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The fluid evolution and reservoir formation model of the ultra-deep gas reservoirs in the Permian Qixia Formation of the northwestern Sichuan Basin are investigated by using thin section, cathodoluminescence, inclusion temperature and U-Pb isotopic dating, combined with gas source identification plates and reservoir formation evolution profiles established based on burial history, thermal history, reservoir formation history and diagenetic evolution sequence. The fluid evolution of the marine ultra-deep gas reservoirs in the Qixia Formation has undergone two stages of dolomitization and one phase of hydrothermal action, two stages of oil and gas charging and two stages of associated burial dissolution. The diagenetic fluids include ancient seawater, atmospheric freshwater, deep hydrothermal fluid and hydrocarbon fluids. The two stages of hydrocarbon charging happened in the Late Triassic and Late Jurassic-Early Cretaceous respectively, and the Middle to Late Cretaceous is the period when the crude oil cracked massively into gas. The gas reservoirs in deep marine Permian strata of northwest Sichuan feature multiple source rocks, composite transportation, differential accumulation and late finalization. The natural gas in the Permian is mainly cracked gas from Permian marine mixed hydrocarbon source rocks, with cracked gas from crude oil in the deeper Sinian strata in local parts. The scale development of paleo-hydrocarbon reservoirs and the stable and good preservation conditions are the keys to the formation of large-scale gas reservoirs.
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Aiming at the scientific problem that only part of dolomite acts as dolomite reservoir, this paper takes the multiple dolomite-bearing formations in the Tarim, Sichuan and Ordos basins as the study object, by means of mineral petrological analysis and geochemical methods including carbonate clumped isotope, U-Pb isotopic dating, etc., to rebuild the dolomitization pathway and evaluate its effects on reservoir formation. On the basis of detailed rock thin section observation, five dolomitic structural components are identified, including original fabric-retained dolomite (microbial and/or micrite structure), buried metasomatic dolomite I (subhedral-euhedral fine, medium and coarse crystalline structure), buried metasomatic dolomite II (allotriomorphic-subhedral fine, medium and coarse crystalline structure), buried precipitation dolomite and coarse crystalline saddle dolomite. Among them, the first three exist in the form of rocks, the latter two occur as dolomite minerals filling in pores and fractures. The corresponding petrological and geochemical identification templates for them are established. Based on the identification of the five dolomitic structural components, six dolomitization pathways for three types of reservoirs (preserved dolomite, reworked dolomite and limestone buried dolomitization) are distinguished. The initial porosity of the original rock before dolomitization and the dolomitization pathway are the main factors controlling the development of dolomite reservoirs. The preserved dolomite and reworked dolomite types have the most favorable dolomitization pathway for reservoir formation, and are large scale and controlled by sedimentary facies in development and distribution, making them the first choices for oil and gas exploration in deep carbonate formations.
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The enrichment characteristics of deep shale gas in the Ordovician Wufeng-Silurian Longmaxi formations in the Sichuan Basin and its surrounding areas are investigated through experiments under high temperature and high pressure, including petrophysical properties analyses, triaxial stress test and isothermal adsorption of methane experiment. (1) The deep shale layers drop significantly in porosity and permeability compared with middle-shallow shale layers, and contain mainly free gas. (2) With higher deviatoric stress and axial strain, the deep shale layers have higher difficulty to fracture. (3) Affected by structural location and morphology, fracture characteristics, geofluid activity stages and intensity, deep shale gas reservoirs have more complicated preservation conditions. (4) To achieve the commercial development of deep shale gas reservoirs, deepening geological understanding is the basis, and exploring reservoir simulation technology befitting the geological features is the key. (5) The siliceous shale and limestone-bearing siliceous shale in the Metabolograptus persculptus-Parakidograptus acuminatus zones (LM1-LM3 graptolite zones) are the high-production intervals for deep shale gas and the most favorable landing targets for horizontal drilling. Deeps water areas such as Jiaoshiba, Wulong, Luzhou and Changning with deep shale reservoirs over 10 m thickness are the most favorable areas for deep shale gas enrichment. It is recommended to carry out exploration and development practice in deep-water shale gas areas deposited deep with burial depth no more than 5000 m where the geological structure is simple and the shale thickness in the LM1-LM3 graptolite zone is greater than 10 m. It is better to increase the lateral length of horizontal wells, apply techniques including high intensity of perforations, large volume of proppant, far-field and near-wellbore diversions to maximize the stimulated deep reservoir volume.
