Enhancing Heavy-Oil Recovery by Using Middle Carbon Alcohol

Mar 30, 2015 - ABSTRACT: Previous studies have shown that the addition of low carbon alcohols can significantly enhance oil recovery in surfactant flo...
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Enhancing Heavy-Oil Recovery by Using Middle Carbon AlcoholEnhanced Waterflooding, Surfactant Flooding, and Foam Flooding Zehua Chen*,† and Xiutai Zhao† †

College of Petroleum Engineering, China University of Petroleum, Qingdao 266580, People’s Republic of China ABSTRACT: Previous studies have shown that the addition of low carbon alcohols can significantly enhance oil recovery in surfactant flooding and alkaline flooding. In this paper, the influence of middle carbon alcohols (including n-butanol, n-pentanol, isoamyl alcohol, and n-hexanol) on waterflooding, surfactant flooding, and foam flooding for enhanced heavy-oil recovery was investigated. Foaming tests, oil viscosity measurements, IFT measurements, emulsification tests, and sandpack flooding tests were conducted to investigate the effects and mechanisms for enhanced heavy-oil recovery by addition of middle alcohols. The results show that the added middle carbon alcohols has no obvious effect on the reduction of IFT between oil and surfactant solution; however, they can reduce the viscosity of heavy oil and can greatly enhance the foaming property and the emulsifying property of surfactant solution. The sandpack flooding results show that the heavy-oil recovery can be enhanced significantly through the addition of middle alcohols in water flooding, surfactant flooding, and foam flooding, especially in foam flooding. The reduction of oil viscosity, the entrapment of emulsified oil droplets, and the enhancement of foaming property by the addition of alcohols contributes to the improvement of sweep efficiency. Meanwhile, the enhancement of emulsifying property of surfactant solution by addition of alcohols facilitates the improvement of displacement efficiency.

1. INTRODUCTION With the continuous consumption and the demand of energy, more exploitation of crude oil is needed urgently. The most widely used oil recovery method, water flooding, can only recover about 40% of the crude oil in reservoirs.1 The reason for the low oil recovery of water flooding is that on one hand, the water−oil mobility ratio is usually highthus the oil is easily bypassed by injected waterand the sweep efficiency is low; on the other hand, the IFT between oil and water is high, and it is difficult for the oil to be emulsified into water phase, leading to a low displacement efficiency. For higher oil recovery, different oil recovery methods,2−5 such as chemical flooding, foam flooding, gas flooding, and microbial flooding, were investigated in laboratory experiments and were adopted in field applications. Among the above recovery methods, foam flooding is an important one, and both laboratory tests and field applications of foam flooding show good effects and prospects.6,7 Foam has a high apparent viscosity. After being injected into the formation, it first enters the highly permeable layers and blocks the water channels effectively. Then the injected fluid is diverted to low permeable layers, and the viscosity fingering is controlled due to the plugging of water channels; therefore, sweep efficiency can be improved greatly through the injection of foam.8,9 As is well-known, surfactant is the most widely used foamer, which can be absorbed on the gas−water interface to stabilize the bubbles. Polymers and nanoparticles are also used to stabilize the foam.10,11 However, there are disadvantages to using polymer as a foamer:12 first, the addition of polymer usually reduces the foaming capacity of surfactant; second, the polymer has weak salt tolerance, and the viscosity of polymer decreases badly under the action of high shear and in reservoir conditions; third, there are parts of pores that a polymer cannot enter; fourth, a polymer can damage the low permeability formation and reduce its production capacity. Nanoparticles are also an important foam stabilizer and have © XXXX American Chemical Society

become a hot research topic now. However, the problem of aggregation of nanoparticles has not yet been solved.13 Furthermore, the cost of nanoparticles is too high for enhanced oil recovery. As an unstable system, foam can rupture successively when migrating in porous media, and the surfactant (foamer) can be released into the water phase or other phase. Surfactant can reduce the oil−water interfacial tension (IFT) and can emulsify the oil into small oil droplets, so the released foamer may enhance oil recovery (reducing residual oil saturation) as a surfactant.14 However, the foamer itself can not reduce the residual oil saturation greatly, so some researchers have added alkaline to the foam system15 (ASF system) to recover oil significantly. There is a good synergistic effect between the foamer and the added alkaline, and they can work with each other to reduce the oil−water IFT to ultralow.3 In addition, the added alkaline can enhance the emulsifying property of the foaming system. Then the crude oil can be easily emulsified into small oil droplets, the oil droplets can be entrained along with the aqueous phase smoothly, and thereby the displacement efficiency can be enhanced significantly. However, the added alkaline can cause severe problems such as scaling, reduction of production capacity, frequent pump inspection, serious emulsification, and so forth,16 which limits its application. Therefore, some substitute for alkaline should be selected for improving displacement efficiency in foam flooding. Alcohol is a kind of special surfactant which is not often used for enhanced oil recovery. First, it can be absorbed on the gas− water interface to reduce surface tension.17 Thus, the alcohols may enhance the foaming property of foamers and improves Received: November 26, 2014 Revised: March 30, 2015

