Analysis And Evaluation Of Water Seal Condition And Surrounding Rock Stability For Huangdao Water Sealed Underground Petroleum Storage Caverns In Rock

In order to analyze and evaluate the stability of water sealed underground petroleum storage caverns, the efficiency of water curtain system and the surrounding rock stability of storage caverns are studied respectively. Several typical laboratory tests are summarized and refined. Through numerical modeling and parameter designing, the mechanism of petroleum leakage is clarified and all related water seal parameters are optimized. The study for surrounding rock stability is carried out through establishing and computing complicated three-dimensional numerical model. And then the value of each indicator for evaluating the surrounding rock stability is achieved, the layout and excavation scheme of storage caverns is optimized. The detailed terms of the study are as follows:(1) After clarifying the engineering geological conditions of storage caverns thoroughly, the orthogonal experiment for determining the values and the weights of mechanical parameters for surrounding rock was carried out with the maximum displacement as test index. The test results reveal that the most important element which affects the maximum displacement is elastic modulus, the next most important elements are Poisson ratio and internal frictional angle, and then tensile strength and cohesive strength take the third place. On the basis of qualified mechanical parameters for surrounding rock, more orthogonal tests were carried out to achieve appropriate geometric parameters, and the maximum displacement, the volume of plastic zone and the vertical hydraulic gradient were taken as test indexes respectively. In addition, hydrogeological parameters were attained according to the result of water lifting test and empirical formula.(2) In accordance with the characteristics of stress field and seepage field, porous equivalent continuum mathematical model was formed for resembling coupled stress field and seepage field. And the calculation procedures of finite element method and finite difference method for the mathematical model were provided.(3) On the basis of summarizing and refining several typical laboratory tests about leakage of water sealed storage caverns, the principle of petroleum leakage was put forward. TOUGH2_TMVOC was employed to resemble the leakage of storage caverns. Propane was taken as petroleum for modeling and four different storage conditions were studied. By making use of PETRASIM, two models for the condition without water curtain and the condition with water curtain were established respectively. The results indicate the following:The artificial water curtain system can retard or prevent leakage of petroleum storage caverns effectively. By operating with appropriate pressure and layout, the gas tightness and water tightness can be ensured. Along highly conductive features such as major fissure and faults (less than 102 times rock permeability), even partially saturated zones possess certain effects that can retard or prevent petroleum leakage, while a fully unsaturated major fissure or fault connected to the storage caverns can quickly cause petroleum leakage. This possibility strongly suggests that ensuring water saturation of the surrounding rock is a very important requirement. Even if an accident should suddenly impair the water curtain, the gas plume of the petroleum does not instantly penetrate the ground surface, the results of numerical simulations reveal that the gas plum takes several years to reach the ground surface.(4) The analytic evaluation for hydrodynamic containment based on natural groundwater or artificial water curtain was carried out respectively, and the related evaluation indexes were brought forth. The leakage parameterψwas introduced to evaluate the efficiency of hydrodynamic containment based on natural groundwater. Meanwhile the influence of geometric parameters on the value ofψwas examined. In order to evaluate the efficiency of the hydrodynamic containment based on artificial water curtain, the minimum overpressure of water curtain was employed. Some empirical rules for the design parameters in practice were put forward. And the design parameters mainly included the coverage and the overpressure of water curtain, the maximum water curtain pressure, the distance between drill holes, the direction of drill holes, and the distance between the roof of caverns and water curtain. Providing that air tightness and water tightness of storage caverns were ensured, the storage capacity of caverns was studied by employing finite element method. The study employed the critical storage pressure as indicator, the geometric parameters of caverns, the layout of water curtain and the depth of storage caverns as impact factors. On the condition of coupled stress field and seepage field, the impact of artificial water curtain on seepage field of surrounding rock was analyzed and the analytical method for estimating the water inflow of storage caverns was put forward.(5) Complicated three-dimensional numerical model with the characteristics of storage caverns was established by software of Solidworks, Hypermesh and FLAC3D. The aim was to analyze the stability of surrounding rock on the condition of coupling stress field and seepage field. Following that simulations for excavating condition with seepage field and without seepage field, and simulation for storing condition were implemented. Three cross sections of connecting tunnels located at three different levels were taken as feature sections. According to the characteristics of stress field, displacement field, hydraulic gradient field and distribution of plastic zone, the results of simulation were studied. In order to ensure optimized schemes for the layout of caverns and the excavating procedure of each cavern, and high stability of surrounding rock, the comparison of schemes was executed with the volume of plastic zones and the maximum horizontal displacement as evaluating indexes. The results reveal that under the current situations of mechanics parameters, geometric parameters, water seal conditions and boundary conditions, the air-water tightness and surrounding rock stability can be ensured.(6) In accordance with the air-water tightness of hydrodynamic containment system, the stability of surrounding rock and the usability of storage caverns, the corresponding evaluating indicators were put forward respectively. The air-water tightness was evaluated through studying the distribution of pore pressure and hydraulic gradient. The evaluating indicators for tightness were composed of leakage parameter, coverage of groundwater depression cone, overpressure, critical pressure, hydraulic gradient, water inflow, gas saturation, oil saturation and mass fraction of dissolved petroleum in water. The stability of surrounding rock was evaluated through analyzing the distribution of stress field, seepage field, displacement field and plastic zone. The evaluating indicators connected to stability included major principal stress, minor principal stress, shear stress, maximum displacement and volume of plastic zone. The usability of storage caverns was evaluated by simulating the long-time leakage and the rheology of surrounding rock. The usability was concerned with the ineffectiveness of hydrodynamic containment and the viscoplastic deformation of surrounding rock, which can be evaluated comprehensively by making use of the indicators for tightness and stability.