Enhancing Sodium Bis(2-ethylhexyl) Sulfosuccinate Injectivity for CO2

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Fossil Fuels 2

Enhancing sodium bis(2-ethylhexyl) sulfosuccinate injectivity for CO foam formation in low-permeability cores: dissolving in CO with ethanol 2

Chao Zhang, Zhaomin Li, Songyan Li, Qichao Lv, Peng Wang, Jiquan Liu, and Jianlin Liu Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b00741 • Publication Date (Web): 17 Apr 2018 Downloaded from http://pubs.acs.org on April 17, 2018

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Enhancing sodium bis(2-ethylhexyl) sulfosuccinate injectivity for CO2 foam formation in low-permeability cores: dissolving in CO2 with ethanol

Chao Zhang a, Zhaomin Li*, a, Songyan Li a, Qichao Lv a, Peng Wang b, Jiquan Liu b, Jianlin Liu*, c

a

School of Petroleum Engineering, China University of Petroleum (East China), Qingdao 266580, Shandong, China

b

Research Institute of Oil and Gas Engineering, Tarim Oilfield Company, Korla 841000, Xinjiang, China

c

College of Pipeline and Civil Engineering, China University of Petroleum (East China), Qingdao 266580, Shandong, China

*Corresponding author at: China University of Petroleum (East China), Qingdao 266580, Shandong, China ; E-mail addresses: [email protected] (Z. Li); [email protected] (J. Liu)

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Abstract The utilization of conventional water-soluble surfactants for the stabilization of CO2 foams has been limited by the low injectability of low-permeability reservoirs. In this study, the anionic surfactant sodium bis(2-ethylhexyl) sulfosuccinate (AOT) was used as the CO2-soluble surfactant to stabilize CO2 foams in the presence of ethanol. The phase equilibrium relationships of the AOT/ethanol/CO2 system and the partition coefficient of AOT between supercritical CO2 (SC-CO2) and water were measured through a fully visible PVT cell. In addition, the effects of the ethanol content on the partition coefficient and interfacial tension (IFT) were studied. Furthermore, a laboratory apparatus was developed to measure the viscosity and injection performance of AOT dissolved in an aqueous or SC-CO2 solution and SC-CO2 foams. The experimental data show that addition of ethanol can significantly improve the solubility of AOT in SC-CO2 and increase the partition coefficient of AOT. The IFT tests show that the ethanol content affects the critical micelle concentration (cmc) and IFT at cmc ( γ cmc ) differently: with ethanol addition, γ cmc decreases, while the cmc first decreases and then increases. The dissolution of AOT in SC-CO2 with ethanol could lead to a threefold increase for SC-CO2 viscosity, while the formation of SC-CO2 foam could result in an increase for viscosity of approximately 50~200 times. Finally, the core flooding results show that dissolved AOT in SC-CO2 could significantly improve the surfactant injectivity in tight rocks and interact with the formation water in situ to form SC-CO2 foams, controlling the mobility of the CO2. In

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addition, the performance of SC-CO2 foams could be governed by altering the ethanol content to adjust the surfactant partition coefficient.

Key words: CO2-soluble surfactant; partition coefficient; CO2 foam; surfactant injectivity; adsorption

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1. Introduction CO2 has become a popular displacement agent for enhanced oil recovery (EOR) because of its numerous attractive recovery mechanisms: swelling effect, viscosity reduction, interfacial tension reduction, solution gas drive, and light-components extraction [1, 2]. Despite of so many advantages, the low density and low viscosity of CO2 result in viscous fingering, gravity overriding, early breakthrough, and consequently poor sweep efficiency [3, 4]. One potential way to overcome this limitation is to use CO2 foam instead of direct CO2 flooding [5, 6]. The CO2 foams can surround the gas in bubbles to reduce the mobility of CO2 in the porous reservoir and stabilize the displacement front of CO2-flooded zones, reducing viscous fingering and thereby improving the sweep efficiency [1]. Several potential injection strategies for robust foam formation have been proposed [7], such as alternating injections of surfactant aqueous solutions and CO2 (SAG) and simultaneously injecting surfactant aqueous solutions and CO2. The former strategy has been most commonly used in field-scale foam applications because of good injectivity [8]. With the shift away from gravity-driven oil developments, unconventional oil and gas resources (i.e., tight oil and shale gas) emerge as crucial alternatives to conventional oil resources [8]. In such low-permeability reservoirs, CO2 flooding is an effective recovery method, but the low injectivity of surfactant aqueous solutions limits the application of CO2 foam, making it difficult to improve the sweep efficiency of CO2 [9, 10].

