In a typical injector string in SAGD operation, steam is injected through tubing placed inside a casing. Heat losses through the string and casing can result in lower quality steam being injected into the formation, and hence reduce the productivity of a SAGD operation. An ideal SAGD operation would be one where no heat losses happen inside the tubing and all the latent heat is released inside the formation to heat up the heavy oil. Several methods have been used to reduce heat losses in the injector tubing. One of the methods is the usage of a vacuum insulated tubing (VIT) placed inside the casing. VIT consists of specially manufactured tubing with a getter coating to maintain vacuum between ID and the OD of the tube. This helps in preventing heat losses compared to normal bare steel tubing.
In an effort to demonstrate the benefits of VIT compared to bare tubing injector string, a comparative 3D Computational Fluid Dynamics (CFD) study was carried out. The objective of the study is to provide a comparison of heat loss in both VIT and bare tubing concentric and eccentric configurations. The simulation accounts for frictional and heat losses throughout the tubing and most importantly, condensation of steam inside the tubing as result of heat loss. The consideration of condensation inside the tubing is a critical component of the simulation as it in turn affects the heat loss through the tubing. The other significant aspect of the study is the consideration of eccentric and concentric placement of tubing string inside the casing and its effect on heat loss. This study uses an Euler-Euler Multiphase flow based approach which accounts for the fluid-fluid interactions such as drag, turbulent dispersion etc. between steam and water inside the tubing. The thermal phase change model in ANSYS CFD accounts for the condensation of steam into water.
The computed results successfully demonstrate the benefits of vacuum insulated tubing over bare tubing in maintaining steam quality in the vertical section of the wellbore. Also, the study shows reduction in steam quality for eccentric position of the bare tubing compared to concentric position. Most importantly this study demonstrates the capability of CFD to solve industrial scale complex problems and provide engineers with an insight into some of the challenging physics associated with a SAGD operation in the field.