Substitute model and CFD Investigations of a Coalescer in a Three-Phase Crude Oil Gravity Separator

The flow structure in a three-phase gas-oil-water separator and its performance was the main objective of the presented investigations, for which the Euler-Euler multiphase model to simulate the flow was used. The main assumption of the model is that secondary phases, consisting of oil and water droplets, are mono-dispersed with no coalescence and breakup. The considered separator is a part of the installation operated by a drilling company. In general, the investigation of separation process is very computationally expensive and time-consuming, therefore it is desirable to search for some simplifications in order to be able to carry out engineering analysis of the processes taking place in a separator. Hence, the three-dimensional coalescer was investigated as a porous element in order to find pressure losses dependence on flow velocity, which was required to simulate the existence of coalescers and baffles. As the next step, a transient Eulerian multiphase simulations were carried out for gas-oil-water mixture in a real horizontal gravity separator for two- and three-dimensional case. Required data for calculations was derived from real exploration well. In the two-dimensional case, the worked out dependence of the pressure drop with respect to velocity was used to model the flow through the porous coalescers. In three-dimensional case, the coalescers and baffles were modelled without any simplifications. It was found that general trends can be predicted despite the simplification of the geometrical model in which coalescer and baffle geometries have been replaced by a porous medium. The calculations confirmed that the complexity of geometry requiring time-consuming calculations can be usually replaced by introducing simplifications allowing for engineering analysis of separator operation that is acceptable by the industry, because the basic parameters regarding the separation process can be determined.