A

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Energy & Fuels Table 1. Basic Properties of Low Carbon Alcohols Used in This Study alcohol type

chemical structure

density at 20 °C (g/cm3)

solubility in water (g/100 g)

n-butanol n-pentanol isoamyl alcohol n-hexanol

CH3(CH2)3OH CH3(CH2)3CH2OH (CH3)2CHCH2CH2OH CH3(CH2)4CH2OH

0.8098 0.817 0.8092 0.8136

9 g/100 g of water at 15 °C 2.7g/100 g of water at 22 °C 2 g/100 g of water at 14 °C 0.6 g/100 g of water at 20 °C

the effect of foam flooding, which was rarely mentioned before. Previous studies has also shown that alcohols can greatly improve oil recovery in surfactant flooding and alkaline flooding through reducing residual oil saturation.18−21 Accordingly, alcohols have potential to improve both the sweep efficiency and the displacement efficiency in foam flooding for enhanced heavy-oil recovery. Alcohols have great tolerance to salt, temperature, and shear. Also, they do not give rise to several problems that polymers and alkaline conditions can cause. Therefore, alcohols are good candidates for enhanced oil recovery. Alpha olefin sodium sulfonate (AOS) is one of the most widely used foamers in foam flooding,22 and in this paper, it is used as the foamer and also as the surfactant in surfactant flooding. The variety of foaming volume and half-life with AOS concentration were measured, and the results are shown in Figure 3. The foaming volume and half-life increase sharply with AOS concentration when less than 0.1 wt %, and then they change little with AOS concentration. n-Butanol, n-pentanol, isoamyl alcohol, and n-hexanol were used as the middle alcohols. Foaming tests, IFT measurements, emulsification tests, oil viscosity measurements, and sandpack flooding tests were conducted to investigate the influence of middle carbon alcohols on waterflooding, surfactant flooding, and foam flooding for enhanced heavy-oil recovery. Also, surfactant flooding was conducted as a comparison with foam flooding, and it was used to investigate the effect of alcohols on improving sweep efficiency and displacement efficiency through emulsification.

20 mL of oil and 80 mL of water solution with different alcohol concentrations were put into a 100 mL cylinder successively, and the cylinder was aged for 10 min at 50 °C. Next, the cylinder was lightly shaken 30 times and was then aged for 10 min. Lastly, the oil was removed with a syringe, and the viscosity of the oil was measured using a viscometer at 50 °C. 2.4. Measurements of Interfacial Tension. The dynamic interfacial tension (IFT) between heavy oil and different chemical solutions was measured with a Texas-500 spinning drop tensionmeter at 50 °C. The dynamic IFT was determined with an image capture system and a calculation software automatically. In the process, it usually took about 10 min to reach equilibrium IFT. 2.5. Emulsification Experiments. To investigate the effect of middle carbon alcohols on the emulsifying property of AOS solution, emulsification tests were conducted, and the minimum emulsifying time (abbreviated as Tmin) for 10 mL of oil to be completely dispersed into 100 mL of AOS solution was determined by the self-designed instrument (see Figure 1).23 The procedure was as follows: first, 100 mL of AOS

2. EXPERIMENTAL SECTION 2.1. Fluids and Chemicals. The alpha olefin sodium sulfonate (AOS) used in this paper is an industrial product, with a purity of >90 wt %, the carbon number of AOS is 14−18, and the critical micelle concentration (CMC) of the AOS in water is about 0.06 wt % at 20 °C. NaCl is AR grade, with a purity of >99 wt %. The middle carbon alcohols used in this study are listed in Table 1. The crude oil was provided by Bohai oilfield in China, with a viscosity of 242 mPa·s at 50 °C. All of the solutions used in this paper were prepared with NaCl solutions in concentration of 0.5 wt %. 2.2. Foaming Ability and Foam Stability Experiments. Foam was prepared using the Warning Blender methods in this paper. A 200 mL AOS solution was put into the blender (GJ3S, Qingdao Senxin Machinery Equipment Co, Ltd., China) and agitated for 1 min at 3000 rpm. Then the foam was poured into a 1000 mL cylinder to monitor the foam volume immediately when the agitation stopped, and the maximum foam volume (defined as foaming volume) was recorded to evaluate the foaming ability of AOS solution. The time taken by the foam volume to decrease to half its size (defined as half-life) was also recorded to assess the foam stability. All of the stability experiments were conducted in an oven at 50 °C. 2.3. Oil Viscosity Measurements. To investigate the effect of middle alcohols on the reduction of heavy-oil viscosity, a new method was adopted. The procedure was as follows: first,

Figure 1. Apparatus used to measure Tmin and Tl. 1 - rheometer; 2 stir bar; 3 - cylinder.

solution and 10 mL of oil were put into the cylinder successively. Next, the height of the agitating rod was adjusted to ensure that the stirring blade was immediately beneath the oil−water interface. Then, the blender was set at a stirring speed of 300 r/min, and the oil was gradually dispersed into the aqueous phase. The time taken from the start of the agitation to the thorough dispersion of oil into aqueous phase is defined as Tmin, and the smaller the Tmin, the stronger the emulsifying ability of the chemical agent. Afterward, the agitation was stopped, and the required time was recorded (defined as Tl) from the moment of stopping agitation to the thorough layering of oil phase and water phase; Tl was used to assess the emulsion stability, and the larger the Tl, the more stable the emulsion. All the emulsification tests were conducted at 50 °C (using a circulator bath). 2.6. Sandpack Flood Tests. The sandpack used in this study was 2.5 cm in diameter and 30 cm in length. Quartz sand with the size of 80−120 mesh was used to ensure that the B

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Figure 2. Schematic of sandpack flooding instruments. 1 - nitrogen cylinder; 2 - pressure pump; 3 - high pressure tank; 4 - gas flowmeter; 5 constant flux pump; 6 - thermostat; 7 - crude oil; 8 - water; 9 - foam generator; 10 - atmospheric valve; 11 - sandpack; 12 - back - pressure valve; 13 oil−water separator; 14 - hand pump.

