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Study on organic alkali-surfactant-polymer flooding for enhanced ordinary heavy oil recovery
Colloids and Surfaces A: Physicochem. Eng. Aspects 508 (2016) 230–239
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Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa
Study on organic alkali-surfactant-polymer flooding for enhanced ordinary heavy oil recovery Lipei Fu, Guicai Zhang ∗ , Jijiang Ge, Kaili Liao ∗ , Haihua Pei, Ping Jiang, Xiaqing Li College of Petroleum Engineering, China University of Petroleum, Qingdao 266580, China
h i g h l i g h t s
g r a p h i c a l
a b s t r a c t
• The • • • •
organic alkali/surfactant/polymer flooding system was established. It is suitable for the reservoir with high content of high-valent metal ions. During the OASP flooding the additional oil recovery was 20%. The recovery increases with the increase in organic alkali concentration. The mechanisms for OASP flooding in normal heavy oil are proven.
a r t i c l e
i n f o
Article history: Received 20 May 2016 Received in revised form 27 July 2016 Accepted 22 August 2016 Available online 24 August 2016 Keywords: Organic alkali Micromodel flooding Alkali/surfactant/polymer flooding W/O emulsion Displacement mechanism Interfacial tension
a b s t r a c t With regard to ordinary heavy oil reservoirs which are not suitable for thermal methods, alkalinesurfactant-polymer (ASP) flooding exhibits great potential for enhancing heavy oil recovery. But for the formation water with high content of Ca2+ and Mg2+ ions, conventional ASP flooding always causes precipitation of a large amount of Ca and Mg salts which are damage to reservoirs. In this study, organic alkali-surfactant-polymer (OASP) flooding system is established, which exhibits good compatibility with the brine containing high-valent metal ions. The interfacial tension tests show that the combination of Shengli petroleum sulfonate (SLPS) and ethanolamine exhibits a good synergistic effect, and acquires an ultralow interfacial tension. Based on micromodel flooding tests, the mechanisms of OASP flooding system are studied as follows: the organic alkali in OASP system reacts with the acidic component of heavy oil and promotes the formation of water-in-oil (W/O) emulsion in heavy oil, thus increasing the flow resistance of flooding liquid and improving the sweep efficiency of normal ASP system. The generated surface active materials and surfactant can decrease the interfacial tension to an ultralow level, which could easily initiate the emulsion dispersion of crude oil, form the O/W emulsion, and improve the oil displacement efficiency. The sandpack flood results demonstrate that the oil recovery is increased by about 20%, and the recovery increases with the increase in organic alkali concentration. Therefore, the organic alkali-surfactant-polymer flooding technology can be developed into a new type of economically and technically feasible compound flooding technology suitable for ordinary heavy oil reservoirs with
∗ Corresponding authors. E-mail addresses: [email protected] (G. Zhang), [email protected] (K. Liao). http://dx.doi.org/10.1016/j.colsurfa.2016.08.042 0927-7757/© 2016 Elsevier B.V. All rights reserved.
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high content of Ca2+ and Mg2+ ions. Moreover, OASP flooding technology shows broad application prospects in improving the recovery of ordinary heavy oil reservoirs in Shengli Oilfield. © 2016 Elsevier B.V. All rights reserved.
1. Introduction The reserves of heavy oil reservoirs is 15 × 108 t in China, and they are mainly distributed in Liaohe, Xinjiang, Shengli, and Henan Oilfield [1]. In China, there are two main methods for ordinary heavy oil production. Water flooding is usually employed to develop the reservoirs in which the viscosity of underground crude oil is less than 5000 mPa s, and this method contributes to 18% of the total ordinary heavy oil production. Thermal methods are used for the reservoirs with the viscosity more than 5000 mPa s, which contributes to 82% of the total ordinary heavy oil production. Due to the high viscosity of the crude oil, the recovery of water flooding is usually less than 25%. Thermal methods are very effective in thick reservoirs or bottom-water reservoirs [2]. However, for the oil sheet reservoirs (less than 10 m), or deep reservoirs (more than 1000 m), or bottom-water reservoirs, thermal methods show poor effect because of the serious loss of heat [3]. In this case, the nonthermal methods are needed to further improve the ordinary heavy oil recovery. Many studies have shown that using chemical flooding after water flooding is an effective method to increase oil recovery for ordinary heavy oil [4,5]. A large amount of movable residual oil still remains in heavy oil reservoir after water flooding [6,7]. To conduct a chemical flooding in heavy oil reservoir after water flooding, following points should be ensured: first, the flooding system should come in contact with the oil-rich zone; and second, the flooding system in oil-rich zone should have higher sweep efficiency [8,9]. The alkalinesurfactant-polymer (ASP) flooding technology can not only increase the viscosity of displacement fluid, improve the water-oil mobility ratio, and further enhance the sweep efficiency, but also significantly reduce the oil/water interface tension and increase the oil displacement efficiency [10]. Therefore, ASP flooding technology is one of the most effective ways for improving the water flooding recovery of heavy oil reservoirs currently [11,12]. However, the inorganic alkali in the ASP flooding system can react with the high valence metal ions present in the formation water, resulting in problems, such as scaling [13]. Therefore, development of an organic alkali system with good compatibility with the brine containing high valence metal ions (i.e., calcium (Ca2+ ) and magnesium (Mg2+ ) ions) is highly desirable [14,15]. Inorganic alkali is sensitive to the divalent ion; therefore, Ca2+ and Mg2+ ions in alkali solution can cause the scaling of both the injection system and the near-well zone of the injection well. Jennings et al. concluded that the hardness of the water in the form of Ca2+ ions could deactivate the in-situ formed surfactants. Therefore, removal of Ca2+ ions from the prepared alkaline solution was highly desirable [16]. Organic alkali is a type of good chelating agent, capable of efficiently complexing with the Ca2+ and Mg2+ ions in the injected water. Berger and Lee studied the effect of replacing the inorganic alkali in ASP flooding with the organic alkali; investigated the effects of organic alkali on the interfacial tension, viscosity, and adsorption of OASP system (Organic alkali-Surfactant-Polymer); and compared them with those of the conventional ASP system. The results indicated that the organic alkali was not affected by the hardness of water, and it strengthened the ability of polymer toward increasing the viscosity of the displacing water to decrease the W/O mobility ratio [17]. The overall effect was better than the conventional ASP flooding. Lakatos et al. used the organic alkali
like organic amine as the pH regulator for preventing scaling [18]. The research results showed that addition of organic alkali not only improved the descaling effect, but also reduced the damage to the formation permeability caused by the injected liquid. Sharma et al. used ammonia instead of sodium carbonate to form ASP flooding system. The research results revealed that ammonia water and formation water exhibited good compatibility, and the addition of ammonia did not lead to the precipitation of divalent ions [19]. In addition, organic alkali-surfactant-polymer flooding system significantly improved the recovery of Berea sandstone oil reservoir. Formation water of Shengli Oilfield contains higher content of Ca2+ and Mg2+ ions; therefore, flooding system prepared using inorganic alkali, in general, results in the precipitation of a large amount of Ca and Mg salts. These salts interfere with the injected oil displacement agent, causing serious alkali consumption, and thus limiting the technology application for this type of reservoir. Therefore, establishing and developing the organic alkali system, instead of the conventional inorganic alkali system, with a good compatibility with the brine containing high valence metal ions (i.e., Ca2+ , Mg2+ ), can solve the problem of Ca and Mg precipitation in the injection system. Further, it can result in the development of a new type of economically and technically feasible compound flooding technology suitable for the ordinary heavy oil reservoirs, thus exhibiting a broad application prospect in increasing the recovery of the ordinary heavy oil reservoir in Shengli Oilfield.
