## 1 Introduction  
As an efficient production logging technology, horizontal well development plays an important role in improving single well productivity and oil recovery [1; 2]. In recent years, most oil fields were in the high water cut stage due to the long-term water injection development of horizontal wells in China [3]. The realization of the water cut measurement has great practical value for optimizing production profile logging technology [4; 5].  
It is necessary to understand the flow characteristics of horizontal oil-water two-phase flow for the water cut measurement. In the process of horizontal profile logging, the fluid exists in the flow state of low yield liquid and high water cut. In this case, because of the difference of oil and water density, the mixture of oil and water exists in the form of a segregated flow. There are many early studies on the flow pattern of horizontal oil-water two-phase flow. [PERSON] et al. [6], [PERSON] et al. [7], [PERSON] et al. [8], and [PERSON] et al. [9] carried out the experiment with a horizontal pipe to obtain the flow patterns. [PERSON] et al. [10] presented the physical model of oil-water two-phase flow pattern transformation in horizontal pipe. It can be deduced that various complex flow structures in the horizontal oil-water two-phase flow bring new challenges to the design of parameter measurement sensors.  
Because the capacitance sensor has the advantages of high sensitivity near the electrode, simple structure, fast response speed, and small flow pattern disturbance, the capacitance sensor is gradually used in liquid holdup measurement of two-phase flows [11; 12; 13]. The capacitance sensors are divided into two types, invasive capacitance sensors and non-invasive capacitance sensors. For invasive capacitance sensors, [PERSON] et al. [14] employed a parallel-wire capacitance probe to compose the cross-correlation velocity measurement and found that the sensor was affected by the flow patterns to some extent. [PERSON] [15] used capacitance probes to investigate the air-oil slug flow parameters by experiments. [PERSON] et al. [16] optimized the concave capacitance sensor and found that the concave capacitance sensor has a good response character for dispersion of water in oil (DW/O) flow and dispersion of water in oil and oil in water (DW/O&DO/W) flow in horizontal pipe. [PERSON] et al. [17; 18] optimized the geometry of the coaxial capacitance sensor (COS) and analyzed the influence of oil bubbles on coaxial capacitance sensors. However, the coaxial capacitance sensor may be submerged in the oil or water phase under the conditions of oil-water two-phase stratified flow, which reduces the resolution of the water cut measurement. When the water cut is more than 0.5, the resolution will be lower, which cannot meet the needs of the actual measurement.  
Although the above invasive capacitance sensors have achieved good results in the liquid holdup measurement of oil-water two-phase flow, they are obviously limited to the fluid structure of the horizontal oil-water two-phase segregated flow. For the non-invasive capacitance sensor, [PERSON] et al. [19; 20] designed a cylindrical capacitance sensor (CYS) and carried out the experimental study by the sensor. However, the cylindrical capacitance sensor loses its resolution when the flow rate is above 20 m\({}^{3}\)/d and below 3 m\({}^{3}\)/d, so the sensor cannot meet the requirements of the water cut measurement in horizontal wells within the application range of the flow rate. The combined capacitance sensor (CCS) is a combination of the cylindrical capacitance sensor and the coaxial capacitance sensor and combines the advantages of both. In this paper, the response characteristics of the CCS under different oil-water two-phase flow patterns were analyzed by simulation and experiments, which verified the validity of the CCS.