Heavy oil transportation through steam heating: An analytical and numerical approach

Abstract

Amongst the innumerable challenges that permeate the production chain of heavy oils, the transportation stage deserves to be highlighted. Due to its high viscosity, heavy oils exhibit considerable flow resistance, requiring auxiliary mechanisms to enable its transportation via pipelines. This study analyzes a concentric pipeline configuration, in which steam flows through the inner pipe and oil flows through the annular space of the oil pipe. The main goal is to investigate the impacts of steam insertion, concentric coupling dimensions, thermal insulation, steam quality and system horizontal length on the main parameters of flow control: oil temperature and viscosity. As a secondary parameter, oil initial pressure required to ensure flow completion is also discussed. In this intent, a numerical approach is applied with software Ansys CFX in a tridimensional, steady state simulation. A second objective is to estimate the main parameters of interest by an analytical approach, using the thermal resistance model associated with the ε – NTU method. The goal is to determine if, given a similar coupling scenario and the absence of computational resources, oil temperature and viscosity can be estimated by direct calculations within an acceptable range of deviation from the numerical approach. Heat transfer rates are also estimated as secondary parameters. Numerical results for a 1-m length system reveal that steam insertion elevates the average oil temperature in 1.2%, reducing its average viscosity and initial flow pressure by 8.7% and 24.2%, respectively. From the previous scenario, a reduction of 24.6% in the oil pipe/steam pipe radius ratio results in an additional 1.2% temperature increase and 4.7% viscosity reduction. On the other hand, reducing steam quality in 30% implicates an average oil viscosity 2.4% higher. In a 4-m system, a convective cell pattern is observed, in which oil heating and viscosity reduction reaches its peak at the central upper region of the annular space, highlighting cooler and denser deposits in the lower section of the system and a nonlinear heating process. Maximum deviations between numerical and analytical approaches in oil temperature and viscosity estimates are 1.3% and 15.6%, respectively, which indicate the latter as a suitable calculation method for small systems in the absence of numerical resources. Deviations are mainly attributed to the neglect of natural convection, gravitational and steam turbulence effects, which contribute for a non-homogeneous heating of oil.