Abstract

A computer procedure and an optimal gas allocation model are presented for gas lift installation design and field wide optimization. The paper outlines the pre-design well performance analysis, system nodal analysis and the design procedure for mandrel spacing and valve sizing. It also describes the algorithm, criterion and implementation of the gas allocation model.

Examples which illustrate the computer design procedure are presented. Results and production improvement after implementing the gas allocation model for the October Nubian field are also discussed.

Special techniques are discussed to accommodate changes in reservoir productivity and operating conditions. A re-design feature is presented for wells at which gas lift valves and mandrels are already installed. An economic and effective operation can be obtained through the redesign procedure. The optimal gas allocation model, which significantly improved field production, is discussed. The integrated study requires 3-D reservoir modeling, system nodal analysis and gas distribution modeling.

Introduction

Ninety percent of the production wells in Amoco's Egyptian operation are under gas lift. Most of them are high capacity wells, therefore efficient design and optimized production of these wells have significant effects on Amoco's overall crude production. Experience has shown that facility design and production optimization without accurate study of the reservoir condition changes could be very costly. Since 1988, a number of comprehensive studies have been carried out to optimize production for the reservoirs, wellbores, and pipeline gathering systems.

Recent work on the October Nubian field in the Gulf of Suez is such a study. The October Nubian reservoir has been under development since 1979. The field is located in the Gulf of Suez, about 210 miles southeast of Cairo. The reservoir is about 10500 feet subsea, with 5500 psig of original pressure. There are 23 producing wells, including some recently developed wells, with a production of more than 100,00 BOPD. In late 1991, Amoco/GUPCO committed to enhance October field production by various engineering and reservoir management techniques. This included limited new development and more importantly, over all field wide optimization.

Production optimization has been an engineering practice since the early 1980s. Historically, the optimized solution was based on a single well system. Some recent work has been done in the area of production optimization and performance analysis for gas lifted wells; however, most of this work concentrated on facility analysis and modeling. This paper outlines a procedure for field wide optimization for gas lifted wells, not only in the above area, but also in the area of reservoir study, combining reservoir simulation with wellbore hydraulics performance.

Reservoir Study

The October Main Nubia reservoir management study was initiated to address the reservoir's declining pressure below bubble point conditions and to determine how best to deplete the light oil reserves. To answer these objectives, a representative performance derived reservoir description was developed by incorporating geologic, petrophysic, and engineering data into Amoco's reservoir simulation model. Table 1 lists pertinent data for the October Main Nubia.

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