Borehole seals of expandable clay with desired water content
by use of “dry water”
Thomas Forsberg a, Roland Pusch b, Ting Yang c, Sven Knutsson ∗
Dept. of Civil, Environmental and Natural resources Engineering,
Luleå University of Technology, 971 87 Luleå, Sweden
Abstract. Dense clay can be used for sealing of boreholes. Keeping the clay confined in perforated tubes it can be
inserted in boreholes of nearly any length and diameter. Expansion of the clay to fill the borehole takes place by
uptake of water and migration of clay through the perforation. The clay thereby exerts a swelling pressure on the
confining rock causing effective sealing. For shallow boreholes to be sealed a very low initial degree of water
saturation of the clay may be valuable since this makes the clay expand and seal the hole quickly, but for certain cases
the clay should have a higher degree of water saturation. This can be required for moderating the rate of clay
densification that may otherwise give too high wall friction for placement in very long holes. Sealing of very deep
holes and holes containing highly radioactive waste makes temperature important: the heat-induced expansion of
initially fully saturated clay can fracture the confining rock. The issue is therefore to prepare the clay inserts with
properly selected water content. The paper describes preparation of clay seals by mixing air-dry clay powder with
nanoparticles of water droplets coated with very thin shells of a hydrophobic silicious substance (“dry water”). It
behaves as dry powder and is easily mixed with dry clay. On compaction to the desired density the shells break into
aggregates of minute fragments while water becomes homogeneously distributed in the mass. Laboratory and benchscale testing verify that the properties of clay prepared in this way are the same as of clay saturated by sorbing water
through a filter, a process that can take hundreds of years for big samples.
Keywords: borehole sealing, clay blocks, degree of saturation, density, “dry water”, water content
1. Introduction
Several concepts for sealing of boreholes and disposal of radioactive waste make use of dense
smectite-rich clay, commonly called “bentonite” (Pusch, 1983; Yong et al, 2010). Physical
characterization of such clay with respect to the rate of ion diffusion, thermal conductivity and
rheological behaviour requires that the degree of water saturation is known. The state of complete
water saturation is particularly important since this is usually the condition after some years or
decades (Ting et al, 2015) in a longer perspective. The most common way to saturate clay samples
is to confine them in cells with filters at the ends and let water be taken up by suction through the
∗
Corresponding author, Professor, E-mail: [email protected]
Research Engineer, E-mail: [email protected]
b
Professor, E-mail: [email protected]
c
Ph.D. Student, E-mail: [email protected]; [email protected]
a
filters but this is a tedious diffusion-like process: saturation a 2 cm thick sample of dense smectiterich clay with uptake from two ends requires about one week, while a 4 cm sample requires 16
weeks and one with 10 cm thickness about 2 years. A 20 cm thick clay block requires about 20
years to be largely saturated. Placing the clay in powder form layer wise in cells and spraying
water on the layers, followed by compaction, is possible but the time for homogenization of the
clay samples will blocks will still be very long and they will not be homogeneous with respect to
the distribution of water an density.
We describe here a quick procedure for preparing any powdered material with homogenously
distributed water content by mixing air-dry or dried clay in powder or granulate form with “dry
water” (DW) consisting of droplets of water coated with very thin shells of a silicious substance
(Forny, 2008). On compaction to the desired density the shells break into fragments that are
smaller than silt grains. The released water becomes uniformly distributed in the mixture.
2. Materials
2.1 “Dry water”
“Dry water” consists of solid water droplets contained in spherical, very thin shells of
hydrophobic, fumed silica particles (Forny, 2009; Bomhard, 2011). The powder is dray and
lyophobic, despite a water content by weight of just about 90%. It flows like flour when poured
into laboratory cells or large containers for compaction to the desired dry density (ratio of mass of
solid substance and total volume including voids). The angle of internal friction is reported to be at
least 44° (Bomhard, 2011). The silica coating repels water and prevents the water droplets from
combining at moderately high and low temperature. The material can be produced on an industrial
scale by exposing volatile chlorosilanes to high temperature by flame hydrolysis and reaction with
methyl chlorosilanes after cooling. Its primary use is as filling agent for plastics and as additive in
food production (Lankes, 2006; Bomhard, 2011). It is available on the market, the chemical
components of the material used in the described project being supplied by Wacker Chemie AG.
The size distribution of the DW grains was 1-10 µm in the present study. The specific surface area
is 20 to 35 % of that of smectite clay. The residual silanol content of the hydrophilic silica is 25 %
and the carbon content about 2.8 % of the solid part of the DW.
2.2 Clay
The tightening component of a borehole seal is dense expandable clay (smectite) that sorbs
water from the confining rock or soil by its potential to bind water between the 1 nm thick Si/O
and Al/Mg/OH lamellae (Pusch & Yong, 2010; Pusch, 2015). It is tightly contained in a perforated
metal tube that is fitted into the hole to be sealed (Fig.1). The clay expands through the perforation
and embeds the tube at a rate that depends on the density and degree of water saturation (Fig.2).
