A New Approach for Continuous Gas Lift Simulation and Optimization

摘要

Continuous gas lift is one of the most typical forms of artificial lift in oil production. Gas lift is rather inexpensive, easy to implement, very effective in a wide range of operating conditions and requires less maintenance in comparison to other alternatives. The basic principle consists on decreasing the pressure gradient in the liquid via the injected gas. The resulting mixture becomes less heavy than the original oil so that it eventually starts flowing. Current gas lift analytical methods applied to individual wells have applied the state of the art on numerical algorithms such as optimization via genetic or polytope algorithms. However, the models have been oversimplified so that several thermodynamic properties are considered constants though mechanistic basis are often employed. This paper develops a model based on mass, energy and momentum balances to produce a set of differential algebraic equations (DAEs). Cubic equations of state are used to solve the phase equilibrium behavior and to estimate other required thermodynamic properties. The problems solved include a) Determination of the outlet conditions, at the surface, when the amount of gas and oil production are fixed and b) Determination of the appropriate amount of gas required to lift the oil but matching the outlet pressure at surface manifold. he DASSL package is used to solve the DAEs problem and KNITRO is used to solve the overall optimization problem. The model allows determination of phase behavior along the well according to the automatic integration step produced within DASSL.