Enhanced oil recovery potential of alkyl alcohol polyoxyethylene ether

Article Cite This: Energy Fuels XXXX, XXX, XXX−XXX

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Enhanced Oil Recovery Potential of Alkyl Alcohol Polyoxyethylene Ether Sulfonate Surfactants in High-Temperature and High-Salinity Reservoirs Rui Liu,† Dai-jun Du,*,† Wan-fen Pu,*,† Jing Zhang,‡ and Xi-bin Fan‡ †

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State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Southwest Petroleum University, Chengdu, Sichuan 610500, People’s Republic of China ‡ Research Institute of Petroleum Exploration and Development, Xinjiang Oilfield Company, PetroChina, Karamay, Xinjiang 834000, People’s Republic of China S Supporting Information *

ABSTRACT: Surfactant flooding has been widely applied in high-temperature and high-salinity reservoirs. In this paper, the enhanced oil recovery potential of alkyl alcohol polyoxyethylene ether sulfonate (CEOS) was investigated in a combined study of surface activity, crude oil−water interfacial tension (IFT) reduction, emulsifying property, wettability improvement, and macroscopic oil displacement efficiency. The results illustrated that CEOS had high surface activity and IFT could be reduced to an ultralow level (10−3 mN/m) at high-temperature and high-salinity conditions. When salinity ranged from 15 × 104 to 22.5 × 104 mg/L and reservoir permeability was ∼10 mD, linear CEOS solution could effectively displace crude oil for its favorable IFT reduction ability. Linear CEOS or CEOS with a benzene ring was optimized for their favorable IFT reduction ability or emulsifying ability when reservoir permeability was ∼50 mD or non-homogeneous. A 0.5 pore volume surfactant flooding and subsequent water flooding could remarkably enhance oil recovery to 16.19−19.38%. All of the results indicated that CEOS has great potential for improving oil recovery in high-temperature and high-salinity oil reservoirs.

1. INTRODUCTION At present, a considerable portion of mature oilfields enter the high-water cut stage after several years of exploring, and stabilizing oilfield production is more and more difficult.1 A serious shortage of crude oil has taken place as a result of the increase in demand; hence, chemical flooding technologies, such as polymer flooding, caustic water flooding, combination flooding (polymer/surfactant flooding and polymer/surfactant/alkaline flooding), and surfactant flooding, are widely applied,2−5 wherein surfactant flooding has been reported in the oil industry as early as the 1960s.6 Surfactant flooding consists of injection water with adding a surfactant to form an emulsion, improving wettability, and reducing interfacial tension (IFT) to mobilize residual oil.7−10 In this way, the capillary force, which traps oil into the pores of the reservoir rocks, can be overcame to improve “microscopic” oil displacement.11,12 Furthermore, ultralow IFT is required to obtain a high capillary number for effective oil displacement from the reservoir rock pore space,13−15 and ultralow IFT can be realized using single or compound surfactants.16−18 The mainly used surfactants in surfactant flooding include anionic surfactants and non-ionic surfactants, such as petroleum sulfonate, alkylbenzenesulfonate, lingosulfonate, alkanolamide, alkyl glycoside, etc.19−22 Even though these surfactants can reduce IFT and enhance oil recovery, they are insufficient for the requirements of surfactant flooding at high-temperature (80− 120 °C) and high-salinity (≥20 × 104 mg/L) conditions.23 Therefore, compound surfactants are used in chemical flooding. Compound surfactants may lead to chromatograph separation during displacement, and the IFT is either not reduced © XXXX American Chemical Society

sufficiently to remove trapped crude oil or the slug may lose efficacy during displacement.24 To overcome the disadvantages of compound surfactants, new types of surfactants were studied, such as anionic−nonionic surfactant (ACS), gemini surfactant, betaine surfactant, etc. Containing an anionic group and non-ionic blocks, ACS simultaneously owns the advantages of anionic surfactants and non-ionic surfactants.25,26 The non-ionic block can enhance the hydration ability of surfactant molecules and weaken the interactions between anionic groups and metal cations in saline solution.27 In comparison to compound surfactants, ACS can not only effectively reduce IFT at high-temperature and highsalinity conditions but also avoid chromatographic separation. ACS also shows favorable wettability improvement ability and emulsifying ability. According to the anionic group type, ACS is divided into three types, including sulfate type, carboxylate type, and sulfonate type,27,28 wherein the sulfonate type has the best salt tolerance, and its activity can be conserved under high Ca2+ and Mg2+ concentration conditions. Moreover, the C−S bond endows the sulfonate surfactant with benign temperature tolerance.29 The main objective of the present work is to identify that the alkyl alcohol polyoxyethylene ether sulfonate (CEOS) surfactant has enhanced oil recovery (EOR) potential in hightemperature and high-salinity reservoirs. Initially, three types of CEOS surfactants were synthesized and evaluated for their Received: August 2, 2018 Revised: November 20, 2018 Published: November 27, 2018 A

