo-ternary phase diagram for six quaternary systems consisting of S DS/propan-2-ol/clove oil/water, S DS/isoamyl alcohol/clove oil/ water, SDS/2-ethyl hexanol/clove oil/water, NaOA/propan -2-ol/clove oil/water, NaOA/iso-amyl alcohol/clove oil/ water and NaOA/2-et hyl hexanol/clove oil/ water are shown in Figure 1. A solid-liquid, SL region is characterized by a solid-like phas e along with a liquid phase. A yellow gel (YG) region is identified by a highly viscous gel phase that is yellow colored while a yellow viscous liquid (YVL) corres ponds to a low viscosity liquid that is yellow colored. A microemulsion (ME ) region consists of a single phase, transparent, low viscosity-homogeneous mixture. It is a thermodynamically stable dispersion of otherwise immiscible water and oil stabilized by surfactant where the micelles are not in order form.

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At low surfactant concentrations, Winsor’s type II (W II) region is recognized by the

presence of milky emulsion phase with excess water and Winsor’s type I (W I) region occurs near t he oil rich zone consisting of excess oil phas e with a milky emulsion phase. A one-phas e emulsion (1E) region is indicated by a milky phase. Near the SL region, a gel (G) region exists, characterized by the presence of a clear, transparent, highly viscous fluid which shows no change in the meniscus aft er tilting a beak er to an angle of 90°.

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Adjacent to this region, a low viscosity gel (LVG) region is present

that is denoted by a transparent-low viscosity fluid. Surfactants or amphiphilic molecules form micro -heterogeneous structures in solutions, namely, 3

micelles, above a Critical Micelle Concentration, CMC. The CMC of SDS and NaOA surfactants are 8.2 and 1.08 mM respectively. The long hydrophobic tail length of NaOA (C18) surfactant participates in the formation of micelles more effectively at a relatively lower surfactant concentration than the SDS (C12 ) surfactant. In the pres ence of propan-2-ol, iso-amyl alcohol and 2-ethyl hexanol as cosurfactants, the CMC of SDS gets reduced to 6.9, 5.5 and 4.4 mM respectively. However, the CMC of NaOA gets reduc ed to 0.98, 0.92 and 0.79 mM respectively. The plots of CMC values for both t he surfactants in the presence of three co -surfactants are given in the s upplementary file. Co-surfactant, 2-ethyl hexanol, having longer carbon-chain lengt h, was more effective in dec reasing the CMC value for both the surfactants. Surfactant molecules usually form spherical micelles above the CMC value, with the hydrophilic head facing towards t he water region and the lipophilic tail towards the oil region as shown in Figure 2. As sodium oleate (NaOA ) surfactant has greater (C 18) hydrophobic tail lengt h than SDS (C12) surfactant, there is larger penetration of the tail into the oil section. The spherical micelles get


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transformed to rod-like micelles on the inc rease of surfactant concentration. With further increase of surfactant conc entration, the surfactant molecules begin t o arrange themselves into loose patterns to form liquid crystal phase having lamellar structures via the hexagonal structures. The lamellae lyotropic liquid crystal phase has a planar interface. The gel region corresponds to the lamellar lyotropic phase that consist of a simple one dimensional crystal composed of alternating layers of water and surfactant bilayers with polycrystalline characteristics that presents rheological properties clearly di fferent from others.

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The

molecular packing consists of double layers with t he water-insoluble tails together while t he hydrophilic part of the molecule in the boundaries with water. These double layers pack parallel to one another and are separat ed from each other by a water layer. The hydrocarbon chains are to some extent disordered; exhibiting essentially fluid-like properties that are able to move in all directions, but this ability is restricted by the interactions of hydrophilic groups with wat er.

