Transient Heat Transfer Model for Wellbore and Formation of Shut-In Offshore Oil Wells

Abstract

Abstract The heat exchange between wellbore and formation is unsteady after the oil well is shut in. As the shut in time increases, the fluid temperature in tubing would decline in a continuous way until it reaches a thermal equilibrium at the ambient temperature. However, there exist the potential risks of wax deposition, crude oil solidifying and poor fluidity in wellbore during the fluid temperature drop, especially for the waxy oil and heavy oil. For resuming production of the well, the prediction of transient temperature is crucial. Taking account of the heat exchange between the fluid in tubing and formation/seawater/atmosphere for the offshore oil wells, the steady-state heat transfer before shut in is modeled based on heat balance. And the transient model for the shut-in stage is developed on the basis of Fourier's law of heat conduction, which can be solved by use of the finite difference method according to the initial condition and boundary conditions. To investigate the variation of transient wellhead temperature (WHT in short), the dimensionless WHT drop is defined. For verifying the reliability and accuracy of the model, the wellhead and flowing temperature of a certain offshore oil well is monitored. The actual monitored WHT is 30.5 °C and the simulated value is 30.9 °C during the steady production stage. After shut in, the relative error of the simulated WHT is less than 5%, showing the model is credible. The variation of transient WHT is studied by applying the model as well. The results present that within just a few shut in hours, the WHT decreases rapidly. And then it goes down gradually to the air temperature. The WHT would almost fall to the atmospheric temperature after shut in 24 hours, even though the air temperature and liquid production before shut in differs greatly. The dimensionless WHT drop declines with the increase of water cut and keeps constant when the water cut is more than 40%. This paper focuses on modeling of transient temperature for the offshore production wells after shut in and summarizes the law of temperature drop in wellbore and formation. It provides an important basis for making the flow assurance plan after the well is shut in.