A SAMPLE-FREEZING DRIVE SHOE FOR A WIRELINE-PISTON CORE SAMPLER by Fred Murphy and W.N. Herkelrath Ground Water Monitoring and Remediation, 1996, v. 16, no. 3. p. 86-90

ABSTRACT

Loss of fluids and sample during retrieval of cores of saturated, noncohesive sediments results in incorrect measures of fluid distributions and an inaccurate measure of the stratigraphic position of the sample. To reduce these errors, we developed a hollow drive shoe that freezes in place the lowest 3 in (75-mm) of a 1.88-in (48-mm)-diameter, 5-ft (1.5-m)-long sediment sample taken using a commercial wireline-piston core sampler. The end of the core is frozen by piping liquid carbon dioxide at ambient temperature through a steel tube from a bottle at the land surface to the drive shoe where it evaporates and expands, cooling the interior surface of the shoe to about -109°(-78°C). Freezing a core end takes about 10 minutes. The device was used to collect samples for a study of oil-water-air distributions, and for studies of water chemistry and microbial activity in unconsolidated sediments at the site of an oil spill near Bemidji, Minnesota. Before freezing was employed, samples of sandy sediments from near the water table sometimes flowed out of the core barrel as the sampler was withdrawn. Freezing the bottom of the core ensured the retention of all material that entered the core barrel and lessened the redistribution of fluids within the core. The device is useful in the unsaturated and shallow saturated zones, but does not freeze cores well at depths greater than about 20 ft (6 m) below water, possibly because the feed tube plugs with dry ice with increased exhaust back- pressure, or because sediment enters the annulus between the core barrel and the core barrel liner and blocks the exhaust.

INTRODUCTION

Because saturated, noncemented, well sorted sands have high permeability and low cohesion, collection of core samples without losing sediment or pore fluids as a sampler is removed to the land surface after driving is difficult. Various devices have been used to reduce sample loss, including pistons or remotely operated valves that maintain a vacuum above the sample (Munch and Killey 1985; Zapico, et al. 1987), inflatable bladders that expand into and seal the lower end of a core barrel (McElwee et al. 1991), movable steel fingers that are driven into the sediment just below the core liner (McElwee et al. 1991), sample- freezing jackets (Durnford et al. 1991) and plastic or sheet metal core catchers that are inserted into the lower end of the sample tube. Since jarring can cause material to drop from a sampler, Munch and Killey (1985) and Zapico et al. (1987) incorporated a left-hand-threaded coupling between the drill string and the core barrel to allow the core barrel to be detached from the drill string and retrieved smoothly and rapidly by means of a wireline; McElwee et al. (1991) employed a quick- disconnect between the sampler and drill string for the same reason.

The hollow drive shoe described in this paper (figure 1) enables freezing in place of the lowest 3 in (75 mm) of a 1.875- in (48-mm)-diameter, 5-ft (1.52-m)-long core after the core sampler has been driven. The hollow drive shoe replaces the standard drive shoe of the Solinst2 wireline-piston core sampler (Zapico et al. 1987). Interstitial ice in the frozen sediment prevents fluid from flowing through it, and friction between the ice-sediment plug and the plastic core liner prevents the otherwise free-flowing sand from leaving the core barrel liner. The purposes of this paper are to describe the design of the sampler and to outline its use in the field.

DESCRIPTION OF THE DEVICE

The sample-freezing drive shoe (figure 2) is a triple-walled steel cylinder that threads onto the Solinst core barrel in place of the standard hardened-steel drive shoe. The device is assembled from brazed and welded mild steel and stainless steel parts. The cutting edge of the drive shoe is faced with a hard alloy for wear resistance. The fittings, fasteners and tubing are stainless steel.

To freeze the bottom end of a core, liquid carbon dioxide is sprayed into the lower end of the inner chamber. Some of the liquid evaporates, and some deposits as solid carbon dioxide (dry ice) at about -109°(-78°C) in the distribution channels of the freezing chamber (figure 2). As the liquid evaporates and the dry ice sublimes the resulting carbon dioxide gas circulates upward through the channels, exhausts though holes in the upper end of the interior wall, moves through the annulus between the plastic core liner and the core barrel, passes from the exterior of the liner into its interior through slots in the upper end of the liner, and leaves the sampler through the cable hole in the core barrel cap (figure 1).

The upper pieces of 1/4-in (6.35-mm) feed tube are interchangeable 5-ft (1.52-m)-long segments, but the two lowest sections of tubing are special. The tube that attaches to the drive shoe extends without a break from the drive shoe to above the core barrel cap; this tube segment includes a thick-walled, threaded lower section about 4 in (10 cm) long that fits a mating part of the drive shoe. A Swagelok brand, 60 micron filter attached to the upper end of the tube prevents particles from entering the drive shoe and plugging the flow-restricting narrow tube in the shoe. The second segment of tubing above the sampler has a U-shaped fold that is formed to fit snugly against the drive rod used to emplace the sampler (figure 3). This double bend makes the feed tube assembly slightly elastic and so prevents breakage of the tube joints when the feed tube scrapes or catches on the auger during driving.

