Ensersa’s drilling fleet is comprised of exploration drill rigs from Atlas Copco. This includes Christensen surface rigs for different depths and capacities, as well as Diamec rigs for underground core sampling work. But it is the newest version of the Christensen rig – the CT20 – that is making the biggest contribution to the project.
On the face of it, this powerful, robust and fairly compact unit, which is capable if reaching depths of 2 450 m (N diameter), was the perfect choice. However, the drill crew soon discovered that the geology at Rio Tinto, mostly in and around the Cerro Colorado pit, was more of a challenge than they had imagined.
Using Hobic and Excore diamond rock drilling tools, the majority of the boreholes were drilled using the HO wire-line system. The underground rock and sediment encountered beneath the surface was hard, varied and littered with layers of quartz, in strong contrast to the smooth, rolling hills of the surrounding Andalucian landscape.
The rough conditions produced an excessive amount of cuttings which threatened to block the core barrel and increase the wear on the drillstring. Furthermore, this was also causing water loss, resulting in excessive wear on the bit.
In an to effort to bring the CT20 up to its full potential, Atlas Copco organized a one week training course for Insersa personnel, and technicians were brought in to study the on-site conditions. The main concern was that the bits were wearing away too quickly and bit changing was making downtime unacceptably high.
After three days the drilling parameters were changed on advice from the Atlas Copco techncians. The rpm, penetration rate, and water flow were all increased. In addition, the crew improved their skill at regulating the drillstring pull and WOB (weight on bit).
Prior to the changes, the drillers reported an average bit life of 50–60 m. After the changes, bit life increased dramatically – to 240 m. Furthermore, the drillers began to achieve their objective: more core in the box in less time and with increased productivity.