Mechanical Rock Properties Prediction: Deriving Rock Strength and Compressibility From Petrophysical Properties Available to Purchase

A database of core-derived mechanical properties measurements has been used to develop new empirical correlations relating rock strength and compressibility to associated petrographic and petrophysical data. Automated fitting procedures translate these empirical observations into predictive algorithms for estimating mechanical properties from geophysical wireline logs. The database comprises ~600 triaxial compressive strength tests of sandstone-to-shale lithotypes and ~275 uniaxial and hydrostatic compaction tests of siliciclastic reservoir rocks (unconsolidated sands to tight gas sandstones). New predictive algorithms derive: shear strength from lithotype (arenite,wacke or shale) porosity and normal stress magnitude; Mohr-Coulomb cohesion and internal friction angle from porosity and total clay weight fraction; pore volume compressibility at initial reservoir stress conditions from elastic moduli. Observed trends in material properties defining the Cam clay elastoplastic constitutive model offer some constraints for approximating the onset of pore collapse and the evolution of rock compressibility with fluid pressure reduction under uniaxial strain boundary conditions.