3D Physical Simulation Experiment of Edge Water Reservoir by Polymer/Surfactant Binary Flooding

To investigate the enhanced oil recovery (EOR) technology by chemical flooding in an edge water reservoir, a 3D physical simulation experimental device for the edge water reservoir was developed, and polymer/surfactant binary flooding experiments were carried out under different edge water energies. In addition, the effect and mechanism of binary flooding on EOR under different edge water energies were comprehensively analyzed. Experimental results show that edge water intrusion considerably affects EOR by binary flooding. )e stronger the edge water energy, the worse the effect of EOR by binary flooding. Edge water possibly diluted the concentration of the chemical agent medium that is injected into the reservoir, and the degree of dilution varied in different regions.)e dilution region was mainly distributed between the injection wells and edge water.)e stronger the edge water energy, the higher the dilution multiple of chemical agent and the greater the recovery loss rate by binary flooding.


Introduction
Edge water reservoir is widely distributed all over the world. e oil production of active edge water reservoir is one of the most challenging problems in petroleum development. In most of these reservoirs, it is likely to form a large water channeling. is phenomenon, which occurs quickly, leads to a rapid increase in the water content of the reservoir. e inflow of water then limits oil recovery, leading to a significant decline in well production [1][2][3]. However, if the edge water can be effectively restrained or utilized in time, it can once again become a positive energy to improve the development efficiency [4].
For many years, the influence of polymer blocks on bottom water reservoir development has been systematically studied [5]. In addition, some scholars have studied the injection of chemical agents to block the dominant channels and transfer the oil displacement agent to the uncleaned areas.
ese reported chemical agents include emulsions, foams, air, nanoparticles, and gels [6][7][8][9]. e shallow reservoir of the Jidong Oilfield is a typical medium-high permeability natural water drive reservoir, with an average porosity of 29% and an average permeability of 494 mD [10][11][12]. e depth of the reservoir is shallow, between 1500 and 2400 m, the fault block area is small, and the average oil-bearing area is only 0.20 km 2 . ere are faults in the reservoir, and some areas are connected with the external natural waters, with a certain amount of edge water energy. Currently, this reservoir is at the ultrahigh watercut stage. e composite water cut is greater than 90%, and the degree of reserve recovery is less than 20%. Hence, enhanced oil recovery (EOR) technologies for this type of a reservoir needs to be investigated [13][14][15][16]. As a mature and effective oil recovery technology, chemical flooding has garnered increasing attention [17][18][19]; however, the invasion of edge water has always been an important factor that reduces the effect of chemical flooding. Hence, it is imperative to explore the oil displacement mechanism of chemical flooding in the edge water reservoir [20][21][22][23].
In this study, a three-dimensional (3D) physical simulation experimental device for the edge water reservoir is developed according to an actual reservoir. In addition, by employing 3D physical simulation experiments of the polymer/surfactant binary flooding under three water energy conditions, i.e., no edge water, weak edge water, and strong edge water, respectively, the EOR mechanism by binary flooding under different edge water energy conditions is comprehensively analyzed.

Model Research.
According to the characteristics of the shallow reservoir of the Jidong Oilfield such as a small oilbearing area and active edge-bottom water, a set of 3D physical simulation experimental devices for the edge water reservoir is developed on the basis of the actual reservoir parameters. e experimental process is shown in Figure 1. e device consists of a model system, a data acquisition and processing system, an automatic control system, a production metering system, and an auxiliary system. Some water intrusion channels are set outside the 3D model, while the edge water permeation plate is set inside the 3D model, which was composed of filling holes and filter mesh. e edge water is injected by an ISCO pump via the outside water invasion channels, infill holes, and filter mesh. And all of these can support the experimental research on the law of water flooding and chemical flooding under the multidirectional edge water invasion condition. e main parameters are as follows. e maximum experimental temperature and maximum experimental pressure are 200°C and 10 MPa, respectively; the internal size of the pressure chamber is 300 × 300 × 200 mm; the number of pressure measuring points is 40; the number of differential pressure measuring points is 30; and the number of resistivity measuring points is 64. e well network in line drive mode is designed on the basis of the shallow reservoir in the Jidong Oilfield (Figure 2). At the water flooding stage, B1-B5 are production wells, which are driven by edge water energy. At the polymer/surfactant binary flooding stage, B1 and B2 are injection wells and B3-B5 are production wells. To dynamically identify the reservoir fluid distribution during the 3D physical simulation, 36 sets of resistivity probes are set up in the model to monitor the changes of resistivity at different locations as well as development phases ( Figure 3). Eighteen sampling ports at different locations are set up to sample formation fluids and analyze the chemical agent concentration.

