Qin Bing, Qiao Fulin, Li Caifu

(SINOPEC Research Institute of Petroleum Processing, Beijing 100083)

Abstract: A novel micro-emulsion was prepared by mixing an oil-soluble viscosity reducer, which was screened to aim at improving the heavy oil properties of Shengli oilfield with water-soluble surfactant and co-surfactant. The static viscosity reduction and oil washing performance of the micro-emulsion were investigated, and the field application of the microemulsion used as heavy oil displacement agent was also reported. Results showed that the micro-emulsion exhibited excellent viscosity reduction performance for the studied heavy oil samples. When heavy oil was mixed with 0.5% of the micro-emulsion, a stable oil-in-water heavy oil emulsion could be formed. After the content of the micro-emulsion was increased to 3.0%, the oil removing rate reached up to 80%. Field application of the micro-emulsion to the Pai-601-Ping-115 well and the Pai-601-Ping-123 well was shown to be effective by increasing the periodic oil production up to 203 tons.

Key words: micro-emulsion, heavy oil displacement agent, field application

1 Introduction

Heavy oil has a high content of asphaltenes and resins,contributing to the characteristics of high viscosity and density of heavy oils, which can greatly hinder the oil recovery process[1-4]. Currently, chemically enhanced steam soaking is one of the common and effective methods for recovery of heavy oil[5-6]. However, most developed heavy oil reservoirs of China are experiencing the stage of multi-cycle huff-n-puff with low oil recovery rate. The oil/gas ratio of some heavy oil reservoirs is less than 0.25, which is below the economic limit of steam soaking. At the same time, the production of new developing reservoirs cannot bridge the demand-supply gap, resulting in a decline of heavy oil production from steam soaking. Thus, developing an effective chemical heavy oil displacement system for steam soaking is exceedingly crucial to enhance the heavy oil recovery.

Viscosity reducing chemicals, including oil-soluble viscosity reducers and water-soluble emulsifiers, are commonly used as heavy oil displacement agents. Oilsoluble viscosity reducers are widely used to improve the heavy oil production performance of steam soaking[1,7-9].However, due to the strong volatility of oil-soluble viscosity reducer, injecting the viscosity reducer before steam injection usually leads to a low viscosity reduction rate. Thus, to achieve effective injection of the oilsoluble viscosity reducer is a key problem that has to be considered. On the other hand, surfactants often act as water-soluble emulsifiers for heavy oil, forming O/W emulsions and greatly decreasing the viscosity of heavy oil[2,10-11]. However, as a kind of water-soluble agents,surfactants are usually injected as an aqueous solution.Therefore, to make surfactant achieve full contact with heavy oil under formation conditions is also a problem to be solved.

Aiming at the problems encountered during the thermal recovery of heavy oil in Shengli oilfield, a novel microemulsion is prepared by mixing an oil-soluble viscosity reducer with water-soluble surfactant and co-surfactant to improve the effect of steam huff-n-puff induced thermal recovery of heavy oil[12]. The micro-emulsion is injected into the heavy oil reservoir together with steam, so that the steam, the oil-soluble viscosity reducer, and the watersoluble surfactant can interact synergistically to greatly improve the oil displacement efficiency and the sweep efficiency of steam.

2 Experimental

2.1 Heavy oil properties

Heavy crude oil samples were supplied by the Chen 373-3 well and the Linzhong well of Shengli oilfield.After being measured with a HAAKE VT 550 rotational viscometer at 50 °C, the viscosity of the Chen 373-3 heavy oil and the Linzhong heavy oil was 29312 mPa·s and 2136 mPa·s, respectively.

2.2 Preparation of micro-emulsion

Surfactant solution at a certain concentration was prepared and transferred into a 50-mL glass tube. The screened oil-soluble viscosity reducer was injected at different proportions and the mixture was fully oscillated.Afterwards, the co-surfactant was added into the mixture,which was subjected to oscillating until the system became clear. Thus, a micro-emulsion was obtained.

2.3 Preparation of oil in water (O/W) emulsion

The O/W emulsion samples were prepared as follows:100 g of heavy oil were placed in a constant temperature water bath at 70 ℃ for 1—2 h, and 43 g of aqueous solution of the heavy oil reducing agent were added. The heavy oil was slowly stirred with a glass rod for about 4 min, resulting in the formation of an O/W emulsion.

