Effects of porestructure on sonic velocity in carbonates

The presence of round pores generally causes a positive deviation from Wyllie's equation (Anselmetti and Eberli 1993, 1999; Saleh and Castagna 2004). However, carbonates contain a large number of different pore types that are not related roundness alone. Three quantitative pore shape parameters derived from digital image analysis are introduced to capture the complicated pore structure of carbonates with the goal to enhance porosity prediction from velocity. The first parameter that describes the roundness of the pores was first introduced by Anslemetti et al. (1998) and called γ. The second parameter Perimeterover-Area (PoA) captures the overall tortuosity of the pores system. The third parameter, Dominant Poresize, is a measure of the dominant pore size within the thin section. Out of these three parameters, PoA is the most dominant factor controlling velocity at a given porosity with Dominant Poresize being second, while roundness alone is the least important factor of the three. We conclude that the roundness of individual pores is not as relevant as the simplicity of the pore system, i.e., the pore system with low tortuosity. Combining all three parameters and porosity in a multivariate linear regression increases correlation to velocity from R² of 0.49 (porosity alone) to R² of 0.78.