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Typically, CO2 is in a supercritical state ( Tc =31.1 °C, Pc =7.38 MPa) under reservoir conditions. In addition, supercritical CO2 (SC-CO2) has been widely used as a solvent for separation and extraction [11-13], creating favorable conditions for the injection and dissolution of surfactant in SC-CO2. This injection strategy will create a higher injectivity and can generate foams in situ when the injected SC-CO2 is mixed with formation water [2, 14, 15]. In 1967, Bernard and Holm first proposed the concept of CO2-soluble surfactant in a patent [16]. Many studies have since been conducted to select and synthesize suitable surfactants and to demonstrate their effectiveness in oilfield applications [17–19]. However, SC-CO2 is a poor solvent for conventional surfactants due to its very low dielectric constant, low polarizability per volume, and correspondingly weak van der Waals forces [20]. Therefore, great efforts have been made to obtain CO2-philic surfactants. Enick et al. [18] determined that nonionic compounds with ethoxylated hydrophiles and hydrocarbon-based or oxygenated hydrocarbon-based CO2-philes were beneficial to improve the solubility of surfactants in SC-CO2. Meanwhile, Xing et al. [21] indicated that branched alkylphenol ethoxylates (Tergitol NP-series of nonionic surfactants) can be dissolved in SC-CO2 and can stabilize CO2 foams, although the required temperature and pressure cannot be met at oil reservoir conditions. Recently, Johnston et al. [22] found commercially available

CO2-soluble

surfactants

can form

CO2

foams

at

high-temperature and under high-salinity conditions. Meanwhile, Johnston et al. [23, 24] and Nguyen et al. [14, 15] found that the partition coefficient of CO2-soluble surfactants would affect the properties of CO2 foams; for example, the increase in the

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surfactant partition coefficient would lower the rate of foam propagation. It is well known that addition of cosolvents or cosurfactants could enhance the solubility of surfactants in SC-CO2 [25–27]. Some studies suggest that alcohol can promote the dissolution of surfactants in SC-CO2 by distributing between the surfactant tails, reducing tail−tail and micelle−micelle interactions. However, other argument mentioned that alcohols act on the interface and reduce the interfacial tension, thus promoting the formation of stable reverse micelles and compelling the surfactants to dissolve in SC-CO2 [28]. Our previous study [29] indicated that sodium bis(2-ethylhexyl) sulfosuccinate (AOT) performs better in CO2 foaming applications, especially when used with nanoparticles. Additionally, AOT has been widely utilized and solubilized in SC-CO2 with a predetermined amount of alcohol to form water-in-CO2 (W/C) reverse microemulsions [30]. As a consequence, the goal of this study is to use ethanol as a cosolvent to promote the dissolution of AOT in SC-CO2, identify the effect of ethanol on the surfactant partitioning between SC-CO2 and water, and evaluate the adsorption and the injectivity of AOT with different injection strategies in tight oil cores. We hope that this study will promote the application of CO2-soluble surfactants in a wide range of reservoir conditions and conveniently select the optimal injection strategies.

2. Experimental section 2.1. Materials CO2 (99.999% purity) supplied by Tianyuan, Inc. (China) was used as received.

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Water was first passed through an Elga reverse-osmosis unit and then a Milli-Q reagent water system. AOT (minimum 99%, MW 444.56) purchased from Sigma Chemical Co., Ltd. (USA) was vacuum dried at 60 °C for 24 h and stored in a vacuum desiccator prior to application. The critical micelle concentration (cmc) in water at 25 °C was 2.5 mM. Ethanol (99.9% analytical pure, MW 46) supplied by Chinese Sinopharm Chemical Reagent Co. Ltd. (China) was used as received.

2.2. Solubility of AOT in SC-CO2 with ethanol The solubility of AOT in SC-CO2 with ethanol was measured by using a fully visible PVT cell (PVT 240-1500, Sanchez Technologies Co., France), as shown in Fig. 1.

Fig. 1 Schematic of the experimental apparatus used for solubility measurement

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In the experiment, the PVT cell is a hollow cylinder with an inner diameter of 60 mm, and a sapphire window is installed on one end of the cylinder to record the inside information of the cylinder with a charge-coupled device (CCD) camera. The cell volume can be adjusted by a movable piston inside the cell, and its value could range from 5 mL to 240 mL. Additionally, the cell could withstand a maximum pressure of 150 MPa. A magnetic stirrer with four blades is installed on the piston, and the stirring speed is set by a computer. An electric heating rod in the PVT cell can be used to control the temperature from 20 °C to 200 °C, with a precision of up to 0.01 °C. Before each measurement, the PVT cell was cleaned, dried, and vacuumed. In order to ensure that the mass of the injected CO2 be consistent for each measurement, the initial volume of the PVT cell is set to 100 mL, and the initial pressure is set to 20 MPa. Each experiment is conducted at 40 °C unless otherwise specified. The density of CO2 was 0.8398 g/mL at 20 MPa and 40 °C. The mass of the injected CO2 could be calculated by means of multiplying its density by its volume. Fig. 2 (1) and (2) show that the CO2 in the experiment transitioned from a gas-liquid coexistence to a supercritical phase. Some AOT with a given volume and ethanol solution was then injected into the PVT cell, as shown in Fig. 2 (3) After being stirred at a speed of 800 r/min for 10 min, the SC-CO2, AOT and ethanol mixture became cloudy, as shown in Fig. 2 (4). Then, the piston was moved to alter the pressure of the PVT cell from 50 MPa to 15 MPa. It was found that conducting the experiments as the pressure changed from a high pressure to a low pressure is beneficial to reduce errors and can lead to a faster establishment of equilibrium. At

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each pressure, additional ethanol was slowly injected into the PVT cell by an ISCO 260D syringe pump until the turbid mixture became a clear single phase, as shown from Fig. 2 (5) to (8). The volume of the additional ethanol injected into the PVT cell can be recorded by the ISCO 260D syringe pump. Thus, the mass of the AOT and ethanol contained in this clear single phase can be calculated. Then, the pressure of the PVT cell was reduced to repeat this experiment. Each experiment was performed at least 3 times, and the results were averaged.

