Global Optimization Techniques in Gas Allocation for Continuous Flow Gas Lift Systems

Abstract

Abstract Continuous flow gas lift is one of the most common artificial lift methods in use. It works by injecting a continuous volume of high pressure gas into the tubing or tubing/casing annulus to lighten the column of oil and thus facilitate extraction. Excessive gas input is costly because of high gas prices and compressing costs. Inefficient gas allocation in a field with limited gas availability also reduces production and profitability. Historically, a sensible allocation was achieved using a technique called the equal slope allocation method. This method uses graphical procedures to optimally allocate gas to a group of wells in order to optimize total oil production. However, there is a disadvantage in this method, as it cannot be applied to wells where there is not an instantaneous response to gas injection. The purpose of this paper is to present a methodology for automatically determining the optimum gas injection rate for a group of wells in order to maximize the total oil production rate for a given total amount of gas. The search algorithm proposed is a combination of stochastic domain exploration and a heuristic calculation of a descent direction, in order to avoid stopping the algorithm at a local optimum. Introduction Continuous flow gas lift works by injecting a continuous volume of high pressure gas into the tubing or tubing/casing annulus to lighten the column of oil and thus facilitate extraction. Ideally, if there is no restriction in the total amount of gas available, sufficient gas could be injected into an individual well until maximum production is achieved. However, in most cases, the total amount of injection gas volume available for the system of wells is insufficient to reach the maximum oil production for every well. It will force the oil operator to approach maximum production cautiously. Inefficient gas allocation in a field with limited gas availability also reduces profitability. Excessive gas input is costly because of high gas prices and compressing costs. Therefore, it is necessary to allocate a limited amount of gas to each well in an optimal way to get the maximum oil production rate from the field. In view of this situation, the development of a procedure to determine the optimal point to produce a group of wells was recognized as having a potential for gas lift design improvement. Kanu et al. established the method of equal slope allocation under both unlimited and limited gas supply. They presented the formulation of the economic slope and the use of this slope to allocate a total amount of gas at the optimal economic point for a group of wells in a step by step procedure. This method works well under the following hypothesis: P. 375