Fine separate layer water injection is the most widely applied development technology in China’ s oilfields. Its core is to apply the “ fine” concept to all aspects of oilfield development, and closely integrate geology, drilling and production processes. Through fine reservoir characterization, the oil-bearing unit is subdivided into the “ single sand body” and “ internal structure” from the “ oil layer” and “ sublayer” , to enable quantitative separate layer water injection, quantified separate layer oil production, quantified remaining oil distribution in single layer and quantified remaining oil distribution in single microfacies^{[3, 4, 5]}. Through fine separate layer water injection and oil production, the control on injection and production systems and multi-directional connectivity of single sand body can be enhanced, and by profile control measures, the conflicts on injection-production profile and plane can be regulated, by dividing water injection layer finely and enhancing the ratio of separate layer water injection, the 3D optimization of reservoir can be realized on the whole to enhance the sweeping efficiency of water flooding^{[5, 6]}. Fine separate layer water injection has shown good performance in China’ s continental reservoirs, for example, in multi-layered sandstone reservoirs of Daqing, the waterflooding recovery efficiency of the main blocks (OOIP recovery ratio, the same below) has exceeded 45%, making significant contribution to the long-term stable production of the oilfield.

At present, fine separate layer water injection is facing the following challenges: (1) In high/extra-high water-cut stage, the strong inter-layer, intra-layer and lateral heterogeneity cause serious inefficient water injection and circulation. (2) Residual oil is further dispersed in both macro and micro scale, leading to difficult identification of the occurrence state and the discontinuity of seepage flow. (3) The loss of recoverable reserves caused by deterioration of injection and production systems. Due to reservoir characteristics and fluid property (such as high temperature, high salinity, developed fractures, and strong edge/bottom water), a large number of medium- and high-permeability reservoirs are difficult to be produced by chemical flooding and other substitutive technologies. Apart from developing suitable tertiary oil recovery technology, it is necessary to continue to improve the fine separate layer water injection technology. Based on better geologic understanding and more accurate characterization of reservoir, and better knowledge on remaining oil distribution, low-cost high-precision directional well drilling and in-situ profile control and water shutoff are to be developed to tap scattered remaining oil, enhance producing degree, and increase water flooding efficiency. Ionic tuning water flooding and a new dispersion system for displacement with adjustable seepage resistance need to be innovated to improve the sweeping volume and oil displacement efficiency further. If evolving into accurate directional water flooding and functional water displacement, fine separate layer water injection is expected to increase recovery ratio by over 5%, which will bring about considerable production potential.

After more than 30 years of research, polymer flooding was put into industrial scale application in the Daqing Oilfield in 1996, making it the main EOR technology for the medium-high permeability mature oilfields: (1) In recent years, under the support of the 973 Program, polymer flooding theory was further developed. It is believed that polymer flooding can not only reduce the water-oil mobility ratio, but also increase sweeping efficiency of the injected water in the oil layer; meanwhile, viscoelastic properties of the polymer can improve displacement efficiency, mainly because compared with Newtonian fluids and power-law fluids, polymer solution has a stronger microscopic force, which leads to residual oil deformation and oil film removal during the flow process, resulting in lower residual oil saturation^{[7, 8]}. (2) A series of polymer products with different molecular weights (medium-low, high, ultra high) and different structures (linear, branched, comb-shaped, star-shaped, associative, etc.) have been developed, and the world’ s largest polymer production capacity with an annual output of over 40 × 10⁴ t has been constructed. (3) The tailor-made optimization designs, separate layer injection with different molecular weights, surface injection allocation and profile control technology were developed.

The theoretical and technological advancement has further enhanced the efficiency of polymer flooding. Compared with water flooding, the technology has increased the recovery ratio by about 12% at present from about 8% in its initial industrial period. Up to date, the annual oil production by polymer flooding in China has exceeded 1 200 × 10⁴t, mature supporting industrial technologies have been developed, including high-concentration viscoelastic polymer flooding and polymer flooding for Class II reservoirs in the Daqing Oilfield, high-temperature and high-salinity polymer flooding for the Shengli Oilfield, and polymer flooding for the conglomerate reservoir in the Xinjiang Oilfield.

At present, polymer flooding is facing several challenges, mainly including limitation of displacement mechanism, limited oil recovery increment, excessive use of polymer, rapid water cut increase and production drop after polymer flooding, scattered remaining oil distribution after polymer flooding, and the lack of effective substitutive technology, so efforts need to be put into research on functional polymer technology, polymer flooding optimized design and enhanced oil recovery technology after polymer flooding.

