1. Field of the Invention
This invention relates generally to oil field production equipment and more particularly to a pumping jack for reciprocating a sucker rod in a well bore.
2. Description of the Prior Art
In deep well pumps of the type used in oil well rigs, the pumping motion of the subsurface pumping equipment may originate at the surface of the ground, or the pumping motion may originate below the surface of the ground as the result of the application of electrical or hydraulic power to the subsurface pump. In the surface powered rig, a vertically reciprocating pump element at the bottom of the well is actuated by a walking beam at the head of the well through a long string of sucker rods. Conventional pumping units of the walking beam type have been mechanically driven through the agency of a Pittman crank and reduction gearing, and the motion produced thereby is characteristically nonuniform throughout the length of the stroke, with equal up-and-down stroke speed. Such mechanically driven units are characterized by relatively short, fast strokes. Consequently, the reduction gearing, Pittmans, bearings and shafts are subjected to severe loading and wear.
It has been determined through experience that the maximum volumetric pump efficiency at reduced power costs, together with greatly reduced wear and breakage of the equipment, are obtained with long, slow strokes of uniform velocity. Among the advantages in favor of the long, slow stroke are the following: (a) the smooth, more uniform motion of plunger and rod tends to reduce the formation of emulsions; (b) there is less tendency for gas lock within the pump; (c) the plunger travel necessary to maintain production is obtained with fewer stress reversals on sucker rods, reducing rod breakage and pumping interruptions; (d) more positive valve action and better filling of the pump on the upstroke usually results; and (e) surface equipment usually shows less wear and greater power economy.
The long, constant velocity stroke is also advantageous in the case of a well which produces substantial quantities of sand, since it greatly reduces surges and agitation at the well pump suction, which causes excessive quantities of sand to be drawn into the pump mechanism. The excessive intake of sand results in binding of the pump mechanism, which is costly and time consuming to repair.
The disadvantages of the mechanically driven, Pittman drive type pumping units may be summarized as follows: (a) the average Pittman type unit operates with a short, fast stroke, resulting in a low volumetric pump efficiency, high power consumption, and low recovery rate; (b) the crank accelerated motion sets up wave stresses within the string of rods, resulting in a pronounced whip which causes premature failure in the rods; (c) the lack of independent control over the up-and-down stroke speeds makes it impossible to meet the particular pumping requirements of a given well; and (d) there is no pause at the end of the down stroke to permit the rods to stretch out from inertia forces and then to recover, but instead, an immediate reversal of motion resulting in high peak stresses and frequent breakage in the rod string.
It has long been realized that the smooth, uniform action of hydraulic actuators is ideally suited to the requirements of oil well pumping. However, widespread use of hydraulic jacks has been limited because of their complexity and initial cost.