Experimental study of three-phase gas-lift

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Abstract

The gas-lift technique is a gravity-based pumping technique used for recovering oil from a production well. Gas injected at the bottom of the production pipe reduces the gravity component of the pressure drop and thereby, stimulates the supply of oil from the reservoir. This results in an enhanced oil production. Previous investigations have underlined the importance of the size of the injected bubble on the efficiency of the gas-lift. For air-water and air-oil flows, it was shown that the injection of small bubbles reduces the pressure drop, compared to the injection of larger bubbles, which then improves the efficiency of the gas-lift. However, in the production pipe, water is often flowing together with the oil, forming a liquid mixture composed of a dispersed phase and a continuous phase, either oil drops in water (O/W), or water drops in oil (W/O). The transition from O/W to W/O, called the {\it phase inversion}, is accompanied by a sharp increase of the mixture viscosity, which, in turn, can give rise to an undesired increased pressure drop. In this thesis we investigate the influence of gas injection on an oil-water flow in a vertical pipe. The study was performed by means of experiments. Special attention has been paid to the phase inversion phenomenon. Pipe flow experiments were conducted. It was shown that gas injection may increase the pressure drop at the inversion point, because the friction component of the pressure overcomes the gravity component. Furthermore, the injection of smaller bubbles leads to an even higher pressure drop, which contradicts the results found for air-water flows. In terms of gas-lift, the beneficial effect of small bubbles was observed in water-in-oil dispersions and dilute oil-in-water dispersions. Local measurements of the wall shear stress as a function of the passage of bubbles close to the wall emphasized the mechanisms involved and gave some explanations on why the friction is increased in the presence of bubbles.