Hazard Evolution And Numerical Simulations Study When Drilling Through Shallow Gas Formations During Deep-water Drilling

Shallow gas,shallow water and gas hydrate have been recognized as the main shallow geological disasters during deep water drilling,and statistically more than 20%of well blowouts were caused by shallow gas.In addition,gas hydrate accumulated under seabed can also be considered as one of the sources of shallow gas.Intensive study on the gas blowout behavior when drilling through shallow gas and hydrate formations is important for the effective control of such disasters.In this thesis,based on geology,reservoir engineering,well-flow dynamic analysis and reservoir simulation methods,different geological models of shallow gas have been established,which can be used to quantitatively analyzes dynamic gas flow and its impact on deep-water drilling safety.The mechanism of overpressure release is cleared and damage of shallow gas is classified.The sensitivity of different factors on shallow gas are evaluated and control plans for deep-sea shallow gas in the South China Sea are put forward.Based on the characteristics of the deep-water shallow hydrate,the hydrate decomposition kinetic model,the hydrate layer heat transfer model,the hydrate-dissociative gas-lift model and the local overpressure evolution model are established to simulate the hydrate decomposition and overpressure process after well cementing.The results show that shallow gas will seriously affect the safety of deep-water drilling.It is more harmful to drilling shallow gas with high pressure gas diapir than layered shallow gas,and theoretical gas blowout rate can reach several hundred thousand cubic meters per hour.According to the simulation results,the shallow gas hazards are divided into four grades,namely slight flow(minor gas invasion),bubble gas flow(early well kick),slug gas flow(well kick)and annular gas flow(blowout).Sensitivity analysis shows that the formation pressure coefficient and the dimension of the shallow gas are the main controlling factors affecting of the gas blowout rate.Using a drilling fluid with a specific gravity of 1.05,drilled through a medium-sized(1000×1000×10m)shallow gas with an overpressure coefficient of 1.22(bottom well pressure difference of 1.53 MPa),the cumulative blowout gas volume can be up to 160×10~4m~3 and the gas blowout rate can be up to 90×10~4m~3/h.It may cause more serious blowout accidents.Increasing the drilling rate can reduce the gas blowout rate,but it has no effect on the total volume of gas blowout.The simulation of shallow gas generated by the decomposition of hydrate shows that when the hydrate is directly drilled,the gas is classified as a slight flow according to the hazard classification of shallow gas while the water blowout rate belong to the middle flow.The main reason of hazard is water blowout and appropriate measures be took according to level of harm.Overpressure simulation after Cementing shows that after 9 days of drilling fluid circulation,the local overpressure in shallow gas due to the hydrate decomposition can reach 2.4MPa within 1.5m around the wellbore,and the local overpressure will increase with drilling.Overpressure state in deep-water drilling must be monitored in real time to take measures in a timely manner.