Published in Vadose Zone Journal 3:1479-1482 (2004)
© 2004 Soil Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA
NOTES
In Situ Freeze-Capturing of Fracture Water using Cryogenic Coring
Grace W. Sua,*,
Joseph S. Y. Wanga and
Kris Zacnyb
a Earth Sciences Division, Lawrence Berkeley National Lab., Berkeley, CA 94720
b Dep. of Material Science and Mineral Engineering, Univ. of California, Berkeley, Berkeley, CA 94720
* Corresponding author (gwsu{at}lbl.gov)
Received 29 January 2004.
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ABSTRACT
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Current methods do not allow for sampling of in situ water from unsaturated fractures in low-moisture environments. A novel cryogenic coring technique based on a previously developed method is used to collect in situ water in unsaturated fractures. This method uses liquid nitrogen as the drilling fluid, which can freeze the fracture water in place while coring. Laboratory experiments are conducted to demonstrate that water in an unsaturated fracture can be frozen and collected using cryogenic coring.
Abbreviations: NAPL, nonaqueous phase liquid
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INTRODUCTION
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SAMPLING OF WATER and contaminants in the subsurface is essential for characterizing flow and contaminant transport. Water sampling in the saturated zone is often performed by pumping water out of wells, while water in unsaturated unconsolidated soils is generally extracted for sampling using a suction lysimeter. Sampling in situ water from unsaturated fractures in low-moisture environments remains a challenge, however, and has not been successful because flow typically occurs along preferential flow paths (e.g., Nicholl et al., 1994; Su et al., 1999; Wang and Bodvarsson, 2003) that are difficult to sample using conventional methods. Obtaining in situ samples of water and contaminants in unsaturated fractured rock is important for a number of applications, including remediation of contaminated sites, site characterization, and recharge in arid environments.
In situ water from soils and rock is also often obtained by removing pore water directly from core samples. In unsaturated fractured rock, pore water from the rock matrix can be extracted from cores, but water in the fractures will likely be displaced or contaminated by the drilling fluid used during coring. Cryogenic coring may be a promising method for obtaining in situ water samples from unsaturated fractured rock. Freezing soil to obtain shallow sediment samples near streams and wetlands has been performed for several decades. The technique has traditionally involved inserting a metal-pointed pipe into the sediment to a depth of about 1 m, and then liquid nitrogen or liquid carbon dioxide is circulated into the pipe (e.g., Walkotten, 1973; Knaus, 1986). The frozen soil adjacent to the pipe is then sampled. Cryogenic coring was also performed in unconsolidated soils by Cavagnaro (1999), using a novel method that was an extension of the cryogenic drilling technique developed by Simon and Cooper (1996). This method uses standard air rotary drilling techniques, but cold nitrogen (–196°C) rather than ambient air is used as the circulation fluid. During drilling, the cold nitrogen freezes and stabilizes the borehole wall. This method has minimal contamination from external sources since the drilling fluid is liquid nitrogen. Another inherent advantage of the cryogenic method is that heat generation while drilling is not an issue. The possibility of extracting clean frozen cores with improved quality over cores extracted using traditional methods exists using cryogenic coring. The advantages of the cryogenic coring technique used by Cavagnaro (1999) over the pipe insertion method are that it can be applied to much greater depths and it can be used over a range of media, including fractured rock. Cryogenic drilling was successfully demonstrated in the field for depths between 7 and 24 m (Simon and Cooper, 1996; Cavagnaro, 1999).
Cavagnaro (1999) examined cryogenic coring in unconsolidated soil but did not investigate this technique in fractured rock. A possible advantage of using liquid nitrogen while coring in fractured rock is that water in the fractures can be frozen in place, allowing for in situ fracture water collection. The fracture water freezes by conduction with the frozen rock and/or convection with the nitrogen gas. No technique currently exists for collecting in situ water samples in unsaturated fractures. Laboratory tests are presented in this paper to examine the effectiveness of cryogenic coring as a method for sampling in situ water from unsaturated fractures.
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MATERIALS AND METHODS
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Cryogenic coring was performed on a sandstone rock with a single horizontal fracture. Two 24 by 12 by 3.5 cm sandstone slabs were placed on top of each other to create the fracture and were clamped together on one end of the rock sample (Fig. 1)
. An aperture gradient was created, with the apertures gradually increasing away from the clamp. The aperture ranged from approximately 0.5 to 3 mm. The rock slabs were initially saturated with water before they were assembled. The sides of the sample were left open to the atmosphere to allow for unsaturated conditions in the fracture.
