Undersaturated oil reservoirs utilized for underground gas storage are devoid of primary gas caps and are characterized by multiphase fluid coexistence and compositional exchange. Currently, they encounter critical challenges, including an inadequate understanding of gas-liquid interactive displacement mechanisms and suboptimal precision in performance indicator design. By analyzing the special characteristics of gas-liquid interaction flooding and fluid properties periodic changes in oil reservoir type UGS, key theoretical and technical challenges have been proposed. Recent key developments have been summarized, and future research directions have been proposed. Research has shown that oil reservoir type UGS faces key scientific issues, such as the expansion mechanism of under gas-liquid interactive displacement and the interphase mass transfer mechanism, as well as technical issues such as secondary gas cap formation and control, index design, and efficient liquid discharge expansion. Special site selection indicators such as water drive efficiency and sweep efficiency have been proposed in oil reservoir type UGS. Simulation experiments reveal that the interactive displacement efficiency of fluid and oil has increased by 10.3 and 12.5 percentage points respectively compared to single gas displacement. Considering the factor of phase mass transfer, the effective storage capacity increases by 17.8%. In the future, oil reservoir type UGS will deepen theoretical and technological research on mechanism of space utilization and phase interaction, UGS construction parameter design, and secondary gas cap formation and control. The research results provide important basis for scientifically guiding the UGS design, achieving peak shaving capacity, and improving the oil recovery.

With the transformation of China's energy structure and the growth of natural gas consumption demand, the coordinated development of oil and gas has become an important development direction. It is of great significance to reconstruct gas storage in old reservoirs to revitalize oilfield assets and improve regional peak shaving and gas storage capacity. In the process of reservoir construction, the formation and migration of secondary gas cap is the key to the synergistic extraction, drainage and expansion of gas storage and the formation of high-speed injection and production capacity in the later stage, which is the core problem affecting the efficient capacity of gas storage. This paper explores the formation conditions and migration rules of secondary gas cap, and provides theoretical support for gas drive expansion scheme. Based on physical simulation, numerical simulation and reservoir engineering methods, the changes of gas saturation and formation pressure are analyzed to determine the critical conditions for the formation of secondary gas cap, and to reveal the influence mechanism of well pattern, formation dip angle, reservoir heterogeneity and other factors on the development and migration of gas cap. The results show that the well pattern and formation dip angle are the main controlling factors for the stability of secondary gas cap, and the strong heterogeneity of reservoir will inhibit the uniform expansion of gas cap. The study clarifies the key control parameters of gas cap evolution, provides a basis for optimizing the injection-production system, and has theoretical and engineering guiding significance for the coordinated development of oil and gas and the efficient construction of gas storage.

To effectively guide the design of the upper limit pressure for the operation of gas storage reservoirs in complex fault-block oilfields and ensure their safe and efficient operation, it is imperative to conduct a comprehensive evaluation of the sealing performance of the geological body. By introducing a multi-scale geomechanical model for simulation, we took the Nanpu 1 Gas Storage Reservoir as an example, on the basis of the single-reservoir geomechanical model, and establish a large-scale multi-reservoir interaction in-situ stress model for the first time. The four-dimensional stress field distribution under different pore pressures was simulated, and the stress changes and stability of faults and cap rocks were analyzed. On the basis of the traditional geomechanical model, a quantitative characterization method for the stress interference effect of multi-reservoir interaction was constructed, to reveal the superimposed influence mechanism of multi-gas storage reservoirs' coordinated injection and production on the regional stress field. A multi-dimensional and multi-index evaluation method for the integrity of geological bodies was proposed, achieving quantitative evaluation of cap rock tensile failure, shear failure and fault slip, and then a full-system monitoring system was deployed. The research results show that the deformation of the Guantao Cap Rock during the operation of the gas storage reservoir is small; there is no risk of fault activation in the early operation of the gas storage reservoir for hydrocarbon-controlling fault F2; the critical pressure for simulating fault slip activation is 34.5 MPa, and there is a risk of instability in the integrity of the gas storage reservoir geological body. The upper limit pressure for operation was determined to be 27.2 MPa based on the combined critical pressures for cap rock tensile failure, cap rock shear failure, and fault slip activation obtained from models of different scales. The research results provide a fine and quantitative assessment of the operational safety of gas storage reservoirs under the influence of dynamic stress fields and scientifically guide the optimization design of the upper limit pressure and monitoring system.

