Earth Sciences Division (ESD) Department of Energy (DOE) Lawrence Berkeley National Laboratory (LBNL)

The Yucca Mountain Project: Laboratory Testing: Liquid Flow in Fractured Rock

Quicktime LogoThe following experiment can be viewed as short Quicktime movies. You need Quicktime and the Quicktime plug-in to view the movies. Because the movie files tend to be large, both large frame and small frame versions of each movies are included. You can seen the phenomena better in the large frame movies, but they may take a reletively long time to download. The data in these movies is preliminary, is for viewing purposes only, and is not to be used for quality affecting work.

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Introduction

Liquid flow in fractured rock is not understood well and is very difficult to predict. The proposed disposal of high level nuclear waste in fractured tuffaceous rock at Yucca Mountain is expected to heat the repository environment to temperatures exceeding the boiling point of water. The seepage of water naturally percolating through the mountain will be affected by the heating, which will boil the water. The boiled water will condense in cooler regions, and continue to flow. To better understand seepage in fractures when liquid boiling occurs, we have performed laboratory experiments in fracture models and replicas.

In the following movies, we will observe some behaviors of a liquid in fractures with a boiling-hot region and a cooler region. One of these fractures is assembled from a natural rock fracture face and a transparent replica of the mating face, one is a transparent replica of the two faces of a natural fracture, and the others are made from rough or flat sandblasted glass. Pentane, which normally boils at 36.1 C, was used instead of water to simplify experimental setup.

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Rock & Replica

We obtained a sample of Topopah Spring Tuff containing a natural fracture from the geologic unit of the proposed repository at Yucca Mountain. In order to see flow in the fracture, a transparent epoxy replica was made of one of the fracture faces and mated to the rock face.

Small Volume Seepage in a Hot Fracture

We heated a portion of the rock to well above the liquid boiling point, attached the replica face, and introduced pentane. The fracture is hot and the pentane quickly boils and imbibes into the dry rock.

Medium Volume Seepage in a Warm Fracture

After some cooling, although still above the pentane boiling point, a larger volume of pentane is introduced. Again boiling occurs, but the pentane fingers much deeper into the fracture before it is imbibed into the rock.

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Experiment Equipment

The equipment used in the following experiments consists of the temperature controller in the lower right, the light source in the upper right, a transparent tank containing warm water supplied by the controller, and a fracture model or replica. The warm water in the tank creates a boiling hot region in the partially submerged fracture model. The fracture model shown here is made from sandblasted "shower-door" glass sealed on the bottom and sides and vented on the top.

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Seepage into Warmed Region

Pentane was introduced into this fracture model made from two sheets of sandblasted "shower-door glass" sealed on the sides and bottom and vented on the top. Heat is applied through the circular devices near the bottom. The temperature within these disks was well above the pentane boiling point causing a boiling region to extend more than a centimeter from the disks.

We'll see three segments from this experiment. In the first segment, pentane is introduced into the model and encounters the warmed region. In the second, no pentane is added and a stable dry-out region is formed. In the third, pentane is again added.

Here we see pentane seeping through the model, reaching the boiling region. When enough pentane builds up above the boiling region, fingers penetrate, and a condensation halo forms both above and below. As more pentane is introduced, the condensation halo increases in size. With even more pentane, fingers deeply penetrate the boiling region.

The amount of light transmitted through the models depends on the amount of pentane in the model aperture, and the lighting conditions. In the brightest regions , liquid completely saturates the aperture. Liquid films present only on the walls of the fracture appear less bright than the saturated islands, with thicker films being brighter than thinner films. Darker regions indicate where the model is dry.

For example, at the top of the model, where the pentane is being introduced, we can clearly see the bright liquid drops or saturated islands where pentane completely fills the aperture. These are surrounded by a less bright region in which liquid pentane films are present on the sandblasted glass surface. Towards the center of the model, we can no longer see the saturated islands due to the lighting conditions and the lack of contrast. We can see another less bright thinner film region between the darker dry glass and the brighter thick film and saturated island region.

Low Flow - Stable Warmed Region

When pentane is no longer introduced, a stable dry region is formed, remaining dry even though liquid continues to move downwards in films.

High Flow - Fingering in Warmed Region

When more liquid is added, fingers again penetrate the boiling region.

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Flow in Films & Intermittent Fingers

Pentane has been introduced into the space between the glass plates in this model. The liquid has boiled out of the lower warmer region, and has condensed above in the condensation halo. In this halo, liquid is flowing in films on the fracture walls back down towards the warmed region. Some liquid remains in films as seen by the two smooth-curved bright areas extending down into the warmed region. (ARROWS) In the condensation halo, where films of liquid from both fracture faces are thick enough to completely fill the aperture, saturated islands of liquid form. Liquid accumulates in these saturated islands until gravitational forces exceed the capillary forces causing the liquid to move. Liquid from one saturated island may flow through other saturated islands adding volume to the moving rivulet. The fingers in the warmed region are the result of these flow events, called intermittent rivulets.

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Intermittent Flow With and Across Microfracture

Here we see intermittent rivulets of pentane flowing in a replica of a natural granite fracture from the Stripa mine in Sweden. Fast moving rivulets from above the diagonal microfracture flow vertically across this feature. Slower-moving rivulets flow diagonally down the microfracture.

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Flow Focused by Funnel-shaped Hetergeneities

Flow in this model made of sandblasted "shower-door" glass is focused by barriers which model flow barriers present in the natural environment. Longer fingers in the center are formed by the focusing of flow from above.

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Rapid Evaporation Events

Rapid evaporation events occur when liquid superheats and violently boils. These events generate large volumes of vapor very quickly, which may alter liquid flow. Here we see frequent rapid evaporation events occurring in a finger on the left side of the model. We see pentane flowing down into this finger and rapid evaporation events occurring at several locations within the finger, while the orb-shaped finger in the center has remained quiet. Keep an eye on the orb-shaped finger.

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Gas Driven Liquid Flow

The liquid evaporating in the boiling region generates large volumes of vapor. This leads to increased pressure which may alter liquid flow. Here we see liquid droplets (left side) being forced upwards by pressurized vapor. We also see alteration of the shape of the saturated islands by the moving vapor(right side).

Conclusion

We have seen several important flow phenomena in boiling-hot fracture models and replicas including condensation halo formation, film flow on fracture walls, intermittent rivulet flow, focused flow, rapid evaporation events, and gas-driven liquid flow. More detailed information on experimental conditions, sample preparation and identification, and equipment calibration is available in LBNL Report 40467 -"Preferential Flow Paths and Heat Pipes: Results from Laboratory Experiments on Heat-Driven Flow in Natural and Artificial Rock Fractures". This report was prepared under the YMP Quality Assurance Program at Lawrence Berkeley National Laboratory.

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Acknowledgements

This work was supported by the Director, Office of Civilian Radioactive Waste Management, U.S. Department of Energy, through the Memorandum Purchase Order EA9013MC5X between TRW Environmental Safety Systems, Inc. and the Ernest Orlando Lawrence Berkeley National Laboratory, under Contract No. DE-AC03-76SF00098.

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For more information, please contact:

Tim Kneafsey
Earth Sciences Division
Phone: 510-486-4414
Email: TJKneafsey@lbl.gov