Abstract

The use of artificial lift methods is generally required at some stage during the life of a reservoir. The phenomenological models for simulating the performance of artificial lift systems for designing and optimizing operations usually assume steady-state flow conditions for calculating the influx from the reservoir. However, the bottomhole flowing pressure in cyclic artificial lift methods, such as intermittent gas lift and plunger lift, is anything but constant. Fluctuation of this pressure, which controls the flow rate of the well, takes place depending on the characteristics of the well and reservoir and the operating conditions of the particular lift method. Accurate prediction of the reservoir performance in the simulation of these artificial lift systems is necessary as it directly affects the behavior of the system itself during production and buildup stages. Using a reservoir simulator, this study investigates the response of the reservoir to cyclical fluctuations of bottomhole flowing pressure expected during commonly used intermittent artificial lift methods.

Introduction

The design of artificial lift systems is an important responsibility of the petroleum engineer. These systems allow a well to produce at rates higher than those achievable under natural flowing conditions and provide a means for a well to produce once it is incapable of flowing to the surface. In designing the artificial lift system, the engineer must make estimates of producing rates, producing pressures (surface and bottomhole), and anticipated production declines utilizing the chosen artificial lift method. These estimates are an integral part of the efficient design and operation of wells.

In most cases, the petroleum engineer estimates the pressure-production performance for an individual well assuming steady-state or semi-steady-state flow conditions. These assumptions may be suitable for many artificial lift applications where the production time is sufficiently long to reach steady production conditions. However, they may not be appropriate for cyclic artificial lift techniques such as intermittent gas lift or plunger lift.

These cyclic lift methods are characterized by regular pressure buildup and drawdown cycles associated with the production and shut-in periods utilized by the lift technique. These periods are usually on the order of minutes and hours and their duration is not sufficient to achieve steady production conditions.

In their analysis of the pressure-production behavior of single well systems, Evinger and Muskatl1,2 pointed out the fact that steady flow conditions never actually occur in a multiphase flow system. This results from the changing fluid saturations and the accompanying changes in relative permeability to each phase which occurs during the production process. However, they do suggest that one can estimate the transient nature of the pressure-production behavior of a single well system by a series of varying steady producing conditions over short time periods.

This suggestion of estimating the transient nature of the pressure-production behavior in multiphase flow systems by a series of steady flow conditions has been invoked by many in designing cyclic artificial lift methods. In this paper, a reservoir simulation study was performed to test the validity of assuming a series of steady-state flow conditions for estimating the short-term transient pressure-production behavior of cyclic artificial lift methods.

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