1. Introduction
Crude oil is widely accepted as a non-renewable energy source and a raw material for numerous chemical products. It is also one of the most important strategic resources in modern world. Its great importance has extended to various fields such as national defense, economic development, energy security, and so on
[1],
[2]. In last decades, China’s demand for crude oil has continued to grow. However, China’s annual crude oil production can only reach the safety baseline of 200 million tons. As the demand is steadily increased, the external dependence of crude oil has exceeded 70% in 2020. Considering the long-term national oil supply security and the current foreign energy situation, it is crucial to maintain stable domestic crude oil production. Developing revolutionary technologies to further enhance oil recovery (EOR) by a large percentage is of great importance. The mature oilfields in China, represented by the Daqing Oilfield and Shengli Oilfield, are mainly distributed in continental sedimentary basins, which has the characteristics of high viscosity and large wax content. At present, most of these fields have entered the ultra-high water cut stage (water cut > 90%), and the remaining oil is scattered, making stable crude oil production increasingly difficult
[3]. Investigations show that oil production and geological reserves in high-water-cut mature reservoirs account for over 70% of China’s oil production in total
[4],
[5]. Even with water flooding and chemical methods such as polymer flooding and alkali–surfactant–polymer (ASP) flooding, the maximum oil recovery rate only reaches 60%. About 19 billion tons of reserves remain unexploited, offering significant potential for further exploration.
In order to improve oil recovery in high-water-cut mature reservoirs, two key aspects are mainly involved: improving oil displacement efficiency and expanding oil swept volume. The existing flooding technologies are often categorized into liquid-phase flooding (water or solutions-based polymer or surfactant) and gas-phase flooding (CO
2, air, nitrogen, etc.). Significant achievements have been made in the theories and practices of liquid-phase and gas-phase displacement, implying a remarkable development of various flooding technologies
[5],
[6],
[7]. The chemical methods mainly including polymer flooding and ASP flooding, have been widely used in Daqing Oilfield, which is being the largest production base in the field of chemical flooding
[8],
[9],
[10],
[11]. However, due to strong reservoir heterogeneity (the maximum permeability contrast can reach to 200 times), obvious difference in pressure gradients and fluid flow velocities, oil displacement front during liquid-phase flooding typically becomes highly uneven. The fluid flow velocity in the middle position between a typical injector–producer pair is extremely low with an average velocity of 0.02 m∙d
−1. Influenced by gravity segregation and decades of water injection, preferential flow paths are widely distributed at the bottom of high-water-cut mature reservoirs, resulting in severe inefficient or ineffective water circulation. This phenomenon aggravates interlayer and intralayer contradictions within reservoir, leading to highly uneven remaining oil distribution
[12]. It is extremely difficult to mobilize this type of remaining oil in high-water-cut mature reservoirs by using the conventional methods.
In last decades, supercritical CO
2 (scCO
2)-EOR technology has gained great attention due to its superior ability to EOR through dissolution, extraction, and molecular diffusion effects and high potential for geological sequestration
[13],
[14],
[15],
[16],
[17]. Since scCO
2 has a lower density than that of crude oil, it preferentially migrates along the upper regions when injected into heterogeneous reservoirs, making it capable of mobilizing remaining oil enriched in the upper low-permeability areas and the upper to middle parts of thick oil layers. This makes the scCO
2-EOR a promising EOR method. However, majority of high-water-cut mature reservoirs in China lack the conditions for CO
2 to achieve miscibility with crude oil. Therefore, great efforts must be made to develop novel methods for enhancing displacement efficiency and oil recovery by improved scCO
2 flooding under non-miscible or partially miscible conditions
[18]. Except that, the high mobility ratio between pure scCO
2 and crude oil often leads to severe “gas channeling” issues during practical applications. This results in limited vertical swept volume of remaining oil and often fails to achieve the expected effect
[19],
[20],
[21]. By introducing an scCO
2 thickener with good solubility and strong dispersion stability, an efficient thickened scCO
2 flooding system is prepared, which can provide an effective alternative to improve the mobility ratio and greatly enhance crude oil recovery.