The primary risk associated with repurposing complex fault-block oil reservoirs into gas storage facilities lies in the potential loss of fault seal integrity under alternating high and low pressure conditions during cyclic injection and production. Therefore, a quantitative assessment of the pressure-bearing capacity of faults is critical to ensuring the safe operation of such gas storage sites. This study, based on seismic data, drilling records, and production dynamics from the Nanpu 1-5 Reservoir, conducts a graded and staged evaluation of faults. Through four-dimensional geomechanical simulations covering the entire lifecycle of reservoir development and gas storage injection-production operations, the safe pressure-bearing capacity of faults is quantitatively evaluated. The results indicate that faults F1 to F6 generally exhibit large throws, characterized by sealing interfaces between gas-bearing layers and tight formations, demonstrating excellent sealing performance. The Nanpu 1-5 Reservoir is divided into four fault blocks: NP1-7, NP105X1, NP1-5X1090, and NP105X5. Critical weak points in these fault blocks are identified as boundary faults F2(NP1-7), F2(NP105X1), F5, and F6, with safe pressure-bearing capacities of 27.4, 28.9, 28.9, and 27.5 MPa, respectively. These findings provide a scientific basis for optimizing the maximum operating pressure of the NP1-5 gas storage facility. The methodology can be extended to enhance the operational efficiency of existing gas storage facilities through pressure optimization.

In order to address the differences in mechanical properties of reservoir and cap rocks in middle and deep UGS, this paper selects mudstone, shale, sandstone, and dolomite cores from four UGS (P, M, X, S) and systematically compares their mechanical properties through uniaxial, triaxial, and fatigue compression experiments. The results are as follows: 1) The mechanical strength and deformation resistance of shale in UGS X are weaker than those of cap mudstone of UGS P. The compressive strength and elastic modulus of dolomite in UGS X are much higher than those of sandstone, and its compressive strength can reach more than twice that of sandstone. 2) For UGS with the same sandstone, the deeper the burial depth, the stronger the compressive strength and deformation resistance, and the strength increase of dolomite due to confining pressure is the greatest. Comparison of mechanical parameters of sandstone and mud stone in P UGS, mudstone has slightly higher strength and deformation resistance than sandstone. 3) An increase in burial depth and a decrease in porosity and permeability can significantly enhance rock strength, with a positive correlation between elastic modulus and compressive strength. Fatigue experiments show that the strength of mudstone in deep UGS only decreases by 10% after 1 000 injections, while the strength fluctuation of mud stone in middle UGS reaches 20%. The deep cap rock is more suitable for high-pressure alternating loads. 4) Microstructure analysis reveals that dolomite has high strength due to dense intergranular cementation, while sandstone's porous structure and weak cementation result in weaker mechanical properties. The research results provide experimental support for long-term safe operation of UGS.

The interlayer heterogeneity in the Nanpu 1-5 Block is strong, which significantly impacts the displacement mechanism and storage capacity formation during the process of converting the reservoir into an underground gas storage facility. Using natural cores and two-dimensional (2D) sand-packed cemented slab models from this block, dual-core parallel longcore flooding experiments and 2D model gas flooding experiments were conducted to investigate the influence of interlayer heterogeneity on enhanced oil recovery coupled with gas storage construction. The results show that both the dual-core long-core flooding and the 2D model gas flooding, as well as cyclic injection-production operations, lowered the mobilization threshold of the low-permeability cores, improved the oil displacement efficiency of the low-permeability cores, reduced the differences in oil displacement efficiency and sweep efficiency between the high and low permeability cores, and effectively alleviated interlayer conflicts. The gas flooding stage is the main phase for oil production and storage capacity creation, contributing 59% of the total oil recovery and 72% of the gas sweep efficiency. The dominant oil displacement mechanisms during this stage include gas cap drive, gravity segregation, and crude oil swelling and viscosity reduction. For UGS construction in heterogeneous multi-layer reservoirs, a slow gas flooding strategy should be adopted to prolong the gas flooding duration, thereby fully leveraging the roles of gas cap drive and gravity drainage to maximize the ultimate oil recovery and storage capacity. This study provides experimental guidance for EOR research during UGS construction in similar reservoirs.