Experimental Materials.
Experimental water is the formation water from the shallow reservoir in the Jidong Oilfield; the type of water is NaHCO 3 ; and the total salinity is 1635 mg/L. e experimental oil is the simulated oil from the shallow reservoir in the Jidong Oilfield, the crude oil viscosity is 5.0 mPa·s at 70°C, the surfactant (betaine) solution concentration is 0.5%, the polymer hydrolyzed polyacrylamide (HPAM) concentration is 1500 mg/L, and the molecular weight is 1200×10 4 -1600×10 4 .

Experimental Scheme.
Based on the similarity criterion, the experimental parameters are calculated according to the PV number. For flow rate and permeability, according to Darcy's law, if other parameters remain unchanged, the flow rate and permeability are reduced by 100 times in equal proportion. On the basis of the reduced flow rate, the prototype and the model are compared to ensure that the PV number of the injected fluid is the same to determine the injection time of the model. e experimental parameters are determined (Table 1), and the experimental scheme is designed as follows: (1) e injection rate of edge water flooding is 2.5 mL/ min (2) When the water cut is up to 98%, the polymer/ surfactant binary flooding under different edge water energy conditions should be carried out, as shown in Table 2 (3) When the polymer/surfactant binary flooding is completed, water flooding is carried out at a rate of 2.5 mL/min until the composite water cut reaches 98%.
It is worth mentioning that, through the numerical simulation part, the intrusion speed of the weak edge water and strong edge water (80 and 500 times, respectively) was determined. By adjusting the relationship between the water body multiple and the bottom-hole flowing pressure drop of the edge water drive, the weak and strong water injection rates were determined. Eventually, the experimental schemes of boundless water chemical flooding, weak edge water chemical flooding, and strong edge water chemical flooding were formed.

Effect of Edge Water on Polymer/Surfactant Binary
Flooding. To examine the effect of edge water on the production effect of polymer/surfactant binary flooding, the oil production under different edge water energy conditions is compared. e data obtained from each experimental stage are summarized in Table 3. Based on the experimental data, the curves of the recovery degree and average water cut curves are shown in Figures 4 and 5, respectively.
It can be drawn from Figures 4 and 5 that the production time of the polymer/surfactant binary flooding with weak edge water is the longest, that without edge water is the second, and that with strong edge water is the shortest. e decrease in the average water cut for the polymer/surfactant binary flooding without edge water is the highest, up to 23.90%, while the corresponding values for the polymer/ surfactant binary flooding with weak edge water and strong edge water were similar, i.e., 10.74% and 12.95%, respectively.
e degree of recovery for the polymer/surfactant binary flooding without edge water is 9.12%, while the corresponding values for the polymer/surfactant binary flooding with weak edge water and strong edge water are 6.67% and 3.76%, respectively. e development effect of the polymer/surfactant binary flooding without edge water is better than that of the weak edge water, and the effect of binary flooding with weak edge water is better than that with strong edge water. Journal of Chemistry Probe distribution layer    show the remaining oil saturation distributions for the polymer/surfactant binary flooding with strong edge water and without edge water, respectively. To more intuitively compare the effects of binary flooding under different edge water energy conditions, the remaining oil saturation of binary flooding with strong edge water is subtracted from that of binary flooding without edge water. Figure 8(a) shows the distribution of the remaining oil saturation of binary flooding under edge water. e difference of the remaining oil saturation between strong edge water and no edge water is divided into positive and negative regions. e negative region is where the edge water action is beneficial to the remaining oil production, and the positive area is the unfavorable one (Figures 8(b) and 8(c)), respectively. From Figure 8, the best area to exploit the remaining oil is confirmed to be between the edge water region and injection wells, while the effect of the edge water is clearly not conducive to produce the remaining oil in the area between injection and production wells. Hence, the presence of edge water is not beneficial to the overall production of the remaining oil, especially the remaining oil between the injection and production wells.

Effect of Edge Water on Polymer Concentration.
With the increase in the polymer concentration, the solution viscosity increases. As a crucial factor in the polymer/surfactant binary flooding, the polymer viscosity can improve oil recovery via the control of the oil-water mobility. e change in concentration of the precursor polymer is the indirect reference to reflect the change in the polymer viscosity. By sampling and analysis, the variation diagrams of the polymer concentration fields with different injection volumes (0.3 PV and 0.6 PV; the following waterflooding is carried out until    the water cut is 98%) in the chemical agent injection process with no edge water, weak, and strong edge water are obtained (Figures 9-11, respectively).
With the increase in the polymer injection volume, the polymer distribution region exhibits a "heart-shaped" expansion potential, and the concentration gradually increases  under the no edge water condition (Figure 9). With the injection of 0.6 PV, the polymer is mainly distributed in the areas near injection well 1 and injection well 2 and between injection and production wells; the following water flooding process drives the polymer outside the "heart shape," i.e., outside the injection-production wells; and the polymer concentration decreases accordingly.
When the same volume of the chemical agent is injected, the distribution area and concentration of the polymer under the three conditions of no edge water, weak edge water, and strong edge water, respectively, decrease in turn (Figures 10 and 11). Under the edge water condition, with the injection of 0.6 PV, the polymer mainly distributes between injection well 1, injection well 2, and production well 2. e range of the polymer distribution under the strong edge water condition is less than that under the weak edge water condition, that is, the "heart" area is smaller and the concentration is lower.