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Based on the practice of oil and gas exploration and the analysis of shallow lithologic reservoirs, combined with the allocation relationship and enrichment law of oil and gas accumulation factors, main controlling factors and models of hydrocarbon accumulation of large lithologic reservoirs in shallow strata around the Bozhong sag are summarized, and favorable exploration areas are proposed. The coupling of the four factors of “ridge-fault-sand-zone” is crucial for the hydrocarbon enrichment in the shallow lithologic reservoirs. The convergence intensity of deep convergence ridges is the basis for shallow oil and gas enrichment, the activity intensity of large fault cutting ridges and the thickness of cap rocks control the vertical migration ability of oil and gas, the coupling degree of large sand bodies and fault cutting ridges control large-scale oil and gas filling, the fault sealing ability of structural stress concentration zones affects the enrichment degree of lithologic oil and gas reservoirs. Three enrichment models including uplift convergence type, steep slope sand convergence type and depression uplift convergence type are established through the case study of lithologic reservoirs in shallow strata around the Bozhong sag.
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Based on field geological survey, interpretation of seismic data and analysis of drilling and logging data, the evolution of geological structures, stratigraphic sedimentary filling sequence and sedimentary system around the Bogda Mountain were analyzed according to the idea of "structure controlling basin, basin controlling facies and facies controlling assemblages". The tectonic evolution of the basin around the Bogda Mountain can be divided into nine stages. The Middle-Late Permian-Middle-Late Triassic was the development stage of intracontinental rift, foreland basin and inland depression basin when lake, fan delta and braided river delta sedimentary facies developed. Early intracontinental rifting, late Permian tectonic uplift, and middle-late Triassic tectonic subsidence controlled the shape, type, subsidence rate and sedimentary system evolution of the basin. The Bogda Mountain area was the subsidence center and deposition center of the deep water lake basin in the Middle Permian with mainly deep-water deposition and local gravity flow deposition. This area had tectonic inversion in the Late Permian, when the Bogda Mountain uplifted to form a low bulge and a series of fan delta sand bodies. In the Middle-Late Triassic, subsidence occurred in the Bogda low uplift, characterized by extensive development of braided river delta deposits.
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Based on comprehensive analysis of seismic, logging, core, thin section data, and stable isotopes composition of carbon and oxygen, the sedimentary filling characteristics of the Lower Cretaceous Barra Velha Formation sequence in H oil field, Santos Basin, are studied, and the high-frequency sequence stratigraphic framework is established, and the spatial distribution of reef-shoal bodies are predicted and the controlling factors are discussed. During the depositional period of the Barra Velha Formation, the study area is a slope-isolated platform-slope sedimentary pattern from southwest to northeast and the change of climate background from rift to depression periods has resulted in the variation of sedimentary characteristics from the lower third-order sequence SQ1 (BVE 300 Member) of low-energy deep water to the upper third-order sequence SQ2 (BVE 200 and 100 members) of high-energy shallow water in the Barra Velha Formation. The activities of extensional faults and strike-slip faults in rift period and the sedimentary differentiation from platform margin to intra-platform in depression period made the sedimentary paleogeomorphology in these two periods show features of “ three ridges and two depressions”. The reef-shoal bodies mainly developed in the SQ2-LHST period, with vertical development positions restricted by the periodic oscillation of the lake level, and developed on the top of each high-frequency sequence stratigraphic unit in SQ2-LHST in the platform. The strike-slip fault activity controlled the distribution of the reef-shoal bodies on the plane by changing the sedimentary paleogeomorphology. The positive flower-shaped strike-slip faults made the formation of local highlands at the margins of and inside the shallow water platforms and which became high-energy sedimentary zones, creating conditions for the development of reef-shoal bodies.