3. RESULTS AND DISCUSSION 3.1. Influence of Middle Carbon Alcohols on Foaming Property of AOS. In field application, the working concentration of AOS is about 0.5−1 wt %.4,11,14 When surfactant solution migrates in the porous media, they can be absorbed on the surface of reservoir rocks, which results in large loss of surfactant. Besides, under the dilution of formation water, the surfactant concentration also becomes lower. Therefore, after migrating for a certain distance in reservoir, the surfactant concentration may be much lower than its initial value. Accordingly, the influence of the addition of middle carbon alcohols on the foaming property of AOS were conducted in its concentration of 0.02, 0.1, and 0.5 wt %, respectively, in this paper (the three concentrations represent low concentration, middle concentration, and high concentration, respectively) (Figure 3). Figure 4 and Figure 5 show

permeability of the sandpacks ranges from 1.8−2.5 Dracy. For each sandpack, fresh sand was wet-packed to guarantee the same wettability for all of the flooding tests. The procedures were as follows: A coreholder filled with formation water was positioned vertically, and the sand was added to fill the coreholder in several steps. In each step, the coreholder was beaten slightly and evenly after the sand was poured in. During this process, the water surface was kept above the top of the sand to ensure that air was not introduced into the sample.3 The schematic of the sandpack flooding experiments is shown in Figure 2. All sandpack flooding tests were conducted horizontally. The procedure are as follows: first, the sandpack was saturated with formation water (0.5 wt % NaCl), and the porosity and the permeability of the sandpack was calculated. Then the crude oil was injected into the sandpacks until water production ceased. The sandpack was aged for 24 h at 50 °C. Next, the sandpack was flooded with formation water (primary waterflooding) until the oil production became negligible (water injection for 2PV and water cut >98%), and then 0.5 PV of surfactant (AOS) solution or foam system (including surfactant solution and nitrogen) was injected, followed by an extended waterflood until the oil production became negligible. A foam generator, which is a porous medium sealed in the flow path, was installed in the upstream of the sandpack. The foam generator is filled with silica sand, with diameters of about 80− 100 μm. Then the foam was generated by passing the gas and surfactant solution simultaneously through the foam generator.11 The injection rate of water and surfactant solution were both 0.5 mL/min (about 0.255 cm/min in Darcy velocity) in waterflooding and surfactant flooding (Run 1−9, 19−20). In foam flooding, the injection rates of foam solution and nitrogen were both 0.25 mL/min (about 0.13 cm/min in Darcy velocity) (Run 10−18). The back-pressure for all of the sandpack floodings were 2.0 MPa. All of these tests were conducted at 50 °C, except where otherwise specified. It should be noted that the static adsorbances of AOS on sand were about 0.008 mg/g, 0.03 mg/g, and 0.2 mg/g, respectively, when AOS concentration is 0.02, 0.1, and 0.5 wt %. The mass of the sand in the sandpack is about 150 g. The absorption loss of AOS is less than 30% of its original value after calculation, and therefore, the foaming property and the emulsifying property cannot be weakened greatly.

Figure 3. Change of foaming volume and half-life with AOS concentration.

the change of the foaming volume and the half-life, respectively, with the concentration of different alcohols when AOS concentration is 0.02 wt %. It can be observed that the foaming volume and the half-life both increase sharply with alcohol concentration when less than 0.1 wt %. The foaming volume and half-life can be doubled at alcohol concentration of 0.1 wt % (except for n-hexanol), and then they change little C

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hydrophilic group of alcohol is uncharged, thus the electrostatic repulsion between different molecules becomes smaller, and the interfacial film becomes more compact), thus the strength of the interfacial film is enhanced, and the foam stability is improved. When alcohol concentration exceeds 0.2 wt %, superfluous alcohol spreads to water phase and does not work, thus the foaming property changes little. In addition, it should be noted that when alcohol concentration exceeds a certain value (for n-hexanol, the value is 0.1 wt %, and for other alcohols the value is 0.1 wt %), the foaming volume and the half-life decrease with alcohol concentration. The reason may be that the excessive alcohol on the gas−water interface hinders the adsorption of AOS. It can also be observed in Figures 4 and 5 that the synergism of n-hexanol is inferior to other alcohols. The reason may be that the solubility of n-hexanol in water phase is lower than other alcohols, so its ability to be absorbed onto the gas−water interface from water phase is stronger; however, the excessive adsorption of n-hexanol on gas−water interface weakens the adsorption of AOS, which therefore reduces the synergism (the higher the ability of alcohol to be absorbed onto the gas−water interface, the more the amount of alcohol to be absorbed on the interface and the larger inhibition of alcohol to the adsorption of AOS). When AOS concentration is 0.1 wt %, the added alcohols shows similar synergism mentioned above (the synergism is weaker than the one when AOS concentration is 0.02 wt %), as shown in Figure 6 (the behavior reported in Figure 6 and 7 was similar for other

Figure 4. Influence of different alcohols on the foaming volume of 0.02 wt % AOS solution.

