Successful Optimization Design for a Gas Lift System in Offshore Oil Field

Abstract

Abstract Gas lift is one of the most typical forms of artificial lift in oil production. Gas lift system apply to achieve big value to several operated company due to: 1) enable wells that will not flow naturally to be produced, 2) to increase production rates in flowing wells and 3) to unload completion fluid from wells that will later flow naturally. Gas lift system is considered inexpensive for operating, easy to implement, very effective in a wide range of operating conditions and requires less maintenance in comparison to other alternatives of artificial lift. The basic principle consists on decreasing the pressure gradient in the liquid via the injected gas. The resulting mixture becomes less heavy than the reservoir oil so that it will create a certain drawdown pressure and eventually starts flowing to the surface. Moreover, gas lift system design is to obtain the optimum gas injection rate. However optimum gas injection rate is important because excessive gas injection will reduce production rate and will increase the operation cost. Some other parameters are playing important roles in a successful gas lift operation and if not chosen properly. This will lead to a major effect on gas lift system operability and production sustainability. These parameters include: 1) gas lift gas injection pressure, 2) well head flowing pressure, 3) depth of operating and unloading valves, 4) valves spacing. Khafji Field has been producing oil since 1961. Most reservoirs are initially at pressures high enough to allow wells to flow naturally. However, the high reservoir pressure drop and continuous increase of water cut through the years, which influenced some of the wells to a greater extent on not flow or producing at lower desired rate. The artificial lift by limited capacity of 25 MMSCf/D of gas lift was introduced in 1988 to operate 65 wells were equipped with gas lift mandrels. Gas with gas lift system was fed from different Flow stations to Gas Lift Station at Offshore where gas lift compressors providing 25 MMSCF/D at discharge pressure 1200 psi. However, both flow stations handling capacity was restricted the lift gas allocation. Gas lift becomes increasingly important to handle the Maximum Sustainable Capacity (MSC) as per Khafji Development Plan (FDP). Therefore, after constructing FDP Phase-1 Facilities to maintain MSC of 300,000 BOPD. Offshore gas lift compression facilitates are demolished and a new Gas plant at onshore side is utilized to feed dry gas to operate gas lift wells at offshore in 2011. Gas lift wells produce more than 60% of total oil production in Khafji Field. Successful operating strategy for finding the optimum gas injection rates for offshore gas lift wells to maximize the total oil production rate is established. Ideally, sufficient gas should be injected into each individual well which is resulted to yield maximum revenues. Due to total amount of gas available feed from gas plant is limited to 50 MMscf/d, it is often necessary to allocate a restricted amount of gas to each well in some optimal manner to achieve maximum oil production from the field. Currently KJO gas lift system consists of gas compression facilities that feed the required gas lift volume to offshore through 12" high pressure gas line then distributed to wellhead jackets by gas network lines with each wellhead jacket having gas lift header and surface control/metering devices. A key concern in Khafji field is the lift gas constraint where the total injection gas volume available for the system is limited to 50 MMscf/d and utilized for operate only 100 out of 184 wells equipped with downhole gas lift mandrels. This volume must be carefully managed to maximize production from the reservoirs given the limited supply to allocate to the wells, so that a proper allocation of injection gas is critical when designing a gas lift system. This successful optimization for gas lift in Khafji field included the practices applied by KJO for designing, operating, monitoring and troubleshooting for gas lift as well as the long term strategy for advanced solutions. The ultimate objective of maximize oil production with lowering operating cost has been achieved in Khafji field due to gas lift optimization.