Methods of velocity prediction tested for North Sea chalk: a review of fluid substitution and vS estimates

Abstract

This study investigates two cases of velocity predictions for North Sea chalk: fluid substitution of compressional velocity ( v P) and estimation of shear velocity ( v S). Both high frequency laboratory and intermediate frequency logging data are included. We test five rock physics methods and two empirical relations that have been established or tested for other lithologies. The predicted velocities are tested against measured velocities or velocities estimated from Gassmann's relations, which are confirmed to give good velocity estimates for water-saturated chalk from data on dry samples. In one case, we test predicted fluid effects on compressional velocity without knowing shear velocity. The fluid substitution effect on the compressional velocity is well estimated from all the four methods: the bound averaging method (BAM), the isoframe method, the method of replacing bulk modulus in the Gassmann's relations with P-wave modulus and the method of decomposition of measured P-wave modulus into shear and bulk modulus. In the other case, we test shear velocity estimated from compressional velocity. We recommend the isoframe method for shear wave estimation for saturated chalk. All methods tend to underestimate shear velocity in dry chalk. For water-saturated chalk, also the empirical second-order polynomial relation between compressional and shear velocity of water-saturated limestone of Castagna et al. [Castagna, J.P., Batzle, M.L., Kan, T.K., 1993. Rock physics—the link between rock properties and AVO response. In: Castagna, J.P., Backus, M.M. (eds.), Offset-Dependent Reflectivity-Theory and Practice of AVO Analysis, Investigations in Geophysics, no. 8. Society of Exploration Geophysicists, 135–171.] gives good shear velocity estimates. The empirical constant ratio between compressional and shear velocity of water-saturated limestone of Pickett [J. Pet. Technol. 15 (1963) 650.] overestimate low shear velocities for high porosity chalk and the shear velocity is systematically underestimated by the method of decomposing P-wave modulus.