Transient flow characteristics of gas lift in underbalanced drilling

The implementation of gas-lift operation directly determines the success or failure of the completion of construction in underbalanced drilling. In particular, dynamic changes in downhole pressure are related to the security of well control in the entire operation process. The essence of gas-lift operation is that under the initial downhole conditions after the stop of well circulation, gas-lift operations are continued by nitrogen; when the gas crosses downhole fluid in a static state, the fluid slips to the borehole and turns to a transient gas-liquid two-phase flow that is different from routine steady flow. A calculation method and a mathematical model are established for numerical simulation of characteristic parameter changes in the downhole transient gas-liquid two-phase flow during continuous gas lift after the stop of well circulation. Characteristic parameter changes in the downhole transient gas-liquid tow-phase flow during continuous gas lift are calculated from measured data of actual well, from which the following patterns are summarized: (1) The speed of nitrogen slipping upward along the borehole rises gradually; (2) The volume of drilling fluid ejected grows over time; and (3) The transient fluctuation in bottomhole flow pressure is greatest at the initial phase. The results contribute to the improvement of wellbore flow model system and pressure control program in gas lift operations after completion of construction in underbalanced nitrogen drilling, provides new insights to the characteristics of the special flow, and ensures the security of well control in gas-lift operations.