Core Drilling at 444 mm Diameter

by Feb 16, 2021

Large Diameter Core Drilling in the Balkans

Constant innovation is a must in every business and that is nowhere more correct then in the mining industry. GEOPS was challenged to execute a large-core drilling program for a major global mining company. The scope of work demanded a borehole 444 mm in diameter and 292 mm of core to be conducted to a depth of 623 m.

        One of the challenges of modern geological research is to identify the quality features of valuable resources at an early stage. The extraction of sufficient material for the initial technological research provides an opportunity to obtain critical data about the properties of the prospective commodities. Expertise in ore enrichment is important for the effective planning of detailed researches and project stabilization, as well as the reduction of investment risk.

In this context, only large-diameter drilling provides sufficient material to carry out research and analyses and obtain technological samples at an early stage of project development. This form of drilling is becoming increasingly popular.

Geops-Bolkan Drilling Services Ltd (‘Geops’) is a Bulgaria-based drilling contractor, with the largest drill-rig fleet in Eastern Europe, serving the mining and mineral exploration sectors. Their extensive operations in the Balkans have been a key factor in the company’s increasing strength, growth, and reputation.

Bespoke Engineering

No suitable equipment was found on the market, so Geops designed and produced something that would accomplish the goal.
The final result was a triple-tube conventional core barrel with an outer diameter of 444 mm that can collect three meters of 292-millimeter diameter core. When empty, the core barrel weighs 1500 kg and when full, 1800 kg. A special drill-bit design with diamond inserts was devised for the job.

To complete the challenging 623-meter large-diameter borehole, a big drill rig was needed. Atlas Copco’s RD20IIIXC drilling machine with a pull-back capacity of 50 tonnes was requested and successfully implemented. A special drilling mud system of 40 m3 was designed and provided by Geops, with two duplex mud pumps NB125 and two 220 kVA generators to supply the necessary power.

All was set and drilling started in August 2018. Choosing a 17 1/2-inch (444.50 mm) drilling diameter for coring allowed the team to switch from mud rotary drilling to core drilling without involving any additional operations, like casing and reaming. This way, the core could be extracted at any desired interval of mineralization and that was very valuable and cost efficient for the client. To cover the overburden an 18 5/8-inch (473.07 mm) casing was installed to a depth of 50 m. The first series of large-diameter conventional coring started at 293 m and was finished off by the geologists to a depth of 335 m. From 335 m to 480 m mud rotary drilling was used with the same 17 1/2-inch (444.50 mm) diameter drill bit and then from 480 m coring was begun again until the end of the hole was reached, which was at 623 m. A total length of 185-meter core with a volume of 12.4 m3 was recovered, weighing approximately 37 tonnes.


The mud rotary drilling was as fast as usual, and the production from the section of coring was around 12 m and 6 m, at shallower and deeper sections respectively. Core recovery of more than 95 % was achieved. At depth of 600 m, the core breaking force reached 28 tonnes (pull-out).

The biggest challenges faced by the company were using a prototype on-site and achieving a high percentage of core recovery. Despite such obstacles and the particular geology, complicated even for wireline drilling, all goals were met.

Drilling was carried out in accordance with the requirements and safety standards of the client and with zero incidents.