In last article, the rock property was introduced, here we will give the introduction of rock drillability.
Rock Mechanical Classification
- Rock Properties that have an overall effect on the drillability are as following:
- Hardness
- Abrasiveness
- Texture
- Structure
- Breaking characteristics
Hardness
- Mohs’ scale and VHN comparison
Mineral Mohs’ scale VHN (Vickers Hardness Number) Diamond 10 4500 Corundium 9 2300 Topaz 8 1680 Quartz 7 1060 Feldspar 6 730 Apatite 5 520 Fluorspar 4 265 Calcite 3 125 Gypsum 2 50 Talc 1 20 Cemented Carbide 6.5
- Uniaxial Compressive Strength(UCS)
- Specimen inserted in the testing machine between the loading platens with deformation measurement equipmentattached on the specimen
- Rock Hardness Classification
Gabbro —– Gniess —— Granite —— Limestone —— Marble —— Quartzite —— Sandstone —— Slate —— Shale —— Taconite Mpa 5 10 15 20 25 30 35 40 Grade soft Medium Medium Hard to Very
Abrasiveness
- Parameter for drill bitwear
- It depends on the mineral composition in the rock
- Quartz(SiO2) content is usually a reliable indicator of drill bit wear
- % silica content: 10 – 20 Non abrasive, 20 – 40 Abrasive, >40 Very abrasive
Rock type Quartz Content % Amphibolite 0 – 5 Limestone 0 – 5 Mica 0 – 35 Pegmatite 15 – 30 Granite 20 – 35 Gneiss 15 – 50 Sandstone 25 – 90 Quartzite 60 – 100 - Cerchar abrasivity index(CAI)
- CAI = 10 x d(mm)
- 10mm in 1 second in rough surface
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- Drilling Rate Index(DRI):
- DRI is a relative measure of penetrationrate and used as a standard test procedure in the industry
- Rating 10 – 100, the lower the number, the harder to drill
- TheDRI is determined on the basis of two parameters:
- Rockresistance to crushing – Brittleness value S20
- Rocksurface hardness – Siewers J value (SJ value)
- Example:S20 Value = 50J-Value = 20DRI = 55
Texture and structure
- Texture refers to grain structure of the rock and will mainlpoy influence drilling speed such as
- grainsize
- porosity
- density
- Structure refers to properties and will mainly influence hole accuracy such as
- faults
- joints
- beddingplanes