力学学报

Theoretical and Applied Mechanics for CO₂ Emission Peak - Carbon Neutrality Strategy

Feng Xiating

2023, 55(3): 565-566. doi: 10.6052/0459-1879-23-125

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DYNAMIC CATASTROPHE AND CONTROL OF OFFSHORE WIND POWER STRUCTURES IN TYPHOON ENVIRONMENT

Wang Lizhong, Hong Yi, Gao Yangyang, Huang Mingfeng, Guo Zhen, Lai Yongqing, Zhu Ronghua, Yang Qinmin, He Ben

The development of offshore wind energy has been playing an important role in contributing to the goal of "3060" of carbon peak and carbon neutralization in China. Upsizing of offshore wind turbine is the main approach for cost reduction and efficiency improvement, which has become an important trend in recent years. At present, the design standard of offshore wind structure and foundation is led by Europe. Differing from the favorable offshore environment and ground conditions in the North Sea in Europe, the offshore wind power structures in China has been facing two major challenges: strong typhoon and soft soil, and are thus prone to dynamic catastrophes. The trend of upsizing offshore wind turbines is likely to further aggravate the risk. The key to disaster mitigation lies in the in-depth understanding of the integrated coupling and intelligent control of aerodynamics, hydrodynamics, structural dynamics and soil dynamics related to offshore wind turbine structures. In this paper, the latest research progresses concerning inter-disciplinary studies on catastrophe and control of offshore wind turbines in typhoon environment (including the advances made by the writer’s research group), have been reviewed. The technical contents mainly include: engineering-scaled characteristics of typhoon and its resulting wave field, aerodynamic and hydrodynamic loads with intelligent control strategies for wind turbines in typhoon environment, the failure mechanism of foundations and turbine structures under the combined multi-directional actions from wind, wave and current, and the integrated analysis and design method considering the coupling between wind-wave-current-structure-foundation-intelligent control. On this basis, the key research areas that need to be addressed are suggested, including but not limited to: deeper insights into the engineering-scaled characteristics of typhoon and typhoon wave, research of control strategy for wind turbine in typhoon environment, development of integrated design theory and industrial software for large-scale offshore wind turbine in typhoon environment. Breakthroughs in the above related fields have both scientific value and practical significance for China to achieve a global leading position in the offshore wind and energy industry.

2023, 55(3): 567-587. doi: 10.6052/0459-1879-22-529

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TIME-FREQUENCY CHARACTERISTICS OF UNSTEADY AERODYNAMIC FORCES FOR FEATHERED WIND TURBINE AIRFOIL UNDER TOWER BLADE INTERACTION

Tang Xinzi, He Wenshuang, Guo Yanwei, Peng Ruitao

Due to the interference between the wind turbine blades and the flow field of the tower, the actual value of the aerodynamic force is quite different from the theoretical value. The difference in aerodynamic force caused by this interference has a non-negligible impact on the reliability of the blade and tower structure. Taking the airfoil DU91-W2-250 as the research object, based on the transient numerical analysis and proper orthogonal decomposition method, considering the interaction between the blade and the tower flow field, the time-frequency characteristics and the influence law of unsteady aerodynamic forces for the feathering airfoil are analyzed, the influence degree of the relative position of the tower blade and the geometric parameters on the mean value of aerodynamic force, the fluctuation range and frequency at different Reynolds numbers are quantified, and the influence mechanism of flow field interference on aerodynamic force are revealed through the analysis of flow field modal energy distribution. Results show that, the vertical and horizontal distances from the aerodynamic center of the airfoil to the geometric center of the tower as well the tower diameter relative to the chord length of the airfoil, which are defined as the dimensionless distance parameters y*, x* and D*, have varying degrees of influence on the aerodynamic force. Among which, y* has the greatest influence on the mean value of lift and drag coefficients, but has no obvious influence on the frequency. The greater the absolute value of y* is, the closer the mean value of C_l is to the C_l value of a single airfoil. The smaller the absolute value of y* is, the greater the fluctuation amplitude of lift and drag coefficients is, and y* increases from −12 to 12, the minimum value of average lift coefficient is −0.48, and the maximum value is 1.16. When x* decreases and D* increases, the mean reverse drag force increases, the fluctuation amplitude increases, and the fluctuation frequency decreases slightly. When x* is less than the critical value 5, the average drag force of the airfoil with tower is reversed. Within the calculation range, compared to the single airfoil, the maximum deviation of the mean value of lift coefficient for the airfoil with tower is −221.94% and its maximum fluctuation is 28.0% of the lift coefficient of single airfoil. While the maximum deviation of the mean drag coefficient of for the airfoil with tower is −1189.3% and its maximum fluctuation is 121.1%. Due to the influence of the high pressure area in front of the tower, the airfoil flow field exhibits obvious symmetrical pulsation excitation, resulting in the deviation and fluctuation of the aerodynamic forces.

