Capacitance probe for water holdup measurement in crude oil-water flows

Abstract

Water holdup in crude oil-water flows is a crucial parameter to identify flow patterns or monitor the flow process in oil production. It has been increasingly important to accurately predict water holdup. Due to the simple structure and robust performance, capacitance probes have been widely used in experimental research and industrial application. In this paper, we proposed a water holdup measurement device based on a capacitance probe for crude oil-water flows. The measurement principle lies in the fact that the output capacitance is proportional to the part of the probe immersed in water. It was found that when inserted into crude oil, the surface of the probes will absorb hydrocarbons and form an oil film covering the probes. The formed oil film will reduce the sensitivity of the output capacitance to the water layer height. The thicker the film the lower the sensitivity. We examined the parameters that affect the thickness of the formed oil film and found that the thickness is mainly dependent on temperature and water holdup. The higher the temperature the thinner the formed oil film. The reason is that an increase of temperature leads to a decrease of viscosity. In addition, as the temperature approached about 50 ​°C, the thickness is nearly unchanged. As for the water holdup, at low temperatures the effect is considerable. At relatively high temperatures, the effect is minor and can be neglected. The experimental results showed that the capacitance probes have the highest sensitivity at about 50 ​°C. We designed and manufactured a prototype of the measurement device. The probes were inserted with heating wires to heat the crude oil near the probes and the current was controlled by an integrated circuit to maintain the temperature at about 50 ​°C. The field test showed that the natural gas entrained in the production fluids caused the device to under-predict the water holdup. After gas-liquid separation, the prediction errors were within ±6%.