Wellbore heat transfer induces wellbore temperature redistribution and can impact wellbore integrity and the ability of the well to perform its required functions effectively as the design intended. Vacuum-insulated-tubing (VIT) is purposed to reduce wellbore tubing fluid heat loss to protect the tubing fluid or the outside components. Accurate prediction of the wellbore operating temperature during the design phase is essential especially for a VIT well because of its natural difference from the regular pipe.
VIT pipe consists of a vacuum section and connector section. The vacuum section has a much better insulation effect than that of the connector, inducing "nonuniform" heat transfer. This generates significant annulus temperature spikes along the axial direction; up to 50°F in half VIT unit length, 20 ft in field observation (Gosch et al. 2004). Conventionally, axial temperature gradient is ignored with regular pipe because it is negligible compared with radial temperature gradient. This is no longer applicable for VIT because these temperature spikes enhance free convection significantly. This paper presents a novel wellbore heat transfer model to provide accurate temperature prediction for a VIT well.
Operators use a higher averaged overall thermal conductivity values over VIT because it yields acceptable conservative incremental pressure in annular pressure buildup (APB) calculation, which matches their field observations. There is still uncertainty about modeling with different thermal conductivity values for the VIT vacuum section and connector in conventional commercial simulators that produce high-temperature spikes and relative unreasonable low incremental pressure. Field case study results with this new model show that compared with conventional simulator results the wellbore average temperature increases and the spikes amplitude in the temperature distribution decreases. In addition, whether the connection is insulated or non-insulated plays a critical role in the overall VIT performance. Good connection insulation will increase the overall insulation effect significantly.
Literature surveys show few studies on modeling wellbore heat transfer with the enhanced free convection effect inherent to VIT. A conventional wellbore temperature simulator ignores this effect and underestimates the overall wellbore temperature. This paper provides a novel solution model for VIT wellbore temperature prediction, which will be integrated into a commercial advanced casing and tubing design software platform.