Numerical study of the performance of a three-phase horizontal separator under varying water outlet pressures

Horizontal gravity separators are important components in the production of unconventional petroleum resources. While tremendous efforts have made on the sizing of separators, relatively few studies have addressed their operational performance under varying outlet conditions. This study aims to investigate the effect of water outlet pressure on the performance of the separator to establish guidelines for its proper operation. Three-dimensional computational fluid dynamics (CFD) simulations are conducted using the Eulerian multiphase model and interfacial concentration area. Four water outlet pressures ranging from 7537.2 to 7421.9 Pa are examined under fixed inlet flow rates. The numerical method is validated by comparing it with experimental results. The numerical results confirm that the separation efficiency improves as the water outlet pressure decreases primarily due to the increased thickness of the oil pad. Notably, the analysis reveals that meeting residence time criteria alone does not guarantee high separation performance, as outlet pressure significantly affects fluid distribution and phase separation. These findings suggest that maintaining the water outlet pressure below 7448.5 Pa is essential for achieving efficient separation in practical operations.