Abstract
In this study, graphene oxide (GO) was prepared by the improved Hummers method, and a synergistically stabilized emulsion of GO and emulsifier was formulated. The best emulsion formula obtained by Response Surface Methodology consists of 1.39‰ GO, and 2.92‰ OP-10; the water-oil ratio is 4:6, achieving an emulsion index of 92.83%. The emulsion still maintained good stability under high temperature and high salt conditions, meeting the environmental requirements of medium and low permeability reservoirs. For injected water flooding, emulsion flooding could increase the oil recovery by 15.41%.
1 Introduction
With the continuous exploitation of oil and gas resources, high permeability reservoirs gradually enter the high water cut stage, and the exploiting proportion of medium and low permeability reservoirs in the oilfield gradually increases [1]. To improve oil recovery, the know-how for effective control of depth profile and flooding for medium and low permeability reservoirs is essential. The traditional polymer flooding [2], [3], [4] is only suitable for low temperature and salinity reservoirs. Steam flooding [5, 6] is not suitable for deeply buried reservoirs, so it is necessary to develop a new oil displacement system.
Emulsion has been widely used in the petroleum industry. Many indoor physical models and field tests showed that emulsion injection into the reservoir significantly improved oil recovery, helping to guide the development of tertiary oil recovery [7], [8], [9]. Haishun et al. [10] found in core displacement experiments and micro-model oil displacement experiments that the emulsification carrying property and the profile control performance could significantly improve oil recovery. Xidao et al. [11] obtained through slim-tube experiments that emulsion can reduce the interfacial tension and control the fingering of heavy oil. Ding et al. [12] prepared an emulsion of Na2CO3 and surfactant. Experimental results showed that the emulsions were effective to recover the by-passed oil in the water-unswept sandpack. Ajay et al. [13] found in a sand flooding experiment that the emulsion significantly reduced the mobility ratio and improved the sweep coefficient.
Emulsifiers play a crucial role in emulsion flooding systems. Cationic emulsifiers are easily adsorbed on the rock surface, while anionic emulsifiers have poor temperature resistance and are not suitable for harsh reservoir environments [14]. The hydrophilic portion of non-ionic emulsifiers mainly contains a certain number of oxygen-containing groups that do not dissociate in aqueous solution and have a certain stability, which explains why they have quickly found application in oil displacement. They are often mixed with other emulsifiers to achieve better salt resistance and resistance to multivalent ions [15, 16].
Among them, octylphenol polyoxyethylene ether-10 (OP-10) is a condensate of alkylphenol and ethylene oxide. The long carbon chain provides a good spatial extension, and the strong polar group makes it capable of being adsorbed at the oil-water interface to form emulsions [17, 18]. It is not uncommon to report that it is compounded (mixed) with other emulsifiers to enhance the application effect [19, 20]. However, as the reservoirs develop, the temperature and salinity of the formation water continue to rise. The emulsion systems formed with conventional surfactants were no longer sufficiently stable and could easily coalesce and break [21]. Therefore, the improvement of emulsion stability, temperature resistance and salt resistance is the research focus for emulsion flooding systems in the future.
In recent years, the development of nano-materials has provided a new idea for developing new efficient oil displacement systems [1, 22], [23], [24], [25]. Compared to the traditional surfactant-stabilized emulsions, the irreversible adsorption of nano-particles on the fluid interface greatly enhances the strength of oil/water film, improves the emulsion resistance in the reservoir environment (high temperature, pressure, shear and salinity) [26], [27], [28], [29], overcomes the limitations of a traditional emulsion, and therefore, nano-particles have a very good application potential in enhanced oil recovery. Han et al. [26] prepared a Pickering emulsion co-stabilized by a TX-100-AlOOH-SiO2 ternary system, which maintained good stability under severe conditions. Hanam et al. [30] found that the O/W emulsion stabilized by nano-silica particles increased the fluidity of oil in the pore network, which is useful for driving out the residual oil in the pores. Han et al. [31] prepared a novel hybrid mesoporous nano-particle-stabilized Pickering emulsion, which could effectively block high permeability channels and enlarge the sweep area.
Graphene oxide (GO), as a nanoscale sheet material, has a huge specific surface area with many surface functional groups such as hydroxyl and epoxy groups and a large number of carboxyl functional groups in the edge region of the lamellae. This makes GO highly hydrophilic, chemically stable, very surface active and highly wettable [32], [33], [34]. In recent years, reports of its use as a filtrate reducer [35, 36], shale inhibitor [37] and lubricant [38], [39], [40], [41] for drilling fluids have attracted considerable attention in the petroleum industry. Kim et al. [42] used GO to prepare a highly stable Pickering emulsion for the first time, which helped to design new functional graphene-based composites. AfzaliTabar et al. [43] found that the nano-porous graphene/silica nanohybrid Pickering emulsion has good stability under neutral or alkaline conditions, and is suitable for chemically enhanced oil recovery. Radnia et al. [44] used GO and asphaltene to prepare a Pickering emulsion. The experiment proved that the GO concentration did not change the type of emulsion. When the asphaltene concentration exceeded 1.5 wt%, the emulsion underwent a phase inversion. Yoon et al. [45] found that GO nanosheets (G-ON) could still form a stable O/W emulsion in a wide acid-base range (pH = 2–10) and high salinity content (5 wt% NaCl), which provided a new idea for enhancing oil recovery in high salinity reservoirs.
