Abstract

The estimation of the liquid column height of a well is one of the most complex problems in the petroleum industry. This complexity is mainly due to the behaviour of the wells' two-phase flow, which varies with pressure, temperature, viscosity, and other factors. Knowing the liquid column height of a well helps determine with higher precision the amount of liquid holdup in the well and to optimise the oil's production per cycle. Although this height can currently be estimated for wells produced with various lift methods, it still cannot be estimated for intermittent gas lift wells.

This paper describes a new methodology to determine the liquid column height of wells produced using intermittent gas lift. Two pressure drop correlations were studied and modified to compute liquid column heights; Hagendorn and Brown's, and a combination of Aziz's and Wallis'. These correlations were programmed and tested using experimental data obtained from twelve wells in Lake Maracaibo. The best solution was based on Aziz's and Wallis' correlations as higher accuracies were achieved in reproducing the experimental data from the field. With this new methodology, the knowledge of liquid column heights will enable more efficient oil production in intermittent gas lift wells.

Introduction

Many wells do not have enough energy to lift all the fluids from the bottomhole to the surface. Artificial lift methods are required to supply hydraulic energy to the fluids. Gas lift is one of the most reliable methods, particularly in offshore fields. Continuous gas lift is used on first stages after natural flow.

Intermittent gas lift has proven to be more efficient when production declines below certain level. It works as a positive displacement mechanism where the gas pushes rapidly the fluid column accumulated after some shut-in time. To determine the best performance for this method it is necessary to know all the wells' characteristics as well as to have an appropriate designing tool. Knowing the liquid column height is one of most important aspect for designing the method in order to inject the correct amount of gas required for lifting the column, increasing oil production, and saving gas lift pressure and flowrate.

Assuming a theoretical height of the accumulated liquid column might as well be not precise for the fluid column composed by liquid and gas. After a literature review and a development of a predicting model, this paper describes a methodology to determine with better precision the height of the fluid column in an intermittent gas lift well. Experimental data from several wells located in Lake Maracaibo were used for tune the model.

LITERATURE REVIEW

Several multiphase flow models have been developed to predict pressure gradients in different problems found in the oil industry. In almost all-vertical wells, gas flows along the producing liquid, and different flow patterns are encountered as the fluids density changes with pressure and temperature throughout the well. The purpose of the optimisation in these cases is to obtain the bottomhole flowing pressure, given the wellhead pressure or viceversa.

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