Through years of technical research and field test, a series of technologies and industrial standards for ASP flooding (alkali, surfactant, and polymer) have been formed in different oilfields represented by the Daqing Oilfield, making China the sole country realize industrial application of ASP flooding^{[7, 9]}. Given the low-acid crude oil property in the Daqing Oilfield, a major breakthrough has been made in the theoretical research of ASP flooding, which has effectively guided the development of supporting technologies for ASP flooding: (1) molecular design theory of oil displacement agents (such as theory of surfactant arrangement on oil-water interface, quantitative molecular structure-property relationship and hydrophilic-lipophilic balance theory), physical-chemical percolation theory (chromatographic separation, emulsification and scale deposition, etc.), ultra-low interfacial tension formation mechanism of low-acid crude oil and main control factors, fine physical simulation and numerical simulation methods, etc.; (2) based on fine cutting and material selection, a series of alkyl benzene sulfonates and petroleum sulfonates were developed to form cheap and efficient strong- and weak-alkali ASP formulations; (3) program optimization design and multi- stage controlling techniques for improving sweeping control have been formed; (4) low-pressure binary (polymer + surfactant)--high-pressure binary (alkaline + surfactant) injection allocation process, which is characterized by uniform preparation and serpararted injection, has been developed; (5) scale- resistant pumps and scale prevention/scale removing agents have been developed, prolonging the pump inspection cycle to over 350 d; (6) crude oil dehydration and demulsification agents and processing equipment have been developed. The Daqing Oilfield started industrial application of ASP flooding in 2014, with oil production exceeding 400 × 10⁴ t in 2017. ASP flooding was mainly performed in the Changyuan Class II reservoirs. The strong- and weak- alkali ASP flooding have improved recovery ratio by over 20% than water flooding^[9](Fig. 1).

Fig. 1.

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	Fig. 1. Field test results of ASP flooding in the Daqing Oilfield.

Compared with strong-alkali (NaOH) ASP flooding, weak- alkali (Na₂CO₃, etc.) ASP flooding has fewer production maintenance troubles, such as scaling, corrosion, and processing problems due to emulsified produced liquid, etc., so it’ s more suitable for large-scale application. However, it is still necessary to further optimize the primary surfactant and production process of surfactant, expand application scope of the formula, and improve product stability; explore different alkali types such as organic alkali, complex alkalis, to reduce the adverse effects of the current alkalis; strengthen the research of alkali-resisting profile control before main slug to reduce the amount of main slug; to optimize the formula, plug combination and stage profile control to improve field application results; and to improve injection and production and processing technique for produced liquid to reduce costs and increase economic benefit. Meanwhile, research on EOR technology after ASP flooding should be stepped up.

The adding of alkali brings about troubles like scaling, corrosion and demulsification. In recent years, with the technological breakthroughs in new high-performance surfactants represented by betaine and anionic-nonionic surfactants, surfactant/ polymerization binary system can achieve ultra-low oil-water interfacial tension with no alkali added and make the oil-water system emulsify moderately, which facilitates the rapid development of SP flooding. At present, field tests have been carried out in a dozen oilfields such as Shengli, Liaohe, Xinjiang, Dagang, Daqing and Changqing. The field test results prove that the SP flooding in Liaohe and Xinjiang oilfields can increase recovery by about 18%. The SP flooding can not only improve recovery ratio, but also simplify surface treatment process, making it a major way to enhance oil recovery of high water-cut mature oilfields. At present, the main issues with this technology are that it is not as effective as ASP flooding in enhancing recovery and the positive effect of alkali (lower interfacial tension, easy emulsification, oil film removing, and lower adsorption) can’ t be completely replaced, so further optimization of the primary SP agents is still the key point. In addition, the surface injection and production supporting technologies need to be improved, and the current process is more like a simplified ASP flooding supporting process. If it can be developed into a matched process like polymer flooding, the construction and operation costs can be reduced by more than half than that of ASP^[10].

The foam flooding is an EOR method using foam as the displacement agent. The foam is composed of foamer, foam stabilizer, gas and water. The foamer selected is often a surfactant with strong foam-generating capability, the foam stabilizer is mostly a polymer (or a biopolymer), a gel, or a nanopowder, and the gas can be air, natural gas, CO₂, N₂, etc. Its oil displacement mechanism includes improving displacement efficiency and expanding sweeping volume.

During the migration in the formation, the foam preferentially enters high-permeability layers or micro-fractures with low oil saturation. Seepage resistance gradually increases due to the Jiamin effect and beading effect etc. With the increase of injection pressure, the foam will gradually flow into the low-permeability layers with high oil saturation, and thus effectively improving sweeping efficiency. Besides, the foamer itself is a kind of surfactant, which can reduce the oil-water interfacial tension and residual oil saturation, and improve oil displacement efficiency^[11]. Foam has unique advantages in treating gas/water channeling during gas/water injection and sweeping remaining oil at the top of thick oil layers. Therefore, it is a promising EOR technology. Through persistent research, we have developed foamer and foam stabilizer combinations with different levels of foaming ability and stability, which are suitable for different formation conditions. The pilot test in Bei 2 block east of the Daqing Oilfield, Gangdong block of the Dagang Oilfield, Hu 12 block of the Zhongyuan Oilfield and Laojunmiao block of the Yumen Oilfield have achieved good initial results. At present, lab evaluation and field testing show quite different results for optimized foam flooding formulas and their performance, so it is necessary to improve lab research evaluation methods as soon as possible. Centering on foam stability, low-cost, high-efficiency and stable foam systems should be developed and supporting technologies like surface injection and controlling should be optimized.

Combination of “ secondary + tertiary” means to optimize the overall deployment of water flooding and EOR well pattern. In the fine water flooding stage, well pattern suitable for EOR should be designed to tap the potential of the oil-bearing, especially thin oil-bearing layers, to realize smooth transition from water flooding to tertiary recovery and maximize the overall economic efficiency^{[10, 12]}. Under the current conditions, the combination of “ secondary + tertiary” is expected to increase the recovery ratio by 3-5% at fine water flooding stage, and by 15-20% at the chemical flooding stage, so that the overall oil recovery ratio will increase by 18-25%.