Fig. 2 Images collected during the solubility measurement.

2.3. Partition coefficient of AOT between SC-CO2 and water The PVT cell was also used to measure the partition coefficient of AOT and ethanol between SC-CO2 and water k ( AOT or Ethanol ) as a function of pressure and ethanol mass at 40 °C and 20 MPa. To determine the equilibrium partition coefficient, equal mass (20 g for each) of CO2 and water, 1 g of AOT and a predetermined mass of ethanol were poured into the PVT cell. After being stirred at a very slow speed (to avoid foaming) of 100 r/min for 10 min, the mixture inside the

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PVT cell was allowed to rest for 24 h to reach the equilibrium state. As shown in Fig. 1, there were two valves on the cylinder of the PVT cell. When the PVT cell was horizontal, one valve was on the top, and the other was on the bottom. Therefore, the upper SC-CO2 phase can be extracted from the top valve to a 2 mL stainless steel sampler. The piston motor was used to maintain constant pressure inside the PVT cell during the sampling process. Then, the sample of the SC-CO2 phase was slowly injected into water of 5 mL to recover all the AOT and ethanol dissolved in the sample. The concentrations of AOT and ethanol in the water were then determined using high-performance liquid chromatography (HPLC) and gas chromatograph, respectively. Then the partition coefficient of AOT and ethanol can be calculated using Eq. (1) k ( AOT or Ethanol ) =

Cgas ( AOT or Ethanol ) Cliquid ( AOT or Ethanol )

,

(1)

where Cgas ( AOT or Ethanol ) is the mass concentration of AOT or ethanol in SC-CO2, Cliquid ( AOT or Ethanol ) is the mass concentration of AOT or ethanol in aqueous solution. The sampling procedures were repeated three times, and the results were averaged.

2.4. Interfacial tension measurements Next, interfacial tension (IFT) was measured through a drop shape tensiometer (Tracker-H, Teclis Co., France), which was described in our previous work [29, 31–33]. First, the view cell was fully filled with CO2 and maintained at 40 °C and 20 MPa. A pendant drop of the AOT ethanol aqueous solution was then formed at the 10


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end of a stainless steel needle attached to a gas-tight syringe. The dynamic IFT was recorded through axisymmetric drop shape analysis, and the droplets of the dispersions were maintained for 10 min to reach a steady effect of IFT. Finally, the IFT between the SC-CO2 and AOT ethanol aqueous solution was calculated. This procedure was repeated three times.

2.5. Surfactant adsorption test The solubility results indicate that AOT can be dissolved in either water or SC-CO2 with ethanol. Therefore, two different types of solutions were prepared, where the first one is the aqueous solution, which was formed through dissolving AOT in water with different concentrations of ethanol, and the second is the SC-CO2 solution, which was formed by dissolving AOT in SC-CO2 with different concentrations of ethanol. 2.5.1. Static adsorption test on surfactant An aqueous solution or SC-CO2 solution with the volume of 10 mL and AOT was mixed with crushed rocks with the mass of 5 g in a stainless steel container. The crushed rocks were sampled from the reservoir sandstones, and the particle size of the crushed rocks was from 160 to 180 meshes. The mixture was kept at 20 MPa and 40 °C with occasional shaking for 72 h. Then, the aqueous solution mixture was centrifuged (GT10-1, Beijing Shidai Beili Centrifuge Co. Ltd., China) at 8000 rpm for 60 min, and the concentration of AOT in the supernatant was analyzed by using a total organic carbon (TOC) technique. For the SC-CO2 solution, SC-CO2 was injected

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into a certain quantity of distilled water at certain time intervals. Then, the concentration of AOT in the distilled water was analyzed by the TOC technique, and the amount of surfactant adsorbed on the crushed rock was calculated by mass balance. 2.5.2. Dynamic adsorption test on surfactant The aqueous solution or SC-CO2 solution with AOT (0.5 wt%) was then injected at a flow rate of 2 mL/min into a sandpack model filled with crushed rocks for 10 pore volumes (PVs). All the experiments were conducted at 40 °C, and the back pressure was maintained at 20 MPa. The effluent fluid (water or gas) was injected into distilled water at certain time intervals. The concentration of AOT in distilled water was analyzed by the TOC technique, and the amount of the surfactant adsorbed on the sandpack model was calculated by mass balance.