2023, 55(3): 588-598. doi: 10.6052/0459-1879-22-554

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NUMERICAL SIMULATION INVESTIGATION OF SEDIMENT COMPRESSION EFFECTS CAUSED BY DEPRESSURIZATION PRODUCTION OF NATURAL GAS HYDRATE

Luan Hengjie, Ma Xianzhuang, Jiang Yujing, Yu Haiyang, Wang Changsheng, Cheng Xianzhen, Liang Wei

The sediment compression during the depressurization production of deep-sea gas hydrates can alter the physical and mechanical characteristics of the reservoir, which can have a significant impact on the production results of the reservoir. In order to reveal the evolution of the physical and mechanical characteristics of the reservoir around the production well under the sediment compression effects, in this paper, a theoretical model considering the sediment compression effects is established, and the evolution of physical and mechanical characteristics of reservoir around well and the production results during depressurization production under different initial intrinsic permeability, initial hydrate saturation and bottomhole pressure conditions is investigated by COMSOL numerical simulation. The results show that the permeability in the hydrate decomposition area increases and then decreases with increasing distance from the well due to the influence of sediment compression. The rates of gas and water production rise immediately from zero to a peak and then decline rapidly, and are lower when sediment compression is compression is considered than when it is not. In the area of complete hydrate decomposition, the magnitude of permeability is negatively correlated with the effective stress and in the undecomposed area the magnitude of permeability is negatively correlated with the hydrate saturation. The lower the bottomhole pressure, the higher the effective stress and the more significant the decrease in permeability around the production well. There is an inflection point for the effect of initial hydrate saturation on gas and water production, and the inflection point for saturation is between 0.25 and 0.35. High hydrate saturation does not mean good reservoir recovery, because the rate of gas production is also related to the permeability of the reservoir. Reservoirs with high hydrate saturation have lower permeability and lower gas production rate, while high initial intrinsic permeability of the reservoir significantly contributes to the production results, but the larger reservoir deformation increases the instability of the reservoir.

2023, 55(3): 599-615. doi: 10.6052/0459-1879-22-465

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NUMERICAL SIMULATION OF MULTI-SCALE COUPLED FLOW IN AFTER-FRACTURING SHALE GAS RESERVOIRS

Xia Yang, Deng Yinghao, Wei Shiming, Jin Yan

Under the national policy background of peak carbon dioxide emissions, shale gas becomes an important transition and energy fulcrum for the transition from traditional energy to green, clean and low-carbon energy. And the fluid flow mechanism of shale gas reservoirs after fracturing becomes a key mechanical problem for the efficient development of shale. In this paper, the small-scale low-conductivity natural fractures are equivalently upgraded to a continuous medium, and a triple organic-inorganic-natural fracture continuous medium model is established. The discrete fracture model is used to portray the large-scale high-conductivity fractures, which are embedded into the natural fracture continuous medium, and a multiple continuous/discrete fracture model (MC/DFM) is constructed. The non-equilibrium nonlinear desorption and surface diffusion of adsorbed gas, viscous flow, and Knudsen diffusion of free gas are integrated to give a nonlinear coupled flow mathematical model of shale gas in the multi-scale complex medium. The multi-scale extended finite element method (MXFEM) is proposed to solve the discrete fractures explicitly, and three types of enrichment functions are innovatively constructed to capture the local flow field characteristics of the discrete fractures, which solves the flow simulation problems of the massive fractures and multi-scale flow channels in the after-fracturing shale. The model and method proposed in this paper can not only accurately characterize the effect of high conductivity fractures on gas flow, but also overcome the problem of the dramatic increase in computational amount due to massive multi-scale discrete fractures. The pressure decay law of each continuous medium is demonstrated by a calculation example, and the pressure/concentration diffusion phenomena of free gas in fracture, free gas in organic medium, and adsorbed gas in the inorganic medium are found to lag in sequence. The analysis focuses on the effects of adsorbed gas surface diffusion coefficient, free gas Knudsen diffusion coefficient, natural fracture continuous medium permeability, and adsorbed gas desorption rate on shale gas production. This paper focuses on the characterization of multi-scale flow channels and modeling of complex coupled flow mechanisms in after-fracturing shale reservoirs and developing an efficient numerical simulation algorithm, which is meaningful for the production assessment of the shale formation after fracturing.