Figure 5. Influence of different alcohols on the half-life when AOS concentration is 0.02 wt %.

with a further increase of alcohol concentration. However, the foaming property of all the middle alcohol solutions (with no AOS) is very poor. The reason for the synergisms of alcohols on the foaming property are the following : (1) The alcohols can be absorbed onto the gas−water interface to reduce the surface tension (see Table 2, and the behavior reported in

Figure 6. Change of the foam property of 0.1 wt % AOS with isoamyl alcohol concentration.

alcohols in particular the n-hexanol). Isoamyl alcohol (0.1 wt %) can enhance the foaming volume and half-life by 15%. Compared with the situation where AOS concentration is 0.02 wt %, the increasing amplitude of the foaimg volume and half-

Table 2. Surface Tension Reduction with Isoamyl Alcohol Concentration When AOS Concentration is 0.02 wt % isoamyl alcohol concentration (wt %)

surface tension (mN/m)

0 0.02 0.05 0.1 0.2 0.3 0.5

39.8 38.1 37.2 36.7 36.6 36.7 37

Table 2 was similar for other alcohols in particular the nhexanol) and makes up the shortage of the amount of AOS for foaming, thus facilitating the formation of more bubbles; (2) The carbon chain of alcohols can interact with the hydrophobic chain of AOS (association between the carbon chain of alcohols and the hydrophobic chain of AOS occurs because of their mutual attraction (similar compatible principle); also, the

Figure 7. Change of the foam property of 0.5 wt % AOS with isoamyl alcohol concentration. D

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Energy & Fuels life through the addition of alcohol is lower when AOS concentration is 0.1 wt %. The reason is that 0.1 wt % has exceeded the CMC of AOS, which is to say that the opportunity for middle alcohols to absorbed into the gas− water interface is reduced. Besides, some alcohols can be solubilized in the AOS micelle, and thus, the synergism of added alcohols was weakened. When AOS concentration is 0.5 wt %, the added alcohol shows no synergism, as shown in Figure 7, indicating that the alcohols are solubilized in the AOS micelle and plays little role. 3.2. Influence of Middle Carbon Alcohols on Oil Viscosity. These viscosity measurements were conducted to simulate the influence of alcohols on the viscosity of heavy oil when injected together with water (water/alcohol flooding). The results are shown in Figure 8, and it can be observed that

Figure 9. Change of Tmin and Tl with AOS concentration.

and then decreases with its concentration. When AOS concentration is less than 0.1 wt %, more surfactant can be absorbed onto the oil−water interface with the increase of its concentration, thus facilitating the formation of more emulsions and enhancing the stability of emulsions. After AOS concentration exceeds 0.1 wt %, there are much micelles formed by AOS molecules at concentration above the CMC. The micelles can solubilize the AOS molecules on the gas− water interface and reduce the amount of AOS on the interface. With a higher AOS concentration, more micelles are formed and more AOS are solubilized in the micelles. Therefore, the emulsifying property of AOS decreases with its concentration when above the CMC. The influence of alcohols on the emulsifying property of AOS solution in concentration of 0.02, 0.1, and 0.5 wt % were examined, respectively, and the results are shown in Figure 10−13. As shown in Figures 10 and 11, the

Figure 8. Influence of different middle alcohols on the viscosity of heavy oil.

the oil viscosity can be reduced significantly with the addition of middle alcohols to water. The oil viscosity decreases with the increase of alcohol concentration. The results illustrate that the alcohols can diffuse into oil phase from water phase, and they can be used as special solvent to dilute the heavy oil, just as thin oil. The viscosity-reducing effect becomes better with the increase of carbon number of middle alcohols. The solubility of alcohol in water phase decreases with carbon number, and the higher the carbon number of alcohol, the more easily the alcohol diffuses into oil phase from water phase. In other words, more alcohol diffuses into oil phase with the increase of carbon number. The larger the amount of alcohol diffusing into heavy oil, the lower the viscosity of heavy oil. Therefore, the ability of alcohols to reduce the oil viscosity becomes stronger with carbon number of alcohols. Therefore, alcohols can be added to water in waterflooding to reduce the heavy-oil viscosity in longdistance reservoirs. In addition, it should be noted that except for alcohols, no other viscosity reducer (solvent kind) can both dissolve to water and oil. 3.3. Influence of Middle Carbon Alcohols on Emulsifying Property of AOS. First of all, the change of Tmin and Tl with AOS concentration were measured as a reference, and the results are shown in Figure 9. The Tmin first decreases sharply with AOS concentration and reaches its minimum value of 290s at AOS concentration of 0.1 wt %; it then increases with a further increase of AOS concentration. The trend of the change of Tl with AOS concentration is contrary to that of Tmin, and Tl reaches its maximum value of 57s when AOS concentration is 0.1 wt %. The results illustrate that the emulsifying property of AOS solution first increases

Figure 10. Influence of different alcohols on Tmin when AOS concentration is 0.02 wt %.

added middle alcohols can greatly reduce the Tmin and enhance the Tl when AOS concentration is 0.02 wt %. It illustrates that the alcohols can significantly improve the emulsifying property of AOS solution in a concentration lower than the CMC. Similar to the results discussed in section 3.1, the added alcohols can be absorbed onto the oil−water interface and insert into the gap between AOS molecules on the oil−water interface, thus making up the shortage of AOS for emulsifying oil and facilitating the formation of more emulsions. Besides, the carbon chain of alcohols can interact with the hydrophobic chain of AOS molecules, thus enhancing the strength of interfacial film and improving the emulsion stability. It can also be observed from Figures 10 and 11 that the sequence of the synergism of different alcohols is n-hexanol > amyl alcohol > nE