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The Ordovician fracture-cavity carbonate reservoirs of Tarim Basin, NW China are featured by developed pores, caves and fractures. The strong heterogeneity results in huge uncertainty when these reservoirs are quantitatively characterized using merely static seismic data. The effective quantitative characterization of the reservoirs has been an urgent problem to be solved. This study creatively proposes the "second quantitative characterization" technique with the combination of dynamic and static data based on the primary static quantitative characterization and fully considering lots of key influence factors when conducting characterization. In this technique, dynamic analysis methods such as well testing, production rate transient analysis, dynamic reserve evaluation and dynamic connectivity evaluation are used to get understandings on this kind of reservoir. These understandings are used as statistical parameters to constrain the inversion of seismic wave impedance to improve the relationship between wave impedance and porosity and determine the fracture-cavity morphology, calculate dynamic reserves, and then a more accurate fracture-cavity model can be selected and used to calculate the oil-water contact inversely based on the results of "second quantitative characterization". This method can lower the uncertainties in the primary quantitative characterization of fracture-cavity reservoirs, enhance the accuracy of characterization results significantly, and has achieved good application results in the fracture-cavity carbonate reservoirs of Tarim Basin.
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The reservoir space, types and distribution characteristics of karst carbonate gas reservoirs in the fourth member of Sinian Dengying Formation (Deng 4 Member) in central Sichuan Basin are analyzed based on the drilling, logging and seismic data. A development model of karst reservoirs is constructed to support the high-efficiency development of gas pools. The research shows that the reservoirs in Deng 4 Member have mainly small-scale karst vugs and fractures as storage space, and can be divided into three types, fracture-vug, pore-vug and pore types. The development patterns of the karst reservoirs are determined. On the plane, the karst layers increase from 65 m to 170 m in thickness from the karst platform to the karst slope, and the high-quality reservoirs increase from 25.0 m to 42.2 m in thickness; vertically, the reservoirs at the top of Deng 4 Member appear in multiple layers, and show along-bedding and along fracture dissolution characteristics. The reservoirs at the bottom are characterized by the dissolution parallel to the water level during the karstification period, and have 3-5 large-scale fracture-cave systems. Based on the reservoir development characteristics and the genetic mechanism, three types of reservoir development models of karst reservoir are established, i.e., bed-dissolved body, fracture- dissolved body and paleohorizon-dissolved body. The construction of karst reservoir development models and seismic response characteristics of the three types of reservoirs can provide parameter for well placement and trajectory design, and substantially improve productivity and development indices of individual wells and gas reservoirs. The designed production capacity of the gas reservoir has enhanced from the initial 3.6 billion to 6 billion cubic meters, making the profit of the reservoir development increase noticeably.
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Based on seismic and drilling data, the reactivation mechanism of the pre-existing basement F4 strike-slip fault in Nanpu sag and its controlling effect on hydrocarbon accumulation difference are systematically studied. By defining fault activation stages, back-stripping fault throw and physical modeling, it is found that the Nanpu No.4 structural zone formed by the Cenozoic reactivity of the F4 fault grew from south to north, with strike-slip deformation dominated in the south and extensional deformation dominated in the north. Faults in the No.4 structural zone and those in the adjacent No.2 and No.3 structural zones were different fault systems, which grew separately, contacted and connected, and finally interwove under the action of unified stress field. By constructing the identification chart of deformation mechanisms of reactivation of pre-existing faults, it is concluded that during the sedimentary period of the Paleogene Shahejie Formation, F4 fault was reactivated by strike-slip faulting, and during the sedimentary period of Paleogene Dongying Formation and Neogene Guantao-Minghuazhen formations, it was reactivated by oblique extension. The controlling effects of Cenozoic reactivation of F4 fault on hydrocarbon accumulation include: (1) As the oil-source fault, it controlled the vertical cross-layer migration of oil and gas. (2) It gave rise to strike-slip transfer zone to control the distribution of sand bodies. (3) It grew upward and interacted with faults in the neighboring area, controlling the formation of two types of traps, and was favorable for oil and gas accumulation.
