Calibrating log-derived UCS profiles in a limestone-shale formation using Equotip non-destructive strength testing

An essential parameter in every geomechanical model is the log-derived UCS profile. Numerous empirical equations exist for different lithologies or different regions to compute UCS from logs. In the absence of laboratory rock strength measurements these equations are mostly used as published. However, since every formation and reservoir is unique in its geological origin and history, a customized UCS correlation is always preferred.

In this study rock strength data was necessary to assess wellbore stability issues for a limestone formation with thin intercalated shales located in Triassic Northern Calcareous Alps basement of the Vienna basin. The lab-derived UCS obtained from index strength measurements (Equotip) on offset wells was used to check and recalibrate the UCS correlations in the existing geomechanical model. Our results show that UCS obtained from Equotip index strength measurements deliver reliable data that can be used to calibrate log-derived UCS profiles in absence of other rock strength data. Furthermore, in our example we were able to create correlations between UCS and mineralogical-geochemical rock composition that could be extended into uncored intervals using compositional data from cuttings.