Pickering emulsion stabilized by graphene-based nano-material has shown many advantages in the petroleum industry, especially in the application of deep strata or reservoirs with complex conditions [46], and therefore, it has great research value. The emulsifier OP-10 has abundant polyoxyethylene ether chains in its structure, which can be combined with oxygen-containing functional groups through hydrogen bonding to improve the shortcomings of graphene-based materials [47]. The combined use of the two produces a synergistic effect, and a better application effect can be obtained.
In this work, the emulsion flooding system with synergistic stability is prepared from GO synthesised by the improved Hummers method and the emulsifier OP-10, with regard to the properties of medium and low permeability reservoirs. The formulation of the emulsion was optimised using Response Surface Methodology (RSM), and the stability of the emulsion was investigated by visual inspection and Turbiscan Lab stability analysis techniques. The stability of the emulsion in medium and low permeability reservoirs was investigated with temperature and salt tolerance tests, and the oil displacement effect was tested with a core displacement experiment. The research in this paper provides new opportunities and ideas for improving oil recovery with graphene-based emulsions.
2 Experimental procedure
2.1 Materials
The graphite used in this study was provided by Tianjin Xiensi Aude Technology Co., Ltd.; Kerosene, H2SO4, HCl, KMnO4, H2O2 are all purchased from Tianjin Jiangtian Chemical Technology Co., Ltd.; OP-10 comes from Tianjin Guangfu Fine Chemical Research Institute; K2S2O8, P2O5, NaNO3 were purchased from Tianjin Kemio Chemical Reagents Co., Ltd. All chemicals are used as received, and all aqueous solutions are prepared with double distilled water.
2.2 Preparation of graphene oxide (GO)
The basic raw material of GO preparation is graphite. The powdered graphite is chemically oxidized by a strong oxidant to generate graphite oxide, namely graphene oxide. Then, the GO is peeled off in water by liquid-phase ultrasonic waves to obtain the water-based dispersion system of GO. In this paper, GO was prepared by the improved Hummers method, and the basic process is shown in Figure 1.
Preparation flow chart of GO.
Pretreatment step: The graphite was added to a dilute HCl solution and stirred. To remove impurities, the graphite-HCl mixture was repeatedly washed with deionised water until it was neutral and then dried in a vacuum oven at 60 °C.
Preoxidation step: 4.7 wt% K2S2O8 and 4.6 wt% P2O5 were added into a three-necked flask containing 86 wt% concentrated H2SO4, placed in a water bath at 80 °C, stirred with a magnetic stirrer for 30 min, 4.7 wt% pretreated graphite was added and stirred for 6 h. After naturally cooling to room temperature, the reaction product was centrifuged, washed with deionised water to neutrality and dried in a vacuum oven at 60 °C.
Reoxidation step: 34.5 wt% concentrated H2SO4 was added to a dry three-neck flask, which was placed in an ice-salt bath at 0 °C. After stabilisation, 0.9 wt% of pre-oxidised graphite was added to the three-neck flask and the content of the flask was continuously stirred. Then 0.4 wt% NaNO3 and 2.6 wt% KMnO4 were added successively. After stirring with a magnetic stirrer for 2 h, the water bath was heated to 35 °C and the reaction mixture was stirred for another 2 h. Then 37 wt% deionised water was added dropwise to the three-neck flask, stirred to remove heat, and the temperature was maintained at 35 °C for 2 h. Then a 24.6 wt% H2O2 solution was added and the system turned light yellow.
Washing and dispersion step: Diluted HCl and deionized water were used alternately for centrifugal washing until the solution of the reaction system contained no SO42− and was almost neutral. It was then dried in vacuo at 60 °C for 48 h to obtain solid GO. GO dispersions with different mass concentrations were prepared by weighing different GO masses, adding them to deionised water and dispersing them with ultrasound (power 100 W) for 2 h.
2.3 Preparation of emulsion under different conditions
The formation conditions of medium and low permeability reservoirs are complex and the emulsion stabilised by a single surfactant often cannot achieve the desired effect. Therefore, it is necessary to combine or mix the emulsion with other substances to improve the performance and efficiency of oil displacement. In this paper, GO and the emulsifier OP-10 were used to produce emulsions synergistically. The emulsion stability was evaluated by the Emulsion Index (EI) given in Eq. (1) [28].
