Empirical relations between rock strength and physical properties in sedimentary rocks

In this study, 31 empirical equations are summarized that relate unconfined compressive strength and internal friction angle of sedimentary rocks (sandstone, shale, and limestone and dolomite) to physical properties (such as velocity, modulus, and porosity). These equations can be used to estimate rock strength from parameters measurable with geophysical well logs. The ability of these equations to fit laboratory-measured strength and physical property data that were compiled from the literature is reviewed. Results from this study can be useful for petroleum industry when a range of geomechanical problems such as wellbore stability and in-situ stress measurements should be addressed without direct strength information available. While some equations work reasonably well (for example, some strength-porosity relationships for sandstone and shale), rock strength variations with individual physical property measurements scatter considerably, indicating that most of the empirical equations are not sufficiently generic to fit all the data published on rock strength and physical properties. This emphasizes the importance of local calibration before one utilizes any of the empirical relationships presented. Nonetheless, some reasonable correlations can be found between geophysical properties and rock strength that can be useful for applications related to wellbore stability where having a lower bound estimate of in situ rock strength is especially useful.