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The relationship between paleogeographic pattern and sedimentary differentiation of evaporite-carbonate symbiotic system is examined based on logging, core and thin section data, by taking the sixth sub-member of fifth member of Ordovician Majiagou Formation (M56) in the central-eastern Ordos Basin as an example. (1) Seven sub-geomorphic units (Taolimiao west low, Taolimiao underwater high, Taolimiao east low, Hengshan high, East salt low, North slope and Southwest slope) developed in the study area. (2) The “three lows” from west to east developed dolomitic restricted lagoon, evaporite evaporative lagoon and salt evaporative lagoon sedimentary facies respectively, the "two highs" developed high-energy grain beach and microbial mound, and the north and south slopes developed dolomitic flats around land. (3) The paleogeographic pattern caused natural differentiation of replenishment seawater from the northwest Qilian sea, leading to the eccentric sedimentary differentiation of dolomite, evaporite and salt rock symbiotic system from west to east, which is different from the classic “bull's eye” and “tear drop” distribution patterns. (4) As the Middle Qilian block subducted and collided into the North China Plate, the far-end compression stress transferred, giving rise to the alternate highland and lowland in near north to south direction during the sedimentary period of M56 sub-member. (5) Taolimiao underwater high and Hengshan high developed favorable zones of microbial mounds and grain shoals in south to north strike in M56 sub-member, making them favorable exploration areas with great exploration potential in the future.
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Interference of thin-interbedded layers in seismic reflections has great negative impact on thin-interbedded reservoirs prediction. To deal with this, two novel methods are proposed that can predict the thin-interbedded reservoirs distribution through strata slices by suppressing the interference of adjacent layer with the help of seismic sedimentology. The plane distribution of single sand bodies in thin-interbedded reservoirs can be clarified. (1) The minimum interference frequency slicing method, uses the amplitude-frequency attribute estimated by wavelet transform to find a constant seismic frequency with the minimum influence on the stratal slice of target layer, and then an optimal slice corresponding the constant frequency mentioned above can be obtained. (2) The superimposed slicing method can calculate multiple interference coefficients of reservoir and adjacent layers of target geological body, and obtain superimposed slice by weighted stacking the multiple stratal slices of neighboring layers and target layer. The two proposed methods were used to predict the distribution of the target oil layers of 6 m thick in three sets of thin-interbedded reservoirs of Triassic Kelamayi Formation in the Fengnan area of Junggar Basin, Northwestern China. A comparison with drilling data and conventional stratal slices show that the two methods can predict the distribution of single sand bodies in thin-interbedded reservoirs more accurately.
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Aiming at the technology of hydraulic fracturing assisted oil displacement which combines hydraulic fracturing, seepage and oil displacement, an experimental system of energy storage and flowback in fracturing assisted oil displacement process has been developed and used to simulate the mechanism of percolation, energy storage, oil displacement and flowback of chemical agents in the whole process. The research shows that in hydraulic fracturing assisted oil displacement, the chemical agent could be directly pushed to the deeper area of the low and medium permeability reservoirs, avoiding the viscosity loss and adhesion retention of chemical agents near the pay zone; in addition, this technology could effectively enlarge the swept volume, improve the oil displacement efficiency, replenish formation energy, gather and exploit the scattered residual oil. For the reservoir with higher permeability, this measure takes effect fast, so to lower cost, and the high pressure hydraulic fracturing assisted oil displacement could be adopted directly. For the reservoir with lower permeability which is difficult to absorb water, hydraulic fracturing assisted oil displacement with surfactant should be adopted to reduce flow resistance of the reservoir and improve the water absorption capacity and development effect of the reservoir. The degree of formation energy deficit was the main factor affecting the effective swept range of chemical agents. Moreover, the larger the formation energy deficit was, the further the seepage distance of chemical agents was, accordingly, the larger the effective swept volume was, and the greater the increase of oil recovery was. Formation energy enhancement was the most important contribution to enhanced oil recovery (EOR), which was the key to EOR by the technology of hydraulic fracturing assisted oil displacement.
