This study aims to analyze the influence of viscosity and interfacial tension (IFT) on the recovery in heterogeneous reservoir and determines the main controlling factors of the polymer-surfactant (SP) flooding. The influence of the salinity and shearing action on the polymer viscosity and effects of the surfactant concentration on the IFT and emulsion behavior between chemical agent and oil were studied through the static and flooding experiments. The results show that increasing the concentration of polymer GF-11 (HPAM) can reduce the influence of the salinity and GF-11 has high shear-resistance property. In the condition of the Jilin Oilfield, the oil/water IFT can reach 10−3 mN/m when the surfactant concentration is 0.3 wt%. The lower the IFT is, the easier the emulsion of SP and oil is formed. Seven flooding experiments are conducted with the SP system. The results show that the recovery can be improved for 5.02%–15.98% under the synergistic effect of the polymer and surfactant. In the heterogeneous reservoir, the contribution of oil recovery is less than that of the sweep volume.
Polymer can increase the viscosity of injected fluid, decrease the mobility ratio of water and oil, and then expand the sweep coefficient [
SP flooding could enhance oil recovery because polymer can increase sweep efficiency and surfactant can improve the oil displacement efficiency. The polymer viscosity influences the sweep efficiency and the displacement efficiency depends on the IFT. The viscosity is influenced by the salinity, temperature, absorption, and shearing action [
According to the characteristics of Jilin Oilfield, a series of static experiments were carried out to study the influence of salinity and shearing action on the viscosity; other experiments are done to analyze the effect of surfactant concentration on the IFT and emulsion. According to the core flooding experiments, the influence of viscosity and IFT on the recovery is discussed. And the main controlling factors of SP flooding are analyzed. The results provide the basis on the optimization of the chemical and the injecting method.
The core holder is 30 cm long, which hold the core with external pressure that is 1-2 MPa more than the inlet pressure. Other equipment includes a flowmeter, DV viscometer, high pressure middle vessel, automatic metering plunger pump, thermotank, pressure acquisition system, and constant flow pump. The pressures are recorded by the pressure acquisition system and the output liquid is collected to calculate the recovery.
The mother liquor of polymer and surfactant are prepared by the injection water; the salinity of injection water is shown in Table
Composition of injected water.
Ions |
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|
|
|
|
|
|
TDS |
---|---|---|---|---|---|---|---|---|
Content mg/L | 2043 | 36 | 39 | 1347 | 24 | 3126 | 135 | 6750 |
The two-dimensional vertical heterogeneous model.
The polymer is GF-11 produced by Daqing Assistant Factory, and GF-11 is partially hydrolyzed polyacrylamide (HPAM). The average relative molecular weight of GF-11 is 2500 × 104, the degree of hydrolysis is 25%, and the solid content is 88%. The surfactant is anionic sulfonate, and the effective content is 40 wt%, 24 wt% is unsulfonated oil, 33 wt% is volatile content, and 3.0 wt% is inorganic salt. The average relative molecular weight is 645.
SP systems were prepared by the polymer whose relative molecular weight is 2500 × 104, the surfactant, and different composition water. The composition of crude oil is shown in Table
Composition of crude oil.
Crude oil | Hydrocarbon | Aromatic Hydrocarbons | Resin | Asphaltene |
---|---|---|---|---|
Content, wt% | 59.9 | 22.4 | 4.3 | 8.0 |
The viscosity of SP system is measured by the HAAKE rotary rheometer, under the shearing rate of 7.31 s−1. IFT is measured by TX500 at 1000 r/min after 120 min under 45°C.
The emulsion is prepared by the chemical and crude oil stirring at the rate of 1500 r/min for 1 minute at 45°C. The volume ratio of oil and water is 3 : 7. The micromorphology of emulsion is observed by the ZEISS SteREO Discovery.V20 microscope. In the emulsion stability experiment, putting the emulsion into the glass tube, the oil and water will separate from each other because of the density difference. The water separating proportion is defined as the ratio of the separated water volume and the original water volume after separating 2 hours and it is used to evaluate the stability of emulsion; the reciprocal of stability constant of Turbiscan Lab Expert (Formulaction Inc.) is used to evaluate the stability of emulsion at the reservoir temperature. The bigger the TSI−1 is, the more stable the emulsion is.
