Abstract

This paper reports the performance of a new type of overrunning clutch that has been developed and installed on sucker rod pumping units. The clutch disengages the power-input shaft of the gear reducer and the unit sheave whenever the torque on the shaft is negative. Based on the results of more than 100 installations, it was found that the pumping speed increased by 9% in average. Liquid production rate increased by up to 30%. Power saving was 23.6% on average. It was also observed that downhole sand and paraffin deposit problems were greatly alleviated. This paper presents pump performance analyses and clutch selection for sucker rod pumping systems.

Introduction

Sucker rod pumping has been widely used as an effective artificial lift method for oil production from low-pressure reservoirs. The most commonly used pumping system is the beam sucker rod system. This system is mechanically simple and has been proven to be durable and economical in operations. However, the efficiency of the system can be improved greatly by careful designs and innovations.

Overrunning clutches installed on the pumping units allow free-motion of sucker rod strings when the pumping system experiences negative net torque. It has been known for more than ten years that the performance of sucker rod pumps can be improved using the overrunning clutches. The clutch eliminates at least a portion of the energy losses in a well pumping system, which are due to "regeneration" during the negative net torque period.1 However, this technology has not been widely used due to the short life of the clutch limited by the fatigue failure of spring elements. Recently, we developed, tested, and installed a new type of overrunning clutch. This type of clutch distinguishes from other types of clutches in that it uses a permanent magnet as a clutch-engaging element.

A clutched pumping unit provides two major benefits to sucker rod-pumping systems:

  1. improved oil production rate; and

  2. saved energy.

Theoretical analyses and field case studies indicate that the effectiveness of the clutched pumping system depends on a number of factors including plunger depth, pumping speed, and stroke length. Results of field applications indicate that the pumping speed of clutched units increased by 9% in average. Liquid production rate increased by 9% to 30%. Power saving ranged from 5% for well-balanced pumping units to 40% for poorly balanced pumping units, averaging at 23.6%. It was also observed that downhole sand and paraffin deposit problems were greatly alleviated. This paper presents field performance data of sucker rod pumping systems with overrunning clutch installations.

The New Overrunning Clutch

A full description of the new overrunning clutch is given in Reference 2. A schematic drawing of the new overrunning clutch is given in Fig. 1. This clutch uses a permanent magnet for engaging and disengaging. The clutch is constructed in the pumping unit sheave (Fig. 2), which can be installed on any pumping unit as shown in Fig. 3.

For a clutched pumping unit, the clutch will overrun and disengage during negative net torque periods. Zero torque will be exerted to the unit sheave shaft. This disengagement allows free-motion of gears, crank arm, pitman arm, walking beam, horse head, and polished rod driven by the weight of sucker rod string during down stroke motion. It also allows free-motion of the system driven by the counterweight during upstream motion. Depending on the negative torque periods, the clutch disengages at least once per stroke cycle. This has been observed by previous investigators1 and in our field operations.

Production Rate Improvement

There are a number of factors affecting the delivering liquid rate of a clutched pumping unit. Guo et al.3 performed a theoretical analysis on the clutched unit and concluded that the increase in the effective plunger stroke is one of the factors. Other factors include flow momentum effect and fluid compressibility effect.

The flow momentum effect may be explained as such that the liquid moves up faster than usual when the system is disengaged during the upstroke motion. Due to inertial effect, it is expected that the traveling valve in the plunger should open earlier than usual at the end of the upstroke, allowing fluid to flow longer through the traveling valve.

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