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204 Part III Underbalanced Drilling Systems

9.3.3 Predicting Fluid Influx
Reservoir fluid influx is another factor that limits the UBD pressure differ-
ential. Several methods have been used for predicting fluid influx in under-
balanced drilling. Garham and Culen (2004) analyzed gas influx from a gas
reservoir to the borehole during UBD. They presented an approach to
predicting expected gas production rates, sizing equipment, and managing
bottomhole pressure to aid in borehole stability and formation damage
control. Single-phase gas flow was assumed in the borehole. Rommetveit
and colleagues (2004) considered multiphase flow in the borehole in their
dynamic model for predicting formation oil influx during UBD.
Mykytiw and colleagues (2004) used a multiphase flow simulator for
UBD applications design with a steady flow reservoir model. Friedel and
Voigt (2004) employed a numerical reservoir simulator to investigate gas
inflow during underbalanced drilling and the impact of UBD on longtime
well productivity considering the non-Darcy flow effect. Haghshenas
(2005) analyzed the effects of drilling parameters and reservoir properties
on the formation oil influx rate and total influx volume in UBD. He used
radial and spherical transient flow models to estimate fluid influx during
drilling. Guo and Shi (2007) and Guo and colleagues (2008) presented
mathematical models for predicting influx rate and volume for planning
UBD horizontal wells. These models can be used for adjusting UBD pres-
sure differentials to fit the capacities of separators and fluid storage tanks.
9.3.4 Constructing a Gas–Liquid Flow Rate Window

A detailed description of a GLRW was presented by Guo and Ghalambor
(2002). This section illustrates the procedure for constructing a GLRW
using a field example.

Illustrative Example 9.5
The following data are given to design aerated liquid hydraulics. Construct a GLRW.
Design basis
Reservoir pressure: 2,300 psia
Desired pressure differential: 300 psi
Collapse pressure: 1,500 psia
Wellbore geometry
Cased hole depth: 5,000 ft
Casing ID: 8.125 in
Open hole diameter: 8 in
Vertical depth: 5,000 ft