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 • Logger’s 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