Phase-shift Detection For Capacitance Sensor Measuring Water Holdup In Horizontal Oil-Water Two-phase Flow

The oil-water two-phase flow widely exists in the petroleum exploration of horizontal wells.The dynamic monitoring of water holdup is of great significance in optimizing the oil exploration scheme and enhancing the oil recovery.However,flow structure of the horizontal two-phase flow is complex and changeable,which brings great challenges to the water holdup measurement.In the paper,we design a new fluid phase-shift detection system based on a four-electrode concave capacitance sensor to measure the water holdup of the horizontal oil-water two-phase flow.First,for stratified oil-water two-phase flow,we set up an equivalent impedance network model of the capacitance sensor,aiming at predicting the measurement characteristics of the capacitance phaseshift sensor under different water holdup.The influence of the water conductivity on the measurement is analyzed based on the model.Besides,we use the model to optimize the structure of the capacitance sensor and the parameters of the measuring circuit.Then,we carry out a horizontal oil-water two-phase flow loop test and obtain the measurement responses of the phase-shift dectection system under different flow patterns.The result shows that when the exciting and the receiving electrodes are respectively the upper and lower ones,the normalized capacitance phase-shift signal is sensitive to water holdup variation of ST,ST&W,ST&MI flow pattern,and the signal is,to some extent,influenced by the flow pattern.In contrast,under the D W/O&D O/W and D O/W&W flow pattern,the normalized phase-shift response is less sensitive to the water holdup in this kind of exciting-receiving mode.It's because that the water film exists in the upper side of the inner pipe wall,which aggravates the leakage of current.In addition,the nomalized phase-shift response is only sensitive to the low water holdup while the exciting and receiving electrodes are respectively the left and right ones.This result arises from that once there is water existing between the left and right electrode,the current leakage and sharp reduction of the impedance will result in no response of the sensor to the water holdup.Notably,the phase-shift response from the left and upper electrodes is similar to that from the upper and lower ones.Moreover,the phase-shift response is seriously influenced by the water impedance while the exciting and receiving electrodes are respectively the left and lower ones,which is less sensitive to the water holdup variation.Finally,we adopt the multi-scale distribution entropy of Poincaré plot to analyze the capacitance phaseshift sensor signals.It reveals that the distribution entropy at high time scale can effectively characterize the evolution characteristics of the horizontal oil-water two-phase flow structrues.