Unstable flow condition is a limitation for production optimization in continuous gas-lift wells due to casing and tubing heading. The instability could be caused due to insufficient gas injection volume, incorrect port diameter size, and variation of gas-lift injection rate due to subcritical flow condition at the point of operation. The traditional square-edge orifice valve is a cylindrical channel used to transport the gas. To reach critical flow condition, the downstream pressure has to be 50% to 40% the upstream pressure, and cannot be used to eliminate instability due to the excessive pressure differential required to achieve this flow regime. Therefore, a new valve was developed, which has the advantage to reach sonic flow velocity when pressure differential is only 10%, due to its converging diverging flow geometry. Comparing the dynamic flow performance of both square-edge and Nova valve, the Nova is independent of the downstream pressure and ideal as means of eliminating upstream instability.
Around 4200 wells currently operates under continuous gas-lift in Lake Maracaibo and 5% to 10% of these operates at unstable flow conditions due to subsonic flow at the injection point. For this reason PDVSA E&P decided to carry out field tests using a new gas-lift stabilization valve called Nova or nozzle-Venturi valve in wells with emulsion problems, high water content or low differential pressure across the operating valve which causes casing and tubing heading. There were installed 7 Nova valves in wells with crude oil gravity ranging from 14° API to 30° API, producing from 300 blpd to 2000 blpd, gas injection volumes of 500 Mscfd to 2000 Mscfd, and as deep as 12000 feet.
The well test results indicate that this new technology could stabilize the gas injection volume eliminating the casing heading and minimizing fluctuations of the tubing pressures. The potential application of this new valve in Lake Maracaibo oscillates between 300 to 400 wells. It is the scope of this work to present the selection criteria used to choose the well candidates, the results and experience gained during the field tests.