Packer fluid whose function is to prevent or tremendously reduce the heat transfer rate which would occur from the production tubing area to the production casing region is being studied in recent decades because of its wide applications in Oil & Gas Industry. Reduction of heat transfer rate can lead to the minimization of trapped annular pressures and reduction of contents of hydrates resolvable in production fluids. This paper utilizing ANSYS Fluent gives numerical solution for the combined convection problem of this packer fluid. Because of the geometry of the tubing-to-casing annulus, it is modeled as vertical and long parallel plates in ANSYS Fluent geometry part where the width of the duct is small comparable to the length of the duct. The flow is assumed to lie in laminar region and ANSYS Fluent laminar flow model is utilized. How different parameters including aspect ratio, temperature difference and inlet velocity will have effects on the convective heat transfer rate are analyzed respectively by measuring and calculating dimensionless parameters including Nusselt number, Reynolds number, Prandtl number, Grashof number and Rayleigh number. Numerical results characterize the convective heat transfer performance of the packer fluid.