2. Experimental 2.1. Materials The oil sample was collected from the Zhuangxi heavy oil reservoir in Shengli oilfield. The is 1550 mPa s at 50 ◦ C and its acid value is up to 2.14 mg KOH/g. The solution used in the experiment was prepared according to the formation water in the field (Shengli Oilfield, China). The content of formation water is shown in Table 1. Five types of surfactants were used in the experiments in order to find the surfactant system suitable for the interfacial tension of heavy oil from Shengli Oilfield. They are Shengli petroleum sulfonate (SLPS, with a purity of 35%, Provided by Shengli oilfield, China), Anqing petroleum sulfonate (WPS, with a purity of 40%, Provided by Sinopec Anqing Branch, China), Xinjiang petroleum sulfonate (XJPS, with a purity of 40%, Provided by Xinjiang oilfield, China), Heavy alkyl benzene sulfonate (NJPS, AR, provided by Nanjing Shihao Chemical, China), ␣-olefin sulfonate (KAS, AR, Aekyuan Chemical CO., LTD, South Korea), Sodium dodecyl benzene sulfonate (SDBS, AR, Sinopharm, China). Ethanolamine was selected as the organic alkali, which was analytical-grade and purchased from Sinopharm, China. Hydrolyzed polyacrylamide (HPAM, 1000 ppm) with a molecular weight of 12 million was selected as the polymer component (Shengli Oilfield, China). Chemicals applied in the
Table 1 Ionic Composition of the formation water. Ion content (mg/L) Cl− 8118.5
HCO3 − 248.2
total salinity (mg/L) Ca2+ 146.7
Mg2+ 49
Na+ 5097.5
13659.9
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experiments, such as NaCl, MgCl2 , CaCl2 , and Na2 CO3 were all analytical-grade and purchased from Sinopharm, China.
10
1
10
0
2.2.1. Determination of interfacial tension Interfacial tension of oil–water system is determined by using a spinning drop interfacial tension instrument TEXAS-500. A densitometer is first selected to measure the densities of the crude oil and the chemical solution. The refractive index of the chemical solution is measured using a WZS-1 Abbe refractometer. Further, an interfacial tensiometer is used to measure the tensile length and diameter of oil droplets in the solution, and a software developed by our research group is used for image acquisition and analysis, in order to calculate the oil–water dynamic interfacial tension. When the length (L) of oil column is more than four times of its diameter (D), the calculation formula of interfacial tension is as follows: 3
= 1.2336(D/n) ω2
(1)
Where ␣, r, ω, D, L, and n are the oil-water interfacial tension (mN/m), the density difference of water and oil phase (g/cm3 ), the rotational velocity (rpm), the diameter of the oil drop (0.0001 m), the length of the oil drop (0.0001 m), and the refractive index of water phase respectively. 2.2.2. Sandpack flood experiment The sandpack is 20 cm in length and 2.5 cm in diameter. The sandpack is prepared as follows: The quartz sands with a size range of 125–150 m and 150–180 m are washed and dried and then evenly mixed in the fixed proportion (125–150 m:150–180 m = 3:1). The sandpack is placed vertically and the moderate amount of quartz sands and formation water are added each time. Subsequently, the sandpack is shaken to compact the sand. In the entire process, it is necessary to ensure that the water level is above the sand surface in order to prevent air from entering. The sandpack flooding experimental steps are as follows: First, the sandpack is saturated with formation water, following which the permeability is measured and the porosity is calculated. Second, the Zhuangxi heavy oil is injected into the sandpack at a constant speed until the water ratio at the outlet is less than 2%, and the saturation rate is then calculated. Third, when the produced fluid by water flooding reaches an oil content of less than 2%, the 0.5 PV chemical slug is then injected and the subsequent water flooding is carried out until the oil content in the produced fluid is negligible (oil content less than 2%). Both formation water and chemical slug are injected at the flow rate of 0.5 mL min−1 . Using sand packs with the similar initial permeability, the flooding tests of each oil displacement formula were conducted three times, and the average of the recoveries was selected. 2.2.3. Micromodel displacement experiment The model for the microscopic flooding experiment is an etched glass model, based on the shape of pores and throats of the core. The glass model is hydrophilic and its size is 25 mm × 25 mm. The steps of microscopic displacement experiment are as follows: First, the glass model is vacuumed and saturated with the formation water; second, the model is saturated with crude oil at the formation temperature and aged for 24 h; third, the chemical solution is injected for displacement at a speed of 0.003 mL min−1 , and the image of the entire displacement process was obtained. Finally, the image of the displacement was analyzed and the recovery was calculated. In order to observe the phenomenon during the displacement, the displacement fluid was stained red with eosin, and each experiment was repeated three times.
IFT/(mN/m)
2.2. Methods WPS NJPS XJPS KAS SDBS SLPS
-1
10
-2
10
0
20
40
60
80
t/min Fig. 1. Dynamic IFT between crude oil and brine with different kinds of sulfonate anionic surfactant.
3. Results and discussions 3.1. IFT behavior of crude oil and various chemicals 3.1.1. IFT behavior of crude oil and surfactant In order to obtain more accurate results, all of the IFT determination tests were repeated three times. The interfacial tensions of the sulfonate anionic surfactant and the Zhuangxi heavy oil were determined, respectively. Fig. 1 shows the results, indicating that the Shengli petroleum sulfonate (SLPS) is the best surfactant to reduce the interfacial tension among the above-mentioned surfactants. The reason lies in that the interfacial tension of oil/water system mainly depends on the number of the surfactant molecules that absorbed onto the oil/water interface and their absorption strength. The main components of the petroleum sulfonate is a mixture of several kinds of alkyl or aryl sulfonate, or to say, the petroleum sulfonate is a kind of mixed surfactants with a range of molecular weight distribution. For crude oil/petroleum sulfonate system, when the system reaches equilibrium, the petroleum sulfonate molecules are distributed in the oil phase, the oil/water interface and the aqueous phase in accordance with the lipophilic from strong to weak. The Shengli petroleum sulfonate (SLPS) is synthesized by using crude oil from Shengli Oilfield, through sulfonation reaction. The SLPS matches with the crude oil from Shengli Oilfield well, and is more easily absorbed onto the oil/water interface. The more number of the surfactant molecules are absorbed, the higher the absorption strength is, as a result, the lower the interfacial tension of the oil/water system is. Therefore, compared with other sulfonate anionic surfactants, the SLPS can significantly reduce the oil/water interfacial tension. Based on the measured results of the interfacial tension tests, the SLPS is selected as the candidate to make complex formulation with ethanolamine, in order to obtain lower interfacial tension system. 3.1.2. IFT behavior of crude oil and OA/S system In chemical flooding, the addition of organic alkali instead of inorganic alkali can not only reduce the IFT, but also can effectively avoid the scaling, clay swelling and migration, and leads to reduction in permeability [20,21]. The SLPS selected from the former experiments was used in the following tests. The interfacial tensions of OA/S system comprising 0.5 wt% ethanolamine and SLPS with different concentrations were determined, and the results are shown in Fig. 2. As shown in Fig. 2, the IFT between oil and OA/S system is lower than that between oil and SPLS, as well as oil and ethanolamine, which are 0.07 mN/m and 0.7 mN/m respectively. At the same time, also from Fig. 2 we can see that the IFT between oil
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233 120
8
0.5% ethanolamine
Chemical flooding Water flooding
Water flooding
7
100
0.1% SLPS
The mass fraction of SLPS
in OA/P system 0.0005% 0.001% 0.025% 0.050% 0.075% 0.100%
0.01
1E-3
Differential pressure/MPa
IFT/(mN/m)
0.1
Water content
80
5 4
60
3 40
Oil recovery
2
20
1
Water content, Oil Recovery/%
6
Differential pressure
0
0
-1 0
1E-4 0
10
20
30
40
50
60
70
80
90
1
2
3
4
5
Pore volume of injection
t/min Fig. 2. IFT curves between crude oil and brine with different concentrations of SLPS and 0.5% ethanolamine compound system.
