**4. Evaluation method based on acoustic wave information**

Many studies have been made to evaluate the rock drillability anisotropy with the core testing method (Gao & Pan, 2006) and the inversion method (Gao et al, 1994). However, as for the core testing method, its result may not reflect the actual rock drillability anisotropy since the experimental conditions are different from the downhole conditions. Moreover, the profile of rock drillability anisotropy along the hole depth can not be established because of the limitation of the core samples. The inversion method needs to work with a bottom hole assembly (BHA) analysis program and some parameters in the inversion model are not easy to obtain so that its applications are limited to some extent. Thus, the evaluation method will be presented in this section so as to predict rock drillability anisotropy of the formation by using the acoustic wave information (Gao et al, 2008).

### **4.1 Acoustic wave velocity of the formation**

The formation studied here is the transversely isotropic formation which is frequently encountered in drilling for oil & gas. Experimental investigation shows that layered rock has the transversely isotropic characteristics.