2.6. Viscosity measurement and injection evaluation According to Darcy’s law, when fluid flows in porous media, the fluid viscosity is crucial in determining the flow behavior [34]. Thus, a large-scale laboratory setup was designed to measure the viscosity and injection performance of fluids under the maximum pressure of 50 MPa and maximum temperature of 100 °C. The advantages of this setup include three aspects, i.e. the first one is that it is useful for us to prepare the SC-CO2 solution by dissolving AOT with ethanol and generating the SC-CO2 foam at a high pressure; the second is that it is helpful to study the flow behaviors of SC-CO2, SC-CO2 solution with dissolved AOT, and SC-CO2 foam in pipes; and the

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last one is that it is beneficial to evaluate the injectability of fluids in tight oil rocks. The schematic of the apparatus is demonstrated in Fig. 3.

Fig. 3 Schematic of viscosity measurement and injection evaluation experimental apparatus

There are three main parts to this apparatus, i.e. the foam generator, the pipeline viscometer, and the injection evaluator. The foam generator part was first used to mix the SC-CO2 and AOT ethanol solution to make the SC-CO2 solution by dissolving AOT with ethanol under high pressures. Then, water was co-injected into the foam generator and mixed with the SC-CO2 solution to generate the SC-CO2 foam. A cooling tank was used to liquify the CO2, and the pressure would be easily increased by the booster pump. The liquid CO2 was injected into a buffer tank, and the AOT ethanol solution was also added into the buffer tank by the ISCO pump. Then, the mixture was stirred in the buffer tank and through a pipe driven by a circulating pump. The temperature of the pipeline was controlled by a water bath, and the oven 13


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thermostats were used to modulate the temperature of the other pieces of equipment, as shown in Fig. 3. The mass flow rate of the fluids in the pipe was governed by a high-pressure mass flow meter. The valve before the foam generator was opened to allow the water and the SC-CO2 solution with dissolved AOT to pass through the foam generator. The foam quality was controlled by adjusting the water injection rate.

2.6.1. Viscosity measurement We first measure the viscosity of the fluid. The length of the pipeline L is 8 m, and its diameter D is 6 mm. When the SC-CO2 solution with dissolved AOT and ethanol or the SC-CO2 foam was circulating in the pipeline, differential pressure sensors were used to record the pressure difference between the two sides of the pipe. The pressure difference ∆P was recorded as a function of the average flow rate ν . The wall shear stress τ w and the apparent shear rate γ w were calculated by using the pressure difference ∆P and average flow rate ν , respectively. Then, the apparent viscosity η was calculated as follows [35]:

η=

τ w D ∆P ( 4 L ) = . γw 8ν D

(2)

2.6.2. Injection scheme evaluation For tight oil reservoirs, due to their low permeability and porosity, it is injectivity that determines whether fluid can be injected into the porous media. To evaluate the injectability of fluids in tight oil rocks, six displacement tests with different injection schemes were carried out, as shown in Table 1.

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Table 1 Parameters of the artificial cores Model No. 1# 2# 3#

Pore volumes (mL) 4.83 4.80 4.86

Porosity (%) 9.85 9.79 9.91

Permeability (×10-3 µm2) 0.71 0.67 0.79

4#

4.74

9.67

0.75

5#

4.68

9.55

0.61

6#

4.83

9.85

0.71

a

Initial water Saturation (%) 0

100

AOT injected method a AOT (0.5 wt%) SC-CO2 solution (with 5 wt% ethanol) AOT (0.5 wt%) aqueous solution (with 5 wt% ethanol) SC-CO2 Second injected First injected AOT (0.5 wt%) Last injected SC-CO2 SC-CO2 solution SC-CO2 (with 5 wt% ethanol) Second injected First injected AOT (0.5 wt%) Last injected SC-CO2 SC-CO2 solution SC-CO2 (with 10 wt% ethanol) Second injected First injected AOT (0.5 wt%) Last injected SC-CO2 SC-CO2 solution SC-CO2 (with 15 wt% ethanol)

The injection speed was 0.5 mL/min during every tests.

To avoid the leakage of fluid from the flange during the experiment, each core sample was wrapped with Teflon tape and heat-shrink tubing before it was inserted into the core holder. The diameter and length of the artificial core sample were 2.5 cm and 10.0 cm, respectively. The physical properties of the artificial core samples are presented in Table 1. The first three cores are dry cores and are used to evaluate the injectability of the AOT SC-CO2 solution, AOT aqueous solution and SC-CO2 into tight oil rocks. The second three cores were first saturated with water. Then, SC-CO2 was injected into each core until the pressure drop between the inlet and outlet stabilized. After injection of the SC-CO2, 0.5 PV of the AOT SC-CO2 solutions with different contents of ethanol was injected into the cores. Finally, SC-CO2 was injected. All tests were conducted horizontally, and a back-pressure regulator (BPR) with an open error less than 0.001 MPa was used to maintain the back pressure at 20 MPa during all the flooding experiments. All these tests were conducted at 40 °C. 15


