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Full-hole cores are cylinders of rock cut by a coring bit and retained in a core barrel behind the bit. Standard core barrels recover 20m or 30m cores although longer barrels have also been developed. There are many types of core barrel that can be used depending on the formation being cut.
Cores provide the best geological samples from a well in that they allow detailed rock, fluid and structural analysis to be carried out. However, core retrieval is an expensive operation due to the need for slow drilling whilst the cutting proceeds and the requirement for repeated trips to recover the core when the core barrel is full. Core barrels also tend to jam, especially in shaley formations, and the barrels are more likely to become stuck than conventional drilling assemblies. Specialist personnel are often engaged to set up the barrels, catch the cores and box them.
Assuming that safety criteria have been satisfied, a decision to stop drilling and take a core is made by the Well-site Geologist. Coring is usually restricted to reservoir formations of particular interest in exploration wells (and only when a good oil show has been detected) or to proven reservoir intervals in appraisal wells and, very occasionally, development wells.
Coring is generally carried out less today than it was in the past both because of improved seismic and log data and the advent of more cost-conscious geologists. For example, the Soviets used to core entire well sections. Sometimes in new exploration areas or in places a long distance from existing wells, a core is required by the government to be drilled at total depth, called the TD core.
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