and OA/S system decreases gradually with the increase in SLPS concentration. When the mass fraction of SLPS reaches 0.1%, the SLPS and 0.5% ethanolamine compound system can reduce the interfacial tension between oil and OA/S system to ultralow. The component in the heavy oil that can react with the alkaline agent is petroleum acid, or naphthenic acid. Petroleum acid is an unspecific mixture of cyclopentyl and cyclohexyl carboxylic acids. The mixture usually has a molecular weight of 120–700, depending on the types of the crude oils. The injected organic alkali reacts with the naphthenic acid in crude oil to produce in-situ surface active materials, which are absorbed onto the oil/water interface and has synergistic effect with the synthetic surfactant (SLPS) to further reduce the oil/water interfacial tension. Ethanolamine itself is a kind of surface active substance. It can be absorbed onto the oil/water interface and reduces the oil/water interfacial tension, playing the similar effect as the surfactant [22]. Because of the synergistic effect between the in-situ surface-active materials and the SLPS, as well as the surface activity of the organic alkali itself, the interfacial tension can be further reduced. In addition, the added organic alkali increases the pH of the chemical slug and makes the reservoir change to a negatively charged environment, reducing the surfactant consumption. In this case, there will be more SLPS which are anionic surfactant distributed on the oil/water interface. Thereby, the displacement efficiency of the OASP system is improved [23]. Based on the above mentioned research, a displacement system with low interfacial tension could preliminarily be formed, i.e., a complex formulation of 0.1 wt% SLPS and 0.5 wt% ethanolamine. 3.2. Sandpack flood test For the Zhuangxi heavy oil with a viscosity of 1550 mPa s at 50 ◦ C, the oil recovery of the compound system consisting of the polymer, surfactant, and organic alkali was evaluated by the sandpack flood tests. For comparative analysis, the enhancement in heavy oil recovery was investigated by using the surfactant flooding, polymer flooding, and OASP flooding. Further, the influence of factors such as the concentration of ethanolamine on the enhanced oil recovery was also comprehensively investigated. In all the experiments, the injected volume was 0.5 PV. The parameters of the sandpack and the experimental results are listed in Table 2. As shown in Table 2, when 0.1% of SLPS is injected, the oil recovery enhanced by surfactant flooding increased only by 4%. While 0.1% of HPAM is injected, the oil recovery of polymer flooding
Fig. 3. Curves of the oil recovery, water content, and differential pressure of OASP flooding (0.5% ethanolamine + 0.1% SLPS + 0.1% HPAM).
increased by 10.7%. These results indicated that using chemical agent (surfactant or polymer) alone does not effectively improve the recovery of heavy oil. This is attributed to the high viscosity of heavy oil, even if the surfactant is capable of decreasing the oil/water interfacial tension down to an ultralow level, a serious fingering phenomenon still exists in the flooding process due to an unfavorable high mobility ratio of water over oil. Therefore, large area of residual oil cannot be touched by water flooding [24]. The polymer can improve the viscosity of the flooding fluid, and to a certain extent reduces the mobility of displacement fluid. However, the oil displacement efficiency of the swept area cannot be improved due to poor interfacial activity of polymer, thus leading to limited increment in oil recovery [25]. In order to better explore the role of surfactant, 0.1% HPAM and 0.1% SLPS were formulated into compound system and a displacement experiment in the sandpack flooding with this system was performed. The results from test No. 3 listed in Table 2 shows that the oil recovery of the compound flooding system consisting of the polymer and surfactant increases by 13.7%; however, the additional recovery is minor, when compared with 0.1% HPAM. Different concentrations of ethanolamine were added into the compound flooding system of 0.1%SLPS and 0.1%HPAM, and the displacement experiments were conducted respectively. The results from test No. 4 to 7 indicate that addition of ethanolamine can significantly enhance oil recovery of the SP compound flooding system. With the increase in ethanolamine concentration in the OASP system, oil recovery is increased significantly. When 0.5 PV of the 0.1% SLPS, 0.1% HPAM, and 0.5% ethanolamine was injected after water flooding, the oil recovery of chemical flooding increased by about 20% and the final recovery reached 54.1%, exhibiting obvious flooding effect. The curves for the recovery, water content, and differential pressure in the process of displacement using the compound flooding system of 0.5% ethanolamine, 0.1% SLPS, and 0.1% HPAM are shown in Fig. 3. Clearly, at the beginning of water flooding, the displacement pressure increases rapidly to a peak value, the injection pressure decreases rapidly after water breakthrough, and continuously decreases down to a minimum. This indicates the formation of water channels in the core. The water content is up to 98% and the oil recovery by water flooding is 33.4% at the moment. And then, the slug of 0.5 PV of the OASP system was injected. The pressure increases rapidly with an obvious peak value and the water content starts to decline, indicating that the original water flow channels created by water flooding were blocked effectively by the injected compound system. The blockage effect enables the displacement
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Table 2 Summary of Chemical flooding in sandpack flood tests for Zhuangxi heavy oil. Test No.
Porosity/(%)
Permeability/(10−3 m2 )
Initial oil saturation/(%)
Chemical flooding system
1 2 3 4 5 6 7
38.2 37.5 38.9 37.5 39.1 37.6 38.2
1350 1380 1410 1410 1450 1350 1380
88.5 87.1 88.5 87.9 88.9 87.0 88.5
0.1% S (SLPS) 0.1% P (HPAM) 0.1% P + 0.1% S 0.1% P + 0.1% S + 0.1% OA 0.1% P + 0.1% S + 0.25% OA 0.1% P + 0.1% S + 0.5% OA 0.1% P + 0.1% S + 0.75% OA
fluid flow into the unswept oil enrichment region, and improves the oil recovery of water flooding. Therefore, the oil recovery can be enhanced by 21.7%, and the final recovery can reach up to 54.1%. 3.3. Microscopic displacement mechanisms of OA/S/P flooding In order to study the mechanism of enhancing ordinary heavy oil recovery by the organic alkali/surfactant/polymer flooding, the microscopic oil displacement experiments were conducted using the compound flooding system of 0.5% ethanolamine, 0.1% SLPS, and 0.1% HPAM. The images of the experiments of the microscopic oil displacement by the direct OASP flooding are shown in Fig. 4. 3.3.1. The sweep efficiency of OA/S/P system Research has shown that the distribution characteristics of the residual oil in heavy oil reservoir after water flooding are completely different from those of the conventional crude oil after water flooding [1]. The residual oil in the former reservoir is not formed by the entrapment of capillary force; however, the significantly high mobility ratio between the injected fluid and the heavy oil with high viscosity leads to bypassing the residual oil of the flooding fluid. Therefore, in these ordinary heavy oil reservoirs, the residual oil is continuously distributed and able to flow. Thus, the key to improving the recovery of such ordinary heavy oil reservoirs is to increase the sweep efficiency of the displacement fluid. Fig. 4(a) and (b) demonstrates that the injected OASP solution does not create a serious fingering phenomenon along the diagonal direction of the model, and it does not break through until 0.68 PV is injected, as shown in Fig. 4(c), indicating that the OASP system exhibits better sweep efficiency. The reason lies in the role of the polymer and the organic alkali in the OASP system. The added polymer improves the sweep efficiency by reducing the water-oil mobility ratio. It is well known that the smaller the water-oil mobility ratio is, the higher the sweep efficiency is [26]. In the OASP flooding system, the polymer reduces the water-oil mobility ratio in two ways. One is increasing the viscosity of the displacing fluid by thickening the surfactant and alkali solution. The other is reducing the water phase permeability in porous media by remaining in the pore space of the polymer [27]. Under the two mentioned effect of the polymer, the polymer in the OASP system increases the sweep efficiency directly. In addition, the polymer can increase the stability of the O/W emulsion consisting of surfactant and alkali, which increases the sweep efficiency indirectly [28]. The microscopic oil displacement tests of SP system containing 0.1% SLPS and 0.1% HPAM are shown in Fig. 5. It can be seen that when 0.5 PV of the SP system is injected, the breakthrough occurs (Fig. 5(a)). The swept area of the SP system is less than that of the OASP system when 2.0 PV of the displacement fluid was injected, as shown in Figs. 4(f) and 5(b). The comparison illustrates that the addition of organic alkali in surfactant/polymer system can inhibit fingering phenomenon and improve the sweep efficiency of the SP system. The organic alkali in the ASP system improves the sweep efficiency can be explained from three aspects.
