Research Status and Development Trend of Air-Foam EOR Technology ()
1. Introduction
Conventional gas injection technology (such as nitrogen, carbon dioxide or natural gas) is an efficient oil recovery technology for the oil field after water injection development, but the problems are insufficient gas sources and high cost. Air injection EOR technology has been paid more and more attention by oil companies both in China and abroad because of its low cost and abundant gas sources [1] . At present, air injection becomes a potential technology which is more economical and feasible to excavate the residual oil in low permeability and medium potential reservoirs. Air-foam injection combines the advantages of air flooding and foam flooding.
For air foam injeciton, the theory of low temperature oxidation kinetics, seepage characteristics, large channel plugging and oil displacement are still not clear, and the reservoir screening criteria need further analysis and verification.
A lot of research for air injection have been carried out such as universities and companies in the United States, Britain, Canada and other countries, China Petroleum University, PetroChina Exploration Institute, Southwest Petroleum University, China University of Geosciences, Tuha Oilfield, Shengli Oilfield and others.
At present, there is no complete mathematical model and simulation method for air injection low temperature oxidation simulation in China and abroad, which can be indirectly used in numerical simulation of air injection foam reservoir.
Since the 1960s, a great deal of work has been done abroad, mainly in the United States, to improve the recovery of light oil reservoirs by air injection. The technology and theory are well studied. Air injection has been applied in many field and the supporting technology of air injection is gradually improved, but there is less study on air foam flooding. So we need to systematically summary the air-foam enhanced oil recovery (EOR) technology from the aspects of oil displacement mechanism, laboratory experiment, numerical simulation and field application.
2. Mechanism of Enhanced Oil Recovery Flooding with Air Foam
Based on lab experiments and mathematical expressions, the mechanisms of enhanced oil recovery flooding with air foam are summarized such as low temperature oxidation, high pressure air injection, air-assisted steam puff heavy oil extraction and so on.
2.1. Air Injection Low Temperature Oxidation to Enhance Oil Recovery Technology
1) Principle of low temperature oxidation
After the air is injected into the reservoir, the oxygen and crude oil react exothermicly, the degree of reaction is related to the characteristics of crude oil, rock and fluid, temperature and pressure, but not to the partial pressure of oxygen; according to the Moore, the reaction degree is related to the characteristics of crude oil, rock and fluid, temperature and pressure [2] [3] [4] . The point view of Moore et al., under the condition of oil layer, the oxidation processes of light oil and heavy oil are different. There are two reactions between injected air and crude oil: low temperature oxidation (LTO) and high temperature oxidation (HTO). The reaction pathway is also divided into two reactions: oxidation reaction and decarbonization reaction. The reaction mechanism of the LTO is certain (especially saturated) hydrocarbons, oxides that consume oxygen to form carbon and oxygen-containing hydrocarbons (such as acids, aldehydes, ketones, alcohols, etc.), while the radical R・, RO・ and HO・ in crude oil can continue to react with oxygen or other hydrocarbons; the oxidation reaction will continue and the oxygen concentration will gradually decrease in a continuous manner; the heat generated increases the oil layer temperature and causes the evaporation of oil light components. So the gas which drive oil finally are the CO, CO2, N2 generated in the reservoir and evaporated light hydrocarbon components and other components of the flue gas.
2) High pressure air injection (HPAI)
The compressed air is injected into the high pressure light oil reservoir, and the oxygen in the air at high temperature (generally higher than 60˚C) reacts with a small part of the crude oil in the reservoir to form CO2. The main flue gas flooding process, including high temperature oxidation mixture air drive (HTO-MAF) and low temperature oxidation mixture air drive (LTO-MAF), does not need to ignite, and the oxidation reaction is maintained in the range of 150˚C - 300˚C.