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increases with n-hexanol concentration, indicating that the added n-hexanol can obviously improve the emulsifying property of AOS solution in concentrations of 0.1 and 0.5 wt %. When AOS concentration is above the CMC, the AOS molecules on the oil−water interface can be easily solubilized into the AOS micelles, and the amount of the AOS molecules on the oil−water interface becomes smaller, which weakens the emulsifying property of AOS (see Figure 9). The added alcohols seem to be able to get rid of AOS micelles and can be absorbed onto the oil−water interface, and the larger the alcohol concentration, the higher the ability of alcohol to be absorbed onto the oil−water interface. Therefore, the emulsifying ability and the emulsion stability become higher with alcohol concentration. 3.4. Sandpack Flooding Study. The sandpack flooding studies in this paper are used to evaluate the effects of waterflooding, water/alcohol flooding, surfactant flooding, surfactant/alcohol flooding, and foam flooding, respectively, for enhanced heavy-oil recovery. More importantly, they are conducted to assess the influence of the addition of middle alcohols on different kinds of floodings for oil recovery. Twenty sandpack flooding tests were performed to evaluate the effectiveness of different floodings, and all the results are listed in Table 3. For each sandpack flooding test, the oil recovery and the pressure drop were monitored and recorded as a function of fluid injected. The oil production was calculated on a mass basis,24and a centrifugal process (used for demulsification and separation of oil and water) was needed. 3.4.1. Influence of Alcohols on the Waterflooding for Oil Recovery. To investigate the effects of alcohols on waterflooding for enhanced oil recovery, two water/alcohol flooding tests (Run 19−20) were conducted to compare with the waterflooding in Run 1. Both 0.5 wt % n-butanol and 0.5 wt % n-hexanol were added to water, respectively, in Run 19 and Run 20, and the water/alcohol system was injected in the primary flooding for two PV, just as waterflooding. It can be observed from Table 3 that the oil recoveries by primary water/n-butanol flooding and water/n-hexanol flooding (35.23% OOIP, 37.12% OOIP) are both higher than the one by waterflooding (30.12% OOIP), and n-hexanol shows a better effect than n-butanol. Figure 14 shows the pressure drop in water flooding (Run 1) and in water/alcohol flooding (Run19−20); it can be observed that the peak value of the displacement pressure in water/nbutanol flooding and water/n-hexanol flooding are both lower than the one in waterflooding. Also, the pressure drop in water/ alcohol flooding decreased more slowly than the pressure drop in water flooding, indicating that the injected fluid breakthrough occurs more slowly in water/alcohol flooding. The effects of alcohols on waterflooding for enhanced heavy-oil recovery can be ascribed to the reduction of oil viscosity corresponding with their addition. The middle alcohols can diffuse from water phase into oil phase and reduce the oil viscosity, thus reducing the water−oil mobility ratio, which is usually high in heavy-oil reservoirs. When oil viscosity decreases, it can flow more easily with lower resistance. Besides, with a lower water−oil mobility ratio, the breakthrough time of injected fluid was prolonged, and a higher sweep efficiency was obtained. 3.4.2. Influence of Alcohols on Surfactant Flooding for Enhanced Oil Recovery. Run 1−9 were conducted to examine the effect of middle alcohols (including n-butanol, n-pentanol, isoamyl alcohol, and n-hexanol) on surfactant (AOS) flooding for enhanced heavy-oil recovery (including surfactant flooding).

Figure 11. Influence of different alcohols on Tl when AOS concentration is 0.02 wt %.

butanol. The reason may be that, the alcohol with a higher carbon number is more easily absorbed onto the oil−water interface from water phase (due to the difference of solubility of alcohols in water). Besides, the interaction between the carbon chain of alcohols and the hydrophobic chain of AOS becomes stronger with the increase of carbon number of alcohols. Therefore, the emulsifying ability and emulsion stability increase with the carbon number of alcohols. The influence of added n-hexanol on the emulsifying property of AOS solution in concentrations of 0.1 and 0.5 wt % were investigated, respectively, and the results are shown in Figures 12 and 13. The change of Tmin and Tl with n-hexanol

Figure 12. Influence of n-hexanol concentration on Tmin and Tl when AOS concentration is 0.1 wt %.

Figure 13. Influence of n-hexanol concentration on Tmin and Tl when AOS concentration is 0.5 wt %.

concentration in Figures 12 and 13 have the same trend. The Tmin decreases with n-hexanol concentration and the Tl F

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Energy & Fuels Table 3. Summary of Flooding of Different Systems in Sandpacks ̀ run no.

porosity (%)

permeability (Dracy)

initial oil saturation (%)

water flood recovery (%)

chemical formula

tertiary oil recovery (% IOIP)

final recovery (% IOIP)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

40.42 40.23 39.48 39.67 40.12 40.32 39.99 39.84 39.36 39.21 39.66 39.52 39.72 39.82 39.83 40.17 39.18 40.13 39.9 40.2

2.12 2.16 2.23 2.08 2.15 2.19 2.05 1.98 2.08 2.03 1.92 2.17 1.89 2.22 2.02 1.96 2.03 2.12 2.09 2.14

87.4 87.3 88.1 87.4 88.2 88.6 87.8 88.1 87.3 88.2 88.4 87.5 87.6 87.6 88.3 86.8 88.2 87.6 88.3 88.5

30.12 32.13 31.41 33.15 30.89 34.21 30.82 31.95 32.65 33.14 31.86 32.94 33.16 30.94 31.95 32.54 32.84 31.76 35.23 37.12