2.4 Viscosity reduction rate

The viscosity of heavy crude oil samples and O/W emulsion samples was measured with a HAAKE VT 550 rotational viscometer. The samples to be tested were injected into the external cylinder of the viscometer, and were stirred at a constant temperature. An appropriate rotor, which was selected according to the viscosity range of the sample to be tested, was inserted into the external cylinder. The shear rate of the rotor was set at a certain speed, and the data was recorded once the data had become stable. The ratio of shear stress to shear rate is the viscosity of the sample. The viscosity reduction rate is calculated based on the followingformula:

Viscosity reduction rate = (Viscosity of heavy oil -Viscosity of water in oil emulsion)/ Viscosity of heavy oil.

2.5 Oil washing performance

A certain amount of quartz sand and oil samples, which were preheated at 60 ℃, were measured and stirred evenly in a beaker. The mixture was kept at 60 °C for 6 h.Then, an aqueous solution of micro-emulsion was added to be subject to soaking at 60 °C for 12 h, and then the free oil was removed by filter. The remaining oil sand mixture was kept at 50 °C for removing the residual water,and the weight of dry oil/sand mixture was measured.The oil washing efficiency was calculated according to the following formula: Oil washing efficiency = (m2-m3+ m1-m4)/m2, where m1is the weight of quartz sand (g),m2is the weight of oil sample (g), m3is the weight of dry oil/sand mixture (g), and m4is the difference of the oil sample weight in the blank test (g).

3 Results and Discussion

3.1 Screening of the oil-soluble viscosity reducer

The presence of asphaltenes and resins is considered as one of the key points to viscosity rise of heavy oil[13-15]. As the heaviest fraction of heavy oil, asphaltene is a complex mixture of different components the exact molecular structure of which has not been well established. The main characteristics of asphaltenes are the presence of a polycyclic aromatic scaffold, and the strong tendency to associate into larger molecular aggregates. The degree of dispersion or aggregation of asphaltenes in crude oil depends not only on its mass fraction but also on other components in crude oil. To give full play to the viscosity reducing agent, an ideal appropriate dispersant is needed to provide synergistic interaction with the viscosity reducer.Therefore, the effects of different kinds of chemicals on the viscosity of asphaltenes were investigated first.

3.1.1 Viscosity reduction performance of oil-soluble viscosity reducers

The test asphaltene sample was first heated to 80 °C to make it flow, then the test oil-soluble viscosity reducers were added at a certain ratio. After being preheated at 60 °C for more than 12 hours, the viscosity test of the mixed sample was carried out.

As shown in Table 1, the viscosity of asphaltenes is reduced obviously with the addition of toluene and xylene, indicating that these solvents can bring about good dispersive effect on asphaltenes. The #5 oil-soluble viscosity reducer, which showed the best viscosity reduction effect, was verified through further screening the appropriate solvents and then the surfactant was mixed with the solvent. By using 5% of the #5 viscosity reducer,the heavy oil viscosity could be reduced by 86.4%.

Table 1 Viscosity reduction effects of oil-soluble viscosity reducers

Figure 1 and Figure 2 show the rheological curves of asphaltenes before and after adding #5 oil-soluble viscosity reducer. After adding the viscosity reducer, the starting pressure gradient is decreased, and the rheological behavior of asphaltenes belongs to a Newtonian fluid.

Herein, the addition of viscosity reducer can destroy the structure of colloid and asphaltenes, and the intermolecular viscous force decreases, leading to disassociation of the aggregates formed by molecular overlapping of asphaltenes. Meanwhile, the aggregate size and the volume of dispersed phase both become smaller, and the volume of continuous phase tends to be larger, causing the reduction of the viscosity and the improved flow properties.

Figure 1 Plots of viscosity versus shear rate at different temperature for asphaltenes

Figure 2 Plots of viscosity versus shear rate at different temperature for asphaltenes with addition of 10% of oilsoluble viscosity reducer◆—90 °C; ■—80 °C; ▲—70 °C; ★—60 °C; ?—50 °C; ●—40 °C

As described above, the screened #5 oil-soluble viscosity reducer shows an excellent effect on reducing the viscosity of asphaltenes. Since the asphaltenes content in heavy oil is very high, the viscosity reducing effect of these oil-soluble viscosity reducers on heavy oil was further studied.