Generally, scCO
2 thickeners mainly include physical thickeners and chemical thickeners
[22]. Physical thickeners
[23] mainly change fluid concentration by adjusting temperature or pressure, or mixing insoluble materials such as quartz sand, ceramic particles, and nanoparticles with scCO
2 to prepare particle–scCO
2 suspensions. Chemical thickeners
[24],
[25],
[26] form stable network structures by molecular interactions such as π–π stacking or entanglement, reduce the migration rate of CO
2 molecules, and achieve thickening purposes. Great efforts have been made to prepare the chemical scCO
2 thickeners, mainly consisting of polysaccharide, silicon-based, fluorine-based, aromatic group-based, ether-based, hydrocarbon-based, and other types according to the functional groups of thickeners. Polysaccharide thickeners
[27],
[28],
[29] are generally water-soluble cellulose or polysaccharide systems, which have excellent salt resistance but poor affinity for organic components. Although silicon-based thickeners
[30],
[31] show superior thickening effect, the complex reticular structures and increased effective viscosity can only be obtained by adding solvents, leading to high production costs. Fluorine-based and aromatic group-based thickeners
[32],
[33] have obvious thickening effects compared to other types, but the degradation of components is slow which might cause environmental burdens. Some fluorine-based thickeners have a large molecular weight and a limited effective dissolution rate in scCO
2. Ether-based thickeners
[34] have strong adaptability to reservoir temperature and mineralization, but it is difficult to control the miscible pressure and there is a potential hazard in production. Hydrocarbon-based thickeners
[35] can easily mix with crude oil, but they have poor adaptability to reservoirs and weak dispersion stability of thickened systems. Different types of thickeners have significantly different thickening effects, and natural high-molecular thickeners can only increase the viscosity of scCO
2 several times, while synthetic fluorine or siloxane thickeners can increase it more than 100 times. It concludes that existing scCO
2 thickeners cannot meet the requirements of eco-friendly, low-cost, and good injectivity with strong thickening effect. The ideal scCO
2 thickeners should minimize the use of chemical elements that does not exist in actual conditions and mainly depend on the functional groups composed of C, H, and O with a small amount or no cosolvent used. The thickening mechanism of acetate-based thickeners newly proposed
[36] is similar to that of hydrocarbon-based thickeners. They can be modified as an ideal scCO
2 thickener without introducing additional elements, and their molecular structure also exhibits excellent lipophilicity, thus showing great potential to prepare the cost-effective scCO
2-thickening agent on a large scale.
Although extensive attempts have been made to prepare various scCO
2 thickeners using polymers, surfactant, and small molecule compounds or nanoparticles, most of the attempts are based on numerical simulation or laboratory with a limited field-scale scenarios, mainly in CO
2 fracturing
[37]. To achieve the desired fracturing effect, large amount of scCO
2 thickener is required. However, the lack of effective, cost-efficient and eco-friendly CO
2 thickener continues to impede the widespread application of thickened scCO
2 flooding
[38]. The majority of field applications of thickened CO
2 flooding have emerged in the United States
[39], where the reservoir conditions differ greatly with that of China. Li et al.
[40] studied the feasibility of thickened scCO
2–alternating-water flooding for EOR in low-permeability sandstone reservoirs. It demonstrates significant technical advantages of polymer-based thickener, but there are still no reports on field applications. Therefore, developing a low-cost, green thickener from a molecular perspective remains a great scientific challenge.
To address the above problems, this paper proposes a novel method for EOR by thickened scCO2 flooding in high-water-cut mature reservoirs. Firstly, the development potential of vertical remaining oil in high-water-cut mature reservoirs is analyzed by massive monitoring data. Using molecular dynamics simulation for optimal design of synthetic route, a copolymer with good solubility, strong dispersion stability and high thickening effect is thereafter synthesized as the scCO2 thickener by modifying vinyl acetate (VAc) polymer with polar functional groups. Two high-temperature, high-pressure oil displacement experiments are then conducted to clarify the underlying mechanism of EOR by thickened scCO2 flooding. The EOR effect by thickened scCO2–alternating-water flooding in a typical high-water-cut mature reservoir is predicted, and future technological advancements and research directions of thickened scCO2 flooding are ultimately discussed.