The combination of “ secondary + tertiary” technology has been applied in industrial scale in Daqing and Xinjiang oilfields, etc., achieving remarkable results. However, the shift timing from water flooding to tertiary recovery, well pattern adjustment, and well pattern layout of combination of “ secondary + tertiary” still need to be investigated, and the implementation plan needs to be optimized to continuously improve the overall results and benefits.

1.7.1. The Dual Parallel Simultaneous Injection and Production (SIP) Completion (downhole oil and water separation) technology

The Dual Parallel Simultaneous Injection and Production (SIP) Completion technology utilizes downhole oil-water separation equipment to separate produced water and oil. The produced oil with reduced water content is lifted to the ground, and the separated water is injected back into the formation to achieve simultaneous injection and production in the same wellbore. This technology can greatly reduce the energy consumption for lifting and cost of produced water treatment, making the development of ultra-high water-cut oilfields profitable, so it has broad application prospects^[13]. The downhole oil-water separation device is the key of this technology. The main separating methods include hydro-cyclone separation and multi-cup equal-flow gravity settlement. In 2016, it was tested in the North 1 fault east of the Daqing Oilfield after polymer flooding. Polymer flooding was started in 1998 in this block. In 2004, the water cut reached 98.2%, and then polymer injection stopped, and the well was shut down. In the early stage of the pilot test, the tested well saw a drop of water cut to 77.2%, and initial increase in liquid and oil production. The technology is currently in the stage of lab R& D and pilot testing, and further research needs to be carried out, including: (1) further improving the downhole oil-water separation technology, to increase the separation efficiency, reduce equipment cost and maintenance cost, and extend the service life of the equipment; (2) integrating reinjection monitoring technology, to accurately measure and monitor the water injection rate and injected layers; (3) strengthening systematic research by combining geology, reservoirs, engineering, and management together, and in particular, optimization of re-injected layers and building injection-production loop systems need deeper study and field test to improve the application and development performance.

1.7.2. Smart nano oil displacement agent

The principle of this technology is: Nano-displacement agent has size small enough to sweep the whole reservoir; strongly hydrophobic and oleophilic, it repels water and attracts oil, realizing self-driving and smart oil seeking; it can get dispersed oil together and capture the dispersed oil to form oil bank or oil-enrichment zone, which will be driven out of the formation. Some ideas have been realized in the laboratory test. The alkane-modified nanoparticles have enhanced hydrophobicity, attracted by the oil phase, they can spontaneously migrate to the oil-water interface and get adsorbed at the oil-water interface, and thus effectively reducing interface tension, so that the water carrying capacity of the oil phase is improved^[14]. The technology is expected to become a strategic subversive EOR technology, enhancing the estimated ultimate recovery ratio (EUR) to over 80%. Widely applicable to various types of reservoirs, it has broad application prospects. A lot of experiment and technical research still need to be done to fulfill the three key functions of this technology.

The medium-high permeability reservoirs represented by the Changyuan block of the Daqing Oilfield are the best hydrocarbon resources in China. Currently, they were largely developed by water flooding and chemical flooding. Although in the stage of “ double high, ” they still have great production potential. With highly dispersed remaining oil, how to continue to effectively tap remaining oil and increase oil recovery is the key point. Through the top-level design, the Daqing Oilfield has realized sustainable development. The 3rd-generation enhanced oil recovery technology represented by fine water flooding, polymer flooding and ASP flooding have enabled the recovery ratio to increase from 30% in early water flooding to 40%, 50% and 60% respectively in main oilfields, so that the recoverable reserves have increased substantially.

According to technical development route for the progresssive replacement of the three generations of main technologies, namely realizing mature technology application within 5 years, technological breakthrough within 5 to 10 years and leading reserve technology within 10 to 15 years, the following strategies are proposed: (1) At current stage, supportive technologies for fine separate layer water injection (accurate directional water injection and functional water flooding) and ASP flooding are to be developed to lower costs and enhance efficiency, which would increase the recovery ratio by 5% and 20% compared with the current water flooding. (2) Efficient chemical flooding, ASP flooding and foam flooding for Changyuan Class I reservoir after polymer flooding, Class II reservoirs after chemical flooding, and Class III reservoirs will be tackled as soon as possible; (3) leading reserve technologies, including SIP Completion and Intelligent Nano Oil Displacement Agent should be researched to provide orderly replacement technologies for the “ Double extra-high” stage.

From development practice, it’ s gradually known that for low-permeability reservoirs, especially extra-/ultra-low permeability reservoirs, the root cause for rapid water cut rise is the dynamic fractures in the formation^{[16, 17]}. It is a new geological characteristic generated by water flooding in extra-/ultra-low permeability reservoirs. The fractures include rock fractures closed in the original state but opened during long-term water flooding, or new fracture initiated due to overpressure by water flooding. Dynamic fractures are controlled by the geo-stress and extend with water injection and pressure increase, which greatly exacerbate reservoir heterogeneity and reduce sweeping efficiency of water injection^[17] (Fig. 2).

Fig. 2.

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	Fig. 2. Directional water channeling caused by dynamic fractures.

