Shale Stability – A Discussion on Lab Strength Testing and Wellbore Stability Predictions

Rock core samples may be tested by destructive methods to determine the material’s mechanical strength. These measurements may however introduce further uncertainties, which often evolve around how to apply the laboratory derived data to subsurface conditions. Indeed, strength discrepancies have been reported between cylindrical plugs and hollow cylinders, which suggest a geometry issue. A scaling concern also presents itself when using small laboratory samples to represent an operationally exposed rock volume. In addition, matching the correct failure model to the lab measurements is often ambiguous. This article reflects on these issues from a wellbore stability perspective. A case is presented where testing datasets (including both triaxial and hollow cylinder tests) are used to characterise the material’s strength suite. Implementing these findings to a wellbore stability calculation shows how an incomplete or poorly interpreted dataset may be misleading, thereby highlighting the inherent uncertainties and difficulties associated with borehole stability modelling.