How is core obtained?
Core is obtained through drilling. In standard subsurface drilling, a hole is drilled into the earth and only chips of the drilled rock are brought back to the surface. A special drill bit is used for coring that excavates and preserves a tubular section of rock. Coring is slower and more expensive than just drilling, so is only used if sections of rock from beneath the surface need to be examined or analyzed. In some cases, a continuous core (in 20- to 30-foot increments) from the surface to the main coal of interest is obtained with a conventional or wireline drilling rig to provide information on all of the rock units (including other coal beds) down to the main coal bed of interest. In other cases, a hole may be drilled to within 50 feet or less of the top of the main coal bed, and then that interval and lower is cored. In most cases, coring stops just beneath the coal bed of interest, or into competent rock beneath the underclay beneath the coal bed.
A difference between coal coring and coring done by the oil and gas industry is that most coal exploration holes are relatively shallow (less than 1,500 feet). This means smaller, more maneuverable rotary-core drilling rigs can be used; most are mounted on trucks. Coal cores are usually obtained in areas where the elevation or depth below the surface to the coal is relatively well known, so drilling depths and coring intervals are typically well constrained.
A coring rig is brought to a site and a tungsten-carbide or diamond-chip drill bit designed for coring is used to cut and retrieve cylinders of rock core from the subsurface and bring them to the surface for description and analysis. Cores are cut to the length of the core barrel, which is generally 20 or 30 feet long. The core barrel is attached to a succession of drill pipes. Each length of pipe is added to the next on a drill string, which is lowered into the hole. In this way, drilling proceeds to the depth needed in 20- to 30-foot increments.
The cut core is brought to the surface inside the core barrel. At the surface, the barrel is opened to reveal the cut core inside. The drilled depths are written on the core. Then the core is removed from the core barrel. Additional drilling pipe is added to the rig and lowered into the hole so the next 20- to 30-ft section can be cored. Each core barrel is brought back to the surface, and the process is repeated until the total core needed is drilled. Usually in coal exploration drilling, the base of the cored interval is the base of a target coal bed, or just below the target coal bed.
For continuous core drilling, the core from each core barrel may be laid out on the ground, described, and the sections sampled for analysis; the remaining core is then disposed of. In most cases, however, the core is at least temporarily preserved. Core is broken or cut into 2- or 3-foot increments so that it can be put in cardboard or wooden core boxes 2 or 3 feet in length. Each box usually has four or five dividers to hold a total of 8 to 10 feet of core. Exact depths for each core run are marked on both the core and the boxes. Care is taken to preserve and keep the core oriented in the boxes.
For oil and gas exploration cores and some department of transportation cores, drawing parallel red and black (or blue) lines in permanent ink or grease pencils down the length of the core is common. The red line is on the right side of the black (or blue) line when the core is oriented so that the top of the core is in the up direction. In other cases, depths are marked on the core (usually in 10-foot increments), or arrows are drawn on the core pieces with the arrow pointing in the up direction. This is done because core boxes and pieces can be dropped or pieces picked up and put back in the wrong orientation. Over time, pieces can also degrade in quality. The technique is not commonly used in coal cores, but is helpful for cores that will be preserved for future analysis.
In core boxes, cores are commonly placed in parallel rows, with the bottom of that core interval on one side of the box and the top on the other. Bottom and top are marked on the box, and in most cases the bottom and top depths of that core interval are marked on the box.
Keeping core in the correct order and orientation is critical so that an accurate record of the subsurface order of rock strata is preserved.
Special core handing
Standard rock and coal sampling does not require special methods or handling. Some special analyses, such as coalbed methane analysis, require that core be sealed in special canisters, or wrapped in plastic or wax as soon as it comes to the surface to prevent gases or moisture from escaping from the core. Planning the type of analysis that will be needed, in advance of coring, to facilitate any special handling and packaging requirements that may be needed, is important.
A variety of terms are used in drilling and for the drill rig. Go to the Coal Core Glossary for definitions of specific terms.