Modeling and Simulation of Annular Pressure Buildup APB in a Deepwater Wellbore with Vacuum-Insulated Tubing
Authors: Kang Yongfeng , Gonzales Adolfo , Liu Zhengchun , Samuel Robello 
Published: March 2017

Annular pressure buildup (APB) is a major concern in high pressure, high temperature (HP/HT) well design. High temperature hydrocarbon production increases wellbore temperature significantly, inducing a large APB as a result of annular fluid thermal expansion. Vacuum-insulated tubing (VIT) can be used to reduce the annulus temperature increase to mitigate APB. However, because of the natural differences between VIT and standard pipe, there is uncertainty about accurately modeling the VIT wellbore temperature distribution and its effect on APB.

The VIT vacuum section has much better insulation than the connector, which induces a nonuniform heat transfer that generates significant annulus temperature spikes along the axial direction, up to 50°F in half-VIT unit length (20 ft) (Gosch et al. 2004). Conventionally, the axial temperature gradient is ignored because it is negligible in comparison with radial temperature gradients. This assumption is no longer applicable with VIT because these temperature spikes significantly enhance natural convection. This study presents a novel wellbore heat transfer model that considers nonuniform heat transfer and enhanced annulus natural convection to provide accurate VIT well temperature predictions and APB estimates.

Operators use overall average thermal conductivity over the VIT because it yields an acceptable conservative incremental pressure in APB calculation that matches field observations. In addition, there is uncertainty about simply modeling VIT in a conventional simulation method with different thermal conductivity values for the vacuum section and connector. This study captures the VIT nonuniform heat transfer behavior. The temperature spikes enhanced natural convection is considered for the overall wellbore heat transfer modeling. Case studies show that temperature spikes in the annulus are observed in which the temperature distribution pattern closely matches the field measurement; the spike amplitude decreases, as compared with a conventional simulation method in which the axial natural convection is ignored. The average annulus temperatures also increase significantly. This result indicates that the conventional simulation method considerably underestimates the temperature and APB. In addition, whether or not the connection is insulated plays a critical role in overall VIT performance. Good connector insulation increases the overall insulation and APB mitigation significantly. The level of the connector insulation on the VIT overall insulation performance and APB mitigation quality are also studied and reveal that the connector thermal conductivity value at 15 times that of the vacuum section is an effective and economic insulation level.

Literature surveys show very few studies that consider this enhanced natural convection effect in VIT wellbore temperature and APB estimation. A conventional wellbore temperature simulator ignores this effect and underestimates the overall wellbore temperature and the APB. This paper provides a novel solution model for VIT wellbore temperature prediction and APB analysis, which will be useful for advanced casing and tubing design.