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The quaternary systems consisting of SDS/propan-2-ol/clove oil/ water, SDS/iso-amyl alcohol/clove oil/water and SDS/2-ethyl hexanol/clove oil/water, the gel formation takes place having a minimum %C/S concentration of 11% (water= 62-77%, oil= 12-15%), 23% (water= 48-66%, oil= 11-19% ) and 17% (wat er= 65-80%, oil= 3-7%) respectively. While for NaOA/propan-2-ol/clove oil/ water, NaOA/isoamyl alcohol/clove oil/water and NaOA/2-ethyl hex anol/clove oil/wat er quaternary systems, the gel formation takes place having a minimum %C/S concentration of 13% (water= 65-76%, oil= 11-17%), 23% (wat er= 47-73%, oil= 4-15% ) and 18% (water= 61-79%, oil= 3-6% ) respectively. As a result, the formation of gel for the development of fracturing fluid is sensitive to the amount of constituents (surfactant, co-surfactant, clove oil and water) and the type of co-surfactant and surfactant used. 3.2. Rheological behavior of V ES fluids. Figure 3(a) shows the variation of shear stress (τ, P a) -1

as a function of shear rate (γ, s ) for the six quaternary systems at 40°C. The developed fluid interprets a non-Newtonian fluid given by the non-linearity relation between shear stress and shear rate. The shape of the flow curves can be represented by a Power law equation (Ostwald-de Waele equation) as given below: τ= k (

(1)

From the power law model, as depicted in equation 1, the parameters for the flow behavior index (n) and consistency coefficient (k) are determined for each sample and are reported in Table 1. The values of the flow behavior index (n) are all less than 1, indicating the pseudo-plastic nature of t he samples. The lower the value of n, the greater is the pseudo-plastic characteristics of the sample.

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A

typical bi-layer structure consists of an alternate polar and apolar regions with the molecules arranged in a head to head, tail to tail packing motif.

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On the application of force, the one-by-one bimolecular

leaflets can slide bet ween bilayers that cause the VES sample t o flow.

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The lamellar liquid c rystal samples presented a decrease in apparent viscosity with increasing -1

shear rate (0.1-1000 s ) that indicates a shear thinning behavior at 40°C as shown in Figure 3(b). The -1

absence of a Newtonian plat eau of viscosity at lower shear rate (0.1-1 s ) indicates the presence of bilayer structures.

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-1

The decrease in viscosity for low shear rate ( 0.1-100 s ) is gradual while for -1

increased shear rat e (100-1000 s ), the viscosity decrease is rapid. The former behavior may be due to the orientation of the liquid crystal structure in the direction of shear as well as the breaking and the



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entanglement of surfactant monomers, constituting the lamellar crystal by the application of shear. The latter case may be due to t he breaking of t he mono-domains constituting the lamellar structure due to the application of high shear. This shear thinning behavior of the developed VES fluids would help in the pumping of the fluid down-hole in a well for conducting a fracturing job. The effect of temperature on the viscosity of the lamellar liquid crystals developed from the six quaternary systems at 100 s

-1

shear rat e is summarized in Figure 4. This shear rate was chosen

because it could be correlated to the operational conditions in which the gels are used to conduct a fracturing job.

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All the six fluids demonstrated a decrease in viscosity on the application of

temperature. The shear viscosity of the six fluids at 40°C is more, than beyond this temperat ure, since at a low temperature t he B rownian movement of the surfactant molecules is slow. This situation is more conducive for aggregation between surfactant monomers that help in the formation of dens elayered structure, contributing to increas ed shear viscosity.

14

However, at increased temperature,

decrease in viscosity can be correlated to the increased distanc e bet ween the adjoining surfactant monomers. The thickness of the bilayers decreases with increasing temperature that may be attributed to the folding of the hydroc arbon chains and/or the tilt of the molec ules in the layers.

15

The

results are in accordance with the SA XS outcome performed by Y ue et. al (2011) and Moran et. al (2004).

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Figure 5 indicates an increas e in the viscosity value with the inc reas e in %C/S concentration of the lamellar liquid crystals developed from any of the six quaternary systems. The increase in t he viscosity value denotes the development of a stronger and structured aggregation of mono -domains constituting the lamellar phase, which can resist shear stronger than the correspondi ng low concentrated lamellar liquid crystals.