Oil recovery/(%) Water flooding
Chemical flooding
Final recovery
32.2 32.2 33.5 33.3 34.4 33.4 33.5
4.0 10.7 13.7 16.1 18.8 21.7 23.4
36.2 42.9 47.2 48.4 53.2 54.1 56.9
For one thing, the organic alkali in OASP system and the acidic component of the heavy oil can react with each other at the interface of oil and OASP solution. The surfactant generated at the interface enables the OASP solution come into the heavy oil to generate plenty of W/O emulsions, as shown in Fig. 6. The flow resistance of the displacement fluid increases due to the high viscosity of W/O emulsion. Therefore, the water-oil mobility ratio is reduced and the sweep efficiency of OASP system is greatly improved [29]. For another, the subsequent OASP solution turns to the untouched area due to the blocking effect of the emulsified oil droplets in the pore throat, improving the sweep efficiency [30]. With the injection of OASP system, a large amount of crude oil emulsified and dispersed under the flow disturbance, and a lot of emulsion droplets with different sizes were formed, as shown in Fig. 7(a). The interfacial tension between the OASP system and heavy oil could reach an ultralow level, due to the synergistic effect of the surface active materials generated by the organic alkali and the naphthenic acid in crude oil and the synthetic surfactant. Therefore, the emulsion and dispersion phenomenon in crude oil could occur with the even slight disturbance action of the displacement fluid which resulted in the formation of plenty of O/W emulsion [31]. In the flow process, some of the emulsion oil droplets (Fig. 7(a)) with smaller sizes are trapped in the pores or throats, such as position 1 (Fig. 7(b)), position 2 (Fig. 7(c)), and position 3 (Fig. 7(d)). When the displacement fluid passes through the pore throats which are filled with captured oil droplets, the flow direction of the fluid turns to the unswept areas, as shown in Fig. 7(b–c), improving the sweep efficiency. Meanwhile, with regard to the emulsified oil droplets with larger diameter, the flow resistance of the emulsion increases dramatically due to the Jamin effect, reducing the mobility of the aqueous phase. Thereby, the water-oil mobility ratio is improved by the Jamin effect of the emulsified oil droplets [32]. To sum up, the reaction between the organic alkali and the heavy oil increases the flow resistance of the displacement fluid through three mechanisms: the generation of W/O emulsion in the crude oil, the capture of the emulsified oil droplets at the pore throat, and the Jamin effect of the oil droplets with larger diameter. Combined with the effect of the polymer, the sweep efficiency of the OASP system is greatly increased.
3.3.2. The displacement efficiency of OA/S/P system Improving the displacement efficiency of the swept area is another aspect to improve the recovery of the ordinary heavy oil reservoirs. The mechanisms for the OASP system improving the displacement efficiency are as follows. Firstly, the oil/water interface tension reaches ultralow level due to the synergistic effect of the generated surface active materials and the synthetic surfactant, meaning the reduction in the work of adhesion of the residual oil. When the displacement fluid passing by, the residual oil encountered on the way is easy to be peeled, and the displacement efficiency is increased, which is illustrated by the displacement of the residual oil in the green circle, as shown in Fig. 8.
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Fig. 4. Microscopic images of direct OA/S/P flooding.
Then, the heavy oil is prone to be emulsified and form lots of oil droplets due to the ultralow oil/water interface tension generated by the synergistic effect of both the produced and synthetic surfactants. Under the flushing effect of the thickened displacement fluid, the residual oil in the pore throat is displaced and carried out
in the form of emulsified oil droplets, illustrated by the change of the residual oil in the blue circle in Fig. 8. What’s more, as the number of the oil droplets displaced from the surface of the reservoir increases, they may collide with each other in the process of moving forward. The oil droplets may get
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Fig. 5. Microscopic images of direct S/P flooding (0.1% SLPS and 0.1% HPAM).
Fig. 6. The W/O emulsion generated during OASP flooding.
together if the collision energy overcomes the electrostatic repulsion between the droplets. Gathering of the oil droplets can form oil belt (Fig. 9(d)). The oil belt continues to expand because of its coalescence with other oil droplets encountered during their migrations, and finally be extracted out. The gathering-forming oil belt mechanism is illustrated in Fig. 9. Finally, the adsorption of the SLPS which is an anionic surfactant onto the surface of both the oil droplets and the reservoirs, increases the surface charge density of the reservoir [33]. Therefore, the increase in the electrostatic repulsion between the oil droplets and the reservoir makes the oil droplets are easy to be displaced, and improves the displacement efficiency. In short, the synergistic effect of the SLPS and the surface active materials produced by the reaction between the ethanolamine and the naphthenic acid, increases the displacement efficiency, and further improves the oil recovery.
4. Conclusions Due to the high content of Ca2+ and Mg2+ ions in the formation water of Shengli Oilfield, organic alkali-surfactant-polymer (OASP) flooding system is established to solve the precipitation of a large amount of Ca and Mg salts in inorganic alkali-surfactant-polymer system. Based on the Zhuangxi heavy oil and the formation water from the field, the formula of the OASP system is determined: 0.5% ethanolamine +0.1% SLPS+ 0.1% HPAM. The interfacial tension tests show that the combination of Shengli petroleum sulfonate and ethanolamine exhibits a good synergistic effect, and forms an ultralow interfacial tension system. The sandpack flooding tests indicate that organic alkali-surfactant-polymer system shows a good displacement effect in ordinary heavy oil recovery, and the oil recovery can be enhanced by 21.7%. The recovery increases with the increase in the alkali concentration. The displacement mechanisms
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Fig. 7. The emulsifying and capturing mechanism of OASP flooding.
Fig. 8. The emulsifying and carrying mechanism of OASP flooding.
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Fig. 9. The emulsifying and gathering-forming oil belt mechanism of OASP flooding.
of OASP flooding are studied by the microscopic displacement tests. For one thing, in addition to the water-oil mobility ratio control ability of the polymer in the OASP system, the surface active materials generated by the reaction between the organic alkali and the acidic component in heavy oil enable the OASP solution come into the heavy oil, which generates plenty of W/O emulsions, increases the flow resistance for the flooding fluid, and finally improves the sweep coefficient of ASP system. For another, the synergistic effect of the surfactant and the surface active materials produced by the reaction between the organic alkali and the naphthenic acid, can decrease the interfacial tension to an ultralow level; therefore, slight disturbance can lead to the emulsion and dispersion phenomenon in the crude oil, which leads to peeling off the residual oil easily, carrying the oil droplets, and forming the oil belt for the oil to be extracted out. Combined with the increase in the electrostatic repulsion between the oil droplets and the reservoir due to the surfactant adsorption, the displacement efficiency is improved. Therefore, the organic alkali-surfactant-polymer flooding system exhibits both high sweep efficiency and high oil displacement efficiency, improving oil recovery of ordinary heavy oil reservoirs with high content of Ca2+ and Mg2+ ions while no salt precipitation is generated.
Acknowledgements This research is financially supported by the Fundamental Research Funds for the Central Universities (Grant 24720156031A, 24720156035A, and 16CX02018A), the National Natural Science Foundation of China (Grant 51574266 and 51474234), and the Shandong Provincial Natural Science Foundation, China (ZR2014EZ002 and ZR2015EQ013).