3) Air-assisted steam puff heavy oil extraction
Ren [5] proposed the process of steam huff and puff and steam drive, air injection is proposed to improve recovery and engineering benefit of heavy oil reservoir. After drilling, the heavy oil layer is completed with casing sand control, artificial bottom hole is built during completion, hot steam is injected into completion (more than 200˚C) section, air can be injected into the same reservoir after the reservoir temperature rises, and then the well is shut down. Oxidation of crude oil and oxygen occurs. A low-temperature oxidation reaction between oxygen and crude oil releases heat and produces CO2 and N2. The oxidation reaction can crack heavy oil to change crude oil composition, improve crude oil fluidity and oil quality, and most of the oxygen in the air can be consumed by oxidation reaction. Rich air resources can reduce economic costs.
2.2. Air Foam Injection Technology for Enhanced Recovery
Air foam flooding technology combines the advantages of air flooding and foam flooding, which can not only improve the sweep coefficient, but also improve the efficiency of oil flooding. Considering the actual situation of heterogeneous reservoir, it is necessary to apply this technology in two stages: air foam profile control and air foam flooding. The former is mainly characterized by regulating gas drive or water drive by air foam plug, selectively plugging hypertonic layer (or large pore) or high water-bearing formation, controlling gas injection/water profile and reducing gas production/water quantity. The latter is mainly to improve oil recovery by using air foam to produce water drive or other displacement methods.
2.3. Reservoir Adaptability and Screening Criteria
According to field test and reservoir screening criteria for low temperature oxidation of air drive [6] [7] , reservoir screening criteria for air foam flooding are recommended combined with the characteristics of foam flooding technology (see Table 1).
3. Indoor Study on Enhanced Oil Recovery with Air Foam Injection
Previous techniques of air-foam injection for oil recovery in light reservoirs have carried out a lot of indoor oil recovery [8] - [18] and security control [19] [20] [21] [22] [23] . However, the research system for air-foam enhanced oil recovery was not formed. The research aims are as follows: 1) selecting suitable reservoirs
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Table 1. Reservoir adaptability and screening criteria.
for air/air foam injection, defining its low temperature oxidation and oil displacement mechanism, studying the oxidation performance and technical feasibility of crude oil in target reservoir; 2) studying oxygen consumption, oxidation reaction rate and its influencing factors during air/air foam injection and its influence on safety control; establishing oxidation kinetics model to evaluate the oxidation activity and property change of crude oil in reservoirs; 3) studying the sealing ability of air foam injection, the influence law of oil displacement efficiency and improving recovery mechanism; the main engineering and production parameters are determined, the reference range of the explosion limit and critical oxygen content of combustible gas are given, and the corresponding field monitoring scheme and safety control measures are formulated.
3.1. Indoor Air Injection Experiment in Light Oil Reservoirs
3.1.1. Low Temperature Oxidation and Composition Analysis
This part mainly includes static oxidation experiment, dynamic oxidation experiment and crude oil composition analysis experiment. In the 1980’s to1990’s, Low temperature oxidation experiments were successfully carried out in West Hackberry, MPHU, Buffalo, Horse Creek oil fields in the United States and Maureen and North Sea oil fields in the United Kingdom (static ARC oxidation and dynamic combustion tube oxidation experiments). In the’90s, at the University of Bath in the United Kingdom, researchers first proposed the concept of low-temperature oxidation oxygen consumption rate. The temperature increment of the sand filling model with the adiabatic reaction was not detected. Calgary University of Canada has carried out 3000 - 4000 air injection experiments, and water-driven crushing core combustion tube experiment. Air injection oil recovery in high water-cut light oil reservoir is feasible, and the residual oil displaced by combustion front is enough to supply the consumption of fuel crude oil.