2. Remaining oil potential in high-water-cut mature reservoirs
Mature oilfields in China, as represented by Daqing and Shengli, have entered an ultra-high water cut stage (water cut exceeding 90%). It demonstrated a sharp increase in inefficient or ineffective water circulation, accelerated production decline, and severely uneven distribution of remaining oil
[41],
[42],
[43],
[44],
[45],
[46]. To address the inter-layer, inner-layer, and planar contradictions in water flooding reservoirs, various adjustment measures such as subdivided extraction, new well infilling, fracturing and perforation have been promoted. While these measures have effectively resolved the inter-layer and planar contradictions, the inner-layer issues remain largely unaddressed due to limited range of profile control and water shutoff treatments. Due to the effects of oil–water gravity segregation and continuous water injection for decades, the lower parts of layers, particularly thick layers, are strongly water washed
[47],
[48],
[49]. To accurately characterize the development potential of vertical remaining oil in high-water-cut mature reservoirs, sealed cores drilled from the main development zone in Daqing Oilfield are utilized. A statistical analysis is conducted on oil displacement efficiency data from the upper, middle, and lower parts of various sand bodies, as listed in
Table 1. Additionally, the dynamic variations in water intake profiles of a typical injector and the oil–water saturation distribution across different layers are analyzed, as shown in
Fig. 1. Results show that after long-term water flooding, severe inefficient or even ineffective circulation was developed in the bottom high-permeability layers and stratified mainstream regions within the main development zone. The remaining oil is predominantly concentrated in low-permeability areas that were unswept or weakly swept by injected water. Notably, the vertical remaining oil mainly enriched in middle and upper parts of oil layers exhibits substantial potential, accounting for approximately 70% of the total remaining oil. This makes it a critical target for further EOR in China’s high-water-cut mature reservoirs.
3. Preparation of thickened scCO2 flooding system
In order to overcome the inherent limitations of easy gas channeling and difficulty to recover the upper remaining oil in high-water-cut mature reservoirs when the traditional scCO
2 flooding was carried out, a polymer–scCO
2 mixed molecular model was constructed by using the GROMACS software. Molecular dynamics simulations were performed to investigate the synthesis, dissolution, and thickening behavior of the polymer–scCO
2 mixed system. By comparing the radial distribution function values of different types of polymer–CO
2 atom pairs and polymer–polymer atom pairs under reservoir temperature and pressure conditions, the dissolution and thickening effects of different types of polymers in scCO
2 were analyzed at the molecular level. Finally, VAc–maleic anhydride (MA)–styrene (St) copolymer
[50] was selected as the synthetic routine for the scCO
2 thickener in high-water-cut mature reservoirs.
Fig. 2(a) [50] shows the idealized copolymer molecular structure of scCO
2 thickener, and
Fig. 2(b) displays the copolymer molecular model after polymerization. The molecular dynamics simulation results indicate that polymers with electron-donating groups such as carbonyl, acetate, and ether groups can interact with scCO
2 molecules at lower temperatures and pressures. The copolymer of VAc, MA, and St can easily dissolve in scCO
2, exhibiting good dispersion stability and thickening effects
[47].
Based on the optimal synthetic routine obtained by molecular dynamics simulation, an amphiphilic copolymer was synthesized under a temperature of 80 °C using the free radical polymerization method. The copolymer has good miscibility with scCO2. The intermolecular interaction force among the copolymer molecules is much higher than that between the copolymer and scCO2. Therefore, the copolymer molecules typically form nano-sized aggregates through association in scCO2, significantly increasing the scCO2 viscosity and acting as the scCO2 thickener. The reagents used in the synthesis experiment include MA, St, VAc, sodium hydroxide, benzoyl peroxide (BPO), 2,2′-azobis(isobutyronitrile), pentamethyldiethylenetriamine, ethylenediaminetetraacetic acid, nitrogen, pure CO2, anhydrous ethanol, and ultrapure water. The equipment required for scCO2 thickener synthesis mainly includes a heat-controlled magnetic stirrer, vacuum oven, Fourier-transform infrared spectrometer, and a high-temperature, high-pressure variable-speed visual PVT reaction vessel.
