The uniaxial compressive strength (UCS) of sedimentary rocks is one of the key parameters in petroleum related geomechanical projects from hydraulic fracturing to the estimation of in-situ stress magnitudes. Due to the scarcity of the rock samples the UCS is often correlated to the interval transient time (equivalent to P-wave velocity) for the field applications, however; the size-dependency of UCS seems to be overlooked in such applications. While the conventional understanding of UCS-size dependency is that the UCS of intact rock decreases with an increase in sample size, recent studies have shown that the UCS increases with an increase in sample size up to a characteristic diameter and then decreases afterward (ascending-descending behavior). Therefore, in this study, a set of uniaxial compressive tests were conducted on shale samples having different diameters. The obtained results were combined with those available in the literature and the Unified Size Effect Law was then fitted to the whole data set. A generalized correlation was proposed for extrapolation of the UCS of rock formations from the USC of rock sample having 25 mm diameter. The calibration of a UCS-interval transient time correlation is also given for the shale sample as a practical example. Finally, it was suggested that the procedure proposed here is adopted as a standard practice in petroleum related geomechanical projects for higher accuracy in design processes.
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