DOI: 10.1021/acs.energyfuels.8b02653 Energy Fuels XXXX, XXX, XXX−XXX

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Figure 1. Structures of (a) C12EO7S, (b) C6BEO7S, and (c) iC12EO7S. Inorganic salts were added to deionized water to simulate injection water used for CEOS solution preparation and physical simulation experiments. 2.2. Fundamental Properties. The equilibrium surface tensions of CEOS solutions prepared with deionized water were measured using the Wilhemy circle method on a JYW-200A tensiometer (China) at 25 °C. 2.2.1. Critical Micelle Concentration (cmc). cmc, which can reflect surface activity, was obtained by the inflection point of the γ−log C curve. 2.2.2. Adsorption Efficiency (pC20). pC20 can be calculated by eq 1

surface activity and IFT reduction ability. Then, the wettability improvement, emulsification property, and adsorptivity of CEOS were evaluated. Finally, the core flooding experiments were conducted to investigate the effect of CEOS type, salinity, and permeability heterogeneity on oil recovery and confirm the optimal hydrophobic chain type of CEOS at different salinity and permeability reservoirs.

2. EXPERIMENTAL SECTION 2.1. Materials. Three types of CEOS, including dodecyl alcohol polyoxyethylene ether sulfonate (C12EO7S), nonylphenol polyoxyethylene ether sulfonate (C6BEO7S), and isododecyl alcohol polyoxyethylene ether sulfonate (iC12EO7S), were synthesized and characterized according to the literature.30−32 The detailed synthesis method and characterization were presented in the Supporting Information. The molecular structures of CEOS were illustrated in Figure 1. The purity of C12EO7S, C6BEO7S, and iC12EO7S was determined using a two-phase titration method, and the values were 92.32, 89.96, and 91.47%, respectively. Three different dehydrated crude oils were obtained from the Yumen oilfield (density of 0.872 g/cm3 and viscosity of 21.9 mPa s at 50 °C), Tahe oilfield (density of 0.849 g/cm3 and viscosity of 17.4 mPa s at 50 °C), and Mobei oilfield (density of 0.859 g/cm3 and viscosity of 21.6 mPa s at 50 °C), respectively. The saturates, aromatics, resins, and asphaltenes (SARA) of dehydrated crude oils were illustrated in Table 1. The composition and distribution of saturated hydrocarbons in crude oils were determined by Agilent 6890 AGC gas chromatography, and the results were presented in Figure 2.

pC 20 = − log C 20

(1)

where C20 is the surfactant concentration related to the surface tension reduced by 20 mN/m. 2.2.3. Adsorption Potency (Πcmc). Πcmc represents the surface tension reduction ability of the surfactant and was calculated by eq 2 Πcmc = γ0 − γcmc

(2)

where γ0 is the surface tension of aqueous water and γcmc is the surface tension of the surfactant solution at cmc. The higher the Πcmc value, the higher the surface activity. 2.2.4. Saturated Adsorption Capacity (Γmax). Γmax represents the compact degree of surfactant molecules arranged at the gas−liquid interface. According to the Gibbs adsorption isotherm, Γmax at the gas− liquid interface was calculated using eq 3

Γmax = −

1 dγ nRT d ln c

(3)

where Γmax is the maximum surface excess concentration (μmol/m ), T is the absolute temperature (K), and dγ/d lnc is the slope of the surface tension isotherm near the cmc. The value of n relies on the type of surfactant and the salinity of the solvent. In this paper, the value of n is 2 in deionized water because of a 1:1 ratio of the ionic surfactant without an extra electrolyte. 2.2.5. Minimum Cross-Sectional Area of the Surfactant Molecule at the Gas−Liquid Interface (Amin). Amin reflects the configuration of a single molecule at the gas−liquid interface. The smaller the value of Amin, the higher the density of the surfactant. Amin could be obtained according to eq 4 2