Compared with conventional underground gas storage construction in depleted gas reservoirs, fluid compositional migration and phase behavior changes are more complex during multi-cycle injection-production in weakly volatile oil reservoirs. To reveal the phase-equilibrium evolution during cyclic injection-production, this study takes the Nanpu 1-5 weakly volatile oil reservoir as the target and establishes a phase-equilibrium experimental procedure and theoretical simulation model for multi-cycle injection-production. The variations in the composition, phase behavior, saturation, and recovery degree of produced and residual fluids were investigated. The results show that after five injection-production cycles, the oil saturation decreased by 2.4%, which is conducive to increasing gas storage space. The methane content of the produced gas increased from 93.8% to 95.3%, while CO₂ and C₂₊ components gradually decreased, indicating a weakening extraction effect of the injected gas. The C₇₊ mole fraction of the residual oil increased by 9.2%; under the upper and lower pressure limits, the solution gas-oil ratio decreased from 185 and 88 m³/m³ to 78 and 35 m³/m³, respectively, and the oil formation volume factor decreased from 1.57 and 1.32 to 1.24 and 1.16, respectively, indicating gradual heavy-end enrichment of the residual oil. This study clarifies the relationships among compositional migration, phase behavior evolution, and gas storage space expansion, providing a basis for optimizing cyclic injection-production strategies in similar oil-reservoir-based underground gas storage projects.

The integrated construction of oil reservoir-type gas storage coupled with natural gas flooding for enhanced oil recovery has emerged as a significant direction in recent years. Through mechanisms such as gravity segregation, miscible displacement, and extraction, this approach can effectively improve crude oil recovery while establishing large-scale seasonal peak-shaving capacity. Taking the PG2 Underground Gas Storage of Jidong Oilfield as an example, this paper systematically conducts geological condition evaluation for gas storage construction, sealing analysis of cap rocks and faults, design of injection-production capacity, and optimization of storage capacity parameters, thereby demonstrating the feasibility of converting such oil reservoirs into gas storage facilities. The results indicate that the target block features thick reservoirs, favorable physical properties, excellent fluid characteristics, and regionally stable cap rocks with strong sealing capability. The comprehensive evaluation shows that the dynamic sealing pressure limit of the geological body reaches 44.9 MPa, providing a solid geological foundation for gas storage construction. The proposed scheme achieves a working gas volume of over 9.0×10⁸ m³, a daily peak-shaving capacity of nearly 0.1×10⁸ m³, and an expected increase in crude oil recovery of approximately 20 percentage points compared to water flooding. These results confirm that converting blocky volatile oil reservoirs into gas storage is technically feasible, economically effective, and the associated methodologies can serve as an important reference for similar reservoirs aiming to simultaneously enhance oil recovery and provide gas storage peak-shaving services.

Underground gas storage (UGS) facilities in China are predominantly developed in gas reservoirs, exhibiting limited diversity, while UGS conversion from depleted oil reservoirs remains in its nascent stage. To address key challenges such as complex post-waterflood fluid distribution, intricate seepage mechanisms, and undefined technical policies for synergistic development, this study focuses on the X Oil Reservoir in the Jidong Oilfield. A comprehensive investigation was conducted to optimize collaborative hydrocarbon recovery enhancement and storage capacity expansion. By integrating high-resolution geological characterization, dynamic evaluation of individual well injectivity/productivity, and optimal well pattern selection, a numerical simulation model was established. Quantitative analyses were performed to evaluate the impacts of full-cycle liquid drainage, timing of transitioning to areal flooding, and staged pressure ramp-up on storage capacity utilization and oil recovery. The results are as follows: 1) Full-cycle liquid drainage significantly enhances drainage rates and working gas volume compared to conventional methods; 2) The optimal transition to areal flooding occurs when the gas front reaches the structural low, effectively mitigating gas breakthrough risks while accelerating working gas formation; 3) A“staged and stepwise pressure ramp-up” strategy is proposed, with the initial pressure increase timed at the shift from single-pattern gas drive to areal flooding, enabling synchronous growth in both storage capacity and drained liquids. Simulation forecasts indicate that the“full-cycle drainage + pressure ramp-up” mode will achieve the target working gas volume of 10.52×10⁸ m³ by 2034—2035, maximizing both storage space and recovery efficiency. These results provide critical technical foundations for realizing the“high deliverability with fewer wells and efficient UGS development” objective in similar depleted oil reservoir conversions.

