Drive or Push Core Samples

The geologist-hydrologist too often relies on a mixture of cuttings to make critical decisions on the selection of screen-size openings and proper



FEET/MINUTE Figure 29.—Pump delivery versus return velocity.

placement of the screens. Also, obtaining an in-situ representative sample is sometimes necessary of the formations being sampled for chemicals or radionuclides and for samples taken for calibrating geophysical-logging tools and so forth. Drive-core sampling readily can be performed in hydraulic-rotary drilled holes; if proper mud control is maintained, contamination of the sample can be held to a minimum.

Variations for taking drive-core samples in a hydraulic-rotary-drilled hole ensure a relatively clean-bottom hole, and allow the driller to use his drill pipe rather than prescribed N rod as a push or drive rod. The procedure is: when the sampling depth is reached, the bit is lifted off bottom a few inches and the drilling mud is circulated until all cuttings that can be removed are removed from the hole. About 50 gal of a clean, high-yield bentonite drilling mud is prepared to a viscosity of between 50-s and 75-s. The drilling mud is then spotted in the bottom 25-50 ft of the hole. This spotting will provide a high-gel-strength drilling mud in the lower part of the hole that will prevent any sand or other cuttings from settling to the bottom. The string of drill pipe is then slowly removed from the hole to prevent swabbing (see p. 23 to 24). The drive-core barrel is now attached to the bottom of the string of drill pipe and run in the hole. Note: If any caved or bridged material is encountered, a description of the wash-in method beginning on page 71 can be used. However, no cuttings will settle through the previously spotted gel, in the bottom of the hole. The sample is then taken, using either the push or drive mode previously described. After the sampler has been removed and dismantled and the sample retainers removed, the sample is inspected to observe the amount of contamination resulting from the filter-cake invasion. If this sampling procedure is carefully followed, sample contamination will be very minimal except for the top 2 or 3 in. of the sample, which, if contaminated, can be discarded.

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