Through long-term field practice, the following techniques have been formed. The first is fracture-based water flooding (matching between fracture and well pattern). The key of the technology is to understand the role of “ fractures” in oilfield development. When the matrix permeability is high enough for effective oil displacement, the scale of high-pressure water injection should be minimized, to reduce the fracture initiation possibility and thus to make matrix displacement more efficient. For extra-/ultra-low permeability reservoirs where the oil in matrix is difficult to be displaced, proper fracturing should be carried out to make full use of fracture conductivity, fracture-based flooding, multiple rounds of injection and well-pattern infilling and flooding direction diversion in well- pattern should be taken to effectively increase the sweeping volume. The second includes supporting technologies such as enrichment zone selection, fracture identification and prediction, well spacing and well pattern optimization, deep-penetrating perforation and fine filtration of injected water. With these technologies, extra-/ultra-low permeability and naturally fractured low permeability oilfields, which were originally believed to be difficult to develop, have been successfully developed. The ideas of fracture-based flooding, the initiation and conversion of complex fracture network will be carried forward through the entire life cycle of low-permeability oilfield development.

The technology needs to be further improved in the following four aspects: (1) dynamic prediction of in-situ stress and fracture distribution, formation mechanism of dynamic fractures; (2) the influence of dynamic fractures on the water flooding swept volume; (3) matching between fracture and well pattern, optimization of hydrodynamic system for injection and production network, optimization of water injection policy; (4) integration of geology, reservoir, and engineering.

Gas injection (including injection of CO₂, N₂, natural gas, air, flue gas, etc.) is an important means of EOR for low-permeability oil reservoirs, especially extra-/ultra-low permeability reservoirs where oil displacement is difficult to be achieved through water flooding. The technology has broad application prospects. Among all kinds of gas injection technologies, the CO₂ storage and flooding technology which can significantly reduce greenhouse gas emissions while enhancing oil recovery, has received wide attention in the world^[18]. Through the research of 973 Program and pilot test, the theoretical knowledge on CO₂ miscible flooding in continental sedimentary reservoir has been further deepened in recent years, which is reflected in the following aspects: (1) Based on the composition characteristics of continental crude oil (Fig. 3) and PVT experiment of CO₂-formation oil, the rela-tionships between CO₂-crude oil miscibility and oil components have been identified, and a new viewpoint that “ C₇to C₁₅ are also important components influencing miscibility of CO₂-crude oil system” has been proposed, and a new understanding that not only C₂ to C₆ but also C₇ to C₁₅ are key hydrocarbon components for forming miscible phase with CO₂ has been suggested^[19]. (2) Based on the new understanding “ oil and gas form a transitional phase firstly due to mass transfer, and then form a miscible phase” , a miscible slug with low interfacial tension is set between CO₂ and oil, which can effectively improve the CO₂-oil miscibility. (3) The characterization of three-phase seepage law of CO₂ flooding has been improved, the three-phase relative permeability of oil, gas and water is measured by means of nuclear magnetic resonance (NMR) and CT scanning, mathematical model and numerical simulation method for CO₂ flooding have been established, and the algorithm for PVT and phase calculation of CO₂-oil system has been optimized^{[20, 21]}. In the meantime, the surface engineering, reservoir engineering, injection and production technologies for CO₂ storage and flooding have been formed. The field test and industrial application in Hei 59, Hei 79 and Hei 46 blocks of the Jilin Oilfield and Gao 89 Block of the Shengli Oilfield resulted in an increase of oil recovery by 7% to 17%. In addition, pilot tests including CO₂ flooding, air flooding, natural gas flooding, and nitrogen flooding have also been carried out in Daqing, Jiangsu, Dagang, Tuha, and Yanchang oilfields, etc., with good recovery results achieved^[21].

Fig. 3.

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	Fig. 3. Comparison of oil components in continental and marine formations[19].

In recent years, the gas injection and stable gravity flooding has been tested indoor and applied in the field. The field tests in the Donghetang reservoir of the Tarim Oilfield, and Liubei block of the Jidong Oilfield showed favorable results, and the core technologies such as gas-liquid interface control are basically mastered. Due to abundant fractures, strong heterogeneity and high miscibility pressure of crude oil in continental sedimentary low-permeability reservoirs, it is necessary to further strengthen research on technologies enhancing injected gas sweeping efficiency, improving miscibility, clarifying the effective CO₂ storage conditions, and improving the capacities of key equipment like compressors, etc., a closed injecting system should be established, meanwhile, it is also necessary to ensure gas supply and improve reservoir management.

In recent years, reservoir stimulation techniques, represented by separate layer fracturing for vertical well and multistage fracturing and volume fracturing for horizontal well, have been widely applied in low-permeability reservoirs. At present, the latter has become the main force for enhancing well production and recovery in ultra-low-permeability and tight reservoirs. Through research and pilot test, a number of fracturing technologies for horizontal well have been developed, such as hydraulic sandblasting, open-hole packer sliding sleeve, fracturing with dual-retrievable packers, pumping-type bridge plug, casing perforations and internal casing packers^[22]. Volume fracturing for horizontal well is to further increase the complexity of fracture system based on multistage fracturing, to initiate fractures in 3D reservoir space and greatly increase the stimulated volume of the formation. Rock mechanics has a great influence on volume fracturing operation of horizontal wells. Complex fracture networks are more easily formed in brittle rocks with natural fractures. Volume fracturing for horizontal well mainly adopts multi-stage fracturing method of “ casing completion + staged multi-cluster perforation fracturing + drillable bridge plug + slick water” . Among them, the staged multi-cluster perforation fracturing generating geostress disturbance to achieve volumetric stimulation is the key technology. At present, horizontal well fracturing technology has achieved important progress in fracturing optimization design and fracturing fluid system design. Large-scale stimulation operations with thousands of cubic meters of sands and tens of thousands of cubic meters of fluids in the tight reservoirs of the Changqing Oilfield have been carried out, reaching a production capacity of 100 × 10⁴ t. Multi-stage, multi- cluster, large-volume, low-cost large-scale stimulations with large displacement and large sand content are the main development directions of horizontal well fracturing. It is necessary to further study and draw on foreign unconventional oil and gas filed development experiences and strengthen the integration of geology, reservoir and engineering, so as to improve development effect, enhance factory-drilling operation management and reduce development costs.