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Contents lists available at ScienceDirect
Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa
Study on organic alkali-surfactant-polymer flooding for enhanced ordinary heavy oil recovery Lipei Fu, Guicai Zhang ∗ , Jijiang Ge, Kaili Liao ∗ , Haihua Pei, Ping Jiang, Xiaqing Li College of Petroleum Engineering, China University of Petroleum, Qingdao 266580, China
h i g h l i g h t s
g r a p h i c a l
a b s t r a c t
• The • • • •
organic alkali/surfactant/polymer flooding system was established. It is suitable for the reservoir with high content of high-valent metal ions. During the OASP flooding the additional oil recovery was 20%. The recovery increases with the increase in organic alkali concentration. The mechanisms for OASP flooding in normal heavy oil are proven.
a r t i c l e
i n f o
Article history: Received 20 May 2016 Received in revised form 27 July 2016 Accepted 22 August 2016 Available online 24 August 2016 Keywords: Organic alkali Micromodel flooding Alkali/surfactant/polymer flooding W/O emulsion Displacement mechanism Interfacial tension
a b s t r a c t With regard to ordinary heavy oil reservoirs which are not suitable for thermal methods, alkalinesurfactant-polymer (ASP) flooding exhibits great potential for enhancing heavy oil recovery. But for the formation water with high content of Ca2+ and Mg2+ ions, conventional ASP flooding always causes precipitation of a large amount of Ca and Mg salts which are damage to reservoirs. In this study, organic alkali-surfactant-polymer (OASP) flooding system is established, which exhibits good compatibility with the brine containing high-valent metal ions. The interfacial tension tests show that the combination of Shengli petroleum sulfonate (SLPS) and ethanolamine exhibits a good synergistic effect, and acquires an ultralow interfacial tension. Based on micromodel flooding tests, the mechanisms of OASP flooding system are studied as follows: the organic alkali in OASP system reacts with the acidic component of heavy oil and promotes the formation of water-in-oil (W/O) emulsion in heavy oil, thus increasing the flow resistance of flooding liquid and improving the sweep efficiency of normal ASP system. The generated surface active materials and surfactant can decrease the interfacial tension to an ultralow level, which could easily initiate the emulsion dispersion of crude oil, form the O/W emulsion, and improve the oil displacement efficiency. The sandpack flood results demonstrate that the oil recovery is increased by about 20%, and the recovery increases with the increase in organic alkali concentration. Therefore, the organic alkali-surfactant-polymer flooding technology can be developed into a new type of economically and technically feasible compound flooding technology suitable for ordinary heavy oil reservoirs with
∗ Corresponding authors. E-mail addresses: [email protected] (G. Zhang), [email protected] (K. Liao). http://dx.doi.org/10.1016/j.colsurfa.2016.08.042 0927-7757/© 2016 Elsevier B.V. All rights reserved.
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high content of Ca2+ and Mg2+ ions. Moreover, OASP flooding technology shows broad application prospects in improving the recovery of ordinary heavy oil reservoirs in Shengli Oilfield. © 2016 Elsevier B.V. All rights reserved.
1. Introduction The reserves of heavy oil reservoirs is 15 × 108 t in China, and they are mainly distributed in Liaohe, Xinjiang, Shengli, and Henan Oilfield [1]. In China, there are two main methods for ordinary heavy oil production. Water flooding is usually employed to develop the reservoirs in which the viscosity of underground crude oil is less than 5000 mPa s, and this method contributes to 18% of the total ordinary heavy oil production. Thermal methods are used for the reservoirs with the viscosity more than 5000 mPa s, which contributes to 82% of the total ordinary heavy oil production. Due to the high viscosity of the crude oil, the recovery of water flooding is usually less than 25%. Thermal methods are very effective in thick reservoirs or bottom-water reservoirs [2]. However, for the oil sheet reservoirs (less than 10 m), or deep reservoirs (more than 1000 m), or bottom-water reservoirs, thermal methods show poor effect because of the serious loss of heat [3]. In this case, the nonthermal methods are needed to further improve the ordinary heavy oil recovery. Many studies have shown that using chemical flooding after water flooding is an effective method to increase oil recovery for ordinary heavy oil [4,5]. A large amount of movable residual oil still remains in heavy oil reservoir after water flooding [6,7]. To conduct a chemical flooding in heavy oil reservoir after water flooding, following points should be ensured: first, the flooding system should come in contact with the oil-rich zone; and second, the flooding system in oil-rich zone should have higher sweep efficiency [8,9]. The alkalinesurfactant-polymer (ASP) flooding technology can not only increase the viscosity of displacement fluid, improve the water-oil mobility ratio, and further enhance the sweep efficiency, but also significantly reduce the oil/water interface tension and increase the oil displacement efficiency [10]. Therefore, ASP flooding technology is one of the most effective ways for improving the water flooding recovery of heavy oil reservoirs currently [11,12]. However, the inorganic alkali in the ASP flooding system can react with the high valence metal ions present in the formation water, resulting in problems, such as scaling [13]. Therefore, development of an organic alkali system with good compatibility with the brine containing high valence metal ions (i.e., calcium (Ca2+ ) and magnesium (Mg2+ ) ions) is highly desirable [14,15]. Inorganic alkali is sensitive to the divalent ion; therefore, Ca2+ and Mg2+ ions in alkali solution can cause the scaling of both the injection system and the near-well zone of the injection well. Jennings et al. concluded that the hardness of the water in the form of Ca2+ ions could deactivate the in-situ formed surfactants. Therefore, removal of Ca2+ ions from the prepared alkaline solution was highly desirable [16]. Organic alkali is a type of good chelating agent, capable of efficiently complexing with the Ca2+ and Mg2+ ions in the injected water. Berger and Lee studied the effect of replacing the inorganic alkali in ASP flooding with the organic alkali; investigated the effects of organic alkali on the interfacial tension, viscosity, and adsorption of OASP system (Organic alkali-Surfactant-Polymer); and compared them with those of the conventional ASP system. The results indicated that the organic alkali was not affected by the hardness of water, and it strengthened the ability of polymer toward increasing the viscosity of the displacing water to decrease the W/O mobility ratio [17]. The overall effect was better than the conventional ASP flooding. Lakatos et al. used the organic alkali
like organic amine as the pH regulator for preventing scaling [18]. The research results showed that addition of organic alkali not only improved the descaling effect, but also reduced the damage to the formation permeability caused by the injected liquid. Sharma et al. used ammonia instead of sodium carbonate to form ASP flooding system. The research results revealed that ammonia water and formation water exhibited good compatibility, and the addition of ammonia did not lead to the precipitation of divalent ions [19]. In addition, organic alkali-surfactant-polymer flooding system significantly improved the recovery of Berea sandstone oil reservoir. Formation water of Shengli Oilfield contains higher content of Ca2+ and Mg2+ ions; therefore, flooding system prepared using inorganic alkali, in general, results in the precipitation of a large amount of Ca and Mg salts. These salts interfere with the injected oil displacement agent, causing serious alkali consumption, and thus limiting the technology application for this type of reservoir. Therefore, establishing and developing the organic alkali system, instead of the conventional inorganic alkali system, with a good compatibility with the brine containing high valence metal ions (i.e., Ca2+ , Mg2+ ), can solve the problem of Ca and Mg precipitation in the injection system. Further, it can result in the development of a new type of economically and technically feasible compound flooding technology suitable for the ordinary heavy oil reservoirs, thus exhibiting a broad application prospect in increasing the recovery of the ordinary heavy oil reservoir in Shengli Oilfield.
