Horsrud-PHI (Claystone, Shale)

 

where

424.7 is in psi.

-0.96 is a non-dimensional constant.

This equation is based on laboratory measurements on North Sea shales. The rocks were mainly high-porosity Tertiary shales with very few compacted and well-consolidated samples. When the equation is applied to the source data, the uncertainty is estimated to be up to 50% for weak samples (UCS< 1,500 psi) and between 10 to 20% for stronger samples (UCS > 7,000 psi). Porosity of shales should be calculated from porosity logs (Dt, rB, NPHI), as almost all of the published porosity equations are for reservoir rocks only and not for shales.  Therefore, there might be an additional error in the calculated UCS data using this formula due to uncertainties in the porosity values for shales. The Horsrud-Vp model is preferred for this reason. Also, Horsrud’s method for measuring porosity may result in overestimates due to clay dewatering that starts to occur close to his drying temperature.