Gas lift wells producing from a subsea template placed 12 km away from the processing facilities is known to be very challenging. Long pipelines give large volumes of gas and fluid which may influence each other, causing slugging or pressure variations in production pipelines and pressure fluctuations in the gas lift line. This again creates variations in rates and pressure into the process plant. Gas lift is the preferred artificial lifting method in subsea completed wells, due to the roboustness.The Yme Beta Øst subsea wells were put on production in the summer of 1996. Following a period of natural production the gas lift was started March 97. Immediately after start up of the gas lift heavy slugging in the system, both subsea and in process units was observed. As a result of the slugging the process was frequently shut down due to the pressure and rate variations. To reduce the slugging the wells were choked back, leading to reduced production. The slugging was investigated and the reason for the slugging was found to be in the production pipelines. The varying pipeline pressure influenced the gas lift system in the wells. In order to reduce this, a new gas lift design was carried out and new gas lift valves installed in one of the Yme Beta Øst wells and later both Beta Vest wells.
This paper presents a new approach to tire optimization problem for a continuous flow gas-lift system. A model was developed which considers the effects on the gas-lift design of variable operation costs of individual wells, limited lift-gas availability, limited liquid handling capacity, and limited gas separation capacity, resulting in maximum daily income. The advantages of this model are:the impacts of variable operation costs on the distribution of lift-gas to individual wells and on the economy of gas-lift can be readily analyzed;the system constraints can easily be incorporated in determining the optimum distribution scheme for the gas-lift system; andsensitivity analysis of the system on variations in the system constraints and the production characteristics can systematically be made. A procedure is also given for sizing surface processing capacities. This model was developed in the context of designing a continuous flow gas-lift system for a North Sea field An example problem consisting of four gas-lift wells was solved to show the solution procedure involved in the model. Introduction Several authors(1–5) have addressed the optimization of continuous gas-lift system. Poettman and Carpenter(1) laid the basic concepts for analyzing gas-lift per performance. Later, Simmons(2), Redden(3) and Kanu(4) combined the gas-lift performance with the economic conditions and suggested procedures to determine optimum gas injection rates and distribution of available lift-gas into individual gas-lift wells in a field. Applications(6–10) of the optimization procedure in the designs and the operations of gas-lift systems prove that the optimization procedure can greatly improve the efficiency of gas-lift systems. This paper addresses, first of all, the optimization problem of daily operation of a gas-lift system. A general optimization model of continuous gas-lift systems has been formulated. This provides a new optimizing procedure to determine the optimum operation of the gas-lift system. The advantages of this procedure are:it considers the impact of water production on the economic performance of gas-lift systems;in addition to the optimum results, it can also give the sensitivities of the optimum to the system constraints: andit deals with limited total liquid production rate, limited total gas production rate, and limited individual well liquid production rates, in addition Lo the limited lift-gas supply. The previous work on gas-lift optimizations considered only the limited lift-gas supply. Secondly, the paper discusses optimization of a gas-lift system. A procedure was given which could provide a rational basis for sizing surface processing capacities from an economic point-of-view. Though the gas injection pressure(5) is one of the most important factors governing the effectiveness of a gas-lift system, it is not explicitly involved in this paper. But as a rule, the deeper the lift-gas can be discharged into the production string, the more efficiently it is used. Optimization of Daily Operations The gas-lift system can be idealized as a system consisting of three elements. Figure I schematically shows the idealized gas-lift system. It consists of wells, production processing installations and high pressure lift-gas sources.
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