English abstract
Wellbore instability issues represented by well collapsing resulted in a
tremendous loss of time and money in the oil industry. This leads to the need
of more critical well drilling plans to prevent such rock failure. The carbonate
rocks are known to be more competent and experience less instability issues
than shale or sandstone. However, the majority of the global hydrocarbons
reservoirs are carbonate. Consequently, a significant number of wells are
drilled every year in carbonate formations. This has raised the amount of
wellbore instability issues encountered while drilling carbonate formations
worldwide.
Rock failure will take place when the rock mass in the field experiences a huge
stress beyond its strength. Building models that can predict the circumstances
under which a rock might fail, would massively help in preventing severe
issues such as borehole collapsing and solids production. Hence, the most
appropriate mud density and borehole trajectory should be known in advance
before drilling through proper geomechanical modeling. In this thesis, a linear
elastic model for the stresses in the field is used together with the Mogi Coulomb failure criterion as a geomechanical stability model. This three dimensional stability model reflects and represents the impact of all existing
principal stresses in the fields. The stability model is used to generate a several
charts to be available and accessible for all drilling engineers and operators in
Oman.
For carbonate rocks, the stability charts provide the minimum allowable mud
density to prevent borehole collapse for all borehole trajectories. The
developed charts are specified for some ranges of rock geomechanical
parameters due to the focus on the carbonate formations of Oman and the most
encountered options. The developed stability charts are categorized according
to the input parameters. The effect of each geomechanical property is studied
to understand the relationships among them.
Real field case studies are conducted utilizing the developed charts for
carbonate formations in Oman and outside the country. The applied mud
density and the associated drilling trajectory in the studied fields was consistent
with the predicted results from the developed charts. These results encourage
the usage of the stability charts to simplify the analysis and reduce the borehole
instability issues represented by borehole collapse in carbonate formations.