2023, 55(3): 616-629. doi: 10.6052/0459-1879-22-489

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STUDY ON THE MECHANISM OF DRAWDOWN MANAGEMENT OF SHALE GAS HORIZONTAL WELL CONSIDERING CREEP EFFECT

Guo Wei, Liu Jiazheng, Zhang Xiaowei, Teng Bailu, Kang Lixia, Gao Jinliang, Liu Yuyang, Luo Wanjing

Considering the characteristics of shale reservoirs such as the low matrix permeability and complex natural fracture development, shale gas is mainly developed by multi-stage fracturing horizontal wells. The production of shale gas wells adopts the two modes: pressure relief production and pressure control production. Currently, scholars generally believe that pressure control production can improve the EUR of shale gas wells, and stress sensitivity is the main reason why the effect of pressure relief production is worse than that of the pressure control production. In this study, besides the stress sensitivity of matrix and fracture, the creep effect of matrix is also considered, and embedded discrete fracture models under the influence of creep are established. The results show that pressure relief production is better than pressure control production when only stress sensitivity is considered, and pressure control production is better than pressure relief production only when creep effect is considered. In addition, inhibiting the creep effect of shale is one of the mechanisms of improving the EUR of shale gas wells through pressure control production. With the increase of pressure control time, the gas well EUR shows a trend of increasing first and then decreasing, and there is an optimal value for pressure control production; The larger the matrix creep parameter, the higher the conductivity of the artificial fracture, the smaller the matrix permeability, the smaller the adsorption volume, and the better the effect of pressure control on gas production This study explains the production increase mechanism through pressure control production, laying a foundation for the optimization of shale gas well production system.

2023, 55(3): 630-642. doi: 10.6052/0459-1879-22-460

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STUDY ON MOVABILITY OF SPONTANEOUS IMBIBITION OIL RECOVERY FROM TIGHT RESERVOIRS BASED ON NUCLEAR MAGNETIC RESONANCE PORE CLASSIFICATION METHOD

Li Teng, Gao Hui, Wang Meiqiang, Feng Yongbing, Wang Chen, Cheng Zhilin

Spontaneous imbibition is an effective method to improve the oil displacement in tight sandstone reservoirs, while the various pore classification methods may lead to variability in the refined quantitative characterization of oil movability. In this study, four typical tight sandstone cores were used to launch the spontaneous imbibition oil displacement experiments in the tight sandstone reservoir of the Yanchang Formation in Ordos Basin. With the refined pore classification method for fluid distribution pores based on nuclear magnetic resonance (NMR) fractal theory, the pore types in tight sandstone cores were distinguished, and the influence of different types of pore structures on oil mobility and spontaneous imbibition displacement rates were clarified. The results show that the oil producing degree of spontaneous imbibition in different types of cores ranges from 22.07% to 33.26%, and the oil producing degree of spontaneous imbibition in NMR T₂ spectra of bimodal cores is higher than that of unimodal core. With the NMR pore classification method, the fluid distribution pores in the typical tight sandstone cores were initially divided into P1 and P2 types, and the P1 type pores could be further classified into P1-1, P1-2 and P1-3 types. It features various spontaneous imbibition oil displacement in P1 and P2 types pores. As the dominant pore type in tight sandstone cores, the ratio of P1-2 and P1-3 type pores in P1 pores would finally determine the spontaneous imbibition oil displacement recovery in tight sandstone cores. The pore structure differences among P1-1, P1-2 and P1-3 pores play a decisive role in the spontaneous imbibition oil displacement recovery. The significance pore structure differences for pores with smaller apertures not only enhance the spontaneous imbibition oil displacement recovery, but also improve the rates of spontaneous imbibition oil displacement recovery. The tight sandstone cores of P1-2 and P1-3 pores with higher fluid mobility index show higher spontaneous imbibition oil displacement recovery.

