Actually, the SE index is corresponded with the drillability of formations and the drilling parameters acted. It cannot explain simply, e.g. under the sam formation or rotary speed, higher penetration speed may cause a higher SE, and vice versa.
measured MSE vs theoretical MSE (~UCS) is an indication of the bid dysfunctions. There is no such thing as "drillability" or "fracability". The lower MSE is the higher ROP will be at a given WOB.
Thanks for your indication . Yeah, under given WOB the MSE is lower under higher ROP. But perhaps, drillability really exists in mining engineering.
`The most wide-used in mining are the classificaitons by drillability, where rocks are systematized by the time spent for drilling one meter of borehole by different machines and tools' A.S. Tanaino, Journal of Mining Science, Vol. 41, 2005.