Study On Occurrence Characteristics Of Shallow Gas In Soft Soil And Vertical Blowout Simulation

With the rapid development of the economy,it is an urgent task for China to expand urban space and improve transportation capacity,and thus many underground exploration and construction projects,such as subways and tunnels,have been carried out.In the process of developing and utilizing underground space,it is common to encounter shallow gas,a high-pressure combustible gas buried in shallow strata,whose presence has been detected in the coastal areas of Jiangsu and Zhejiang and the Yangtze River Delta in China.Despite its resource value,shallow gas is often regarded as a potential geological hazard in civil engineering construction.Due to the occurrence pressure of shallow gas,vigilance is required during exploration operations in gas-bearing areas to avoid shallow gas blowouts caused by uncontrolled bottomhole pressure.This has been the case with the Hangzhou Bay Bridge project,where exploration operations were once disrupted and even stopped for a time because of a strong blowout.Therefore,research on occurrence characteristics of shallow gas and the regularity of blowout accident is of great importance for disaster prevention and mitigation in engineering construction.In this paper,geochemical analysis and testing are carried out on sediment samples collected from exploration sites,and theoretical and numerical simulation studies are carried out on the jet flow laws of single-phase and dual-phase blowouts.Details and results of the research are as follows:1.Basic physical and geochemical tests are carried out on the sediment samples collected from shallow gas-bearing areas to explore the characteristics of gas components,genesis types and migration,and the occurrence characteristics of the soft soil shallow gas are presented,showing that:(1)The shallow gas in the sampling area is mainly methane,with very little heavy hydrocarbon content.(2)The?¹³C₁ distribution of the sample CH₄ ranges from-85.6225‰?-70.273‰,and the value of C₁/(C₂+C₃)is greater than 150,indicating that it is biogenic gas.(3)CH₄ in the sampling area does not migrate in a large scale vertically.(4)There are three occurrence types of shallow gas in soft ground:free gas,water-soluble gas and adsorbed gas.2.Based on the theory of unidimensional unsteady isentropic flow,the single-phase blowout law of lenticular shallow gas reservoirs in soft soil is analyzed.It is concluded that when the gas reservoir pressure is greater than 1.84 times the atmospheric pressure,the blowout includes two processes:sonic blowout and subsonic blowout,and the calculation formulas of sonic and total blowout time of methane gas are also deduced;the study uses the software“FLUENT”to simulate the well blowout jet diffusion process under different initial reservoir pressure,reservoir gas volume and blowout pipe diameter.The results show that:(1)When the blowout pipe diameter and other conditions are constant,the duration of the blowout venting process increases with the increase of reservoir pressure and reservoir volume;the maximum height of the danger zone formed by methane diffusion increases linearly with increasing reservoir pressure,and non-linearly with increasing reservoir volume.(2)When the gas reservoir pressure and gas volume are constant,the larger the diameter of the well blowout pipe,the shorter the blowout release time;the maximum height of the danger zone formed by methane diffusion increases linearly with the increase of the diameter of blowout pipe.(3)A predication formula for the maximum height of the methane hazard zone considering the initial reservoir pressure,reservoir gas volume and blowout pipe diameter is established.3.The height of blowout liquid column can provide guidance for inversion of bottom hole parameters.The effects of wellhead pressure,blowout pipe diameter and wellhead gas holdup on the height of gas-liquid dual-phase blowout liquid column are studied through simulation,and the results show:(1)When the diameter of the blowout pipe is constant,the height of the gas-liquid dual-phase blowout liquid column increases with the increase of the wellhead pressure and the wellhead gas holdup,and the relationship curve between the height of the blowout liquid column and the wellhead pressure shows a linear change trend,while the relationship curve between the height of the column and the wellhead gas holdup shows an exponential change trend.(2)When the wellhead pressure and gas holdup are constant,the height of the dual-phase blowout liquid column decreases with the increase of the blowout pipe diameter,and the two show a non-linear relationship.(3)The mathematical model of the relationship between wellhead gas holdup,wellhead pressure,blowout pipe diameter and blowout liquid column height is developed.