2.4.1. Air foam flooding

Learning from foam flooding technology for medium-high permeability reservoirs, air foam flooding technology can also be applied in low-permeability naturally fractured reservoirs to block micro-fractures and expand the sweeping volume of following injected fluids. At present, field pilot tests have been conducted in Wuliwan blocks of the Changqing Oilfield and Da’ an blocks of the Oilfield, with some initial results achieved.

2.4.2. Viscoelastic surfactant flooding

Traditional chemical flooding methods, such as polymer flooding and ASP flooding, generally utilize the viscous characteristics of high molecular polymers to increase the sweeping volume of the injected fluids, but it is difficult to inject polymer into low-permeability reservoirs. Viscoelastic surfacetant flooding system, which has viscosity to some extent and can improve displacement efficiency, can reduce interfacial tension by using surfactant and adding less or no polymer. This type of surfactant features a certain degree of self- adaptability. In fractures or large pores, due to the high concentration, monomer molecules aggregate to form a 3D network structure, exhibiting viscous and viscoelastic characteristics. In the formation matrix, as the molecular size of the surfactant is much smaller than pore throat radius, it can easily enter the matrix, thus the “ pore throat plugging” phenomenon will not occur. In addition, due to the large shear rate in the near wellbore area, the 3D network structure is broken into monomer molecules, so that viscoelasticity drops, to facilitate injection^[23]. From its mechanism, we can tell it can be developed to a chemical flooding technology suitable for low-permeability reservoirs. The viscoelastic surfactant system developed in the lab has a viscosity of above 20 mPa• s at a concentration of 1000 mg/L, and features shear reversibility. In view of the large specific surface area and large adsorption capacity of low-permeability formation, we need further develop high- efficiency and low-adsorption agents to reduce the dosage, reduce adsorption loss and accelerate pilot tests.

2.4.3. Nanoparticle-assisted water flooding

For extra-/ultra-low permeability reservoirs, by injecting hydrophilic modified nano-systems to weaken the interaction forces between water molecules, ordinary water is turned into smaller molecules, which can reduce the pore entering resistance, so that the pores that have not been swept can now be swept by water, and thus greatly increasing displacement volume of extra-/ultra-low permeability reservoirs^[24].

Low-permeability reservoirs are the main contributor for increase of oil reserves and production both at present and in the future. Low-permeability reservoirs, represented by the Changqing Oilfield, the peripheral areas of the Daqing Oilfield and Jilin Oilfield, are mainly developed by water flooding. The keys for technical development are further increasing well productivity, updating development methods and reducing operation costs.

At present, fracture-based fine water flooding technology should be further improved, which is expected to increase recovery ratio by more than 5%; the development of miscible/non-miscible gas flooding, stable gravity flooding, imbibition oil recovery, and low molecular weight SP flooding technologies are expected to increase recovery ratio by more than 10%; replacement EOR technologies including foam flooding, viscoelastic surfactant flooding, nanoparticle-assisted water flooding, and microbial flooding are to be developed for backup. Gas flooding (with CO₂, hydrocarbon gas, air and N₂) technology plays a very important role in stabilizing production of such reservoirs, so relevant field tests should be stepped up and application should be expanded.

Steam huff and puff has been applied industrially in various heavy oil reservoirs, which currently face problems such as annual increase of operating costs and decline of oil-steam ratio after cycles of injection. For some heavy-oil reservoirs difficult to be produced by other modes, the key is to improve cyclic stimulation performance, and study how to effectively reduce viscosity and increasing sweeping volume by horizontal wells and additives, including CO₂ and N₂^[26]. For blocks that can be recovered by new methods, substitutive development methods such as steam flooding, SAGD, and fire flooding should be selected as soon as possible and adopted at proper moment.

Steam flooding is a mature thermal recovery technology, which has been successfully applied in heavy oil reservoirs with better formation connectivity, and realized large-scale industrial application, and it’ s an effective replacement EOR method after cyclic steam stimulation. In recent years, thermal recovery theory and development mode combining horizontal well with vertical gravity drainage have been proposed, high- temperature large-volume lifting, high-temperature high-pressure surface gathering, and waste heat recovery systems are integrated into a matching set of thermal recovery technology for medium-ultra deep heavy oil reservoirs. Thanks to the technology, bottom hole steam dryness has increased by 20%, and the depth limit for steam flooding has increased from 800 m to 1 400 m, and that for gravity drainage has increased from 600 m to 1 000 m. This makes effective development of heavy oil resources in the medium-ultra deep layers possible, with the recovery ratio increasing from 20%-25% of steam huff-and-puff to above 50%. In recent years, multi-medium steam flooding technology has also been developed. The multi-media system refers to a high-efficiency oil displacement system composed of gas, chemical agent and steam, which has the synergistic function of inhibiting steam channeling/overlap, reducing viscosity, replenishing formation energy, improving oil displacement efficiency, expanding sweeping volume and reducing steam usage^[27]. In the later stage of steam flooding, if multi-medium steam flooding is adopted, which is expected to further increase oil-steam ratio by 30% and recovery ratio by more than 10%. The reservoir in the late stage of steam huff-and-puff can be directly converted to multi-media steam flooding, which is expected to increase recovery by more than 30%.

