Simulation of Shallow Gas Invasion Process During Deepwater Drilling and Its Control Measures

Shallow gas is considered one of the most serious geological hazards in deepwater drilling because it has the characteristics of suddenness and is difficult to deal with.To perform a quantitative evaluation of shallow gas risk during deepwater drilling,a numerical model for calculating gas invasion volume is established based on gas-water two-phase flow theory.The model considers the effect of the dynamic drilling process,and the influencing factors which affect the gas invasion volume are analyzed.Results indicate that the gas invasion rate and accumulated gas invasion volume increase with increasing bottom-hole pressure difference.A linear relationship exists between gas invasion volume and bottom-hole pressure difference.The duration of gas invasion increases as the shallow gas zone thickness increases,and the accumulated gas invasion volume grows as shallow gas zone thickness increases.The increase in formation permeability,water depth,and rate of penetration will enhance the gas invasion rate.However,these three factors can hardly affect the accumulated gas invasion volume.The gas flow rate increases significantly with increasing burial depth of shallow gas.On the basis of influencing factor analysis,a series of methods that consider different risk levels is proposed to control shallow gas,which can provide a reference for the prevention of shallow gas disasters during deepwater drilling.

Drill string dynamic characteristics simulation for the ultra-deep well drilling on the south margins of Junggar Basin

Improper drilling parameters may cause severe vibration of drill string which leads to reduce the rate of penetration and drilling tool premature failure accidents in the drilling process of ultra-deep well.The study on dynamic characteristics of drill string plays an important role in increasing the safety of drilling tool and optimizing the drilling parameters.Considering the influences of real borehole trajectory,interaction between bit and formation,contact between drill string and borehole wall,stiction of drilling fluid and other factors,a comprehensive drill string dynamic model was established to simulate the changes of wellhead hook load,torque,equivalent stress of drill string and BHA(bottomhole assembly)section acceleration and motion trajectory with time at different WOBs(weights on bit)and rotary speeds.The safety factor and overpull margin of wellhead drill string were calculated and the strength of drilling tool in ultra-deep well was checked using the fourth strength theory.The analysis results show that,in the drilling process of ultra-deep well,the transverse motion amplitude of the drill string near the wellhead is relatively small and vibration of drill string mainly occurs in the lower well section.As the rotary speed increases,the number of collision between lower drilling tool and borehole wall increases,wellhead transverse stress increases,change in torque is not large and change in wellhead equivalent stress is relatively small.As the WOB increases,wellhead torque will increase,axial load and equivalent stress will decrease and vibration acceleration of BHA will increase sharply.Wellhead overpull margin and safety factor will decrease with the increase of rotary speed and increase with the increase of WOB.Wellhead safety factor of S135 drilling tool in an F190.5 mm ultra-deep well on the south margins of Junggar basin changes around 1.8.The drilling tool is safe and has relatively sufficient ability to deal with the downhole accidents if a large size high steel grade drill string(F139.7 mm S135)is used.However,in view of BHA safety,neither rotary speed shall be too high nor WOB shall be too large.