In recent years, The static constant temperature cycle experiment of air injection for crude oil from Tuha Shanshan oilfield, the thermal analysis experiments of high pressure injection air acceleration, pressure differential thermal analysis (PDSC) and thermogravimetric and differential thermal analysis (TG/DSC) were carried out by Southwest Petroleum University in China; Ren Shaoran et al. in China University of Petroleum did the static oxidation, dynamic oxidation and dynamic displacement experiments of air and air foam with different oil samples such as crude oil in Hu12 block of Zhongyuan oilfield, in Bin 425 block of Shengli oilfield and in Xinjiang oilfield. The indoor evaluation technique of air injection and air foam injection is studied systematically and comprehensively. Study on low temperature oxidation model of Cao Tai high condensate oil and block leng-1 heavy oil was proposed in the Changjiang University. If the temperature is higher than 120˚C, 6.86% - 10.24% crude oil can be produced by air injection with Cao Tai high condensate oil. Experiment on static low temperature oxidation of light crude oil in Liaohe Oilfield was conducted. The content of aromatic hydrocarbons decreased obviously and colloid content increased significantly. The physical properties of crude oil become worse. Experiment on low temperature oxidation and displacement of air injection with crude oil in Xindu 1 block in Liaohe Oilfield was reported. It is proved that aie injection technique is an effective method to replenish formation energy. The simulation experiment of low temperature oxidation process under reservoir conditions (90˚C, 13 MPa) shows that the content of carbon, hydrogen and nitrogen in crude oil has not changed and the oxygen content has increased. The simulation study of low temperature oxidation process of air injection was carried out by China University of Geosciences according to the actual reservoir conditions of Daqing, which proved that the change of crude oil composition in the oxidation process did not adversely affect the flow performance of crude oil.
3.1.2. Dynamic Displacement Experiment
The results show that the oxygen content in the air and reservoir temperature have few effects on the oxygen consumption. The effect of yield air drive is higher than that of nitrogen drive. A number of displacement experiments of air and air foam have been carried out at the University of Bath and the University of Petroleum of China, which prove the feasibility of air injection in different reservoirs.
3.2. Air Foam Injection for Oil Recovery
The micro and macro oil displacement mechanisms of foam have been studied detailly in China, but there is few study of air foam enhanced oil recovery. Some studies were reported by China University of Petroleum, Xi’an University of Petroleum, Zhongyuan Oilfield, Changqing Oilfield and other units. The results show that Hu12 block has good oxidation activity. The oxidation rate is related with the temperature, pressure, water saturation and clay content. Air foam has influence on static oxidation of crude oil, but not on dynamic oxidation under reservoir conditions. From air foam displacement experiments we find that when the gas-liquid ratio is between 1:1 to 2:1, the foam stability and plugging ability are improved with low temperature, high pressure and permeability. The air foam flooding can improve the oil displacement efficiency by 13% - 24%, which is suitable for improving the oil recovery in heterogeneous severe strata. Through laboratory experiments, Xi’an University of Petroleum evaluated the feasibility of air foam composite flooding in low permeability reservoirs to improve oil recovery, and expanded the application of air foam injection. The foaming property, foam stability and salt resistance of the foam agent are evaluated by the compound air foam flooding test of Chang-6 reservoir in Wuliwan Changqing Oilfield.
3.3. Air Foam Safety Control Experiments
The characteristics of gas corrosion and explosion were also studied in China Petroleum University, Daqing Oilfield and Zhongyuan Oilfield. The results show that the explosion range increases with the increment of temperature, pressure and oxygen content and the critical oxygen content decreases with the increment of temperature and pressure. Lu Xin et al. estimated the explosion limit of natural gas, which proved that the air foam flooding technology was safe and reliable. Daqing Oilfield Exploration Institute designed an explosion limit measuring device for combustible gas and studied the explosion limit of output gas after low temperature oxidation. The results show that the mixed gas prepared according to the proportion of output gas components did not explode under reservoir conditions. Aiming at the corrosion problem in the process of air foam flooding, Zhongyuan Oilfield Oil production Institute simulated the field test conditions, studied the effects of gas-liquid alternate injection frequency, air injection parameters (temperature, pressure, humidity, flow rate) and different foam dispensing systems on corrosion rate and also investigated the corrosion inhibition effect of corrosion inhibitor and sacrificial anode. The relevant corrosion rules and anticorrosion technology of air foam injection is reported. From the lab corrosion experiment we can conclude that the corrosion inhibition rate of the synergistic corrosion inhibition process between the inhibitor and the sacrificial anode is 92.4%.
