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CENTER SAMPLE RECOVERY (CSR) DRILLING

Center sample recovery drilling is a continuous sampling method that provides uncontaminat-ed chip samples. This method is also known as reverse air or reverse dual-tube drillingbecause it uses a double-wall drill pipe. The drilling fluid is usually compressed air, butwater, foam or bentonite mud can also be used.

Compressed air is forced between the inner and outer tube down to the bottom of the hole.The air cleans the face of a bit and brings the sample cuttings through the center hole ofthe inner tube up to the surface. Samples are then directed through side inlet swivel andtop-drive rotary head into the discharge cyclone. After the cyclone, the samples can be splitand collected as required (FIGURE 9.2.-2.).

The drill rig for CSR drilling is often a standard rotary machine equipped with a side inletswivel, cyclone and dual-tube drill pipes. Drilling can be performed with either a tricone bitor DTH hammer depending on the formation. Standard bits and hammers can be used in thedrilling process, but tools designed especially for CSR drilling give better results. The normalbit size used in CSR drilling is 5 1/8” to 5 1/2” (130 mm - 140 mm). The dual-tube drill pipediameter is only slightly smaller than the bit diameter (1/2” - 1”), so that cuttings can not

CONVENTIONAL WIRE LINE

DRILL ROD

INNER TUBERECOVERY HEAD

SPRING LATCH

LANDING RING

INNER TUBESWIVEL BEARINGS

CORE LIFTER

CORE BIT

FIGURE 9.2.-1. Conventional and wire-line core barrels.

9.1. GENERAL

The objective of exploration drilling is to gather information about the formations belowground surface. This data is used to locate ore bodies and determine the mineral content andquantity in the subsurface layers. Methods used in exploration drilling include:

- Diamond coring- Dual-tube reverse circulation drilling- Cable tool drilling- Hollow-stem auger drilling- Rotary air and mud drilling

Cable-tool drilling, rotary air & mud drilling, and auger drilling methods were discussed earli-er in chapter 8. The first two, samples were taken from the drilling cuttings. Auger drillingprovides a soil sample. Diamond coring and dual-tube reverse circulation drilling are methodsdeveloped specially for exploration drilling.

9.2. METHODS

DIAMOND CORE DRILLING

Diamond coring is a method that provides core samples. The name of the method refers tothe drill bit, which has small diamond inserts. These bits may also have tungsten carbideinserts or even rollers instead of diamonds. However, the method is still referred to as dia-mond coring. Core drills perform best in consolidatedformations.

A hollow coring bit is attached to a core barrel that collects sample into an inner tube asthe bit penetrates the formation (FIGURE 9.2.-1.). The inner tube is swiveled so that it staysstationary at all times. There are two ways to get the core sample out when the inner tube isfull. In conventional coring, the complete core barrel must be drawn up to the surface,which means that all drill rods must be pulled out of the hole. This is very time consumingsince it must be done every time the inner tube is full. In wire-line coring, the inner tube islifted to the surface and lowered back to the barrel through the drill rods with a winch. Thismethods saves time because the drill rods stay in the hole.

Diamond coring drills are small in size compared to rotary drills and are therefore well suitedfor remote exploration sites. Some are small enough to be transported by helicopter to areaswithout access roads. The rig consists of a high-speed rotation unit (over 1000 rpm), a feedsystem, which provides smooth and consistent pressure on a bit, and a diesel engine thatpowers the unit. Drill rod handling is manual because it uses shorter and lighter drill pipesthan rotary drilling. Coring drills are equipped with a water pump for flushing cuttings to thesurface between the hole wall and drill pipe.

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Table 9.2.-1. Comparison between diamond coring and CRS drilling in mineral exploration.

METHOD DIAMOND CORE CRS DRILLINGDRILLING

SAMPLE - Core sample- Uncontaminated

or contaminated- Provides complete

range of information - Chip samples- Uncontaminated - No information on rock structure

and physical properties FORMATION - Consolidated

formations - All formation types

DRILLING FLUID - Water - Compressed air- Water, foam or bentonite mud

optionalDRILLING SPEED - Slow (intermittent

sampling) - Fast (continuos sampling,

possibility to use DTH hammer)COST - Low investment costs

- High drilling costs due to slow speed and expensive bits

- High investment costs- Low drilling costs due to

fast speed and longer life of drilling tools

TRANSPORTABILITY - Excellent, due to OF EQUIPMENT size of equipment

- Good in developed areas - Poor in remote areas

be flushed out between the hole wall and drill pipe. Additional tricone bit shrouds or ham-mer sleeves are used to block the annulus.

The superior drilling speed over core drilling has been one of CSR drilling’s key success fac-tors. CSR drilling provides samples approximately 10 times faster than coring. There are,however, cases where core drilling is considered the only alternative in sampling. The follow-ing table compares the benefits of both coring and dual-tube reverse circulation drilling:

FIGURE 9.2.-2. Dual-tube reverse circulation drilling.