2. Experimental 2.1. Materials The oil sample was collected from the Zhuangxi heavy oil reservoir in Shengli oilfield. The is 1550 mPa s at 50 ◦ C and its acid value is up to 2.14 mg KOH/g. The solution used in the experiment was prepared according to the formation water in the field (Shengli Oilfield, China). The content of formation water is shown in Table 1. Five types of surfactants were used in the experiments in order to find the surfactant system suitable for the interfacial tension of heavy oil from Shengli Oilfield. They are Shengli petroleum sulfonate (SLPS, with a purity of 35%, Provided by Shengli oilfield, China), Anqing petroleum sulfonate (WPS, with a purity of 40%, Provided by Sinopec Anqing Branch, China), Xinjiang petroleum sulfonate (XJPS, with a purity of 40%, Provided by Xinjiang oilfield, China), Heavy alkyl benzene sulfonate (NJPS, AR, provided by Nanjing Shihao Chemical, China), ␣-olefin sulfonate (KAS, AR, Aekyuan Chemical CO., LTD, South Korea), Sodium dodecyl benzene sulfonate (SDBS, AR, Sinopharm, China). Ethanolamine was selected as the organic alkali, which was analytical-grade and purchased from Sinopharm, China. Hydrolyzed polyacrylamide (HPAM, 1000 ppm) with a molecular weight of 12 million was selected as the polymer component (Shengli Oilfield, China). Chemicals applied in the
Table 1 Ionic Composition of the formation water. Ion content (mg/L) Cl− 8118.5
HCO3 − 248.2
total salinity (mg/L) Ca2+ 146.7
Mg2+ 49
Na+ 5097.5
13659.9
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experiments, such as NaCl, MgCl2 , CaCl2 , and Na2 CO3 were all analytical-grade and purchased from Sinopharm, China.
10
1
10
0
2.2.1. Determination of interfacial tension Interfacial tension of oil–water system is determined by using a spinning drop interfacial tension instrument TEXAS-500. A densitometer is first selected to measure the densities of the crude oil and the chemical solution. The refractive index of the chemical solution is measured using a WZS-1 Abbe refractometer. Further, an interfacial tensiometer is used to measure the tensile length and diameter of oil droplets in the solution, and a software developed by our research group is used for image acquisition and analysis, in order to calculate the oil–water dynamic interfacial tension. When the length (L) of oil column is more than four times of its diameter (D), the calculation formula of interfacial tension is as follows: 3
= 1.2336(D/n) ω2
(1)
Where ␣, r, ω, D, L, and n are the oil-water interfacial tension (mN/m), the density difference of water and oil phase (g/cm3 ), the rotational velocity (rpm), the diameter of the oil drop (0.0001 m), the length of the oil drop (0.0001 m), and the refractive index of water phase respectively. 2.2.2. Sandpack flood experiment The sandpack is 20 cm in length and 2.5 cm in diameter. The sandpack is prepared as follows: The quartz sands with a size range of 125–150 m and 150–180 m are washed and dried and then evenly mixed in the fixed proportion (125–150 m:150–180 m = 3:1). The sandpack is placed vertically and the moderate amount of quartz sands and formation water are added each time. Subsequently, the sandpack is shaken to compact the sand. In the entire process, it is necessary to ensure that the water level is above the sand surface in order to prevent air from entering. The sandpack flooding experimental steps are as follows: First, the sandpack is saturated with formation water, following which the permeability is measured and the porosity is calculated. Second, the Zhuangxi heavy oil is injected into the sandpack at a constant speed until the water ratio at the outlet is less than 2%, and the saturation rate is then calculated. Third, when the produced fluid by water flooding reaches an oil content of less than 2%, the 0.5 PV chemical slug is then injected and the subsequent water flooding is carried out until the oil content in the produced fluid is negligible (oil content less than 2%). Both formation water and chemical slug are injected at the flow rate of 0.5 mL min−1 . Using sand packs with the similar initial permeability, the flooding tests of each oil displacement formula were conducted three times, and the average of the recoveries was selected. 2.2.3. Micromodel displacement experiment The model for the microscopic flooding experiment is an etched glass model, based on the shape of pores and throats of the core. The glass model is hydrophilic and its size is 25 mm × 25 mm. The steps of microscopic displacement experiment are as follows: First, the glass model is vacuumed and saturated with the formation water; second, the model is saturated with crude oil at the formation temperature and aged for 24 h; third, the chemical solution is injected for displacement at a speed of 0.003 mL min−1 , and the image of the entire displacement process was obtained. Finally, the image of the displacement was analyzed and the recovery was calculated. In order to observe the phenomenon during the displacement, the displacement fluid was stained red with eosin, and each experiment was repeated three times.
IFT/(mN/m)
2.2. Methods WPS NJPS XJPS KAS SDBS SLPS
-1
10
-2
10
0
20
40
60
80
t/min Fig. 1. Dynamic IFT between crude oil and brine with different kinds of sulfonate anionic surfactant.
3. Results and discussions 3.1. IFT behavior of crude oil and various chemicals 3.1.1. IFT behavior of crude oil and surfactant In order to obtain more accurate results, all of the IFT determination tests were repeated three times. The interfacial tensions of the sulfonate anionic surfactant and the Zhuangxi heavy oil were determined, respectively. Fig. 1 shows the results, indicating that the Shengli petroleum sulfonate (SLPS) is the best surfactant to reduce the interfacial tension among the above-mentioned surfactants. The reason lies in that the interfacial tension of oil/water system mainly depends on the number of the surfactant molecules that absorbed onto the oil/water interface and their absorption strength. The main components of the petroleum sulfonate is a mixture of several kinds of alkyl or aryl sulfonate, or to say, the petroleum sulfonate is a kind of mixed surfactants with a range of molecular weight distribution. For crude oil/petroleum sulfonate system, when the system reaches equilibrium, the petroleum sulfonate molecules are distributed in the oil phase, the oil/water interface and the aqueous phase in accordance with the lipophilic from strong to weak. The Shengli petroleum sulfonate (SLPS) is synthesized by using crude oil from Shengli Oilfield, through sulfonation reaction. The SLPS matches with the crude oil from Shengli Oilfield well, and is more easily absorbed onto the oil/water interface. The more number of the surfactant molecules are absorbed, the higher the absorption strength is, as a result, the lower the interfacial tension of the oil/water system is. Therefore, compared with other sulfonate anionic surfactants, the SLPS can significantly reduce the oil/water interfacial tension. Based on the measured results of the interfacial tension tests, the SLPS is selected as the candidate to make complex formulation with ethanolamine, in order to obtain lower interfacial tension system. 3.1.2. IFT behavior of crude oil and OA/S system In chemical flooding, the addition of organic alkali instead of inorganic alkali can not only reduce the IFT, but also can effectively avoid the scaling, clay swelling and migration, and leads to reduction in permeability [20,21]. The SLPS selected from the former experiments was used in the following tests. The interfacial tensions of OA/S system comprising 0.5 wt% ethanolamine and SLPS with different concentrations were determined, and the results are shown in Fig. 2. As shown in Fig. 2, the IFT between oil and OA/S system is lower than that between oil and SPLS, as well as oil and ethanolamine, which are 0.07 mN/m and 0.7 mN/m respectively. At the same time, also from Fig. 2 we can see that the IFT between oil
L. Fu et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 508 (2016) 230–239 1
233 120
8
0.5% ethanolamine
Chemical flooding Water flooding
Water flooding
7
100
0.1% SLPS
The mass fraction of SLPS
in OA/P system 0.0005% 0.001% 0.025% 0.050% 0.075% 0.100%
0.01
1E-3
Differential pressure/MPa
IFT/(mN/m)
0.1
Water content
80
5 4
60
3 40
Oil recovery
2
20
1
Water content, Oil Recovery/%
6
Differential pressure
0
0
-1 0
1E-4 0
10
20
30
40
50
60
70
80
90
1
2
3
4
5
Pore volume of injection
t/min Fig. 2. IFT curves between crude oil and brine with different concentrations of SLPS and 0.5% ethanolamine compound system.
