Etude des instabilités dans les puits activés par gas-lift

— Gas-lifted wells often present unstable behaviors that cannot be described by hydrostatic laws. In this thesis, we aim at analyzing the well dynamics, especially when its production is irregular. We end up by designing control solutions. We begin with a brief description of the gas-lift activation technique. The negative impact of instabilities on oil production is underlined. Two main mechanisms are at birth of the unstable behaviors. On-site real time records serve as illustrative examples. We explain the first mechanism, well referenced in the literature, thanks to mass balances equations. Analyzing the vector field properties allows us to interpret the observed phenomenon as a limit cycle. Then, we expose the main contribution of this thesis. It lies in the description and analysis of the second mechanism. We show that the instability arises from out-of-phase effects induced by the propagation delay in the well. A distributed parameter model is used. Then, we gather our results and present a complete and compact model of the well dynamics. This model is a first order stable system interconnected with a distributed parameters system. Thanks to the small gain theorem, the instabilities root-causes appear as two possibly positive feedback loops. This model inspires the design of control solutions that suit the physical structure of the well. Moreover, these solutions fit with the operational constraints. Realistic simulation results illustrate the efficiency of our strategies. Some of the solutions have been tested on line on a production site. Some of the obtained results are presented.