Diamond Coring, Exploration Drilling Q & A (From Boart Longyear expert)
Q. For readers who may not be familiar with diamond coring (e.g., strict water well guys or HDD utility installers), can you give me a quick definition?
A. Diamond coring is a drilling technique that uses a diamond impregnated bit to cut a “core” sample, which is then typically assayed for a valuable mineral or chemical content. As such, it is used primarily in the mineral exploration business, gold and copper being the two most prevalent commodities for which Boart Longyear is contracted to drill for in the United States.
Q. What challenges are specific to diamond coring that drillers might not encounter in other types of drilling?
A. In diamond coring, we have a very small annular space between the drill rod and the drill hole. At best a couple tenths of an inch on each side. Special care must be taken to maintain the stability of the borehole is in such tight quarters. Drilling fluid properties and dynamics are a key here.
Q. Talk a bit about fluids for diamond coring. Is it just water to maintain the “purity” of the sample? Or do you use bentonite and/or polymers?
A. The makeup of the drilling fluid is very dependent on the formation in which you are operating, just like water well drilling. As in water well drilling, the primary purpose of the mud program is three-fold:
• Lubricate and cool the bit.
• Return the cuttings to surface.
• Condition the borehole for stability and water loss.
Drilling fluid does not contaminate the sample from a mineralogical perspective and therefore does not affect the purity of the sample. Drilling with straight water is not advisable; only in rare cases can and/or should you drill with only water, especially in the western U.S.
Q. What areas attract the most interest from the exploration community right now? That is, what draws the most attention and investment—ores, minerals, rare earths, oil—and where are those materials being looked for?
A. Typically, areas that have a history of production for the particular commodity you are looking for are viewed favorably, especially in this operating climate where funds for grassroots or green field exploration have been significantly cut.
That said, there are plenty of opportunities to find new deposits in previously under-discovered areas; it just takes knowledge and a bit of luck. At the moment, political stability, favorable tax strategies, and governments that have a tried and true permitting process are drawing the majority of the exploration dollars. Every continent but Antarctica has on-going mineral exploration today.
Q. What makes a good, quality core?
A. A good quality core is one that is intact, clean, free from artificial breaks, and properly boxed and labeled as to depth and recovery. In the diamond coring business, we measure percent recovery on every hole; that is the amount of core retrieved relative to the total depth of the hole. Good companies with solid mud programs and competent crews will achieve 95 percent recovery with little to no issues on average.
Q. Describe proper management of core samples. What are best practices to maintain samples, particularly in remote or difficult areas to work in?
A. Drill core is the product we as core drillers produce, therefore the handling and presentation of it to the client is key. Typically, a segmented cardboard box is used that holds 10 feet. These are remarkably stout and work for 80 percent of the normal surface drilling. In remote locations where helicopter transport is used or in underground applications, a wooden box is preferred due to its strength and more robust nature. Keeping the core sample in its linear order from which it was removed from the ground is key, because without knowing the exact location where the core was retrieved from, makes the core information almost useless.
Q. Can you talk a little bit about technology for core extraction? Are there advancements to be made in that area that will help in more fractured formations?
A. Bit technology within Boart Longyear has made a significant leap in technology over the last 5 years. We are now using bits with crown heights—the amount of diamond impregnate material—two to three times what we used in the past.
Having a higher crown height allows the bit to last longer, thereby reducing tripping time (the amount of time necessary to pull all the drill rods out of the hole, replace the bit, and get back to bottom of the hole and begin drilling again) and increasing overall productivity. In addition, the materials and geometries incorporated into the bits are more robust, especially in fractured formations.
This line of bits in particular are called Ultramatrix (UMX) diamond core bits. The family of UMX bits is designed to drill efficiently in both soft and hard ground.
The 07UMX bit is chosen to handle anything from limestone to taconite (3 to 8 on the Mohs scale of mineral hardness) and the 09UMX tackles slate to iron ore (4 to 9 on the Mohs scale). The UMX bits are designed to self-sharpen through the innovative crown matrix, which cuts away during drilling to expose new, large synthetic diamonds. This allows the drillers to go deeper without having to switch out the bit.
The 10UMX is designed to penetrate the hardest rock formations encountered (8 to 9 on the Mohs scale) and offers the freest-cutting matrix available. By completing the range of the UMX bits, drillers now have a high-performance, efficient bit for any type of ground condition.
The UMX bits also feature the Boart Longyear patented Stage waterway design, crown heights up to 25 millimeters and Twin-Taper window design. With the unique Razorcut technology, UMX bits can begin cutting right out of the box, even in overburden.
Q. Can you talk about a project that had unexpected results? For example, rigs hired to confirm the presence of a mineral deposit finding oil instead.
A. We don’t typically run across drastically different results such as finding oil instead. Before we ever drill a hole, the level of geologic knowledge from other means has already qualified the potential target for the particular commodity. The unexpected things we do get to see on occasion, and that every client hopes for, is a nice sequence of high-grade intercepts. For mineralization like gold and copper, this can make for exciting times on the rig. It is rare, but on occasion you can see gold in the core; that makes for good day!
Q. Describe best practices for maintenance of rigs involved in diamond coring.
A. Like all other drilling machinery, rig uptime is key. A proper parts and maintenance program that allows for replacement of components based on useful life cycle versus catastrophic failure is a key to maximizing the drills potential. ‘If you are not turning to the right, you are not making money!