Multi-electrode capacitive and inductive sensing applied to level measurement of multiphase fluids

Multiphase gravity separators are widely used in the petroleum industry to separate the produced stream of oil, water and natural gas into pure (single phase) streams. These equipment work based on the density differences of each fluid which tend to settle into layers when dwelled for some time in the separator. It is fundamental to monitor the levels of these phases inside the vessel, so that this information can be used in control strategies in order to increase efficiency and safety of the process. There are different sensing technologies for multiphase level monitoring, based, for instance, on ultrasound, ionizing radiation and electrical impedance. However, none can be seen as general and each solution present some disadvantage. In this work, we present a novel multiphase level sensor based on capacitance and inductance measurements of planar multi-electrodes and multi-coils. The sensor is low-cost, fast and does not apply ionizing radiation, being therefore simple to operate. The prototype sensor was constructed in standard PCB technology and highly sensitive capacitance and inductance measurements are acquired with modern ICs. Capacitive sensor highly depends on the water salinity and therefore we include inductance measurements to compensate such dependence. We have tested the prototype at varying water salinity and for different water/oil emulsions. Therefore, the novel sensor is well able to differentiate each one of the produced fluids, i.e. salty water, oil and gas, as well as the interfaces between them.