At the temperature of 45°C and the constant flow of 0.1 mL/min, the experiments of water flooding, surfactant flooding, polymer flooding, and SP flooding are conducted. The experiment steps are as follows:
(1) Measure the dry weight, length, and diameter of the core; the permeability is measured by
(2) Vacuum the core, saturate it with the reservoir water, weigh the wet weight, and calculate the porosity. Then, saturate the core with oil and calculate the original oil saturation.
(3) Displace the oil in the core with the reservoir water; measure the output oil, water, and pressure until the water cut of the output liquid is 98%. After the water flooding, the chemical flooding is done until the water cut is 98%.
The SP solution is prepared by the injection water of the block, and the salinity of injected water is less than that of the original formation water. Although after long-term water flooding, the salinity of formation water is nearly that of the injected water. But due to mobility control of the polymer, the injected chemical agents can sweep the area which is unsweep during water flooding. These areas are still high salinity, so the viscosity of polymer would reduce and then oil recovery decreases. Therefore it is necessary to evaluate the influence of salinity on the viscosity of polymer solution.
Figure
Viscosity of SP solution with different salinity when polymer concentrations are 1500 mg/L and 2000 mg/L.
In order to reduce oil-water mobility ratio effectively and enhance oil recovery, polymer solution in the formation needs high viscosity. But when polymer solution is injected, it has to pass through pump, valve, pipeline, perforation, and the pore and throat of rock and the shearing action during flooding will change the size of polymer molecule. In order to simulate the effect on the viscosity when the polymer solution is affected by the mechanical shear, inject polymer solution into one-meter long cores with the concentration of 1500 mg/L, 2000 mg/L, 2500 mg/L confected by injected water and formation water. Take a sample at the exit of the core, determine the viscosity of produced fluid at 45°C, and evaluate the shear resistance of polymers.
The results are shown in Table
The viscosity of the polymer solution under different conditions.
Polymer concentration mg/L | Injected water | Formation water | ||
---|---|---|---|---|
Viscosity before shearing, mPa⋅s | Viscosity after shearing, mPa⋅s | Viscosity before shearing, mPa⋅s | Viscosity after shearing, mPa⋅s | |
1500 | 39.67 | 28.96 | 30.14 | 20.29 |
2000 | 51.64 | 37.26 | 42.31 | 28.86 |
2500 | 72.46 | 54.16 | 59.47 | 42.41 |
As SP system has not the synergistic effect of alkali, surfactant is the key to enhance oil recovery. Excellent surfactant can keep the IFT ultralow (10−3 mN/m) [
The results are shown in Figure
Minimum IFT between surfactant solution and oil with different surfactant concentrations.
Emulsification is a common phenomenon in the SP flooding and also an important mechanism for EOR [
In Figure
Relationship of TSI−1 and separated water ratio when surfactant concentration varies. The polymer concentration is 2000 mg/L.
Figure
Micrographs of emulsions with different chemical agent: (a) 0.1 wt% surfactant + 2000 mg/L polymer; (b) 0.2 wt% surfactant + 2000 mg/L polymer; (c) 0.3 wt% surfactant + 2000 mg/L polymer.
Distribution of emulsion droplet with different chemical agent.
Under the reservoir temperature of 45°C, the heterogeneous cores of three layers are used for the SP flooding experiments. The chemical agents were prepared by the injection water. The results of polymer flooding, surfactant flooding, and SP flooding are shown in Table
Experiment data of oil displacement efficiency.