Table 2 The effect of adding 10% of oil-soluble viscosity reducers on reduction of heavy oil viscosity

As shown in Table 2, the #5 and #2 viscosity reducer samples exhibit an obvious viscosity reducing effect on Chen 373-3 heavy oil, achieving a viscosity reduction rate of up to 85% with the addition of 10% of viscosity reducer. In contrast, gasoline and other light hydrocarbons are less effective. According to the dissolution mechanism of asphaltenes in heavy oil, the self-association of asphaltenes in non-aqueous system could be weakened and the asphaltenes could be highly dispersed with the addition of viscosity reducer, contributing to the reduction of heavy oil viscosity.

Effect of the viscosity reducer content on heavy oil viscosity was investigated, with the results shown in Table 3.

Table 3 Effect of the viscosity reducer content on viscosity reduction rate of heavy oil

As shown in Table 3, the viscosity reduction rate of Chen 373-3 heavy oil reaches 89.3% with the addition of 10%of the #5 viscosity reducer, indicating that the excellent viscosity reducer for asphaltenes could also achieve superior viscosity reduction performance on heavy oil.The addition of aromatic oil-soluble viscosity reducer into the oil phase could decrease the concentration of asphaltenes, and the dispersion of asphaltenes is greatly improved, resulting in the reduction of crude oil viscosity.This mechanism is reminiscent of the dilution of crude oil by using light oil.

3.1.2 Effect of temperature on viscosity reduction performance

The viscosity of Chen 373-3 heavy oil at 30—80 °C after adding different oil-soluble viscosity reducers was investigated. As shown in Table 4, the temperature has little effect on the viscosity reduction performance, #2 and #5 viscosity reducers both demonstrate high viscosity reduction rate at low and high temperatures.

3.1.3 Effect of formation water on viscosity reduction performance

The viscosity reduction performance was evaluated using water-containing Chen 373-3 oil samples (23.27% ofwater), with the results shown in Table 5. The salinity of formation water has little effect on the viscosity reducing performance of heavy oil, indicating the excellent compatibility of the viscosity reducer with the formation water.

Table 4 Effect of temperature on viscosity reduction performance

3.1.4 Effect of oil-soluble viscosity reducer on viscosity and rheological properties of heavy oil

Figure 3 The viscosity-temperature curves of heavy oil samples before and after the addition of 5% of #2 viscosity reducer▲—Chen 373-3 heavy oil; ●—Chen 373-3 heavy oil with 5% viscosity reducer;◆—Chen 375 heavy oil; ■—Chen 375 heavy oil with 5% viscosity reducer

The viscosity-temperature curves of heavy oil samples before and after adding 5% and 10% of #2 viscosity reducer are shown in Figure 3 and Figure 4, respectively.Regression analysis of viscosity-temperature curves of heavy oil samples before and after the addition of viscosity reducer showed that these curves are in good agreement with the Arrhenius equation: η=AeΔE/RT=AeB/T,where η is the apparent viscosity (Pa·s), R is the universal gas constant, T is the absolute temperature (K), ΔE is the activation energy (J/mol) of the reaction, A is a constant,and B is the ratio of ΔE and R.

Table 5 Effect of formation water on viscosity reduction performance

Figure 4 The viscosity-temperature curves of heavy oil samples before and after the addition of 10% of #2 viscosity reducer▲—Chen 373-3 heavy oil; ■—Chen 373-3 heavy oil with 10% viscosity reducer;●—Chen 375 heavy oil; ◆—Chen 375 heavy oil with 10% viscosity reducer

Herein, the activation energy is a measure of the internal friction of the fluid molecules, which depends on the polarity, molecular weight, and molecular configuration of the fluid molecules. In order to form the pore which is large enough for the transportation of the molecules,the molecules of the flowing fluid must overcome the activation energy barrier.

