February 26, 2019
Australasian Drilling: Creating Value Through Superior Core
Good, quality core is often difficult to retrieve. From the initial setup of a run, to the placement in the box, many factors affect a final core sample. It’s important to understand common mistakes to avoid, to troubleshoot where possible, and use best practices in order to obtain quality core – core that’s clean, labeled, and accurate.
What can go wrong?
Even experienced drillers can make mistakes when retrieving core. Before core recovery, there are a few factors when drilling that can negatively impact the core, such as the type of bit used and the location of the drill site. For example, if one is drilling in a soft formation, like sand, it’s a good idea to use a TT bit with a closed center so the water is not directly contacting the core before it gets into the inner tube. By doing so, more core will be recovered than with a bit that allows water contact with core samples.
Another factor is core loss at the beginning of a run, due to drilled material left from a previous run. This material rolls around in front of the bit and breaks up the core as drillers attempt to drill their new run.
One of the most critical steps to remember is the core placement within the box. This includes how it’s labeled, the order, and the specific direction of the sample. One common pitfall is failing to adapt to the client’s needs. It might be standard practice for a team to put the core in a particular direction, and may be confusing if the client requires a different placement. If the core is positioned backward inside the box, it’s difficult to decipher the correct direction, affecting the information. It’s essential to quickly learn the client’s expectations and train teams accordingly to produce accurate representative samples.
Troubleshooting Tips for Core Recovery
Many different methods contribute to better intact core recovery. One easy practice is to shorten the length of the runs. For example, if one is drilling five-foot runs as a standard for his/her shift and core is lost, the run can be shortened. This will cause less stress for the core to enter the tube and core recovery will increase. It adds up to significantly more runs for the day, and less footage per shift, but the goal of retaining quality core samples is more likely to be achieved.
Another practice is coating the inner tube or splits with EZ-Mud®* or polymer, which helps keep some of the core in the tube. One can also pump Enviroplug®* or bentonite pellets down the hole after each run, which creates a clay that sticks the broken rock together.
Retrieving Quality Core
One approach to retrieving quality core is the use of splits in the inner tubes. When pumping the splits out, the core remains intact and appears just as it was in the ground. Then, as the core is placed in boxes, it’s less likely to break which results in a cleaner presentation. Overall, splits provide a more consistent and representative core sample.
When drilling with a standard HQ™ core, splits aren’t an option. Drillers must meticulously retrieve the core from the inner tube without breaking the core. Using hammers and other tools are not recommended as they tend to break up the core, making it difficult to neatly store the sample in the box. Instead, it’s more effective to pump or push the core out with alternative methods like a hydraulic press system.
Quality core does not solely depend on the percentage of core recovery rate, but also considers the core being washed, placed inside the box, and transported. Once the core is retrieved, many times clients may come to the site to pick up the core sample. However, often it’s the responsibility of the company to transport the core. Teams must be aware of the potential of core breaking en route. It’s crucial to drive carefully and slowly, and always avoid bumps or divots in the road.
There’s no book that can teach all of life’s experiences. Every hole drilled becomes a new life experience that can’t be captured in a textbook. With the experience of surface coring, there’s a lot to be learned about the importance of quality core, difficulties encountered when retrieving core, and best practices for obtaining quality core samples. If everyone knew as much as a geologist to piece core back together, everyone would have consistent results in the field. Since this isn’t possible, however, it’s important to learn best practices and troubleshoot when necessary to recover quality core on a consistent basis.
*EZ-Mud is a registered trademark of Baroid Industrial Drilling Products. Enviroplug is a registered trademark of Wyo-Ben, Inc.
Q is a trademark of Boart Longyear