and OA/S system decreases gradually with the increase in SLPS concentration. When the mass fraction of SLPS reaches 0.1%, the SLPS and 0.5% ethanolamine compound system can reduce the interfacial tension between oil and OA/S system to ultralow. The component in the heavy oil that can react with the alkaline agent is petroleum acid, or naphthenic acid. Petroleum acid is an unspecific mixture of cyclopentyl and cyclohexyl carboxylic acids. The mixture usually has a molecular weight of 120–700, depending on the types of the crude oils. The injected organic alkali reacts with the naphthenic acid in crude oil to produce in-situ surface active materials, which are absorbed onto the oil/water interface and has synergistic effect with the synthetic surfactant (SLPS) to further reduce the oil/water interfacial tension. Ethanolamine itself is a kind of surface active substance. It can be absorbed onto the oil/water interface and reduces the oil/water interfacial tension, playing the similar effect as the surfactant [22]. Because of the synergistic effect between the in-situ surface-active materials and the SLPS, as well as the surface activity of the organic alkali itself, the interfacial tension can be further reduced. In addition, the added organic alkali increases the pH of the chemical slug and makes the reservoir change to a negatively charged environment, reducing the surfactant consumption. In this case, there will be more SLPS which are anionic surfactant distributed on the oil/water interface. Thereby, the displacement efficiency of the OASP system is improved [23]. Based on the above mentioned research, a displacement system with low interfacial tension could preliminarily be formed, i.e., a complex formulation of 0.1 wt% SLPS and 0.5 wt% ethanolamine. 3.2. Sandpack flood test For the Zhuangxi heavy oil with a viscosity of 1550 mPa s at 50 ◦ C, the oil recovery of the compound system consisting of the polymer, surfactant, and organic alkali was evaluated by the sandpack flood tests. For comparative analysis, the enhancement in heavy oil recovery was investigated by using the surfactant flooding, polymer flooding, and OASP flooding. Further, the influence of factors such as the concentration of ethanolamine on the enhanced oil recovery was also comprehensively investigated. In all the experiments, the injected volume was 0.5 PV. The parameters of the sandpack and the experimental results are listed in Table 2. As shown in Table 2, when 0.1% of SLPS is injected, the oil recovery enhanced by surfactant flooding increased only by 4%. While 0.1% of HPAM is injected, the oil recovery of polymer flooding
Fig. 3. Curves of the oil recovery, water content, and differential pressure of OASP flooding (0.5% ethanolamine + 0.1% SLPS + 0.1% HPAM).
increased by 10.7%. These results indicated that using chemical agent (surfactant or polymer) alone does not effectively improve the recovery of heavy oil. This is attributed to the high viscosity of heavy oil, even if the surfactant is capable of decreasing the oil/water interfacial tension down to an ultralow level, a serious fingering phenomenon still exists in the flooding process due to an unfavorable high mobility ratio of water over oil. Therefore, large area of residual oil cannot be touched by water flooding [24]. The polymer can improve the viscosity of the flooding fluid, and to a certain extent reduces the mobility of displacement fluid. However, the oil displacement efficiency of the swept area cannot be improved due to poor interfacial activity of polymer, thus leading to limited increment in oil recovery [25]. In order to better explore the role of surfactant, 0.1% HPAM and 0.1% SLPS were formulated into compound system and a displacement experiment in the sandpack flooding with this system was performed. The results from test No. 3 listed in Table 2 shows that the oil recovery of the compound flooding system consisting of the polymer and surfactant increases by 13.7%; however, the additional recovery is minor, when compared with 0.1% HPAM. Different concentrations of ethanolamine were added into the compound flooding system of 0.1%SLPS and 0.1%HPAM, and the displacement experiments were conducted respectively. The results from test No. 4 to 7 indicate that addition of ethanolamine can significantly enhance oil recovery of the SP compound flooding system. With the increase in ethanolamine concentration in the OASP system, oil recovery is increased significantly. When 0.5 PV of the 0.1% SLPS, 0.1% HPAM, and 0.5% ethanolamine was injected after water flooding, the oil recovery of chemical flooding increased by about 20% and the final recovery reached 54.1%, exhibiting obvious flooding effect. The curves for the recovery, water content, and differential pressure in the process of displacement using the compound flooding system of 0.5% ethanolamine, 0.1% SLPS, and 0.1% HPAM are shown in Fig. 3. Clearly, at the beginning of water flooding, the displacement pressure increases rapidly to a peak value, the injection pressure decreases rapidly after water breakthrough, and continuously decreases down to a minimum. This indicates the formation of water channels in the core. The water content is up to 98% and the oil recovery by water flooding is 33.4% at the moment. And then, the slug of 0.5 PV of the OASP system was injected. The pressure increases rapidly with an obvious peak value and the water content starts to decline, indicating that the original water flow channels created by water flooding were blocked effectively by the injected compound system. The blockage effect enables the displacement
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Table 2 Summary of Chemical flooding in sandpack flood tests for Zhuangxi heavy oil. Test No.
Porosity/(%)
Permeability/(10−3 m2 )
Initial oil saturation/(%)
Chemical flooding system
1 2 3 4 5 6 7
38.2 37.5 38.9 37.5 39.1 37.6 38.2
1350 1380 1410 1410 1450 1350 1380
88.5 87.1 88.5 87.9 88.9 87.0 88.5
0.1% S (SLPS) 0.1% P (HPAM) 0.1% P + 0.1% S 0.1% P + 0.1% S + 0.1% OA 0.1% P + 0.1% S + 0.25% OA 0.1% P + 0.1% S + 0.5% OA 0.1% P + 0.1% S + 0.75% OA
fluid flow into the unswept oil enrichment region, and improves the oil recovery of water flooding. Therefore, the oil recovery can be enhanced by 21.7%, and the final recovery can reach up to 54.1%. 3.3. Microscopic displacement mechanisms of OA/S/P flooding In order to study the mechanism of enhancing ordinary heavy oil recovery by the organic alkali/surfactant/polymer flooding, the microscopic oil displacement experiments were conducted using the compound flooding system of 0.5% ethanolamine, 0.1% SLPS, and 0.1% HPAM. The images of the experiments of the microscopic oil displacement by the direct OASP flooding are shown in Fig. 4. 3.3.1. The sweep efficiency of OA/S/P system Research has shown that the distribution characteristics of the residual oil in heavy oil reservoir after water flooding are completely different from those of the conventional crude oil after water flooding [1]. The residual oil in the former reservoir is not formed by the entrapment of capillary force; however, the significantly high mobility ratio between the injected fluid and the heavy oil with high viscosity leads to bypassing the residual oil of the flooding fluid. Therefore, in these ordinary heavy oil reservoirs, the residual oil is continuously distributed and able to flow. Thus, the key to improving the recovery of such ordinary heavy oil reservoirs is to increase the sweep efficiency of the displacement fluid. Fig. 4(a) and (b) demonstrates that the injected OASP solution does not create a serious fingering phenomenon along the diagonal direction of the model, and it does not break through until 0.68 PV is injected, as shown in Fig. 4(c), indicating that the OASP system exhibits better sweep efficiency. The reason lies in the role of the polymer and the organic alkali in the OASP system. The added polymer improves the sweep efficiency by reducing the water-oil mobility ratio. It is well known that the smaller the water-oil mobility ratio is, the higher the sweep efficiency is [26]. In the OASP flooding system, the polymer reduces the water-oil mobility ratio in two ways. One is increasing the viscosity of the displacing fluid by thickening the surfactant and alkali solution. The other is reducing the water phase permeability in porous media by remaining in the pore space of the polymer [27]. Under the two mentioned effect of the polymer, the polymer in the OASP system increases the sweep efficiency directly. In addition, the polymer can increase the stability of the O/W emulsion consisting of surfactant and alkali, which increases the sweep efficiency indirectly [28]. The microscopic oil displacement tests of SP system containing 0.1% SLPS and 0.1% HPAM are shown in Fig. 5. It can be seen that when 0.5 PV of the SP system is injected, the breakthrough occurs (Fig. 5(a)). The swept area of the SP system is less than that of the OASP system when 2.0 PV of the displacement fluid was injected, as shown in Figs. 4(f) and 5(b). The comparison illustrates that the addition of organic alkali in surfactant/polymer system can inhibit fingering phenomenon and improve the sweep efficiency of the SP system. The organic alkali in the ASP system improves the sweep efficiency can be explained from three aspects.
