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Gas and Liquid Injection Rates 195

yield the same flowing bottomhole pressure being equal to the formation
pore pressure minus a minimum UBD pressure differential. The right
boundary of the GLRW is defined by a locus of gas‒liquid rate combina-
tions that yield the same circulation break bottomhole pressure which is
equal to the formation collapse pressure at different gas injection rates.
The lower boundary of the GLRW can be defined based on the cuttings
carrying capacity of the fluid mixture. Different criteria for cuttings trans-
port can be used, depending on the types of fluids.
A conservative criterion for aerated liquid is the minimum kinetic
3
energy, which assumes that a minimum unit kinetic energy of 3 lbf-ft/ft
is required for drilling fluids to effectively carry drill cuttings up to the
surface in normal drilling conditions. For calculation simplicity, it is safe
to assume that the gas phase has no contribution to the carrying capacity
of the mixture. This means that the minimum kinetic energy of an aera-
ted fluid can be conservatively estimated based on the liquid flow rate.
The upper boundary of the GLRW can be defined based on the well-
bore washout constraint. Because no design method is available for this
issue, a good practice is to look at calliper logs and use experience gained
from past local drilling operations.
Determining the right and left boundaries of the GLRW requires
knowledge of the UBD pressure differential and borehole stability against
a hole collapse. These issues are discussed in the following sections.

9.3.1 The Underbalanced Drilling Pressure Differential
The UBD is mainly used for reducing formation damage during drilling.
It is believed that the higher the pressure differential between the forma-
tion and the wellbore, the less the formation damage and thus the
higher the well productivity. However, the pressure differential is also
responsible for some drilling complications such as borehole collapse and
excess formation fluid influx. Guo (2002) discussed the balance between
formation damage and wellbore damage. Guo and Ghalambor (2006)
presented a guideline for optimizing pressure differential in UBD and
reported the following:
• The pressure differential should be high enough to counter the
capillary pressure force responsible for liquid imbibition into the pay
zone causing formation damage.
• The pressure differential should be low enough to ensure that the
entire section of the open hole will not collapse during drilling.