Dilution Multiple.
To compare the concentration difference of the polymer flooding front under different edge water energy conditions, the dilution multiple concept is defined, which is the ratio of the chemical concentration with no edge water and with edge water; the dilution multiple characterizes the dilution degree of the edge water to the chemical agent. e dilution multiple of the polymer concentration under strong and weak edge water conditions is converted into field diagrams (Figures 12 and 13). At the chemical injection stage, the invasion of edge water dilutes the polymer concentration. e stronger the edge water energy, the higher the dilution multiple. e area percentage of the strong edge water dilution to the chemical agent is 83% and that of the weak edge water dilution to the chemical agent is 48%.
e dilution degree is different in different areas, and the dilution area is mainly distributed in the area between the injection well and edge water (Figures 12 and 13). e dilution multiples of different regions are quantitatively evaluated by taking the average value ( Figure 14). e dilution multiple between the injection well and edge water in a strong edge water reservoir is 2.2, while that between injection wells and production wells is only 1.3. In the weak edge water reservoir, the dilution multiple between the injection well and edge water is 1.6, and the dilution multiple between injection wells and production wells is 1.  dilution multiple between the injection well and edge water is greater than that between the injection wells and production wells. e dilution multiple and development effect of the polymer/surfactant binary flooding under different volumes of water invasion and different edge water energy conditions is compared (Figures 15 and 16). e invasion multiple of edge water considerably affects the dilution degree of the polymer. With the increase in the edge water invasion, the dilution multiple of the polymer gradually increases and the recovery degree for chemical flooding slowly increases. With the increase in the water invasion volume from 0.05 PV to 0.4 PV, the polymer dilution multiple increases from 1.08 to 1.72 in the strong edge water reservoir, and the polymer dilution multiple increases from 1.05 to 1.36 in the weak edge water reservoir. e dilution degree of strong edge water is more obvious. e dilution of the edge water to the polymer leads to the decrease in the effective polymer concentration.
e oil recovery loss rates for the binary flooding in the strong edge and weak edge water reservoirs are 58.77% and 26.86%, respectively. e stronger the edge water energy, the higher the recovery loss rate for the binary flooding.

Practical Guidance
e dilution multiple reflects the dilution degree of the edge water to the chemical agent. In a majority of the reservoir areas, the chemical agent concentration is diluted by edge water, especially in the region between the injection well and edge water. e improvement of the well pattern of chemical flooding near the water/oil contact and the maintenance of a reasonable injection-production ratio and injection rate are effective methods to control the invasion of edge water and improve chemical flooding EOR.
By taking the Ng8 reservoir of the Gao104-5 fault block in the North Gaoqian area of the Jidong Oilfield as an example, it is seen that the reservoir is a typical layered structural reservoir with edge water. After several years of development, the recovery degree and composite water cut are 28.3% and 96.0%, respectively. From the quantitative evaluation of the remaining oil, the residual remaining oil accounts for 66%, and the retained remaining oil accounts for 34%, which is mainly enriched in the top region among wells and the bypass areas along the water flooded path. e EOR technology should be adopted to improve the displacement efficiency. According to different occurrence states of the remaining oil, the differential design and combination of various displacement methods are carried out: binary flooding is implemented at the waist to improve the sweep efficiency and displacement efficiency; profile control is applied at the edge of the reservoir to plug the high capacity channel and adjust the invasion direction and invasion rate of edge water; and CO 2 huff and puff is carried out in the area beyond the well pattern control. Currently, the program is being implemented, and oil recovery is predicted to be improved by 10.22%.

Conclusion
(1) Experimental results revealed that the recovery degree for binary flooding without edge water is the highest, and the one with strong edge water is the lowest. e reduction of the watercut during binary flooding without the edge water is the highest, while there is marginal difference between the weak edge water and strong edge water. e stronger the edge water energy, the worse the effect of binary flooding. (2) e concentration and distribution range of the chemical agent under the no edge water condition is clearly greater than that under the edge water condition, indicating that the chemical agent concentration can be "diluted" by edge water. e dilution degree is different in different areas, and the dilution is mainly located in the area between the injection wells and edge water. (3) With the increase in the invasion volume of edge water, the dilution ratio of the chemical agent gradually increases. e dilution of edge water to the chemical agent leads to the decrease in the effective chemical concentration. e stronger the edge water energy, the higher the dilution multiple, and the greater the recovery loss rate by binary flooding.

Data Availability
e data used to support the findings of this study are included within the article.

Conflicts of Interest
e authors declare that they have no conflicts of interest.