High Volume Pumping with Sucker Rods

Abstract

Abstract Recent studies have shown that often large fluid volumes can be lifted by sucker rods with great effectiveness from shallow to medium-depth wells. Wherever feasible, significant redaction in cost of elevating a barrel of fluid may be realized by the operator. The advent of longer stroke units with improved geometry, larger speed reducers and bottom-hole pumps, as well as the new high-strength sucker rods, make high volume rod pumping a practical and economical achievement. Past applications of this type generally have been considered only for bottom-hole centrifugal pumps. This paper reviews some field studies and emphasizes the practicality of high volume production with sucker rods. Introduction A century ago the almost universal mechanism for artificially lifting fluid in an oil well was the standard rig-front. A wooden walking beam drove a string of hickory sucker rods, often called "well poles", as many as ten 15-in.- strokes/minute with the maximum tensile stress of the rods about 12,800 psi. The bottom-hole pump was cast iron or brass with the barrel approximately 1 1/4-in. in diameter, and the well depth ranged from 500 to 1,000 ft. The torque capacity of the band wheel and flat-belt speed reducer ran only a few thousand inch-pounds, and the unit's structural capacity was from 1,000 to 1,500 lb. Today, the structure of a modern pumping unit with its 240-in. maximum stroke reaches higher than the wooden drilling and servicing derricks of those early days. Its structural capacity exceeds 47,000 lb, with a torque rating greater than 2.5 million in. lb. The plunger diameter of some bottom-hole pumps (casing-type) runs as high as 5 3/4 -in. Perhaps the greatest improvement is in the sucker rod, which now has a tensile stress of some 140,000 psi. Today practice sucker rod pumping approaches 13,000 ft, and capacities of 5,000 and 6,000 B/D from shallow to medium depths are handled with ease. Volumes as high as 9,500 B/D are entirely practical with modern sucker rod pumping equipment. Comparing the modern sucker-rod system with its counterpart of 100 years ago produces some startling figures. The structural capacity of the modern unit has increased nearly fifty-fold, the torque capacity perhaps a thousand-fold, the area of the bottom-hole pump over 20 times; the stroke length nearly twenty-fold; and maximum rod tensile stress nearly 12 times. With the increased stroke length, even the maximum rate of polished rod travel per minute has increased five or six times. Since all of these are compounding increases, the sucker-rod pumping unit of today has the capacity for producing massive fluid volumes from relatively great depths. General The ability of a sucker-rod pumping system to produce fluid (relatively incompressible fluid is assumed) is limited by one or more of the following:the unit's stroke length,the maximum rate of rod fall for a given well,the plunger diameter of the bottom-hole pump,the strength of the sucker rods, andthe unit's torsional and structural capacity, and, to a slight degree, its geometry.* A brief discussion of these items should be helpful for gaining a thorough understanding of high volume sucker-rod pumping. Stroke Length and Maximum Rod Fall Critical pumping speed** is the speed on a given well (under a fixed set of pumping conditions) at which the carrier bar just begins to leave the rod clamp during the downstroke; it is the highest pumping speed at which the minimum polished rod load (downstroke) becomes zero. For any given pumping unit geometry, critical pumping speed is controlled by two variables, (1) stroke length pumping speed is controlled by two variables, (1) stroke length and(2) the well forces, such as friction, buoyancy, etc., that retard rod fall. Table 1 shows the relationship between stroke length, critical pumping speed, and polished rod travel for the conventional pumping unit dropping rods in air. (This discussion considers only the symmetrical conventional unit, though other types of pumping unit geometry behave in a similar and proportionate fashion.) The stroke length range, from 64 to 168 in., is considered representative. Table 2 illustrates the effect that the buoyancy of water has upon critical pumping speed. All frictional and other retarding forces are disregarded. JPT p. 1355