The coring equipment consisted of a pillar-mounted drill press that was converted for coring of soils and rocks (Fig. 1). A side entry swivel used in place of a chuck allowed for the introduction of liquid nitrogen into the interior of a diamond-tip core barrel (approximately 25 mm, 1 inch) that was attached to the swivel. A hose connected a cylinder of liquid nitrogen directly to the swivel fitting, and the sandstone rock was placed directly below the core barrel. Coring was performed after the liquid nitrogen had cooled the core barrel, which was evident by the formation of frost on the outer surface of the core barrel. Liquid nitrogen, which has a boiling point of –196°C, was supplied at a pressure of 0.207 MPa (30 psi). Nitrogen gas was observed to flow out the edges of the experimental apparatus. The time for each core to be extracted was between 1 and 3 min.
The drilling apparatus, including the inner (rotating) part of the swivel, was made almost entirely out of carbon steel. The exception to this was the outer or nonrotating part of the swivel, which was made out of brass. Carbon steel is subject to the ductile-to-brittle transition at low temperatures, but brass is not. Fracturing of the carbon steel was not observed in our experiments, most likely because the loading was not excessive. The coefficient of thermal expansion of the brass and steel found in the swivel are of the same order of magnitude, and we did not observe any excessive wear or ceasing of the swivel while drilling.
Three experiments were performed to test cryogenic coring as a tool for collecting water in fractured rock. Experiment 1 was conducted where no water was injected into the fracture, but the rock matrix was saturated. Experiments 2 and 3 were conducted with a finite volume of water injected into the fracture before coring. The fracture was nearly saturated with water in Exp. 2 and 3, but since the sides of the sample were not sealed, some of this water exited the fracture before the rock was cored.
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RESULTS AND DISCUSSION
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In Exp. 1, no water was injected into the fracture, and the core extracted from the matrix had small pieces of frost on the surface, as shown in Fig. 2
. The frost could be due to condensation from the air or because some of the matrix water was driven out as the sample was cored. No additional evidence of frozen water was observed on the core or on the fracture surfaces. This observation will be used as a baseline to compare with the cores extracted from the other experiments.
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Fig. 2. Photograph of the core extracted in Exp. 1 where the rock matrix was saturated, but the fracture was dry. Frost on the surface is caused by matrix water that was driven out while coring or condensation from the air. The core has an approximately 25-mm (1-inch) diameter.
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Two cores were extracted from the partially saturated fracture in Exp. 2. The first core extracted did not have any evidence of ice on the fracture surfaces and looked similar to the core extracted in Exp. 1. A second core was subsequently extracted adjacent to the first core, and a portion of one of the fracture surfaces was covered with a small sheet of ice, as shown in the circled region in Fig. 3a
. The presence of a translucent sheet of ice on the fracture surface was indicative of frozen fracture water. Samples collected without fracture water had only small pieces of frost on the fracture surface (Fig. 2). The fracture in Exp. 2 was opened after the second core was extracted to examine the fracture surface. Ice near the cored areas was observed as well as an unfrozen water film further away from the cored regions where the clamp was located.
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Fig. 3. Photographs of cores extracted in Exp. 2 and 3 where the rock matrix was saturated and the fracture was partially saturated. Circled regions show evidence of ice due to the fracture water freezing while coring. The cores have an approximately 25-mm (1-inch) diameter.
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To ensure the repeatability of extracting ice from the fracture, Exp. 3 was conducted. The first core extracted in this experiment had ice on both fracture surfaces as shown in Fig. 3b, which is a photograph of the two fracture halves. Condensation had already occurred on the surfaces by the time the photograph was taken. Frost began to form on the core surface shortly after the core was left at room temperature. This observation indicated that the cores must be stored below freezing temperatures immediately after extraction to minimize condensation.
In Exp. 2, fracture water was not captured on the first core extracted but was present on the second core. The absence of ice on the core may have resulted from the core being drilled through a region of the partially saturated fracture that contained very little or no water. The lack of water on the core may have also resulted from some of the fracture water being displaced during coring by the nitrogen pressure at the core barrel tip. Cavagnaro (1999) investigated water movement caused by cold nitrogen gas flowing through a partially saturated sand core and measured water losses up to 20% at higher water contents. Compared to water in larger apertures, water in smaller apertures is less likely to be displaced by the nitrogen gas since the water will freeze faster in the smaller apertures. In our experiments, the fracture had an aperture gradient owing to a clamp being placed on one end of the sandstone rock sample. Observation of the fracture surface after the cores were extracted in Exp. 2 indicated that condensation was present in the region with the largest apertures, but little if any ice was present in that part of the fracture. Some ice was observed, however, in the middle region of the fracture where the apertures were smaller.
