| The construction of underground water-sealed storage has gradually become the preferred choice for maintaining strategic petroleum reserves worldwide owing to the large storage scale,high degree of safety,economical land and environmental protection use associated with this approach.Underground water-sealed storage are generally built in granite bodies.The permeability of the surrounding rock exhibits spatial variability due to the effect of magma intrusion,tectonization and weathering.The containment performance of the underground watersealed storage shows obvious uncertainty due to the influence of the spatial variability of the permeability of the surrounding rock.The artificial water-sealed function has been regarded as a necessary measurement for gas tightness safety.With a water curtain,the groundwater pressure around the caverns should be strong enough to prevent any petroleum leakage.Therefore,performance of the water curtain system has become the main focus in the safety assessment of underground water-sealed oil storage.In this study,reliability evaluation of water-sealed was studied for China's large-scale underground water-sealed oil storage.With help of theoretical analysis,engineering geological survey,field test,numerical simulation,considering the spatial variability of hydraulic conductivity of the surrounding rock,the evaluation method of watersealed reliability for the underground oil storage was established,and the water-sealed performance of the water curtain system was studied.The main research content and results are as follows:(1)By coupling random field theory with the finite element method within a Monte Carlo framework,a probability-based reliability analysis method for underground water-sealed storage was proposed.The interface program between the reliability analysis of underground water-sealed oil storage and the general finite element software COMSOL Multiphysics was compiled using MATLAB,and the integration of finite element analysis and reliability analysis of underground water-sealed oil storage under the condition of heterogeneous surrounding rock was realized.An important step has been taken for the reliability theory to be used in the probability design of underground water-sealed oil storage.(2)Using a stochastic finite element analysis model considering the anisotropic heterogeneity of hydraulic conductivity of surrounding rock,the influence of the spatial statistical of hydraulic conductivity on the water-sealed reliability of underground oil storage was studied.Compared with the horizontal spatial correlation of the hydraulic conductivity,the water-sealed reliability of underground oil storage is more sensitive to change of the vertical spatial correlation of the hydraulic conductivity.Based on the analysis of the spatial variability of the hydraulic conductivity of the surrounding rock,the probability analysis of the water-sealed performance of different water curtain system schemes and the optimization of the water curtain system design parameters were carried out.The results showed that an excessively large pressure of water curtain boreholes provided a small contribution to improving water curtain performance.Compared with the conventional fixed interval,installing water curtain boreholes with changeable intervals may improve the utilisation efficiency of the water curtain system.(3)Combining random field theory with geostatistical theory,a conditional random field considering spatial location and the anisotropic heterogeneity of hydraulic conductivity of surrounding rock was established.The concept of conditional random field was introduced into the reliability analysis of underground water-sealed oil storage,the water-seal performance of the water curtain system was evaluated based on the field data,and the design optimization of the water curtain system under different surrounding rock conditions was provided.For areas with weak permeability of the surrounding rock,simply increasing the number of water curtain holes(reducing the water curtain hole spacing)has limited contribution to the improvement of the watersealed effect of the water curtain system.In areas with strong permeability of the surrounding rock,the best distance between the water curtain hole and the main cavern can be determined by the spatial correlation of the surrounding rock.This study providing valuable insights and a theoretical basis for quantitative analysis of the reliability of underground water-sealed oil storage and the optimisation of water curtain system design parameters for underground water-sealed oil storage in the face of the spatial variability of hydraulic conductivity. |
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