Criteria for Gas-Lift Stability

Abstract

Summary Severe flow instability (heading or annulus heading) is known from operations of gas-lift systems. Here, two simple stability criteria are developed and compared with reported field data. The stability problems experienced for the cases examined would have been identified with these criteria and corrected at the design stage. Introduction The currently used principles for gas-lift design were established during the early 1950's. They provide relations betweengas injection pressure and the most efficient point of injection andgas injection rate and the production rate to be expected. From these relations, standardized procedures for gas-lift design have been worked out. Works on application and optimization of the procedures have provided further insight into the interrelations between gas-lift design and economic performance. Often-unstated assumptions of gas-lift design are that it will be possible to inject gas at a constant downhole rate and that the resultant production rate will be stable. This is not necessarily true; severe flow instability is well known in the actual operation of gas-lift systems. Variations in pressure and flow rate are observed in all multiphase flow systems, even in pumping wells, because of redistribution of gas and liquids. They cause relatively small short-duration pressure and flow changes. Alone, this has little effect on the continuity of production. In a gas-lift system, however, it may trigger system instabilities. API recommends that, for the sizing of pipes receiving gas-lifted production, a "surge factor" of 40 to 50% should be added to the estimated steady-state flow rate, compared with 20% for naturally flowing wells. Intended as guidelines for cases where more definite information is lacking, these numbers may indicate something about the uncertainties concerning the flow instabilities during gas lift. Bertuzzi et al. observed that when the lift-gas input rate was reduced below a certain minimum, violent heading would occur and the liquid production would eventually cease. They postulated that "a sudden drop in pressure in the tubing brought about a sudden surge of gas into the tubing. The volume of gas surging into the tubing is dependent on the pressure and volume of gas in the annular space. If the pressure in the annular space dropped too much, gas ceased to flow into the tubing." More recently, gas-lift insta-bilities have led to shutdowns of wells in the Claymore field. This was amended by replacement of the downhole injection valve by a fixed orifice. Flow instabilities have also been observed and analyzed for simple air-lift pumps. This is related to gas-lift instability. However, the inflow mechanisms of a gas-lift system are considerably more complicated than for an air-lift pump. Besides, the friction dampening will be much larger in a gas-lift system because of order-of-magnitude-larger flow length. Thus, the dominating mechanisms of instability will be quite different.