Effect of CO_(2)flooding in an oil reservoir with strong bottom-water drive in the Tahe Oilfield,Tarim Basin,Northwest China

The dissolution and diffusion of CO_(2)in oil and water and its displacement mechanism were investigated by laboratory experiment and numerical simulation for Block 9 in the Tahe oilfield,a sandstone oil reservoir with strong bottom-water drive in Tarim Basin,Northwest China.Such parameters were analyzed as solubility ratio of CO_(2)in oil,gas and water,interfacial tension,in-situ oil viscosity distribution,remaining oil saturation distribution,and oil compositions.The results show that CO_(2)flooding could control water coning and increase oil production.In the early stage of the injection process,CO_(2)expanded vertically due to gravity differentiation,and extended laterally under the action of strong bottom water in the intermediate and late stages.The CO_(2)got enriched and extended at the oil-water interface,forming a high interfacial tension zone,which inhibited the coning of bottom water to some extent.A miscible region with low interfacial tension formed at the gas injection front,which reduced the in-situ oil viscosity by about 50%.The numerical simulation results show that enhanced oil recovery(EOR)is estimated at 5.72%and the oil exchange ratio of CO_(2)is 0.17 t/t.

Utilization mechanism of foam flooding and distribution situation of residual oil in fractured-vuggy carbonate reservoirs

The development of fractured-vuggy carbonate reservoirs is extremely difficult because of the complex fractured-vuggy structure and strong heterogeneity.Foam flooding is a potential enhanced oil recovery(EOR)technology in fractured-vuggy carbonate reservoirs.Based on the similarity criterion,three types of 2D visual physical models of the fractured-vuggy structure were made by laser ablation technique,and a 3D visual physical model of the fractured-vuggy reservoir was made by 3D printing technology.Then the physical analog experiments of foam flooding were carried out in these models.The experimental results show that foam can effectively improve the mobility ratio,control the flow velocity of the fluid in different directions,and sweep complex fracture networks.The effect of foam flooding in fractures can be improved by increasing foam strength and enhancing foam stability.The effect of foam flooding in vugs can be improved by reducing the density of the foam and the interfacial tension between foam and oil.Three types of microscopic residual oil and three types of macroscopic residual oil can be displaced by foam flooding.This study verifies the EOR of foam flooding in the fractured-vuggy reservoir and provides theoretical support for the application of foam flooding in fractured-vuggy reservoirs.

Mechanism investigation of steam flooding heavy oil by comprehensive molecular characterization

Steam flooding is a widely used technique to enhance oil recovery of heavy oil.Thermal viscosity reduction and distillation effect are considered as two main displacement mechanisms in steam flooding process.However,the molecular composition understanding and contribution for oil production are still unclear.In this study,the composition analysis of the heavy oil was investigated in the core scale steam flooding process with the temperature from 120 to 280℃.The crude oil,produced oils and residual oils were characterized comprehensively by gas chromatography and high-resolution mass spectrometry.It is found that steam flooding preferentially extracts aromatics and remains more resins in the residual oil.Viscosity reduction is the dominant mechanism when steam is injected at a low temperature.Large molecular heteroatoms with high carbon number and high double bond equivalent(DBE)are eluted into the produced oil,while compounds with low carbon number and low DBE are remained in the residual oil.As the steam temperature rises,the increased distillation effect results in the extraction of light hydrocarbons from the residual oil to the produced oil.More small heteroatoms with low carbon number and low DBE enter into the produced oil,especially in the none water cut stage.The compositional difference of produced oils is characterized in DBE versus carbon number distribution of the N and O containing compound classes.This work uses a variety of composition analysis methods to clarify the steam flooding mechanism and provides a novel understanding of steam flooding mechanisms with various temperatures and production stages from the molecular perspective.

