Screening of microemulsion properties for application in enhanced oil recovery

Fuel 121 (2014) 198–207

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Screening of microemulsion properties for application in enhanced oil recovery Achinta Bera, T. Kumar, Keka Ojha, Ajay Mandal ⇑ Department of Petroleum Engineering, Indian School of Mines, Dhanbad 826004, India

h i g h l i g h t s  Phase behavior of microemulsion was studied by water solubilization method.  IFTs of crude oil and brine, surfactant and microemulsion system were determined.  Effect of salinity on particle size distribution of microemulsion was analyzed.  Flooding tests were carried out with microemulsion slugs at different salinities.  Adsorption of surfactant on the sand particles at different salinities was studied.

a r t i c l e

i n f o

Article history: Received 27 February 2013 Received in revised form 16 December 2013 Accepted 17 December 2013 Available online 31 December 2013 Keywords: Microemulsion phase behavior Interfacial tension Particle size distribution Microemulsion flooding Enhanced oil recovery

a b s t r a c t Microemulsion in enhanced oil recovery is an efficient tool because of its high level of extraction efficiency of residual oil from natural oil reservoir. In the present paper the effect of salinity on anionic microemulsion phase behavior has been investigated by water solubilization method. Interfacial tensions between crude oil and brine, surfactant solution and microemulsion have been measured and it was found that the microemulsion has high ability to reduce interfacial tension. Pseudoternary phase diagram has been established at optimum salinity and composition of single-phase microemulsion has been determined from the curve. Laser light scattering was used to monitor particle size of several microemulsion formulations at different salinities. A series of flooding experiments have been performed using the prepared microemulsions. The additional recoveries were calculated by material balance. Encouraging results with additional recovery more than 25% of original oil in place above the conventional water flooding have been observed. Adsorption characteristics of the surfactant on the sand particles (adsorbent) at different salinities have also been studied. Ó 2013 Elsevier Ltd. All rights reserved.

1. Introduction The application of microemulsion in enhanced oil recovery (EOR) has been investigated with incorporation of new mechanism when the conventional techniques start to become unprofitable. Microemulsions are isotropic, transparent or translucent, thermodynamically stable dispersions of surfactant, alcohol, oil and water (brine), and have been used in various fields, such as EOR, pharmaceutics, nanoparticle synthesis, liquid–liquid extraction, cosmetic, detergency and other chemical engineerings, due to their very low interfacial tension (IFT), nanometer-sized droplets, and good solubilization capacity [1–4]. Along with these properties, particle size, interactions, and dynamics are of fundamental importance since they control many of the general properties of microemulsions. In particular, the size distributions of microemulsions give ⇑ Corresponding author. Tel.: +91 326 2235485; fax: +91 326 2296632. E-mail address: [email protected] (A. Mandal). 0016-2361/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.fuel.2013.12.051

essential information for a reasonable understanding of the mechanism governing both the stability and penetration into porous media. Microemulsion in EOR is an efficient tool because of its high level of extraction efficiency of residual oil from natural oil reservoir [5–8]. Ultra low IFT can be obtained by creating a middle phase microemulsion using brine, oil, surfactant and cosurfactant. The phase behavior of surfactant–brine–alcohol–oil systems is of immense importance for surfactant flooding in EOR due to the well-established relationship between the IFT and microemulsion phase behavior [9–13]. The phase behavior of surfactant–brine– alcohol–oil system is one of the key factors in interpreting the performance of chemically EOR by microemulsion process [14]. The tertiary oil recovery is mainly dependent on the properties of oil–water-rock interfaces. These are capillary forces, contact angle, wettability, viscous forces and IFT. These properties are related to a dimensionless quantity, called Capillary number, NC, that is a measure of the mobilization of the occluded oil to enhance the oil recovery and well represented by as:

