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Blocked Core in Dry Hole Drilling

posted by Rod McCoremick, Blog author, Fordia Powered by Epiroc

11 de marzo de 2020

In our last blog, we looked at the expression “pushing a block”, what it meant and why drillers should never try to advance in a hole when the core is blocked. However, in many parts of the world, especially where dry hole drilling is common, or when the hole is very deep, different advice and procedures should be followed. We are going to look at these different situations and the recommended procedures.What is important to understand is that core can block whether you are in a shallow hole or a deep hole, whether you are drilling in Nevada or in South Africa, and whether you are drilling with water or without water. In the case of drilling without water, drillers who are accustomed to these conditions often do not use shut off valves. Shut off valves and dry holes or lost circulation don’t always go together for the following reason. If a driller drilling in a dry hole waits for the shut off valve to cause the water psi gauge to increase, thereby indicating to the driller that the core is blocked, it may already be too late. In all likelihood, the driller has already burned his core bit or has his drill rods stuck in the bore hole.

As we described in our earlier blog, if you are drilling in unconsolidated or broken ground and the core bit and outer tube are advancing in the hole, the broken and wedge-like pieces of core can become jammed as the core enters the inner tube. The driller should notice a spike in his water pressure due to the expansion of the shut off valves on the head assembly.  The shut-off valves are compressed due to the feed force being exerted. This compression causes them to expand in diameter until they spread out and touch the walls of the drill rod. This keeps the drill water from flowing freely but when there is no water, the water pressure will not increase and there will be no indication - or if there is, it will be too late.

In dry hole drilling, the driller must pay attention to the drill and not the water pressure gauge. If you are paying attention, the drill will tell you what is happening in the hole before the water gauge does. As the core starts block, the motor on the drill will begin to speed up and you will hear a change in the sound it is making, like it is spinning its wheels. The pressure is no longer on the bit, and is transferred to the inner tube, bearings and spindle of the head assembly.  The feed gauge and the torque gauge will also give the driller a good indication of what is going on in the hole because the torque and weight will both decrease as the weight comes off the bit.

Another option is to replace the rubber shut off valves with steel washers. When you hit a fracture and the bore hole goes dry, it also creates suction in the hole.  This suction will cause rubber shut off valves to compress causing your inner tube to be sucked down onto the bevel of the inside of the bit. This causes a wet pull and prevents water or mud from passing through by the bit. Using steel washers allows you to avoid this issue. Of course, if you use steel washers, you still need to monitor the gauges closely to avoid pushing blocked core.

So what should you do if your core is blocked and you are drilling in a dry hole? The solution is the same whether water is involved or not – you stop advancing.  When you encounter a core block, you must pull the tube, remove the core and try again.

A lot of successful diamond drilling comes from learning how to read your instruments and the “feel” of the equipment. This type of situation is a perfect example of knowing how to read your drill. As I have said often, in this industry, patience is an asset and the ability to finesse a problem rather than jamming through is a valued skill. Wherever you are drilling and in whatever kind of conditions, our goal is improve drilling performance one drill site at a time. If you need advice on any drilling problems, you should know that our technical support team has a depth of knowledge and experience that can help guide you.

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