Investigation of feasibility of alkali-cosolvent flooding in heavy oil reservoirs

Cold production is a challenge in the case of heavy oil because of its high viscosity and poor fluidity in reservoir conditions.Alkali-cosolvent-polymer flooding is a type of microemulsion flooding with low costs and possible potential for heavy oil reservoirs.However,the addition of polymer may cause problems with injection in the case of highly viscous oil.Hence,in this study the feasibility of alkali-cosolvent(AC)flooding in heavy oil reservoirs was investigated via several groups of experiments.The interfacial tension between various AC formulations and heavy crude oil was measured to select appropriate formulations.Phase behavior tests were performed to determine the most appropriate formulation and conditions for the generation of a microemulsion.Sandpack flooding experiments were carried out to investigate the displacement efficiency of the selected Ac formulation.The results showed that the interfacial tension between an AC formulation and heavy oil could be reduced to below 1o-3 mN/m but differed greatly between different types of cosolvent.A butanol random polyether series displayed good performance in reducing the water-oil interfacial tension,which made it possible to form a Type Il microemulsion in reservoir conditions.According to the results of the phase behavior tests,the optimal salinity for different formulations with four cosolvent concentrations(0.5 wt%,1 wt%,2 wt%,and 3 wt%)was 4000,8000,14000,and 20000 ppm,respectively.The results of rheological measurements showed that Type Ill microemulsion had a viscosity that was ten times that of water.The results of sandpack flooding experiments showed that,in comparison with waterflooding,the injection of a certain Ac formulation slug could reduce the injection pressure.The pressure gradient during waterflooding and AC flooding was around 870 and 30-57 kPa/m,respectively.With the addition of an AC slug,the displacement efficiency was 30%-50%higher than in the case of waterflooding.

Layered injection technology for chemical flooding of Class-Ⅲ oil reservoirs in the Daqing Oil Fields complex,Songliao Basin,Northeast China

The Class-Ⅲ oil reservoirs of Lasaxing oilfield in the Daqing Oil Fields complex have geological oil reserves of 1.86 billion tonnes,an oil recovery of 39%,with remaining reserves accounting for more than 45%of the total geological reserves of the oilfield.Therefore,they have considerable potential for future oil production.The current layered injection technologies fail to achieve effective control over the low single-layer injection rates since they can only produce low throttle differential pressure under low injection rates(5-20 m^(3)/d).In this study,a symmetrically-structured double-offset-hole injection allocator and a novel throttling component were developed.Their spatial layout was constructed and mechanical parameters were optimized using finite element analysis,which allows for expanding the flow rate range at low injection rates.According to experimental results,the throttle differential pressure increased from 0.2 MPa to 0.8 MPa at an injection rate of 5 m^(3)/d,and the range of the single-layer flow rates expanded from 20-70 m^(3)/d to 5-70 m3/d.The field test results show that the effective production of oil layers with medium and low permeability was achieved and that the ratio of producing oil layer thickness to the total reservoir thickness increased by 9.7%on average.Therefore,this study provides valuable technical support for the effective chemical-flooding-based development of Class-Ⅲ oil reservoirs.

Necessity and feasibility of improving the residual resistance factor of polymer flooding in heavy oil reservoirs

The efficiency of water flooding in heavy oil reservoirs would be improved by increasing the viscosity of the displacing phase, but the sweep efficiency is not of significance due to the low mobility of the vicious oil. On the basis of mobility control theory, increasing the residual resistance factor not only reduces the water-oil mobility ratio but also decreases the requirement for viscosity enhancement of the polymer solution. The residual resistance factor caused by hydrophobic associating polymer solution is higher than that caused by polyacrylamide solution in brine containing high concentrations of calcium and magnesium ions. The results of numerical simulations show that the polymer flooding efficiency improved by increasing the residual resistance factor is far better than that by only increasing solution viscosity. The recovery factor of heavy oil reservoirs （70 mPa·s） can be enhanced by hydrophobic associating polymer solution of high residual resistance factor （more than 3） and high effective viscosity （24 mPa·s）. Therefore, increasing the residual resistance factor of the polymer solution not only decreases the requirement for the viscosity of polymer solution injected into heavy oil reservoirs but also is favorable to enhanced oil recovery during polymer flooding.