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An increase in viscosity of the lamellar liquid crystals was observed with t he addition of sodium hydroxide alkali (0. 1% NaOH) for all t he six quaternary systems as presented in Figure 5. Electrostatic interactions exist between the anionic surfactant head -groups in aqueous solutions that +

may limit the growth of micelles. The addition of Na c ounter-ions in the presenc e of co-s urfactant reduces the electrostatic repulsive forces that induce closer packing of the surfactant monomers in the aggregates. A stronger network and a more steady internal structure of the lamellar liquid crystals is developed, that increases the lamellar-lamellar interactions causing the micelles to be packed more closely due to which these fluids can resist shear more strongly. As a consequence, the viscosity of the lamellar phase is improved wit h the increase in %C/S concentration and addition of alkali, while the viscosity decreased by the application of temperature. For hydraulic fracturing t reatments, the developed gels should have a viscosity greater t han 90 cP at a shear rate of 100 s

-1

for any operating temperature.

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However, 0.1% NaOH+24% C/S fluid

developed from NaOA/2-ethyl hexanol/clove oil/water system pres ented enhanced rheological properties as it presented a viscosity of 92.71 cP at 115.3°C while the other VES fluids offered lower temperature values for a corresponding minimum viscosity value of 90 cP. 3.3. Effect of lipophilic tail length of surfactant. Figure 5(a) denotes the variation of viscosity with temperature at 100 s

-1

shear rate for the lamellar liquid crystals developed from NaOA/2-ethyl

hexanol/clove oil/water and SDS/2 -ethyl hexanol/clove oil/water system at 24% C/S. The results


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denoted superior outcomes in the presenc e of NaOA unlike S DS surfactant. The rheological behavior of the lamellar liquid crystals developed from NaOA and SDS surfactant can be explained through Figure 6(a) and (b) respectively. NaOA surfactant has a large number of hydrophobic side chain group (C18 ), which increases the hydrophobicity as compared to SDS t hat has C 12 at oms only. The long hydrocarbon (hydrophobic) tail of NaOA molecule penetrates deeper into the non-polar (oil) region of the oil/ water interface as indicated by Figure 6(a).

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This leads to the presence of a stronger

network and a more steady internal structure of the lamellar liquid crystals that causes an increased lamellar-lamellar interaction. The surfactant molecules are packed more closely due to which t hese fluids can resist shear and temperature more strongly than the ordinary sample. On the other hand, the lamellar liquid crystals developed from S DS surfactant as presented in Figure 6(b) offered less penetration of the hydrophobic tail in the oil region that resulted in low lamellar-lamellar int eractions, leading to a low viscosity outcome. The explanation is the same in the presence of propan-2-ol as cosurfactant at the same %C/S that is revealed in Figure 5(c). Thus, lamellar liquid crystals developed from NaOA presented better thermal stability than SDS surfactant even at same % C/S for any specific co-surfactant. Figure 5(b) shows the effect of % C/S for lamellar liquid c rystals developed from S DS/iso-amyl alcohol/clove oil/water (32% C/S) and NaOA/iso-amyl alcohol/clove oil/water (28% C/S). The results indicate that the lamellar liquid crystals developed from SDS even at high %C/S presented a lower viscosity profile over the entire investigated temperature range. The lamellar lyotropic phase developed from SDS (C12) system has fewer numbers of hydrophobic chains in comparison to NaOA surfactant that favors the formation of less structured lamellar liquid crystals so as to have low thermal stability. As a result, even with increased % C/S of lamellar liquid crystals obtained from SDS surfactant, the rheological properties were poor in comparison to NaOA surfactant. 3.4. Effect of lipophilic tail length of co-surfactants. The lipophilic moiety of the three alcohols (propan-2-ol, iso-amyl alcohol and 2-ethyl hexanol) as co-surfactants also play a crucial role in t he rheological behavior for bot h the surfactants (NaOA and SDS) as revealed in Figure 5. The lamellar liquid crystals prepared from propan-2-ol, having short carbon-chain length, presented, a low viscosity profile and inferior t hermal stability compared to the longer carbon-chain lengt h co-s urfactants (isoamyl alcohol and 2-ethyl hexanol) for either of the surfactants. Propan-2-ol has a backbone carbonchain length of C3 only while iso-amyl alcohol and 2-ethyl hexanol has C4 and C6 atoms respectively. The presenc e of small length carbon-chain alcohols leads to low penetration of the lipophilic tail into the oil region as indicated in Figure 6(c). This marks the formation of a less rigid network structure that cannot provide a high sustaining viscosity at increased temperature. Convers ely, increased viscosity was presented in the presence of 2 -ethyl hexanol for bot h the surfactants that indicat ed t he formation of more rigid and structured mono-domains constituting the lamellar structure due to great er penetration of the lipophilic tail of alcohols into the oil region as presented in Figure 6(a). Therefore, longer carbon-chain co-surfactants participate in the development of a rigid lamellar liquid crystal that has more tolerance to temperature and shear. 3.5. Effect of nano-particle s. The application of VES fluids in reservoirs provides the disadvant age of low t hermal stability and inability to form a filter cake on the formation face that



