Summary

Allocation of gas to wells on continuous lift can affect profitability. 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 profitability. To alleviate the problem of excessive gas usage, an economic slope that relates liquid production and gas injection to cost and profit has been developed. To resolve the problem of gas allocation in a field with limited gas, a method to allocate gas to wells efficiently under this situation is presented and a step by step procedure is given. An example problem consisting of a six-well field is solved to illustrate the procedures presented.

Introduction

The current energy situation and the increasing costs of gas lift are forcing oil operators to consider economic operations first rather than desired maximum production. As the market price of oil continues to increase, it is understandable that oil operators would prefer gas lift wells to produce at a maximum. In fact, optimization in continuous gas lift has come to mean the injection of gas until maximum production is achieved. On the other hand, gas prices and compression costs continue to increase, forcing the oil operator to approach maximum production cautiously. In view of this situation, the development of a procedure to determine the optimal economic point to produce a well or a group of wells was recognized as having a high potential for gas-lift design improvement. The economic slope concept offers a good approach to the problem. This slope relates the well and reservoir parameters to cost and profit. The performance of a well on continuous gas lift can be described by a typical gas requirement plot (liquid flow rate vs. gas injection) as shown in Fig. 1. Any point of tangency to the curve is unique and describes a certain situation. For instance, the point of tangency that occurs at 0 deg. slope indicates the gas injection that will yield maximum production. If this slope coincides with the economic slope, the determination of the economic optimum is clear-cut since only one maximum exist. However, the economic slope usually occurs at a slope greater than 0 deg., so obtaining the optimal economic point can become tedious. In fact, efforts made in the past yielded procedures that are very cumbersome. This paper, therefore, presents (1) the formulation of a simple slope, (2) the use of this economic slope in a simple procedure to allocate gas, at the optimal economic point, to a well or a group of wells(given an unlimited gas situation), and (3) the determination of total gas required for a field, thus simplifying compressor sizing. Another important aspect of gas allocation that deserves attention is the area of limited gas. A method is presented that uses a simple graphical procedure to allocate gas to wells efficiently in a field operating under such a condition. A six-well field is used to illustrate the procedures presented.

Economic Formulation

The gas requirement plot (Fig. 1) is indispensable in any useful analysis of gas allocation in continuous gas lift because it relates liquid production to gas injection. This liquid production/gas injection relationship is the cornerstone of any economic approach. To obtain an economic slope, it is necessary to formulate mathematically a function of gas injection and liquid production on one hand and cost and profit on the other.

JPT

P. 1887^

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