Dynamic dissolution of halite rock during flow of diluted saline solutions

Authors

  • N. Weisbrod,

    Corresponding author
    1. Department of Hydrology and Microbiology, Zuckerberg Institute for Water Research, Blaustein Institutes for Desert Research, Ben-Gurion University of the Negev, Midreshet Ben-Gurion, Israel
      Corresponding author: N. Weisbrod, Department of Hydrology and Microbiology, Zuckerberg Institute for Water Research, Blaustein Institutes for Desert Research, Ben-Gurion University of the Negev, Midreshet Ben-Gurion, 84990 Israel. (weisbrod@bgu.ac.il)
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  • C. Alon-Mordish,

    1. Department of Hydrology and Microbiology, Zuckerberg Institute for Water Research, Blaustein Institutes for Desert Research, Ben-Gurion University of the Negev, Midreshet Ben-Gurion, Israel
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  • E. Konen,

    1. Diagnostic Imaging Department, Chaim Sheba Medical Center, Tel Hashomer, Israel
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  • Y. Yechieli

    1. Department of Hydrology and Microbiology, Zuckerberg Institute for Water Research, Blaustein Institutes for Desert Research, Ben-Gurion University of the Negev, Midreshet Ben-Gurion, Israel
    2. Geological Survey of Israel, Jerusalem, Israel
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Corresponding author: N. Weisbrod, Department of Hydrology and Microbiology, Zuckerberg Institute for Water Research, Blaustein Institutes for Desert Research, Ben-Gurion University of the Negev, Midreshet Ben-Gurion, 84990 Israel. (weisbrod@bgu.ac.il)

Abstract

[1] The dynamic dissolution pattern of halite salt rocks taken from coreholes near the Dead Sea was studied in laboratory-scale experiments. When unsaturated solution (with respect to halite) flowed through salt cores, dissolution developed along preferential flow pathways in a channel structure. The channel structure was related to the salt's properties and internal heterogeneities, flow velocity and impact of gravity. Preferential dissolution pathways developed in areas of minimum resistance to flow, such as large-pore networks and cracks. Nevertheless, in many cases no structural heterogeneity was observed along the dissolution channels prior to the experiments. The initial formation of channels took place above a critical flow velocity; below this threshold, dissolution developed as a slowly propagating front. In these cases, salt re-precipitation resulted in clogging and cessation of flow through a few of the salt cores. Solution density was found to be important, as evidenced by the fact that more channels developed upward than downward, due to gravitational fractionation. The development of dissolution channels could have very important implications for the overall permeability of the salt layer in general, and the use of salt formations for industrial waste storage and the development of sinkholes along the Dead Sea shore in particular.

1. Introduction

[2] The kinetics of halite dissolution in the presence of flowing unsaturated (with respect to halite salt) solutions is not well understood. Interactions between salt rock and solutions have been examined for salt rheology and permeability, mainly for industrial waste storage applications [Spiers et al., 1987; Stormont, 1997; Cosenza et al., 1999; Schulze et al., 2001]. Dissolution rates have been measured in the laboratory with NaCl solutions on compressed halite powders [Simon, 1981; Alkattan et al., 1997] and on natural salt rock slices with diluted Dead Sea (DS) water [Stiller et al., 2007]. In general, the calculated dissolution rates were determined in ideal batch experiments [Alkattan et al., 1997; Stiller et al., 2007] assuming that the salt solution is mixed with the surface area of the salt rock/grains. However, if the propagating solution changes the structure of the salt rock, it may affect the dissolution rate as flow is occurs.

[3] The dissolution rate of halite was determined by Wagner [1949] from the weight loss of thin plates of halite rock suspended in water to be 5.3 × 10−2 ± 25% mol/m2 s. Alkattan et al. [1997] examined dissolution rate as a function of temperature, halite saturation degree, disk rotation speed, and concentrations of aqueous trace metals, and was found to range between ∼1 × 10−1 and ∼9 × 10−3 mol/m2 s.

