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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 when pressure differentials of only 10% occur. 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.
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