Directional and wireline core drilling Jwaneng diamond mine

April 22, 2020

by Filip Slavtchev, Sales Representative at MICON-Drilling GmbH

German-based global drilling company, MICON-Drilling GmbH (MICON) offers directional drilling and coring services, including development and manufacturing of complete drilling bottom hole assemblies (BHAs). Projects are unique and therefore tailored to meet client expectation – as was the case with a Botswana diamond exploration project in collaboration with Dewet Drilling Botswana (DWD).

Buffalo 90 rig on-site

The Jwaneng story

Jwaneng is a local Setswana word meaning ‘where a small stone is found’. It’s also the name of a small yet abundant mine situated south-central to Botswana and owned by Debswana – a partnership between De Beers and the Botswana government. High extraction rates of quality diamonds, fetching excellent per weight price deem Jwaneng the world’s richest diamond mine by value of recovered diamonds. Dubbed ‘The Prince of Mines’ upon its discovery in 1972, the mine remained unused for a decade until operations began in 1982. The resource itself consists of three separate kimberlite pipes that run north, south and central, with two additional small kimberlite bodies intersecting the pit. The pipes erupted through Transvaal strata and overlying Karoo sediments dating back to approximately 245 million years ago.

Scope of work

The deposit’s location below an existing operational open pit mine ruled out certain exploration methods including vertical drilling. Wireline coring and directional drilling was then applied from outside the open pit to enable exploration of the kimberlitic pipes. The mine’s dimensions are approximately 2500 m x 2000 m (8202 ft x 6562 ft), with an estimated kimberlitic pipe diameter below the mine measuring 2500 m x 2000 m (8202.10 ft x 6561.68 ft). Boreholes were positioned as close to the open pit as possible to maximize core retrieval.

Current excavation before the project reached 400 m (1312 ft) measured vertical depth from the surface. The distance between 400 m and 850 m (2789 ft) had already been explored, or was to be explored through different methods. The target was located between 850 m (2789 ft) and 1000 m (3280 ft) from the surface. To reach the deposit and minimize drilling through ineffective or previously planned ground, the team utilized MICON’s downhole motor and MWD system for controlled directional drilling, which would then enable wireline coring down the same hole – adding to what was already ‘challenging’ circumstances.

Figure 1 – An example of mother and daughter holes.

Tight directional control was necessary while drilling to ensure a safe operation once the ‘mother’ or initial hole was complete. After reaching the target, the directional drilling assembly was replaced with wireline coring equipment. For increased efficiency, the directional drill string and wireline coring drill rods’ inside diameters had to be equal, so that only the BHA required replacement. Once coring was complete, the wireline coring BHA was tripped out, and the hole was cemented along the entire coring distance – 20 m (66 ft) below the casing shoe. The remaining holes (fig. 1) were then side-tracked to a new path from the top of the cement plug. All drilling operations were undertaken in accordance with client requirements and safety standards, with zero incidents.

Figure 2 – MICON’s hot-pressed core bit.

Figure 3 – MICON’s SK 4 ¼ B wireline core barrel.


High performance equipment and tools

The project was split into two exploration methods – wireline coring and directional drilling. Both methods were equally important and undertaken in extreme conditions, which meant turning to MICON’s robust and reliable wireline coring utilizing Christensen’s SK 4¼ B and 5½ B-series (fig. 3). Christensen’s SK wireline core barrels are heavy duty and intended for deep coring applications in rough or difficult conditions. The system proved highly reliable in every type of formation – even when coring through difficult rocks such as kimberlite or sticky sediment. SK wireline core barrels performed with outstanding reliability and efficiency. Importantly – deep coring projects can utilize the 9 m (30 ft) core barrel to significantly reduce the number of trips. Christensen’s unique latching system ensures consistent and effective coring, while SK wireline core barrels are complemented by durable drill rods with friction-welded, high-grade tool joints, and hard facing pin tool joints for high abrasion resistance. Additionally, the core barrel was supplemented by a unique, diamond-impregnated, hot-pressed insert core bit (fig. 2) with 12 straight profile blades and T2A TSD gauge protection.

An aerial view of Jwaneng diamond mine

Directional drilling utilized MICON’s Positive Displacement Motor (PDM) series, including MICON 100 (standard power section), MICON 300 (equidistant power section II) and the Measurement While Drilling (MWD) tool. The trio of tools delivered precise and reliable directional drilling through accelerometers and magnetometers measuring the well bore’s inclination and direction. Data was directly transmitted to the surface through positive mud pulse technology, allowing real-time determination of the well bore path and three-dimensional positioning. MICON’s MWD tool is equipped with a generator turbine and battery to prevent data loss in the event of mud flow interruption. The added combination of these two energy sources allows extended in-hole operation time. All electronic components are protected by a pressure-resistant case of ultra-high strength alloy steel, while the resin-impregnated material protects against shock and vibration.


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