4. Numerical Simulation of Enhanced Oil Recovery with Air Foam Injection
There is no complete mathematical model and simulation method indirectly used in numerical simulation of air injection foam reservoir [24] - [34] .
The softwares used most often are Canadian CMG thermal production module STARS and American SSI company thermal production reservoir simulator THERM. The numerical simulation is mainly used to study the enhanced recovery mechanism of air-foam injection, determine the necessary oxidation kinetic parameters, fit the indoor experiment of air displacement (oxidation kinetics experiment, core displacement experiment), predict the dynamic and design of field test and improve the reliability of air injection reservoir simulation and the feasibility of field implementation. At abroad, some researches about the over-injection air by using the black oil mixed-phase displacement simulator were proposed by S. Sakthikumar, BATH University of England, Horse Creek and West Hackberry oil fields in the United States and Maureen oil reservoirs in the United Kingdom. In China, the numerical simulation of air-foam injection was carried out by China Petroleum University, PetroChina Exploration Institute and Tuha Oilfield Exploration Institute.
The simulation process contains 1-D lab oil displacement experiment simulation, low temperature oxidation reaction simulation, air foam injection simulation et al.
4.1. LTO Kinetic Parameters with Matching Experimental Results
S. Sakthikumar and other researchers establish a one-dimensional model for indoor LTO experiment. According to the Arrhenius equation, the kinetic parameters of low temperature oxidation were determined by fitting the experimental results. In University of BATH, software CMG-STARS is used to establish 1D experimental model, 2D plane model and 3D reservoir model based on SBR test and oxidation tube test. It proves that the model has a good match with the experimental results and the sensitivity of the influence factors is analyzed. In Horse Creek oilfield, many works were done to determine the relevant process parameters, such as fitting of experimental results of combustion tube, simulation of radial displacement of air injection single well and simulation of whole oil field using numerical simulator of thermal production. SSI company’s THERM simulator was used to simulate the air injection gravity flooding process in the U.S. West Hackberry oil field. British Maureen reservoir simulated air injection using black oil mixed drive simulator, and two types of gas injection after water flooding were carried out in oil field for dynamic prediction of oil recovery.
On the basis of crude oil oxidation experiment, a low temperature oxidation kinetic model is established and the main influence factors are analyzed by the researchers in China University of Petroleum. At the same time, the improved low temperature oxidation reaction model is proposed. The low temperature oxidation kinetic parameters of the improved model are obtained by ARC experiment and the oxidation tube (OT) experiment results are fitted and verified by thermal mining numerical simulation method. CNPC applied numerical simulation method to fit the process of low permeability reservoir model experiments (core flooding, low temperature oxidation of slender tube and combustion tube experiment), and the model which has a good match with the experimental results is used to study the sensitivity of oxidation reaction parameters and the mechanism of air flooding. Air injection can build the reservoir pressure more efficiently than water injection. The contribution of nitrogen flooding to the total oil recovery of air flooding is 69% and the contributions of temperature rise and carbon dioxide to oil recovery are 26.7% and 4.3% respectively. The oil recovery of air flooding is 21.5% for 30 years. The development mechanism of air injection in light oil reservoirs, represented by Tuha Shanshan oilfield, was studied and elucidated by using the numerical simulation method of thermal production (low temperature oxidation reaction process, phase state change and distribution characteristics, reservoir energy change law).
4.2. Air Foam Simulation
The characteristics of air and foam are combined to achieve foam plugging and oil displacement. At present, the common numerical simulation methods of foam displacement are: semi-empirical relation model, total foam balance model, critical capillary force model, resistance coefficient model, component method, seepage and network model, semi-total balance model and so on. This paper focuses on the application of foam empirical models. In the foam empirical models, the foam mobility is characterized as a function of surfactant concentration, gas flow rate (or capillary number) and oil saturation, etc., and the principle of which is mainly by reducing the gas relative permeability, that is, gas phase permeability multiplied by the factor of interpolation dimensionless FM. In order to reduce the flow rate and adjust the displacement, three regions with different surfactant concentration are considered: no foam, weak foam and strong foam, which characterize the decrease of gas flow rate. The characteristics of foam plugging hypertonic layer in the model can be investigated by the change of gas mobility, pressure field and foam flow rate in the foam region.