A. Bera et al. / Fuel 121 (2014) 198–207

NC ¼

lv c

ð1Þ

where l is the dynamic viscosity of the liquid, v is the velocity, and c is the IFT between oil and water phases. In any enhanced oil recovery process the overall efficiency depends on both the macroscopic sweep efficiency and microscopic displacement efficiencies. While the gravity overrun and rock heterogeneity affect the macroscopic sweep efficiency, the microscopic displacement efficiency is influenced by the interfacial interactions involving interfacial tension and dynamic contact angles. Volumetric sweep is a macroscopic efficiency which is defined as the fraction of reservoir invaded by the injected fluid. Volumetric sweep efficiency of a displacement process is influenced by four factors like (a) the properties of the injected fluids, (b) the properties of the displaced fluids, (c) the properties and geological characteristics of the reservoir rock and (d) the geometry of the injection and production well pattern. In microemulsion systems, a variety of phases can exist in equilibrium with another phase, with each phase having different structure. Microemulsion phases are changed from Winsor type I to Winsor type II through Winsor type III by systematic variation of salinity at a particular temperature and pressure [15,16]. The commonly observed Winsor-type systems indicate that the microemulsions can remain in equilibrium with excess oil, excess water, or both. The factors that affect the phase transition between different types of systems and physicochemical properties include salinity, temperature, molecular structure and nature of the surfactant and cosurfactant, nature of the oil and the water–oil ratio (WOR) [17–20]. Under adequate conditions, the microemulsion system is miscible with both oil and water. Optimum salinity and the amounts of solubilized oil and water contained in a microemulsion play important roles in obtaining low IFTs and higher oil recoveries in chemical EOR. Since IFTs are having minimum value at optimal salinity and solubilization parameters are related to IFT, estimation of both properties is of great importance in designing economical microemulsion flooding [21,22]. The use of microemulsions is of high interest in many aspects of crude oil exploitation, but none more so that in EOR. In cases where the pressure exerted by gushing sea water on the oil phase is not able to overcome capillary forces sufficiently, microemulsions are the key to extracting more than just a minor portion of crude oil. Properly balanced microemulsions are able to do so by drastically reducing the IFT to the magnitude of 102–103 mN m1. This decrease in IFT allows spontaneous emulsification and displacement of the oil [23,24]. In microemulsion flooding a small chemical slug (5–30% pore volume) is injected into oil reservoir during the process. This slug is displaced through the reservoir by a polymer bank, which in turn is displaced by drive water. Adsorption of surfactants from aqueous solutions in porous media is very important in EOR of oil reservoirs because surfactant loss due to adsorption on the reservoir rocks impairs the effectiveness of the chemical solution injected to reduce the IFT of oil– water and render the process economically unfeasible [25–27]. Surfactant adsorption at solid/liquid interface has been studied for several decades. A number of studies have been conducted on the adsorption of ionic and nonionic surfactants [26,28–34]. The solid surfaces are either positively or negatively charged in the aqueous medium by ionization/dissociation of surface groups or by the adsorption of ions from solution onto a previously uncharged surface. Microemulsion flooding prefers a high concentration of surfactant solution to form micelles that can solubilize or dissolve the reservoir oil and the process takes place via incorporation of small oil droplets in micelle core, effectively causing miscibility in the system [35]. Some researchers carried out flooding experiments with microemulsions and they observed a linear

199

relationship between the values of injected pore volume (PV) and the oil recovery, typically reaching 40–50% of residual oil recovery by injecting 5–10 PV of microemulsion [5,36,37]. The objectives of the present study are the formation and characterization of microemulsion that are stable at the reservoir conditions. The formation of stable microemulsion using minimum amount of surfactant is the biggest challenge for its use in EOR. Thus, a complete study on phase behavior and physicochemical properties of microemulsion comprising of sodium dodecyl sulfate, brine, propan-1-ol and heptane have been investigated as a function of salinity. Relative phase volumes of different components in microemulsions are very sensitive to the salinity. IFTs between crude oil, and brine, surfactant solution and microemulsion have been studied as a function of time. Effect of salinity on IFT has also been investigated. Pseudoternary phase diagrams have been drawn to identify the microemulsion region. Laser light scattering was used to monitor particle size of several microemulsion formulations at different salinities. A series of flooding experiments have been performed using the prepared microemulsion. The additional recoveries were calculated by material balance. Encouraging results with additional recovery more than 25% of original oil in place above the conventional water flooding have been observed. Another series of batch experiments were carried out to determine the adsorption isotherms of surfactants on the sand particles (adsorbent) at different salinity. The results of the adsorption study show that as salinity of the solution increases adsorption capacity of adsorbent also increases. 2. Experimental section 2.1. Materials Anionic surfactant, Sodium dodecyl sulfate (SDS) of 98% purity was purchased from Fisher Scientific, India. Sodium Chloride (NaCl) with 98% purity procured from Qualigens Fine Chemicals, India, was used for preparation of brine. Reverse osmosis water from Millipore water system (Millipore SA, 67120 Molshein, France) was used for preparation of solutions. Propan-1-ol (98% pure) was used as cosurfactant. In formation of microemulsion, cosurfactant (medium chain alcohol) is used for improving microemulsion property. Cosurfactant increases the solubility of the solid surfactant which is less soluble in the system hence makes the microemulsion formation very feasible. Cosurfactant improves the viscosity of microemulsion which is important in flooding purpose. Cosurfactant also helps to reduce interfacial tension with combination of surfactant. Addition of cosurfactant reduces the adsorption of costly surfactant on reservoir rock. It was supplied by Otto-kemi Pvt. Ltd., India. In this study n-heptane (with purity >99%) was used as oil and purchased from MERCK, India. The entire chemicals were used without further purification. 2.2. Experimental procedures 2.2.1. Determination of water solubilization and phase boundary The solubilization of water in the microemulsion region was determined by conventional titration method of microemulsion with brine or water under satisfied conditions until the opaqueness of the microemulsions was obtained. Here the opaqueness is especially defined as the turbid, densed milky appearance of that system through which nothing can be seen. For different cases different colored translucent mixture was not considered the required opaqueness of the system. However, the end point of the titration was considered the actual transition point of the clear, transparent and isotropic microemulsions to a birefringent phase where the boundary was determined as the onset of the cloudiness