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3. Results and discussion 3.1. Solubility of AOT in SC-CO2 with ethanol If AOT is soluble in the CO2 phase with auxiliary alcohol, its injection through the CO2 phase (instead of a more conventional aqueous phase) is beneficial for enhancing sweep efficiency [9, 10], especially in tight oil reservoirs where it is difficult to realize water injection. In this process, AOT moves with the CO2 phase and lowers the mobility of CO2 by forming foams when CO2 is mixed with water in the reservoir. The temperature and pressure of almost all the oil reservoirs are higher than those of the supercritical temperature and pressure of CO2, and previous studies have also shown that alcohols can greatly improve oil recovery and enhance the foaming property [36]. Therefore, the solubility of AOT in SC-CO2 with ethanol was tested at 40 °C and various pressures. Clearly, AOT was completely insoluble in SC-CO2 [37]. If a certain amount of ethanol cosolvent is added to the SC-CO2, AOT can be solubilized in the SC-CO2. The amounts of ethanol required to make various AOT concentrations dissolved in SC-CO2 under pressures ranging from 15 to 50 MPa are shown in Fig. 4. It was observed that SC-CO2 with higher dissolving pressures exhibits higher solubility of AOT when the addition of ethanol was held constant. This result indicates that the solubility of SC-CO2 can be adjusted by changing the pressure, presumably because of the enhanced density of the CO2 phase, as the decrease in distance between the CO2 molecules increases the force between CO2 molecules and therefore favors solvation. Meanwhile, the rising pressure increased the dielectric constant of the CO2 and improved the strength of the CO2 polarity, thereby

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improving the solvent effect of SC-CO2 [30]. However, when the concentration of ethanol was low, improving only the pressure of the SC-CO2 to enhance its solubility did not suffice. Therefore, it is feasible to add ethanol as a cosolvent to SC-CO2 to improve its solubility. At each pressure, the mass of the AOT dissolved into the SC-CO2 increased with the ethanol added into SC-CO2. Due to the mixing of the ethanol molecules and CO2 molecules, the dielectric constant and polarizability of the SC-CO2 and ethanol mixture can be significantly improved. Moreover, the ethanol molecules can insert itself between the AOT molecular tails, reducing the AOT tail-AOT tail interactions, promoting the dispersion of the AOT molecules in the SC-CO2 phase [26]. For instance, the solubility of AOT in SC-CO2 was markedly increased with the increase in the injected ethanol concentration at the pressure of 15 MPa. When the pressure increases to 50 MPa, increasing the injected ethanol concentration from 3.09% to 5.85% can enhance the solubility of AOT in SC-CO2 from 0.5% to 3.5%. Therefore, the higher the pressure is, the more apparent is the ethanol-assisted solubilization.

Fig. 4 Phase equilibrium curves of the ternary system including CO2, ethanol and AOT (40 °C) 17


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3.2. CO2-water partition coefficient The CO2-water partition coefficients of AOT and ethanol in CO2 and water, with different mass fractions of ethanol, at 40 °C and 20 MPa were presented in Fig. 5. The curves in Fig. 5 can be described as “S”-type curves with three regions. In region I, the H atom in the ethanol hydroxyl can form a hydrogen bond with the O atom in the CO2, forming a stable SC-CO2 and ethanol dispersion system, therefore slightly increasing the CO2-water partition coefficient of ethanol. When the initial concentration of ethanol was 4%, there was enough SC-CO2 in the ethanol dispersion system to promote the miscibility of AOT with SC-CO2, and the partition coefficient of AOT began to increase. In region II, there was a more significant increase in the partition coefficient of ethanol, which would increase the dielectric constant and polarizability of the SC-CO2. Then, the solubility of SC-CO2 improved, and the partition coefficient of AOT clearly increased from 0.11 to 0.52. Moreover, the increase in the ethanol solubilized in the SC-CO2 would insert itself between the AOT tails to reduce the AOT tail-AOT tail interactions, which reduces the miscibility of AOT with SC-CO2. In region III, the partition coefficient of ethanol reached a plateau value, indicating the dissolution equilibrium of ethanol. However, a slight decrease in the partition coefficient of AOT occurred from 0.52 to 0.43, possibly because of the micellization of AOT in the SC-CO2 with ethanol and the solubilization of water, which would reduce the concentration of AOT in SC-CO2.

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Fig. 5 Partition coefficients for AOT and ethanol

3.3. Interfacial tension at CO2-water interface As the ethanol content will influence the critical micelle concentration (cmc) and the IFT at the cmc ( γ cmc ) of surfactant, the IFT between the SC-CO2 and AOT aqueous solution at 40 °C and 20 MPa in the presence of ethanol was determined, as shown in Fig. 6.

Fig. 6 IFT between SC-CO2 and AOT aqueous solution with different mass fractions of ethanol (a: 0%, b: 5%, c: 10%, d: 15%, e: 20%, f: 25%, and g: 30%; the stars represent the inflection points for each curve) 19


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For each ethanol concentration, the IFT value decreased gradually with the increase of AOT concentration and plateaus above the cmc. As the ethanol concentration increases, the IFT decreases significantly. The value of γ cmc and cmc values of these systems can be determined from the inflection points as along the curves in Fig. 6; this process is illustrated in Fig. 7. The values of γ cmc and cmc influenced by the ethanol show different trends: the γ cmc values decreases with the increase of ethanol content, while the cmc values first decreases to a minimum at a certain ethanol content and then increases with ethanol content. The curves in Fig. 7 can be divided into three regions, and the schematic diagram for the effect of ethanol on the IFT is shown in Fig. 8.