0.02 wt % AOS 0.1 wt % AOS 0.5 wt % AOS 0.02 wt % AOS + 0.3 wt % isomyl alcohol 0.5 wt % AOS + 0.3 wt % isomyl alcohol 0.1 wt % AOS + 0.3 wt % n-butanol 0.1 wt % AOS+0.3 wt % n-pentanol 0.1 wt % AOS + 0.3 wt % isoamyl alcohol 0.1 wt % AOS+0.3 wt % n-hexanol 0.02 wt % AOS + N2 0.1 wt % AOS + N2 0.5 wt % AOS + N2 0.02 wt % AOS+0.3 wt % isomyl alcohol+N2 0.5 wt % AOS + 0.3 wt % isomyl alcohol + N2 0.1 wt % AOS + 0.3 wt % n-butanol + N2 0.1 wt % AOS + 0.3 wt % n-pentanol + N2 0.1 wt % AOS+0.3 wt % isoamyl alcohol+N2 0.1 wt % AOS + 0.3 wt % n-hexanol + N2 water/n-butanol flooding in primary flooding water/n-hexanol flooding in primary flooding

2.52 4.48 4.32 5.89 8.22 7.12 9.21 9.56 10.48 10.12 14.34 15.34 16.87 23.86 20.86 23.45 25.58 23.12 0 0

32.73 36.61 35.74 39.04 39.11 41.33 41.03 41.51 43.13 43.26 46.2 48.28 50.13 54.8 52.81 55.99 58.42 54.88 35.23 37.12

emulsidied into small oil droplets, they can block the water channels (big pores) through Jamin Effect, then the injected fluid was diverted to the unswept area (small pores), leading to an increase of sweep efficiency. As shown in Figure 15, the

Figure 14. Pressure drop with injected fluid volume in waterflooding or water/alcohol flooding.

For each surfactant/alcohol flooding, different alcohol was added to the AOS solutions, respectively. The alcohol concentration used in surfactant/alcohol flooding is 0.3 wt %, which is proper for improving foaming property and emulsifying property of AOS significantly (see Figures 4−6, 10−13). As shown in Table 3, the incremental oil recovery of surfactant flooding is low (Run1−3, about 2.5%−4.5% OOIP), and the oil recovery can be enhanced by 3%−6% OOIP with the addition of 0.3 wt % of different alcohols (including nbutanol, n-pentanol, isoamyl alcohol, and n-hexanol) in surfactant floodings (Run 4−9). When AOS concentration is 0.1 wt %, the sequence of the synergisms of alcohols for enhanced oil recovery is n-hexanol > amyl alcohol > n-butanol. It can be concluded that there is good corresponding relationship between incremental oil recovery and the emulsifying property of chemical solutions in surfactant flooding and surfactant/alcohol flooding. A higher incremental oil recovery is usually accompanied by good emulsifying ability and good emulsion stability (see Figures 9−13 and Table 3). Emulsification is an important mechanism in surfactant flooding25 for enhanced oil recovery. After the oil was

Figure 15. Pressure drop with injected fluid volume in surfactant flooding or surfactant/alcohol flooding.

pressure drop in surfactant/alcohol flooding is higher than the one in surfactant flooding. The reason is that the added alcohol can improve the emulsifying property of AOS solution. Thus, in surfactant/alcohol flooding, more emulsions were created to block the water channels, leading to a higher pressure drop. The emulsifying property of AOS/n-hexanol system is stronger than the one of AOS/isomyl alcohol system, and thus, the pressure drop in AOS/n-hexanol flooding is higher. Another mechanism of emulsification for enhanced oil recovery is that the emulsified oil droplets can be entrained along with water phase and flow through the porous media easily, leading to an increase of displacement efficiency. A stronger emulsifying property means that more small oil droplets were created and that the emulsion was more stable. The smaller the emulsified oil droplets and more stable the emulsions, the more easily the emulsions flows through the porous media. Therefore, a stronger emulsifying property usually contributes to a higher G

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Energy & Fuels

more important for enhanced displacement efficiency when IFT is high (>1 mN/m). 3.4.3. Influence of Alcohols on Foam Flooding for Enhanced Oil Recovery. In order to investigate the effect of middle alcohols on foam flooding for enhanced heavy-oil recovery, 9 foam floodings (Run 10−18) were conducted, with the surfactant concentration and the alcohol concentration in foam flooding are the same as the one in surfactant flooding (Run1−9 were corresponding with Run 10−18, respectively, in surfactant and alcohol concentration), and the results are shown in Table 3. The incremental oil recovery of foam flooding is much higher than the one of surfactant flooding, and the incremental oil recoveries of foam floodings all exceed 10%. In addition to an increase of displacement efficiency as in surfactant flooding, the sweep efficiency can also be enhanced greatly through foam flooding. Compared with foam flooding, surfactant flooding cannot obviously improve the sweep efficiency through the entrapment of emulsified oil droplets, and thus, the incremental oil recovery of surfactant flooding is lower than foam flooding. Between 8% and 11% OOIP of extra oil recovery can be obtained through the addition of different alcohols (including n-butanol, n-pentanol, isoamyl alcohol, and n-hexanol) in foam flooding. Furthermore, the magnitude of the increase of oil recovery by addition of different alcohols in foam flooding is higher than the one in surfactant flooding, which is about 3%−6% OOIP, as described in section 3.4.2. The reason is that, the added alcohols can only enhance the emulsifying property of AOS solution in surfactant flooding, which mainly improves the displacement efficiency. However, in foam flooding, the added alcohols not only raise the displacement efficiency by improving the emulsifying property of AOS solution but also enhance the sweep efficiency by improving the foaming property of AOS solution. As shown in Figure 17, when AOS concentration is 0.02 or 0.1 wt %, the

displacement efficiency. From Figures 10 and 11, we can know that the sequence of the emulsifying property of AOS/alcohol system is AOS/n-hexanol > AOS/amyl alcohol > AOS/nbutanol, which has the same sequence as the one of incremental oil recovery of AOS/alcohol flooding with different alcohols (see Table 3, Run 6−9). This is because a stronger emulsifying property contributes to a higher sweep efficiency and a higher displacement efficiency, which result in a higher incremental oil recovery. Although the added alcohols can both improve the sweep efficiency and the displacement efficiency in surfactant flooding, it is difficult to determine the influence degree that sweep efficiency and displacement efficiency exerts on oil recovery in present conditions. The micrographs of the subnatant liquor of the produced fluid in AOS flooding and AOS/n-hexanol flooding are shown, respectively, in Figure 16

