posted by the Epiroc Exploration & OreBody Solutions team
February 5, 2019
In a previous blog, we discussed some of the issues you encounter when drilling in soft rock. This time, we’ll look at the opposite end of the spectrum: hard rock. Hard rock is defined as being over 6 on the Mohs scale of hardness. Common types of hard rock included kimberlite, granite, chert, quartz and silicified rock.
To find out how hard the ground is, you can perform a scratch test using a hardness scale and compare the results with the Mohs scale. This is the easiest most reliable way to determine rock hardness. Most drillers find drilling in harder ground conditions to be more challenging, but there are guidelines that can make it easier.
The right bit
Choosing the right core bit when you are drilling in hard rock is one of your most important decisions. Hard rock requires a free cutting bit with a softer matrix, and that means a core bit with a higher number. A bit with a higher number has a matrix that will allow diamonds to be exposed efficiently. The exposed diamonds will cut the rock better. A core bit that was developed for hard rock, such as a Hero 9 or Hero 11 is a good choice as they have great self-sharpening abilities.
You should also consider the configuration of the bit face. A turbo configuration is a good choice as it will flush and cool the bit face effectively. This configuration has less surface area in contact with the rock resulting in a very free cutting bit.
Watch your drilling parameters
In hard rock you need to pay attention to your drilling parameters such as feed pressure, water flow and rotation speed. You will want to avoid polishing the bit face. On the other hand, you want to avoid overexposing your diamonds and risk having them pop off or break. Ideal wear on a core bit is attained when the wear on the matrix and diamonds is balanced and even, and the full depth of impregnation is evenly consumed.
Not enough water flow and you can burn the bit and too much can keep the bit from sharpening. Rotation speed that is too low can cause excessive wear on the matrix and rotation speed that is too high can polish the diamonds.
If you are not advancing, it could be that your bit is polished and you need to sharpen the bit to expose the diamonds. Sharpening a bit requires some experience and knowledge, there is no easy, one-size-fits-all solution.
Consider drilling fluid additives
In hard rock, you want to make sure the face of your core bit is cooled. Using a polymer such as DD-2000 raises the viscosity of the drilling fluids so that cuttings can be more easily flushed out. An added benefit is that it is non-abrasive. If you mix it with Torqueless, it will help cool and protect the core bit, resulting in longer bit life. Torqueless is an environmentally safe product can be used alone or in combination with other additives, to help bind polymers, lubricate, reduce wear and rusting of the rods and cool the core bit. Using Torqueless enhances all other polymers and should be added to every mix. It ensures proper mixing with the water by encapsulating each grain of the polymer to ensure a thorough mix.
Finding the right bit configuration and the best drilling parameters may take some trial and error. What works in one region may not work in another. You can also contact our technical support team for additional guidance. Our goal is to improve your drilling performance and accelerate the transformation of our industry.