Table 1. SARA of Crude Oil component

saturate (%)

aromatic (%)

resin (%)

asphaltene (%)

Yumen oil Tahe oil Mobei oil

41.61 36.74 50.48

44.98 31.92 18.41

9.39 28.95 30.52

1.73 3.29 0.61

A min =

1 ( × 1023) NA Γmax

(4)

where NA is Avogadro’s number. 2.2.6. Hydrophilic−Lipophilic Balance (HLB) Value. The HLB values of CEOS were calculated according to the cardinal formula, as shown in eq 5. The cardinal numbers of the hydrophilic group and lipophilic group were shown in Table 2.

∑ (radix of hydrophilic group) − ∑ (radix of hydrophobic group)

HLB = 7 +

(5)

2.3. Measurement of Oil−Water IFT. The IFT between crude oil and CEOS solution was measured using a spinning drop video tensiometer (SVT20) from Data Physics Instruments GmbH, Filderstadt, Germany. 2.4. Wettability Improvement. The wettability improvement of CEOS solution prepared with injection water (containing 22 × 104 mg/

Figure 2. Saturated hydrocarbon composition of crude oil. B


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T ij ∫0 V (t ) dx yzzz jj j zz × 100% j SI = jj1 − zz jj zz Tvw k {

Table 2. Cardinal Numbers of Hydrophilic Groups and Lipophilic Groups

V Te

v water‐liberating rate = t × 100% vw

(8)

where Se is the emulsification rate (mL/min), V is oil volume emulsified (mL), Te is the emulsification time (min), vt is the water volume liberated (mL), vw is the total volume of the water phase (mL), T is the water-liberating time (min), and V(t) is the function of water liberated with time (mL min). 2.6. Adsorptivity. The adsorptivity of CEOS was measured with the following procedures. First, the optimal wavelength of CEOS solutions was optimized by an ultraviolet and visible spectrophotometer. Then, the relationship between the CEOS concentration and absorbance was obtained. Third, a batch of experiments were performed by equilibrating 10.0 g of quartz sand (100−200 mesh) in 100 mL of CEOS solutions ranging from 500 to 3500 mg/L. The bottles were then shaken intermittently at 85 °C. After 48 h, the absorbance of CEOS solutions was measured at the optimal adsorption wavelength. The capacity of CEOS adsorbed on the quartz sand surface was determined from the difference of the CEOS concentration before and after adsorption. The adsorptive capacity could be calculate by eq 9. Moreover, the effect of salinity on adsorption of CEOS was studied at 85 °C

L NaCl and 4162.5 mg/L CaCl2) on the hydrophilic surface and hydrophobic surface at 85 °C was evaluated by the contact angle method using a HARKE-SPCA contact angle measuring instrument (Haako, China). The hydrophilic surface was prepared by immersing a quartz plate in diluted hydrochloric acid for 1 week, then washing by deionized water, and drying in a drying oven. The hydrophobic surface was prepared by immersing a quartz plate in silicone oil for 1 week. Before measurement, the silicone oil on the surface was erased by lenswiping paper. Yumen oil was used in the experiment. 2.5. Emulsifying Property. The emulsifying property of CEOS was evaluated with the following procedures. First, CEOS solution prepared with injection water and Yumen oil was added to penicillin bottles at the volume ratios of 1:1 and 3:7, respectively. The temperature of the penicillin bottle was heated to 85 °C, and then CEOS solution and oil were mixed by a turbine mixer. The time of the system when completely emulsified (Te) was recorded to calculate the emulsification rate. The emulsifying rate reflects the emulsifying ability. The higher the emulsification rate, the easier it is for the dispersion phase to be dispersed in the continuous phase, and the emulsification rate could be calculated by eq 6. The emulsion type was determined by the electrical conductivity method. The electrical conductivities of crude oil, injection water, and emulsion were measured. If the electrical conductivity of the emulsion is closer to the electrical conductivity of the crude oil, the type of emulsion is water-in-oil (W/O); otherwise, the type of emulsion is oil-in-water (O/W). The distribution of the droplet size of the emulsion was tested by a Leica microscope. The viscosity of the emulsion was determined by Brookfield DV-III under the conditions of a shear rate of 7.34 s−1 and temperature of 85 °C. Then, the penicillin bottle was put in an 85 °C oven, and the volume of the liberated water phase at different times was recorded to calculate the water-liberating rate (eq 7) and emulsifying stability index (SI) (eq 8)

