Chapter Nine - Gas and Liquid Injection Rates

Abstract

This chapter describes the principle of selecting the gas and liquid injection rates for underbalanced drilling operations. Multiphase fluids used in UBD operations include aerated liquids and foams. These fluid systems consist of gas, liquid, and solid. Multiphase flow is much more complicated than single-phase flow due to the variation of flow regimes or flow patterns. Fluid distribution changes greatly in different flow regimes, which significantly affects pressure gradients inside and outside the drill string. The liquid phase always flows slower than the gas phase in upward flow streams and faster in downward flow streams. The differences in phase velocities cause the in situ volume fractions of fluids to be different from the volume fractions at the injection point. The combination of liquid flow rate and gas injection rate should be carefully designed so the flowing bottom-hole pressure is less than the formation pore pressure under drilling conditions and the circulation-break bottom-hole pressure is greater than the formation collapse pressure. Other considerations in designing liquid and gas flow rates include the cutting-carrying capacity of the fluid mixture and the wellbore washout.