In the scCO2 thickener synthesis experiment, a water solution of MA–St with a monomer concentration of 15%–30% was firstly prepared and stirred until completely dissolved. Then, it was transferred to a 500 mL four-necked flask. While stirring in a water bath, the system was thereafter heated to above 80 °C. After the temperature was stabilized, an appropriate amount of initiator BPO was added, followed by the addition of the third functional monomer VAc. To ensure the concentration of VAc monomer in the system and reduce the risk of self-polymerization, the flow rate of the monomer in the constant pressure separatory funnel was controlled, and VAc was added at a constant speed within 1 h. The three-component system was continuously stirred to ensure perfect mixing and then kept at a constant temperature for 2 h. The reaction was stopped and the heating equipment was turned off. After the temperature was dropped to 50 °C, the crude product in the bottle was then poured into room-temperature anhydrous ethanol, left to stand for 12 h, and the solvent and unreacted monomers were separated. The product was washed three times with anhydrous ethanol during the suction filtration process and then placed in a freeze dryer for drying. Finally, a loose white powder-like final product was obtained.
Fig. 3 displays the detailed procedures for the synthesis of the scCO
2 thickener. Since the reagents VAc and St often contain inhibitors, these two reagents need to be distilled first to prepare inhibitor-free monomers before the scCO
2 thickener synthesis experiment; after that, under a pure nitrogen environment, free radical polymerization of the three functional monomers VAc, St, and MA was carried out, resulting in the synthesis of three scCO
2 thickeners with St:MA:VAc mass ratios of 1:4:1, 1:4:2, and 1:4:2.5, named P-1, P-2, and P-3, respectively. To validate the inter-connectivity, proportion, and morphology of the monomer units within the copolymer molecules, the synthesized VAc–St–MA copolymers were systematically characterized, including infrared spectroscopy testing, scCO
2 solubility test and thickening effect evaluation.
Fig. 4 shows the Fourier transform infrared spectra of the three copolymers synthesized in this study. It can be observed that the peak positions of the transmittance curves are similar but with different intensities, indicating that the three copolymers contain similar functional groups but with certain differences in synthesis ratios. At around 3000 cm
−1, there is a stretching vibration absorption peak of C–H on the carbon skeleton, while at around 1700 cm
−1, there is a stretching vibration absorption peak of the VAc ester group C=O, which indicates that VAc and MA exist in the molecular chain in the form of acetate and unopened MA groups, respectively. Near 1500 cm
−1, there is a stretching vibration absorption peak of the –COOH group on VAc, and around 1400 cm
−1, there is a stretching vibration absorption peak of the aromatic ring connected to the carbon skeleton on the St group. The peaks around 1200 and 1000 cm
−1 correspond to the anti-symmetric stretching vibration peak and stretching vibration peak of the C=O group on MA, respectively, and the broad absorption peak at 1200 cm
−1 indicates that the hydrolysis of MA does not occur and it still maintains a cyclic structure. The absorption peak around 800 cm
−1 corresponds to the stretching vibration absorption peak of the benzene ring C–H. A good match between the absorption peaks and the target products indicates that the three synthesized products are the copolymers of MA, St, and VAc.
Fig. 5 displays the gradual change in PVT reactor pressure at 50 °C after adequate mixing of a thickener with a mass concentration of 0.2% and scCO
2. The visual window allows for visualizing the dissolution state of the thickener in scCO
2. From
Fig. 5, it can be observed that when the pressure is 13.0 MPa, the visual window is pure and transparent, indicating complete dissolution of the thickener in the scCO
2. As the pressure decreases from 13.0 to 11.7 MPa, the visual window starts to become cloudy. Furthermore, as the PVT reactor pressure continues to decrease to 7.0 MPa, the visual window becomes completely cloudy, indicating a significant decrease in the solubility of the thickener in the scCO
2. The pressure of 11.7 MPa is the turbidity point pressure for scCO
2 and the thickener.