The expansion, which can be up to about 3 times is caused by establishment of one, two or three
water molecules thick intraparticle hydration layers. The expansion is uniquely controlled by the
dry density and access to water.
Fig.1 Borehole sealing by use of dense smectite clay. Left: Sealing stages (Pusch, 1983). Right: Installation
of borehole seal in 500 m deep borehole.
Fig.2 Appearance of 24 hour old clay plug after removing part of the clay skin formed by clay migrated
from the dense clay core in the perforated tube to the narrow space between tube and rock (Pusch,
2008).
2.3 Interaction of clay and DW
Fig.3 shows schematically how DW droplets are linked in the microstructure of smectite clay
after mixing dry clay powder and DW. The strongly hydrophilic clay considered in this paper will
suck up water given off from crushed DWs that are uniformly distributed in the DW/clay mixture.
Fig.4 indicates how DW is distributed and integrated in the clay matrix.
Fig.3 Microstructural voids with DW droplets between 3-7 nm thick stacks of smectite lamellae in
uncompacted air-dry smectite clay (Pusch, 2015).
Compression of clay with DW droplets causes breakage at a pressure of 40-80 kPa and
subsequent, successive homogenization (cf. Fig. 4). The obtained degree of water saturation can be
100 % or lower, depending on the needs; the required amount of water is calculated and the
corresponding amount of DW added by mixing. Pre-saturated blocks of dense clay for borehole
sealing in a repository with canisters holding high-level radioactive waste transfer the produced
heat effectively to the rock, while blocks with natural water content can give unacceptably high
temperature of the canisters and clay seals. An important question dealt with in the present paper,
is whether the remainders of the crushed silicious shells of the droplets can significantly affect the
physical properties of the DW clay and make them deviate from those of conventionally saturated
clay.
Fig.4 Redistribution of DW water at compaction (left), and subsequent maturation of the clay matrix (right).
3. Experimental
3.1 Clay material
The clay used in the study a mixed-layer clay1 belonging to a Paleocene formation of Tertiary
age (Henning and Kasbohm, 1998) with 55 % expandable minerals, mainly montmorillonite. The
mineralogical and chemical compositions are given in Tables 1 and 2. Determination of the size
distribution of the granules, by sieving after drying at 60°C, gave 2 mm at maximum and 40 %
smaller than 0.063 mm. The content of uniformly distributed residual silanol and carbon makes up
less than one weight percent of the clay sample. The majority of the silicious part is believed to
behave like the 20-30 % amount of the silica-rich accessory minerals quartz, feldspars.
Table 1 Accessory minerals in weight percentages of smectite-rich Holmehus clay identified by XRD and
CEC techniques (Pusch, 2015).
Muscovite Chlorite
Quartz
Plagioclase
K-feldspar
Gypsum
Pyrite
<1
15
5-8
0.3
Table 2 Chemical composition of Holmehus clay (weight percentages); (Kasbohm et al., 2013).
Oxides
wt %
SiO2
58.6
Al2O3
15.3
Fe2O3
6.5
CaO
0.7
MgO
2.2
K2O
2.8
Na2O
1.4
3.2 DW material
The DW material was prepared by adding 50 g pyrogenic silica powder (Whacker HD
K2000) to 500 g of distilled water in a mixer and agitating it for about 2 minutes. By ru
nning the mixer at 20,000 rpm for 2 minutes the DW got a suitable form –it appeared as
light, dry powder (Fig.5) - for being mixed with clay.
Fig. 5 Manufacturing of DW by mixing 10 % (weight percentage) of distilled water with 90 % pyrogenic
silica.
Fig.6 illustrates the size of the droplets and the thickness of their coatings (“shells”) of
1
Illite/smectite clay provided by Dantonite A/S, Denmark
hydrophobic silica. Since the surface of DW particles is entirely covered by hydrophobic material
the material is felt dry and performs as dry powder of sugar or flour.
Fig. 6 Silicious DW droplet with coating (“shell”).
The hydration energy of DW droplets is mirrored by the release and evaporation of water from
them in different environments. Fig.7 shows the outcome of laboratory experiments indicating that
droplets with 90 to 97 % water content lose water much slower than (free) water in a glass beaker.
The loss is because the confined water diffuses slowly through the coatings of the droplets, a
process that successively releases the water contained in them. In practice, only freshly prepared
DW should be used for avoiding non-uniform wetting of the clay.
Fig.7 Rate of loss of water from a mass of DW droplets with different water contents compared with the
evaporation of ordinary water from the same volume of free water.
3.3 Mixed clay and DW
DW material was prepared for reaching complete saturation of clay after compaction in
oedometers. Samples with different dry densities, 1370, 1800 and 1900 kg/m3, were made for
determining the hydraulic conductivity and expandability. The conductivity was determined by
applying a hydraulic gradient of 67 m/m (meter water pressure difference per meter flow length) at
percolation with distilled water. The filters confining the samples at each end were connected to
burettes and the pressure was adjusted to maintain constant clay volume. The oedometer cells were
mounted in a compression apparatus for recording the expandability in the form of swelling
pressure.