5. Field Application of Air Foam Injection EOR Technology
In China, field tests and field applications of air foam flooding have been applied in Guangxi Baise, Zhongyuan, Shengli, Daqing, Qisheng, Yanchang and Changqing oil fields [35] - [40] . The oil reservoir parameters and oil displacement effect of air and air foam flooding are shown in Table 2. The test applications contain many types, such as air flooding, flue gas test, nitrogen foams, air foam injection. The injection pressure is 4.3 - 32.4 MPa and oil viscosity is 0.5 - 9 mPa・s.
The air flooding and air foam flooding technology in sandstone, limestone and carbonate reservoirs are applied successfully. It is suitable for low permeability reservoirs, high permeability heterogeneity and high temperature reservoirs and high salt reservoirs (such as Hu 12 block in Zhongyuan Oilfield).
The injectors with formation dip angle can improve the vertical sweep efficiency; there is good Foaming effect when the gas-liquid ratio of foam flooding is determined to be 1:1 to 1:2, and the foaming method is mainly foaming in the ground; and injection mode is the mixed air foam-air-water; the injection slugs are mainly multi round small slugs, and no safety accident occurred in the test.
6. Problems and Trends
6.1. Problems
1) There are potential safety problems, such as oxygen corrosion in injection wells, CO2 corrosion in production wells; and the, well explosion. It is necessary to pay more attention to the field safety monitoring, even no such safety accidents have been reported;
2) The mechanism of air-foam enhanced oil recovery is not well studied;
3) The foam injection system for the target reservoir is unstable and it is difficult to form a reliable system;
4) The influence of low temperature oxidation on reservoir crude oil is not clear;
5) The field size scale test is few and it is difficult to popularize air foam injection.
6.2. Trends and Directions
1) Air foam profile control or flooding in ultra-low permeability, low permeability and low temperature reservoirs;
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Table 2. Oil reservoir parameters and oil displacement effect of air-foam flooding field test.
2) Low-temperature oxidation technology for heavy oil reservoirs;
3) Air injection assisted steam huff and puff heavy oil recovery;
4) Mathematical model and numerical simulation of air foam injection for oil recovery;
5) Study on enhanced oil recovery seepage characteristics of air foam;
6) Study on the kinetic characteristics of low temperature oxidation of air;
7) Effect and influence of steam on oxidation kinetics at relatively low temperature in high pressure air injection (HPAI);
8) Assessment of reservoir potential of air foam flooding technology;
9) Air foam injection EOR technology system;
10) Oil recovery mechanism and reservoir adaptability evaluation of air foam;
11) Field practice and experience in air foam injection for oil recovery.
7. Conclusions
1) Air foam flooding technology combines the advantages of air flooding and foam flooding, which can not only improve the sweep coefficient, but also improve the displacement efficiency.
2) The explosion range increases with the increment of temperature, pressure and oxygen content and the critical oxygen content decreases with the increment of temperature and pressure, and air foam injection with oxygen reducing is safe to be conducted for a field pilot.
3) The air flooding and air foam flooding technology in sandstone, limestone and carbonate reservoirs are applied successfully. It is suitable for low permeability reservoirs, high permeability heterogeneity and high temperature reservoirs and high salt reservoirs.
Acknowledgements
This work is funded by SINOPEC Science and Technology Project P18080, the project funded by China Postdoctoral Science Foundation (2022M723499), and by a National Energy Administration Research and Development Center project for the “Sustainable Development of Mature Continental Sandstone Oilfields”. We thank the technology and translation supports from Dr. Yao Zhang, Dr. Quanqi Dai, Dr. Peng Wang, and Dr. Chuanjie Cheng.