2023, 55(3): 643-655. doi: 10.6052/0459-1879-22-566

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PRESSURE TRANSIENT MODEL OF MULTI-STAGE FRACTURED HORIZONTAL WELL WITH INDUCED FRACTURE FOR HETEROGENEOUS STIMULATED RESERVOIR VOLUME

Xu Youjie, Xiang Zuping, Zhang Xiaotao, Yu Mengnan, Yu Xingying

In order to simulate accurately wellbore pressure transient performance of multi-stage fractured horizontal well (MFHW) with the complex fracture network and heterogeneous in tight oil reservoirs, an unsteady seepage mathematical model of the MFHW with induced fractures in rectangular heterogeneous reservoir is established. The wellbore pressure solution is obtained by coupling the fracture model, the reservoir model and heterogeneous reservoir interface model. The wellbore pressure accuracy of the two heterogeneous reservoir models is verified by numerical solutions, boundary element method and previous model. By flow stages analysis and parameters sensitivity analysis, the following results can be obtained. Compared with the wellbore pressure derivative curve of the MFHW in homogeneous reservoir, the unique flow stages of this model in the ideal case include: the complex linear flow stage, the "supply" stage from induced fractures to fractured fractures, the linear flow stage and the pseudo boundary control flow stage. The increase of induced fracture number extends the duration of the complex linear flow stage and reduces the fluid seepage resistance. Therefore, the pressure curve of the early stage is lower. If the conductivity of induced fracture and fractured fracture is constant, the greater the conductivity is, the longer the duration of the bilinear flow regime is. When all fractured fractures are different region, the two end low permeability region along the wellbore weakens the "supply" stage. Therefore, the lower the permeability along the wellbore is, the higher the pressure curve at the early stage is. When all fractured fractures are in same area, the permeability change only affects the pressure curve shape after the radial flow stage. The lower the permeability of the outer area is, the higher the pressure curve after the early radial flow stage is. The practicability and accuracy of the model and method are demonstrated by the field example.

2023, 55(3): 656-668. doi: 10.6052/0459-1879-22-514

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RECENT PROGRESS ON NUMERICAL RESEARCH OF KEY MECHANICAL PROBLEMS DURING UNDERGROUND COAL GASIFICATION

Liu Yuewu, Fang Huijun, Li Longlong, Ge Tengze, Zheng Taiyi, Liu Danlu, Ding Jiuge

Clean and efficient coal utilization becomes an important direction and new research topic under the dual-carbon background. Recently, underground coal gasification (UCG) develops very fast and shows great potential in this area. However, the laboratory and field experiments, which are usually used to investigate the gasification mechanisms and optimize the operating parameters, are quite expensive. Thus, there is a strong demand for the numerical approach that is low-cost, easy operation, and short-cycle. Currently, the numerical approach faces challenges in terms of mathematical modelling and numerical method for solving the nonlinear system because of the complexity of the gasification process. To deal with that, we have done the work as follows: clarified the materials and key problems in each space based on a detailed analysis and revealed the essence of the UCG; summarized four kinds of key mechanical issues including fluid dynamics, thermodynamics, material mechanics, and chemical reaction kinetics; reviewed the development history of numerical research for key mechanical problems in detail and introduced the latest results; illustrated the status of engineering application of numerical research and pointed out the development trends. The work in this paper has positive theoretical significance for the development of the numerical technique for UCG and guiding the design and implementation of UCG trials in China.

2023, 55(3): 669-685. doi: 10.6052/0459-1879-22-331

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ULTRA-LOW FRICTION TIME-CHANGE MODEL AND ENERGY CONVERSION OF DEEP COAL-ROCK INTERFACE

