Contamination of Core
Problems of core contamination from the air-rotary coring method are rarely covered in the literature; as a consequence, those problems are ignored. Contamination resulting from filter caking or core invasion that may occur if some of the previously mentioned additives are used in the air are not as great a cause for concern because they can be seen and possibly even corrected. However, there is a need to be concerned with the contamination processes that cannot be seen or even understood.
If geoscientists wish to obtain a core sample from a formation for measuring moisture content, they would probably specify air-rotary coring rather than mud-rotary coring, because mud-rotary coring would contaminate the sample and air probably would not. They are partly right: air might not contaminate the sample with foreign materials; however it will possibly dry or blow a percentage of the moisture out of the sample. An experiment conducted by the research-drilling project provides an example of air-drying of moisture in a core. A 20-ft interval of a soft sandstone with medium-sand grain-size, was cored, using three coring methods: (1) air rotary, (2) mud rotary, and (3) drive core. Drive-core samples were used as a control assuming no moisture change. Mud-rotary cores were analyzed, and results showed moisture increases in the range of 2 to 6 percent. Air-rotary cores were analyzed and showed moisture decreases ranging from 22 to 45 percent. Not only was the moisture dried or driven out of the air-rotary cores, but those cores also indicated temperatures in the range between 110°F and 123°F, even though they had been collected from a formation having an ambient temperature of about 60°F.
The same result has been observed elsewhere. It is easy to understand the possibility of the air, even under low pressure, driving fluid out of a fairly permeable material; however, heating of the core is contrary to the literature. Supposedly, the air that is heated as it moves through the compressor, the swivel, and down the drill pipe should cool as it expands. As it leaves the bit restrictions and moves into a greater volume area, it should cool and provide cooling to the bit. But, this principle does not occui; at least in many instances, in air-rotary coring. Possibly, the observed heating effect occurs, because, although much of the air is returned to the surface of the borehole, some of it goes into even more restricted pore openings in the formation, and the hot air emitting from the bit retains or even generates additional heat.
Whatever the reasons for these problems, they do exist, and the geoscientist should consider them when planning the intended uses and analyses of the core. If considering only the physical properties of the rock—porosity, permeability, and density-drying or driving out moisture has no particular deleterious effect. However, if looking for certain chemical constituents or radionuclides that may be contained in the pore water of a permeable rock, then the risk is that that constituent also may be driven out.
Air-rotary coring generates a considerable amount of dust and cuttings during the process of coring. Usually, this dust is blown through a cyclone separator, where the larger particles are separated out; the dust is directly discharged to the atmosphere, sometimes through a large hose or pipe located some distance from the drill rig but often right at the separator. This dust poses health problems that should not be tolerated. If air-rotary coring is being conducted in contaminated environments, no dust should be allowed to discharge to the atmosphere. We have referred to and praised the efforts of the 1975 Radioactive
Waste Management Complex Core Drilling Program, but the cited example of dust handling (released to the atmosphere downwind) is not a good one (Burgus and Maestas, 1975).
Any specifications written for air-rotary coring should include suggested requirements similar to the following for dust control:
Was this article helpful?