Strength Prediction and Evolution in Shales

Shale strength is an important parameter for wellbore stability, trap integrity, hydrofracturing and various types of subsurface geological storage. However, strength of shales is not well constrained due to limited available data. This study is in two parts: 1. Empirical correlations between physical and petrophysical properties of a range of shales from basins worldwide to static and dynamic elastic properties. Here, good correlations were found between porosity and normalised cation exchange capacity (CEC) to cohesion and unconfined compressive strength. CEC normalised to density proved a useful parameter in these empirical correlations. Friction was harder to derive an empirical relationship for, although a complex relationship was found with good correlation although of dubious statistical merit. 2. Evolution of static and dynamic mechanical properties with compaction and diagenesis. Investigations of a sequence of shales from a well in the Officer Basin in Western Australia and showed significant increases in static mechanical properties with chemical compaction likely the dominant mechanism. P- and S-wave velocities both parallel and normal to bedding also increase with depth and the anisotropy of velocity decreases. While these results are somewhat intuitive, this is believed to be the first laboratory-based study to show such a result.