Fig. 7 Curves of cmc and γ cmc of AOT with SC-CO2 as a function of the mass fraction of ethanol

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Fig. 8 Schematic diagrams for the effect of ethanol on the IFT

In region A, the ethanol content is low (15 wt%), which is beneficial for forming the SC-CO2 and ethanol enrichment zone and solubilizing more AOT molecules. In particular, water molecules can also be dissolved in the enrichment zone when the solubilizing quantity of the AOT molecules reaches a threshold, as in SC-CO2 microemulsion. Meanwhile, large quantities of ethanol molecules decrease the dielectric constants of the mixtures, causing the electrostatic force of the polar head groups in the micelle to increase. Furthermore, the hydrophobic interaction between hydrophobic chains of AOT is gradually reduced, leading to the decrease in the AOT micellization potential. These two effects result in a higher cmc and a minimum ethanol concentration. Simultaneously, the decrease in the AOT micellization would promote the AOT molecules to transfer from the bulk to the surface, inducing a more significant dissolution of AOT in the enrichment zone. Then, the AOT micelles begin to form in the SC-CO2 with ethanol and solubilized water, making the drop slightly contract and have an irregular pear shape (Fig. 8 (c)). It is understandable that the γ cmc values decrease more significantly during this stage.

3.4. Static adsorption test on surfactant The adsorption isotherms of AOT on the crushed rock were characterized using static batch adsorption tests with dissolution in an aqueous solution or a SC-CO2 solution at 20 MPa and 40 °C, as shown in Fig. 9.

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(a)

(b) Fig. 9 Effect of (a) AOT content and (b) ethanol content on the static adsorption of AOT on the crushed rock with AOT dissolution in an aqueous solution or a SC-CO2 solution

As shown in Fig. 9(a), the static adsorption of AOT onto the crushed rock surfaces increases as the dissolved AOT concentration in the solution increases. The adsorption plateau is ~5.7 mg/g-rock. To understand the adsorption mechanism

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between the AOT and crushed rock, the three-phase contact angle of crushed rocks before and after a static adsorption test was measured by the squash method, which was used in our previous publications [29, 41]. After the adsorption of dissolved AOT onto the rock surface, the contact angle decreased from 105 o to 95 o , indicating that the surfactants were adsorbed on the crushed rock surface as individual molecules with hydrophilic groups toward the solution; therefore, the hydrophobicity of the rock surface decreased. It was known that the alkyl chains of the surfactant would form a hemimicelle or admicelle structure to adsorb onto the solid surface when the hydrophobic attractive interaction between surfactant hydrophobic groups was sufficiently high and a trend of surfactant escaping from water was exhibited [42], and this process was confirmed by Gaspar Gonzalez [40]. When 10 wt% of ethanol was added into the aqueous solution, the adsorption of the AOT onto the crushed rock significantly decreases, and the adsorption maximum is reduced to ~3.4 mg/g-rock. First, ethanol influenced the adsorption of AOT by changing the property of the solution. Fig. 7 shows that a low concentration of additional ethanol ( < 15 wt%) promotes micelle formation and the reduction of cmc values. This effect will decrease the amount of surfactant that escapes from the water and the adsorption of the AOT onto the rock surface, as shown in Fig. 9 (b). Secondly, ethanol is a kind of surface active matter that will be adsorbed onto the rock surface. Moreover, the hydrophobic interaction between twin hydrocarbon tails of AOT will decrease as the ethanol concentration increases. Both of these effects decrease the adsorption of AOT onto the rock surface.

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Furthermore, the isotherms of the AOT adsorption on the crushed rock with dissolution in the SC-CO2 solution assisted by ethanol was also studied. When the concentration of the AOT dissolved in the SC-CO2 solution with 10 wt% ethanol did not exceed 0.25 wt%, no adsorption of AOT onto the rock surface was observed. With a continued increase in the AOT concentration, the adsorption increased slightly. In particular, the adsorption of AOT onto the rock surface decreased with an increase in the ethanol concentration in the SC-CO2, as shown in Fig. 9 (b). This result is mainly due to the changing property of the SC-CO2 through the addition of ethanol. By mixing the ethanol molecules and CO2 molecules, the dielectric constant and polarizability of the SC-CO2 and ethanol mixture can be significantly improved, and the solubility of AOT in the SC-CO2 increased. Moreover, non-ionizing AOT molecules existed in the SC-CO2 solution. Therefore, the hydrophobic interaction between the AOT tails will reduce, promoting the dispersion of the AOT molecules in the SC-CO2 phase and decreasing the adsorption of AOT onto the rock surface [26].

3.5. Dynamic adsorption test on surfactant During the application of surfactant in the oil field for EOR, the adsorption of surfactant would be further influenced by the hydrodynamic forces during flow [22]. Hence, the dynamic adsorption of AOT was thus studied at 20 MPa and 40 °C. 10 PVs of AOT aqueous solution with and without 10 wt% ethanol was injected into a sandpack model filled with crushed rocks at a rate of 2 mL/min. Then the dynamic adsorption of the AOT on the crushed rock sandpack was calculated to be

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0.19 mg/g-rock and 0.57 mg/g-rock, respectively. For the SC-CO2 solution with ethanol under the same flow procedures, a dynamic adsorption below 0.02 mg/g-rock was found. The addition of ethanol to the water strongly contributed to the formation of AOT micelles, increasing the penetration distance of the AOT, favoring a low dynamic adsorption. Meanwhile, the extraction and solvation of AOT in SC-CO2 was relatively strong when ethanol was added, and the highly diffusible characteristic of SC-CO2 was conducive to the mass diffusion of AOT, resulting in an extremely low dynamic adsorption. In general, the dynamic adsorption was fairly low and favors dissolving AOT in SC-CO2 to produce enough AOT to reach the region of treatment in the application.

