Research On Capacitance Array Tool

Water holdup of oil-water two phase flow in well is an important evaluation parameter of petroleum exploration and development, which provides a real-time and accurate data for formation evaluation and downhole operation. Capacitance method is the most important way in water holdup measurement. In recent years, with widespread application of highly-deviated well and horizontal well technologies, the sensor of the log tool to be an array structure in detection section is required. Difficulties mainly lie in following aspects:miniaturizing the sensor to deploy detection points as many as possible; avoiding the impact from the distributed capacitance on the connection line between sensor and detection.To solve above problems, this paper briefly introduces the relationship between the water holdup and equivalent dielectric constant of oil-water two phase flow, analyzes the relationship between the capacitance value and dielectric constant of filling medium between the electrodes, studies how to measure capacitance value when the oil-water two phase flow flowing through the capacitance sensor, exposit the basic principles of capacitance method of water holdup measurement. On this basis, according to the requirement of capacitance sensor miniaturization, two new types of sensors have been designed; for avoiding the impact from the distributed capacitance on the connector line between sensor and circuit board, a detect scheme has been proposed, which integrates the sensors with the front-end detection circuit together; in order to enhance the anti-interference ability, a solution has been used, which directly transforms capacitance to frequency at the sensing points instead of converting capacitance value to a voltage value. In order to reduce the connecting lines between sensor array and circuit board, a signal extracting circuit from power supply has been desgined. Finally, this paper proposes the overall desgin scheme of capacitance array tool and develops the corresponding measuring circuits. The above solution has advantages of minimum size sensor, strong anti-interference performance, small measurement error and good channel consistency, it meets the requirements of the predetermined specification.In conclusion, the research of this paper solves several key problems of capacitance array tool.The development tool has high definition when water hold up is not above 50%, having important reference value for developing array capacitance based log tool for water holdup measurement.