Using anisotropic effective medium theories to quantify elastic properties of sandstone-shale laminated rocks

Abstract

We have developed a new rock-physics model based on a combination of effective medium theories to reconstruct rock fabric, and we have estimated the elastic properties of laminated hydrocarbon-bearing shales. This innovative model is based on the separate treatment of isotropic sandstone and vertical transverse isotropic (VTI) shale volumes of each layer; the model reproduces many typical properties of laminated shales, such as alignment of cracks, alignment of clay platelets, distinction between isolated and connected porosity, and presence of fractures. Moreover, the model enables the simulation of Stoneley velocity based on the estimated stiffness of the rock in addition to compressional and shear velocities commonly calculated with rock-physics models. Finally, it is a joint model that also calculates vertical and horizontal electrical resistivity while keeping a consistent microstructure for the elastic and resistivity simulations, thereby decreasing nonuniqueness of the solution. Nonuniqueness is an inherent flaw of effective medium modeling and has rarely been addressed in previous studies. Our simulations assumed VTI elastic behavior and suggested the presence of compliant horizontal pores and natural fractures. Our model was then validated by comparing computed properties against compressional and shear velocity and, when available, Stoneley velocity and resistivity logs. Our simulation method is implemented in two wells in the Haynesville Shale and in one well in the Barnett Shale, giving rise to accurate estimates of wave velocities, with errors lower than 5.9% for wells in the Haynesville Shale and 3.8% difference for the well in the Barnett Shale. Identification of suitable depth zones to drill a horizontal well or to initiate fractures is then based on the inferred elastic properties of the rocks.