Se =

Article

Γ1 =

V (c0 − c) m

(9)

where Γ1 is the static adsorption capacity (mg/g), V is the solution volume (L), c0 is the surfactant concentration before adsorption (mg/ L), c is the surfactant concentration after adsorption (mg/L), and m is the quality of quartz sand (g). 2.7. Core Flooding Experiments. The core flooding experiments were conducted in artificial sandstone cores at 85 °C with 1500 mg/L CEOS solution prepared with saline water, and the procedures were as follows. First, the basic parameters of artificial sandstone cores were measured. Then, after the cores were saturated with saline water, the artificial sandstone cores were flooded by Yumen crude oil until water production ceased under ring pressure conditions (the ring pressure was always greater than the injection pressure by 2 MPa). Subsequently, the water flooding (WF) was conducted by injecting injection water until the water cut was 98%. Then, 0.5 pore volume (PV) surfactant solution slug was injected (SF). Finally, the subsequent water flooding (SWF) was conducted until the water cut reached 98%. The flow rate was 0.5 mL/min for all of the displacement tests in the study. The schematic diagram of the experimental apparatus was shown in Figure 3.

(6)

3. RESULTS AND DISCUSSION 3.1. Surface Performance. The equilibrium surface tension curves of CEOS solution prepared with deionized water were

(7)

Figure 3. Schematic diagram of the core flooding experiments. C


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Figure 4. Curve of γ−log C (temperature, 25 °C).

Figure 5. IFT versus CEOS concentration.

shown in Figure 4. When the CEOS concentration was lower than cmc, with the increase of the concentration, the surface tensions decreased sharply and then tended to be constants when the concentration exceeded cmc. The surface performance of CEOS were calculated and shown in Table 3. The cmc of CEOS was almost 2 orders of magnitude lower than that of a common surfactant [sodium dodecyl sulfate (SDS)]. The introduction of an ethoxyl group in CEOS chains enhanced its hydrophobic property, making micelles easier to form, and the cmc value decreased evidently. The surface tension of C6BEO7S solution was higher than that of C12EO7S because the hydrogen atom and carbon atom in the benzene ring were coplanar, and a regular hexagon of the benzene ring made C6BEO7S molecules difficult to arrange closely at the gas−liquid interface. The cmc value of iC12EO7S solution was higher than that of C12EO7S because the molecules with branched chains had better water solubility, and the effect of the carbon atom in branched chains on the cmc value was only half of the carbon atom in straight chains. 3.2. IFT between CEOS and Crude Oil. The equilibrium IFT between Yumen crude oil and CEOS solution prepared with deionized water was measured at 85 °C and normal pressure, and the results were shown in Figure 5. With the increase of the CEOS concentration, the IFT decreased sharply first and then tended to be constant when the CEOS concentration surpassed 1500 mg/L. When the CEOS concentration was less than 1500 mg/L, with the increase of the concentration, the adsorption capacity of CEOS at the oil−water interface increased, resulting in the evident reduction of IFT. When the CEOS concentration exceeded 1500 mg/L, CEOS molecules adsorbed on the oil− water interface arranged closely and tended to be saturated, and the increase of the CEOS concentration had no effect on IFT reduction. For the branched chains, iC12EO7S had the best

Figure 6. Effect of NaCl on IFT (CEOS concentration, 1500 mg/L; temperature, 85 °C).

ability to reduce the IFT, and the IFT could be reduced to 10−2 mN/m. The effect of NaCl on IFT was shown in Figure 6. For iC12EO7S and C12EO7S, with the increase of the NaCl concentration, the IFT decreased first and then tended to be constant. When the NaCl concentration ranged from 15 × 104 to 22 × 104 mg/L, iC12EO7S and C12EO7S could reduce IFT to 10−4 and 10−3 mN/m, respectively. For C6BEO7S, with the increase of the NaCl concentration, the IFT decreased first and then increased and the IFT could be reduced to 10−1 mN/m. NaCl could reduce IFT distinctly as a result of Na+ screening sulfonate ions, resulting in a repulsion force among hydrophilic