Advanced treatment of oil recovery wastewater from polymer flooding by UV/H_2O_2/O_3 and fine filtration

In order to purify oil recovery wastewater from polymer flooding （ORWPF） in tertiary oil recovery in oil fields, advanced treatment of UV/H2O2/O3 and fine filtration were investigated. The experimental results showed that polyacrylamide and oil remaining in ORWPF after the conventional treatment process could be effectively removed by UV/H2O2/O3 process. Fine filtration gave a high performance in eliminating suspended solids. The treated ORWPF can meet the quality requirement of the wastewater-bearing polymer injection in oilfield and be safely re-injected into oil reservoirs for oil recovery.

Low gas-liquid ratio foam flooding for conventional heavy oil

The recovery of heavy oil by water flooding is 10% lower than that of conventional crude oil, so enhanced oil recovery （EOR） is of great significance for heavy oil. In this paper, foam flooding with a gas-liquid ratio （GLR） of 0.2：1 for the Zhuangxi heavy oil （325 mPa.s at 55 ℃） was performed on cores, sand packs and plate model. In sand pack tests, polymer enhanced foam flooding increased oil recovery by 39.8%, which was 11.4% higher than that for alkali/surfactant/polymer （ASP） flooding under the same conditions. Polymer enhanced foam flooding in plate models shows that the low GLR foam flooding increased oil recovery by about 30%, even when the extended water flooding was finished at 90% water cut. Moreover, it was discovered by microscopy that foam was more stable in heavy oil than in light oil. These results confirm that low GLR foam flooding is a promising technology for displacing conventional heavy oil.

Study on Oil Displacement Efficiency of Binary Compound Flooding in Heterogeneous Reservoir

Heterogeneous reservoir characteristics for oilfield, choose HS-1 non-ionic surfactant and polymer formation in binary combination flooding system can significantly improve the rate of production of low permeability reservoir in heterogeneous reservoir. According to the core flooding experiment analyzed longitudinal heterogeneous models, single surfactant and a single polymer and polymer flooding of table binary complex drive effect. Studies show that binary combination flooding recovery effect is best, followed by polymer flooding, minimum of surfactant flooding, in heterogeneous reservoir.

Oxidization characteristics and thermal miscible flooding of high pressure air injection in light oil reservoirs

Physical modeling,numerical simulation and field case analysis were carried out to find out the subsurface thermal oxidation state,thermal oxidation front characteristics and production dynamic characteristics of high pressure air injection thermal oxidation miscible flooding technology.The lighter the composition and the lower the viscosity of the crude oil,the lower the fuel consumption and the combustion temperature are.The thermal oxidation front of light oil and volatile oil can advance stably,and a medium-temperature thermal oxidation stable displacement state can be formed in the light oil reservoir under high pressure conditions.With strong thermal gasification and distillation,light oil and volatile oil are likely to form a single phase zone of gasification and distillation with thermal flue gas at the high-temperature and high-pressure heat front,finally,an air-injection thermal miscible front.In light oil reservoirs,the development process of high-pressure air-injection thermal miscible flooding can be divided into three stages:boosting pressure stage,low gas-oil ratio and high-efficiency stable production stage and high gas-oil ratio production stage.Approximately 70%of crude oil is produced during the boosting pressure stage and low gas-oil ratio high-efficiency and stable production stage.

Treatment of Oil Field Wastewater after Chemical Flooding by Fenton Oxidation Combined with Biodegradation

Wastewater after chemical flooding is difficult to be treated by biological methods due to the residual chemicals in the wastewater. Fenton oxidation, which has been widely applied to detoxifying hazardous organic compounds and improving the biodegradability of these compounds, can solve this problem. So, in this paper, Fenton oxidation was used to enhance the biodegradability of the wastewater after chemical flooding. The op- timum operating conditions for the Fenton oxidation process were 2.5 g/L Fe2 ＋ , 600 mg/L H202 and 30 rain of treatment time. The wastewater pre- treated by Fenton oxidation was treated by aerobic activated sludge. It was found that a small amount of H202 can significantly increase both the biodegradation rate and extent of the wastewater. The effluent CODc, and HPAM contents were 260 and 94 mg/L respectively after the aerobic bio- logical process with the biodegradation time of 24 h. After Fenton oxidation combining with biodegradation, the total CODc, removal efficiency was up to 90%, and polymer and oil degrading efficiencies were 95% and 92% respectively.