Depleted and water-flooded oil and gas reservoirs constitute the primary construction carriers of underground gas storage (UGS) facilities in China. Such gas storages are universally plagued by engineering challenges including massive residual formation water volume, severe interlayer reservoir heterogeneity, and reservoir pore clogging induced by long-term cyclic injection-production operations, which restrict the utilization efficiency of gas storage capacity as well as the capacity for rapid commissioning and stable production. Based on an analysis of technical difficulties associated with liquid drainage in the offshore shallow blocks of Jidong Oilfield in the Bohai Bay Basin, this paper proposes an efficient gas-lift liquid drainage engineering technology for UGS converted from oil and gas reservoirs. A two-stage hermetically sealed zonal injection-production gas-lift drainage process and a series of high-flow-rate liquid drainage processes for old wells are designed, with matching downhole tools developed accordingly. Large-scale field application of this technology at Nanpu N Gas Storage has yielded remarkable results: cumulative formation liquid drainage reached 68.70×10⁴ t, effective utilization of the remaining enclosed gas space, increases the effective storage capacity. The research outcomes provide an engineering reference for the renovation of UGS converted from complex water-flooded old oil and gas reservoirs in the Bohai Bay Basin, and also offer critical technical references and practical experience for liquid drainage, pressure reduction, precise capacity expansion and rapid commissioning of similar depleted water-invaded oil and gas reservoirs nationwide.

A dynamic productivity design method for gas wells based on equivalent seepage theory and multiphase flow dynamic coupling is proposed to address the challenge of insufficient accuracy in forecasting well deliverability caused by dynamic gas saturation evolution during the capacity expansion and production ramp-up of oil reservoir-based gas storage facilities. Unlike traditional gas storage models relying on well test data, this approach establishes a conversion model between absolute gas permeability under low confining pressure in surface cores and effective oil permeability under irreducible water conditions at formation pressure. Combined with gas-oil relative permeability curves, it creates a dynamic mapping relationship between gas saturation and effective gas permeability. Using nodal analysis, the method quantifies the impact of gas saturation in the secondary gas cap zone on gas flow capacity during injection-production cycles, innovatively deriving a binomial dynamic deliverability equation. Research demonstrates that this technique overcomes the dependence on production data in existing methods, achieving effective conversion from surface permeability parameters to in-situ multiphase flow conditions, and reveals the nonlinear influence of gas saturation variation on well productivity during storage operations. Compared with conventional steady-state methods, dynamic predictions show well productivity fluctuations reaching 1.5~4.0 times steady-state values, verifying the necessity of dynamic modeling. This research provides crucial theoretical support for well type selection, pattern optimization, and dynamic parameter adjustment during cyclic operations in oil reservoir-based gas storage facilities, filling the technical gap in dynamic productivity prediction while significantly enhancing peak-shaving capacity and project economics.