Table 3. Surface Activity of Surfactants in Deionized Water surfactant

cmc (mmol/L)

cmc (mg/L)

γcmc (mN/m)

HLB value

pC20

Πcmc (mN/m)

Γmax (μmol/m2)

Amin (nm2)

C12EO7S C6BEO7S iC12EO7S SDSa

0.085 0.097 0.153 8.100

54.57 65.09 99.14 2332.8

30.1 32.4 34.4 31.9

14.94 14.70 14.47 40

2.08 1.85 1.63 0.21

41.9 39.6 37.6 40.1

1.25 1.21 1.19

1.32 1.37 1.40

a

From ref 33. D


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The effect of CaCl2 on IFT was shown in Figure 7. For iC12EO7S and C12EO7S, with the increase of the CaCl2 concentration, the IFT decreased first and then increased. When the CaCl2 concentration was 4162.5 mg/L, the IFT reached a minimum. When the CaCl2 concentration was less than 4162.5 mg/L, on the one hand, chelate between Ca2+ and ethyoxyl was formed, resulting in the decrease of the interaction between Ca2+ and SO32−. On the other hand, Ca2+, as a counterion, could weaken the repulsion force among SO32− ions, resulting in the increase of the adsorption capacity of CEOS molecules on the oil−water interface and the decrease of IFT. When the CaCl2 concentration surpassed 4162.5 mg/L, excessive Ca2+ destroyed the hydration film near hydrophilic heads, promoting CEOS molecules at the oil−water interface to transfer into the oil phase. The loss of CEOS molecules at the interface resulted in the increase of IFT. For C6BEO7S, Ca2+ had little effect on IFT for the rigid benzene ring in hydrophobic chains. The IFT between crude oil and CEOS solution prepared with injection water was shown in Figure 8. For iC12EO7S and C12EO7S, the IFT decreased sharply first and then tended to be constant, and the ultimate IFT was reduced to an ultralow level (10−3 mN/m), which revealed the favorable anti-salt performance. Meanwhile, the long-term stability of the surfactant under high temperature (85 and 110 °C, respectively) and high salinity was investigated (Figure 9). After aging 90 days, the IFT between iC12EO7S (C12EO7S) and crude oil could be maintained at 10−2 mN/m, which illustrated the favorable chemical stability. For the effect of crude oil type on IFT, the IFT between crude oil and CEOS solution was shown in Figure 10. CEOS had different capabilities on IFT reduction for different crude oils. CEOS had strong interaction with Tahe crude oil and Yumen crude oil. The medium components (C5−C15) of Tahe crude oil, Yumen crude oil, and Mobei crude oil were 57.66, 55.87, and 42.55%, respectively. According to the principle of the similar dissolve mutually theory, Yumen crude oil and Tahe crude oil had better solubility with CEOS, and the IFT could be reduced to a lower level than that of Mobei oil. Moreover, Tahe crude oil and Yumen crude oil had higher asphaltene content (Tahe crude oil, 3.29%; Yumen crude oil, 1.73%; and Mobei crude oil, 0.61%), and asphaltene, as a natural interface active substance,

Figure 7. Effect of CaCl2 on IFT (CEOS concentration, 1500 mg/L; temperature, 85 °C).

Figure 8. Effect of time on IFT (CEOS concentration, 1500 mg/L; temperature, 85 °C).

heads being weakened and CEOS molecules being arranged more closely.

Figure 9. Long-term stability of IFT (CEOS concentration, 1500 mg/L). E


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Figure 10. Effect of crude oil on IFT (left, CEOS solution prepared with injection water; right, CEOS solution prepared with deionized water).

Figure 11. Effect of crude oil components on IFT (a, C12EO7S solution; b, iC12EO7S solution; and CEOS concentration, 1500 mg/L).

Figure 12. Contact angle versus adsorption time (surfactant concentration, 1500 mg/L; temperature, 85 °C).

Figure 13. Adsorption of CEOS molecules on the surface of the quartz plate. F


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Energy & Fuels Table 4. Emulsification Results of CEOS sample

system

oil−water ratio

emulsifying rate (mL/min)

SI (%)

emulsion type

emulsion viscosity (mPa s)

1 2 3 4 5 6

C12EO7S iC12EO7S C6BEO7S C12EO7S iC12EO7S C6BEO7S

1:1 1:1 1:1 3:7 3:7 3:7

7.00 10.00 13.04 11.50 16.36 25.71

18.21 26.88 31.51 29.02 45.31 39.73

O/W O/W W/O O/W O/W O/W

12.7 15.4 26.3 7.2 9.5 11.5

Figure 14. Aspect of emulsion.