Figure 16. Micrographs of the subnatant liquor of the produced fluid: (a) in flooding with 0.1 wt % AOS; (b) in flooding with 0.1 wt % AOS + 0.3 wt % isoamyl alcohol; (c) in flooding with 0.1 wt % AOS + 0.3 wt % n-hexanol).

(magnified 100×), and it can be observed that the number of the emulsified oil droplets in AOS/isoamyl alcohol flooding and AOS/n-hexanol flooding were far more than the one in AOS flooding, indicating that AOS/isoamyl alcohol system and AOS/n-hexanol system have stronger emulsifying property than AOS system. In addition to the effect of emulsification, the reduction of oil viscosity by the addition of alcohols may also help enhance oil recovery. It should be noted that there is no obvious relationship between the IFTs and the incremental oil recovery in surfactant flooding (see Table 3 and Table 4). The IFTs between oil and AOS system and AOS/alcohol system are all high (1.4−4.5 mN/m, see Table 4), and the effect of reduction of IFT on enhancing displacement efficiency through addition of alcohols is seemingly not obvious in this paper. Maybe emulsification is Table 4. IFTs between Oil and Different Chemical Solution formula of solution brine (0.5 wt % NaCl) 0.02 wt % AOS 0.1 wt % AOS 0.5 wt % AOS 0.02 wt % AOS + 0.3 wt % n-hexanol 0.5 wt % AOS + 0.3 wt % n-hexanol 0.1 wt % AOS + 0.1 wt % n-butanol 0.1 wt % AOS + 0.1 wt % n-pentanol 0.1 wt % AOS + 0.1 wt % isoamyl alcohol 0.1 wt % AOS + 0.1 wt % n-hexanol 0.1 wt % AOS + 0.3 wt % n-butanol 0.1 wt % AOS + 0.3 wt % n-pentanol 0.1 wt % AOS + 0.3 wt % isoamyl alcohol 0.1 wt % AOS + 0.3 wt % n-hexanol

temperature 50 50 50 50 50 50 50 50 50 50 50 50 50 50

°C °C °C °C °C °C °C °C °C °C °C °C °C °C

IFT (mN/m)

Figure 17. Pressure drop with injected fluid volume in foam flooding in the presence and absence of alcohol.

22.4 2.8 2.2 4.5 2.0 3.2 2.6 2.0 2.4 1.7 2.8 1.9 2.6 1.5

added middle alcohols can significantly enhance the pressure drop in foam flooding. Middle alcohols can greatly improve the foaming ability and the foam stability in foam flooding, which is discussed in section 3.1. Therefore, the alcohols (including nbutanol, n-pentanol, isoamyl alcohol, and n-hexanol) can raise the plugging performance of foam (plugging the large pores and diverting the injected fluid to small pores) and result in a further improvement of sweep efficiency. Among the alcohols, isoamyl alcohol has the best synergism on enhanced oil recovery in foam flooding (incremental oil recovery is 25.58% OOIP), it is the results of the joint action of the synergisms of H

