Economical Distribution Of Gas In At Continuous Gas-Lift System Subject To Variable Cost And System Constraints
Authors: Gang Xu Zheng , Herfjord Hans J. , Ramstad Helge J. 
Published: March 2010

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.