This paper provides the design and operating considerations for Progressive Cavity Pumping (PCP) systems in Coal Bed Methane (CBM) wells based on field experience and illustrates their optimization with case studies of C-Fer software simulation and field trial. The challenge is to effectively lift water throughout the CBM well life with the same PCP system while accommodating pressure head increments and flow rate decrements with time. This paper presents a novel idea to optimize PCP system at different stages of CBM well life with minimal cost implications to the company.

The major inputs for artificial lift design are seam depth and well productivity, while PCP head and flow rate, down-hole string sizing and motor sizing are the required outputs. To take a case study of PCP system optimization, we have Well – A, where the bottom perforation is at 600 m and the water production is expected to start with a peak of 150 m3/day at 100 m water level and reach 100 m3/day at 600 m water level in about 10 months. The authors chose a system with flow capacity of 76 CMD at 100 RPM and lift capacity greater than 800 m. Tubing size of 2.875″ is chosen and sucker rod size of 1″ is chosen. The surface motor of 22 kW is selected based on the maximum power required by the PCP system at the maximum lift condition. The process of selection is depicted clearly in this paper.

Although, the PCP system is designed as per the design considerations, the maximum RPM in the initial CBM well life may be limited by the pulley ratio (driven pulley diameter: driving pulley diameter) while the torque imparting capability of the motor at later stages may be limited by the motor frequency at the later stages of the well life. Hence, simulations for different run scenarios (water levels and flow rates) were done in C-Fer PC-Pump software for two cases taking Pulley Ratio as 4.74 and 6.22. The authors have presented trends comparing important parameters namely Fluid Level-Surface Motor loading, Fluid Level-Maximum Rod Torque and Fluid Flow Rate-Pump Speed. Based on the interpretation of these results, it was decided to experiment with two different driving pulleys for changing the pulley ratio during the well life to optimize the PCP system instead of upsizing the motor. This field trial was successfully conducted in Well – B and the parameters and the results are clearly depicted in the paper. Changing the pulley ratio over CBM well life will fulfill the torque and power requirements at the required pump RPM with significant cost savings by eliminating the need to install a new motor.

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