Multiphase flow behavior for acid-gas mixture and drilling fluid flow in vertical wellbore

Abstract

The transition laws of multiphase flow in a wellbore during acid-gas mixture influx are important for the hydraulic parameters design and wellbore pressure control in acid gasfield drillings. Herein, the phase change of acid-gas mixture in a wellbore was investigated by the experimental analysis. The results indicate that the acid-gas mixture in the supercritical phase exists under conditions of certain wellbore temperature and pressure, which makes its physical properties change abruptly near the critical point. Moreover, considering the phase change and dissolution of acid-gas mixture in drilling fluids, the multiphase flow may go through in turn the single phase flow, supercritical-liquid two phase flow, liquid-liquid two phase flow and gas-liquid two phase flow (including bubbly flow, slug flow, churn flow and annular flow) with the mixed fluids rising from bottom hole to wellhead. Meanwhile, the flow transition criteria, drift flux models and friction factor calculating methods for this type of multiphase flow were proposed. Finally, the multiphase flow characteristics in a wellbore during acid-gas mixture influx were analyzed using the new method with an example. The results suggest that during the acid-gas mixture rising up along the wellbore, the flow pattern transforming from the supercritical-liquid two phase flow or the liquid-liquid two phase flow into gas-liquid two phase flow can cause a large volume expansion, increasing the blowout risk.