3.4 Results
3.4.1 Chemical constitution
Fig.8 shows the atomic composition of dried DW-clay indicating presence of iron, calcium,
sulphur, silica and aluminum. The latter element represents clay particles and silica shells of DW.
Pd and the strong peak to the left are signals from the detector. Fe is present both as sorbed
exchangeable ions in and on the clay minerals (cf. Xiaodong, Prikryl, Pusch, 2011), in shell fragments,
and in precipitated complexes in the natural clay. The atomic spectrum of the remainders of
crushed droplet shows Si and Fe as important cationic components. The role of chlorine from the
pyrogenic substance, if still present, is unimportant because of its coupling to the silicious
component and because it makes up a very small fraction of the solid mass.
Fig.8 EDX spectrum of DW-saturated clay (Warr, Greifswald University, Germany).
3.4.2 Microstructural constitution
Fig. 9 illustrates the typical microstructural appearance without visible residual shell fragments.
The photo reveals the strong variation in mineral composition (quartz grains are white, feldspars
brown and clay minerals greenish). The row of small black dots are organic remainders. Open
voids cannot have been larger than 20 µm. The micrograph was taken of the surface of a section
exposed by layerwise tape peeling.
Fig. 10 shows a totally fractured but still coherent DW-grain embedded in a dense matrix of
smectite particles having compressed it (cf. Fig.4, left). Such fractured and compacted shell
fragments were few and remained in their original positions. The large majority of the shells were
strongly fragmented and their only impact on the bulk physical properties would be to tighten very
small voids and channels.
Fig.9. Optical micrograph of moist DW-saturated smectite-rich Holmehus clay with a density of 1570 kg/m3
(magnification 250x).
Fig. 10 SEM micrograph showing remnants of a crushed DW particle with a size of about 2 µm.
3.4.3 Hydraulic conductivity and swelling pressure
The hydraulic conductivity of DW-saturated clay and clay saturated by inflow of distilled water
in oedometer cells is given in Table 3.
Table 3. Comparison of hydraulic conductivity (K, m/s) and swelling pressure (ps) for tests of DW-saturated
Holmehus clay and samples prepared by conventional wetting, i.e. suction of air-dry clay powder
compressed and confined in oedometer cells for saturation and percolation with distilled water (RW).
Wetting type
Dry density,
Saturated density
K, m/s
kg/m3
kg/m3
DW
1430
1900
1E-13
DW
1270
1800
7E-13
DW
980
1570
1E-10
RW1)
1430
1900
2E-12
RW1)
1270
1800
2E-11
RW2)
1065
1670
1E-10
1)
Ting, 2015; 2) Equivalent clay with slightly higher montmorillonite content
ps,
MPa
3.2
2.2
0.3
2.7
1.3
0.3
It is obvious that the DW-saturated clay was consistently somewhat less conductive than the
conventionally wetted clay. This is believed to be caused by a more uniform distribution of water
and a more homogeneous microstructural constitution of the DW-clay. The swelling pressure
exhibits a similar pattern: the values are consistently somewhat higher for the DW samples than
for the conventionally saturated clay. As for the conductivity this is believed to be caused by a
more uniform distribution of water and a more homogeneous microstructural constitution of the
DW-clay. The higher values for DW clay proved that the very fine fragments of silicious shells did
not hinder the smectite stacks to hydrate and expand. They were confined in small voids in the
clay.
4. Discussion and conclusions
The following major conclusions can be drawn from the study:
•
At saturation by DW technique the water added to air-dry clay material by thorough
mixing and subsequent compaction becomes uniformly distributed and complete homogeneity
of the mixture is reached early. The technique can be used for preparing seals of boreholes and
deposition holes for radioactive waste,
•
The very thin silicious coatings of DW droplets break on compaction under less than 100
kPa pressure and create numerous fragments smaller than 1 µm, which assemble on site in
small clay voids and channels in the microstructure. This does not cause any reduction of the
expansion potential of the clay but has a clogging effect that explains the low bulk hydraulic
conductivity compared to that of clay that is water saturated by inflow in clay confined in cells
like oedometers,
•
The physical properties of clay saturated by DW technique are slightly different from
those of clays wetted by conventional one-dimensional uptake of water by suction in oedometer
cells. The hydraulic conductivity is lower for DW-clay because of filling of voids and channels
with shell fragments. The swelling pressure is higher since expansion and loss in density of the
clay particles by expansion into microstructural voids is hindered by such fillings, an effect that
contributes to the lower hydraulic conductivity of DW clay.
•
DW wetting gives immediate saturation since it takes place in conjunction with the
compaction of the clay to give blocks. The advantage of the method is that the samples
instantly reach a state of uniform distribution of porewater at any desired degree of water
saturation. The technique is very attractive from economical and practical points of view,
especially for preparation of blocks of large dimensions for which conventional ways of
saturation require many years or decades.
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