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CHAPTER TEN
Underbalanced Drilling
Operations
Contents
10.1 Introduction 213
10.2 Aerated Liquid Drilling 213
10.2.1 Aeration Methods 214
10.2.2 Pressure Stability Control 218
10.3 Foam Drilling 223
10.3.1 Foam Formulations 223
10.3.2 Foam Stability Control 224
10.3.3 Handling the Returned Foam 229
Summary 230
10.1 INTRODUCTION
Underbalanced drilling is commonly carried out using aerated liquids
and foams. Since the operations are mostly for improving hydrocarbon
recovery from depleted reservoirs, well control is usually not a major con-
cern as long as the required types of well control equipment are installed.
When drilling with aerated liquids, pressure stability and directional drilling
problems may occur. Foam stability and foam disposal are important issues
in foam drilling. This chapter provides some basic knowledge of unbalanced
drilling (UBD) while focusing on minimizing drilling complications.
10.2 AERATED LIQUID DRILLING
Aerated liquid drilling is also referred to as aerated mud drilling
or gasified drilling. Gas (air or nitrogen) is mixed with liquid (water, mud,
or oil) in the borehole to reduce the bottomhole pressure for achieving
underbalanced conditions. The level of bottomhole pressure and its stability
mainly depend on the aeration method and the gas injection rate.
Applied Drilling Circulation Systems. DOI: 10.1016/B978-0-12-381957-4.00010-3
© 2011 Elsevier Inc. All rights reserved. 213
