Abstract

It has become increasingly clear to the oil and gas community that earth stresses at depth in sedimentary basins have a profound effect on wellbore stability. Drilling problems frequently occur due to severe mechanical instabilities at the borehole wall where stress amplification can exceed the strength of the rock. This is because the rock surrounding the hole must support the stress previously supported by the material removed in the drilling process. Drilling problems associated with lost circulation often occur where the borehole has intersected critically-stressed natural fractures that are inherently prone to high fracture permeability. In order to design a drilling and completion programme that eliminates or minimises these mechanical instabilities in the borehole, it is essential to understand the interaction between the stress field, pore pressure, natural fractures, rock strength, mud weight, and borehole trajectory.

In some cases wellbore performance can be maximised by selecting an optimal trajectory through the reservoir that can be drilled near balanced or under-balanced to minimise the formation damaging effects of mud infiltration, while other trajectories may require more aggressive drilling parameters. In these situations a well-constrained stress field is essential for determining the appropriate mud window to control compressive failure leading to the development of wellbore breakouts and, at the same time, prevent catastrophic tensile failure leading to formation breakdown or fluid losses through natural fractures.

This paper serves to illustrate how a well-constrained geomechanical model can be used to address a suite of drilling and completion problems. Case studies to be reviewed include wellbore stability and completion practices in extended reach wells (North West Shelf), wellbore stability in vertical and deviated wells (North Sea), drilling and completions in complex geological environments associated with steeply-dipping bedded shales (Colombia), and lost circulation in highly fractured regions (Timor Sea).