Enhanced oil recovery mechanisms of polymer flooding in a heterogeneous oil reservoir

Taking reservoir rocks and fluids of the Daqing,Dagang and Changqing oilfields as research objects,the EOR mechanisms and technical approach of polymer flooding were discussed.By comparing the displacement performances of ordinary polymer,glycerol,polymer in"sheet-net"structure and heterogeneous weak gel at the same viscosity and concentration,the relationship between the viscosity of polymer displacement agents and displacement performance was demonstrated,and the method of improving polymer flooding effect was worked out.The main mechanism of polymer flooding to increase oil recovery is the swept volume expansion of water injection due to polymer retention in porous media.The viscosity of polymer agents has no positive correlation with polymer flooding effect.Although polymer of"sheet-net"structure has strong capacity in increasing viscosity,it has poor compatibility with pore throat structure of reservoir rock,low injectivity and low shear resistance.Heterogeneous weak gel system has higher adsorption and capture capacity in porous media,which is easy to retain in porous media,and can effectively establish seepage resistance in high permeability layers(zones).Compared with polymer solutions with the same viscosity or concentration,it has stronger ability to expand swept volume.Long term injection of polymer flooding agents will inevitably lead to fluid entry profile reversal,and thus worsening of polymer flooding effect.Alternate injection of high retention and low or non-retention displacement agents can further improve the displacement effect of polymer flooding agents.

Research and Application of CO₂Flooding Enhanced Oil Recovery in Low Permeability Oilfield

This paper discusses the new progress and field application of CO2 flooding in low permeability reservoirs enhanced oil recovery. The study shows that CO2 flooding can improve the oil recovery rate of low permeability oilfield by more than 10%. The practice shows that the liquid CO2 injection in low permeability reservoir is easier than water injection, and the reservoir generally has better CO2 storage.

Improving oil recovery in the CO2 flooding process by utilizing nonpolar chemical modifiers

By means of experiments of C02 miscibility with crude oil, four nonpolar chemicals were evaluated in order to enhance the miscibility of C02 with crude oil. Through pre-slug injection and joint injection of toluene in CO2, crude oil displacement experiments in the slim-tube were conducted to investigate effects of the toluene- enhanced C02 flooding under simulated subterranean reservoir conditions. Experimental results showed that toluene can enhance extraction of oil into C02 and dissolution of C02 into oil with the increment of 251% and 64% respectively. Addition of toluene can obviously improve the oil recovery in either pre-slug injection or joint injection, and the crude oil recovery increased with the increase of the toluene concentration. The oil recov- ery can increase by 22.5% in pre-slug injection with the high toluene concentration. Pre-slug injection was recom- mended because it can consume less toluene than joint injection. This work could be useful to development and application of the CO） flooding in the oil recoverv as well as CO2 emission reduction.

Steam Flooding after Steam Soak in Heavy Oil Reservoirs through Extended-reach Horizontal Wells

This paper presents a new development scheme of simultaneous injection and production in a single horizontal well drilled for developing small block reservoirs or offshore reservoirs. It is possible to set special packers within the long completion horizontal interval to establish an injection zone and a production zone. This method can also be used in steam flooding after steam soak through a horizontal well. Simulation results showed that it was desirable to start steam flooding after six steam soaking cycles and at this time the oil/steam ratio was 0.25 and oil recovery efficiency was 23.48%. Steam flooding performance was affected by separation interval and steam injection rate. Reservoir numerical simulation indicated that maximum oil recovery would be achieved at a separation section of 40-50 m at steam injection rate of 100-180 t/d; and the larger the steam injection rate, the greater the water cut and pressure difference between injection zone and production zone. A steam injection rate of 120 t/d was suitable for steam flooding under practical injection-production conditions. All the results could be useful for the guidance of steam flooding projects.

