•Rock drillability is defined as the penetration of a drill bit into the rock expressed in metres per minute •Rock drillability is a function of the intact rock strength, mineralogy, abrasiveness, degree of jointing, and grain size. •It is normally determined in the lab by assessing the drilling rate index (DRI) and the bit wear index (BWI). For most rocks the Bit Wear Index and Drilling Rate Index are inversely proportional; if a rock has a low BWI, the DRI is high, and vice versa. Table 1: Classification of Drilling Rate Index and Bit Wear Index for rock formations with quartz content of 10-40% | Drilling Rate Index, DRI | Bit Wear Index, BWI | Extremely Low | 21 | Extremely High | 63 | Very Low | 28 | Very High | 53 | Low | 37 | High | 43 | Medium | 49 | Medium | 33 | High | 65 | Low | 23 | Very High | 86 | Very Low | 13 | Extremely High | 114 | Extremely Low | 3 |
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