[4] Dissolution of halite rock from Sedom Mountain, located on the west coast of the DS, in diluted DS brines was measured by Stiller et al. [2007]. Experiments were performed for a large range of DS dilutions (10 to 90% volume of DS water). As expected, the dissolution rate was correlated to the initial salinity of the solutions and was double in experiments with stirring compared to experiments without. After about 2 h, the rates became similar. The experiments showed that dissolution rates of halite in DS solutions are relatively rapid and even in 50 to 90% DS water can be ∼10−2 mol/m2 s. Although DS chemical composition is very different (in g/l: Cl ∼ 225, Na ∼ 36.4, Mg ∼ 45.9, Ca ∼ 17.3, K ∼ 7.8, Br ∼ 5.6, SO4 ∼ 0.4, TDS ∼ 338.7; [Gavrieli, 1997]) from pure NaCl solution, dissolution rates found by Stiller et al. [2007] were within the range presented by Alkattan et al. [1997]. From a hydrological point of view, the structure and propagation of the dissolution can have very important implications as it may dramatically change matrix permeability.

[5] Dissolution morphologies, both occurrence of fingering and dissolution shape, in salt rock can be estimated by the dimensionless Peclet (Pe) and Damkohelr (Da) numbers and their multiplication [Kang et al., 2003]: Pe is the ratio between flow velocity and solution dispersion velocity, and Da is the ratio between reaction rate and flow rate [e.g., Steefel and Lasaga, 1990; Kang et al., 2003; Shalev et al., 2006]. Generally, a planar reaction front is expected for Da < 1 where flux is high and reaction is limited. For Da > 1, channel dissolution morphologies will be formed. The shape of the dissolution depends on the predominant transport process and the Pe number: at Pe < 1, convection is insignificant and diffusion is the main transport process and dissolution will form in the shape of short and wide fingers. At Pe > 1, convection is the main transport process and dissolution mostly forms along the main flow path, creating long and narrow fingers [Békri et al., 1995; Kang et al., 2003; Shalev et al., 2006].

[6] Morphologies such as fingering, wormholes and channels can develop as a result of small interruptions in the rock-solution interface, or of initial heterogeneity in rock structure [e.g.,Chen and Liu, 2004]. Dissolution develops via positive feedback between the flow rate and the chemical reaction that accelerates dissolution and enhances the porosity and permeability [Steefel and Lasaga, 1990; Chen and Liu, 2002; Shalev et al., 2006]. For example, when fingering morphology is formed, fluid flows preferentially to the fingers' tips, accelerating its advance [e.g., Weisbrod et al., 2002]. It is worth mentioning that new tools, such as computerized tomography (CT) that was used in this research, has become more popular in the last few years for accurate high resolution determination of matric texture and structure and various soil and rock morphologies [e.g., Nachshon et al., 2011].

[7] Beyond pure scientific interest, dissolution dynamics in subsurface salt layers is important for practical reasons. For example, it contributes to our understanding of the processes of cavity and sinkhole formation attributed to the exposure of salt layers to unsaturated solutions [e.g., Martinez et al., 1998]. In the DS area, a decrease in groundwater salinity near the subsurface salt layer and its partial dissolution have caused the development of over 2000 sinkholes along the DS shore [Yechieli et al., 2006]. The other practical aspect is possible use of the salt layer for storage of industrial or even radioactive waste materials [e.g., International Atomic Energy Agency, 2003].

[8] The objectives of this study were: (1) to explore the dynamics and rate of natural salt rock dissolution under flow of unsaturated solutions (with respect to halite), and (2) to explore the dynamics of dissolution hole expansion during flow through it. Data from a series of laboratory flow experiments in which unsaturated solutions were forced to flow through natural salt cores are presented together with CT analyses of dissolution channel structure. The expansion of an existing pipe/channel in salt cores during flow of diluted solution is also explored. Both visual observations and CT analyses are presented.

2. Materials and Methods

[9] Halite salt cores (∼67 mm diameter) were taken from coreholes that were drilled along the DS shore from depths of 27 to 65 m at two drilling sites [Alon-Mordish, 2010]. All cores consisted of natural salt rocks composed of halite crystals. Two groups of salt cores were defined based on their texture, color, and morphologies as observed in CT images: “white massive salt” (WMS) and “crystalline transparent salt” (CTS). 8 WMS and 6 CTS cores were prepared. Some cores were composed of microscopic (<8 μm) halite crystals, others of up to 2 cm halite crystals imbedded in some fraction of clay and calcite (∼1% by weight). For comparison with the natural salt cores, the permeability of artificial agricultural salt lick blocks (KNZ 100, AkzoNobel, The Netherlands) was also examined. This salt is produced from 100% NaCl salt (raw >99.7% pure salt). Two sets of experiments were carried out: experiment A and experiment B. The permeability of the salt was calculated from the saturated hydraulic conductivity which was determined from flow experiments with DS water saturated with respect to halite (i.e., no dissolution).

