Use of the Automatic Free Piston in Oil Well Production Problems

Abstract

Introduction Before discussing the use of the free piston, it may be well to explainbriefly the type of gas lift valves with which the tool is designed tofunction. The gas lift valve is nothing more or less than a small back pressureregulator. Its construction consists of an outer housing covering a seamlessmetallic bellows in such a way as to form a sealed pressure vessel or pressuredome. To the bellows is attached a stem, and to this stem is attached a seatingmember. When the valve is run into the hole, a pre-determined pressure chargeis placed in the pressure dome. As shown in Fig. 1 the valve is furnished infour different major assemblies, to accomplish good results in different typesof wells. The valve is mounted on a pup joint (Fig. 2-a); the complete assembly isentered into the tubing string at the desired location. Each valve is furnishedwith a reverse flow check valve (Fig. 2-b) to eliminate any back wash throughthe mechanism. Valve action is as follows: At any time the casing pressure is allowed to exceed the pressure forceswithin the pressure dome of the valve, the valve simply opens and passes gasfrom the casing annulus into the tubing. When the casing pressure forces areallowed to bleed to a force less than the pressure force within the dome of thevalve, the valve closes. With this type of construction, it is possible to open and close the valveor valves at will from the surface, through the medium of the gas pressure inthe casing annulus. The cycle of operation is governed by a time cycle motorvalve, commonly referred to as a gas lift intermitter or gas life surfacecontroller (Fig. 3). In actual practice, after the well is unloaded only onevalve is in operation… all other valves remain in the closed position. Onpre-determined time cycles, it is possible to open and close this operating valve at will, and to pass thedesired amount of gas through this valve whenever needed. This method of gas lift is widely used and has been found to be veryefficient. However, it has long been known that even greater efficiency wouldbe obtained if it were possible to place a piston between the injected gas andthe fluid head. The free piston which will be discussed here was designed toaccomplish this end and has been found to do so with very satisfactoryresults. T.P. 3583