This work presents a comprehensive experimental study on the effect of gas on the performance of oil-water bulk separation in a multi-parallel pipe separator (MPPS). Several flowrates of gas, up to 10% volume fraction were tested and values of separation efficiency and water cut ratio were estimated from measurements. Also, the three-phase flow pattern upstream the separator inlet is reported.

Exxsol D60 spiked with 700 ppm of crude oil, 3.4 wt% NaCl saltwater, and air were used in steady-state and transient tests on a transparent 6.1 m long, 15.24 cm diameter, separator prototype. Preliminary values of separation efficiency and water cut ratio were measured over a variety of total flow rates (300 to 700 L/min), water cuts (WC, 30%, 50%, 70%, and 90%), and water extraction rates (ER, 50% to 90% the rate of inlet water)

It is observed that the gas phase accumulates at higher sections of the separator (i.e. at the inlet section after the splitting and at the exit section where water rich fluid is tapped from the bottom of the pipe). It seems the reduction in separation performance is due to turbulence caused by the gas and reduced cross section for the oil-water mixture to flow. The higher the gas flow rate, the lower the separation efficiency and water cut ratio. The presence of gas impacts more flow conditions with low water cuts including 30 and 50%. The presence of gas has less negative impact on separator performance when the liquid flow rate is high. Results show even small amounts of gas entering the separator have a significant detrimental effect on oil-water separation.

Pipe oil-water separators have several advantages over heavy gravity vessels, including cost, compactness, ease of deployment and enable energy saving by separating close to the source. However, this study shows that the presence of gas can impact dramatically their performance. Thus, the performance of upstream gas separation must be studied closely.

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