3.6. Viscosity of SC-CO2 foam As shown in Fig. 3, CO2, AOT and ethanol were first co-injected into the pipeline to circulate several times to ensure that the AOT was completely dissolved in the SC-CO2. Water was then injected into the pipeline and mixed with the SC-CO2 solution, dissolving the AOT and generating SC-CO2 foam. Hence, the viscosity of the SC-CO2 solution with dissolving AOT was first measured. 3.6.1. Viscosity of the SC-CO2 solution with dissolving AOT and ethanol Fig. 10 presents the viscosity variation in SC-CO2 with and without AOT dissolving under different concentrations of ethanol at 20 MPa and 40 °C. The addition of ethanol to SC-CO2 can slightly increase its viscosity, which can be ascribed to the miscibility of ethanol in SC-CO2 and agrees with the “mixing rule” of

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the viscosity of liquid mixtures [10]. Additionally, SC-CO2 with a slightly higher solubility of AOT exhibited a significantly higher viscosity when the addition of ethanol was held constant. Therefore, the viscosity of the SC-CO2 increased significantly while the AOT and ethanol were simultaneously added. In this case, the ethanol molecules can insert themselves between the AOT molecular tails, promoting the formation of AOT reverse micelles in SC-CO2, as a W/C reverse microemulsions. The presence of AOT reverse micelles result in the increase in the inner flow resistance, contributing to the increase in the SC-CO2 viscosity.

Fig. 10 Variation in the SC-CO2 viscosity with and without AOT dissolving under different concentrations of ethanol at 20 MPa and 40 °C

3.6.2. Viscosity of the SC-CO2 foam After the SC-CO2 solution with dissolving AOT with ethanol was mixed with water, the SC-CO2 foams were generated and showed considerable increases in viscosity compared to the SC-CO2 phase. The apparent viscosity of the SC-CO2

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foams as a function of the shear rate and ethanol concentration at 20 MPa and 40 °C are shown in Fig. 11 (a). The SC-CO2 foams show a typical non-Newtonian property with and without ethanol. As the shear rate increased, the apparent viscosity of the foams decreased. The apparent viscosity was also related to the foam quality. When the foam quality increased from 40% to 80%, the viscosity was increased significantly. Moreover, the viscosity of the high-quality foams (80%) was approximately 100 to 150 times higher than that of the SC-CO2 under the same conditions as shown in Fig. 10, demonstrating feasibility of using foam to enhance the viscosity of SC-CO2. Moreover, no significant viscosity difference was observed between the SC-CO2 foams with and without ethanol, indicating that the addition of ethanol did not change the viscosity property of the SC-CO2 foams. Therefore, the surfactants will dissolve in the SC-CO2 with ethanol and interact with the formation water in situ to form SC-CO2 foams that improve the viscosity of CO2, resulting in a decrease in the mobility and an increase in the sweep efficiency of the CO2.

(a) 29


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(b) Fig. 11 Apparent viscosity of the SC-CO2 foams (a) with and without 10 wt% ethanol as a function of the shear rate and (b) with 10 wt% ethanol as a function of the foam quality at 20 MPa and 40 °C

The effect of foam quality on the apparent viscosity of the SC-CO2 foams generated by the mixing of water and SC-CO2 solution with dissolving AOT assisted by ethanol at shear rates of 200 and 800 s-1 at 20 MPa and 40 °C are shown in Fig. 11 (b). As reported by Princen et al. [43], foam viscosity is dependent on the foam bubble size, the interfacial tension, and the continuous phase viscosity. The two curves in Fig. 11 (b) show a similar general trend: the apparent viscosity first increased to a maximum at a certain foam quality (90%) and then decreased with increasing foam quality. This behavior can be ascribed to the changes in the bubble size due to different foam qualities. When the foam quality increased from 40% to

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90%, increasingly more were bubbles generated. The interactions between the bubbles became more significant, increasing the internal friction and the flow resistance of the bubbles. Therefore, the apparent viscosity of the SC-CO2 foams increased. When the foam quality continued to increase (>90%), the SC-CO2 foam became unstable because the foam films were fragile and sensitive to disturbances such as interactions and pressure fluctuations, leading to a decrease in the number bubbles. Thus, the apparent viscosity of the SC-CO2 foam decreased drastically when the foam quality increased beyond 90%.

