Research On The Annular Pressure Buildup And Mitigation Mechanism In Deepwater Wells

During the stage of production in the deepwater wells,the fluid in the annulus is easily heated up due to the large temperature difference between the hot formation fluid and the low temperature fluid in the annulus.So the annular pressure buildup(APB)occurs.APB can be very high and severely threatens the well integrity,casing safe and sustaining production.The semi-steady state wellbore temperature model is established considering the existence of multi-annuli.Based on the semi-steady state temperature model,the APB prediction model is developed taking the nonlinear relationship between the temperature and the ratio of isobaric thermal expansion and the isothermal compressibility.The traditional APB prediction model thinks the ratio of isobaric thermal expansion and the isothermal compressibility is constant or using the value of water,which induce great errors.The transient wellbore heat transfer model is established according to the difference of heat transfer process in the well and then solved by Crank-Nicolson finite difference scheme.The transient APB prediction model is developed based on the transient wellbore temperature model considering the annular volume change caused by the casing thermal effect and pressure change inner and out of the casing.Crank-Nicolson finite difference scheme is unconditional stable and better than explicit scheme and implicit scheme on the accuracy,stability and calculation efficiency.The APB is over-estimated in the early stage of production because of it overlooks the heat transmission time from the tuber to the out edge of the wellbore.The influence of producing time,production rate,formation thermal gradient,well depth and length of annulus on APB is analyzed.The results indicate that: the APB is mainly occurs in the early stage of production;the production rate influence the APB by changing the convection heat transfer and the energy available participated in the heat transfer.Knowing how the factors influence the APB is helpful to make a good wellbore configuration and mitigation strategy in advance.The mechanism of APB mitigation is analyzed.The model of APB prediction model is established considering the lack of study on compressibility foam to mitigate the APB.The calculation example shows that installing the compressibility foam can mitigate the APB effectively.