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

Harness

  • Mohs’ scale and VHN comparison
  • Uniaxial Compressive Strength(UCS)
    • Specimen inserted in the testing machine between the loading platens with deformation measurement equipmentattached on the specimen

 

 

 

 

  • Rock Hardness Classification

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
  • Cerchar abrasivity index(CAI)
    • CAI = 10 x d(mm)
    • 10mm in 1 second in rough surface

 

 

 

 

  • 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

 

 

 

 

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