Where Ve is the volume of the final stable emulsion and V0 is the total volume of the phase.
GO content, emulsifier content and water-oil ratio are the main factors that determine the properties of the emulsion. Therefore, in this paper, the factors influencing the emulsion stability were evaluated by changing the experimental conditions. Firstly, a certain amount of OP-10 ([1–6])‰ was added into GO dispersion ([0.5–3.0])‰ and both were evenly mixed by stirring and ultrasonication to complete the preparation of the aqueous phase. After the system was cooled to room temperature, the oil phase (kerosene, water-oil ratio of 1:9–9:1) was added and stirred at high speed to form a Pickering emulsion.
2.4 Experiment optimization
Response surface methodology (RSM) was used to optimize the experimental condition. Design-Expert.V8.0.6.1 software was used for experimental design, data analysis and optimization design. The emulsion prepared in this experiment was a cooperative and stable system of solid particles GO and emulsifier OP-10. Combined with the RSM model, Box-Behnken Design (BBD) was established, in which GO content, OP-10 content and water-oil ratio were the main factors affecting the stability of the emulsion. At the same time, the emulsification index EI was used as the response value.
The response model for predicting the optimal conditions and describing the relationship between response values and factors can be expressed according to Eq. (2) [48].
Where Y m is the predicted response value; B0, b i , b ii and b ij are offset term, linear coefficient, quadratic coefficient and interaction coefficient, respectively. X i and X j are independent factors in coding units.
2.5 Characterization
The structure, elemental composition and chemical state of the products, GO or raw materials were characterized by Fourier transform infrared (FTIR) spectrometer (Germany, Bruker Company, ALPHA), Raman spectrometer (Bruker, EQUNIOX 55, Germany) and X-ray photoelectron spectroscopy (XPS, Kratos, Axis Supra). The morphology of the products was detected by field emission scanning electron microscope (FESEM, Japan, Hitachi, SU8010) and transmission electron microscope (TEM, FEI Company, Netherlands, Tecnai G2 F20). The type of emulsion was determined by the dilution method.
2.6 Emulsion stability test
The stability of emulsion was evaluated by studying the backscattering spectrum and kinetic index of all-around stability using the Turbiscan Lab stability analyser. Take about 20 mL of emulsion and put it into a glass bottle with a length of 55 mm matched with the instrument. Insert the sample tube into the sample tank, set the test temperature at 60 °C, and scan the sample once every 40 µm height. Scan once at regular intervals and obtain the backscattered light spectrum after scanning for 24 h. The stability dynamic index TSI is calculated by the Turbiscan Easy Soft software provided by the instrument. TSI is an index to evaluate the stability of the sample obtained by accumulating the light intensity change values measured twice before and after scanning at all heights of the sample. The instrument software automatically draws the TSI value as the ordinate, and time as the abscissa by summing the absolute values of the values, obtaining the dynamic Atlas reflecting the stability changes of different samples. The calculation method of TSI is shown in Eq. (3).
Where h and H are the height of the scanning point and the total height of the sample respectively.
2.7 Core experiment
A core displacement test can evaluate the oil displacement performance of the emulsion. The experimental procedure is briefly described as follows: Firstly, fill sand pipe cores with three pieces of artificial quartz sand, inject air into the cores for a pressure test, set vacuum and saturate the simulated formation with water after air leakage detection, and measure pore volume and porosity. Next, put the core holder in a 60 °C (simulated formation temperature) constant temperature box and heat it. Water flooding was carried out at a rate of 0.5 mL/min until the injection pressure remained stable, and the water phase permeability was obtained. Then, 3–5 PV experimental oil is injected and the oil is used to drive out water until there is no water at the outlet of the model, so as to obtain the original oil saturation. The core is aged at a constant temperature of 60 °C for 6 h. At the displacement rate of 0.45 mL/min, water drives oil to the model outlet with a water cut of 98%, and the water drive recovery ratio is calculated. Inject the 2 PV emulsion for displacement at the same speed, and calculate the recovery ratio of emulsion flooding.
3 Results and discussion
3.1 Basic characterization of GO
3.1.1 FT-IR spectrum
The FT-IR spectrum of the product GO is shown in Figure 2. It can be seen that there is a strong absorption peak at 3659.65 cm−1 (νO–H) caused by the stretching vibrations of the hydroxyl group and a peak at 1639.72 cm−1 (νC=O) attributed to the carbonyl group, confirming the existence of oxygen-containing functional groups. Thus it can be concluded that GO was successfully synthesised by oxidation of graphene.
Infrared spectrum of GO.
3.1.2 Raman spectrum
Figure 3 shows the Raman spectrum of GO, in which two characteristic GO peaks around 1347 cm−1 and 1580 cm−1 can be seen.
Raman spectrum of GO (532 nm).