The sealing of old wells is one of the core tasks in converting depleted oil and gas reservoirs into underground gas storage, and the quality of sealing directly determines the operational safety of the gas storage. Old wells with sidetracked open-hole structures, due to their complex processes and prominent construction difficulties, are faced with engineering bottlenecks such as difficulty in exploring sidetracked wellbores, poor stability of open-hole walls, and difficulty in ensuring sealing quality, thus becoming the key and difficult points in old well sealing projects. To solve these problems, by optimizing the direct reentry open-hole connection method and By-pass trajectory design, an integrated scheme based on passive magnetic guidance trajectory control and open-hole tracking has been constructed, which improves the wellbore trajectory control accuracy to the centimeter level. Meanwhile, by integrating the anti-collapse and lubricating drilling fluid system and high-efficiency cementing technology for complex wellbores, a key technical scheme for open-hole sealing of old sidetracked wells has been formed. This technology has been applied to 7 multi-well old wells with different well conditions in the Jidong Oilfield gas storage. All sidetracked open holes have been successfully re-entered and sealed, with a 95% pass rate of cementing quality in the open-hole sections, and the construction period is 30 d shorter than designed, meeting the requirements for optimal and rapid disposal of difficult old well plugging. This technology provides a mature technical means for the disposal of old sidetracked wells in gas storage and also offers practical experience and a reference for similar difficult well plugging projects in China.

Frequent wellbore instability poses a significant challenge during the drilling of large-diameter wellbores in the hard and brittle mudstone of the Dong 3 Member in the Pugu 2 Gas Storage Reservoir. Based on the scale effect theory in rock mechanics, this study conducted comparative mechanical degradation experiments on intact and crushed mudstone samples immersed in drilling fluid. A wellbore collapse prediction model considering the time-dependent seepage damage was subsequently established to reveal the fracture-fluid coupling damage mechanism. The results are as follows: 1) The well-developed micro-fracture network in the mudstone transforms singular hydration diffusion into a severe hydration-seepage coupling damage. 2) The rock degradation exhibits a significant scale effect and nonlinear time-dependent characteristics. During the initial immersion phase (0~4 d), the strength decay rate of the crushed samples is 1.54 times that of the intact ones. 3) A wellbore collapse pressure prediction model considering the time effect of fracture seepage damage was established. The calculation results show that, the critical collapse density of the 9.5 in. well position exceeded the safety warning line on the second day, and compared with the 12-day stable period of the 8.5 in. conventional section, the safe operation window was shortened by more than 80%. Based on this, a collaborative anti-collapse strategy of micron-scale rigid plugging and film-forming agents and a 48 h rapid drilling and completion operation suggestion were proposed, providing a theoretical basis for safe drilling in complex sections of gas storage reservoirs.

Gas storage is the core infrastructure for natural gas peak shaving, supply guarantee and energy security. Aiming at the complex wellbore problems in the reconstruction of oil reservoirs into gas storage, such as casing deformation and damage, falling object sticking, substandard caprock cementing quality and insufficient reliability of reservoir plugging, this paper studies the key technologies and supporting tools for wellbore disposal, and constructs three technical systems: electric submersible pump (ESP) fishing for long fish in high-deviated wells, expansion pipe casing milling disposal, and high-efficiency section milling of long casing sections. An integrated disposal scheme is formed through the optimal design of special fishing tools, scientific assembly of fishing strings and accurate analysis of fish top morphology. The high-efficiency split pilot milling shoe adopts a multi-channel circulation structure and reasonable cutting taper, which significantly improves the operation safety of channel making in high-deviated wells. The six-blade section mill cooperates with the reaming technology, and optimizes the blade geometric parameters and arrangement, effectively improving the section milling efficiency and cutting quality. Field application results show that from 2020 to 2025, the technical system has successfully disposed 8 complex wells in Jidong Oilfield gas storage, with a 100% construction success rate, an average reduction of 32% in operation period, a reduction of more than 90% in sticking risk and a reduction of 28% in construction cost. It realizes safe, efficient and low-cost disposal of complex wellbores, and can provide technical support and engineering reference for the disposal of old wells in similar oil reservoirs reconstructed into gas storage.