A high-pressure-resistant, acid-resistant, and temperature-controllable Haake rheometer is thereafter used to measure the torque resistance of scCO2 mixtures with copolymer completely dissolved under certain pressure, temperature, and shear rate conditions. The higher the torque resistance, the greater the mixture viscosity. The test accuracy reaches 0.0001 mPa·s. In the experiment, the system is pre-sheared for 10 min to ensure uniform mixing of the copolymer and scCO2 with a shear rate of 1000 s−1. After the pre-shear process, the shear rate is adjusted to 170 s−1, and the system is subjected to stepwise temperature increases at pressure of 10, 15, and 20 MPa, respectively to measure the variation of the copolymer–scCO2 mixture shear viscosity. To ensure the accuracy of mixture shear viscosity, the tests near the critical point are repeated three times, and the blank group is pure CO2.
Fig. 6 displays the dynamic variation of scCO
2 viscosity at different temperature and pressure after adding 0.2% mass concentration of three copolymer thickeners. It can be observed that the P-3 copolymer exhibits good thickening effects at pressure of 10, 15, and 20 MPa, respectively. Under the condition of 10 MPa pressure and 50 °C temperature, the viscosity of scCO
2 increases by 39.4 times. When the pressure is 20 MPa and the temperature is 110 °C, the system viscosity is still 13.1 times that of pure scCO
2 viscosity. Furthermore, the viscosity of the thickened scCO
2 system and the pure scCO
2 system both decrease as the temperature increases, while the viscosity significantly increases with higher pressure. The thicker the copolymer (in terms of total number of atoms), the better the thickening effect will be. Analysis suggests that the VAc–MA–St copolymer molecules contain a large number of C=O groups with good affinity to CO
2. These groups are connected to CO
2 through van der Waals forces, which are much stronger than those between CO
2 molecules, thus inhibiting molecular motion and increasing system viscosity. However, with increasing temperature, both molecular collisions and internal energy increase, weakening the constraint ability of van der Waals forces and allowing more movement space between copolymer molecules and CO
2 molecules, enhancing system flowability, resulting in a decrease in system viscosity. At lower pressures, the copolymer chains are more extended and the free volume of CO
2 is larger, leading to less noticeable changes in system viscosity. Increasing pressure compresses the molecular free volume, resulting in higher molecular friction and increased viscosity, as reflected by the rising system viscosity.
4. EOR mechanism of thickened scCO2 flooding
Two types of high-temperature, high-pressure scCO2 flooding experiments are designed and carried out to explore the EOR mechanism of the thickened scCO2 flooding system. The mechanism of swept volume enlargement was investigated by visual 3D displacement experiments. Another gas–alternating-water displacement experiments within natural core are conducted to specifically explore the mechanism of inhibiting gas channeling by the thickened scCO2 flooding system. The pressure and temperature are maintained at 18.0 MPa and 70 °C, respectively. The experimental oil is a 1:1 mixture of kerosene and white oil with a density of 0.884 g∙cm−3 and a viscosity of 2.718 mPa·s at standard temperature and pressure. The experimental water is deionized water while the mass concentration of polymer solution is 1500 mg∙L−1, and CO2 with the purity of 99.8% is used. The scCO2 thickener is the P-3 copolymer synthesized above. The scCO2 viscosity is increased by 28.6 times when the mass concentration of the thickener is 0.2%.
4.1. Visual 3D displacement experiments
A visual 3D model, which is a vertically placed cylindrical sand-filling model with a diameter of 5.0 cm and a height of 12.5 cm, is employed. The sand-filling material consists of transparent glass beads with diameter ranging from 0.5 to 0.6 mm. The permeability of this model is measured around 5000 × 10−3 μm2. A polydimethylsiloxane (PDMS) material layer with a thickness of 2–3 mm was pre-coated on the wall of the sapphire window before the experiment starts. This is primarily to mitigate the extrusion-induced damage of the sand particles on the window. After the application of pressure, the sand particles became embedded within the surface of the PDMS material, thereby precluding the channeling of gas and liquid along the wall. Additionally, the cured PDMS material occupied a fraction of the cavity volume, which effectively reduced the spatial distance of the sand filling and enhanced the light transmission of the particles. A 25000 Lumen (LM) ultra-strong light flashlight is utilized as the light source to guarantee a satisfactory visual experimental result. The pure sand-filling glass beads exhibited a light blue hue upon the refraction of white light. Sudan III is employed to dye the simulated oil in order to expand the contrast between the oil and water phase.
