Improving pumping system efficiency in low production wells has been a big challenge for most of mature fields in China. These wells often cause high energy consumption and additional mechanical wear because of relatively high pumping speed and nonoptimalpower input for motor drive, even though operators respond it by simply turning down the stroke per minute (SPM) or adopting variable intermittent production scheme based on the well conditions. Therefore, for low production wells, it is important to produce more oil with lower energy input or operational cost. In recent years, researchers proposed a few high-efficiency intelligent control strategies used for these wells, but rarely could be effective and widely used in the field due to the lack of behavior improvement and adaptability. Based on the results from previous research, this paper provides a new solution on flexible variable frequency drive to address the aforementioned challenges.

To obtain high system efficiency for low production well, firstly new solution, under a preset and fixed SPM, manage to change pumping speed within each cycle. This can be done utilizing the pumping net load data which is calculated by controller algorithm. Then, it will regulate the SPM based on the real time production data (mainly for fluid rate) which is automatically collected by a specialized wellhead flow meter. Since flow rate is the most effective and direct data to demonstrate reservoir capacity, it can guarantee an accurate and reasonable pump SPM. All adjustments above would be done through continuously regulating the motor electrical parameters, such as voltage and current, using converter and control algorithm built in the flexible controller. Real-time matching to the lifting load and well capacity can realize the best motor drive pattern and avoid the waste of energy. Tow set of process control makes up the new control strategy and fulfills a more ideal pumping operation.

For different well capacity, new technology allows the pump frequency to be regulated from 0.5 SPM to 12 SPM under a stable running condition. Each stroke has an adjustable rate process to match inconstant net load. For those low production wells, relatively low and constant-changing pumping speed not only largely decrease producing energy needs, but also reduce rod-on-tubing wear. Thus, the test wells have improved system efficiency and prolonged mean time between failures (MTBF). This paper would describe the control principle of flexible variable frequency drive system in detail and demonstrate its application through two well cases.

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