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due to a lack of a strong turbidity. For determination of the effect of surfactant to cosurfactant ratio on the solubilization of water in microemulsion, different amount of surfactant and cosurfactant were added in the microemulsion systems and then titrated with water till the turbidity appeared. Phase behavior of different systems was investigated by visual observation of phase changes and corresponding readings. In all cases, more than three successive measurements were carried out, and the standard deviation did not exceed ±0.1 g. All the experiments were performed at 300 K. 2.2.2. IFT measurement The IFT between crude oil and water interface was measured with a Spinning Drop Video Tensiometer (DataPhysics, Model No. SVT 15 N) with variation of salinity at 313 ± 1 K. During the measurements each sample was rotated at 5000 rpm speed. The IFT values were recorded until the equilibrium was established. The equilibrium was considered to be obtained when successive values agreed within 0.01 mN/m. The equilibrium IFT (c) was obtained from the Eq. (2) as follow:

c¼

x 2 R 2 Dq 4

ð2Þ

where x is the angular velocity, R is the radius of the drop, and Dq is the difference in density between crude oil and tested solutions. The IFT of each point was repeated at least twice. IFT between crude oil and surfactant/microemulsion system has also been measured by the same procedure. 2.2.3. Phase diagram of microemulsion system The pseudo-ternary phase diagram for oil–water–surfactant system was obtained at 303 K. In the present study, a fixed cosurfactant (CS) to surfactant (S) ratio of 2 with different amounts of heptane were taken and respective volumes of brine (4 wt% NaCl) were added from micropipette till the turbidity appeared; the number and type of phases separated were identified at equilibrium and the ternary phase diagram was constructed. All mixtures were prepared by weight; and compositions are expressed as weight percent to draw the phase diagram. The selection of CS/S ratio for the construction of pseudoternary phase diagram has been made on the basis of solubility of powder surfactant in oil–water– surfactant–cosurfactant mixture. It is important to note that to maintain transparency of microemulsion which is unique feature of this solution; cosurfactant has been taken always higher quantity than that of surfactant. There are several literatures are available where this CS/S ratio is used [38–42]. 2.2.4. Particle size distribution analysis of microemulsion The droplet size distribution for the microemulsions at different salinities was measured after 4 h equilibrium by a laser diffraction method of Zetasizer Ver. 6.00 (Malvern Instruments Ltd., Worcestershire, UK). The droplet size distribution of the dispersed particles can be obtained by the in build software of the instrument. The software uses a reflective index (RI) of 1.465 (SBO) and a dispersant RI of 1.33 (water) during the measurement. Drops of microemulsion were introduced into the cell until the volume reached the optimum one, indicated by the instrument. All the experiments were conducted at 298 K. 2.2.5. Experimental apparatus and methods for microemulsion flooding The experimental apparatus is composed of a sand pack holder, cylinders for chemical slugs and crude oil, positive displacement pump, measuring cylinders for collecting the samples. The detail of the apparatus is shown in Fig. 1. The displacement pump is

one set of Teledyne Isco syringe pump. Control and measuring system is composed of different pressure transducers and a computer. The accuracy of the pressure transducer is 0.1%. The physical model is homogeneous sand packing model vertically positive rhythm. The model geometry size is L = 35 cm and r = 3.5 cm. The core holder was tightly packed with uniform sands (60–100 mesh) and saturated with 1.0 wt% brine. It was flooded with the brine at 25 psig and the absolute permeability was calculated from the flow rate of the through sand pack. The sand pack was then flooded with the crude oil at 400 psig to irreducible water saturation. The initial water saturation was determined on the basis of mass balance. The crude oil used in the flooding experiments was collected from Ahmedabad oil-field (India). The oil is having total acid number of 0.038 mg KOH/g, gravity of 38.86° API and viscosity of 11.9 Pa.s at 303 K. Water flooding was conducted by placing the coreholder horizontally at a constant injection pressure at 200 pisg. After water flooding, when water-cut reached above 95%, around 1.0 pore volume (PV) of microemulsion slug was injected followed by chasing water. The water flooding has been stopped at 95% for economic feasibility. Above 95% water cut the viscous fingering may be a major problem because of increase in relative permeability to water. After 95% water cut if we continue to inject water to recover the remaining oil the process may not be economically feasible due to marginal increase in recovery. On the other hand if chemical injection is done at this stage, it is possible to recover the additional oil within a short span of time. The experiments were repeated using different microemulsion slugs. The additional recoveries were calculated by material balance. The effective permeability to oil (ko) and effective permeability to water (kw) were measured at irreducible water saturation (Swi) and residual oil saturation (Sor), respectively, using Darcy’s law equation. The permeability of the sand packs was assessed with the Darcy equation, Eq. (3), used for fluid flow in porous media. For a horizontal linear system, flow rate is related with permeability as follow:

q¼

kA dp l dx

ð3Þ

where q is volumetric flow rate (cm3/s), A is total cross-sectional area of the sand pack (cm2), l is the fluid viscosity (cp), dp is the dx pressure gradient (atm/cm) and k is Permeability in Darcy. The recovery factor is obtained by summing up the amounts of oil recovered in each step (secondary and tertiary oil displacement process) and is expressed in percentage (%)

RFTotal ¼ RFSM þ RFTM where RFTotal is the total recovery factor (%), RFSM is the recovery factor obtained by secondary method (%), RFTM is recovery factor obtained by tertiary method (%). 2.2.6. Adsorption isotherms A series of batch experiments were carried out to determine the adsorption isotherms of different types of surfactants on the adsorbent. 8 g of clean sand particles were added to a set of 50 ml surfactant solutions in a 100 ml glass vials and allowed to conduct the experiments by constant shaking at 303 K for 24 h on a horizontal shaker mode. After adsorption, the surfactant solutions were isolated by centrifugation. The equilibrium or residual concentrations of the surfactant solutions were determined by Chemical Oxygen Demand (COD) measurement of the solutions. The detailed procedure has been given in supplementary content. The adsorption capacity of surfactant on the adsorbent, Ci (mg/g), was calculated by a mass balance relation (4):