Firstly, the visual 3D sand-filling model is saturated with water and oil successively. Oil displacement experiments involving water flooding, polymer flooding, scCO
2 flooding, and thickened scCO
2 flooding is thereafter carried out with an injection rate of 0.8 mL∙min
−1. A mode of low-injection and low-production is employed for water and polymer flooding while the mode of high-injection and high-production is adopted for traditional and thickened scCO
2 flooding.
Fig. 7 shows the fluid distribution within the 3D sand-filling model under different flooding methods. The thickened scCO
2 flooding exhibits a pronounced effect in enlarging swept volume from
Fig. 7. The overall recovery efficiencies are determined to be 28% for water flooding, 42% for polymer flooding, 58% for traditional scCO
2 flooding, and 67% for thickened scCO
2 flooding. It demonstrates that the thickened scCO
2 flooding can further increase crude oil recovery by 9% in contrast to the traditional scCO
2 flooding.
4.2. ScCO2–alternating-water flooding experiments
To evaluate the effect of the thickened scCO2 flooding system on inhibiting gas channeling in heterogeneous reservoirs, a natural ultra-low permeability core is selected, which has a permeability of 0.25 × 10–3 μm2, a diameter of 3.8 cm, and a length of 5.0 cm. Under the experimental conditions of 70 °C and 18 MPa, after being saturated with water and oil successively, the traditional and thickened scCO2–alternating-water flooding experiments are carried out within the natural core at an injection rate of 0.1 mL∙min–1. The gas–water slug ratio used in this study is 15:1, and the flooding experiment is ended until almost 8 pore volumes (PV) of fluid are injected.
Fig. 8 shows the dynamic variations of oil recovery rate and gas production rate during scCO
2–alternating-water flooding experiments. The thickened scCO
2–alternating-water flooding with a mass concentration of 0.2% could achieve a higher oil recovery rate, reaching up to 89.3% in comparison with the traditional scCO
2–alternating-water flooding. After the addition of the scCO
2 thickener, the gas–oil mobility ratio is significantly improved. Moreover, the scCO
2 gas channeling in the ultra-low permeability core is effectively inhibited, with the gas production rate at breakthrough decreasing from 12.00 to 6.07 mL∙min
–1∙MPa
–1.
5. Prediction of EOR effect by thickened scCO2 flooding
In order to evaluate the future potential of EOR by the thickened scCO2 flooding, a typical heterogeneous block, Hei-46, distributed in Jilin Oilfield, PetroChina, is selected as a pilot area for this study. The pilot area is 10.6 km2, with geological reserves of 5.7 million tons, initial formation pressure of 23.9 MPa, reservoir temperature of 97.8 °C, average porosity of 12.8%, average permeability of 5.6 × 10−3 μm2 and crude oil viscosity of 3.53 mPa·s. Initially, a 160 m × 480 m reverse 9-point well pattern was utilized for oil displacement by water flooding, with a total of 27 injectors and 141 producers. Until 2014, water cut reached 83.75%. From 2014, water injection continued in the northern area while scCO2–alternating-water flooding was implemented in the southern area with a cumulative gas injection of 0.3 hydrocarbon pore volume (HCPV). However, due to the insufficient supply of CO2, the formation pressure gradually decreases. The current formation pressure is only 18.0 MPa, which is far lower than the minimum miscibility pressure of 22.3 MPa, so miscible displacement is unable to be achieved. In addition, as the traditional scCO2–alternating-water flooding continues, reservoir properties such as porosity and permeability changed drastically due to water–rock reaction, reservoir heterogeneity aggravated and scCO2 spread unevenly, resulting in large differences in development effect and a rapid decline in oil production.
