oa Shear wave velocity prediction using improved effective medium model

Seismic prospecting and evaluation of the rock mechanical properties require knowledge of shear-wave (S-wave) velocity. However, frequently the S-wave logs are not available, especially in past years. Prediction of Swave velocity is still attracting researchers’ attention because of its importance in petroleum exploration and engineering application.

A new rock physics model of clay-sand mixture is developed based on Self-Consistent Approximations (SCA) and Gassmann model. The model assumes that total pore space consists of two parts: (1) pores from sand grains and (2) pores from clays. The elastic moduli of clay-fluid mixture and sand-pore mixture are obtained by SCA. The clay-fluid mixture first fills up the total porosity, then replace the matrix (sand grains), taking the form of structure clay. The model can explain the distinctive changing relations between clay content and compressional velocity of the sand-clay mixture observed by Marion.

The model can be applied to predict compressional and shear wave velocities of the rock using conventional logs such as acoustic velocity, density and gamma ray logs. Parameters of the sand clay mixture model are chosen according to the fitness between predicted and measured compressional wave velocity. After that, we use the improved model to predict shear wave velocity, and the computational results indicate that the predicted shear wave velocities agree well with the measured velocities. The reliability of the shear wave velocity estimation can be validated by the predicted and measured compressional wave velocities fitness.