Steam assisted gravity drainage (SAGD) refers to displacing ultra-heavy oil by using high-dryness steam as a heat source and gravity force of asphalt and condensate as driving force. Implemented in forms of dual horizontal well or combination of vertical well and horizontal well, this technology has become the main technology for the development of ultra-heavy oil reservoirs. At present, the SAGD recovery mechanism, numerical simulation, production prediction, and production scheme design optimization have been basically mastered. Combined with fine reservoir characterization, SAGD dynamic performance monitoring and regulation technology has been established, supportive techniques like ultra-shallow dual horizontal well drilling and completion, high-temperature large-volume lifting, surface high-efficiency steam injection, sealed treatment of high-temperature produced fluids, and integrated automatic surface-reservoir monitoring, have been developed. The technology has been working well in Du 84 block in the Liaohe Oilfield, Chong 32 and Chong 37 blocks in Fengcheng of the Xinjiang Oilfield. By the end of 2015, the SAGD vertical well-horizontal well pairs in Liaohe Oilfield had reached 48 pairs, with an annual oil production of over 90 × 10⁴t, and the recovery efficiency over 25% higher than steam huff-and-puff; the SAGD dual horizontal well pairs in Fengcheng of the Xinjiang Oilfield had reached 150 pairs, with the annual oil production exceeding 72 × 10⁴t^[28].

SAGD flooding is currently facing a series of problems such as strong formation heterogeneity and uneven expansion of the steam chamber. Therefore, the tasks of the next step are to focus on multi-medium assisted SAGD and SAGD application in wells with different structures. In the production process of SAGD, non-condensable gases, solvents (chemicals) and steam are injected into the oil layer together to accelerate viscosity reduction, development of the steam chamber, and reduce energy consumption and steam usage^[29]. It is expected to increase the oil-steam ratio by 30% and recovery ratio by more than 15% than conventional SAGD.

Fire flooding is an important thermal recovery method for heavy oil, in which air is continuously injected into the formation through the gas injection well and the oil layer is ignited to achieve heavy crude oil cracking, heavy component combustion and crude oil viscosity reduction in-situ. The cracked oil is propelled from the gas injection well to production well. It is accompanied by complex heat transfer, mass transfer as well as physical and chemical changes. It shares some common mechanisms of steam flooding, hot water flooding, and flue gas flooding^[30]. The advantages of the technology include the following aspects: (1) air injection cost is lower than that of steam injection; (2) it is higher in displacement efficiency and ultimate recovery ratio, which can reach 80% and 70%, respectively^[30]; (3) through subsurface high-temperature pyrolysis, in-situ crude oil property upgrading can be achieved to some extent; (4) good reservoir adaptability; (5) low energy consumption.

In recent years, the technology has achieved remarkable progress in fundamental theory, lab simulation and filed test. The fire flooding test in Hongqian-1 of the Xinjiang Oilfield was carried out in an abandoned reservoir with a recovery ratio of nearly 30% after steam cyclic stimulation and steam flooding. By the end of 2016, it had been in operation for 7 years continuously, with an additional recovery ratio of 25.2%, and average annual oil recovery rate of 3.6%. According to the numerical simulation, the estimated ultimate recovery rate (EUR) can be 65.1%, which is 30% higher than that by steam flooding^[31]. From experimental results, it can be seen fire flooding has obvious characteristics of high-temperature combustion. The analysis of samples cored from fire flooded formation shows that the remaining oil saturation in the oil layer is only 2.6% (Fig. 4). The crude oil property was significantly modified, the mass fraction of saturated hydrocarbon increased by 7%, and that of colloidal bitumen and asphaltene decreased by 2.5%; viscosity of the produced oil by steam flooding was 16 500 mPa• s at 20 ° C, and 3 381 mPa• s by fire flooding, 79.5% lower than previous value. At present, key technologies such as high-temperature fire flooding, string anti-corrosion, fire flooding monitoring, and front control need to be improved, and supporting technologies need to be perfected, and the scale of field test and application should be expanded.

Fig. 4.

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	Fig. 4. Samples cored from fire flooded formation[31].

The principle of in-situ crude oil viscosity reduction and quality upgrading is to crack big molecules which increase viscosity of crude oil into small molecules by reduction, oxidation and bio metabolism reactions in the formation, to achieve irreversible viscosity reduction, and thus greatly increase the mobility, recovery ratio, and energy efficiency of heavy oil. A number of technical solutions have been proposed, including hydrogen donor catalytic upgrading, oxidation catalytic upgrading and microbial upgrading^[32]. The catalytic viscosity-reducing agent sample has been developed, which can reduce viscosity at the operation temperature of above 250 ° C. The viscosity-reducing ratios of extra-heavy and ultra-heavy oil are over 90%. For this technology, researches on reduce the threshold temperature of cracking initiation and dispersed agent injection technology need to be studied further and field test should be started as soon as possible.

