|dc.description.abstract||Underbalanced drilling (UBD) has gained popularity during the recent years as it
provides a method to prevent formation damage, minimize lost circulation risks, and
increase the rate of penetration. However, drilling with a bottomhole pressure less than
the formation pore pressure will often increase the risk of borehole instability due to the
shear or compression failure of the rock adjacent to the wellbore. The extent of rock
failure is related directly to the pressure in the annulus between the drillpipe, collars and
the wellbore which can only be calculated through modelling multiphase flow in the
drilling system. The relationship between rock failure and wellbore hydraulics becomes
more complex due to the appearance of the influx formation fluid in UBD. Therefore,
the aim of this research is to describe methods to solve the complex interaction of
wellbore stability, rock yielding, collapse, wellbore hydraulics, and production capacity
during UBD operations.
To achieve the aim, analytical and numerical solutions have been codified into
two programs WELLST, and UBDRILL. Commercial software packages such as
ABAQUS, PERFORM, HYMOD were also used to model the process. Field data from
granite basement reservoirs of Basin X, Vietnam were used as the parameters input into
the model to calculate. This research includes:
• An analysis of the influences of time dependence, thermal and hydraulic
diffusivity, wellbore pressure changes, inclination and azimuth variation,
poroelastic and thermo-poroelastic deformation, cooling and heating effects
on wellbore stability in UBD.
• An analysis of pressure, temperature, fluid properties distribution in the
annulus and inside the drillpipe while UBD.
• An analysis of the suggested liquid gas rate window (LGRW) which gives
field engineers flexibility in the selection of liquid and gas injection rates on
the drilling site when UBD is applied.
• An estimation of production capacity in UBD operations.
These results were obtained by analyzing the field data of granite basement
formations and clastic formations of Basin X, Vietnam which is under compression in a