Determination of Primary and Secondary Porosity in Carbonate Formations Using Acoustic Data

Abstract

Abstract A technique for quantitative estimation of crack or vug porosity using acoustic well-log data is proposed. The estimation is based on a double-porosity rock model that consists of a solid matrix with primary porosity and inclusions which represent the secondary pore system. We applied the method of effective medium approximation (EMA) to calculate P- and S-wave velocities in the double-porosity formations. This method allows modeling multicomponent systems composed by grains of different mineral components, primary pores and secondary inclusions. All these components are approximated by spheroids with different aspect ratios. The technique includes two steps. At the first step the matrix P- and S-wave velocities as functions of the primary porosity are calculated. For this procedure two groups of parameters are required: 1 - solid-skeleton elastic moduli that are defined by laboratory measurements or calculated using formation mineralogical composition; 2 - aspect ratios for grains and primary pores. We determined these aspect ratios by the inversion of the published regression equations for P- and S-wave velocities to the microstructure parameters. At the second step, using the obtained previously matrix parameters, the value and aspect ratio of secondary pores are determined by the joint inversion of P- and S-velocities (minimizing the difference between the experimental data and velocities predicted for the double-porosity model). The obtained aspect ratio indicates the type of secondary-pore system (cracks or vugs). Examples of secondary-porosity determination using acoustic core and well log data are presented.