Why core for coal?
Cores are collected in coal exploration to obtain several different types of data. Not all of these data are collected from all cores. Because coring is expensive, specific reasons are usually needed for specific types of data at a particular location in the subsurface. Some of the data collected from coal coring is:
- Coal samples: Samples of the coal to be mined for thickness and chemical analysis. Proximate analysis is common, but many types of analysis can be done, depending on need.
- Coalbed-methane sampling: For coal beds that are gassy and may be potential gas reservoirs, cores may be taken to analyze the coal’s gas content and the type of gas it contains. Special containers are used to seal the coal when the core is brought to the surface so that the gas does not escape.
- Exploration: If a coal bed or several coal beds are suspected beneath the surface on a particular property, a continuous core to a certain depth may be drilled to determine exactly what coals are in the subsurface at that location, and to provide samples for analysis.
- Rock description: Data are collected on the thickness and types of roof and floor rocks above and below a coal for correlations of coal-bearing units, mine planning, and environmental issues. In underground mine planning, understanding the rock units that will need to be supported in the potential mine roof is important. Maps are often generated of key rock types and changes in rock types to plan for appropriate roof support.
- Rock sampling and analysis: Sometimes samples of rock strata above and below the coal or coals of interest are needed for testing. For surface mines, chemical analysis is needed to determine acid-producing or acid-mitigating potential of rock that will be disturbed by mining. For underground mines, mechanical strength properties may be tested to aid in planning roof support. In some mines with thick underclays, samples of clays beneath the coal may be analyzed to determine their moisture content. Clays that are prone to swelling because of high moisture may lead to floor heave, which is a hindrance to mining.
- Depths: Depths to the coal bed and key rock intervals in the floor or roof are obtained for correlation and mine planning. The interpreted elevation of the coal seam can be mapped from core data. This helps in planning for the overall dip of the coal seam, and for anticipating rolls or changes in coal elevation.
- Fractures, faults, and slickensides: Observations about the occurrence and spacing of fractures, faults, and slickensides (glassy surfaces formed from pressure and movement of rock strata at depth) can be made from cores, and are useful in mine planning.