Transient-Nonisothermal-Multiphase-Wellbore-Model Development With Phase Change and Its Application to Producer Wells

Abstract

Summary This paper focuses on modeling nonisothermal multiphase outflow of high-temperature producer wells in Shell's in-situ-upgrading process (IUP). Subsurface heating and in-situ upgrading of bitumen involves installing heaters into the subsurface and raising reservoir temperatures to higher than 325°C. Consequently, flow conditions at the wellhead and along the tubing for a typical IUP producer well exceed pressure and temperature ratings of conventional equipment, particularly during peak production periods. Thus, the ability to reasonably predict pressure and temperature along the wellbore over the entire production cycle is important for designing IUP production wells and associated production facilities. A nonisothermal multiphase computational model has been developed for predicting the performance of IUP producer wells. Complex multiphase transport phenomena occur inside an IUP producer well during the production of high-temperature, upgraded hydrocarbon products. These include gas/oil/water three-phase flow; turbulent convective heat transfer between the tubing wall and the surrounding formation; pressure drop along the wellbore caused by gravity, friction, and acceleration; and phase changes caused by condensation and evaporation caused by variations in pressure and temperature along the well. These processes are strongly coupled, and accurate analysis demands a coupled modeling approach. Pressure and temperature variations result in changes in mass density and velocity, which have a significant influence on convective-heat-transfer rates. Mass-flow rates in the wellbore vary significantly with time because of production requirements during the life of a producer well (5 to 8 years). Long durations of high production rates can raise the temperature of the wellbore in the overburden and lower overall heat-loss rates. Sustained periods of low or no flow can cause the wellbore to cool and result in different flow and heat-transfer characteristics upon reopening of the well. Therefore, conductive time scales in the near-well formation are important to accurately predict flow tubing temperatures and pressures. An advanced wellbore model is developed for coupling the multiphase flow, heat transfer, and phase change phenomena in a high temperature, unconventional oil producer well. Vapor/liquid/ liquid (VLL) three-phase flash calculations are used to describe phase condensation and evaporation caused by changes in temperature and pressure along the wellbore. The model is formulated by use of k-values that are consistent with the CMG STARS reservoir model (STARS 2007) used for thermal simulation of Shell's IUP process. The drift-flux model is used to describe gas/liquid two-phase flow, and multiple transient energy equations are used for the wellbore, casing strings, and surrounding formation. The overall pressure gradient in the two-phase flow is formulated as the sum of gravitational, friction, and acceleration components. All transport equations are implicitly coupled for stable efficient transient calculations The model is validated with published data and simplified analytical solutions for limiting flow conditions. Computational results are compared with data from an IUP producer well in the oil sands of Alberta, Canada. Reasonable temperature and pressure matches were obtained, demonstrating that the model can predict transient and axial profiles of pressure, temperature, phase volume fraction, phase mass density, and component composition in a high-temperature flowing producer well during the entire production cycle.