[10] Porosity and bulk density were measured with a porosimeter (Corelab, Core Laboratories Inc., Houston, TX) at the Geological Survey of Israel (porosity 11 ± 2%; bulk density 2 ± 3% g/cm3). Cores were sliced to 5-cm long pieces and casted at the center of PVC pipes (8 cm long × 8 cm diameter) using epoxy (EP 501 + EPC 304, Polymer G'vulot Ltd., Israel) that filled the gap between the core perimeter and the PVC pipe. A column cover was cut from a 3-mm thick Perspex plate and glued to the PVC pipe. The space between the Plexiglas cover and the core edge formed a cell that was later filled with DS solution. Plastic pipe connectors were screwed into the PVC tubes (Figure 1a). Of the 14 cores prepared, seven cores were used in this set of experiments (3 WMS and 4 CTS) while 7 cores were used in experiment B (5 WMS and 2 CTS).

Figure 1.

Experimental setups for (a) the flow-through experiments and (b) that examining the dissolution process of a pre-existing channel. On the right, the schematics of the salt core setup are enlarged.

2.1. Experiment A

[11] Laboratory-scale flow experiments were performed under atmospheric pressure and a fixed temperature of 25 ± 2°C. In a set of column experiments (Figure 1a), saturated DS solution was forced through the cores under a constant head of 80 cm using a Marriott bottle [McCarthy, 1934]. When flux through the core was stable, the natural permeability of the matrix was determined. Next, the solution was switched to an unsaturated solution with dilutions of 70 to 95% DS solution (where % DS represents the volumetric percentage of DS water mixed with distilled water). Outflow solutions were continuously weighed (GF-2000, A&D Weighing, San Jose, CA), and permeability and flow velocity were calculated based on Darcy's law [Freeze and Cherry, 1979]. Dissolution patterns at the inflow (front) face were monitored by digital camera (UI-1545, IDS uEye, Germany) operated by RsCom computer program. Images of the front face of the core were taken from every few seconds to 2 h, depending on the experiment stage. The concentrations of Na and Cl were also analyzed to estimate the amount of halite dissolution. Once a dramatic change in flow rate was observed, the flow experiment was terminated. The cores were taken for examination of internal dissolution patterns by CT (ICT 128, Philips, The Netherlands). To analyze the CT scans, 3D image processing software was used (AVIZO 5, by Mercury, Burlington, MA). A high-resolution protocol was used for all experiments, with images of 768 × 768 pixels. Scans were performed under an excitation voltage of 120 kV with voxel dimensions of 0.26 × 0.26 mm and slice thickness of 0.33 mm. Note that termination of the experiments as soon as a rapid increase in flow rate was observed, to prevent further fast dissolution, enabled 3D mapping of the internal dissolution channels with the CT.

2.2. Experiment B

[12] In another set of column experiments performed under the same environmental conditions as experiment A (Figure 1b), the dissolution process was studied for pre-existing channels drilled into seven cores. Prior to the flow experiment, a 2.5-mm diameter channel was drilled along the entire core. Next, 70% DS solution was forced to flow through the drilled channel at a constant flow rate of 0.25 or 0.1 ml/min via a peristaltic pump (Minipuls 3, Gilson, Middelton, WI). During flow, images of the core's inlet face were taken at 15-min resolution with a digital camera to follow channel growth. Experiments were terminated when the diameter of the drilled channel at the inlet face had about doubled (semi-quantitative). In all but one experiment, the cores were placed horizontally (as inFigure 1b). In one experiment, the core was placed vertically and the flow was forced from bottom to top. Prior to and following these experiments, all cores were scanned with a CT and analyzed as described for experiment A, to compare channel size and shape with its initial parameters. At the end of each experiment, the columns were emptied to prevent further dissolution.

3. Results and Discussion

3.1. Salt Permeability

[13] The measured initial permeability in the eight salt cores ranged between 5 × 10−13 and 5 × 10−9 cm2(flow rates of ∼0.0005-1 mL/min). Although the number of cores tested did not allow for statistical analysis, the data shows grouping; different slat structures seem to have a similar permeability range (Figure 2), with higher values from the CTS.

Figure 2.

Initial salt permeability of the CTS and WMS salt cores used in the experiments described in this paper and of commercial salt, shown in a box plot curve (50% of the values are within the gray box). Cores and commercial salt are designated “C” and “Ch”, respectively. Permeability is on a logarithmic scale.