Ci ¼ ðC 0  C s Þ

v m

ð4Þ

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Fig. 1. Schematic of experimental set-up for microemulsion flooding in sandpacks.

where C0 and Cs are the initial and equilibrium concentrations of surfactant solutions (mg/g) respectively, V is the volume of the surfactant solution (L), and m is the weight of the sand particles (g) (adsorbent) used. The effects of NaCl concentrations on the adsorption capacity of the adsorbent to the anionic surfactant, SDS were also investigated. 3. Results and discussion 3.1. Water solubilization and phase behavior Phase behavior and phase boundary of microemulsion systems can be determined by water solubilization method with variation of salinity. Typical plot of water solubilization as a function of salinity in the microemulsions consisting of heptane, SDS along with propan-1-ol is shown in Fig. 2. It was found from the experimental results that as salinity increases water solubilization also increase up to a certain value and then decreases. Initially at low salinity solubilization of brine was high due to partitioning of alcohol at the interface. Brine solubilization decreases with at high salinity due to an increase in interfacial rigidity. The salinity at which water solubilization is highest is generally termed as ‘‘optimal salinity’’ for the microemulsion system. In the present study salinity varies from 1 to 10 wt% and it was found that brine solubilization was maximum at 4 wt% NaCl. At optimum salinity the middle-phase microemulsion has the ability to solubilize equal amounts of oil and brine [43–45]. After optimum salinity the water solubilization decreases with increase in concentration of brine. As salinity increases microemulsion phase changes from Winsor

type-I (designated as ‘‘eo’’) to Winsor type-II (designated as ‘‘ew’’) through middle phase of Winsor type-III (designated as ‘‘mp’’) shown in the figure. These phenomena can be illustrated on the basis of attractive interdroplet interaction and interfacial bending stress. As the salt concentration is increased, some of the water molecules are attracted by the salt ions, which decreases the number of water molecules available to interact with the charged part of the surfactant. As a result of the increased demand for solvent molecules, the interactions between the hydrophilic head groups become stronger than the solvent-solute interactions; the surfactant molecules are precipitated by forming hydrophobic interactions with each other. The interfacial film curvature turns from positive value to zero to negative one, corresponding to phase transition from oil–water (O/W) to bi-continuous phase to water– oil (W/O) structure [16,46].

3.2. Effect of salinity on IFT The effects of salt concentrations on IFT have been shown in Fig. 3. The dynamic IFTs are also shown in the same figure. In case of crude oil/water system IFT was found to be quite high around 48 mN/m. When NaCl was used the change in IFT takes place noticeably. From Fig. 3 it is clear that with increase in salt concentration IFT decreases initially and after a certain concentration it increases. The minimum IFT can be obtained only when the concentration of salt reaches to an optimal value. In the present system the optimal salinity was found to be 4 wt% and at this salinity the IFT value is near about 7 mN/m. As IFT decreases therefore it is confirmed that there must be some mechanism of

8 70

7

Turbid Phase

Brine, g

5

mp mp

4

eo

mp

eo

ew

3

eo

Clear Phase

2

ew

1

Interfacial Tension, γ (mN/m)

60

6

50

40

30

20

10

0 0

2

4

6

8

10

Salinity, wt % NaCl

0 0

1

2

3

4

5

6

7

8

9

10

Salinity, wt% NaCl Fig. 2. Relative phase volumes of sodium dodecyl sulfate/brine/propan-1-ol/ heptane systems as a function of salinity, wt% NaCl.

Fig. 3. Effect of salinity on interfacial tension between crude oil–water systems.

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reduction of IFT. Since surface active materials are responsible for reducing IFT between crude oil and water system therefore in presence of NaCl surface active agents are produced and they reduce the IFT. Different research works have been reported in literatures regarding the reduction of IFT in presence of salt [47–52]. The presence of salts in the aqueous phase has strong ability to increase the accumulation of the surface-active species which are available in crude oil at the crude oil/water interface and thereby reduce the IFT. When NaCl concentration increases to exceed 4 wt%, it prevents the surface-active material from dissolving into the aqueous phase due to increase of the repulsive electrostatic double-layer forces and repulsive hydration forces. Therefore, the amount of produced surfactant from the crude oil has been reduced and it could not further reduce the IFT. Dynamic IFTs were also determined with different time intervals. Fig. 4 shows dynamic IFTs of saline water, surfactant solution and microemulsion. With increase in time IFT decreases and after certain time the equilibrium value was reached in all cases. This phenomenon indicates that interaction between the test solutions and crude oil takes place during the measurements. In saline water with elapse of time in situ formation of surfactant from crude oil in presence of salt takes place and after certain time the accumulation of surfactant from crude oil stops and the equilibrium value has been found. In case of surfactant and microemulsion the lowering of IFT is higher than the brine solution. It is well known that surfactant molecules can transfer from the bulk solution to the oil/water interface and make the IFT decrease [53–55]. In surfactant and microemulsion systems the hydrophobicity of both the systems becomes stronger because the electrostatic repulsion and hydration are weakened. As a result, the molecules of the systems will adsorb quickly and arrange close together at oil/water interface, acting to reduce the IFT. In case of crude oil-microemulsion system, ultralow IFT is known to be closely related with phase behavior of the system near critical point like plait point where two liquid phases are indistinguishable. As the two immiscible liquid phases are indistinguishable therefore the IFT between these two phases goes to zero. The ultralow IFT between microemulsion and crude oil is due to the chemically indistinguishable phases near the tricritical point around optimal salinity. 3.3. Phase diagram of microemulsion system The pseudo ternary phase diagram of sodium dodecyl sulfate/ brine/propan-1-ol/heptane system has been constructed and shown in Fig. 5, where water and NaCl salt considered as a single