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causes a large amount of fluid loss into the formation. With technological advancement, these fluids have been cross-linked with nano-particles to overcome the aforementioned shortcomings. For t hese reasons, the effect of adding SiO2 nano-particle on t he rheological behavior of lamellar liquid crystals for 0.1% NaOH+24% C/S developed from NaOA/2-ethyl hexanol/clove oil/water system was only analyzed, as this fluid exhibited superior rheological properties among the six developed fluids. The results summarized in Figure 7 indicated a marginal increase ( G″ in the investigat ed frequency range (0.01-10 rad/s). However, G′ present ed about one order of magnitude higher than G″ throughout the whole frequency range (0.01-10 rad/s) indicating the presence of lamellar liquid crystals.

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Whenever external forces

act on the parallel structure of the lamellar liquid crystal phase, it gets compressed and removing t he external force can restore it. Hence, the elastic property of the system is closely rel ated t o the tightness of the arranged surfactant mono-domains.

41

It is noted that the response of the lamellar

phase to shear components perpendicular to the planes of the lamellae is viscoelastic, with very long relaxation times as the lamellar phase has virt ually zero radius of curvature.

34

The relaxation time

corresponding to t he inverse of cross-over frequency has high values, such that it falls outside of t he spectrum of frequencies being investigated. As a result, no cross-over frequency point was observed between both the viscoelastic functions (G′ and G″) in the frequency range studied, which denotes the presence of lamellar liquid crystals.

30,33,42

In the presence of the same co-surfactant, lamellar liquid crystals developed from NaOA surfactant offered superior G′ and G″ values than obtained from SDS surfactant; while for any corresponding surfactant (S DS or NaOA), lamellar liquid crystals prepared from 2-ethyl hexanol pres ented enhanced G′ and G″ values in comparison to the other t wo co-surfactants (propan-2-ol or iso-amyl alcohol). The experimental results indicate that 0.1% NaOH+24% C/S VES fluid prepared from NaOA/2-ethyl


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hexanol/clove oil/wat er system, offered the best viscoelastic properties among the six developed fluids. The results were in agreement with the steady shear rheological properties. The improvement of G′ and G″ can be related to a system getting more and more structured. G′ increase is related to the compactness of the surfactant molecules in the interface layer while G″ is ascribed to the sliding between the layers constituting the lamellar liquid crystal.

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500 ppm SiO2+0.1% NaOH+24% C/S

VES fluid developed from NaOA/2-ethyl hexanol/clove oil/ water quaternary system, offered t he highest G′ and G″ values that is due to the participation of nano-particles in the formation of a more stable int ernal structure and the presence of increas ed interaction bet ween the mono-domains of t he lamellae structure. 3.7. Miscibility te st. Miscibility test was conducted to determine the breaking characteristics of the developed fluids with formation fluids (water and hydrocarbon) that help in predicting the recovery of the fracturing fluid aft er a fracturing job is completed. The test was conducted for 500 ppm SiO2+0.1% NaOH+24% C/S VES fluid developed from NaOA/2-ethyl hexanol/clove oil/ water quaternary system, as this fluid presented the best viscoelastic properties. Experiments indicate that the fluid was miscible with water in contrast to diesel oil at any definite mixing proportion. The viscosities of the homogeneous mixtures of the developed fluid in the presence of water at any definite mixing proportion are summarized in Table 2 that suggests low viscosity values. The reduction in viscosity value can be c orrelated to the transformation of the highly viscous lamellar structures int o low viscosity spherical micelles.

