Procedures of Sampling in Hydraulic Rotary Drilling

As apparent from the discussion of mud and hole-control problems when the hydraulic-rotary system is used for drilling, taking representative samples of penetrated formations is difficult and relies on the skill and experience of the driller. We describe the normal collection of cuttings samples, as well as taking drive or push-core samples to provide more representative samples in a hydraulic-rotary drilled hole.

Samples from the Hole

The washing action of the drilling mud as it moves bit-generated cuttings up the borehole obviously is going to contaminate the samples to varying degrees. Even though there is a concentrated effort to maintain good mud control, some separation of particles will occur. Coarse particles will not be transported through the ascending mud as fast as medium or fine particles are transported, and these separations have to be recombined if a representative sample is to result. These problems are much more pronounced when drilling in unconsolidated materials. The principal concerns, when the mud-rotary method is used in drilling consolidated formations, are the ascending velocities of the mud and recombining of the samples from the drilled interval. Washing samples out of upper erodable formations will not be a problem.

Two common methods of collecting cuttings samples during the hydraulic-rotary drilling process are the following: 1. Samples are collected continuously (with a sieve, colandei; or shovel) while drilling progresses. Samples collected by this method must be combined through the judgment of the driller or the person taking the sample. In this method, the ascending velocity of the mud carrying the cuttings to the surface must be considered (figs. 28 and 29). The velocities shown in the figures are based on the use of 2 3/8-in. and 2 7/8-in. drill pipe; any other diameter of drill pipe will change the annular area in the hole, resulting in a change of ascending mud velocity. 2. A deeper and wider section is cut in the ditch that carries the mud from the hole to the first major settling pit; this pit is referred to as the cuttings collector or temporary settling pit. As the mud flows through this first settling pit, its velocity slows and part of the cuttings drop out. The proper procedure for this method of sampling is to circulate the drilling mud with the string of drill pipe lifted slightly off the bottom of the hole, turning slowly until no cuttings are coming up with the mud (all cuttings are out of the hole). Now, clean out the temporary cuttings pit and drill a certain increment, maybe 5 ft; again, lift the bit off

4x5 59 GPM

UP HOLE VELOCITY -SLOW--

3-7/8 HOLE-153 FPM

3-7/8 HOLE-185 FPM

4-1/4 HOLE-140 FPM

4-3/4 HOLE-103 FPM

IN FEET PER MINUTE

IDEAL RANGE

1 50 FPM

TO 200 FPM

1 50 FPM

TO 200 FPM

3-1/2 HOLE —338 FPM

3-7/8 HOLE —239 FPM

4-1/2 HOLE-153 FPM

Up hole velocities are for 2-3/8" O.D. drill rods

4-1/2" x 5" and 4" x 6" pumps show the same output. But 6" stroke pumps deliver higher pressures and are used for deeper drilling to overcome friction losses

4-1/2 HOLE-153 FPM

4-3/4 HOLE-132 FPM

3-7/8 HOLE —292 FPM

4-1/4 H0LE-220 FPM . ,

4-1/2 HOLE-188 FPM

I

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I

Figure 28.—Cuttings velocity.

Figure 28.—Cuttings velocity.

the bottom and circulate until all cuttings are out of the hole. Cuttings that accumulate in the settling pit are placed in a bucket or tub and allowed to settle, later the excess fluid is poured off. This procedure is repeated for desired increments as the hole is deepened. The added time needed for drilling and circulating out all cuttings of successive increments is a slower method of sampling, particularly at greater depths; economics may dictate using the first method.

Whichever method of collection is used, it needs to be noted on the sample container and the sample bagged and correctly labeled after draining excessive fluid. These samples can be used later to construct a lithologie or geologic log of the formations penetrated by the drill bit. However, the geologist-hydrologist must rely on the driller's log or geophysical logs to construct the geological log because the driller's feel of the materials may result in a better lithologie log than using only the description of the cuttings.

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