The characteristic peak near 1350 cm−1 is caused by the symmetrical stretching vibration (radial breathing mode) of sp3 carbon atoms in aromatic rings, which is called the D-peak. This peak can be used to measure the degree of chemical change of graphene. The stronger the peak, the worse the integrity of graphene, and vice versa. The intensity of D-peak of GO at 1347 cm−1 is quite distinct, indicating that the graphene plane was modified by oxygen-containing functional groups during the oxidation process, resulting in a significant reduction in the size and integrity of the sp2 hybrid region in the graphene plane.
The characteristic peak at 1580 cm−1 is called the G-peak, which is caused by the tensile vibration of the sp2 carbon atoms. Sending a vibrational mode is the most important feature of graphene materials that can describe the conjugated system of sp2 hybrid orbitals in the plane of graphene materials, i.e. conjugated large π-bond. The G-peak of GO is at 1580 cm−1, while that of graphite was at 1596 cm−1. After oxidation of graphite, the G-peak shifted to the blue and broadened, indicating that GO retained the conjugated π-bond of graphite.
3.1.3 X-ray photoelectron spectroscopy
Figures 4 and 5 show the full XPS spectrum and the carbon spectrum (C 1s) of GO. In the full spectrum two obvious peaks can be seen that belong to the oxygen spectrum and the carbon spectrum respectively. If we then divide the peaks of the carbon spectrum of GO separately using the peak division software, four characteristic peaks are obtained located at (284.8, 286.3, 287.1 and 288.5) eV. This shows that GO contains many oxygen-containing functional groups, which makes GO hydrophilic. Therefore, GO solid particles can be prepared to aqueous suspensions with different contents before being used to form a Pickering emulsion.
X-ray spectrum of GO.
C1s XPS spectrogram of GO.
3.1.4 Environmental scanning electron microscope
The environmental scanning electron microscope image of GO is shown in Figure 6. Figure 6(a) shows the wrinkled surface and curled edges of GO. From Figure 6(b), it is clear that GO exists in a fluffy state with interlayer expansion and some interlayer space. This is because more oxygen-containing functional groups were generated on the surface and edge of GO lamellae after the oxidation reaction of graphite, and the force between GO lamellae was weakened, resulting in interlamellar fluctuation.
Environmental scanning electron microscope images of GO at different scales: (a) 50 μm; (b) 500 μm.
3.1.5 Transmission electron microscope
The transmission electron microscope image of GO is shown in Figure 7. Figure 7(a) shows that the surface and edge of GO are wrinkled and folded. This is due to the fact that the formation of oxygen-containing functional groups destroyed the structure of the C=C bond during the oxidation process. The two-dimensional structure was transformed into a stable three-dimensional structure due to energy instability. As a result, a wrinkled morphology was formed and the free energy of the system was reduced, allowing it to disperse stably. In Figure 7(b) and (c), it can be seen that GO is gauze-like and transparent, indicating that GO has been completely exfoliated and that the GO in dispersion has fewer layers and a smaller thickness, and its thickness has reached the nanometer level.
Transmission electron microscope images of an aqueous GO dispersion at different scales: (a) 300 nm; (b) 200 nm; (c) 80 nm.
3.2 Screening of factors affecting emulsion stability
By recording the emulsion index (EI) and observing the droplet morphology under the microscope, it was found that the GO content has a significant influence on the emulsion stability. The emulsion type was assessed by the dilution method. For this purpose, a small number of emulsion droplets were transferred into a test tube. If the droplets are still in the form of an emulsion in the oil phase (kerosene in this study), but in a dispersion and dissolution of distilled water, an O/W type emulsion is present, otherwise it is W/O type [44].
With the migration of GO from the water dispersion system to the oil-water interface, the surface energy of the system decreased, forming a relatively stable emulsion system. Figure 8 shows the influence of different GO contents on the stability of the emulsion. Figure 9 displays the microscopic image of the emulsion at different GO contents. The emulsion type was evaluated by the dilution method.
Stability of emulsions with different GO content.
Microscopic images of emulsions with different GO contents and results of the evaluation of the emulsion types by dilution method: (a) 0.5‰; (b) 1.0‰; (c) 1.5‰; (d) 2.0‰; (e) 2.5‰; (f) 3.0‰.
When the GO content increased from 0.5‰ to 1.5‰, the EI increased and the droplet gradually became smaller. We observed that within 24 h only a small water phase separated from the emulsion with a GO content of 1.5‰. After one week of storage at room temperature, the EI was still more than 80%, indicating that the emulsion has good stability at room temperature and can be stored for a long time. This may be due to the increased total surface area of the interfacial film formed by the arrangement of GO on the surface of the emulsion droplets. This blocked the collision between droplets in space, increased the mutual repulsive force, effectively prevented the droplets from deformation and coalescence, and maintained the stability of the emulsion [49].