3.7. Displacement test To study the fluid injectivity in tight oil cores, scenarios 1# to 3# were carried out. AOT was injected into the dry core by dissolving the AOT in the SC-CO2 solution with 5 wt% ethanol in scenario 1# and by dissolving the AOT in the aqueous solution with the same content of ethanol in scenario 2#. Scenario 3# was used to perform a comparison by injecting SC-CO2 into the core. As shown in Fig. 12 (a), the pressure drop increased rapidly when dissolved AOT was injected into the core in the aqueous solution and reached a plateau value at 3.15 MPa, indicating a low injectivity in the tight oil core. However, the pressure drop during the injection of dissolved AOT in the SC-CO2 solution peaked at 0.58 MPa and stabilized around at 0.35 MPa, which was slightly higher than that during the SC-CO2 injection. This result implied that the AOT injectivity of the tight oil cores can be significantly improved by dissolving the AOT in a SC-CO2 solution.

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(a)

(b) Fig. 12 Pressure drops with different (a) fluid injections of SC-CO2 and (b) slug injections of AOT SC-CO2 solutions with different ethanol contents (insert: the RF changes during the slug injection of the AOT SC-CO2 solution and SC-CO2)

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In scenarios 4# to 6#, the cores were first saturated with water to ensure that the AOT injected by dissolving the AOT in SC-CO2 can interact with the formation water in situ to form SC-CO2 foams. Fig. 12(b) shows the changes in the pressure drops for SC-CO2 solutions with different ethanol contents. In these three tests, SC-CO2 broke through with the first 0.4 PV of SC-CO2 injected. After SC-CO2 breakthrough occurred, the pressure drops decreased and stabilized at approximately 0.6 MPa. During the injection of the AOT SC-CO2 solution, the pressure drop increased significantly, indicating that the formation of foams in the cores could reduce the mobility of the CO2 and prevent the channeling of CO2. To quantitatively analyze the mobility control of the foams, the resistance factors (RFs) were calculated from the pressure drop ratios at the same injection rate:

RF =

∆P2 , ∆P1

(3)

where ∆P1 is the pressure drop during the CO2 injection process, and ∆P2 is the pressure drop during the foam injection process and the later CO2 flooding. The calculated results of the RF for these three tests are presented in Fig. 12 (b). The maximum RF, as an indicator of strong foam, occurred at 1.3 PV in scenario 4#, whereas more than 1.8 PV was needed in scenario 6#. Meanwhile, the maximum RF decreased from 22 in scenario 4# to 12 in scenario 6#. Ren et al. [2] studied the effect of surfactant partitioning on mobility control during CO2 flooding. They demonstrated that an increase in the surfactant partition coefficient will cause a further delay of strong foam propagation into the core. As shown in Fig. 5, the partition coefficient of the AOT between the SC-CO2 and water will increase with increasing ethanol content. 33


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This reaction will lead to a considerable spreading of the AOT concentration distribution along the main flow direction, which promotes the propagation of AOT in the tight oil cores but delays strong foam appearance. Hence, the partition coefficient of AOT can be adjusted by changing the ethanol content in the SC-CO2 solution, changing the performance property of the SC-CO2 foam.

4. Conclusions In this study, the anionic surfactant sodium bis(2-ethylhexyl) sulfosuccinate (AOT) was used as the CO2-soluble surfactant to stabilize CO2 foams in the presence of ethanol. AOT cannot be solubilized in SC-CO2 until a certain amount of ethanol is added and the pressure is sufficiently high. Ethanol plays a significant role in enhancing the dissolving capacity of SC-CO2 because it increases the dielectric constant and polarizability of CO2, promoting the dispersion of AOT molecules in the SC-CO2 phase by inserting itself between the AOT molecular tails. An increase in pressure can also improve the dissolving capacity of SC-CO2, although the effect is limited. The partition coefficient of AOT between SC-CO2 and water can be adjusted through the addition of ethanol. At ethanol concentrations greater than 4%, enough ethanol is present to interact with the SC-CO2 and promote the dispersion of the AOT in the SC-CO2, and the partition coefficient of the AOT begins to increase with the ethanol concentration. In addition, the IFT between the SC-CO2 and aqueous surfactant solutions was reduced because of the addition of ethanol, and the γ cmc decreased while the cmc first decreased and then increased with the increase in

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ethanol. The dissolution of AOT in SC-CO2 could significantly decrease the adsorption of AOT on rock surfaces and improve the injectivity of AOT in tight rocks. The core flooding tests indicate that AOT dissolved in SC-CO2 can interact with formation water in situ to form SC-CO2 foams and control the mobility of CO2. When the partition coefficient of the AOT increases due to an increase in the ethanol content in the SC-CO2 solution, AOT can be transported deeper into the tight oil cores, but the strong foam appearance was delayed.

Author Information Corresponding Author *E-mail: Zhaomin Li [email protected] Jianlin Liu [email protected]

Notes The authors declare no competing financial interest.

Acknowledgments This work was financially supported by the Natural Science Foundation of Shandong Province, China (ZR2017BEE059), the China Postdoctoral Science Foundation (2016M600572), the National Science and Technology Major Project of China (2017ZX05072005-004), and the Fundamental Research Funds for the Central Universities (18CX02160A). The authors sincerely thank Professor Mingyuan Li’s group at the Enhanced Oil Recovery Institute of China University of Petroleum (Beijing) for helping with the experiments.

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