A Method to Predict S-Wave Velocity from Wire-Line Logs for Organic- Rich Shales

Summary The absence of S-wave velocity in logging adversely affects pre-stack seismic inversion, brittleness index calculation, and reservoir prediction for organic rich reservoir. This paper presents a simple but effective method for S-wave velocity estimation from P-wave velocity based on the combination of several rock physics equations and simulated annealing (SA) non-linear global optimization algorithm. In the proposed method, the critical porosity-consolidation coefficient (CPCC) model, Gassmann equation and Voigt-Reuss-Hill(VRH) average equation are integrated to establish the relationship between velocity and two effective parameters (critical porosity and consolidation coefficient). The S-wave velocity of organic-rich rock can be related to P-wave velocity through the critical porosity and consolidation coefficient in the integrated model. Thus, the critical porosity and consolidation coefficient can be inverted from P-wave velocity by the unified model. Finally, S-wave velocity of saturated organic rich rock is calculated with the inverted parameters. The proposed method is applied to the measured data in laboratory. And the results show that the predicted data are in good agreement with the measured data, demonstrating the validity and applicability of the method for organic rich rock. What’s more, compared with two single-parameter adaptive methods, the prediction results show that the proposed method is superior.