Li Liping, Yu Honghao, Zhang Haitao, Pan Yishan

The ultra-low friction impact ground pressure of deep coal rock is essentially a time-varying process in which a large amount of coal rock mass is instable and sliding along the coal-rock interface, during which the friction and friction coefficient of the coal-rock interface change with time, and at the same time, the energy conversion characteristics of releasing energy from the impact kinetic energy of the coal-rock interfacial with the frictional force of the coal-rock interface. In order to quantitatively describe the energy conversion law of coal rock interface, the dimensional analysis method is introduced, and the elastic coefficient, damping coefficient and pending coefficient of coal rock are experimentally determined, and the expression of the friction coefficient of deep coal rock interface is given. Taking Shenyang Hongyang Three Mines as the research object, through the combination of experimental research and engineering practice, a new index of impact kinetic energy conversion rate is defined, the reliability of the built model is verified, and the law of coal-rock interface friction work to coal-rock impact kinetic energy conversion is quantitatively described. The results show that the interfacial friction coefficient of deep coal rocks decreases linearly with the increase of the amplitude of the impact load, and increases linearly with the increase of the frequency of the impact load. When the impact load amplitude is 5000 N and the impact load frequency is 500 Hz, the ultra-low friction effect occurs when the friction force of deep coal rock interface decreases by 97% and the reduction rate is 38.9 kN/ms ~ 41.38 kN/ms. For the first time, the ultra-low friction effect is quantitatively described in terms of friction reduction amplitude and reduction rate. Combined with the experimental and engineering actual analysis, it is found that the average experimental result of the energy consumption ratio is 0.441, and the calculation result of the "11.11" impact ground pressure of Hongyang Three Mines is 0.488, which is relatively close, which further proves the rationality of the proposed model.

2023, 55(3): 686-698. doi: 10.6052/0459-1879-22-467

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RETHINKING OF THE BUILDING ENERGY SYSTEM TOWARDS THE CARBON NEUTRAL TARGET

Liu Xiaohua, Zhang Tao, Liu Xiaochen, Jiang Yi

The building sector is a key sector to achieve the double carbon target, and under the guidance of the double carbon target, the building energy system needs to make innovation. Therefore, the development tasks of building energy system are discussed in depth in this paper, and the development direction of building energy system facing the dual carbon target is proposed. The traditional building energy system is mainly to meet the basic energy needs of the building itself, such as cold, heat and electricity. Under the dual carbon target, there needs to be a shift from energy efficient buildings to new targets for low-carbon buildings. The building energy system needs to make changes in reducing the energy demand of the building body, fully electrifying the building energy system, improving the energy efficiency level of the building energy system, realizing flexible adjustment and becoming an adjustable load of the energy system with flexible adjustment ability. It needs to shift from being a simple consumer of energy systems to a complex that integrates energy production, consumption, regulation and storage. To build low carbon building energy system as the goal, the trend of research of building energy systems is discussed: the need to further meet the demand of the building energy to make a full understanding of the building energy requirements, e.g. the energy requirements for building thermal environment is a key section for building energy consumption and energy conservation, need further integration architecture and traffic, electricity and other fields, from the monomer building to multiple scales, such as architecture, urban construction as the carrier to build new energy system between urban and rural areas. The current research provides a useful reference for building energy system to realize its own role transformation and accelerate the realization of energy system reform under the dual carbon target.

2023, 55(3): 699-709. doi: 10.6052/0459-1879-22-462

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ANALYTICAL SOLUTION OF LIMIT STORAGE PRESSURES FOR TUNNEL TYPE LINED GAS STORAGE CAVERNS

Wang Zhechao, Jia Wenjie, Feng Xiating, Wang Jingkui

Tunnel lined cavern gas storage is a new energy storage method, which helps balance supply and demand, promotes the continuous transition from fossil energy to green energy, and facilitates the realization of national goal of "carbon neutralization and carbon peak". In this paper, the ultimate equilibrium method and the elastoplastic analysis method are used to derive the analytical solution of the ultimate storage pressure of tunnel lined rock cavern gas storage. In the ultimate equilibrium method, the self-weight of the overlying surrounding rock, the force of the fracture surface and the ultimate storage pressure are considered, the rigid cone model is selected, and the upper limit pressure expression is derived. In the elastoplastic analysis method, according to the stress distribution law and shear and tensile strength in the surrounding rock, the upper and lower pressure expressions under elastoplastic conditions are derived. Finally, the analytical solution of the ultimate pressure is determined with considering the results obtained by the two methods. The results show that the relationship between the upper limit pressure and the buried depth is quadratic function, and increases with the increase of lateral pressure coefficient; The upper limit pressure and lower limit pressure determined by the elastoplastic analysis method are linear with the burial depth, and the lower limit pressure decreases with the increase of the lateral pressure coefficient, and the lower limit pressure is not considered for the lined gas storage under the condition of class I surrounding rock. When the lateral pressure coefficient is 1.2, the upper limit pressure is the largest, so the tunnel type gas storage should be built as far as possible under the surrounding rock condition with the lateral pressure coefficient of 1.2. Finally, the recommended pressure ranges of lined rock caverns are given according to the upper and lower limit pressure curves under typical working conditions.

