Numerical Analysis On The Annular Pressure Buildup And Safety Control At Subsea Wellhead Under Deepwater Condition

Huge reservoirs are buried underneath the deep ocean and are getting more and more attentions at home and abroad.However,limited by deepwater condition,it is very challenging to install conventional pressure relief device on these underwater wellheads to release the annular pressure generated by fluid thermal expansion,which may lead to casing collapse accidents in severe cases.In this paper,a semi steady-state multi-layer annular temperature prediction model was established based on heat conduction between deepwater well structure,formation and multiphase flow.The model was solved by semi-state method and implemented to investigate the influence of output,production time,geothermal gradient,cement and annular fluid heat conduct coefficient on wellbore temperature distribution.The result shows that annular temperature rises rapidly in the early production stage and gradually stabilizes afterwards.Besides,annular temperature is dependent on output at low output,but the dependence gradually vanishes as output increases.Casing displacement under internal and external pressure was calculated based on elastic mechanics and combined with temperature-pressure coupling effect,nonlinear thermal expansion coefficient and isothermal compression coefficient to predict annular pressure by iterative method.The model was implemented to investigate the influence of output,production time,geothermal gradient,cement and annular fluid heat conduct coefficient on annular pressure buildup.The result shows that the annular pressure will increase with production rate and tends to be stable beyond threshold value.At this time,slight product reduction cannot help to control the annular pressure.Fluid expansion coefficient,annular fluid thermal conductivity is positively related with annular pressure.Besides,the stronger the cement thermal conductivity,the lower the annular pressure.It may require special consideration in practice that greater geothermal gradient may result in higher annular pressure for B and C but inversely for annulus A.Theoretical analysis on insulated tubing and nitrogen injection is carried out based on this model,and the reliability of these two methods is verified.