Efficient Core Recovery

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Vital information

Recovered core samples are used to extract vital information where chips or other data gathering techniques do not provide an elevated level of confi-dence required to make decisions, be they related to mineralization or ground

mechanics. The quality of recovered core samples is of paramount impor-tance, and is influenced by the person-nel, capital and support equipment, diamond products, tools and accesso-ries used in given ground conditions.

In The Hole (ITH) tools represent only a small component of total project cost, but their selection and use are hugely significant to the outcome and overall success of a drilling project. Understanding the end user, his goals, and in particular his application, influ-ences how efficient coring and com-mercial solutions are developed and implemented.

Core recovery

Core can be generally defined as a volu-metric cylinder of material, created by the advancement of a rotating hollow centred diamond drill bit through an

in-situ formation, and subsequently removed.

Core recovery is a quantifiable mea-surement defined as the total linear amount of physical core sample extracted over the total linear advance in a bore- hole, expressed as a percentage. Reco- very is often measured against a section of advance, typically in the target zone and/or for the entire borehole.CR (%) = Length of core X 100

Length of advanceThe core being created is encapsu-

lated within, and subsequently extract-ed by, a retrievable sampling device called a core barrel. The core barrel is a mechanically designed device consist-ing of many interconnected engineered components. It is connected to a con-sumable core drilling bit, typically made with synthetic diamonds, which is the core cutting tool. As the drill bit penetrates through the material, it

Efficient Team: Geologist, Driller, Atlas Copco representative.

efficient core recovery

Core in the boxThe end users of exploration core drilling products are generally in the business of, and compensated for, providing representative and quality physical core samples to their customers, so nobody wins without “putting core in the box”. Core drilling is typically carried out either by contracted drilling com-panies, or directly by some mining houses, consulting companies or governmental bodies who have their own drilling departments.

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creates a core in its wake, entering the core barrel until it’s recipient tube is full, or the core’s entry is impeded, at which time the sample recipient tube is removed, emptied of its core, replaced and drilling resumed.

Drilling diameters

A drilling application comprises the borehole starting point, or collar, the target and the path in between. Bore- holes ranging from 48 mm to 146 mm- diameter, and depths of 3 m to 3 000 m share the following: diamond core drilling rig; flushing circuit; drill rod string; core barrel; and diamond tools. Selecting which diameter to drill for a given application is dependent on a variety of factors, most of which are not economic drivers and are beyond the scope of this basic overview, but capital and tools must be adequately sized and suited for each other and the objective to yield efficient recovery. As a general rule: the larger the core diameter, the better the core recovery.

Core drilling rigs come in many shapes and sizes. The rig, properly an-chored at the borehole collar, primarily needs to transmit rotation, thrust and pullback forces. As a general rule: the smoother the transmission of rotational force, the better the core recovery.

Fluid flushing

The fluid flushing circuit can vary signi-ficantly, but typically consists of one or more pumps, the fluid media itself, the drill rod string, the available borehole annular area and the peripheral acces-sories for controlling delivery, treatment and handling. Fluid is primarily required to cool and flush the cuttings from the advancing diamond core bit, and evacu-ate them from the borehole. The flushing medium can be clear water, or include additives or muds to condition borehole integrity and complete the circuit. As a general rule, with deteriorating ground conditions, focus on the flushing circuit to improve core recovery.

Drill rods

Drill rods play an important role in effi- cient core recovery. Their tool joints are

Drill rod

Conventional

Reaming shell

Bit

Core sample

Core barrel

Drill rod

Casing shoe

Flushing circulating

Drill rod

Casing tube

Bit

Core sample

Wireline

Drill rod

Core barrel

Reaming shell

Fig 1. Typical application.

