With the growing demand for oil energy and a decrease in the recoverable reserves of conventional oil, the development of viscous oil, bitumen, and shale oil is playing an important role in the oil industry. Bohai Bay in China is an offshore oilfield that was developed through polymer flooding process. This study investigated the pore-scale displacement of medium viscosity oil by hydrophobically associating water-soluble polymers and purely viscous glycerin solutions. The role and contribution of elasticity on medium oil recovery were revealed and determined. Comparing the residual oil distribution after polymer flooding with that after glycerin flooding at a dead end, the results showed that the residual oil interface exhibited an asymmetrical “U” shape owing to the elasticity behavior of the polymer. This phenomenon revealed the key of elasticity enhancing oil recovery. Comparing the results of polymer flooding with that of glycerin flooding at different water flooding sweep efficiency levels, it was shown that the ratio of elastic contribution on the oil displacement efficiency increased as the water flooding sweep efficiency decreased. Additionally, the experiments on polymers, glycerin solutions, and brines displacement medium viscosity oil based on a constant pressure gradient at the core scale were carried out. The results indicated that the elasticity of the polymer can further reduce the saturation of medium viscosity oil with the same number of capillaries. In this study, the elasticity effect on the medium viscosity oil interface and the elasticity contribution on the medium viscosity oil were specified and clarified. The results of this study are promising with regard to the design and optimum polymers applied in an oilfield and to an improvement in the recovery of medium viscosity oil.
According to BP’s “Statistical Review of World Energy” data, oil prices have decreased from 2014 [
Based on a core experiment and a pilot test, in 2008, Aktas and Clemens used two-dimensional etched-silicon micromodels simulating a porous medium, carrying out brine, conventional polymers, and associative polymers at the same concentration displacement as medium viscosity oil (200 mPa·s), the results of which indicated that the associative polymers did not act as a plug and showed a stable front; therefore, an associative polymer can improve the sweep efficiency compared with brine and conventional polymer flooding process [
In this study, we carried out microvisual and core flooding experiments, comparing the polymer displacement characteristics in medium viscosity oil with glycerin flooding, using the same viscosity of the polymer solutions, and analyzed the mechanism of the elasticity effect on medium viscosity oil recovery, concluding that the elasticity contributes to oil recovery after water flooding at a different water sweep efficiency. Furthermore, these results were validated using reliable core experiments. The purpose of this study is to reveal the mechanism of elastic polymer enhancing medium viscosity oil recovery and identify the quantitative contribution of elasticity on the oil recovery. All the findings will provide a theoretical basis for the design of an optimum polymer solution applied to an oilfield.
In this present study, the displacement performance of medium viscosity oil with viscoelastic polymer was experimentally investigated. The general sketch of this experimental study can be illustrated in Figure
General sketch of this experimental study.
In this study, we used two etched-glass models to observe the phenomena affecting viscous oil displacement. Figure
Etched-glass physical models (both of the models are saturated with oil, the dark area represents oil, and the light area represents glass).
Dead-end model
Microsimulation model
As shown in Figure
Schematic illustration of the equipment used in the microvisual displacement experiments.
In this study, refined oil was collected, which is mixed with Bohai Bay crude oil without gas or diesel oil, and the viscosity is 70 mPa·s at 30°C. The synthetic brine was prepared using redistilled water, simulating the mineral composition of the formation water in the Bohai Bay offshore oilfield. The salinity of the brine is 9947.8 mg/L. The displacement fluids include brine, glycerin, and AP-P4 hydrophobically associating water-soluble polymers with a relative molecular weight of 1.1 × 107 Da. The polymer solution was created using a mother liquid dilution method, with prepared concentrations of 1200, 1000, and 900 mg/L and a viscosity of 75, 40, and 20 mPa·s, respectively.
We investigated polymer and glycerin displacement medium viscosity oil, in which the viscosity of both displacement fluids is 40 mPa·s. The detailed procedure was as follows: after the micromodel was vacuumed, it was saturated with oil and undergoing isothermal aging; then polymer flooding process was performed; the flow rate was designed as 1.0 pore volume per hour (PV/h) and was applied until the oil can no longer be produced. Finally, the micromodel was washed and the other displacement fluid experiment was repeated.