By fitting these viscosity-temperature curves of heavy oil samples, following equations are derived for different heavy oil samples:

Chen 373-3 heavy oil:

Chen 373-3 heavy oil after the addition of 5% of viscosity reducer:

Chen 373-3 heavy oil after the addition of 10% of viscosity reducer:

Chen 375 heavy oil:

Chen 375 heavy oil after the addition of 5% of viscosity reducer:

Chen 375 heavy oil after the addition of 10% of viscosity reducer:

Herein, as a measure of the activation energy, the ΔE/R value usually increases with an increasing heavy oil viscosity. As shown in equations (1)—(3), the ΔE/R value of Chen 373-3 heavy oil decreases from 11 624 to 10 642 and then to 9112.8 with the addition of 0, 5%, and 10% of viscosity reducer, respectively. Correspondingly,equations (4)—(6) show that with the addition of 0, 5%,and 10% of viscosity reducer, the ΔE/R value of Chen 375 heavy oil decreases from 11 554 to 10 124 and then to 8 896, respectively. Obviously, after the addition of the viscosity reducer, the activation energy barrier for the flowing of fluid molecules becomes smaller due to the decrease of the molecular weight of fluid molecules.Thus, with the addition of oil-soluble viscosity reducer,the asphaltene-resin aggregates disassociate into small molecules, achieving the effective reduction of heavy oil viscosity.

The rheological curves of Chen 373-3 crude oil before and after the addition of oil-soluble viscosity reducer are shown in Figures 5 and 6.

Figure 5 Plots of shear force versus shear rate at different temperature for Chen 373-3 heavy oil■—40 °C; ▲—50 °C; ●—60 °C; ◆—70 °C; ▼—80 °C

Following equations are obtained by linear regression of the rheological curve at 50 °C:

The rheological equation of Chen 373-3 heavy oil:

y-0.055=29.25x

The rheological equation of Chen 373-3 heavy oil after adding 5% of #2 viscosity reducer:

y-0.71=12.169x

Figure 6 Plots of shear force versus shear rate at different temperature for Chen 373-3 heavy oil with the addition of 5%of 2# viscosity reducer■—40 °C; ▲—50 °C; ●—60 °C; ◆—70 °C; ▼—80 °C

As shown in Figure 5 and Figure 6, the flow performance of heavy oil is improved and the starting pressure is decreased due to the reduction of heavy oil viscosity after adding oil-soluble viscosity reducer.

3.2 Preparation of the micro-emulsion

Due to the strong volatility of oil-soluble viscosity reducer, injecting the viscosity reducer before the steam injection usually leads to a low viscosity reduction rate.Thus, to achieve effective injection of the oil-soluble viscosity reducer is a key problem that has to be solved.Meanwhile, the oil-soluble viscosity reducer is commonly used at a high concentration, and constant attention should be paid on how to achieve a highly efficient viscosity reducing performance at lower concentration.Herein, the oil-soluble viscosity reducer and the surfactant are mixed rationally to obtain a novel product[16]. Thus,synergism between different components can be achieved,and the field application of the product can be carried out conveniently.

Figure 7 shows the phase boundaries of the microemulsion, which were derived from the ternary phase diagram of surfactants, co-surfactants and oil-soluble viscosity reducer.

3.3 Performance of micro-emulsion as oil displacement agent

3.3.1 The static viscosity reduction performance of the micro-emulsion

Figure 7 Ternary phase diagram of surfactants, cosurfactants and oil-soluble viscosity reducer

Table 6 Viscosity reduction performance of the microemulsion

Figure 8 Microscopic images of the O/W emulsion for Chen 373-3 heavy oil with the addition of 1%of micro-emulsion

The viscosity reduction performance of the microemulsion is shown in Table 6. The viscosity of Linzhong heavy oil is reduced by more than 90% with the addition of 0.5% of micro-emulsion.Size distribution of the O/W emulsion for Chen 373-3 heavy oil with the addition of 1% of micro-emulsion was determined by a microscope. As shown in Figure 8, with the addition of 1% of micro-emulsion, the size of the oil phase droplets in the O/W emulsion for Chen 373-3 heavy oil is about 5—100 μm.

3.3.2 Oil washing performance of the micro-emulsion

The washing performance of the micro-emulsion for Chen 373-3 heavy oil is shown in Table 7.