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leak proof, permitting the f lushing medium to efficiently travel over great distances within the string to discharge through the face of the bit under remo- tely controlled volumes and pressures. Connected to the drilling rig, a straight rod string combined with smooth rota-tion and borehole conditioning will turn vibration free in the hole while trans- mitting the feed pressure to the cutting tool. As a general rule: proper care and handling of rods and a vibration free rotation are key to improved core recovery.

Core barrel

The core barrel is most critical to effi-cient core recovery. As a mechanically engineered device consisting of dozens of inexpensive individually intercon-nected and interdependent components, predictive and preventative mainte-nance is the easiest way to maximize recovery.

The core barrel is sandwiched be-tween the drill rods and the diamond bit. (see Fig 1) Whether conventional or wireline, the design will typically be configured as a double tube system con- sisting of an outer tube and inner tube. Triple tube systems using a split inner tube are common in broken ground.

While the outer tube rotates with the drill rod, the inner tube is meant to remain stationary during advance-ment though the in situ material. As a general rule: to improve core recovery, ensure that components are serviced and bearings regularly greased.

Core lifterThe drilling fluid circuit operates with-in the available annular area provided. More specifically, after travelling through the drill rods, the fluid enters the core barrel. There it is channelled between the inner and outer tubes, exiting via the throat of the diamond bit, and back out between the outer tube and borehole wall to the collar (see Fig 2).

At the bit end of the inner tube, core lifter case adjustment and core lifter selection are of great importance. The core lifter case needs to be placed close enough to the throat of the bit to allow for efficient core breakage, but The objective is 'core in the box'.

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its adjustment gap should not create an undesirable increase in fluid pressure and/or sample washing from the drill-ing fluid. By design, the tapered core lifter slides along the inside of the core lifter case, wedging and securing itself onto the core during core recovery and breaking. As a general rule: to improve core recovery, choose an optional fluted core lifter in difficult ground.

Core breaking

During core breaking, the drill string is raised slowly. The spring-loaded

inner tube assembly, by virtue of the core being firmly wedged by the lifter, remains stationary until the bit throat adjustment gap is closed.

At this point, the core lifter case nests into the bit, transferring the substantial lifting force from the inner tube to the drill string until the core breaks away. As a general rule: the adjustment gap should never exceed the available com-pression spring travel to improve core recovery.

In broken or friable ground, the flu- shing medium’s pathway can wash away some of the sample before it makes its

way into the inner tube with consequen-tial effect on measurable recovery.

In these poorer ground conditions the core barrel can be configured to accept a third ‘split’ inner tube. The split tube is located within the regular inner tube and uses core lifter cases and diamond bits designed to direct the drilling fluid though the bit blank instead of though the bit throat, preventing sample wash- ing (see Fig 2). As a general rule: choose the largest triple tube system possible in bad ground to improve core recovery.

A core sample will enter the inner tube until the tube is filled, or the sample is impeded, or "blocked", from entering the tube, usually because of a fracture in the sample. When this happens, continuing to advance the drill string will likely result in grinding and degradation of the desired core sample. As a general rule: avoid deformations, damaging blows and mishandling to the inner tube in order to improve core recovery.

Diamond tools

The diamond tools are the business end of the string, and the bit, in particular, is a metallurgical marvel. The bit cuts through the material that makes its way into the core barrel.

Crown profile, flushing, and gauge design characteristics react differently with changes to rotation, thrust and flu- shing parameters. As a general rule: to improve core recovery, choose bit de-signs that protect ID run out, and that minimize back-pressure and sample washing.

The fundamentals for diamond core drilling have not greatly changed since 1887, when Per Anton Craelius started in the business. We still push, turn, cool, flush, cut and collect.

However, advances by Atlas Copco in tooling and equipment have conti-nuously improved the efficiency of core recovery.

Peter Balen

Solid ground Broken ground

Flushingmedium

Reaming shell

Inner tube

Split tube

Stop ring

Core lifter case

Gap clearance

Minimal clearance

Bit

Sample

Core lifter

Double tube Triple tube

Fig 2. Bit /ground interface.