With increasing GO content, GO was no longer adsorbed at the oil-water interface but wrapped on the surface of the spherical droplets. A thicker GO layer formed and the GO particles in the dispersed phase decreased, so that the colour of the water phase at the bottom of the emulsion system gradually became light to colourless. The EI value did not change significantly and the droplets were more dispersed. At the same time, using the dilution method, it was found that changing the GO content did not change the nature of the emulsion. GO contains many oxygen-containing functional groups, such as hydroxyl groups, epoxy groups, etc., especially the marginal carboxylic acid groups, which made it very hydrophilic. GO tended to stabilise the O/W emulsion. In the experiment, the emulsion became O/W type by changing the GO content. The type, emulsifying effect, stability and droplet shape of the emulsion did not change significantly when the GO content was further increased, but the rheological properties of the emulsion did. A further increase of the GO content therefore only leads to an increase in the cost of the emulsification process. In summary, the optimal GO content was (1–2)‰.
Figure 10 shows the stability of emulsions with different contents of octylphenol polyoxyethylene ether-10 (OP-10). Figure 11 shows the corresponding microscope photos and the results for the emulsion type determined by the dilution method. From the results, it can be seen that when the OP-10 content was increased, the emulsion droplets became slightly smaller and the stability of the emulsion was improved as long as the OP-10 content was below 3‰. This could be because OP-10 has a strong polar structure and was adsorbed at the oil-water interface, which reduced the surface tension and at the same time filled the defects of the interfacial film formed by GO through hydrogen bonding. This resulted in a denser and firmer interfacial film and the emulsion became more stable. When the OP-10 content reached 3‰, excess emulsifier molecules were adsorbed on the surface of GO, which reduced the surfactant content in the stable emulsion droplets and decreased the stability of the emulsion. The water phase containing GO and OP-10 was separated and stability was no longer improved by increasing the emulsifier content. It was also found by microscopic observation that the emulsion droplets remained essentially unchanged after the OP-10 content reached 3‰. At the same time, the result of the dilution experiment showed that the OP-10 content could not convert the emulsion type into an O/W type. In summary, (2–4)‰ is the optimal range for the OP-10 content.
Stability of emulsion with different OP-10 contents.
Microscopic images of emulsion with different OP-10 contents and result chart of judging emulsion type by dilution method: (a) 1.0‰; (b) 2.0‰; (c) 3.0‰; (d) 4.0‰; (e) 5.0‰; (f) 6.0‰.
Figure 12 shows the stability of the emulsion at different water-oil ratios, and Figure 13 shows the corresponding microscopic images and photos of the emulsion type assessed by the dilution method. At a water-oil ratio of 1:9, a relatively stable emulsion cannot be formed due to the high proportion of the oil phase, and the morphology of the emulsion did not change after being left to stand. Based on microscopic observation and dilution experiments, it was found that when the oil phase proportion was more than 70%, the emulsion was of the W/O type. When the water phase fraction was increased, the solid particles adsorbed at the oil-water interface were not sufficient to prevent coalescence of the droplets and a phase reversal occurred. All emulsions with a water-oil ratio from 4:6 to 9:1 were of the O/W type. As the water ratio increased, the stability decreased and the droplets became smaller, while the distribution became more disperse. The W/O emulsion had a high viscosity and required a high water injection pressure, which is the reason why we chose the O/W emulsion. In summary, the best water-to-oil ratio was between 4:6 and 6:4.
Stability of emulsion at different water-oil ratios.
Microscopic image of emulsion under different water-oil ratios and result chart of judging emulsion type by dilution method: (a) 1:9; (b) 2:8; (c) 3:7; (d) 4:6; (e) 5:5; (f) 6:4; (g) 7:3; (h) 8:2; (i) 9:1.
3.3 Optimization of emulsion formulation
In this experiment, the response surface method (RSM) was used to optimize the preparation conditions of emulsions. The RSM was obtained by analysis of variance (ANOVA) to determine the maximum value of the objective function. By screening the individual factors (see: Section 3.2), the factors for the experimental design and the horizontal coding values of the reaction surface were obtained and presented in Table 1. The experimental design and results are shown in Table 2.
Levels and codification of the factors (independent variables) used in the optimization of emulsion formulation.
| Factor | Name | Low actual (−1) | Central actual (0) | High actual (1) |
|---|---|---|---|---|
| A | GO (‰) | 1 | 1.5 | 2 |
| B | OP-10 (g/L) | 2 | 3 | 4 |
| C | Water oil ratio | 4:6 | 5:5 | 6:4 |
Experimental design and response results on optimizing emulsion formulation conditions.