Heavy oil reservoirs represented by the Liaohe and Xinjiang oilfields have made significant progress in research on new technologies, but problems including high energy consumption and high costs still affect the profitable development of these oilfields. Therefore, further improvement of thermal efficiency and conversion of development mode are the keys for them to realize profitable development. At present, water flooding and chemical flooding for ordinary heavy oil reservoirs are to be improved, which are expected to increase recovery ratio by 5% and 10% than current water flooding, respectively. Steam huff and puff, steam flooding, multi-medium steam flooding, SAGD and multi-medium-assisted SAGD technology for extra-/ultra-heavy oil reservoir are expected to increase recovery ratio by 5%, 25%, and 35%, respectively than conventional huff and puff. Multi-component thermal fluid flooding and fire flooding technologies are expected to achieve ultimate recovery ratio of above 70%. Leading reserve technologies such as in-situ viscosity reduction and upgrading technology need to be researched, including stepping up field test and upgrading of main technology, to continuously improve the production and economic effectiveness.

In recent years, the secondary development technologies widely applied in fault block oilfields such as Dagang and Huabei oilfields, and the three-dimensional development or equilibrium displacement of complex fault block reservoirs in the Shengli Oilfield are all based on the fine reservoir characterization. Through multi-disciplinary integrated fine reservoir characterization, we can get a deeper understanding and re-build the cognition on the underground reservoir system.

Based on high-resolution 3D seismic data, high-density well pattern data, and production monitoring data, hierarchical fine reservoir characterization methods have been established by combining well-log and seismic data, dynamic and static data. Major progress has been made in prediction of remaining oil distribution, small-scale fault interpretation, and single sand body characterization, which laid the foundation for tapping the potential of fault block oilfields. Stagewise control and stepwise simulation fine geological modeling for complex fault block oilfields have been developed following the procedures: the fault models were established step by step from high-order to low-order faults, and horizon models were constructed according to structural interpretation from standard layers to single sand layers. The modeling order is firstly fault model then layer model. Three-dimensional well-log and seismic data combination methods are used to determine the levels of faults by comprehensively considering drilling, coring, and tectonic development history, the breakpoint, fault throw, inclination, dip angle, and disconnected horizon, to form a fault model of the entire region^[34]. Using the geological grid technique^[35], the technical difficulties in building a complex structural model and strong heterogeneous reservoir model are solved. The grid is not required to be parallel to faults and can be arbitrarily cut by the fault. There is also no need to maintain a consistent number of grids in the strata held between faults. Therefore, some limitations of traditional grids are eliminated, grid distortion is resolved, and faults in the region can be accurately characterized.

In light of the difficulties in development and production of small fragmented complex fault block reservoirs and small- scale remaining oil areas, the Shengli Oilfield has integrated geology, reservoir, drilling, and production processes together to form a three-dimensional development technology for complex fault block reservoirs. In geological study, detailed identification of faults lower than 5^th-order has been realized, and the identification of small breakpoints of 3-5 m and prediction of small fault of 5-10 m are now possible. In drilling engineering, directional wells with multiple targets are used to connect multiple vertical fragments, cross-block horizontal wells are employed for the development of planar adjacent fault blocks. Design and trajectory optimization techniques for four types of complex structure wells, multi-target directional wells, horizontal wells near faults, horizontal wells crossing faults, and horizontal wells avoiding water coning zone, have been worked out. Fine well trajectory control, self-repairing cement slurry completion systems, and other supporting technologies for drilling and completion have been developed. In production engineering, technology of separate layer water injection, metering and adjustment and injection layer change by fluid pressure control have been realized.

There are 11 small fault blocks in the test area of Yong 3-1 extremely complex fault block of the Yong’ an Oilfield, with geological reserves of 71 × 10⁴t. A total of 12 wells with complex structures were deployed, including horizontal wells avoiding water coning zone, cross-fault horizontal wells and multi-target directional wells, the daily oil production increased to more than 120 t and cumulative oil production reached 4.84 × 10⁴t after implementation of relevant measures, with an oil increment of 3.8 × 10⁴t and oil recovery increase of 9.6%^[36].

Various EOR tests have been carried out in fault block oilfields^{[33, 37]}. In the Shengli and Henan oilfields, polymer flooding, chemical combination flooding, and heterogeneous combination flooding after SAP flooding have been carried out for the medium-high permeability block reservoirs. These reservoirs generally have the characteristics of high formation temperature (65-120 ° C), high formation water salinity (5 000-100 000 mg/L), and high crude oil viscosity (50-130 mPa• s). A series of high temperature resistant and salt tolerant polymer products (including modified polymer) and corresponding supporting processes have been developed. By the end of 2015, polymer flooding had enhanced oil recovery by 6%-12%, and binary SP flooding enhanced oil recovery by 8%-15%^[37]. Heterogeneous combination flooding is mainly applied after polymer flooding, based on the binary flooding system, viscoelastic particles are added to form a heterogeneous system. The continuous phase of the system is “ polymer + surfactant” solution, the disperse phase comprises of particles with higher viscoelasticity, which can increase friction coefficient, strengthen flow redirection, and increase sweeping volume compared with binary composite flooding. Pilot test conducted in Zhong 1 block of Gudao is expected to increase oil recovery by 7.3% after polymer flooding.

