Special Bauer drilling rigs in Canada

The primary deposits of diamonds are the so-called kimberlite pipes of volcanic origin, although actually only a small number of these pipes are diamondiferous. In the Fort ‡ la Corne province in Canada over 70 kimberlite pipes are known to exist of which about 50 have been shown to contain diamonds. The kimberlite craters of the Fort ‡ la Corne field extend over an area of several hundred hectares and, in contrast to most of the known primary deposits worldwide, have not been worn away by glaciation but have only been flattened and covered by thick glacial deposits which today have an average thickness of 90 metres and comprise mainly sands, mudstone and glacial till. For the requirements of future exploration of mineral resources, modern large-diameter drilling technology is opening up new alternative methods for collecting bulk samples from great depths.

When assessing the ore value of a diamond deposit, it is necessary to determine not only the average value of diamonds contained in the kimberlite (US$ per carat), but also the diamond content or grade (carat per tonne of kimberlite). In order to be able to make the final decision for taking the project forward to developing a diamond mine, it is imperative to obtain further information about diamond grade and value across the entire anticipated field in addition to all the very localised results to date. This evaluation is being carried out at present by way of a large number of 1.2 m diameter boreholes which are arranged on a 100 m grid across the projected deposit.

The new combined kelly and air-lift drilling technique

The requirements of Shore Gold Inc. on the Canadian drilling company NUNA Drilling FALC to construct the boreholes were defined as follows:

– Construction of drill holes in 1.2 m diameter to full terminal depth of 360 m.

– Borehole stability throughout the entire drilling operation with a minimum of overbreak

– Selection of the least destructive drilling system to minimise damage to the diamonds

– Pre-treatment of the collected kimberlite and filling into BigBags

– All-year-round drill hole construction, even in the harsh Canadian winter.

In just four months after having been awarded the contract, the entire drilling system was specified, designed and manufactured by Bauer Maschinen GmbH in Schrobenhausen, Germany. The equipment concept combines the advantages of the dry drilling technique in mixed soils and the air-lift or reverse circulation (RC) drilling technique to excavate hard rock formations with the use of rock roller bits. A great deal of attention has focused on ensuring that operating the equipment was as simple and reliable as possible in order to prevent accidents, particularly during the winter season.

The construction of cased boreholes to a depth of about 35 m

Due to the presence of loose sands in the upper section to a depth of about 25 m and also as a protection of the boreholes against fluctuating slurry levels during kelly and reverse circulation drilling, it is necessary to case the boreholes to a depth of about 30 to 35 m.. Drill casing with an outer diameter of 1320 mm was selected for the specified uncased borehole diameter of 1200 mm. The double-walled drill casing with an overall wall thickness of 40 mm is installed in lengths of 1 to 5 m. These are screw-jointed together to make up the required length for each borehole. When required, the BV 1320 hydraulic casing oscillator which generates a torque of 1070 kNm supports the rotary drive of the BG 36 drilling rig with a torque of 367 kNm, although in most cases the torque of the rotary drive is adequate for the installation of the drill casing with the use of the automatic casing adapter. In the kelly drilling mode the soil inside the casing is drilled out with the drilling bucket concurrently with the installation of the casing. Extraction of the casing even after a prolonged period of time in the ground is ensured with the help of the BV 1320 casing oscillator.

Advancing the slurry-stabilised borehole to 85 m

The geology clearly indicates that the upper sand formations are underlain by glacial till comprising clays with inclusions of gravel and boulders. The boulders, up to 50 cm in diameter, are concentrated at depths ranging roughly between 70 and 110 m. As both the clay formations with their highly variable compressive strengths and the cobble and boulder inclusions are extremely difficult to drill through with the air-lift drilling technique, it was decided to drill as deep as possible using the dry drilling process with kelly bar and drilling bucket. The quadruple telescopic kelly bar, which together with a 30 tonne winch is optimised for the stability of the drilling rig, is capable of drilling to a depth of 85 m.

Extending the kelly borehole to 125 m

During the drill run of the first borehole it emerged that, contrary to information available prior to the award of the contract, the change from glacial till to kimberlite occurred at a far greater depth than originally anticipated. After several attempts of drilling through the glacial till and its boulders with the air-lift drilling technique, it was decided to increase the capacity of the kelly system by way of kelly extensions to a maximum attainable depth of 125 m. This is achieved by 13 m long buoyancy-compensated kelly extension sections which are installed and extracted by the auxiliary winch.

Drilling to the terminal depth with the air-lift or reverse circulation drilling technique

After reaching the kimberlite horizon, the kelly drilling rig is converted to air-lift or reverse circulation (RC) drilling in approximately 2 shifts and the drilling mud treatment plant is connected up. For this purpose, the upper sledge with the main hoist rope reeving is locked onto the mast. After mounting the air-lift kelly, the main hoist rope is reeved 4-fold to be able to activate a 100 tonne pull-down force and 120 tonnes for drill string extraction and recovery. Afterwards, the drilling platform for suspending the drill string is mounted on top of the drill casing and connected up to the hydraulic system of the drilling rig. To transfer the high tensile forces safely into the ground, the mast is supported by an hydraulically operated support frame mounted on the platform.