Aiming at the problems of vibration instability and strength failure easily occurring in the injection-production string of deep directional wells in hydrocarbon reservoir gas storage under alternating temperature-pressure loads and high-rate injection-production conditions, this paper takes the deep directional wells in Jidong N Gas Storage as the research object, establishes a fluid-structure coupling finite element model, analyzes the vibration response law of the string and the influences of gas injection rate, well depth and string outer diameter on its stress and deformation, identifies the sensitive factors of vibration failure and puts forward safety control strategies. The results show that the vibration of the injection-production string is divided into two stages: unsteady fluctuation (0~6 s) and steady oscillation (7~15 s). The water hammer effect at the initial stage of well opening leads to a sharp rise in stress, with the maximum Mises stress reaching 886.45 MPa, posing a risk of buckling failure. The stress amplitude at the wellhead is the largest, the transverse vibration in the middle of the well is the most significant, and the transverse vibration is much stronger than the longitudinal vibration. The increase of gas injection rate significantly raises the stress level of the string. For the string with an outer diameter of 101.6 mm, the stress exceeds the yield strength at the initial stage of vibration when the gas injection rate reaches 83×10⁴ m³/d. Increasing the outer diameter of the string will raise the initial stress fluctuation amplitude, but can inhibit the increase of transverse vibration. Based on the coupling relationship between “gas injection rate and pipe diameter”, a safety control strategy is proposed to optimize the injection-production gas rate and focus on monitoring the stress mutation at the initial stage of gas injection and the vibration amplitude in the middle of the well. The research results can provide technical support for the safe operation of injection-production strings in deep directional wells of gas storage.

To address the constraints of limited water depth and poor riser safety during the landfall of subsea pipelines for injection, production and transmission of offshore gas storage reservoirs in shallow coastal waters, key control technologies for pipeline pullback in land-to-sea directional drilling are investigated. Three pullback operation modes are classified according to the layout of pipeline prefabrication and traction equipment, and the optimal construction scheme is selected based on site conditions. A novel four-section model for calculating the total length of pullback pipelines is established, with calculation criteria defined for each section. Referencing GB/T 50423 and NEN 3650 standards, a three-stage formula for pullback force is derived by taking full account of resistances on land, inside boreholes and on seabed. The combined prefabrication on land and traction via drilling rig on anchored barge is verified as the optimal solution. Calculations with a safety factor of 3.0 yield a maximum design pullback force of 4 160.92 kN, matching a 500 t offshore drilling rig. Comparison of the applicability of fixed platforms, jack-up platforms and anchored barges confirms the 8-point mooring barge as the preferred operating platform. A calculation method for anchor cable touch-down points is developed based on the catenary equation, alongside comprehensive protection measures for existing subsea facilities. The proposed technologies provide technical support for similar landfall projects of subsea pipelines for offshore gas storage reservoirs via directional drilling in shallow coastal waters.

Gas injection compressors serve as the core power equipment for pressurized gas injection in gas storage, and the unit configuration scheme directly determines the peak-shaving capacity, operational stability and energy consumption level of gas storage. To clarify the adaptation law of centrifugal compressors with different structures in gas storage engineering, optimize the type selection and configuration of compressors, and reduce the project operation cost, a study on the type selection and application of centrifugal compressors for gas storage is carried out. Adopting the methods of literature investigation, structural comparative analysis and engineering economic comparison, this paper systematically summarizes the application status and technical development trend of compressors in domestic gas storage facilities. The structural characteristics, technical bottlenecks and applicable working conditions of three mainstream centrifugal compressor units, namely single-cylinder single-stage, single-cylinder two-stage and high-low pressure cylinder types, are compared and analyzed. Combined with actual gas storage engineering projects, quantitative economic comparison of different compressor configuration schemes (single-cylinder single-stage and single-cylinder two-stage) is conducted from the dimensions of equipment cost, operating energy consumption and maintenance cost. The research results show that the single-cylinder single-stage unit features a simple structure and is suitable for low-pressure conventional gas injection scenarios, while the single-cylinder two-stage centrifugal compressor matches the injection operating conditions of medium-to-high-pressure gas storage and delivers optimal overall performance. The research conclusions define the adaptation boundaries of different types of centrifugal compressors as well as the bottlenecks and improvement directions of domestic centrifugal compressors, which can provide theoretical reference and technical guidance for the type selection, structural optimization and engineering configuration of compressors in gas storage facilities.