This document discusses core recovery methods used in drilling. It describes rotary core drilling where a tube with teeth is rotated to cut a cylindrical core sample from rock. Parameters for measuring core recovery include total core recovery and solid core recovery. Rock Quality Designation (RQD) is also discussed, which evaluates core pieces over 100mm in length. Technical factors like bent drilling equipment and geological factors like soft or fractured rock can negatively impact core recovery.

1. TOPIC:CORE RECOVERY
PRESENTED BY:NARENDRA LAMBA

2. INTRODUCTION

3. DRILLING METHODS
• Rotary core drilling
• Rotary open hole drilling
• Percussion boring with cable tools
Percussion “down hole
hammer”(jackhammer)
• Probing

4. ROTARY CORE DRILLING
• A tube is armed at its base with teeth of a
material harder than the rock to be drilled,is
rotated and pushed downward , a circular
annulus will be cut out of the rock and a
cylindrical core will pass into the tube.
• Commonly the teeth consists of diamond set
into a metal matrix to form core bit.
• Flushing is provided

6. MEASUREMENT OF CORE RECOVERY
These are the parameters of core recovery
Total core recovery:
TCR =Total length of core recovered × 100
/Drilled length
Solid core recovery:
SCR=Total length of core in pieces > core
diameter × 100/ Drilled length

7. RQD
• Rock Quality Designation (RQD) is a modified
core recovery percentage in which the lengths
of all sound rock core pieces over 100 mm in
length are summed and divided by the length
of the core run. Pieces of core that are not
hard and sound should not be included in the
RQD evaluation even if they are at least 100
mm in length.

8. DIA:RQD

9. Technical factors affecting core
recovery
• (i) Bent inner tube so that: (a) the core will not travel up the tube and will be
subject to grinding; (b) it rotates with the outer tube again disturbing and grinding
the core; and (c) it fails to seat properly in the outer barrel resulting in total core
loss
• (ii) Failure of back-end bearing resulting in: (a) loss of core due to grinding; and (b)
grinding of core leaving flat faces
• (iii) Bent outer tube of core barrel resulting in: (a) failure of the inner tube to latch
in and thus loss of core; and (b) less than full diameter cores
• (iv) core spring missing, displaced, damaged, worn or not lubricated
• (v) badly worn or damaged crowns
• (vi) diamonds inside kerf damaged, worn or displaced causing core to jam in inner
tube
• (vii) worn stabilisers
• (viii) vibration induced by poor equipment, insecure rig mountings and hole
deviation
• (ix) blocked waterways (x) inadequate flow/pressure of flushing medium and
unsuitable flushing medium

10. Geological factors affecting core
recovery
• (i) soft friable ground due to alteration, weathering or leaching
• (ii) unconsolidated materials
• (iii) broken ground with clay infill
• (iv) soluble components removed by unsuitable flushing medium
• (v) low intersection angles with rock discontinuities (cleavage, open
bedding, joints, schistosity, foliation, etc.), particularly joints
following the core axis, and cleavage disking leading to a ‘rasher of
bacon’ effect in the inner tube
• (vi) high frequency of discontinuities per metre
• (vii) unexpected fault zones
• (viii) secondary porosity or vug development due to karstic solution
or dolomitisation or hydration of anhydrite
• S(ix) cavities induced by karstic weathering along joints and faults
and also mining (stopes and caved zones)