Rock Coring

Obtain undisturbed samples of solid, fractured, or weathered rock

formations

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

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

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

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

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

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

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

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

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

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

Borehole Logging

• Written record of all field work

• Prepared in the field

• Eventually converted to a final report log

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

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

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

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

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

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