What is rock drillability?
Drillability of rock refers to the ability of rock to resist pressure and crushing during drilling, it also indicates the efficiency of footage, and it is also an index of rock solidity that indicates the difficulty of drilling blastholes (Alicia, 2023). Drillability, in other words, may be measured by the drilling speed (centimeter per minute) at which a drill bit penetrates the rock mass. A drillability factor has been determined for all drilling conditions from a performance study of rock drilling jobs both in the field and in the laboratory (Singh & Goel, 2011).
Which parameters affect rock drillability?
Rock drillability is influenced by a number of parameters, including rock hardness, texture and density, rock fracture pattern, general structure of the formation/rock mass.
The features of the drilling equipment are not taken into consideration by these metrics. Every property on the list that influences drillability is taken into account independently. A skilled driller is able to predict how a rock will drill. The important thing to know is how fast it will drill.
Hardness
The Mohs hardness scale can be used to determine a mineral’s hardness. A higher number indicates that the mineral is more difficult than the one after it. Higher-numbered minerals have the ability to scratch others with the same or lower number. A piece of rock should be tested multiple times to determine its average hardness because rocks might include multiple minerals. In the field, the Mohs hardness test kit for minerals can also be utilised.
Texture and density
Fine-grained rocks with tightly packed mineral grains generally offer higher resistance to drilling due to their cohesive structure, reducing drill bit penetration rates. Coarse-grained rocks may exhibit variable drillability depending on grain bonding strength. Grain orientation and interlocking patterns further affect energy transfer during drilling, influencing efficiency. Porous rocks often require less mechanical energy to fracture, as void spaces reduce overall rock cohesion. Dense, low-porosity rocks demand higher drilling forces due to stronger intergranular bonds (Askaripour et al., 2022).
The type and strength of mineral cement (e.g., silica, calcite) directly impact drillability. Strongly cemented rocks with quartz or carbonate binders resist bit penetration more effectively than weakly bonded formations.
Rock fracture pattern
The arrangement, orientation, spacing, and continuity of fractures or joints within a rock mass—plays a significant role in determining how easily a rock can be drilled. Rocks with closely spaced fractures or joints generally exhibit higher drillability because the fractures act as planes of weakness, making it easier for drilling tools to break and remove material (Xu et al., 2024). Crack propagation during drilling is greatly influenced by the orientation (dip angle) of fractures with respect to the drilling direction. While some angles can improve drillability by facilitating crack extension and fragmentation, others may make the operation more difficult.
Formation/rock mass
Formation describes the condition of rock mass structure. A high drilling rate is possible in massive rocks, whereas slow drilling is obtained in blocky and seamy rock masses.
How to quantify rock drillability?
Drilling Rate Index (DRI)
Developed in Norway, the DRI integrates two laboratory tests:
- Sievers’ J-miniature drill test: measures surface hardness by drilling a 1/10 mm hole depth after 175–200 revolutions.
- Brittleness test (S₂₀): quantifies resistance to impact by measuring the percentage of material passing through an 11.2 mm mesh after 20 hammer drops.
DRI is determined graphically using these two parameters, providing a standardized measure for comparing rock types.
Schreiner Hardness Test
This method applies increasing load via a cylindrical punch until rock rupture. It correlates hardness with sonic velocity and specific disintegration (volume of rock broken per unit work). Results can predict drillability when combined with sonic log data, adjusted for mineral composition.
Reference
Alicia. (2023, August 10). What is the drillability of rock? – Bestonedrill – 10+ Years Chinese Rock Drilling Golden Supplier. https://www.bestonedrill.com/what-is-the-drillability-of-rock/
Askaripour, M., Saeidi, A., Mercier-Langevin, P., & Rouleau, A. (2022). A Review of Relationship between Texture Characteristic and Mechanical Properties of Rock. Geotechnics, 2(1), Article 1. https://doi.org/10.3390/geotechnics2010012
Singh, B., & Goel, R. K. (2011). Chapter 22—Rock Drillability. In B. Singh & R. K. Goel (Eds.), Engineering Rock Mass Classification (pp. 287–292). Butterworth-Heinemann. https://doi.org/10.1016/B978-0-12-385878-8.00022-7
Xu, P., Zou, J., Huang, X., & Han, J. (2024). Numerical analyses of the influences of rock properties and joints on rock fragmentation in shaft sinking by drilling method. Scientific Reports, 14, 24870. https://doi.org/10.1038/s41598-024-75767-6
