Transient Nonisothermal Multiphase Wellbore Model Development with Phase Change and Its Application to Insitu Producer Wells

Abstract

Abstract This paper focuses on modelling non-isothermal multiphase outflow of high temperature producer wells in unconventional oil applications, specifically Shell's in-situ upgrading process (IUP). Subsurface heating and in-situ upgrading of bitumen (IUP) involves installing downhole heaters and raising reservoir temperatures above 350°C. Consequently, flow conditions at the wellhead and along the tubing for a typical IUP producer wells are expected to exceed pressure and temperature ratings of conventional equipment particularly during peak production. Thus, the ability to reasonably predict pressure and temperature along the wellbore over the entire production cycle is important for designing the IUP production well and associated production facilities. A non-isothermal 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 well-bore due to gravity, friction and acceleration, and phase changes due to condensation and evaporation caused by variations in pressure and temperature along the well. These processes are strongly coupled and accurate analysis demands a coupled modelling 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 well-bore vary significantly with time due to production requirements. Long durations of high production rates can raise the temperature of the well-bore in the overburden and lower overall heat loss rates. Sustained periods of low or no flow can cause the well-bore 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. An advanced well-bore model is developed for coupling the multiphase flow, heat transfer and phase change phenomena in a high temperature, unconventional oil producer well. Vapour-liquid-liquid three-phase flash calculations are used to describe phase condensation and evaporation due to changes in temperature and pressure along the well-bore. The drift-flux model is used to describe gas-liquid two-phase flow and multiple transient energy equations are used for the well-bore, 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 located in Peace River oil sands. 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, components composition in a high temperature flowing producer well, particularly during the blowdown phase.