Study On Seepage Channel Identification And Stability Of The Large-scale Water-sealed Underground Oil Storage Caverns Subjected To Excavation

With the advantages of large storage capacity.high safety.strong emergency capability.low cost and conservation of land resources,the large water-sealed underground oil storage caverns(WUOSC)have been one of the main methods for oil or gas storage around the world.They are often excavated at a certain depth below the groundwater level to realize hydrologic containment for oil or gas.Therefore,seepage flow of groundwater and the stability of surrounding rock masses are the basic scientific problems during the construction period.Because of strong unloading induced by excavation,the surrounding rock masses are prone to problems such as groundwater leakage and rock instability.In essence,these problems are caused by the cascade and inter-coupling of different scale failures distributed hierarchically in the surrounding rock masses.By combining three-dimensional digital photogrammetry,RFPA(Rock Failure Process Analysis)and numerical simulation,this paper proposed a multi-scale equivalent method for the analysis of multi-scale deformation of the surrounding rocks.Meanwhile,temporal and spatial distribution of the surrounding rock microcracks during the excavation process have been studied based on the microseismic monitoring results of the WUOSC.From this,the dominant seepage channels for groundwater were identified and the instability mechanism and its precursors were revealed,which can provide theoretical and technical support for leakage and instability warning of the WUOSC.The major achievements are as follows:(1)With the aid of digital photogrammetry and joint network simulation,the probability distribution and its characteristic parameters of joint occurrence in the surrounding rock masses of the Jinzhou WUOSC were determined so that a three-dimensional random joint network was generated.Then,meso-mechanical parameters of rocks were back analyzed by RFPA.On this basis,numerical tests of jointed rock masses with different sizes were implemented,and the REV(Representative Elementary Volume)and its equivalent mechanical parameters were obtained by analyzing size effects of jointed rock masses.Considering the mesoscopic heterogeneity of rocks and the macroscopic random distribution of joints,a multi-scale equivalent method was proposed based on the macro-mesoscopic equivalence principle of rocks to analyze mechanical responses of surrounding rock masses of the WUOSC.(2)The sensor array for microseismic monitoring system(MMS)were optimized by the integration of expected error estimation and actively triggering test and the first construction MMS for WUOSC was established in Jinzhou,China.The average positioning accuracy of the MMS reached 7.5 m.The microcracks of surrounding rock masses caused by excavation-induced unloading have been monitored continuously in real time.Moreover,the relations between the microseismicity and excavation were studied.The range of the blasting influence zone was determined to be 120 m which is basically consistent with the result obtained by the empirical formula.(3)Breaking through the traditional method for groundwater leakage analysis and pre-warning based on the apparent groundwater level and quantity,this paper focused on the connectivity and growth of microcracks in the surrounding rock masses and proposed a three-dimensional identification method for dominant seepage channel of the WUOSC based on microseismic monitoring.The attitudes and sizes of excavation-induced microcracks were obtained by the moment tensor analysis,and the connectivity of those microcracks was analyzed based on the graph theory model and priority traversal method.Furthermore,the groundwater pore pressure calculated by numerical simulation was adopted to determine search priority,and five dominant seepage channels in the study area were identified and the results were verified by water curtain hole water supply data and on-site reconnaissance.(4)Based on the energy dissipation principle in rock failure process,the energy conversion and evolution patterns,especially the energy accumulation,release and transfer phenomenon(3E phenomenon)of the surrounding rock masses subjected to excavation at the Jinzhou WUOSC were discussed,and the microseismic energy was proved feasible to investigate energy evolution and stability of the surrounding rock masses.According to the evolutionary process of the microseismic energy density subjected to excavation.3E phenomenon was tracked and potential hazardous areas at the WUOSC were delineated.Integrating the stress and deformation analysis of the surrounding rock masses by the proposed multiscale equivalent method,it is found that the phenomenon of rapid growth of the accumulated apparent volume and significant increase of microseismic energy density are the precursors before the instability of the surrounding rock masses.