Wellbore Instabilities: An Integrated Geomechanical Model Across the Silurian Shale Formation in Saudi Arabia

Many wells were drilled through a thick Silurian Shale Formation in Saudi Arabia. The wells do not usually present major drilling problems but some wells encountered severe wellbore instabilities and have to be sidetracked or abandoned. Silurian Shale Formation is controlled predominantly by the mud weight density, time and number of casing strings that isolates shallower formations. Currently, the wells being drilled in some regions of Saudi Arabia have significant intervals of the Silurian Shale Formation combined with the underlying Ordovician clastic rock unit in a single hole section.

A detailed analysis was conducted to optimize the drilling performance and mitigate wellbore issues. Geomechanics study and monitoring of drilling behaviors indicates that there are three interrelated factors affecting the wellbore stability across the Silurian Shale Formation: drilling-time dependency, mineralogy and formation anisotropy.

This paper will present the detailed modeling of the Silurian shale instability and how it is utilized to overcome the unstable behavior of the Silurian Shale Formation. This model gives the capability to select suitable mud weight density to drill through the Silurian Shale at any inclination with the maximum time required to deliver the well successfully.