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The increase in temperature at any mixing proportion caused a further

decrease in viscosity that facilitate the formation of s pherical micelles. Consequently, water can be used to break the highly viscous fracturing fluid into a low viscosity fluid, to bring back the well into production. 3.8. Static Particle Settling Test. Proppants are pumped along with fracturing fluids to keep the created fractures open upon t he cessation of pumping.

44

Static particle settling test was conducted for

500 ppm SiO2+0. 1% NaOH+24% C/S VES fluid developed from NaOA/2-ethyl hexanol/clove oil/wat er system and the average settling velocities of all the particle sizes lumped together were recorded at increasing temperatures (100, 110 and 120°C) that are reported in Table 3. Results indicate that the particle settling velocity increased with temperature rise, as the effective viscosity of the fluid decreased. This could be associated with the formation of less stable lamellar structure that leads to the dec rease in the long range interactions responsible for the liquid crystal order. The consequences are the weakening of proppant-carrying capability of the fluid. good sand suspension capability of the VES fluid up to 120°C.

4.

45

Nevertheless, the results indicated

27

CONCLUSION

A comparative study on the phase behavior and rheological properties of lamellar liquid crystals developed from six different quaternary systems are investigated in details. Gel region was identified from the plotted pseudo-ternary phase diagram for the development of fracturing fluid. The gel formation depends on the type of surfactant (NaOA and S DS), co-surfactant (propan-2-ol, iso-amyl alcohol and 2-ethyl hexanol) used and is sensitive to the amount of surfactant, co -surfactant, organic phase and aqueous phase present. The developed lamellar liquid crystals basically manifested



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pseudo-plastic nature that exhibited shear-thinning behavior on the application of shear. These qualities of the fluids would help in pumping of the fluid downhole in an oil well. Unlike SDS surfactant, lamellar liquid crystals developed from NaOA surfactant always present ed better viscoelastic properties for any corresponding co-surfactant. 2-Ethyl hexanol as a co-surfactant constantly presented s uperior rheological properties for bot h the surfactants in contrast to the other t wo alcohols (propan-2-ol and iso-amyl alcohol). Consequently, lamellar liquid crystals prepared from NaOA/2-ethyl hexanol/clove oil/water system offered the finest viscoelastic properties among t he developed fluids as denoted by static and dynamic rheological results. Improvement in viscoelastic properties was obtained with the addition of alkali and nano-particle. In conclusion, our study revealed that 500 ppm SiO2+0.1% NaOH+ 24% C/S VES fluid prepared from NaOA/2-ethyl hex anol/clove oil/wat er quaternary system could behave as an efficient fracturing fluid for an operating temperature of 117°C as it offered satisfactory viscoelastic properties and proppant suspension's ability. The fluid was miscible with water, therefore water c ould be used to break the highly viscous fracturing fluid int o a low viscosity fluid for easy recovery during a post-fracturing job. ■ ASSOCI ATED CONTENT S Supporting Information Figure 1(a) and (b) in the Supporting Information document presents the plots of Int erfacial Tension, mN/m versus surfactant concentration, % wt. for S DS and NaOA surfactants in the presence of propan-2-ol, iso-amyl alcohol and 2-ethyl hexanol. Table 1 summarizes t he CMC values of t he surfactants. This material is available free of charge via the Internet at http://pubs.acs.org/. ■ AUTHOR INFORMATION Corre sponding Author *E-mail: [email protected]. Phone: +91-326-2235484. Fax: +91-326-2296632. Note s The authors declare no competing financial interest. ■ ACKNOWLEDGMENTS The authors thank the Council of Scientific and Industrial Research, India, for financial assistant [CSIR Project 22 (0625)/13/EMR-II] and the Department of Petroleum E ngineering, Indian School of Mines, Dhanbad, India. ■ REFERENCES (1)

