Effect of R Ratio on Performance of Injection-Pressure-Operated Gas-Lift Valves

Abstract

This article, written by Special Publications Editor Adam Wilson, contains highlights of paper SPE 186110, “Effect of R Ratio on Performance of Injection-Pressure-Operated Gas-Lift Valves,” by K.L. Decker, SPE, Decker Technology. The paper has been peer reviewed and published in the February 2018 SPE Production & Operations journal. An injection-pressure-operated gas-lift valve’s closing force comes from a nitrogen charge acting on the effective area of the bellows (Ab) or a spring force. The opening force is the production pressure acting on the area of the port (AP) plus the injection pressure acting on the bellows effective area minus the area of the port. The ratio AP / Ab is referred to as the R ratio, which traditionally has been considered constant for every valve of the same type. The actual R ratio, however, is not a constant. This paper describes the consequences of using an assumed R ratio in a gas-lift design that is not the same as the actual R ratio of the valve. Introduction The force from a spring or a nitrogen-charged dome acting on the effective area of the bellows creates a contact area be-tween the ball and port. When using a square-edge seat, this contact area is de-fined by an outer seal diameter and the port diameter. The contact area is an annular ring on the surface of the port where neither production nor annulus pressure is acting. The width of this area changes with the valve-dome pressure, strength of port material, port size, and lap band. Fig. 1 shows a typical nitrogen-charged gas-lift valve. For chamfered ports, the ball makes contact on the chamfered surface. In this case, an outer sealing area and an inner sealing area exist because of the lap band. The outer diameter of the lap band defines the annulus-pressure sealing area. The inner diameter of the lap band defines the production-pressure sealing area. The R ratio can be obtained by an equation that uses properties of the valve (please see the complete paper for the equation). It generally is not possible to measure the seal area or the lap-band width directly; therefore, the R ratio determined by means of the equation should be considered an estimate. The most reliable and accurate method of determining the R ratio is by means of pressure testing. Both opening- and closing-pressure tests must be performed. Please see the complete paper for the equation to calculate R ratio using the opening and closing pressures. Discussion The opening pressure is commonly used as the basis for preparing a valve for service in a well. The reason is the ease with which it can be tested and the idea that the valve acts as a backpressure regulator. The gas-lift technician strives to keep the opening pressure within ±5 psi of what is requested. The dome pressure is normally charged to a pressure slightly higher than intended. The opening pressure is adjusted by lightly tapping the core valve of the nitrogen dome, releasing a small amount of nitrogen until the desired pressure is achieved. The practical ability of adjusting a very high pressure to an accuracy of ±5 psi by the tap of a hammer on the core valve is generally not within the skill set of gas-lift technicians.