Vernik (Vernik, 1993, modified)

UCS = 145 ´ (A – B ´ Vclay)(1 – C ´ Φ)²

145 is a conversion factor from Mpa to psi.

A, B and C are non-dimensional constants.

Default values: A = 254; B = 204; C = 2.7.

The modified Vernik equation gives reasonable results for consolidated sandstones (Phi < 0.30) that are grain-supported (Vclay < 0.30). It was calibrated using a wide variety of measurements of samples in the lab and works best in consolidated sandstones. The equation requires values for porosity and Vclay, both of which can be calculated from well logs. The choice of values for the constants A, B, and C in the equation is empirical. A is UCS in a zero porosity clean sample. B is UCS in shale and C may be adjusted according to core data (lower C value for Phi > 0.30). The original form of the Vernik equation did not have a Vclay correction (i.e., B ´ Vclay = 0), but applying a clay correction has been found to improve the results. Alternatively, different values for the constants A and C could be used to account for varying shale content, particularly when the equation is calibrated against core data.