The ESPCP is a new rodless artificial lift, which completely gets rid of rod tubing wearing problems. Because of the higher efficiency of the system, the advantage of energy saving makes the new system used more and more widely in low production wells in CNPC. However, the working condition of system at the bottom of the well is hard to monitor. The pump and the motor are in danger of burning out when the dynamic liquid level is lower than the inlet.

In this paper, we introduce a closed loop control system with capillary tube to solve these problems. The system can monitor the pressure at the bottom of the well, and calculate the dynamic liquid level using downhole pressure and wellhead casing pressure. The system can adjust the speed of the motor according to the dynamic liquid level. In this way, it achieves the closed loop speed control of the motor. The pressure measurement cylinder was located above the pump preventing sand burial. The cylinder and the tube were filled with nitrogen, so the pressure was transmitted through a capillary tube to the sensor on the ground.

We have tested the capillary tube measurement system in the lab, the results of the experiments showed that the error of the measured pressure value was less than 5%. In this design, there is no semi-conductor device at the bottom of the well. So the measurement no longer suffered from the temperature drift of the sensors and the expensive price of high temperature resistance electronic devices. We have used this measurement system with ESPCP in Jilin and Jidong oil field in two wells. The results of the field experiments showed that the system can monitor the downhole pressure continually and it has been running for 4 months. Moreover, we could get two key parameters, downhole pressure and dynamic liquid level, by using this measurement system. It would be helpful for further system failure diagnosis.

The capillary tube downhole pressure measurement system was a cost-saving and stable system. It achieved the closed loop control and prevented the evacuation problem in low production wells. Also it makes contribution to the popularization of Electronic Submersible Progressing Cavity Pump (ESPCP). This paper will present the details of this design and experiments.

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