Radial water movement from the warmer core center to the colder edge in contact with the core barrel may also occur because of the temperature gradient. This phenomenon was observed by Cavagnaro (1999) in clay cores, where a layer of ice formed along the edge where the clay contacted the core barrel. In fractured rock, this phenomenon may occur while coring through the rock matrix, but water movement in the fractures will most likely result from the nitrogen pressure displacing the water while coring.
Potential Applications
Our laboratory experiments demonstrate that cryogenic coring is a technique that can freeze and collect in situ water in unsaturated fractures. The observations from our experiments demonstrate that the presence of a translucent sheet of ice on the fracture surface, rather than small pieces of frost, is an indicator of an active fracture. This technique could have a range of applications for characterizing in situ water and contaminant distributions in fractured porous media. Other currently used coring techniques do not allow for fracture water sampling without contaminating the core or displacing the fracture water. In fractured rocks contaminated with nonaqueous phase liquids (NAPLs), traditional coring methods may not be effective for determining NAPL distributions since the NAPL may remobilize or drain during sampling. Cryogenic coring may, however, be a promising tool for extracting undisturbed samples from fractured rocks contaminated with NAPLs. Characterization of water and contaminants in the subsurface can also be difficult because of the heterogeneous conditions, but another advantage of cryogenic coring is that it can be used over a range of media. This technique could also be used for obtaining undisturbed cores for microbial and geochemical analyses. Fracture-matrix processes may also be investigated, since simultaneous sampling of water and contaminants in the fracture and matrix is one of the intrinsic advantages of using cryogenic coring.
One limitation of cryogenic coring is that water in larger apertures may become displaced while coring by the nitrogen pressure at the core barrel tip. Therefore, the absence of ice on the fracture surface does not necessarily indicate that the fracture is inactive. The possibility of water displacement by the nitrogen pressure also makes quantification of the fracture water saturation difficult. To minimize fracture water displacement, the rate of coring can be reduced to allow more time for the fracture water in the larger apertures to freeze by conduction with the rock. Contamination of the samples by condensation is another challenge, but this can be minimized by keeping the samples in liquid nitrogen after core samples are retrieved.
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ACKNOWLEDGMENTS
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This work was supported by the Director, Office of Civilian Radioactive Waste Management, U.S. Department of Energy, through Memorandum Purchase Order EA9013MC5X between Bechtel SAIC Company, LLC, and the Ernest Orlando Lawrence Berkeley National Laboratory (Berkeley Lab). The support is provided to Berkeley Lab through the U.S. Department of Energy Contract No. DE-AC03-76SF00098. The authors would like to thank Rohit Salve, Dan Hawkes, and two anonymous reviewers for their comments on this manuscript. The work presented also benefited from discussions with George Cooper.
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REFERENCES
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- Cavagnaro, P. 1999. The improvement and environmental soil sampling through the application of cryogenic drilling and coring. M.S. thesis. Univ. of California, Berkeley.
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- Nicholl, M.J., R.J. Glass, and S.W. Wheatcraft. 1994. Gravity-driven infiltration instability in initially dry nonhorizontal fractures. Water Resour. Res. 30:2533–2546.
- Simon, R.D., and G.A. Cooper. 1996. Cryogenic drilling: A new drilling method for environmental remediation. Ground Water Monit. Rev. 16(3):79–85.
- Su, G.W., J.T. Geller, K. Pruess, and F. Wen. 1999. Experimental studies of water seepage and intermittent flow in unsaturated, rough-walled fractures. Water Resour. Res. 35:1019–1037.
- Walkotten, W.J. 1973. A freezing technique for freeze sampling streambed sediments. USDA, Forest Service Reserve, PNW-205. Pacific NW Forest Range Exp., Portland, OR.
- Wang, J.S.Y., and G.S. Bodvarsson. 2003. Evolution of the unsaturated zone testing at Yucca Mountain. J. Contam. Hydrol. 62–63:337–360.
TOP
ABSTRACT
INTRODUCTION
