Optimization of production in continuous gas lift wells is difficult to achieve when unstable flow (tubing or casing heading) causes gas lift injection rates to fluctuate. This problem, which occurs when there are variations in casing and tubing pressures, is particularly prevalent in a field with multiple wells drawing upon a single source for injection pressure. Square-edged orifice valves with a simple cylindrical channel have traditionally been employed as operating valves to transport the gas. Gas flow through the channel is usually in the subcritical flow regime. The injection rate from the casing to the tubing fluctuates with the tubing pressure, even with a constant casing pressure, allowing injection rates to be affected by both the casing and tubing pressures. With this type of symmetric flow geometry, critical flow (known as sonic flow velocity) will occur when the down-stream pressure is 40% to 50% less than the upstream pressure.
A new injection valve has been developed to ensure constant injection rate from the casing to the tubing with constant casing pressure, even when tubing pressure is only 10% less than casing pressure. The laterally asymmetric internal geometry of the nozzle-Venturi creates an injection valve that reaches critical flow velocity with pressure differentials of only 10%. At critical flow velocity, the injection flow rate becomes constant and is controlled by casing pressure only. The new design is a 1-inch or 1-1/2-inch OD valve that fits into any standard side pocket mandrel and can be deployed with standard slickline. A computer software program has been developed to determine the proper size orifice to output a specific flow rate at the given well conditions.
Initial usage of the valve has shown that injection flow rates will be constant if source pressure remains constant and tubing pressure is 10 to 100% less than the casing pressure.
Unstable flow (casing heading) is a common occurrence in continuous gas lift systems and can develop because the characteristics of the system are such that small perturbations can degenerate into huge oscillations in flow parameters.
Tubing Heading. Gilbert and Grupping et al. were the first to describe the mechanisms by which these unstable conditions are generated. In many wells, the operating gas lift valve is simply an orifice valve and operates in the subcritical flow regime. Under this flow condition, a temporary variation in tubing pressure at the operating valve depth can result in an increase in the gas injection rate through the gas lift valve, decreasing the density of the production fluid. This in turn, decreases the tubing pressure at the valve depth and increases the differential across the valve, causing more gas to flow through the valve. The flow from the reservoir will also increase as a result of the reduced pressure in the tubing. This positive feedback process accelerates until the casing pressure drops sufficiently, causing the injection flow rate through the gas lift valve to decrease. As a result of this process, the density of the fluid in the tubing string increases, causing the production pressure to increase, and subsequently, a reduction of reservoir fluid entering the wellbore. These conditions remain until the pressure in the annulus increases sufficiently and the rate of gas injection through the gas lift valve once again increases.
Operating a well under these cyclical conditions has several disadvantages. First, gas and liquid flow rate surges (or slugs) can occur. Coupled with pressure surges in the production facilities, these surges may be so large that severe operational problems, which include difficult operation of the low pressure separator or compressor shutdown, can occur. Second, the full lift potential of the gas is not used, resulting in an inefficient operation that consumes excessive quantities of gas. Third, in traditional prevention of gas lift instability, either 1) more gas is injected than needed or 2) the flow is choked at the well head, which reduces the inflow from the reservoir. Fourth, production control and gas allocation become very difficult because of casing and tubing pressure fluctuations. P. 235
