Rock Coring

1
Rock Coring

• Obtain undisturbed samples of solid, fractured,
  or weathered rock formations

2
Purpose

• Exploration for structures
• Exploration for mineral deposits, rock quarries,
  and structural studies
• Test for load bearing capacity, hydraulic
  conductivity, porosity, and mineral or chemical
  content

3
Conventional Coring Systems

• Use a core barrel attached to the end of a drill
  rod string
• Rod string and core barrel assembly are removed
  after each cutting run to recover the sample
• The core barrel is emptied and returned to the
  borehole for the next run

4
Wire-line Coring Systems

• More efficient that Conventional Coring Systems
• Use an outer barrel and a core casing
• An inner barrel can be lifted through the drill
  string to the surface
• The inner barrel is lifted to the surface with a
  wire-line device, then lowered back in

5
Drilling Fluids

• Diamond or Carbide bits require cooling and
  circulation fluids
• Void spaces in formations can cause loss of fluid
  circulation, indicated by loss of return fluid
• Cuttings can accumulate in void spaces, resulting
  in a loss of or damage to equipment
• Filled void spaces affect borehole testing

6
Problems

• Rock mass surrounding borehole not
  self-supporting
• Protruding material in borehole
• Small diameter, vertical boreholes are generally
  more stable
• Friable sandstone and solution channeled
  limestone or dolomite are more susceptible to
  wall failure

7
Solutions

• High density drilling fluids plug fractures and
  temporarily stabilize the borehole wall
• Installing casing in the borehole and reducing
  the size of the hole beneath the casing
• Cement hole and redrill after the cement has
  hardened

8
Minimizing Core Losses due to erosion of material
from fluid circulation

• Bit discharge directed away from core
• Drilling fluid use kept to a minimum
• Polymer compounds added to drilling fluid
• Vibration of drill rod minimized
• Controlled advancement of the drill rod
• Core-catching devices installed

9
Rock Core Logs

• To record all relevant information about the core
  and to make a field description
• Contains
• Percent core recovery
• Amount and location of core loss
• Depth of the beginning and ending of each core
  run

10
Handling Procedures

• Properly label and identify all samples
• Sample packaging depends on sample types
• Samples for chemical analysis should remain
  undisturbed, subsampled in the field and sealed
  immediately
• Chain of custody forms

11
Rock Core Samples

• Rigid container required for transport
• Core placed into box left to right like top to
  bottom so it reads like a book
• Mechanical breaks in core should be distinguished
  from fractures
• Core can be labeled on core surface
• Depth, core loss points, fractures

12
Borehole Logging

• Written record of all field work
• Prepared in the field
• Eventually converted to a final report log

13
Log Heading Information

• Project name and number
• Property location
• Surface elevation
• Surface conditions
• Type of drilling rig or pumping equipment
• Bit size and type
• Loggers name
• Borehole coordinates

14
Log Completion Information

• Should include the time and date that drilling
  started
• Time and date of completion
• Total depth drilled
• Total depth cased
• Abandonment procedures used, if any
• Final water level measurement

15
Sample Information

• Sampling method
• Number of blows required to advance split-barrel
  sampler
• Size and type of sampler
• Sample number and depth
• Lengths of drives, pushes, or core runs
• Length of recovered sample and percent of
  recovery
• Rock quality designation (RQD)
• Portion of the sample saved or submitted to lab

16
Soil and Rock Descriptions

• Sample description should be done in field
• Rock and Soil classification schemes are widely
  studied and very important when classifying
  samples
• Knowledge of locality of sampling is very valuable

17
Drilling Information

• Records of drilling information should be made
  for contract records, future site work, and cost
  evaluation
• Borehole logging form should have separate
  drilling information and sample description
  columns

18
Drilling Information

• Changes in penetration rate and drill rig
  conditions
• Mechanical performance and maintenance
• Conditions of circulation fluids and cuttings
• Penetration rate and bit pressure
• Changes in personnel