CS/S=2 0.00

1.00

0.25

0.75

0.50

0.50

1φ 0.75

0.25

2φ 1.00

Brine

0.00

0.00

0.25

0.50

0.75

Oil

Fig. 5. Pseudoternary phase diagram of oil–brine–surfactant–cosurfactant system for low salinity microemulsion.

pseudo-component and (S + CS) is another single component so that the apexes represent oil, brine and (S + CS). The importance of the construction of this type of phase diagram is to determine the composition of the microemulsion. Fig. 5 depicts the phase diagram of surfactant, cosurfactant, oil and brine at a constant CS/S ratio of 2. It is also important to prepare microemulsions with low concentrations of surfactant from the economical point of view. In Fig. 5, the shaded areas represent the two-phase microemulsion region and out of the shaded areas imply single-phase microemulsion regions (Winsor IV). From Fig. 5 the composition of the microemulsion has been determined to prepare a large amount of microemulsion for flooding experiments. A typical composition in the microemulsion zone with CS/S- 20%, oil-53%, and 27% brine was selected to prepare the microemulsion for flooding experiment.

3.4. Particle size distribution Size of the dispersed particles in microemulsions plays an important role in EOR. As the size decreases, the colloidal interaction between the dispersed particles increases; stability of the microemulsion increases and the ability of the emulsion migrate through the pore throats in sedimentary rocks increases. Particle size distributions in microemulsions at 2, 4 and 9 wt% NaCl solutions are shown in Fig. 6. The mean particle diameter and

20

1

1x10

ME in 2 wt% NaCl ME in 4 wt% NaCl ME in 9 wt% NaCl

18 16 14

0

1x10

Intensity (%)

Interfacial tension, mN/m

1.00

-1

1x10

12 10 8 6 4

6 wt% NaCl SDS solution Microemulsion

-2

1x10

2 0 0

0

10

20

30

40

50

60

5

10

15

20

25

30

35

40

Size (d. nm)

Time (min) Fig. 4. Dynamic interfacial tension of brine, surfactant and microemulsion systems.

Fig. 6. Intensity wise particle size distribution of microemulsion at 2, 4, and 9 wt% NaCl solutions.

A. Bera et al. / Fuel 121 (2014) 198–207

polydispersity have been calculated from intensity, mass and number bimodal distribution. However, in each case only % intensity vs. particle size distribution curves has been shown for clarity. The Zaverage diameter of dispersed droplet is the mean hydrodynamic diameter and is calculated according to the International Standard on dynamic light scattering ISO13321. The Z-average diameter is intensity weighted and is therefore sensitive to the presence of large particles. Z-average diameter is found to be strong function of the salinity of the microemulsion. Interestingly it has been found that the Z-average diameter of the microemulsion at optimal salinity (4 wt% NaCl) is smaller than that of lower (2 wt% NaCl) and higher (9 wt% NaCl) salinities. It has also been found that the distribution of the dispersed droplet size is narrower for microemulsion around optimal salinity. Particle size and IFT of the microemulsion is dependent on the salinity and hence oil recovery by microemulsion flooding is also function of salinity of microemulsion. At a particular salinity oil recovery increases due to formation of smaller sized emulsion droplets which are effective for oil recovery. IFT between crude oil and microemulsion produces lower value at a particular salinity which is called optimal salinity. At that optimal salinity microemulsion also produces smaller droplet sized emulsion. Thus particle size distribution and salinity have significant role together on oil recovery. The results of the particle size measurements of the microemulsions at different salinities have been depicted in Table 1. 3.5. Microemulsion flooding and recovery The sand pack was flooded with microemulsion slug after water flooding and Fig. 6 shows the oil recovered with pore volume injected into sand pack. Fig. 7 shows an early breakthrough and channel flow which caused much lower oil recovery in oil recovery by waterflood. Surfactant flooding or surfactant/polymer flooding is being used in pilot scale is commercially for enhanced recovery of oil after primary and secondary recovery particularly in USA and Table 1 Results of the particle size measurements of the microemulsions at different salinities. Microemulsion salinity (wt% NaCl)

Z-Average diameter (nm)

