Study On The Law Of Gas Eruption In The Middle And Shallow Layers Of Deepwater Drilling

At present,the demand for oil is increasing day by day.The South my country Sea is rich in offshore oil resources.In recent years,the proportion of newly discovered oil and gas resources in the deepwater area has increased,and it has gradually become a replacement area for oil and gas resources.However,in the deepwater area,there is a high probability of subsea shallow gas.Drilling a pilot hole is usually used to relieve the pressure of the shallow gas.During the shallow gas eruption,it is very easy to cause risks such as wellbore collapse and subsidence of the surface conduit,which brings huge hidden dangers to the safety of deepwater drilling.In this thesis,the numerical simulation method is used,combined with the properties of shallow gas,a two-phase flow model of gas and drilling fluid in the wellbore is established when shallow gas is encountered in second spud seawater drilling conditions.The eruption process of shallow gas is studied,and the eruption time and bottom hole pressure of shallow gas are analyzed.The influence of formation pressure coefficient,drilling fluid displacement and the density of killing fluid used after drilling on the eruption of shallow gas is evaluated,and the hazard of shallow gas is quantitatively classified according to the numerical simulation results.The prevention and control measures for shallow gas disasters with different risk levels under the second spud condition are proposed.Studies have shown that after the shallow gas intrudes into the wellbore,the gas will be broken during the rising process,and the flow pattern will change;with the development of the gas blowout,the flow pattern at the same position will change from bubbly flow to annular flow,and at the same time,the bottom hole will change.Pressure fluctuations occur.The formation pressure coefficient has the greatest influence on the eruption time.When the formation pressure coefficient is 1.1,it takes 3.181 s for the gas to rise 5m in the annulus.When the formation pressure coefficient is 1.3,the time required for the gas to rise 5m is 1.238 s.The larger the displacement,the slower the gas phase rises,but the influence is small;when the well killing is required,the increase of the killing fluid density can balance the formation pressure and also reduce the gas phase rising speed.According to the simulation results,the formation pressure coefficient,gas phase rising velocity and flow state are used as quantitative judgment criteria to supplement the existing shallow gas risk assessment criteria,and improve the shallow gas risk level assessment method.Starting from the geological prediction stage,the dynamic killing operation stage,the subsea equipment control stage and the cementing operation stage,the factors that cause the risk of shallow gas disasters are analyzed,and the shallow gas prevention and control schemes of different risk levels are put forward: “risk-free area” “Strengthen pre-drilling identification,real-time monitoring,and accelerate drilling”,“real-time monitoring,switch to high-density mud,and observe while drilling”in low-risk areas,“lead hole drilling and reaming after safety” in medium-risk areas,highrisk areas “Uncontrollable well position shift,controllable dynamic well kill”.Based on the on-site formation stress conditions and numerical simulation results,the optimization of drilling fluid displacement and killing fluid density after drilling under different shallow gas risk levels is carried out.This study has reference significance for reducing the hazard of shallow gas.