Abstract

The assessment of Intermittent Gas Lift (IGL) by computer calculations using the mechanistic models found in the literature usually approaches the IGL's operating cycle as a sequence of independent non-interacting stages, occurring back-to-back, without overlapping in time; that approach restricts the system analysis to the limited range of operational conditions when such particular behavior indeed exists. In fact, out of such range, some stages of the IGL cycle may be rather simultaneous than sequential, as previously assumed. This paper presents a new IGL mechanistic model and simulation scheme to consider the possible occurrence of both sequential and simultaneous stages throughout the IGL cycles. The dynamics of the IGL cycles is assessed through a simultaneous and coupled variable set of non-linear algebraic and time-differential equations, interactively defined on the run and solved according to the ongoing stages of the IGL cycle. A case study for a typical IGL well, producing from a low productivity reservoir partially depleted, at different operational conditions, shows how the IGL computer simulator can help the operator to set up the IGL parameters to maximize the well production or its economic gain.

Introduction

The Intermittent Gas Lift (IGL) is an artificial lift method employed to produce oil when the reservoir is somewhat exhausted or its productivity is too low to use a higher producing method. A high-pressure gas supply provides the supplement of energy necessary to intermittently lift the reservoir's liquids (oil and water) up to the surface.

The IGL cyclic operation is controlled by setting up the cycle period and the gas injection period on the timer controller of the injection motor valve at the surface, and by pressure-charging the dome of the operating lift-valve located inside the tubing string, near to the casing bottom.

The IGL assisted wells can produce within a somewhat wide range of flow rates provided that the operator sets the aforementioned control parameters to achieve the goals designated by the company, which may be maximizing the oil production or the financial profit.

The operator must understand the behavior of an IGL welland how sensitive is the well response to its control parameters. The complexity of the calculations involved in the optimization of IGL cycles often encourage the use of simplistic correlations, lacking in generality, and of questionable validity. Hence a good computer simulator would be a helpful aid to the operator.

Literature review

The early works on IGL modeling were presented by Brown and Jessen[1], White et al.[2], Brill et al.[3] and Neely et al.[4]. Those authors described the main patterns of the IGL, based on field measurements of test wells. Semi-empirical models were derived to predict some variables of the IGL operation. The simplicity of such models for handy calculations is advantageous. Nevertheless, most of them are based on results recorded just for the first IGL cycle, still under the influence of transient effects. Those models also lack generality and lead to a fragmented analysis of the IGL behavior, since some aspects of the IGL are not considered.

Schmidt et al.[5] presented a dynamic model for the conventional IGL cycle, based on the conservation equations of mechanics.Comparisons between numerical calculations and measurements in an experimental test facility supported the validity of the model.

Liao et al.[6] developed a comprehensive mechanistic model for the conventional IGL cycle, obtaining results in good agreement with the former experimental works. The conventional IGL cycle was divided into 4 sequential stages, each one with its own complete set of ordinary differential equations. The stages were simulated in a standalone fashion, through an iterative numerical procedure.

Santos et al.[7] improved Liao's modeling approach, and extended it to other variants of the IGL: the IGL with chamber, the IGL with plunger (ideal case) and the IGL with pig (pig-lift); including the gas injection stage on the simulation.

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