PDI

Intercept

Peak vol. % r.nm

2 4 9

10 3 6

0.159 0.181 0.111

0.899 0.829 0.847

99.93 50.00 50.00

China. Microemulsions are isotropic, translucent mixtures of medium chain alkane, surfactant, brine and cosurfactant (medium chain alcohol). Microemulsion can be injected commercially. The advantages to use microemulsion slug over surfactant slug are as follows: (a) Low adsorption of surfactant on rock surface takes place during flooding and (b) It attains lower IFT than simple surfactant solution. A successful microemulsion flood pilot test has been completed by Exxon in a watered-out portion of the Weiler sand, Loudon Field, Fayette County, Illinois, USA. Surfactant solution can be injected at appropriate salinity for oil recovery purpose. But experimental results show that if microemulsion is used then additional oil recovery is higher than surfactant flooding. The additional recovery in case of microemulsion flooding is higher than surfactant flooding due to attainment of ultralow IFT and higher viscosity than general surfactant solution. During injection of microemulsion slug, water-cut declines gradually, and then again reaches 100% at the end of flooding. After the injection of microemulsion slug, the trapped oil droplets or ganglions are mobilized due to a reduction in IFT between oil and water. The coalescence of these drops leads to a local increase in oil saturation. Behind the oil bank, the surfactant now prevents the mobilized oil from being retrapped. The effectiveness of the microemulsion at different brine concentration on EOR was tested with three sets of flooding experiments performed in the sandpack systems. The concentration of brine to be used for microemulsion flooding is related to the optimal salinity concept which has been determined from the phase behavior and solubilization parameter study. In the present work the experiments were carried out in sandpack, the water flood recovers almost 40% of the original oil in place (OOIP) because of higher porosity (37%) and permeability. During water flooding, as the water cut reaches above 95%, it was subsequently flooded with microemulsion slugs, followed by chase water. The recovery of oil and water cut with pore volume (PV) injected for three different microemulsion systems is presented in Fig. 7. It has been found that water begins to break through when the injected volume of water reaches 0.15 PV, and then water cut sharply increases above 90% for each case. During injection of microemulsion slug, the water cut declines gradually and then reaches 100% at the end of flooding. Fig. 7 also shows that the cumulative oil recovery by microemulsion flooding is higher in the case of microemulsion

% of Oil Recovery and water cut

100 90 80 70 60

% Oil Recovery (ME at 2 wt% NaCl) % Water cut (ME at 2 wt% NaCl) % Oil Recovery (ME at 4 wt% NaCl) % water cut (ME at 4 wt% NaCl) % Oil Recovery (ME at 9 wt% NaCl) % Water cut (ME at 9 wt% NaCl)

50 40 30 20 10

ME

Brine

0 0.0

0.5

1.0

1.5

203

Chase water 2.0

2.5

Pore Volume Injected Fig. 7. Production performance of microemulsion (ME) flooding at different salinities.

A. Bera et al. / Fuel 121 (2014) 198–207

3.6. Adsorption of surfactant on sand particles The adsorption of surfactants on rock/soil/sediment solid matrix may result in the loss and reduction of their concentration, which may render them less efficient or ineffective in EOR. The net adsorption of surfactant in an EOR process strongly depends on the presence of oil and the flow field. When a surfactant slug is injected as displacing fluid, it undergoes partitioning into oil and water and lowers the interfacial tension between oil and water thereby increasing the capillary number. As a result, the trapped immobile oil becomes mobile. At the same time, an oil-in-water emulsion is formed which blocks the larger pores leading to an improvement in the effective mobility ratio. Otherwise the injected surfactant solution flows through the highly permeable zone bypassing the trapped oil in smaller pores. The injected surfactant continues to mobilize oil, until the surfactant is diluted or otherwise lost due to adsorption on the rock surface [58–60]. Consequently the surfactant solutions with lower concentration could not be able to lower the interfacial tension and mobilize oil. At that point, the process degenerates into a water flood. Hence to design a surfactant flooding for EOR, it is very important to have a complete knowledge of adsorption of the specific surfactant on the reservoir rock under the reservoir conditions. Adsorption studies have employed the surfactant depletion method with the results being presented as isotherms which are simply plots of the amount of surfactant adsorbed per gram of solid or per surface area of solid

0.8

ME at 2 wt% Brine ME at 4 wt% Brine ME at 9 wt% Brine

0.7 0.6

Flow Rate, ml/sec

prepared at optimal salinity because of significant physicochemical properties of microemulsion at that salinity. The microemulsion shows ultra-low IFT between oil and water at optimal salinity. If the IFT is low enough, residual oil in a water-wet core could be emulsified and move downstream. The mechanism results in a reduced water mobility that improves both vertical and areal sweep efficiency. The additional recovery of oil over water flooding is 27.63%, 29.88%, and 25.87% when microemulsion slugs with 2 wt%, 4 wt% and 9 wt% brines were used respectively. However, the proper design of the microemulsion slug should be performed by feasibility analysis, considering the operating costs and market value of the crude. In the present study the microemulsion is highly effective in oil recover and show better results. The previous work on microemulsion flooding for oil recovery [5] shows the oil recovery of 78.5% for other surfactant system. In their experiment they injected higher amount of pore volume (more than 1 PV). In the present work the variation of salinity and a correlation with salinity effect has been shown and the additional oil recovery is more than 25% and the total recovery is 80% by applying on 0.5 PV microemulsions as chemical slug. The additional oil recovery in the present study is also significantly higher than other reported surfactant flooding [56,57]. A relationship between the microemulsion at different salinity and the flow rate across the sand pack is shown in Fig. 8. The three runs had almost the same flow rate for the initial waterflood stage. However, during microemulsion injection the flow rate was found to decrease drastically though the injection pressure was maintained constant. The decrease in injection rate may be due to the formation of oil bank and consequent displacement of oil with lower mobility. The higher drop in flow rate was observed for microemulsion of optimal salinity which results higher recovery at that salinity. Fig. 9 depicts the effect of pore volume injected on oil cut by microemulsion flooding at different salinities. Microemulsion at optimal salinity shows maximum oil cut for a certain oil cut. In case of water flooding oil cut decreases as efficiency of water flooding decreases as initial oil saturation decreases. When microemulsion applied then again oil cut increases for enhanced efficiency of microemulsion to recover the trapped oil in porous media.