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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.
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Shale gas wells frequently suffer from liquid loading and insufficient formation pressure in the late stage of production. To address this issue, an intelligent production optimization method for low pressure and low productivity shale gas well is proposed. Based on the artificial intelligence algorithms, this method realizes automatic production and monitoring of gas well. The method can forecast the production performance of a single well by using the long short-term memory neural network and then guide gas well production accordingly, to fulfill liquid loading warning and automatic intermittent production. Combined with adjustable nozzle, the method can keep production and pressure of gas wells stable automatically, extend normal production time of shale gas wells, enhance automatic level of well sites, and reach the goal of refined production management by making production regime for each well. Field tests show that wells with production regime optimized by this method increased 15% in estimated ultimate reserve (EUR). Compared with the development mode of drainage after depletion recovery, this method is more economical and can increase and stabilize production effectively, so it has a bright application prospect.
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The effects of trapping number on enhanced oil recovery by schizophyllan biopolymer flooding in carbonate reservoirs were investigated by running several 1D simulations using measured reservoir rock and fluid data. Sensitivity analysis was performed on different uncertain parameters to history match the oil recovery obtained in the core flooding experiment. These parameters include inaccessible pore volume (IPV), biopolymer adsorption, permeability reduction factor, shear rate coefficient, hardness of injection water, and trapping number. The IPV, biopolymer adsorption, permeability reduction factor, shear rate coefficient and hardness of injection water have negligible effects on oil recovery by biopolymer flooding. Also, history matching of oil recovery data was not possible when these parameters were varied within their typical range of values. When trapping number effect was considered through capillary desaturation curve (CDC), residual oil saturation was reduced from 25.1% without considering its effect or under low trapping number to 10.0%, and the fitting effect for recovery was better. Therefore, we can’t neglect the trapping number effect during biopolymer flooding simulation in the carbonate reservoirs.
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To find out the relationship between the oil-based mud, the formation fluid and the extracted gas, we use a thermodynamic approach based on static headspace gas chromatography technique to calculate the partition coefficients of 47 kinds of light hydrocarbons compounds between nC5 and nC8 in two kinds of oil-based mud-air systems, and reconstruct the original formation fluid composition under thermodynamic equilibrium. The oil-based drilling mud has little effect on the formation fluid compositions in the range of nC5-nC8 (less than 1% for low-toxicity oil-based mud and less than 10% for oil-based mud). For most light hydrocarbon compositions, the partition coefficients obtained by vapor phase calibration and the direct quantitative methods have errors of less than 10%, and the partition coefficients obtained by direct quantitative method are more accurate. The reconstructed compositions of the two kinds of crude oil have match degrees of 91% and 89% with their real compositions, proving the feasibility and accuracy of reconstructing the composition of original formation fluid by using partition coefficients of light hydrocarbon compositions between nC5 and nC8.
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Based on the elastoplastic model of cement sheath considering the influence of three-dimensional principal stress and the stress field model of interface fracture, a mechanical performance design method of cement sheath is established to meet the wellbore sealing requirements during fracturing. This method takes the failure types of the cement sheath, such as tensile failure, plastic yield, interface fracture propagation along interface and zigzag propagation into account. Meanwhile, the elasticity modulus and Poisson's ratio quantitative design charts of cement sheath are constructed based on this method, and the safety and risk areas of wellbores are defined, which quantify the yield strength and tensile strength indexes of cement sheath. The results show that decreasing elasticity modulus, increasing yield strength and Poisson's ratio of cement sheath can avoid plastic deformation of cement sheath; increasing the tensile strength of cement sheath can prevent its tensile failure; increasing elasticity modulus and Poisson's ratio of cement sheath is good for shortening the length of the interface fracture, but will increase the risk of interface fractures zigzagging into cement sheath. The model calculation and case verification has proved that the method in this paper can give accurate calculation results and is convenient for field application.