2023, 55(3): 710-718. doi: 10.6052/0459-1879-22-474

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STRATEGIC AND GEODYNAMIC ANALYSES OF GEO-SEQUESTRATION OF CO₂ IN CHINA OFFSHORE SEDIMENTARY BASINS

Li Chunfeng, Zhao Xueting, Duan Wei, Wu Tao, Yao Zewei, Chen Guoxin, Li Gang, Peng Xi

Focusing on the national strategic goal of "Carbon Peaking and Carbon Neutrality", this paper comprehensively analyzes the strategic conditions and targets suitable for large-scale CO₂ geo-storage in the China offshore basins, from the perspectives of fault activity, basin pressure, tectonic subsidence, seismicity, and geothermal gradient. It is considered that the East China Sea Shelf Basin, Pearl River Mouth Basin, eastern Qiongdongnan Basin, and the central South China Sea basin are the best geological storage areas for CO₂, although this does not exclude suitable targets in other unfavorable sedimentary basins since a specific geo-sequestration target is small in area. The suitable CO₂ storage strata in the East China Sea Shelf, Pearl River Mouth, and Qiongdongnan Basins include the bottom salt-water layer of the late rapid subsidence sediments in the open-sea environment and the hydrocarbon-bearing units in the thermal subsidence sedimentary sequences. Between 800 and 4000 m depths beneath the seafloor, the porosity is greater than 10%, and the hydrostatic and lithostatic pressures vary from ~ 8 to ~ 40 MPa and from ~ 13 to ~ 83 MPa, respectively. In this pressure and suitable geothermal gradient ranges, CO₂ exists in a supercritical state, and its density is relatively stable with temperature and pressure changes, which is beneficial to the flow and permeation of CO₂. The scale and number of mafic magmatic rock formations in the basins also provide good conditions for CO₂ geological sequestration and permanent mineralization. Although operationally difficult and expensive, CO₂ storage in the central South China Sea basin is very safe. CO₂ injected deep into the oceanic basalt can undergo basalt mineralization, but if CO₂ is escaped as the mineralization process is relatively slow, escaped CO₂ can be further trapped by multiple other storage processes, including pyroclastic rock mineralization, seafloor sediment sequestration, seabed sediment CO₂ hydrate storage, carbonate neutralization reaction, seabed carbon lake, ocean dissolution, etc. The existing six International Oceanic Discovery Program (IODP) boreholes that have encountered basement basalt in the central basin of the South China Sea can provide a good scientific and engineering foundation for the pilot CO₂ storage experiment in the South China Sea basin.

2023, 55(3): 719-731. doi: 10.6052/0459-1879-22-384

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CO₂ SEQUESTRATION IN UCG CAVITIES: RESEARCH PROGRESS AND FUTURE DEVELOPMENT TRENDS

Li Longlong, Fang Huijun, Ge Tengze, Liu Yuewu, Wang Feng, Liu Danlu, Ding Jiuge, Yu Yueyu

Carbon capture and storage (CCS) could help a lot to achieve carbon peaking and carbon neutrality goals and is an effective way to deal with the Greenhouse effect. Among the geologic sequestration formations, the cavities resulting from deep underground coal gasification (UCG) become a hot topic in the research area of geologic CO₂ sequestration. However, compared with conventional sequestration methods, the related work is still in the theoretical exploration stage and lack of trial tests. To promote the development of UCG cavity sequestration, we have done the work as follows. First, we introduce the research progress of UCG and post UCG cavity sequestration, and divide the development of the latter one into three stages including the early stage of conception, stage of quantitative assessment and feasibility analysis, and stage of mechanism analysis. Currently, it is still in a stage of theory exploration. Second, we compare the UCG cavity sequestration with the conventional sequestration options in detail from the perspective of injectivity, sealing capacity, economy, storage capacity, and trapping mechanism. The results show that the UCG cavity sequestration holds an excellent injectivity, has a similar sealing capacity to the unmined coal seams but more complex, is capable to reduce transport cost a lot, has a great potential in storage capacity, and has complex trapping mechanisms, owing to the additional effects of cavity morphology, wall properties, and interactions between supercritical CO₂ and in-situ fluid on the injection and storage processes. Third, we point out the key scientific and engineering issues, and basic future development trends of the UCG cavity sequestration. Based on the above work, we suggest that the government introduces some policies to encourage and support the development of UCG and post cavity sequestration which could enrich the CCS family and promote the clean and low-carbon utilization of coal resources.

