A New Approach to Gas-Lift Allocation Optimization
Authors: Dutta-Roy Kunal , Kattapuram James 
Published: December 2007

Analytical methods for determining the optimal injection rate in a well operating under gas-lift are based on balancing the buoyancy improvements from the additional gas in the production stream with the corresponding increase in the flowline back-pressure and its negative impact on the overall deliverability. Such an approach, while useful in determining the maximum production improvement achievable for an individual well, ignores a number of practical and relatively significant aspects that tend to complicate the real-world application. One of the key aspects to the gas-lift problem is the interaction between the wells in the gathering network. The back-pressure from the additional gas in the flowline adversely affects production from all the other connected wells, whether they be on gas-lift or naturally flowing. As such, the "optimal" gas-lift injection rate determined from existing methods is invariably optimistic.

Another major complication arises from the limitations associated with the available compressors. Typically, these facilities are designed to handle the requirements identified when gas-lift is first introduced. As the field depletes, the requirement for lift gas increases. Inevitably, the operator is challenged with the classical gas-lift allocation problem: how does one allocate limited available injection gas to maximize overall field production?

Other factors that influence the economics of a field-wide gas-lift strategy include the reservoir depletion behavior, the effect of varying water-cut, the impact of both capital and operating costs, and the detailed performance characteristics of the compressor units.

This paper presents a novel approach to the simulation and optimization of the overall gas-lift allocation problem, using a rigorous pressure-balance based multiphase flow network solving technique, coupled with a robust sequential quadratic programming (SQP) approach for the non-linear, constrained optimization. The new technique is applied to typical fieldwide problems, and the results obtained are compared with conventional analyses methods.

Introduction

Continuous gas-lift is one of the more common forms of artificial lift in oil production. Gas-lift is effective over a wide range of operating conditions, is relatively inexpensive and simple to install, and requires less maintenance especially when compared to alternatives such as electrical submersible pumps.

The mechanism of gas-lift is fairly straightforward. Gas at a relatively high pressure in the casing is injected into the tubing string to lighten the fluid column by aeration, until the reduction in the flowing bottom hole pressure creates the pressure differential across the sand face needed to achieve the desired production rate.

This paper first analyzes the flow mechanism of a single-well gas-lift system, evaluating the effect of various operating and design parameters. The analysis then proceeds to a two-well system with a common flowline to highlight its significant impact on single well performance parameters. The interaction effects are next extend to a production network with multiple wells sharing a system of common gathering lines leading to separation and treatment facilities at a fixed pressure.

Next, the paper addresses the problem of limited gas allocation. The unavailability of adequate injection gas is a common problem with older gas-lift installations. P. 685^