0.5 0.4 0.3 0.2 0.1 0.0 0.0

0.5

1.0

1.5

2.0

2.5

PV Injected Fig. 8. Effect of injected pore volume on flow rate in microemulsion flooding at different salinities.

100

OilCut (ME at 4% Brine) OilCut (ME at 2% Brine) OilCut (ME at 9% Brine)

90 80 70 60

% Oil Cut

204

50 40 30 20 10 0

0.0

0.5

1.0

1.5

2.0

2.5

PV Injected Fig. 9. Oil cut by microemulsion flooding at different salinities.

versus the equilibrium surfactant concentration at a constant temperature. Adsorption isotherms for surfactant solution at different salinities have been shown in Fig. 10. At the interface between surfactant and sand particles, there is always an unequal distribution of electrical charges. This unequal charge distribution gives rise to a potential across at the interface and forms a so-called electrical double layer [61]. With increase in NaCl concentration, the electrical double layer on the surface of adsorbent is compressed and electrostatic repulsion between the adsorbed surfactant species decreases, which results in the increase of adsorption capacity. When surfactant concentration reaches CMC, micelles starts to form and exist in the bulk solution and act as chemical potential sink for additional surfactant added to the system. As a result, surfactants cannot adsorb onto the surface and plateau of the adsorption isotherm shown in Fig. 10 is characterized by little or no increases in surfactant adsorption with increasing surfactant concentration. Adsorption is a unit operation in which dissolved constituents is removed from the solvent by interphase transfer to the surface of an adsorbent particle. In chemical flooding, surfactants are inevitably adsorbed on the surface of reservoir rock by the rock/oil/brine interaction. Surfactant adsorption in porous media is a typically complex phenomenon (e.g., mass transfer and reaction). Adsorption in

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A. Bera et al. / Fuel 121 (2014) 198–207

2.1

0.8

2.0

0 wt% NaCl 2 wt% NaCl 4 wt% NaCl

1.9

0.7

1.8 1.7

0.6

Γi (g/mg)

0.5

−1

Adsorption Density, Γi (mg/g)

2.2

0.9

0.4

1.6 1.5 1.4 1.3

0.3

1.2

0 wt% NaCl 2 wt% NaCl 4 wt% NaCl

0.2 0.1

1.1 1.0 0.9

0.0 0

200

400

600

800

1000

0.8

1200

0.000

0.002

0.004

Equilibrium Concentration, Ce(mg/L)

0.006

0.008

0.010

0.012

0.014

-1

Ce (L/mg)

Fig. 10. Adsorption isotherm of surfactant on sand surface at different salinities of brine solutions.

Fig. 11. Langmuir equation fitting for adsorption isotherms of SDS at different NaCl salt concentrations at 303 K.

-0.7

Ci ¼

Cmax K L C s

ð5Þ

1 þ K L Cs

where C is the amount adsorbed (mg/g); Cmax is the maximum amount adsorbed (mg/g); KL is the Langmuir equilibrium constant (L/g); Cs is the equilibrium aqueous concentration (g/L). Eq. (5) can be rewritten in the well known simplified linearized expression of the Langmuir model as follows:

1

Ci

¼

1 C s K L Cmax

þ

1

Cmax

ð6Þ

0 wt% NaCl 2 wt% NaCl 4 wt% NaCl

-0.6 -0.5 -0.4 -0.3

log i

porous media is a phenomenon in which transport of surfactant molecules takes place from bulk phase onto the interface at rockfluid boundary. In adsorption process, the interface is energetically favored by the surfactant molecules compared to the bulk phase.’’ The above discussion has been added to the revised manuscript. At low surfactant concentrations the adsorption is due to electrostatic interaction between the head group of the surfactants and charged sites on the sand surface. This electrostatic attraction typically described in terms of the interaction of the charged surfactant ion with the electrical double layer of the sand particle. According to Henry’s law the adsorption increases linearly with concentration and the slope of the curve is approximately one [62]. In Fig. 10 there is a sudden increase in adsorption isotherm as concentration of the surfactant increases. The sudden increase in adsorption isotherm may be described in terms of formation of surface aggregates, known as ‘‘hemi micelles’’ of the surfactant molecules on the sand surface due to lateral interaction between hydrocarbon chains. This lateral attraction force generates an additional driving force, and with the still existing electrostatic attraction, makes the adsorption isotherm curve in this stage exhibit a sharp increase. However, it is important to note that the exact shape of the isotherm will be dependent on some factors including the type of surface, and the presence or absence of additional compounds such as electrolytes, co-surfactants, and hydrotropes. Figs. 11 and 12 show that the adsorption equilibrium data of the adsorbent towards SDS at different salinities for Langmuir model and Freundlich model respectively. The Langmuir equation relates solid-phase adsorbate concentration Ci, the uptake, to the equilibrium liquid concentration at a fixed temperature. The equation was developed by Irving Langmuir in 1916 [63]. The Langmuir equation is expressed as:

-0.2 -0.1 0.0 0.1 0.2 1.8

2.0

2.2

2.4

2.6

2.8

3.0

3.2

logCe Fig. 12. Freundlich equation fitting for adsorption isotherms of SDS at different NaCl salt concentrations at 303 K.