2023, 55(3): 732-743. doi: 10.6052/0459-1879-22-538

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EXPERIMENTAL STUDY ON FRICTIONAL PROPERTIES OF FLUID-BEARING SANDSTONE FRACTURES AT DIFFERENT TEMPERATURES

Shen Nao, Li Xiaochun, Wang Lei

Seismic activity caused by fluid injection in sandstone reservoirs has been associated with the frictional properties of embedded faults or fractures. In order to study the frictional characteristics of fluid-bearing sandstone fractures under different temperature conditions, velocity stepping tests were carried out at varying temperature and pressure conditions (a temperature range of 25 °C ~ 140 °C and an effective normal stress range of 4 ~ 12 MPa) on dry, water saturated and CO₂ injected sandstone fractures (obtained by saw cutting), respectively. The experimental results show that: (1) For dry sandstone fractures, increasing effective normal stress and increasing temperature can both increase the initial friction coefficient of fractures, while varying effective normal stress has no obvious effect on the frictional stability of fractures. An increase in temperature is found to enhance the frictional stability of fractures. (2) For sandstone fractures saturated by water, the initial friction coefficients of fractures also increase with the effective normal stresses, but they can be weakened by the rising temperatures, and increasing effective normal stress and temperature can both favor the frictional instability of fractures; (3) For the CO₂ injected sandstone fractures, the initial friction coefficients of fractures are affected by the change in effective normal stress and temperature, which is opposite to that of water-saturated sandstone fractures. The frictional stability of fractures is affected by the ambient temperature, seemingly independent of the effective normal stress. To sum up, these experimental results suggest that the frictional characteristics of sandstone fractures are jointly controlled by the effective normal stress, temperature and the injected fluid type. These experimental results may provide a better understanding of earthquakes induced by fluid injection.

2023, 55(3): 744-754. doi: 10.6052/0459-1879-22-400

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RESEARCH ON CO₂ MICROBUBBLE DISSOLUTION KINETICS AND ENHANCED OIL RECOVERY MECHANISMS

Jia Haowei, Yu Haiyang, Xie Feifan, Yuan Zhou, Xu Ke, Wang Yang

CO₂ microbubble is a promising enhanced oil recovery and carbon sequestration method. In this paper, based on microbubbles porous media generation method, a self-designed microbubble generator featuring the porous ceramic membrane was developed. The morphology and dissolution characteristics of CO₂ microbubbles at different initial CO₂ concentrations were experimentally investigated. The results showed that the CO₂ microbubbles prepared at 10 MPa were distributed in the range of 10 ~ 70 μm with an average bubble diameter of 34.43 μm. At 15 MPa, CO₂ microbubbles with smaller diameter were generated, with an average bubble radius of 25.03 μm. However, under high salinity condition, microbubbles with average diameter of 277.17 μm were produced. The brine salinity decreased microbubbles stability, which leading to bigger bubble. In a word, the microbubbles diameter was highly affected by the pressure in microbubbles porous medium generation method. Then, the static and dynamic dissolution kinetics of microbubbles in the porous media were investigated by microfluidics. The results of dissolution experiments showed that microbubbles had excellent dissolution efficiency. When contacting with formation water, microbubbles would rapidly dissolve and the undissolved microbubbles were still migrating the porous media in the form of bubbles. CO₂ microbubbles could form a migration mode with carbonated water in the front and microbubbles in the rear, after microbubble were injected into the reservoir. For the first time, the enhanced oil recovery mechanisms of CO₂ microbubbles were studied under high-temperature high-pressure conditions, which mainly include: ①Microbubbles carry residual oil on the pore wall during migration; ②Microbubbles carry residual oil droplets out of the pores with dead ends through dissolution and oil swelling; ③Break the capillary force balance of residual oil droplets and promote the flow of oil droplets; ④Block the high permeability channel to improve the sweep efficiency. This paper provides valuable guidance for CO₂ microbubble to enhanced oil recovery and carbon sequestration.