Figure 15. Change of the emulsion bleeding rate.

only dissolve in oil and change the polarity of kerosene; therefore, the adsorption of C12EO7S was affected and IFT increased. The higher the resin and asphaltene contents, the lower the IFT. Most acidic components with a strong interfacial activity in crude oil were in resin, and asphaltene was a natural active substance. They synergistically decreased IFT with C12EO7S molecules. For iC12EO7S solution (Figure 11b), saturate and aromatic had a similar effect on IFT with C12EO7S solution. The addition of resin and asphaltene could remarkably enhance IFT. iC12EO7S molecules could be closely arranged at the oil−water interface for the branched chains and evidently reduce IFT. When resin, asphaltene, and iC12EO7S molecules adsorbed on the oil−water interface, the adsorption capacity reduced for the spatial effect of resin and asphaltene and adsorbed molecules arranged loosely in the oil−water interface, resulting in the increase of IFT.

could synergistically adsorb at the oil−water interface and reduce IFT. For the effect of crude oil components on IFT, first, after the separation of four components of Yumen crude oil (the separation method referred to the Standards of Petrochemical Industry of the People’s Republic of China test method for the separation of asphalt into four fractions, NB/SH/T 0509-2010) was conducted, simulated oil was prepared by mixing components with kerosene. Then, the IFT between simulated oil and CEOS solution prepared with injection water was measured, and the results were shown in Figure 11. For C12EO7S solution (Figure 11a), when the ratio of component/kerosene was 1:25, saturate and aromatic had little effect on IFT reduction, and when the addition of saturate and aromatic was increased (ratio of component/kerosene was 1:5), the IFT increased sharply. The polarity of saturate and aromatic was low. Their interfacial activity was inferior, and these molecules could G


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Figure 16. Images of emulsion.

3.3. Wettability Improvement. The relationship between the contact angle and CEOS adsorption time was shown in Figure 12. For the hydrophilic surface, the surface of the quartz plate had high energy, water was easily spread on the surface as a result of the formation of hydrogen bonds between SiOH groups on the hydrophilic surface and water molecules, and the initial contact angle was 31.7°. After the quartz plate was immersed in CEOS solution, with the increase of the immersion time, the contact angle increased first and then decreased. When the immersion time reached 18 h, the contact angle tended to be constant. The long adsorption equilibrium time was ascribed to the repulsion force between CEOS and the negatively charged surface. When CEOS contacted with the hydrophilic surface, the hydrogen bonds between ethyoxyl and the quartz plate surface and the hydrophobic effect among hydrophobic chains made “flat-shape” CEOS molecules distribute on the surface (Figure 13a). With the increase of the contact time, the adsorbing capacity of CEOS molecules on the hydrophilic surface increased, hemispherical micelles formed as a result of hydrophobic association among hydrophobic chains, and “upright” CEOS molecules arranged on the hydrophilic surface (Figure 13b), resulting in a decrease of the contact angle. For the hydrophobic surface, the initial contact angle was 153.4°. With the increase of the contact time, the contact angle decreased

sharply. When the contact time reached 18 h, the contact angle tended to be constant. When CEOS contacted with the hydrophobic surface, the silicone oil that adsorbed on the quartz plate surface was solubilized by CEOS micelles first (Figure 13c), then CEOS molecules adsorbed on the surface (Figure 13d), and finally the hydrophobic surface turned into a hydrophilic surface. The results illustrated that CEOS had favorable wettability improvement ability. 3.4. Emulsifying Property. The results of the emulsification rate and emulsion stability were shown in Table 4. For the same type of CEOS, the lower the oil−water ratio, the higher the emulsifying rate as a result of more CEOS molecules existing in the system. For different types of CEOS, iC12EO7S and C6BEO7S had better emulsifying properties than C12EO7S, and the emulsifying rate of C6BEO7S was the fastest. According to the aspect of emulsion after demulsifying (Figure 14), a lot of scattered oil droplets dispersed in the aqueous phase, which illustrated the favorable emulsification ability of C6BEO7S. Moreover, the branched chains of iC12EO7S and the rigid benzene ring of C6BEO7S enhanced the emulsion ability. The viscosity of the emulsion formed by iC12EO7S solution and C6BEO7S solution was higher than that of the emulsion formed by C12EO7S solution; hence, the diffusion resistance was larger, H


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Figure 17. (a) Absorbance wavelength curve of CEOS, (b) standard calibration curve of CEOS, (c) CEOS adsorbing capacity prepared with injection water, and (d) effect of salinity on CEOS adsorption (CEOS concentration, 1500 mg/L).