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The improved delayed detached eddy simulation method with shear stress transport model was used to analyze the evolution of vortex structure, velocity and pressure of swirling jet. The influence of nozzle pressure drop on vortex structure development and turbulence pulsation was investigated. The development of vortex structure could be divided into three stages: Kelvin-Helmholtz (K-H) instability, transition stage and swirling flow instability. Swirling flow could significantly enhance radial turbulence pulsation and increase diffusion angle. At the downstream of the jet flow, turbulence pulsation dissipation was the main reason for jet velocity attenuation. With the increase of pressure drop, the jet velocity, pulsation amplitude and the symmetry of velocity distribution increased correspondingly. Meanwhile the pressure pulsation along with the axis and vortex transport intensity also increased significantly. When the jet distance exceeded about 9 times the dimensionless jet distance, the impact distance of swirling jet could not be improved effectively by increasing the pressure drop. However, it could effectively increase the swirl intensity and jet diffusion angle. The swirling jet is more suitable for radial horizontal drilling with large hole size, coalbed methane horizontal well cavity completion and roadway drilling and pressure relief, etc.
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Focusing on the extending length restriction of the completion screen pipe resistance running into ultra-short radius horizontal well, this paper proposed technology of hydraulic drive completion tubular string running into ultra-short radius horizontal well. Innovative hydraulic drive tools and string structure are designed, which are composed of guide tubing, hydraulic drive tubing and non-metallic completion screen pipe from inside to outside. A novel mechanic-hydraulic coupling model is established. Based on the wellbore structure of an ultra-short radius horizontal well for deep coalbed methane, the numerical calculations of force and hydraulic load on tubular strings were accomplished by the mechanic-hydraulic coupling model. The results show that the extending length of completion tubular string with the hydraulic drive is 17 times that of conventional completion technology under the same conditions. The multi-factor orthogonal design is adopted to analyze the numerical calculations, and the results show that the extending length of the completion tubular string is mainly affected by the completion tubular string structure and the friction coefficient between the non-metallic composite continuous screen pipe and the wellbore. Two series of hydraulic drive completion tubular string structures suitable for ultra-short radius horizontal wells under different conditions are optimized, with the extending limits of 381 m and 655 m, respectively. These researches will provide theoretical guidance for design and control of hydraulic drive non-metallic composite continuous completion screen pipe running into ultra-short radius horizontal wells.
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Carbon dioxide capture, EOR-utilization and storage (CCUS-EOR) are the most practical and feasible large-scale carbon reduction technologies, and also the key technologies to greatly improve the recovery of low-permeability oil fields. This paper sorts out the main course of CCUS-EOR technological development abroad and its industrialization progress. The progress of CCUS-EOR technological research and field tests in China are summarized, the development status, problems and challenges of the entire industry chain of CO2 capture, transportation, oil displacement, and storage are analyzed. The results show a huge potential of the large-scale application of CCUS-EOR in China in terms of carbon emission reduction and oil production increase. At present, CCUS-EOR in China is in a critical stage of development, from field pilot tests to industrialization. Aiming at the feature of continental sedimentary oil and gas reservoirs in China, and giving full play to the advantages of the abundant reserves for CO2 flooding, huge underground storage space, surface infrastructure, and wide distribution of wellbore injection channels, by cooperating with carbon emission enterprises, critical technological research and demonstration project construction should be accelerated, including the capture of low-concentration CO2 at low-cost and on large-scale, supercritical CO2 long-distance transportation, greatly enhancing oil recovery and storage rate, and CO2 large-scale and safe storage. CCUS-EOR theoretical and technical standard system should be constructed for the whole industrial chain to support and promote the industrial scale application, leading the rapid and profitable development of CCUS-EOR emerging industrial chain with innovation.