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

C/S= 0.5 0.00

1.00

0.25

0.75

SL

0.50

ME

0.75

E 1.00

0.50

0.25

LVG

G

YG

YVL

WII

0.00

0.25

Water

WI

0.00

0.50

0.75

1.00

(a)

C/S= 0.5 0.00

1.00

0.25

SL

0.50

ME

0.00

Water

0.50

LVG

E G

0.75

1.00

0.75

0.25

YVL YG

WII 0.25

WI 0.50

0.75

0.00 1.00

(b)


Oil

Oil

Page 14 of 22

Page 15 of 22

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


C/S= 0.5

0.00

1.00

0.25

0.75

SL 0.50

0.50

LVG 0.75

0.25

G YVL YG

ME E

1.00

WI

WII

0.00

0.25

Water

0.50

0.75

0.00 1.00

Oil

(c)

C/S=0.5

0.00

1.00

0.25

0.75

SL

0.50

ME

0.75

E 1.00

Water

0.00

ME

0.50

G YVL

0.25

LVG YG WI

WII

0.25

0.50

0.75

0.00 1.00

(d)


Oil


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

C/S=0.5 0.00

1.00

0.25

0.75

SL

0.50

0.50

LVG G

0.75

E ME

0.25

YVL YG

WII

1.00 0.00

0.25

Water

WI 0.50

0.00 1.00 Oil

0.75

(e)

C/S= 0.5 0.00

1.00

0.25

0.75

SL

0.50

0.50

LVG 0.75

0.25

YVL

G ME

E

YG WI

1.00 WII 0.00

Water

0.25

0.50

0.00 0.75

1.00

Oil

(f) Figure 1. Pseudo-ternary phase diagram of (a) SDS/propan-2-ol/clove oil/water; (b) S DS/iso-amyl alcohol/clove oil/water; (c) SDS/ 2-ethyl hexanol/clove oil/ water; (d) NaOA/propan-2-ol/clove oil/water; (e) NaOA/iso-amyl/clove oil/wat er; (f) NaOA/2-ethyl hexanol/clove oil/ water. SL: Solid-liquid, LVG: Low Viscosity Gel, YVL: Y ellow Viscous Liquid, G: Gel, YG: Yellow Gel, 1E: Emulsion, ME: Microemulsion, WI: Winsor’s type I, WII: Winsor’s type II.


Page 16 of 22

Page 17 of 22

Figure 2. Schematic represent ation of (a) spherical micelles formed from NaOA and SDS surfactants; (b) rod-like micelles; (c) lamellar structures.

450 22% C/S SDS+ 2-ethyl hexanol 30% C/S SDS+ iso-amyl alcohol 18% C/S SDS+ propan-2-ol 22% C/S NaOA + 2-ethyl hexanol 26% C/S NaOA + iso-amyl alcohol 18% C/S NaoA + propan-2-ol

400 350 300 250

, Pa

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


200 150 100 50 0 0

200

400

600

, s

800

-1

(a)


1000


, cP

10000

1000

22% C/S SDS+ 2-ethyl hexanol 30% C/S SDS+ iso-amyl alcohol 18% C/S SDS+ propan-2-ol 22% C/S NaOA + 2-ethyl hexanol 26% C/S NaOA + iso-amyl alcohol 18% C/S NaOA + propan-2-ol

100

0.1

1

10

, s

100

1000

-1

(b) Figure 3. Variation of (a) shear stress, τ and (b) viscosity, η as a function of shear rate, γ at 40°C.

22% C/S SDS+ 2-ethyl hexanol 30% C/S SDS+ iso-amyl alcohol 18% C/S SDS+ propan-2-ol 22% C/S NaOA + 2-ethyl hexanol 26% C/S NaOA + iso-amyl alcohol 18% C/S NaoA + propan-2-ol

1000

, cP

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 22

100

10

40

50

60

70

80

90

100

110

120

T, C

-1

Figure 4. Variation of viscosity, η as a function of temperature, T at 100 s .


