Page 211 -

CHAPTER NINE

Gas and Liquid Injection Rates

Contents
9.1 Introduction 173
9.2 Multiphase Flows in UBD Systems 173
9.2.1 Flow Regimes 174
9.2.2 Liquid Holdups 174
9.2.3 Multiphase Flow Models 176
9.3 The Gas–Liquid Flow Rate Window 194
9.3.1 The Underbalanced Drilling Pressure Differential 195
9.3.2 Collapse Pressure Analysis 198
9.3.3 Predicting Fluid Influx 204
9.3.4 Constructing a Gas–Liquid Flow Rate Window 204
Summary 209

9.1 INTRODUCTION
Most underbalanced drilling operations are performed using aerated
liquids or foams. Injecting the right amounts of gas and liquid into the
well is crucial for the success of unbalanced drilling (UBD) operations.
This is because the injection rates determine the borehole pressure that
is responsible for formation damage and borehole damage (borehole
collapses and washouts). The injection rates also affect hole cleaning. This
chapter presents multiphase flow models and their applications to design-
ing gas and liquid injection rates for UBD operations using aerated liquids
and foams as drilling fluids.

9.2 MULTIPHASE FLOWS IN UBD SYSTEMS

Multiphase fluids used in UBD operations include aerated liquids
and foams. These fluid systems consist of gas (normally air or nitrogen),
liquid (normally water or oil), and solid (cuttings). The liquid constitutes
the continuous phase, with gas appearing as discontinuous bubbles. Both
steady-state and transient flow models are available for multiphase flow
drilling hydraulics calculations. Unfortunately, the results from these

Applied Drilling Circulation Systems. DOI: 10.1016/B978-0-12-381957-4.00009-7
© 2011 Elsevier Inc. All rights reserved. 173