Field tests of polymer flooding with salt water, surfactant/polymer flooding, microbial flooding, air foam flooding and gas injection and stable gravity flooding have been conducted in complex fault block oilfields such as Dagang, Huabei, and Jidong, resulting in the increase of oil recovery ratio of 7%-15%. The crude oil of Ba 19 fault block in Baolige Oilfield has poor petrophysical properties, high oil-water viscosity ratio, and the fault block has prominent interlayer heterogeneity. By applying gel-assisted microbial flooding, a stable microbial field has been established in the formation, with the concentration of bacterial maintaining at 1× 10⁶ cells/mL and crude oil viscosity decreasing by an average of 48.1% and the oil recovery increasing by 9.5%, which has effectively improved the oilfield development effect^[38].

The complex fault block reservoirs in the Shengli, Dagang, Huabei, Henan and Jidong oilfields in the Bohai Bay Basin have already entered into the “ double extra-high” stage, while water flooding is still the main development method, pilot tests of polymer flooding, binary compound flooding, microbial flooding, and air foam flooding have been started. Some technologies have been put into industrial application. However, due to the limitation of reservoir conditions, the performance and economic effectiveness of enhanced oil recovery still need to be further improved. Corresponding EOR technologies need to be developed based on reservoir characteristics. For the fault blocks where fine injection-production well patterns can be deployed, fine water flooding, subdivision of strata, and well-pattern infilling should be applied, which is expected to increase the recovery ratio by 3%-5%. Based on the experience of the Daqing Oilfield, chemical flooding, CO₂ flooding, gas injection and stable gravity flooding, microbial flooding and other technologies are tackled, which are expected to increase oil recovery by 10%-15%. Moreover, high-temperature and high-salinity (high calcium and magnesium) reservoir foam flooding and other EOR technologies have been explored. For fault blocks where fine injection well patterns are difficult to be deployed, it is necessary to develop technologies such as complex structure well drilling, hydrodynamic methods, three-dimensional oilfield development, single-well injection and production, to continuously increase producing degree and sweeping volume and further increase recovery ratio.

Through multi-discipline joint research, a number of key technologies such as geophysical descriptions for fracture- cavity reservoirs, multi-scale facies-control fracture-cavity modeling, and fracture-cavity reservoir numerical simulation have been developed. Based on high-solution seismic data, comprehensive logging evaluation and seismic paleogeomorphology studies have improved the reliability of fracture-cavity reservoir and fluid identification. Fractured-cavity reservoirs has been classified into several types, namely cave, vug, fracture and matrix types. Different methods were used for modeling of different reservoir types, and then the models were merged together into a 3D geological model of the fracture-cavity reservoir, which improves the accuracy of fracture-cavity reservoir modeling^{[28, 39]}. A mathematical model of fluid flow coupling equivalent continuous medium and discrete medium of fracture-cavity reservoir has been established, and a complex simulation technique that considers cave flow, fracture flow, and matrix seepage has been developed. Based on the progresses made in the fracture-cavity reservoir identification and characterization, combined with various dynamic and static data obtained in production, we have a deeper understanding on the seepage mechanism of carbonate reservoirs, and a better idea on the remaining oil distribution pattern and follow-up potential tapping measures.

Various EOR researches and tests have been carried out, including water injection to suppress water coning, hydrodynamic methods, gas injection, gas injection and stable gravity flooding and chemical flooding. The mechanisms of enhanced oil recovery by water injection and gas injection include replenishment of formation energy, gravity differentiation (resulting in bottom water or gas top), volume expansion and viscosity reduction, which enables the “ attic oil” to be effectively developed. Tahe Oilfield is a fracture-cavity carbonate reservoir with a burial depth of 5 300 to 7 000 m. In 2012, N₂ injection was started in this oilfield, with N₂ generated by skidded mobile membrane nitrogen generator (purity 95%). The water alternating gas (WAG) injection was taken to control the injection pressure. By the end of 2016, N₂ injection had been conducted in 281 wells, with the cumulative oil production of 28.9 × 10⁴t^[37].

In Xinggu 7 metamorphic buried hill reservoir of the Liaohe Oilfield, industrial natural gas flooding test has shown initial results. Xinggu 7 is a giant deep thick bottom water metamorphic naturally fractured reservoir. The storage space and seepage channels are mainly structural fractures and weathered fractures, followed by secondary dissolution pores and secondary metasomatic dissolution pores, and the formation crude oil has a viscosity of less than 5 mPa• s. The reservoir has been developing by natural depletion, and the production wells at the bottom of the buried hill have been water flooded and shut down. The test is largely top gas injection and stable gravity flooding, assisted by gas injection in the middle and lower part of the buried hill, which has increased nearly 2 × 10⁴t of oil since the gas injection, and is expected to increase recovery ratio by 18%.

The special lithologic reservoirs, represented by the Tahe/ Tarim carbonate reservoir, Liaohe buried hill reservoir and Xinjiang volcanic reservoir, are mainly developed by natural depletion and water flooding. As identification and seepage law of these reservoirs have not yet fully mastered, technologies for effective energy replenishment have not yet been established either. At current stage, the main targets for these reservoirs are to improve fracture-cavity reservoir identification and develop water-injection energy replenishing technology, which is expected increase recovery ratio by 5%-7%. The work will be focused on gas injection and stable gravity flooding technologies, which are expected to further increase the recovery ratio by more than 10%. Smart oil drive technologies should be developed in advance. The test results show that gas injection and stable gravity flooding technology is expected to take the lead in achieving breakthroughs and becoming the main substitutive technology for effective oil recovery.