Based on the detailed geological analysis and production history matching of water flooding and scCO
2–alternating-water flooding in the Hei-46 block, a typical unit with 4 injectors and 21 producers is intercepted from the southern gas injection area of the targeted high-water-cut mature reservoir as the research object, and numerical simulations under different scCO
2 thickening multiple scenarios are carried out to predict the EOR effect until year 2040 by thickened scCO
2–alternating-water flooding. The typical reservoir unit adopts a corner grid system with a grid number of 68 × 104 × 46 and a grid size of 20 m × 20 m × 0.2 m. Three-dimensional permeability distribution of the typical reservoir unit intercepted from actual heterogeneous reservoir is shown in
Fig. 9. A five-spot well pattern is used with a well spacing of 106 m × 212 m. For this study, the thickened scCO
2–alternating-water injection is implemented where gas injection lasts for two months and water injection lasts for one month at each cycle, implying an alternating ratio of 2:1. Water injection rate is 50 m
3∙d
−1, and the surface injection rate of CO
2 is 16 629 m
3∙d
−1. The annual cumulative injection volume of CO
2 is 0.08 HCPV. The minimum bottomhole pressure of producers is maintained at 12.0 MPa, and the maximum bottomhole pressure of injectors is controlled at 40.0 MPa to ensure the stable operation of the thickened scCO
2–alternating-water flooding in high-water-cut mature reservoir.
The EOR effects of thickened scCO
2–alternating-water flooding in the typical unit are compared under five scenarios of thickening multiples of 0, 25, 50, 75, and 100, as shown in
Fig. 10. It demonstrates that compared with the traditional scCO
2–alternating-water flooding, The higher the scCO
2 thickening multiples, the more remarkable the EOR effect by the thickened scCO
2–alternating-water flooding. After increasing the scCO
2 viscosity by 25 times, an additional 19.7% of crude oil recovery can be achieved; when scCO
2 is thickened by 50 times, crude oil recovery is increased by 23.13%. However, if CO
2 viscosity is further increased to a threshold (100 times of pure scCO
2 viscosity for this study) close to the viscosity of crude oil, a piston-like front movement can be achieved. Further increasing the scCO
2 viscosity will weaken the yielding effect of vertical remaining oil in high-water-cut mature reservoirs, and significantly increase the cost of thickening agents and the capability requirements of surface injection equipment. Therefore, there exists a reasonable scCO
2 thickening multiple in the pilot practice of high-water-cut mature reservoirs by the thickened scCO
2–alternating water flooding.
In order to further clarify the main reason for why crude oil recovery is enhanced by a large percentage when the thickened scCO
2 flooding is carried out, both the planar and vertical fluid distributions during the thickened scCO
2–alternating-water flooding in the typical high-water-cut mature reservoir unit are analyzed here, as shown in
Fig. 11. It indicates that compared with the traditional scCO
2–alternating-water flooding, both the planar and vertical sweep efficiency in high-water-cut mature reservoirs can be expanded due to the thickening effect. The closer the viscosity of the thickened scCO
2 is to the viscosity of crude oil, the larger the vertical remaining oil swept volume, and the more remarkable the effect of EOR by the thickened scCO
2–alternating-water flooding in high-water-cut mature reservoirs, indicating a great potential of the proposed EOR method to practical application.
6. Discussion
In view that the existing scCO2 thickeners typically contain fluorine or silicon and have poor reservoir adaptability, thus unable to meet the technical requirements of environmentally friendly, low cost and field-scale application in high-water-cut mature reservoirs, this paper utilizes molecular dynamics simulation to optimize the synthetic route of scCO2 thickener. Using the free radical polymerization method, an amphiphilic copolymer with strong solubility, excellent dispersion stability and superior thickening effect is synthesized by introducing polar functional groups (MA and St) onto the VAc polymer molecules, serving as an efficient scCO2 thickener. Laboratory experiments and reservoir numerical simulations demonstrate that the copolymer-based thickened scCO2 flooding system proposed in this study significantly improves the EOR effect of high-water-cut mature reservoirs triggered the synergistic mechanisms between expanding the vertical sweep volume and effectively inhibiting gas channeling. When the viscosity of scCO2 is increased by 50 times, crude oil recovery can be further enhanced by 23.1% for a typical high-water-cut mature reservoir, implying that the thickened scCO2 flooding is becoming a revolutionary technology for further EOR by a large percentage in high-water-cut mature reservoirs.
Although the potential for EOR by thickened scCO2 flooding in high-water-cut mature reservoirs is considerable, the field-scale application of this technology still faces major challenges, mainly in the following respects:
(1) The application of scCO2 thickeners is still mainly at the laboratory stage. It is of great importance to develop a series of low-cost, green scCO2 thickeners from a molecular perspective to adapt to different high-water-cut reservoir conditions. Further investigations are needed for scCO2 thickening mechanism, especially the action of CO2, polymers, and oil–water phases reaction kinetics in formations, significantly affecting the uniform dispersion and thickening effect of the scCO2 thickeners.