Table 5. Parameters of Cores core sample

Φ (%)

permeability (mD)

So (%)

surfactant

concentration (mg/L)

WF recovery (%)

EOR (%)

total recovery (%)

1 2 3 4 5 6

20.72 20.89 19.28 19.01 18.54 19.67

8.14 8.45 7.60 55.23 56.89 53.23

47.02 45.63 42.78 66.32 65.06 65.18

iC12EO7S C12EO7S C6BEO7S iC12EO7S C12EO7S C6BEO7S

1500 1500 1500 1500 1500 1500

50.00 50.00 59.72 48.94 51.42 53.98

13.95 19.12 13.89 13.82 16.19 17.69

63.95 69.12 76.31 62.76 67.61 71.67

3.5. Adsorptivity. The adsorption wavelength related to the optimal adsorption peak of CEOS was shown in Figure 17a. The standard curve of the CEOS concentration versus absorbance was shown in Figure 17b. The relationship between the CEOS concentration and adsorbing capacity was shown in Figure 17c. The relationship between salinity and adsorbing capacity was shown in Figure 17d. It could be clearly concluded from Figure 17c that, with the increase of the CEOS concentration, the adsorption amount increased first and then tended to be constant. When the CEOS concentration reached 2000 mg/L, the equilibrium adsorption capacity for iC12EO7S, C12EO7S, and C6BEO7S was 0.911, 1.082, and 0.724 mg/L, respectively. In addition, with the increase of salinity, the adsorption amount increased first and then tended to be constant. When CEOS solution contacted the quartz sand, an electric double layer was

the collision frequency was smaller, the water-liberating rate was slower (Figure 15), and the property of emulsion was better.34,35 The morphology of emulsion was observed, and the mean diameter of emulsion droplets was calculated by ImageJ software. The results were shown in Figure 16. When the oil− water ratio was 1:1, iC12EO7S solution and C12EO7S solution formed O/W emulsions with crude oil, and the mean diameter of oil droplet in the C12EO7S system was larger than that of the iC12EO7S system, while a W/O emulsion formed in the C6BEO7S system. When the oil−water ratio was 7:3, O/W emulsions were formed, and the diameter of the droplet for C12EO7S, C6BEO7S, and iC12EO7S was 47.88, 20.61, and 43.98 μm, respectively. The results illustrated that the smaller the average particle size, the more stable the emulsion. I


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Figure 18. Oil recovery, water cut, and pressure history.

Table 6. Parameters of Cores core sample

Φ (%)

permeability (mD)

So (%)

surfactant

salinity (mg/L)

WF recovery (%)

EOR (%)

total recovery (%)

1 7 8

20.72 19.38 17.83

8.14 10.33 11.56

47.02 51.57 59.87

iC12EO7S iC12EO7S iC12EO7S

22.5 × 104 1.5 × 104 10.5 × 104

50.00 36.76 45.97

13.95 8.83 12.65

63.95 45.59 58.62

formed, while Na+ in solution compressed it, resulting in the repulsive force among sulfonate groups decreasing, CEOS molecules being arranged closely, the active sites that could be adsorbed by CEOS increasing, and the adsorption amount increasing. When salinity reached 20 × 104 mg/L, the electric

double layer could not be compressed and the adsorption amount tended to be constant. 3.6. Core Flooding Experiments. Core flooding experiments were conducted to evaluate the EOR potential using 1500 mg/L CEOS solution. J


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Table 7. Parameters of Three-Layer Heterogeneous Cores core sample

permeability (mD)

Φ (%)

So (%)

surfactant

concentration (mg/L)

WF recovery (%)

EOR (%)

total recovery (%)