Three-Dimensional Imaging of Pore-Scale Water Flooding Phenomena in Water-Wet and Oil-Wet Porous Media

The penetration of water during water flooding has been observed over many years using several methods. A microfocused X-ray computed tomography scanner can be used to directly observe 3D water flooding in a nondestructive manner. To eliminate the possibility of false images being produced because of X-ray broadening effects, we developed a visualization method by arranging the brightness distribution of all phases involved. Water flooding experiments were conducted using oil-wet and water-wet porous media. The water phase was injected upward into packed glass beads containing an oil phase, and the process was scanned every minute until steady state was reached. Using this scheme, real-time, the water invasion pattern and oil trapping process in clusters of pores and individual pores can be observed clearly. By eliminating false images, the boundary of each phase could be identified with high precision, even in a single pore. Porelevel phenomena, including snap off (which has never before been captured in a real 3D porous medium), piston-like displacement, and the curvature of the interface, were also observed. Direct measurement of the pore throat radius and the contact angle between the wetting and nonwetting phases gave an approximation of the capillary pressure during the piston-like displacement and snap-off processes.

Factors influencing oil recovery by surfactant-polymer flooding in conglomerate reservoirs and its quantitative calculation method

This study aims to clarify the factors influencing oil recovery of surfactant-polymer(SP)flooding and to establish a quantitative calculation model of oil recovery during different displacement stages from water flooding to SP flooding.The conglomerate reservoir of the Badaowan Formation in the seventh block of the Karamay Oilfield is selected as the research object to reveal the start-up mechanism of residual oil and determine the controlling factors of oil recovery through SP flooding experiments of natural cores and microetching models.The experimental results are used to identify four types of residual oil after water flooding in this conglomerate reservoir with a complex pore structure:oil droplets retained in pore throats by capillary forces,oil cluster trapped at the junction of pores and throats,oil film on the rock surface,isolated oil in dead-ends of flow channel.For the four types of residual oil identified,the SP solution can enhance oil recovery by enlarging the sweep volume and improving the oil displacement efficiency.First,the viscosity-increasing effect of the polymer can effectively reduce the permeability of the displacement liquid phase,change the oil-water mobility ratio,and increase the water absorption.Furthermore,the stronger the shear drag force of the SP solution,the more the crude oil in a porous medium is displaced.Second,the surfactant can change the rock wettability and reduce the absorption capacity of residual oil by lowering interfacial tension.At the same time,the emulsification further increases the viscosity of the SP solution,and the residual oil is recovered effectively under the combined effect of the above two factors.For the four start-up mechanisms of residual oil identified after water flooding,enlarging the sweep volume and improving the oil displacement efficiency are interdependent,but their contribution to enhanced oil recovery are different.The SP flooding system primarily enlarges the sweep volume by increasing viscosity of solution to start two kinds of residual oil such as oil droplet retained in pore throats and isolated oil in dead-ends of flow channel,and primarily improves the oil displacement efficiency by lowing interfacial tension of oil phase to start two kinds of residual oil such as oil cluster trapped at the junction of pores and oil film on the rock surface.On this basis,the experimental results of the oil displacement from seven natural cores show that the pore structure of the reservoir is the main factor influencing water flooding recovery,while the physical properties and original oil saturation have relatively little influence.The main factor influencing SP flooding recovery is the physical and chemical properties of the solution itself,which primarily control the interfacial tension and solution viscosity in the reservoir.The residual oil saturation after water flooding is the material basis of SP flooding,and it is the second-most dominant factor controlling oil recovery.Combined with the analysis results of the influencing factors and reservoir parameters,the water flooding recovery index and SP flooding recovery index are defined to further establish quantitative calculation models of oil recovery under different displacement modes.The average relative errors of the two models are 4.4%and 2.5%,respectively;thus,they can accurately predict the oil recovery of different displacement stages and the ultimate reservoir oil recovery.