(2) Future studies should also consider incorporating more than one scCO2 thickening agent, specifically, nanoparticles can be modified with polymers to increase the scCO2 viscosity and EOR. In order to achieve a structural balance between CO2-philic/CO2-phobic groups to enhance the solubility of the thickener and increase the viscosity of CO2, the molecular structure of the polymer-based scCO2 thickener should be critically designed by molecule dynamics simulation.
(3) The majority of high-water-cut mature reservoirs in China are continental clastic formations, characterized by severe reservoir heterogeneity. It is generally believed that chemical flooding such as polymer or polymer–surfactant flooding will be primarily promoted in the high-water-cut development stage. The potential synergistic effects of thickened scCO2 flooding with other EOR methods, such as thermal recovery and chemical flooding should be in-depth investigated. Additionally, how to evaluate future environmental impact and sustainability challenges such as strategies for CO2 capture and utilization remains a significant challenge.
7. Conclusions
In order to exploit the large-scale remaining reserves enriched in high-water-cut mature reservoirs, a novel EOR method by thickened scCO2 flooding is proposed. The main conclusions are summarized as follows.
(1) A detailed analysis of dynamic data from Daqing Oilfield reveals that severe inefficient and ineffective water circulation occurs in the bottom high-permeability layers of high-water-cut mature reservoirs. Remaining oil is mainly concentrated in low-permeability layers and weakly swept zones. Notably, the development potential of vertical remaining oil is substantial, making it a primary target for further EOR in high-water-cut mature reservoirs.
(2) Based on molecular dynamics simulation for optimal design of synthetic routine, three functional monomers, namely, VAc, MA, and St, are selected for free radical polymerization to synthesize a amphiphilic copolymer without fluorine or silicon as a scCO2 thickener, which is economical, eco-friendly, and highly efficient. It demonstrates that the synthesized copolymer exhibits good solubility in scCO2, strong dispersion stability, and greatly increases scCO2 viscosity. Under an ambient pressure of 10 MPa and a temperature of 50 °C, the addition of the thickener with a mass concentration of 0.2% can increase scCO2 viscosity by 39.4 times.
(3) High-temperature, high-pressure oil displacement experiments are conducted to explore the mechanism of EOR by thickened scCO2 flooding. Compared to the traditional scCO2 flooding, it exhibits good effects in expanding vertical swept volume and inhibiting gas channeling. At a mass concentration of 0.2%, the thickened scCO2 flooding can increase oil recovery rate by 9%, while gas breakthrough rate reduces from 12 to 6.07 mL∙min–1∙MPa–1.
(4) Taking a typical reservoir unit as example, reservoir numerical simulation is performed to predict the EOR effect by thickened scCO2 flooding. As CO2 viscosity gradually increases, the EOR effect tends to be more remarkable. When scCO2 is thickened by 50 times, crude oil recovery can be further enhanced by 23.1%. The main reason is that the thickened scCO2 flooding can expand both planar and vertical sweep efficiency as well as effectively inhibiting gas channeling. If scCO2 viscosity is further increased to a threshold close to crude oil viscosity, a piston-like front movement can be achieved. The proposed thickened scCO2 flooding is being recognized as a revolutionary technology for EOR by a large percentage in high-water-cut mature reservoirs.
CRediT authorship contribution statement
Kaoping Song: Methodology, Funding acquisition, Conceptualization. Daigang Wang: Writing – original draft, Formal analysis. Fengyuan Zhang: Validation, Data curation. Hong Fu: Visualization, Software, Investigation. Mingxing Bai: Validation, Software. Hamid Emami-Meybodi: Writing – review & editing, Supervision.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This research was funded by the National Natural Science Foundation of China (U22B6005, 52174043, and 52474035), the Beijing Natural Science Foundation (3242019), and the China National Petroleum Corporation (CNPC) Innovation Foundation (2022DQ02-0208 and 2024DQ02-0114).
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