As shown in Table 7, the washing efficiency is 66%with the addition of 3% of oil-soluble viscosity reducer and is 80% with the addition of 3% of microemulsion. Meanwhile, the removal ratio of heavy oil is 82% with the addition of 5% of oil-soluble viscosity reducer and is 89% with the addition of 3%of micro-emulsion. Therefore, relative to using oilsoluble viscosity reducer alone, adding the microemulsion, which is prepared by mixing the oil-soluble viscosity reducer with surfactants and co-surfactants,could achieve higher washing efficiency. By focusing on breaking the strong cohesion of heavy oil, a hydrophobic microdomain could be formed by the oilsoluble viscosity reducer, in order to reduce the heavy oil viscosity through dispersion and solubilization of asphaltenes and resins. Moreover, the heavy oil is converted into O/W emulsion through emulsification by the surfactant. Thus, oil washing efficiency is effectively improved by the synergistic interactions of different ingredients in micro-emulsion.

Table 7 Oil washing performance of the micro-emulsion.

3.4 Field application of the micro-emulsion as oil displacement agent

The block Pai-601 is located in the western margin of Junggar Basin in Xinjiang Uygur Autonomous Region.Crude oil of the block Pai-601 belongs to a super heavy oil at a formation temperature of 28 °C, and the viscosity of degassed crude oil falls into the range from 50 000 to 90 000 mPa·s. The concentration of chloride ions in formation water is 21 380 mg/L, and the total salinity is 34 911 mg/L.

Field applications of micro-emulsion were carried out in two wells of the block Pai-601 based on the above experiments.

(1) Implementation of pre-slug of micro-emulsion for thermal oil recovery in Pai-601-Ping-115 well.

Before thermal recovery, 5 tons of micro-emulsion product were diluted into 200 m3of aqueous solution,which was injected into the well by pump truck at a speed of 10 m3/h. Then, thermal recovery was carried out by injecting 1 500 m3of steam.

As shown in Figure 10, after the introduction of the micro-emulsion, the daily fluid level of the producing well was 40 m3/d and the daily peak oil level reached 10 t/d, both indicators were significantly improved compared with the previous cycle. The cumulative oil production in this cycle was 472.8 tons, while the cumulative oil production from last cycle was 269 tons, resulting in an ideal periodic oil increase of 203 tons.

(2) Implementation of pre-slug of micro-emulsion for thermal oil recovery in Pai-601-Ping-123 well.

Before thermal recovery, 5 tons of micro-emulsion product was diluted into 200 m3of aqueous solution,which was injected into the well by a pump truck at a speed of 10 m3/h. Then, thermal recovery was carried out by injecting 1 500 m3of steam.

The production curves before and after thermal recovery is shown in Figure 11. After the implementation of the micro-emulsion, the daily fluid level of the producing well was 40 m3/d and the daily peak oil level reached 20 t/d, both indicators were obviously improved compared with the previous cycle. The cumulative oil production in this cycle was 592 tons, while the cumulative oil production from last cycle was 489 tons, resulting in an ideal periodic oil increase of 103 tons.

4 Conclusions

(1) Oil-soluble viscosity reducer suitable for thermal recovery of Shengli heavy oil is screened. With the addition of the screened oil-soluble viscosity reducer,the viscosity and rheology of heavy oil are obviously improved, and the initial starting pressure of the investigated heavy oil is decreased significantly.

(2) A novel micro-emulsion was prepared by mixing the oil-soluble viscosity reducer with surfactant and cosurfactant. The thermal recovery performance of Pai-601 heavy oil is effectively improved with the addition of the micro-emulsion at 150 °C and 5 MPa, achieving a maximum displacement efficiency of 85.4%.

Figure 10 Production curves before and after thermal recovery in Pai-601-Ping-115 well

Figure 11 Production curves before and after thermal recovery in Pai-601-Ping-123 well

(3) Field application of the micro-emulsion used as the heavy oil displacement agent in block Pai-601 was shown to be effective. After the injection of the micro-emulsion,the cycle production time and peak oil production both increased obviously, and the periodic oil production of Pai-601-Ping-115 well and Pai-601-Ping-123 well was increased by 203 t and 103 t, respectively.

Acknowledgements: This work was supported by the 13th Fiveyear Plan National Key Project of China (No. 2016ZX05011-003-004 and No. 2017ZX05049-003-008).