High Rate Gas-Lift at Limited Injection Pressure – BelidaGas-Lift Challenges
Andi A Wibowo
ConocoPhillips Indonesia Ltd
Abstract
Belida is a mature oil field located in South Natuna Sea, Offshore Indonesia. The field is typical for high productivity and high water cut production environments. Therefore high gas-lift injection rates are required to get optimum gas-lift performance. Typical, gas injection rate is as high as 4 mmscfd with available differential pressure between 100 to 150 psi at the deepest possible mandrel depth. The well has to be operated at a reasonable injection pressure to keep the upper unloading valve closed during normal operations. Unfortunately in the available market, there is no orifice size which can provide the required injection rate. Consequently, some of the wells were producing below optimum condition.
This paper describes the results of dual mandrel application and orifice valve bullnose modification techniques that have been implemented in Belida wells to achieve optimum injection rate at relatively low available differential pressure. Dual mandrel application at the expected point of injection has become standard for wells completed with 1” gas-lift pockets. It has been observed that the actual amount of gas through a single 1” orifice was only about 50% of the required volume. The dual mandrel application has been providing the capability to inject more gas with lower operating injection pressure to keep the upper unloading valves in the closed position during normal production operations.
For the high productivity wells that were completed with 1.5” gas-lift pocket, the orifice bullnose modification technique has become standard to lift at the deepest possible mandrel with optimum injection rate. The modification has been done on the customized orifice with a port size of 44/64” that had been made by the gas-lift vendor. During the initial trial it was found that this orifice size required unexpected differential pressure to inject the required lift gas injection rate. In most cases this had led to multipoint injection as the casing pressure exceeded designed injection pressure. Modeling indicated that this orifice acted like 26/64” port instead of 44/64”. Based on this observation, a subsequent inspection of the back check and bullnose cap determined that the mechanical design of these parts restrictedgas-lift injection through the port. A modification to the back check and bull nose designs were performed to remove the mechanical restriction. The result of this modification delivered a greater gas-lift injection rate which translated to well performance improvement.