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CHAPTER EIGHT

Equipment in Underbalanced
Drilling Systems

Contents
8.1 Introduction 167
8.2 Surface Equipment 167
8.3 Downhole Equipment 170
Summary 171

8.1 INTRODUCTION

Liquid foam, aerated liquid, and oil are used as the circulating fluids
in underbalanced drilling (UBD) operations. The same types of equipment
are used in UBD with oil as for normal liquid drilling, except more reliable
well-control equipment is emphasized. The equipment in foam and aera-
ted liquid UBD systems includes gas compressors and separators in addition
to the liquid drilling systems. This chapter provides a brief introduction to
the equipment used in foam and aerated liquid UBD systems.

8.2 SURFACE EQUIPMENT

Figure 8.1 shows a layout of surface equipment in a closed system
using aerated liquid as the drilling fluid. The liquid pump is usually the
same pump as for mud drilling. The nitrogen pumpers are gas compres-
sors. The nitrogen gas is usually obtained using nitrogen generators, as
shown in Figure 5.4. Liquid nitrogen has been employed in offshore
UBD operations. A 4-phase separator (Figure 8.2) separates gas, oil, dril-
ling fluid, and drilling solids. The rotating head used in gas drilling is not
popular in UBD operations due to its low pressure rating. Rotating
blowout preventers (BOPs) are often used in UBD (Figure 8.3) (GRI,
1997). They can handle up to 2,500-psi wellhead pressure.

Applied Drilling Circulation Systems. DOI: 10.1016/B978-0-12-381957-4.00008-5
© 2011 Elsevier Inc. All rights reserved. 167