Well Control Safety Operation Cycle during Typhoon at Deep Waters of South China Sea

Typhoons frequent deep waters of South China Sea in summer.Drilling fluid in the well will stay for a long time when evacuating from platform during typhoon landing,and gas channeling into wellbore and rising up to the storm valve can bring high risk to well control during well unseal and open operations.Therefore,in view of the characteristics of well control operations during typhoon in the South China Sea,a variety of factors that affect the rising speed of the gas were considered,including drilling fluid viscosity,density,surface tension,bubble diameter and wall effect,etc.The drag coefficient model was modified to calculate the rising speed of the gas in the wellbore during typhoon.The model that calculates the rising speed of the gas was verified and the results showed a good agreement with field data.Factors that affect the rising speed of gas were analysed by applying the model,the results showed that gas rise velocity would go up with depth decrease and the drilling fluid density increase,and would go down with the increase of drilling fluid viscosity,but increase with formation porosity increase.According to the actual situation in the South China Sea,plots of safe operating cycle under different borehole conditions were drawn so that field staff could determine from these plots whether storm valve traps high-pressure gas,and take pertinent measures of well control.