A 500 ft vertical well was used to study slip velocity of air in mud and pressure gradients through a 2.93" annulus (5.43 – 2.50") during continuous two phase flow in flowing liquids and in stagnant liquid columns. The well was instrumented to measure liquid- and air flow rate, surface back pressure and annular pressure gradients.

Tests were undertaken with a broad range of air and liquid rates, different liquid properties, and with the injection of air slugs at different rate combinations. It was possible to detect these slugs as they passed the pressure transducers in the annulus. Results were applied to determine gas rise velocity.

Correlation has been developed for gas rise velocity, which was used to estimate gas and liquid hold up. The in situ gas velocity and terminal settling velocity were determined for both dispersed bubbly flow and slug flow. The resulting pressure gradients have been compared to estimates from 8 different empirical correlations. The best results were obtained by using the Zuber & Findley correlation for holdup estimation with a gas holdup of 0.6 to distinguish the boundary between bubble and slug flow. This high transition value was mainly caused by the geometry of the well (tool joints) and partly by the rheology of the mud. A very good agreement between recorded and estimated downhole pressure was achieved, with a mean error of approximately 1% and a standard derivation of 2.9%.

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