Water cut measurement of crude oil slug flow with double-tube capacitance sensor

An online method for measurement of water cut in the crude oil slug flow in vertical pipes is presented. The water cut is obtained from the water film length, which is measured by a double-tube capacitance probe sensor, on the surface-insulated tube. The double-tube capacitance probe sensor consists of two stainless steel tubes, one tube is covered with an insulating film and the other is not. There is a heating wire in each tube, which provides a certain amount of heat to prevent the formation of waxes on the surface of the tube. The output of water cut variations is in the form of capacitance signal, which is independent of water salinity and electrical properties of the liquid. The water film length, capacitance and water cut are linearly proportional to each other. The field experiments indicate that the measurements are affected by both the natural gas entrained in the liquid and the flow patterns. A comparison with manual sampling measurement suggests that the mean water cut obtained by the sensor is lower than that of manual sampling value in the slug flow and the relative deviation is 11.58%. The measured values are revised according to the water holdup of the slug flow, and the relative deviation is reduced to 8.04% after revising.