Figures
Distribution characteristics of residual oil at dead ends after viscoelastic polymer flooding (the dark area represents oil, and the lighter the color is, the lower the saturation is).
#1 Dead end
#2 Dead end
Distribution characteristics of residual oil at dead ends after glycerin flooding (the dark area is oil, and the lighter the color is, the lower the saturation is; the light area represents the displacing fluid).
#1 Dead end
#2 Dead end
As can be observed from Figures
A series of similar microvisual experiments were performed using the microsimulation model as mentioned above (Figure
The experiments for identifying the elastic contribution on oil recovery were first carried out. Both glycerin and polymer solutions with the same viscosity were employed in this experiment, and the detailed procedure is the same as the above dead-end model experiments. Figures
Oil distribution with viscoelastic polymer flooding in a simulation model (the dark area is oil, and the lighter the color is, the lower the saturation is; the light area represents the displacing fluid).
Initial saturation with oil
Displacement process
Residual oil after polymer flooding
Oil distribution with glycerin flooding in a simulation model (the dark area is oil, and the lighter the color is, the lower the saturation is; the light area represents the displacing fluid).
Initial saturation with oil
Displacement process
Residual oil after glycerin flooding
As can be seen in these images, the residual oil saturation is reduced during the displacement process. Comparing the residual oil saturation in viscoelastic polymer flooding with that in glycerin flooding, we can see that the oil saturation distribution after viscoelastic polymer flooding (Figures
We can further use image processing method to calculate the medium viscosity oil recovery quantitatively, as shown in Table
Displacement results of glycerin and viscoelastic polymer with the same viscosity (40 mPa·s).
Displacement fluid | Flow rate (PV/h) | Oil recovery (%) | Incremental oil recovery by elasticity (%) |
---|---|---|---|
Glycerin solution | 1.0 | 50.0 | — |
Viscoelastic polymer solution | 56.0 | 6.0 |
The actual viscous oil is developed on offshore oilfield by switching the injection polymer after water flooding, and therefore, based on the above direct polymer or glycerin flooding microvisual experiments, we carried out microvisual experiments for polymer or glycerin displacement of viscous oil at water flooding stages with different sweep efficiency of 20%, 40%, and 60%, and the elastic contribution to oil recovery after water flooding at different water sweep efficiencies was further identified.
Figure
Residual oil distribution of glycerin flooding and viscoelastic polymer flooding with the same viscosity (40 mPa·s) at a water displacement sweep efficiency of 20% (the dark area is oil, and the lighter the color is, the lower the saturation is; the light area represents the displacing fluid).
Water flooding
Glycerin flooding
Polymer flooding
Residual oil distribution of glycerin flooding and viscoelastic polymer flooding with the same viscosity (40 mPa·s) at a water displacement sweep efficiency of 60% (the dark area is oil, and the lighter the color is, the lower the saturation is; the light area represents the displacing fluid).
Water flooding
Glycerin flooding
Polymer flooding
As shown in these images, the residual oil saturation after viscoelastic polymer flooding is reduced more than that after glycerin flooding under the same water displacement sweep efficiency. Similarly, as shown in Table
Displacement efficiency of glycerin and viscoelastic polymer with the same viscosity (40 mPa·s) at different water displacement sweep efficiency levels.
Water displacement sweep efficiency (%) | Ultimate oil recovery (%) | Elastic contribution to oil recovery (%) | |
---|---|---|---|
Glycerin flooding | Viscoelastic polymer flooding | ||
20 | 48.94 | 54.12 | 9.57 |
40 | 47.26 | 51.59 | 8.39 |
60 | 45.89 | 49.02 | 6.39 |
The results are in agreement with the existing understanding that the viscoelastic behavior of the polymer plays an indispensable mechanism in chemical flooding EOR process. The previous studies reported the elastic contribution on low-viscosity oil through an etched-glass experiment comparing the recovery after glycerin flooding and showed that the oil recovery after polymer flooding increases 20% and that the ratio of elastic contribution on the low viscosity recovery can reach 10.61% [
Based on the microvisual experiments, the core flooding experiments were further carried out, and the understanding on the viscoelastic behavior of polymer flooding in medium viscosity oil recovery were validated, where the displacement fluids were also glycerin and polymer solutions of the same viscosity.