| Run | Factor | Emulsification index (%) (Y) | ||
|---|---|---|---|---|
| GO (A) | OP-10 (B) | Water Oil ratio (C) | ||
| 1 | 1 | −1 | 0 | 77.52 |
| 2 | 0 | 1 | −1 | 80.19 |
| 3 | −1 | 0 | −1 | 85.03 |
| 4 | 0 | −1 | 1 | 73.45 |
| 5 | −1 | −1 | 0 | 72.06 |
| 6 | 0 | 0 | 0 | 85.00 |
| 7 | 0 | 1 | 1 | 65.58 |
| 8 | −1 | 1 | 0 | 64.20 |
| 9 | 0 | 0 | 0 | 85.74 |
| 10 | −1 | 0 | 1 | 70.24 |
| 11 | 0 | 0 | 0 | 84.88 |
| 12 | 0 | −1 | −1 | 87.01 |
| 13 | 0 | 0 | 0 | 85.36 |
| 14 | 1 | 1 | 0 | 70.08 |
| 15 | 1 | 0 | −1 | 91.11 |
| 16 | 1 | 0 | 1 | 76.08 |
| 17 | 0 | 0 | 0 | 85.11 |
The data were fitted to a quadratic model, and ANOVA was used to test the significance and sufficiency of the model. The variance analysis results of the final response regression model are shown in Table 3, and the quadratic polynomial regression model of the emulsification index is given by Eq. (4). The reliability of the model for mapping the influences between the parameters can be correctly depicted by the coefficient of determination (R2 = 0.9992) [50]. The accuracy of response prediction of the model can be evaluated by the predicted R2: If the predicted and adjusted R2 values are within approximately 0.20 then it is reasonable for the model or the data [51]. In this experiment, the predicted R2 (0.9930) and the adjusted R2 (0.9981) were reasonably consistent. A signal-to-noise ratio of four or more (adequate precision) was a measure of the predicted response relative to the associated error [52]. The signal-to-noise ratio value of 7.096 in this experiment indicated that the signal was sufficient. The relatively low coefficient of variation (0.45%) was well below 2%, which means that the experiment had higher precision and reliability [53]. The p-value without fitting is 0.4091, which indicated that the F-value was not significant, and the experimental results fit well with the mathematical model [54]. The results showed that the model equation can accurately predict the emulsification index. In addition, the pure error value was low, which indicated that the data had good reproducibility. At the same time, it can be seen from Table 3 that the linear effect and the quadratic term of coefficients A, B and C are all less than 0.05, which shows that they are all important factors affecting the emulsification index [44].
ANOVA for response surface quadratic model of emulsion formulation.
| Source | Sum of squares | DFa | Mean square | F-Value | p-Value |
|---|---|---|---|---|---|
| Model | 1085.48 | 9 | 120.61 | 943.36 | <0.0001 |
| A | 67.63 | 1 | 67.63 | 528.96 | <0.0001 |
| B | 112.43 | 1 | 112.43 | 879.35 | <0.0001 |
| C | 420.36 | 1 | 420.36 | 3287.86 | <0.0001 |
| AB | 0.044 | 1 | 0.044 | 0.34 | 0.5754 |
| AC | 0.014 | 1 | 0.014 | 0.11 | 0.7470 |
| BC | 0.28 | 1 | 0.28 | 2.16 | 0.1855 |
| A2 | 109.42 | 1 | 109.42 | 855.84 | <0.0001 |
| B2 | 352.92 | 1 | 352.92 | 2760.41 | <0.0001 |
| C2 | 1.03 | 1 | 1.03 | 8.06 | 0.0251 |
| Residual | 0.89 | 7 | 0.13 | ||
| Lack of fit | 0.43 | 3 | 0.14 | 1.23 | 0.4091 |
| Pure error | 0.7 | 4 | 0.12 | ||
| Cor total | 1086.38 | 16 |
-
aDegrees of freedom (DF) refers to the number of independent or freely varying data in the sample when estimating the parameters of the population by the statistic of the sample.
Figure 14 shows the three-dimensional response surface of the interaction of various variables. The influence of each factor and its interaction on the response value can be seen intuitively in the response surface diagram. As shown in Figure 14(a), with increasing both, the GO content and the OP-10 content, the emulsification index first increases and then decreases, indicating that an excessive amount of any one of these factors is not conducive to the formation of a stable emulsion. The process of emulsion formation is shown in Figure 15. As the GO content and the OP-10 content increase, the emulsifier OP-10 causes more GO particles to adhere to the spherical droplets by electrostatic adsorption, resulting in a synergistic effect and the formation of a denser interfacial film, which stabilises the emulsion more. When the content of solid particles and emulsifier is increased, solid particles will adhere to the surface of droplets, and excessive emulsifier will adsorb solid particles, which will reduce the stability of emulsion.
(a) Response surface diagram of the interaction between GO content and OP-10 content when the water-oil ratio is kept at 1:1; (b) GO content and water-oil ratio when OP-10 content is kept at 3‰; (c) the OP-10 content and the water-oil ratio when the GO content is kept at 1.5‰.
Scheme of the emulsion forming process.
