Multiphase hydrodynamic model predicts important phenomena in air-drilling hydraulics

Wellbore hydraulics poses difficult problems for the drilling engineer designing a drilling program. Intricate interactions between the drill cuttings, the transport fluid (mud or air), the wellbore, and the drillstring cause the difficulties. Lack of understanding of the physics involved and the lack of a fundamental descriptive capability inhibit the development of an appropriate predictive model. This problem is more apparent in air drilling because only limited data are available on which empirical correlations can be based. This paper addresses wellbore hydraulics with a fundamental hydrodynamic multiphase-flow model. The model incorporates the fundamental physics involved in the pneumatic transportation of solids cuttings in the drillstring/wellbore annulus. This model forms the basis for a predictive tool for the optimal lifting velocity, an essential ingredient in the optimal design of an air-drilling program. Available correlations are, at best, gross approximations; more important, they fail to account for the physical phenomena observed in the pneumatic transport involved in air drilling (e.g., choking and clumping). The model accommodates nonuniformity in particle sizes. Extensive parametric analysis of the system explores the predictability of some of the phenomena associated with air drilling. The model is capable of predicting the pressure-drop profile in the annulus under various simulated drilling conditions. Also, results demonstrate that the model can predict many phenomena associated with lifting cuttings out of the hole during air drilling. Model prediction shows very good agreement with experimental data. Finally, the model possesses good scale-up capability.