Experimental Study On Shallow Gas Intrusion Of Seabed Sediments

Shallow gas is one of the most typical marine geological hazard,which is widely distributed and frequently encountered in the Huanghai Sea,Bohai Sea,East China Sea and South China Sea,as well as in the worldwide marine environment and inland areas.Shallow gas is a huge potential risk in the field of marine engineering and can possibly cause severe destructions to civil engineering structures such as offshore oil drilling platforms,undersea optical cables and oil pipelines.At the present stage,while researches concerning shallow gas usually focus on its origin and occurrence state as well as the ways by which it can be detected,knowledge about the transportation of the shallow gas in sediments is relatively deficient because of the restrictions of experiment methods.Under the background of extensive exploitation of marine resources in China,it is of significant economic and scientific merit to find out the rules obeyed by shallow gas during transportation in sediments.In this paper,based on the research background of gas invasion fluidization in shallow gas-rich sandy seabed,taking the unbonded quartz material as the research object,the GDS stress path triaxial test system is modified to increase the gas injection function.The eruption process of point source gas in seafloor sediments is simulated.at the same time,different experimental stages are scanned by means of high precision industrial CT,and the pore pressure and initial effective confining pressure are controlled.The research work on the law of shallow gas invasion and migrationbehavior in seafloor sandy sediments is carried out.the main contents are as follows:1.During test,the cohesionless sandy materials were used with back pressure and initial effective confining pressure being the controlled variables.The initial gas injection pressure was kept equal to pore water pressure and then increased step by step and maintained for a period of time.The pressure of gas and volume of pore water drained was recorded during tests in order to study the behaviour of gas injected.2.By changing the particle size distribution of samples and effective confining pressure,their influences were studied.The results show that when the gas injection pressure reaches the capillary entry pressure,the gas invades the sample when the gas injection pressure reaches the capillary pressure,and the gas mainly invades the sample by driving the pore water in the capillary channel.In the fine particle silicon powder sample,the gas enters the sample before the pressure reaches the capillary entry pressure,and the gas mainly invades the sample by forming a crack pipe in the sample.Through the gas injection test of silicon powder samples under different effective confining pressures,it is found that the pressure value of gas invading silicon powder samples increases gradually with the increase of effective confining pressure,which is consistent with the existing research results.3.In order to study the meso-mode of gas intrusion behavior,high precision industrial CT was used to scan the samples in different gas injection stages,and the scanned samples were reconstructed in three dimensions to extract the internal pores of the samples in different scanning stages.The meso-model of gas intrusion behavior was studied and the pore development of samples in different stages was compared.The results show that the invasion behavior of the gas is consistent with the results of the laboratory triaxial gas injection test,that is,in the coarse quartz sand sample,the main invasion mode of the gas is capillary entry,and in the fine particle silicon powder sample,The main mode of gas invasion is splitting entry.With the increase of gas injection pressure,there are no obvious cracks in sand particles,indicating that the gas is mainly transported through capillary channels between particles.The thickness and range of cracks expand gradually with the increase of gas injection pressure in the sample until the whole sample is completely cracked,which indicates that the gas migrates mainly through the cracks,and the cracks are centered at the gas injection point and propagate diagonally upward.The extracted cracks are "bowl-shaped" inside the sample.Figure 48 table 10 reference 110.