From Figure 14(b) and (c) it can be seen that the water-oil ratio is also an important factor affecting emulsion stability. With the increase of the oil phase, the stability of the emulsion is improved regardless of the content of GO and OP-10. This is because the polar components in the oil phase can be adsorbed on the surface of the solid particles, resulting in a transition of the wettability of the particles from strongly hydrophilic to moderately wettable. Particles with excessive hydrophilicity form large emulsion droplets that are unstable and coalesce, while particles with moderate wettability have moderate droplet size and high coalescence stability, improving the stability of the oil-in-water emulsion [55]. According to the results of the Box-Behnken design (BBD) experiments, the best reaction conditions are as follows: GO content = 1.39‰, OP-10 content = 2.92‰, and water to oil ratio = 4:6. The experimental results show that the emulsification index is 92.83%, which is close to the predicted value (92.26%), indicating that the predicted result has high reliability and accuracy.
3.4 Performance test of emulsion
Following the optimisation results in Section 3.3, the emulsion with the best preparation conditions was selected for the performance test to determine its suitability for medium and low permeability EOR deposits.
Figure 16 shows the diagram of the backscattered light from the emulsion. The abscissa indicates the height of the sample cell, the ordinate on the left the intensity of the scattered light and the ordinate on the right the sampling time corresponding to the curves of different colours in the diagram. The curves in the diagram were sampled at regular intervals of (0–24) h. The left side of the diagram shows the liquid separated from the bottom of the sample tube. The intensity of the backscattered light gradually decreased with increasing time, indicating that a dark aqueous solution was separated from the bottom of the sample cell. The right-hand side shows the liquid separated from the top of the sample tube, and the intensity of the backscattered light was slightly increased, indicating that there was less colourless oil phase in the top of the sample cell. The light curves were essentially consistent with the middle part of the sample cell, indicating that the particles in the emulsion had not agglomerated and the emulsion remained stable. Figure 17 shows the system stability index TSI as a function of time. The curve also shows that the emulsion has good stability at 60 °C.
The backscattering spectrum of emulsion.
The relationship between TSI and time.
As the depth of drilling increases, the formation temperature and salinity of formation water also increase. Therefore, it is necessary to investigate the tolerance of emulsion under high-temperature and high-salt conditions.
The main ions contained in oilfield formation water are Na+ and Ca2+. Defined amounts of solid NaCl and CaCl2 were added to the water phase to adjust the salinity to (2500, 5000, 7500, 10,000, 12,500, 15,000, 17,500, 20,000, 22,500, and 25,000) mg/L. Then the oil phase was added, stirred and poured into containers. After the containers were tempered in a water bath at 60 °C for 48 h, they were quickly dried and the stratifications of the emulsion were recorded. The results can be seen in Figure 18. It is obvious that when the salt content is less than 10,000 mg/L, the emulsion index increases slightly. The reason for this is that according to the DLVO theory, a sufficient amount of electrolyte in the water phase increases the number of counterions in the vicinity of GO and shields the electrical double layer, resulting in a decrease in electrostatic repulsion between GO and the micro-flocculation of GO. This has a positive effect on the stability of the Pickering emulsion. At the same time, the oil-water interface increases to GO, GO is adsorbed at the interface to form a solid interfacial film, and the stability of the emulsion is slightly increased. However, if the electrolyte is too strong, the solid particles agglomerate and the solubility of the surfactant decreases due to the salting out effect, which greatly decreases the content of active particles in the stable interfacial film and the stability of the emulsion. At a salt content of 25,000 mg/L, the emulsification index of the emulsion system can still be maintained at about 80%, showing that the emulsion system has good salt tolerance and can meet the requirements of oil displacement at high salt formation for profile control and as a flooding agent.
Stability of emulsion at constant temperature of 60 °C and at different salinities.
In order to investigate the temperature stability of the emulsions, the prepared emulsions were placed in a water bath whose temperature had been set to (30, 40, 50, 60, 70, 80, 90, and 100) °C. After 48 h, the containers were quickly dried and the stratifications recorded.
Figure 19 shows the stability of the emulsion at the different temperatures. As the temperature increased, the stability of the emulsion decreases slightly. This is due to the fact that the temperature increase destroys the molecular structure of some surfactants, and damages the originally compact interfacial film. When the temperature was increased to 100 °C, the emulsification index was still larger than 75%. The reason for this is that the adsorption of solid GO at the water-oil interface is irreversible, which greatly increases the strength of the interfacial film and improves the temperature stability of the emulsion, allowing its use in high-temperature reservoir conditions.
Stability of emulsion at different temperatures.
3.5 Emulsion flooding
According to the core displacement experiment, the emulsion flooding can increase the oil recovery by 15.41% compared to flooding with injected water (see Table 4). We believe that the reason for the good oil displacement effect can be analysed under the two aspects of emulsion composition and stability.