Oil recovery/(%) Water flooding
Chemical flooding
Final recovery
32.2 32.2 33.5 33.3 34.4 33.4 33.5
4.0 10.7 13.7 16.1 18.8 21.7 23.4
36.2 42.9 47.2 48.4 53.2 54.1 56.9
For one thing, the organic alkali in OASP system and the acidic component of the heavy oil can react with each other at the interface of oil and OASP solution. The surfactant generated at the interface enables the OASP solution come into the heavy oil to generate plenty of W/O emulsions, as shown in Fig. 6. The flow resistance of the displacement fluid increases due to the high viscosity of W/O emulsion. Therefore, the water-oil mobility ratio is reduced and the sweep efficiency of OASP system is greatly improved [29]. For another, the subsequent OASP solution turns to the untouched area due to the blocking effect of the emulsified oil droplets in the pore throat, improving the sweep efficiency [30]. With the injection of OASP system, a large amount of crude oil emulsified and dispersed under the flow disturbance, and a lot of emulsion droplets with different sizes were formed, as shown in Fig. 7(a). The interfacial tension between the OASP system and heavy oil could reach an ultralow level, due to the synergistic effect of the surface active materials generated by the organic alkali and the naphthenic acid in crude oil and the synthetic surfactant. Therefore, the emulsion and dispersion phenomenon in crude oil could occur with the even slight disturbance action of the displacement fluid which resulted in the formation of plenty of O/W emulsion [31]. In the flow process, some of the emulsion oil droplets (Fig. 7(a)) with smaller sizes are trapped in the pores or throats, such as position 1 (Fig. 7(b)), position 2 (Fig. 7(c)), and position 3 (Fig. 7(d)). When the displacement fluid passes through the pore throats which are filled with captured oil droplets, the flow direction of the fluid turns to the unswept areas, as shown in Fig. 7(b–c), improving the sweep efficiency. Meanwhile, with regard to the emulsified oil droplets with larger diameter, the flow resistance of the emulsion increases dramatically due to the Jamin effect, reducing the mobility of the aqueous phase. Thereby, the water-oil mobility ratio is improved by the Jamin effect of the emulsified oil droplets [32]. To sum up, the reaction between the organic alkali and the heavy oil increases the flow resistance of the displacement fluid through three mechanisms: the generation of W/O emulsion in the crude oil, the capture of the emulsified oil droplets at the pore throat, and the Jamin effect of the oil droplets with larger diameter. Combined with the effect of the polymer, the sweep efficiency of the OASP system is greatly increased.
3.3.2. The displacement efficiency of OA/S/P system Improving the displacement efficiency of the swept area is another aspect to improve the recovery of the ordinary heavy oil reservoirs. The mechanisms for the OASP system improving the displacement efficiency are as follows. Firstly, the oil/water interface tension reaches ultralow level due to the synergistic effect of the generated surface active materials and the synthetic surfactant, meaning the reduction in the work of adhesion of the residual oil. When the displacement fluid passing by, the residual oil encountered on the way is easy to be peeled, and the displacement efficiency is increased, which is illustrated by the displacement of the residual oil in the green circle, as shown in Fig. 8.
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Fig. 4. Microscopic images of direct OA/S/P flooding.
Then, the heavy oil is prone to be emulsified and form lots of oil droplets due to the ultralow oil/water interface tension generated by the synergistic effect of both the produced and synthetic surfactants. Under the flushing effect of the thickened displacement fluid, the residual oil in the pore throat is displaced and carried out
in the form of emulsified oil droplets, illustrated by the change of the residual oil in the blue circle in Fig. 8. What’s more, as the number of the oil droplets displaced from the surface of the reservoir increases, they may collide with each other in the process of moving forward. The oil droplets may get
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Fig. 5. Microscopic images of direct S/P flooding (0.1% SLPS and 0.1% HPAM).
Fig. 6. The W/O emulsion generated during OASP flooding.
together if the collision energy overcomes the electrostatic repulsion between the droplets. Gathering of the oil droplets can form oil belt (Fig. 9(d)). The oil belt continues to expand because of its coalescence with other oil droplets encountered during their migrations, and finally be extracted out. The gathering-forming oil belt mechanism is illustrated in Fig. 9. Finally, the adsorption of the SLPS which is an anionic surfactant onto the surface of both the oil droplets and the reservoirs, increases the surface charge density of the reservoir [33]. Therefore, the increase in the electrostatic repulsion between the oil droplets and the reservoir makes the oil droplets are easy to be displaced, and improves the displacement efficiency. In short, the synergistic effect of the SLPS and the surface active materials produced by the reaction between the ethanolamine and the naphthenic acid, increases the displacement efficiency, and further improves the oil recovery.
4. Conclusions Due to the high content of Ca2+ and Mg2+ ions in the formation water of Shengli Oilfield, organic alkali-surfactant-polymer (OASP) flooding system is established to solve the precipitation of a large amount of Ca and Mg salts in inorganic alkali-surfactant-polymer system. Based on the Zhuangxi heavy oil and the formation water from the field, the formula of the OASP system is determined: 0.5% ethanolamine +0.1% SLPS+ 0.1% HPAM. The interfacial tension tests show that the combination of Shengli petroleum sulfonate and ethanolamine exhibits a good synergistic effect, and forms an ultralow interfacial tension system. The sandpack flooding tests indicate that organic alkali-surfactant-polymer system shows a good displacement effect in ordinary heavy oil recovery, and the oil recovery can be enhanced by 21.7%. The recovery increases with the increase in the alkali concentration. The displacement mechanisms
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Fig. 7. The emulsifying and capturing mechanism of OASP flooding.
Fig. 8. The emulsifying and carrying mechanism of OASP flooding.
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Fig. 9. The emulsifying and gathering-forming oil belt mechanism of OASP flooding.
of OASP flooding are studied by the microscopic displacement tests. For one thing, in addition to the water-oil mobility ratio control ability of the polymer in the OASP system, the surface active materials generated by the reaction between the organic alkali and the acidic component in heavy oil enable the OASP solution come into the heavy oil, which generates plenty of W/O emulsions, increases the flow resistance for the flooding fluid, and finally improves the sweep coefficient of ASP system. For another, the synergistic effect of the surfactant and the surface active materials produced by the reaction between the organic alkali and the naphthenic acid, can decrease the interfacial tension to an ultralow level; therefore, slight disturbance can lead to the emulsion and dispersion phenomenon in the crude oil, which leads to peeling off the residual oil easily, carrying the oil droplets, and forming the oil belt for the oil to be extracted out. Combined with the increase in the electrostatic repulsion between the oil droplets and the reservoir due to the surfactant adsorption, the displacement efficiency is improved. Therefore, the organic alkali-surfactant-polymer flooding system exhibits both high sweep efficiency and high oil displacement efficiency, improving oil recovery of ordinary heavy oil reservoirs with high content of Ca2+ and Mg2+ ions while no salt precipitation is generated.
Acknowledgements This research is financially supported by the Fundamental Research Funds for the Central Universities (Grant 24720156031A, 24720156035A, and 16CX02018A), the National Natural Science Foundation of China (Grant 51574266 and 51474234), and the Shandong Provincial Natural Science Foundation, China (ZR2014EZ002 and ZR2015EQ013).
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