The core barrel is assembled (figure 1) with a 5.75-ft (1.75-m)-long piece of polycarbonate liner fitted between shoulders in the drive shoe and in the core barrel cap. Polycarbonate is used because it is tougher than other transparent plastics. The piston, which is attached to the wireline, is passed through the liner and set in the nose of the drive shoe, and the core barrel cap is threaded onto the sampler. A hollow-stem auger is advanced to the top of the interval to be sampled, using a basket or center bit to exclude sediment from the bore. The core sampler is lowered on the drill string, through the augers, to the bottom of the hole. 5-ft (1.52- m)-long segments of feed tube are added as the sampler is lowered. Once the sampler is in place at the bottom of the bore, the wireline is pulled taut, and the sampler is hammered 5 ft (1.52 m) into the ground.

DISCUSSION

Use of the sample-freezing drive shoe takes more time and effort than using a standard wireline-piston core sampler. It is not necessary to use a piston to retain sediment in the core barrel when the end of a core is frozen. However, a piston does prevent loose material and fluid, particularly oil floating on water in the bore, from entering the core barrel liner and so contaminating the liner or the sample as the core barrel is lowered into, or lifted from the bore. Judging from the preservation of laminae and sharp interfaces in the cores that we have retrieved, samples recovered using the sample-freezing drive shoe appear to be no more disturbed than those collected by means of the standard drive shoe with the Solinst sampler.

At the Bemidji site we attempted to recover a continuous core from the land surface to about one-half meter below the water table. In fact we recovered only about 80 percent of the driven length regardless of whether we used the freezing sampler or the standard drive shoe. Poor sample recovery had several causes. Sometimes a cobble would plug the drive shoe opening. Other times the material through which the barrel was being driven would be compressed or forced aside by the drive shoe and not enter the core barrel. Sometimes the end of the core barrel liner would fold into the interior of the drive shoe and prevent sediment from entering. The sample-freezing drive shoe ensured only that all material that entered the core barrel was retained and lessened the redistribution of fluids in the sample.

Cores that were of most interest came from about 3 ft (1 m) above to 1.5 ft (0.5 m) below the interface between the water and an overlying oily zone, and so were liquid-saturated in their lowest parts. With the standard sampler we recovered smaller fractions of the core as the liquid content in the lowest segment approached saturation. We also retrieved less of cores taken from depths greater than 16 ft (5 m). Saturated sand or gravel flows freely; samples from greater depths took more time to recover and were more likely to be jarred loose while being raised than were samples taken from shallower depths.

SUMMARY AND CONCLUSIONS

REFERENCES

Cozzarelli, I.M., M.J. Baedecker, G. Aiken, and C. Phinney, 1994,

Small-scale chemical heterogeneities in a crude-oil contaminated aquifer, Bemidji, Minnesota, In, U.S. Geological Survey Toxic Substances Hydrology Program -- Proceedings of the Technical Meeting, Colorado Springs, Colorado, September 20-24, 1993: ed. D.W. Morganwalp, and D.A. Aronson, U.S. Geological Survey Water Resources Investigations Report 94-4015.

Durnford, D., J. Brookman, J. Billica, and J. Milligan, 1991,

LNAPL distribution in a cohesionless soil: A field investigation and cryogenic sampler: Ground Water Monitoring Review, v. 11, no. 3, p. 115-122.

Essaid, H.I., M.J. Baedecker, and I.M. Cozzarelli, 1994,

Use of simulation to study field-scale solute transport and biodegradation at the Bemidji, Minnesota, crude-oil spill site, In, U.S. Geological Survey Toxic Substances Hydrology Program -- Proceedings of the Technical Meeting, Colorado Springs, Colorado, September 20-24, 1993: ed. D.W. Morganwalp, and D.A. Aronson, U.S. Geological Survey Water Resources Investigations Report 94-4015.

Essaid, H.I., W.N. Herkelrath, and K.M. Hess, 1993,

Simulation of fluid distributions observed at a crude-oil spill site incorporating hysteresis, oil entrapment and spatial variability of hydraulic properties: Water Resources Research, v. 29, no. 6, p. 1753-1770.

Hess, K.M., W.N. Herkelrath, and H.I. Essaid, 1992,

Determination of subsurface fluid contents at a crude-oil spill site: Journal of Contaminant Hydrology, v. 10, p. 75-96.

Hult, M.F., 1984,

Ground-water contamination by crude oil at the Bemidji, Minnesota research site--an introduction, in Hult, M.F., ed., Ground-water contamination by crude oil at the Bemidji, Minnesota research site--U.S. Geological Survey Toxic Waste--Ground-water Contamination Study: U.S. Geological Survey Water-Resources Investigation Report 84- 4188, p. 1-15.

McElwee, C.D., J.J. Butler, and J.M. Healy, 1991,

A new sampling system for obtaining relatively undisturbed samples of unconsolidated coarse sand and gravel: Ground Water Monitoring Review, v. 11, no. 3, p. 182-191.

Munch, J.H. and R.W.D. Killey, 1985,

Equipment and methodology for sampling and testing cohesionless sediments: Ground Water Monitoring Review, v. 5, no. 1, p. 38-42.

Zapico, M.M., S. Vales, and J.A. Cherry, 1987,

A wireline piston core barrel for sampling cohesionless sand and gravel below the water table: Ground Water Monitoring Review, v. 7, no. 3, p. 74-82.