The oil used in this experiment is the same with that in the above microvisual experiments, with a viscosity of 70 mPa·s at 65°C. The displacement fluid includes AP-P4 hydrophobically associating water-soluble polymers and glycerin solutions, where the viscosity of both fluids was 40 mPa·s at 65°C, and the particulates and agglomerates were filtered by a sand core funnel of G1 type with apertures of 20~30
To obtain oil saturation with different numbers of capillaries, in this study, the number of capillaries was defined as
Considering that the interfacial tension for brine oil was 4 mN/m as tested using a rotating drop method in this study, the interfacial tension for glycerin oil and polymer oil was 5.8 mN/m and 4.8 mN/m, respectively. The capillary desaturation curves can be acquired. As shown in Figure
Capillary desaturation curves for the core flooding experiments.
Both etched-glass and core flooding experiments were conducted in this study, and with regard to the elasticity of the polymer solution and the elastic contribution on the medium viscosity oil recovery were revealed, the following can be concluded:
In the microvisual experiment, a clear asymmetrical “U” shape of the residual oil interface after polymer flooding can be seen downstream and moving deeper to the dead end owing to the elastic effect. However, the residual oil interface after glycerin flooding is different and shows a symmetrical “U” shape The incremental oil recovery by elasticity can reach 6.0%, and the ratio of elastic contribution on the oil recovery is 10.7% based on the direct polymer flooding experiment using etched-glass model. In the experiments on the injection of polymer or glycerin after water flooding, the oil recovery after polymer flooding is higher than that after glycerin flooding owing to the elastic effect at different water sweep efficiencies. The ratio of elastic contribution on the oil recovery increases with the decrease in the water sweep efficiency. Compared with previous knowledge, the elastic contribution to medium viscosity oil is lower than that for low-viscosity oil At the same number of capillaries, a reduction in the residual oil saturation can be realized through a better mobility ratio and shear-thickening owing to the elasticity effect. Compared with the oil recovery using brine flooding, the ratio of elastic contribution on medium viscosity oil reached 15.51% during the core flooding experiments. It is greater than that in the pore-scale and core-scale models, and this understanding is significant for promoting the application of viscoelastic polymer flooding process in medium viscosity oil reservoir and accelerating nonconventional petroleum development and production efficiently Intuitively, the elasticity effect on the medium viscosity oil can be observed in the microvisual experiments, and the results are comparable with the findings from previous low-viscosity oil experimental study. It can be concluded that the elasticity can improve medium viscosity oil recovery. It is essential to employ a polymer solution with greater elasticity in the actual medium viscosity oil development. Nevertheless, it should be noted that there is a nonnegligible repeatability standard deviation in micromodel experiments, and the numerical simulation method is potential to further understand the underlying mechanism and the role of viscoelastic polymer flooding in enhancing medium viscosity oil recovery
Both the displacement efficiency data and the oil saturation data used to support the findings of this study are included within the article. The microvisual experimental data used to support the findings of this study are currently restricted by the National Natural Science Funds for Young Scholars of China in order to protect the privacy of the related patients. However, all these data are available from Huiying Zhong (Email:
The authors declare that they have no conflicts of interest.
This work presented in this paper was financially supported by the National Natural Science Funds for Young Scholars of China (Grant no. 51604079) and the Natural Science Foundation of Heilongjiang Province (Grant no. E2017012). The Natural Science Foundation of Heilongjiang Province (Grant no. E2016015) and the University Nursing Program for Young Scholars with Creative Talents in Heilongjiang Province (Grant no. UNPYSCT-2016126) are gratefully acknowledged.