DOI: 10.1021/ef502652a Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels

(7) Liu, H.; Ye, P.; Liu, Y.; Wang, X. Nitrogen foam injection technique and its application in reservoirs with high water cut. Acta Pet. Sin. 2010, 31 (1), 92−95. (8) Kim, J. S.; Dong, Y.; Rossen, W. R. Steady-state flow behavior of CO2 foam. Society of Petroleum Engineers (SPE): Richardson, TX, 2004; SPE Paper 89351. (9) Llave, F. M.; Chung, F. T.; Louvier, R. W.; Hudgins, D. A. Foams as mobility control agents for oil recovery by gas displacement. Society of Petroleum Engineers (SPE): Richardson, TX, 1990; SPE Paper 20245. (10) Phong, N.; Hossein, F.; David, S. Pore-scale assessment of nanoparticle-stabilized CO2 foam for enhanced oil recovery. Energy Fuels 2014, 28 (10), 6221−6227. (11) Sun, Q.; Li, Z. M.; Li, S. Y. Utilization of surfactant-stabilized foam for enhanced oil recovery by adding nanoparticles. Energy Fuels 2014, 28 (4), 2384−2394. (12) Wang, D. M.; Cheng, J. C.; Wu, J. Z.; Wang, G. Application of polymer flooding technology in Daqing Oilfield. Acta Petrolei Sinica 2005, 26 (1), 74−78. (13) Rohallah, H.; Nashaat, N. N.; Pedro, P. A. Transport behavior of multimetallic ultradispersed nanoparticles in an oil-sands-packed bed column at a high temperature and pressure. Energy Fuels 2012, 26 (3), 1645−1655. (14) Simjoo, M.; Dong, Y.; Andrianov, A. CT scan study of immiscible foam flow in porous media for enhancing oil recovery. Ind. Eng. Chem. Res. 2013, 52 (18), 6221−6233. (15) Farajzadeh, R.; Ameri, A.; Faber, M. J. Effect of continuous, trapped, and flowing gas on performance of alkaline surfactant polymer(ASP) flooding. Ind. Eng. Chem. Res. 2013, 52 (38), 13839− 13848. (16) Wang, H.; Zhang, B.; Zhang, J.; Tu, W. Scale inhibitors for the alkaline−surfactant−polymer flooding system in Shengli oilfield. Oilfield Chem. 2005, 22, 252−254. (17) Makoto, A.; Takayuki, T.; Takeo, S. Dihedral angle of lens and interfacial tension of air/long chin alcohol/water systems. Langmuir 1997, 13 (7), 2158−2163. (18) Salter, S. J. The influence of type and amount of alcohol on surfactant-oil-brine phase behavior and properties. Presented at the SPE Annual Fall Technical Conference and Exhibition, Denver, Colorado, U.S.A., Oct 9−12, 1977; SPE 6853. (19) Salter, S. J. Selection of pseudo-components in surfactant-oilbrine-alcohol systems. Presented at the SPE Symposium on Improved Methods of Oil Recovery, Tulsa, Oklahoma, USA, Apr 16−17, 1978; SPE 7056. (20) Fortenberry, Kim. D. H.; Nizamidin, N. Use of cosolvents to improve alkaline/polymer flooding. SPEJ, Soc. Pet. Eng. J.; SPE Paper 166478-PA, 2014. (21) Pei, H. H.; Zhang, G. C.; Ge, J. J. Effect of the addition of low molecular weight alcohols on heavy oil recovery during alkaline flooding. Ind. Eng. Chem. Res. 2014, 53 (4), 1301−1307. (22) Farajzadeh, R.; Krastev, R.; Zitha, P. L. Foam films stabilized with alpha olefin sulfonate (AOS). Colloids Surf., A 2008, 324, 35−40. (23) Ding, B. D.; Zhang, G. C.; Ge, J. J. Research on mechanisms of alkaline flooding for heavy oil. Energy Fuels 2010, 24, 6346−6352. (24) Dong, M. Z.; Ma, S.; Liu, Q. Enhanced heavy oil recovery through interfacial instability, a study of chemical flooding for Brintnell heavy oil. Fuel 2009, 88, 1049−1056. (25) Johnson, J. C. E. Status of caustic and emulsion methods. J. Can. Petrol. Technol. 1976, 28, 85−92.

alcohols on improving foaming property and emulsifying property. Morever, the alcohols can also diffuse into oil phase and reduce the oil viscosity, which may also contributes to the raise of oil recovery.

4. CONCLUSION (1) The added middle carbon alcohols cannot significantly reduce the oil−water IFT, but can greatly reduce the viscosity of heavy oil; in addition, they can also improve the foaming property and the emulsifying property of surfactant solution (AOS solution) obviously. (2) 4%−6% OOIP of extra heavy-oil recovery can be obtained through the addition of middle alcohols in waterflooding; 3%−6% OOIP of extra heavy-oil recovery can be obtained through the addition of middle alcohols in surfactant flooding; and 8%−11% OOIP of extra heavy-oil recovery can be obtained through the addition of middle alcohols in foam flooding. (3) The middle alcohols can reduce the viscosity of heavy oil, and thus, they can reduce the water−oil mobility ratio and improve sweep efficiency in waterflooding. Middle alcohols can enhance the emulsifying property of AOS solution, which can both improve the sweep efficiency and the displacement efficiency in surfactant flooding. More importantly, middle alcohols can both enhance the foaming property and the emulsifying property of AOS solution and thus can both raise the sweep efficiency and the displacement efficiency, which contributes to higher extra oil recovery in foam flooding.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support by National Science and Technology Major project of China (Grant 2011ZX05002-005) and the Fundamental Research Funds for the Central Universities (Grant 27R1202006A) is greatly acknowledged.

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REFERENCES

(1) Green, D. W.; Willhite, G. P. Enhanced Oil Recovery; SPE Textbook Series, Vol. 6; Society of Petroleum Engineers (SPE): Richardson, TX, 1998. (2) Knatib, A. K.; Earlongher, R. C. CO2 injection as an immiscible application for enhanced recovery in heavy oil reservoirs. SPEJ, Soc. Pet. Eng. J.; SPE Paper 9928-MS, 1981. (3) Pei, H. H.; Zhang, G. C.; Ge, J. J.; Tang, M. G.; Zheng, Y. F. Comparative effectiveness of alkaline flooding and alkaline−surfactant flooding for improved heavy-oil recovery. Energy Fuels 2012, 26 (5), 2911−2919. (4) Andrianov, A.; Farajzadeh, R.; Mahmoodi, N. M. Immiscible Foam for Enhancing Oil Redcdovery: Bulk and Porous Media Experiments. Ind. Eng. Chem. Res. 2012, 51 (5), 2214−2226. (5) Cheng, M. M.; Lei, G. L.; Gao, J. B.; Xia, T.; Wang, H. S. Laboratory experiment, production performance prediction model, and field application of multi-slug microbial enhanced oil recovery. Energy Fuels 2014, 28 (2), 6655−6665. (6) Casteel, J. F.; Djabbarah, N. F. Sweep improvement in CO2 flooding by use of foaming agents. SPE Reservoir Eng. 1988, 3 (4), 1186−1192. I

DOI: 10.1021/ef502652a Energy Fuels XXXX, XXX, XXX−XXX