Study On Flow Field And Simulation Leaching Of Horizontal Cavern For Underground Energy Storage In Bedded Salt

Due to high consumption and foreign-trade dependence of petroleum and natural gas in China,developing underground energy reserves is an important measure to ensure energy security.Since the salt rock has low permeability and self-healing capability,salt cavern is widely used as storage for natural gas and petroleum.In China,as supporting projects for long-distance natural gas transmission pipelines,several salt gas storage have been constructed.However,the salt formations of China are most bedded deposits with high impurity and many interlayers.Some problems were encountered in cavern construction using traditional single-well-oil-blanket method,the site selection and construction of salt caverns gradually slow down.The horizontal cavern construction technology can build salt caverns with larger volume in thin salt layer,and it can also avoid thick interlayers.Therefore,the method will have broad applications in China.But,relative studies about flow field and cavern development are rare.The engineering experience is insufficient.For this purpose,lab tests and numerical methods have been carried out to research the flow field in horizontal cavern leaching process.Based on the characteristics of brine flow and salt mass transport,the salt cavern are divided into four regions.A mathematical model describing the multi-step horizontal leaching method is introduced.It contains several main physical change equations during salt cavern development.Finally,a series of simulations of building cavern have been conducted for technological parameter optimization using the model.The research can provide relatively theoretical basis and technology reference for safe efficient mining of salt rock.Main study contents and results are as follows:(1)Based on similarity theory,a physical test platform for simulating the flow field of horizontal cavern was established,the dyeing water flow tests and PIV tests were carried out.The flow field formed by the fresh water horizontally flowing into brine was observed.When fresh water jet enters brine environment,a stable buoyant jet area with higher flow velocity will be formed.The jet trajectory is curved upwards,the jet shape is symmetrical about the trajectory centerline,and its cross section expands with the increment of flow distance.The length of jet trajectory and the horizontal distance between jet end point and jet outset increase with the water injection rate,but the horizontal influence range is limited.(2)The influence of insoluble sediments on the flow field was explored through laboratory experiments and numerical simulations.Fresh water flows into salt cavern with sediments,it flows through sediment zone into the fluid space forming a submerged buoyant jet,its overall direction is vertical upward.The larger the water injection rate,the larger the floating plume area above sediment zone and the larger the fluid velocity.The results show that sediments reduced the convection intensity in cavern.The high sediment with small porosity decrease obviously the mass transfer rate.Therefore,when encountering salt layers with high insoluble content,a large water injection rate should be used to improve the construction efficiency.(3)Through laboratory tests,the flow process of fresh water injected into different salt cavern models was observed,and the characteristics of salt transport were analyzed based on previous experience.With reference to related researches,the flow field of salt cavern is divided into four main areas as: buoyant jet zone,diffusion area,boundary zone and saturation zone.The flow state and concentration characteristics of each area were analyzed separately.It is found that sediments significantly affects the convection and diffusion characteristics of fluid in each area of cavern.(4)A numerical model is introduced for the multi-step construction of horizontal salt cavern.The model is based on a composite structural mesh.It includes many equations about main physical processes such as concentration field,velocity field,salt dissolution and cavern wall movement,and insolubles accumulation and redistribution.A C++ program is developed for model implementation.The accuracy of the model are verified by comparisons with laboratory test and engineering data in published literature.The results show that the software can predict cavern development well,the calculated results are also consistent with actual data.(5)The horizontal multi-step leaching method can be used for mining thin salt layers.A series of simulations according to the geological conditions of Huasu salt mine have been conducted.The technological parameters including borehole length,step distance and flow rate were considered.The salt recovery rate,the salt and water ratio and salt production rate were used as evaluation parameters.Results can provide engineering suggestions for the safe and efficient mining of thin salt layers.