9 10 11

200/500/1000 200/500/1000 200/500/1000

17.58 18.21 17.63

65.39 67.28 66.64

C6BEO7S C12EO7S iC6EO7S

1500 1500 1500

29.08 25.52 27.93

19.38 18.90 14.17

48.46 44.42 42.10


Figure 18. When core permeability was ∼10 mD, C12EO7S solution had the highest EOR because of its favorable ability on IFT reduction and capillary resistance reduction. It evidently increased the capillary number, resulting in the oil droplet

3.6.1. Effect of CEOS Type on Oil Recovery. Six artificial sandstone cores were used to investigate the effect of CEOS type on oil recovery. The diameter and length of cores were 3.8 and 7.0 cm, respectively. The results were shown in Table 5 and K


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Energy & Fuels

4. CONCLUSION Three types of CEOS were synthesized and evaluated. The synthesized CEOS showed high surface activity, favorable IFT reduction ability, wettability improvement, and emulsifying property. CEOS had favorable adaptability with Tahe crude oil and Yumen crude oil. The resin and asphaltene could synergistically reduce IFT with C12EO7S, while the IFT increased when C6BEO7S was applied. C6BEO7S had favorable emulsifying ability, and the diameter of the oil droplet in the emulsion was 20.61 μm. In homogeneous cores, when the CEOS concentration was 1500 mg/L and salinity was 22.5 × 104 mg/L, 0.5 PV linear C12EO7S flooding and subsequent water flooding could remarkably enhance oil recovery of 16−19%, C6BEO7S with a benzene ring could evidently enhance oil recovery of 13.89− 17.69%, and branched iC12EO7S could distinctly enhance oil recovery of ∼13%. In three-floor non-homogeneous square cores, C6BEO7S had favorable displacement performance for its benign emulsifying property. The formed emulsion could effectively regulate core heterogeneity and improve the mobility ratio. All of the results indicate that CEOS has great potential in high-temperature and high-salinity oil reservoirs.

Figure 21. Image of produced water for core sample 9 in the subsequent water flooding.

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ASSOCIATED CONTENT

S Supporting Information *

passing through tiny holes in the core. When core permeability was ∼50 mD, C6BEO7S flooding and subsequent water flooding could remarkably enhance oil recovery by 17.69% because of its favorable emulsifying property. The viscosity of the emulsion was the highest (Table 4); hence, the mobility ratio could be effectively reduced, and the “fingering effect” could be evidently weakened. As shown in panel 6 of Figure 18, after C6BEO7S solution was injected, the pressure slightly increased, which illustrated the formation of the emulsion and the increase of the sweeping volume. 3.6.2. Effect of Salinity on Oil Recovery. Three cores were used to investigate the effect of salinity on oil recovery. The diameter and length of cores were 3.8 and 7.0 cm, respectively. The results were shown in Table 6 and Figure 19. With the increase of salinity, the EOR increased because the increase of salinity is beneficial to IFT reduction. When the salinity is 1.5 × 104, 10.5 × 104, and 22.5 × 104 mg/L, the IFT was 0.088, 0.009, and 0.003 mN/m, respectively. The lower the IFT, the higher the capillary number and the more residual oil activated. 3.6.3. Effect of Permeability Heterogeneity on Oil Recovery. Three three-floor heterogeneous square cores were used to investigate the effect of permeability heterogeneity on oil recovery. The gas permeability of the layers was 200, 500, and 1000 mD, respectively. The width and length of cores were 4.5 and 30.0 cm, respectively. The results were shown in Table 7 and Figure 20. After surfactant solution was injected, the pressure increased sharply and then decreased, which illustrated the formation of the emulsion. The formed emulsion could effectively regulate core heterogeneity and improve the mobility ratio. Figure 21a showed the image of produced liquid, and an obvious emulsification could be observed. After 30 min, a clear oil−water interface with water and oil at the bottom and top was observed, respectively (Figure 21b), which demonstrated the breaking of the emulsion. The results illustrated that CEOS surfactants had favorable displacement performance in heterogeneous cores.

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.8b02653.

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Detailed synthesis method and characterization (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Rui Liu: 0000-0001-6723-1856 Dai-jun Du: 0000-0002-6782-7374 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are thankful to the support received from the State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Southwest Petroleum University.

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REFERENCES

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Enhanced oil recovery potential of alkyl alcohol polyoxyethylene ether

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