A plot of 1/Ci versus 1/Cs yields a slope is equal to 1/(CiKL) and an intercept of 1/Cmax from which Cmax and KL can be computed. Table 3 shows the parameters obtained from Eq. (6). The Freundlich isotherm assumes that if the concentration of the solute in the solution at equilibrium, Cs, is raised 1=n to the power C 1/n, the amount of the solute adsorbed being C, the sC is constant at given temperature:

C ¼ K F C 1=n s

ð7Þ

where KF is the Freundlich equilibrium constant (L/g); Cs is the equilibrium aqueous concentration (g/L); n is the Freundlich constant. Freundlich isotherm has been derived by assuming an exponentially decaying sorption site energy distribution. This equation can be rearranged in the linear form by taking the logarithm of both sides as:

log C ¼ log K F þ

1 log C s n

ð8Þ

KF and 1/n are the Freundlich constants related to sorption capacity and sorption intensity, respectively. The intercept and the slope of the linear plot of log C versus log Cs at a given experimental conditions provide the values of KF and 1/n, respectively. The Freundlich

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A. Bera et al. / Fuel 121 (2014) 198–207

Table 2 Porous media property and oil recovery by microemulsion flooding. Expt. no.

Porosity (%)

S1 S2 S3

36.42 35.72 35.36

Permeability, k (Darcy) kw (Sw = 1)

ko (Swi)

1.273 1.336 1.264

0.229 0.239 0.201

Design of microemulsion slug for flooding

Recovery of oil by water flooding at 95% water cut (%OOIP)

Additional recovery (% OOIP)

1.0 PV ME (2% brine) + Chase water 1.0 PV ME (4% brine) + Chase water 1.0 PV ME (9% brine) + Chase water

47.9 42 39.25

27.63 29.88 25.87

% Saturation Swi

Soi

Sor

20.38 20.79 20

79.62 79.21 80

17.86 23.86 27.90

Table 3 Adsorption isotherm parameters of SDS at different salinities. Salinity (wt% NaCl)

0 2 4

Langmuir parameters

Freundlich parameters

Cmax (mg/g)

KL  102 (L/mg)

R2

KF (mg/g)

1/n

R2

0.763 1.011 1.031

2.141 1.472 1.642

0.951 0.993 0.988

0.231 0.216 0.345

0.173 0.218 0.151

0.937 0.894 0.899

constant (1/n) is related to the adsorption intensity of the adsorbent. When, 0.1 < 1/n < 0.5, adsorption is favorable; 0.5 < 1/n 6 1, it is easy to adsorb; 1/n > 1, there is a difficult to adsorb [64]. The correlation coefficients (R2) for the linear equation fittings at different salinities lie between 0.950 and 0.988 for Langmuir isotherm model whereas; the values for Freundlich isotherm are less than 0.950. Therefore, from R2 value it may be concluded that Langmuir isotherm model is best fitted for the system. The adsorption parameters are shown in Table 2. Further, the adsorption capacity increased with the increase in salinity of the system at a constant temperature of 303 K. These facts imply that the adsorption of SDS on sand particle adsorbent is favored at high salinity and therefore the adsorption process is seen to be a chemical process with increasing salinity.

celles that exist in the bulk solution and act as chemical potential sink for additional surfactant added to the system. It was found that Langmuir adsorption isotherm model is the best fitted model for the present study.

4. Conclusion

Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.fuel.2013.12.051.

Phase behavior and physicochemical properties of microemulsions comprising of sodium dodecyl sulfate/brine/propan-1-ol/ heptane and its structural changes with the salinity have been investigated for the application of microemulsion in EOR. Salinity plays important roles on the relative phase volume and solubilization parameters in microemulsions. The analysis of Pseudoternary phase diagram of microemulsion is very important for selecting the typical single phase microemulsion for its use in EOR. The phase behavior study shows that as salinity changes from low to high, phase transition takes place from Winsor I to Winsor II via Winsor III. Highest water solubilization was found to be at 4 wt% NaCl for the composed microemulsion system. This salinity is termed as optimal salinity. Salinity plays an important role on IFT. As salinity increases IFT decreases and after certain value it decreases. Microemulsion shows higher ability to reduce IFT than other solutions. At optimal salinity particle size distribution shows an interesting result with smaller Z-average particle size than the other salinities of the microemulsions. A series of flooding experiments have been performed using the prepared microemulsion. The additional recoveries were calculated by material balance. Encouraging results with additional recovery more than 25% of original oil in place above the conventional water flooding have been observed. Microemulsion at optimal salinity (4 wt% brine) shows highest efficiency to recover the additional oil recovery. The results of the adsorption study show that as salinity of the solution increases adsorption capacity of adsorbent increases. Adsorption density of surfactant on sand surface increases up to a near CMC value of the surfactant and then further adsorption is not possible due to formation of mi-

Acknowledgments The authors gratefully acknowledge the financial assistance provided by University Grant Commission [F. No. 37-203/ 2009(SR)], New Delhi to the Department of Petroleum Engineering, Indian School of Mines, and Dhanbad, India. Thanks are also extended to all individuals associated with the project. Appendix A. Supplementary material

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