Number | Polymer concentration | Surfactant concentration | Viscosity, mPa⋅s | IFT mN/m | Water flooding Recovery, % | chemical flooding Recovery, % | Incremental Recovery, % |
---|---|---|---|---|---|---|---|
(1) | 1500 | 0 | 39.67 | — | 39.98 | 49.21 | 9.23 |
(2) | 0 | 0.3 | 0.67 |
|
40.42 | 43.35 | 2.93 |
(3) | 1500 | 0.3 | 40 |
|
40.15 | 53.55 | 13.40 |
(4) | 2000 | 0.3 | 51.64 |
|
41.11 | 55.09 | 13.98 |
(5) | 1500 | 0.2 | 39.47 |
|
38.97 | 50.08 | 11.11 |
(6) | 1500 | 0.4 | 38.79 |
|
40.34 | 53.86 | 13.52 |
(7) | 1500 | 0.3 | 28.97 |
|
38.92 | 43.97 | 5.02 |
The oil displacement experiments results of different chemical are shown in Table
Relationship between enhanced recovery, maximum pressure gradient, reduction of water cut, and viscosity.
The SP system with polymer concentration 1500 mg/L and the surfactant concentration 0.3 wt% was injected into the core to simulate shearing during the injecting process in oilfield and then the SP system shared was used to displace oil. Comparing experiments (3) and (7), it can be concluded that the viscosity changes a lot after shearing and the recovery has an obvious decrease from 13.4% to 5.02%. This indicates that the system has a weaker mobility.
In order to find the influence of the surfactant concentration of SP system on the EOR, 4 experiments were conducted. The range of surfactant concentration is 0–0.4 wt% and the slug size of injection chemical is 0.6 PV. It can be concluded from Figure
Relationship of IFT and incremental oil recovery when surfactant concentration varies. The polymer concentration is 1500 mg/L.
Relationship of separated water ratio and incremental oil recovery when surfactant concentration varies. The polymer concentration is 1500 mg/L.
The micromorphology of the output liquid in the process of chemical flooding is shown in Figure
Micrographs of produced fluids in different production periods when chemical solution was injected: (a) in the earlier production period; (b) in the later production period.
According to the traditional theory of chemical flooding, the basic methods of chemical flooding for EOR are enlarging the sweep volume and improving cleaning efficiency [
Comparing experiments number (1), (2), and (3), it can be concluded that different kinds of chemical have different effects on oil recovery. The SP flooding has the largest oil recovery increases, followed by the polymer flooding, and the last is surfactant flooding. Comparing with the polymer flooding and SP flooding, the surfactant flooding has better oil displacement efficiency, but as surfactant solution viscosity is low, the flow resistance does not rise and the injection pressure is not higher than the water flooding (Figure
Relationship between inject pressure of different agent and pore volume.
SP flooding not just improves oil displacement recovery but also emulsifies oil into different sizes of drops which play the role of deep steering, improve the sweep factors of the following injection liquid, and reach the highest recovery. The oil drops are relatively large and hard to go through the throat. This is called the resistance effect (Jamin effect). The accumulation of resistance effect will result in improving the resistance factor that the liquid passes through the high permeability throat and raises the sweep factor of the following liquid. Meanwhile, under the stable effect of polymer, the emulsion can play the role of deep steering for a long time, compensating for the lack of polymer profile control ability.
(1) Salinity and shearing have a negative effect on both viscosity of polymer solution and oil recovery, so increasing the concentration of the polymer to maintain high viscosity is necessary.
(2) Different from the polymer flooding and SP flooding, the surfactant solution can only flow in the water channel and cannot improve the recovery effectively. The improving recovery of SP flooding is higher than the sum of improving recovery of the polymer and surfactant. It shows that the polymer carries surfactant into more pore volume and the oil displacing action gets full play, forming synergistic effect. It is a further explanation that the SP flooding is an effective method after the polymer flooding.
(3) The main factor of SP flooding in the heterogeneous reservoir is the mobility control action. As a result, when choosing the SP system, the viscosity should be considered first and the IFT then comes second. When the system reaches the ultralow IFT, the emulsion is easy to form and it is advantageous to the start of residual oil.
The authors declare that they have no conflicts of interest.
The authors would like to express their appreciation for the financial support received from the National Natural Science Foundation of China (51604243) and School of Petrochemical and Energy Engineering of Zhejiang Ocean University for permission to publish this paper.