Page 19 of 22

, cP

1000

100

22% C/S SDS 24% C/S SDS 0.1% NaOH+ 24% C/S SDS 22% C/S NaOA 24% C/S NaOA 0.1% NaOH+ 24% C/S NaOA

10

40

50

60

70

80

90

100

110

120

100

110

120

T, C

(a)

1000

, cP

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


100

30% C/S SDS 32% C/S SDS 0.1% NaOH+ 32% C/S SDS 26% C/S NaOA 28% C/S NaOA 0.1% NaOH + 28% C/S NaOA

10

40

50

60

70

80

90

T, C

(b)



18% C/S SDS 20% C/S SDS 0.1% NaOH+ 20% C/S SDS 18% C/S NaOA 20% C/S NaOA 0.1% NaOH+ 20% C/S NaOA

1000

100

, cP

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 22

10

40

50

60

70

80

90

100

110

120

T, C

(c) Figure 5. Variation of viscosity, η as a function of temperature, T at 100 s

-1

in the presence of (a) 2-

Ethyl hexanol (b) Iso-amyl alcohol (c) Propan-2-ol.

Figure 6. Schematic representation of lamellar structures developed from surfactant and cosurfactant having different lipophilic tail length.


Page 21 of 22

0.1% NaOH 500 ppm SiO2+ 0.1% NaOH

, cP

1000

100

40

60

80

100

120

T, C

Figure 7. Variation of viscosity, η as a function of temperature, T for 24% C/S VES fluid developed -1

from NaOA/2-ethyl hexanol/clove oil/water system at 100 s .

1000

G & G, Pa

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


100 G G G G G G G

10 0.01

G 24% C/S SDS+2-ethyl hexanol G 32% C/S SDS+ iso-amyl alcohol G 20% C/S SDS+ propan-2-ol G 24% C/S NaOA+ 2-ethyl hexanol G 28% C/S NaOA+ iso-amyl alcohol G 20% C/S NaOA+ propan-2-ol G 500 ppm SiO2+ 24% C/S NaOA+ 2-ethyl hexanol

0.1

1

10

, rad/s

Figure 8. Variation of storage modulus, G′ & loss modulus, G″ with angular frequency, ω of various VES fluids at 40°C.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 22

Table 1. Parameters of the Power Law model for various developed VES fluids. Serial

% C/S

Flow behaviour index, n

no.

Consistency

R²

index, k

1.

22% C/S SDS+ 2-ethyl hexanol

0.4772

10.418

0.9899

2.

30% C/S SDS+ iso-amyl alcohol

0.5601

4.7207

0.9868

3.

18% C/S SDS+ propan-2-ol

0.5867

3.3812

0.9959

4.

22% C/S NaOA +2-et hyl hexanol

0.5500

9.9259

0.9873

5.

26% C/S NaOA+iso-amyl alcohol

0.5714

7.7246

0.9913

6.

18% C/S NaOA + propan-2-ol

0.5920

5.3252

0.9883

Table 2. Values of viscosity for different mixing ratios (Gel:Water) for 500 ppm SiO2+0.1% NaOH +24% C/S developed from NaOA/2-ethyl hexanol/clove oil/water system. Temperat ure, °C

Viscosity, cP 1:3

1:2

1:1

2:1

3:1

80

2.213

3.867

4.872

7.561

13.125

90

1.887

3.189

4.369

6.623

10.245

100

1.454

2.731

3.878

5.897

8.565

110

0.947

2.192

3.245

4.456

6.745

120

0.745

1.745

2.789

3.654

5.515

Table 3. A verage settling velocities of particles for 500 ppm SiO2+0.1% NaOH+24% C/S VES fluid prepared from NaOA/2-ethyl hexanol/clove oil/water system. Serial No.

Temperat ure, °C

Settling velocity, mm/s

1.

100

0.0089

2.

110

0.0234

3.

120

0.1311


Study on the Thermal Stability of Viscoelastic Surfactant-Based Fluids

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