RE:What isI've been on wells where the ROP (rate of penetration) was as low as 1 - 2 meters per hour with the bit changed every 30 to 40 hours of drilling. Usually in highly abrasive, well cemented sandstones, siltstones. By cement you geologists refer to the pore space between grains being filled with something that was once liquid and is now solid. Carbonate compounds I think.
.Anyway if the formations have silicates like sand or silt which are very hard on the Mohs hardness scale where the grains are cemented together it will wear the bit out faster because more weight has to be put on the cutters in a PDC bit to crush the rock rather than scrape it out like the cutters do with a poorly cemented rock.
Dolomite and limestone are also a pain to drill but the worst is chert as reported by drillers.