Estimating Petrophysical Parameters from Acoustic Impedance and P to S Wave Velocity Ratio Using a Simple Rock Physics Model

In reservoir characterisation, rock physics analysis has been frequently used to describe petrophysical properties of rocks based on their elastic behaviour. Recently, a new rock physics model that facilitates direct estimation of porosity and fluid saturation from acoustic impedance (AI) and P to S wave velocity ratio (VP/VS) was introduced. The method has the flexibility to adapt the rock physics model to the data by simply adjusting two parameters without the need to consider the detailed elastic moduli. In this study, a slight modification is proposed by considering the influence of fluids on VP/VS ratio to better comply with the Gassmann equation. A workflow for obtaining optimal model parameters for the observed data by incorporating the curved pseudo elastic impedance (CPEI) approach is also demonstrated. The modified method successfully produces a model that fits the data. The proposed workflow effectively finds optimal model parameters, resulting in better estimation of petrophysical properties. However, a detailed examination of the results shows that variations in shale volume should be incorporated to obtain better petrophysical estimation results. This method offers a simple yet reliable way to estimate porosity and fluid saturation directly from AI and VP/VS ratio typically obtained from the pre-stack seismic inversion.