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PART THREE

Underbalanced Drilling

Systems

Underbalanced drilling is used mainly for drilling oil and gas recovery
wells. Liquid foam, aerated liquid, and oil are the popular fluids used in
unbalanced drilling (UBD). Since these fluids have densities lower than
water, they are utilized for drilling oil and gas pay zones with subnormal
or depleted pressures. The main purpose of UBD is to protect these pay
zones from being damaged by drilling fluids.
This part of the book provides drilling engineers with basic informa-
tion about UBD systems and the techniques used to improve UBD per-
formance. Materials are presented in three chapters:
Chapter 8: Equipment in Underbalanced Drilling Systems
Chapter 9: Gas and Liquid Injection Rates
Chapter 10: Underbalanced Drilling Operations

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