2023, 55(3): 755-764. doi: 10.6052/0459-1879-22-507

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DEVELOPMENT OF DIGITAL INTELLIGENCE FLUID DYNAMICS AND APPLICATIONS IN THE OIL & GAS SEEPAGE FIELDS

Song Hongqing, Du Shuyi, Wang Jiulong, Lao Junming, Xie Chiyu

The growth of big data and artificial intelligence technologies has driven the rapid development of digital intelligence fluid mechanics. Digital intelligence fluid mechanics combines fluid mechanics, big data and artificial intelligence, to establish a new research paradigm oriented to specific scenarios of fluid mechanics, with "data" as the basis, "intelligence" as the core, and arithmetic power as the support. Its connotation is to establish a "data + physics" co-driven digital intelligence model, which is mainly data-driven and incorporates prior knowledge such as physical information and expert experience, to solve practical problems in different scenarios. Digital intelligence fluid dynamics has very obvious advantages in modeling flexibility, computing efficiency, and computational accuracy, whose application potential has been proven in multi-scale flow, multi-field coupling, and flow field modeling. In terms of the construction of digital intelligence models, data governance is indispensable since the data quality improved by governance enables intelligent algorithms to perform preferably. There are four main mechanisms for introducing "data + physics" co-driving in intelligent algorithms, which are input data-based embedding mechanism, model architecture-based embedding mechanism, loss function-based embedding mechanism and model optimization-based embedding mechanism. Taking oil & gas field applications as an example, a series of research advances in the prediction of physical parameters, evaluation of fracturing effects and optimization of injection parameters by digital intelligence fluid dynamics are introduced. Future diversified research models can take advantage of the efficient and rapid modeling of digital intelligence fluid dynamics, but also ensure physical interpretability and extrapolation in both classical and computational fluid dynamics. Therefore, digital intelligence fluid mechanics is an inevitable trend in the future development of fluid mechanics, and it is necessary to take the scenario demand as the guide, deeply integrate physical information and prior knowledge, actively explore new intelligent theories and methods, and attack the complex and changing scientific problems in fluid mechanics.

2023, 55(3): 765-791. doi: 10.6052/0459-1879-22-484

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STUDY ON CONNECTIVITY AND ENTROPY SCALE OF THREE-DIMENSIONAL FRACTURE NETWORK

Fan Xincheng, Ye Zuyang, Huang Shibing, Cheng Aiping

The fracture network is complex in engineering rock mass, both the geometric characteristics and connectivity have an important influence on its permeability. In order to comprehensively quantify the influence of fracture trace length, dip angle, spacing and aperture on the connectivity and permeability of fracture network, basing on the principle of information entropy, the geological entropy theory and connectivity index entropy scale of three-dimensional fracture network is proposed. Compared the entropy scale with other traditional connectivity indexes, the rationality of entropy scale in evaluating the connectivity and permeability of three-dimensional fracture network is verified. The results show that the trace length of the fracture is negatively correlated with the entropy scale and the permeability coefficient. The fracture spacing and aperture are positively correlated with the entropy scale and the permeability coefficient. The dip angle of the fracture has little effect on the entropy scale and the permeability coefficient. The nonlinear relationship between entropy scale and permeability coefficient approximately satisfies quadratic polynomial. Basing on the statistical distribution of the fractures on the Left Bank Slope Jinping Hydropower Station, a numerical calculation method of three-dimensional fracture network seepage is established. By analyzing the relationship between the three-dimensional fracture network geometric characteristics and fracture areal intensity, dimensionless percolation density, entropy scale and permeability coefficient, The following conclusions are obtained: when the volume ratio is constant and the influence of aperture is considered, the fracture areal intensity and dimensionless percolation density cannot quantitatively characterize the influence of fracture network geometric characteristics. The length of fracture is negatively correlated with entropy scale and permeability coefficient. The fracture spacing and aperture are positively correlated with entropy scale and permeability coefficient. The dip angle of fracture have little influence on entropy scale and permeability coefficient. The nonlinear relationship between entropy scale and permeability coefficient approximately satisfies the quadratic polynomial.

2023, 55(3): 792-804. doi: 10.6052/0459-1879-22-579

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2023 Vol. 55, No. 3

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