Effects of pore structure on surfactant/polymer floodingbased enhanced oil recovery in conglomerate reservoirs

To understand the displacement characteristics and remaining oil displacement process by the surfactant/polymer(SP) flooding in cores with different pore structures, the effects of pore structure on the enhanced oil recovery of SP flooding was investigated at the pore, core and field scales through conducting experiments on natural core samples with three typical types of pore structures. First, the in-situ nuclear magnetic resonance core flooding test was carried out to capture the remaining oil variation features in the water flooding and SP flooding through these three types of cores. Subsequently, at the core scale, displacement characteristics and performances of water flooding and SP flooding in these three types of cores were evaluated based on the full-size core flooding tests. Finally, at the field scale, production characteristics of SP flooding in the bimodal sandstone reservoir and multimodal conglomerate reservoir were compared using the actual field production data. The results show: as the pore structure gets more and more complex, the water flooding performance gets poorer, but the incremental recovery factor by SP flooding gets higher;the SP flooding can enhance the producing degree of oil in 1-3 μm pores in the unimodal and bimodal core samples, while it produces largely oil in medium and large pores more than 3 μm in pore radius in the multimodal core sample. The core flooding test using full-size core sample demonstrates that the injection of SP solution can significantly raise up the displacement pressure of the multimodal core sample, and greatly enhance recovery factor by emulsifying the remaining oil and enlarging swept volume. Compared with the sandstone reservoir, the multimodal conglomerate reservoir is more prone to channeling. With proper profile control treatments to efficiently enlarge the microscopic and macroscopic swept volumes, SP flooding in the conglomerate reservoir can contribute to lower water cuts and longer effective durations.

Characteristics and mechanism of smart fluid for sweep-controlling during CO_(2) flooding

A smart response fluid was designed and developed to overcome the challenges of gas channeling during CO_(2)flooding in low-permeability,tight oil reservoirs.The fluid is based on Gemini surfactant with self-assembly capabilities,and the tertiary amine group serves as the response component.The responsive characteristics and corresponding mechanism of the smart fluid during the interaction with CO_(2)/oil were studied,followed by the shear characteristics of the thickened aggregates obtained by the smart fluid responding to CO_(2).The temperature and salt resistance of the smart fluid and the aggregates were evaluated,and their feasibility and effectiveness in sweep-controlling during the CO_(2)flooding were confirmed.This research reveals:(1)Thickened aggregates could be assembled since the smart fluid interacted with CO_(2).When the mass fraction of the smart fluid ranged from 0.05%to 2.50%,the thickening ratio changed from 9 to 246,with viscosity reaching 13 to 3100 mPas.As a result,the sweep efficiency in low-permeability core models could be increased in our experiments.(2)When the smart fluid(0.5%to 1.0%)was exposed to simulated oil,the oil/fluid interfacial tension decreased to the level of 1×10^(-2)mN/m.Furthermore,the vesicle-like micelles in the smart fluid completely transformed into spherical micelles when the fluid was exposed to simulated oil with the saturation greater than 10%.As a result,the smart fluid could maintain low oil/fluid interfacial tension,and would not be thickened after oil exposure.(3)When the smart fluid interacted with CO_(2),the aggregates showed self-healing properties in terms of shear-thinning,static-thickening,and structural integrity after several shear-static cycles.Therefore,this fluid is safe to be placed in deep reservoirs.(4)The long-term temperature and salt resistance of the smart fluid and thickened aggregates have been confirmed.

Technologies of enhancing oil recovery by chemical flooding in Daqing Oilfield, NE China

By tracking and analyzing the research and practices of chemical flooding carried out in the Daqing Oilfield, NE China since the 1970 s, the chemical flooding theory, technology adaptability and existing problems were systematically summarized, and directions and ideas of development in the future were proposed. In enhanced oil recovery by chemical flooding, the Daqing Oilfield developed theories related to compatibility between crude oil and surfactant that may form ultra-low interfacial tensions with low-acidity oil, and a series of surfactant products were developed independently. The key technologies for chemical flooding such as injection parameter optimization and numerical simulation were established. The technologies of separation injection, cleansing and anti-scaling, preparation and injection, and produced liquid processing were developed. The matching technologies of production engineering and surface facilities were formed. Through implementation of chemical flooding, the Daqing Oilfield achieved outstanding performances with enhanced recovery rate of 12% in polymer flooding and with enhanced recovery rate of 18% in ASP flooding. To further enhance the oil recovery of chemical flooding, three aspects need to be studied:(1) fine characterization of reservoirs;(2) smart and efficient recovery enhancement technologies;(3) environment friendly, high-efficiency and smart matching processes.