On completion of the mechanical and hydraulic conversion, the electronic control system of the rig is finally switched from kelly drilling mode to air-lift drilling mode. Apart from the manual mode, which enables crowd pressure, rate of penetration and rotary speed to be determined by the rig operator, the automatic mode can be selected in which the borehole is drilled in load-dependent mode or constant rate of penetration-dependent mode under the supervision of the rig operator.

The drill string

The drill string consists of screwed double-walled drill pipes with the inner air-lift pipe having an internal diameter of 196 mm. If required, drill pipes with stabilisers can be installed to guide and centralise the drill string inside the borehole. A cross-over sub connects the screwed drill pipe to the rill collars and the large diameter drill head Every second drill collar is also fitted with a stabilizer to guide and centralise the large diameter drill head inside the borehole. The drill head is equipped with 6 roller cutters. Depending on the soil conditions encountered, roller cutters can be equipped teeth or discs armoured with tungsten carbide inserts or with surface-mounted tungsten carbide button inserts.

In addition to the actual rate of penetration, the time required for installing and extracting the drill string is an important factor in achieving a high level of productivity in air-lift drilling of deep boreholes

In the exploration of kimberlite, drilling tool selection is based on their ability to produce the largest possible cuttings in combination with a good rate of penetration. This minimises the risk of damage being inflicted on the diamonds embedded in the hard rock formation. By fitting banks of automatic jetting nozzles, air can be injected into the drill pipe at specific depths and at controlled pre-determined volumes and pressures. This alone makes it possible to control the rate at which the mud flush rises up through the air-lift pipe and to establish an optimum between often conflicting requirements:

– High rise rate of mud flush for solids removal in line with rate of penetration and large cuttings, although this does require a high separation capacity at ground level;

– Low rise rate of mud flush to minimise the dynamic forces that can affect the diamonds.

Drilling data acquisition

All important drilling and machine operating data are acquired every second and displayed for the rig operator graphically processed on different pages of the screen. Apart from standard operating screens for kelly and air-lift drilling, other screen pages with machine operating data and automatic fault analysis are also available. A further display screen mounted at the discharge of the mud treatment plant displays the most important data for the geologists, e.g. the current drilling depth for positive allocation of the recovered kimberlite samples.

The data collected during the operation is also stored on a RAM card inside the rig and can be downloaded onto any PC, after transfer to the office by hand or by radio link, and processed using special BTronic software. This makes it possible to obtain a detailed and accurate analysis of each phase of the drilling operation.

Mud flush circulation

The support slurry, which consists of both polymer and bentonite mud, is mixed and stored in a central mixing and storage plant. As long as outside temperatures permit, the drilling fluid is pumped to the drill hole position, in winter it is delivered by tanker lorry. The drilling fluid circulation at the borehole has deliberately been kept very short in order to ensure that the effects of ice formation on the drilling process during the winter season are reduced to a minimum. The amount of drilling fluid required for providing an optimal carrying capacity within the mud circulation is around 500 m=/h. The mud treatment plant, which is set up immediately adjacent to the drilling rig, separates all solids larger than 60 microns out of the mud flush and returns the processed mud to the borehole via a gravity feed.

The drill spoil itself is put through a screening and jetwash unit where all fine particles are removed by water jets. The kimberlite rock material with a grain size larger than 1 mm is subsequently filled into sample bags in the attached BigBag handling unit.

For logistic reasons, the drilling mud is mixed in a central mixing plant and stored in two holding tanks each with a capacity of 70 cu m. To protect the tanks against the effects of extreme weather conditions they are contained within a housing. From here, drilling mud is delivered to the borehole positions either by mud pipe or, during the frost period, by heatable tankers.

Winter measures

Temperatures in Saskatoon fall below the zero centigrade mark as early as the end of October and generally remain there until April. In order not to lose 5 months of drilling time out of a whole year, i.e. almost half the annual production, the drilling system had to be designed and set up for a continuous drilling operation even in temperatures below minus 20 degrees centigrade. In addition to the usual modifications of the drilling rig for arctic operation, particular emphasis was given to the protection of the mud flush circulation.

As already mentioned earlier, to this end the mixing and storage plant was set up at a distance of just 5 metres from the drilling rig. Permanent sag formation in the mud pipe was prevented by automatically dumping the charged mud contained within the return mud hose pipe into the mud treatment plant every time the drill head is raised for adding or removing drill pipes. The mud treatment plant itself is also completely enclosed and permanently heated.

Conclusion

Combining the know-how in specialist foundation construction & equipment technology (Bauer Maschinen GmbH of Schrobenhausen, Germany), well drilling & mud flush technology (Prakla Bohrtechnik GmbH of Peine, Germany) and arctic mining technology (NUNA LOGISTICS LIMITED of Vancouver, Canada) set the stage for the cost-effective construction of large-diameter boreholes (1200 mm) with a combination of kelly and air-lift or reverse circulation drilling technique in the Fort a la Corne region of Saskatchewan, Canada. Even in temperatures below -20 C, continuous drilling operations can be maintained with good rates of penetration. The large quantities of kimberlite cuttings which are continuously transported to the surface by the mud flush technology from depths of up to 340 m and separated out in the treatment plant provide a clear indication of the diamond content and their statistical distribution in the Star Kimberlit . On the basis of this information a reliable decision can be taken at the end of the drilling campaign on the development of a diamond mine.