Experimental results of emulsion profile control and flooding.
| Material | Core permeability (D) | Initial oil saturation (%) | Recovery ratio (%) | Enhanced recovery ratio (%) | |
|---|---|---|---|---|---|
| Water drive | Finally | ||||
| Emulsion | 0.153 | 79.44 | 20.23 | 35.64 | 15.41 |
From the analysis of the emulsion composition, it appears that the emulsion contains GO particles, some of which are adsorbed in the pores of the reservoir rock, changing the rock pore surface from lipophilic to hydrophilic. In addition, the washout of the displacing agent peels off some of the crude oil adsorbed from the rock surface itself, reducing the remaining oil saturation and thus improving the oil recovery rate. The O/W emulsion can disperse the dispersed oil droplets in the continuous water phase, and a large amount of the water phase carries dispersed oil droplets into the reservoir. During the flow process, it flows in the porous medium with the remaining oil that has not been recovered in secondary oil recovery and stirs at a high speed. Finally, the remaining oil and the oil-in-water system form a new oil-in-water system, which can greatly improve the macro sweep efficiency. When the droplets formed by the emulsion pass through the large pore channel, they produce the Jamin effect, which increases the additional resistance of the emulsion after passing through the throat, and plays the role of temporarily blocking the throat of high permeability pores.
Simultaneously, the stable emulsion tested in Section 3.4 can meet the requirements of an oil displacement agent for high temperature and salty environments at medium and low permeability and remain stable for a long time when flowing after injection, thus achieving a significant improvement in oil recovery.
4 Conclusions
Graphene oxide (GO) was prepared by the improved Hummers method, and the synergistically stabilized emulsion properties of GO and emulsifier and its application in chemical flooding for enhanced oil recovery were studied. Single factor experiment proved that the best content range of GO was (1–2)‰; the optimum content range of OP-10 was (2–4)‰; the optimum range of water-oil ratio was 4:6–6:4. The content of GO and emulsifier did not change the type of emulsion. All emulsions were O/W type. When the proportion of oil phase exceeded 70%, the emulsion turned into W/O type. Taking the emulsification index (EI) as the quantitative index, the response surface method (RSM) was used to model and optimize the best formula of emulsion: When GO content was 1.39‰, OP-10 content was 2.92‰, and the water-oil ratio was 4:6, the emulsification index reached 92.83%. The stability of the emulsion with the best formula was tested and its temperature and salt resistance were investigated. Analysis with Turbiscan Lab stability analyser methods showed that the emulsion had good stability. Even at a salt content of 25 g/L, the emulsification index can still be kept at about 80%. When the temperature reached 100 °C, the emulsification index can still reach more than 75%, which met the environmental requirements of high temperature and salt content in medium and low permeability reservoirs. The core displacement experiment showed that emulsion flooding can increase oil recovery by 15.41%, and the experimental results provided a new idea for the application of graphene-based materials in profile control and flooding of medium-low permeability, high temperature and salt reservoirs.
About the authors
Xin Li is a graduate student in the Department of Chemistry, School of Science, Tianjin University. Areas of scientific interest include emulsifier flooding, and flooding from medium and low permeability reservoirs.
Yang Chen is a graduate student at the Department of Chemistry, School of Science, Tianjin University. Areas of scientific interest include the application of polyacrylamide in oil flooding, and graphene oxide flooding.
Yuqin Tian is a researcher at the Petroleum Engineering Technology Research Institute of Sinopec Shengli Oilfield Branch. Areas of scientific interest include deep profile control and enhanced oil recovery.
Shuang Zheng is a graduate student at the Department of Chemistry, School of Science, Tianjin University. Areas of scientific interest include the modification of graphene oxide and its application in oil fields.
Rongjiao Zhu is an associate professor at the Department of Chemistry, School of Science, Tianjin University. Areas of scientific interest include synthesis and application of oilfield chemicals, controllable chemical transformation of nanomaterials, and energy catalysis.
Xia Feng is an associate professor at the Department of Chemistry, School of Science, Tianjin University. Areas of scientific interest include nano-self-assembled drug delivery systems, preparation and application of nano-functional materials.
Cunhui Liu is a researcher at Tianjin Dagang Oilfield Bingang Petroleum Technology Group Co. Ltd. Areas of scientific interest include chemical enhanced oil recovery, preparation of flood control agents.
Yichen Zhang is an undergraduate student at the Department of Chemistry, School of Science, Tianjin University.
Jingyi Chen is an undergraduate student at the Department of Chemistry, School of Science, Tianjin University.
Acknowledgements
The authors acknowledge Tianjin University for offering chemicals and allowing researchers to use laboratories and working instruments and devices.
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Author contributions: All the authors have accepted responsibility for the entire content of this submitted manuscript and approved submission.
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Research funding: This work was supported by Sinopec’s Key Scientific and Technological Research Project “Development of Modified Graphene Oxide Control and Displacement System” (No. 2020GKF-0657).
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Conflict of interest statement: The authors declare non conflicts of interest regarding this article.
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