posted by the Epiroc Exploration & OreBody Solutions team
May 29, 2019
It's been a while since we did a blog on matrix troubleshooting where we examine the different wear patterns on a bit crown. By looking at the wear profile, you can get clues as to why your core bit may be underperforming. There are many variables that can cause a really good core bit to underperform. It can be a change in ground conditions, drilling parameter or simply a lack of experience.
You may be able to improve drilling performance by making simple changes like adjusting your drilling parameters. We have already looked at a few different problems like burnt bits or polished diamonds but we didn’t tackle problems with gauge loss on the inner diameter.
Remember that the ideal wear on a core bit is attained when the wear on the diamonds and matrix is balanced and even, and the full depth of impregnation is evenly consumed. This allows the driller to get as much footage as possible on the bit. When you are experiencing gauge loss on the inner diameter, you will notice a wearing down of the outside diameter and inside ringing resulting in a wear pattern as shown in the photo below. We’ll look at some of the possible reasons for this and suggested solutions.
Drilling pressure too high
Bit pressure or the weight on the bit (WOB) that is too high can result in premature wear of the mechanical components of the drill, the drill rods, the core bit and the core barrel. If the inside diameter is wearing out too quickly, your drilling pressure may be too high. The solution is to lower your drilling pressure but you must combine this with the right rotation speed. Rotation speed should be chosen carefully based on the diameter of the system you are using. If you are lowering your drilling pressure, you should be increasing your rotation speed. Refer to this guide on drilling parameters for advice on this.
Water flow too low
The water flow should be as high as possible but must be related to the bit size and type of rock to be drilled. For example, in soft or fractured rock, the water flow must be high. If the inner diameter is wearing down, it could be due to fractured ground and you may need to increase the flow. You should always make sure there is proper and sufficient water flow to the bit so a check that your rods don’t leak is also a good idea. Check our guide for suggested water flow.
Matrix too soft
Excessive wear on the inside of the diameter can mean that the matrix chosen may be too soft for the type of ground in which you are drilling. Consider changing to a lower series (lower number) matrix. A lower number on a core bit represents a harder matrix which is suitable for softer ground.
Core left in the hole
Drilling through pieces of core that are in the hole is a surefire way to damage your inner diameter. To avoid this, make sure your core lifter spring is the right size, is in good condition and not worn out. Also, take care when using new core bits. Always thread them by hand and not by the rotation chuck, and avoid catching them in the chuck of the foot clamp. If you damage the bit even slightly, messing up its inner or outer diameters, your core will not be caught by the core lifter system and the core will fall back in the hole.
Make sure you check out our previous blogs on matrix troubleshooting, including outside diameter gauge loss and overly exposed diamonds. Reviewing the wear patterns on your bit crowns is an excellent way to improve your drilling performance and make your life easier. It provides critical information that helps you solve problems quickly. If you want to know more about the wear patterns on your core bit, don’t hesitate to contact a member of technical crew for tips or advice.
