A rock physics analysis based on inversion of poroelastic Brown and Korringa parameters in Vaca Muerta shale: Theoretical aspects and applications

Abstract

In their classical paper, Brown and Korringa (1975) developed a theory to describe the elastic behavior of porous saturated rocks with microheterogenous frames, valid for isotropic and anisotropic media. However, it has been almost unused for practical applications in geophysics due to the difficulties in the determination of the parameters involved. With these ideas in this work we propose and test a simple modeling workflow for composite porous rocks which does not require detailed petrophysical information about mineral volume fractions and their corresponding elastic properties. This is particularly convenient when dealing with multiphase rocks, such as organic-rich shales, which are formed by many different minerals, organic matter, and pore fluids. The physical parameters of such amount of constituents may introduce errors and uncertainty in the computations. To overcome this problem we show that it is possible to calibrate the model coefficients using a numerical inversion procedure without using a detailed description employing Brown and Korringa (1975) and Gassmann (1951) formulations. We illustrate the application of the procedure using real data of the Vaca Muerta shale formation. We analyze appropriate search ranges for the different model coefficients, with special attention to the feasibility of the inversion of the unjacketed pore modulus from velocity data. The overall goodness of fit between real and synthetic velocities is quantified and the statistical distribution of the inverted coefficients, as well as the determination of the critical porosity in this shale, are also discussed.