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Core Drilling

Core Drilling

TonaTec Management has over 125 combined years of core drilling experience in every application and condition. Core drilling services are the backbone of our operations and most other services and specialty are offered in conjunction with core drilling.

The core barrel assembly consists of an inner and outer barrel, bit, reaming shell, latch and rotating head assembly, overshot, wire line cable and an adapter sub to match the threads on the drilling rod (pipe). Core Drilling utilizes a hollow centered coring bit, much like a donut. The bit kerf (or the donut) has cutters, usually very small diamonds, which are set on the surface or embedded in the matrix of the bit kerf. As the bit is forced against the bottom of the hole and rotated, the diamonds on the bit kerf cut through the rock. This creates a round ring, or channel, around the rock to be sampled called the “core.” As the core bit drills ahead, the retrievable inner barrel slides over core (undisturbed rock) which is still attached to the bottom of the drill hole. After the inner barrel is full or blocks off when broken pieces or swelling core becomes lodged inside the inner barrel and will not allow additional core to enter, it is brought to the surface typically by an overshot via wire-line cable which is lowered or pumped into the hole and latched onto the top of the inner barrel for retrieval of the core sample.

Following is a breakdown of the Core Drilling process:

Step 1

The drilling string (pipe) is rotated which transfers rotational force to the outer barrel, the reaming shell, and the core bit.

Step 2

The core bit cuts a circular channel around the core, preserving a sample of the rock being drilled.

Step 3

The reaming shell is attached to the lower end of the outer barrel (between the outer barrel and the core bit) and has a cutting surface which keeps the diameter of the drill-hole in gauge (the correct size). As the outside diameter of the core bit gradually begins to wear, it will drill a smaller diameter hole and the reaming shell will correct for this wear allowing a new core bit to be placed into the hole without having to ream it into the hole.

Step 4

The inner barrel, in theory, does not rotate while drilling ahead; it slides over the core as the outer barrel, reamer shell, and bit rotate around it. The inner barrel has a catcher on the bottom (core lifter) which sits inside the bit and lets the core enter the barrel but will not let it fall back out.

Step 5

The inner and outer barrels are connected by the latching / rotating head. It has latches that keep the inner barrel locked in place inside the outer barrel and a bearing spindle assembly which allows the outer barrel to rotate while the inner barrel remains stationary so it can slide over the core with as little interference as possible.

Step 6

The overshot has a latching mechanism that connects to spear point on the top of the latching head to permit retrieving the inner barrel from the bottom of the hole without the entire drill string having to be removed.

Step 7

The wire-line is a small diameter cable that is used for lowering and/or pumping the overshot into the hole to latch onto and retrieve the inner barrel containing the core.

Step 8

Once the inner barrel containing the core sample is retrieved from the hole an empty inner barrel is then dropped, lowered, or pumped (depending on the angle of the hole or the amount of drilling fluid column in the hole) from the surface through the drill string and latched into the outer barrel and the entire process is repeated.

The most commonly used inner barrel is 10 feet long so this cycle would be repeated every 10 feet unless the core sample is badly broken and will not slide the entire length inside the inner barrel. Shorter core barrel assemblies are commonly used in broken or fractured rock which blocks or jams going into the inner barrel more frequently. Longer barrel assemblies of 20 feet or more can be used in competent rock or in ultra deep-hole situations where the inner barrel retrieval times are excessively long.