3.2. Permeability Changes Due to Dissolution

[14] Replacement of the saturated DS solution with a diluted solution resulted in an increased amount of dissolved NaCl in the outflow (Figure 3). The permeability of the different salt cores either increased dramatically (four cores) or decreased (three cores) [Alon-Mordish, 2010]. An example of each case is shown in Figures 3a and 3b, respectively. The increase in permeability was related to the development of dissolution channels in the salt cores (visually observed at the inflow and outflow faces during the flow experiments). Once there was a channel or a set of connected channels through the core, the permeability increased dramatically. In few cases, where the experiments were continued, the permeability increase by several orders of magnitude. In most case, the flow experiment was terminated at this stage and the cores were taken for CT analysis. Examples of the developed dissolution channels are shown in Figure 3c. The 3D CT images revealed different structures where the channels combine and split off in the salt bulk (Figure 3c). Where dissolution occurred, the channel network consisted of two to five channels with a maximum diameter of 2 cm.

Figure 3.

(a) Permeability and NaCl dissolution over 67 pore volumes (PV) in core 4. Note the sharp increase in permeability. Note also that once the exponential increase in flow started, the experiments were terminated to retain the channel geometry for the CT analysis. (b) Permeability and NaCl dissolution over 47 PV in core 11. Note the decrease in permeability. The insets in Figures 3a and 3b demonstrate the mechanism governing the increase and decrease in flow rate, respectively. (c) Two examples of dissolution channel morphology as defined by the CT analysis following the flow experiments. The black and white images are cross sections and the purple ones are 3D images. On the right, the core position of each image is given. Flow direction was from left to right in both cases. Note that due to technical limitations, once permeability started to increase rapidly measurements were ceased. Very rough calculation of dissolution rate was calculated for core 4 to be 0.4 g/day or 1.26 × 10−3 g/g*day, taking into account the volume of the core (176 cc), assumed porosity (0.1), salt density (2 g/cm3), and core weight (300 g).

[15] Under the experimental conditions, dissolution channels in the salt cores formed above a critical flow rate of ∼0.01 ml/min. At a lower flow velocity, no channels were created and the flow rate was either stable or decreased. Nevertheless, this flow rate threshold should be considered with caution as the differences between flow rates were very small and the number of repetitions was also small. It is interpreted that the reduction in permeability (e.g., Figure 3b) was due to re-precipitation of salt in the pores, resulting in pore clogging and flow reduction.

[16] As already noted, dissolution shape is expected to depend mainly on the values of the Da and Pe numbers and their multiplication. In general, a planar front dissolution is expected when Da < 1, and a dissolution channel is expected when Da > 1. However, in some cases, the obtained morphologies were not as expected based on Da number: both morphologies (planar and channel) were formed for Da numbers larger than 1 (Da ∼ 1000; a range of 40–5000 was found in all the experiments of Alon-Mordish [2010]) representing the case in which reaction rate is higher than flow rate and channels are expected to form. While in some cores channels did form, in others, a dissolution front developed. It seems, therefore, that a dissolution front can be formed even when Da > 1. Pe numbers could not be estimated in these experiments since the dispersion velocity was unknown.

[17] Taken together, our results showed that under the experimental conditions used in this study, dissolution channels develop under an unsaturated flow rate of about 0.01 ml/min or higher (examined up to 0.2 ml/min). It is likely that the cutoff velocity between the development of flow channels and a progressing dissolution front will differ according to salt properties, head differences and solution chemistry. The importance of flow velocity and its effect on fingering and channel formation was also explored in a theoretical work on sinkhole development in the DS coastal area [Shalev et al., 2006], showing that fingering does not occur at relatively small flow velocities. It is interesting to note that, according to that theoretical study, high velocity are expected to create a dissolution front without fingering [Shalev et al., 2006].

3.3. Impact of Density on Dissolution Pattern

[18] Figure 4shows dissolution patterns from pre-existing artificial channels (experiment B) in salt cores in 2D and 3D CT images. Under a forced flow of 70% DS solution, dissolution was quite similar in all cores. Massive dissolution was measured near the area of solution inlet and dissolution volume decreased along the flow path.Figures 4a–4cclearly shows that in horizontal flow, channels expanded upward relative to the initial artificial channel location. Dissolution expansion was in the shape of small bumps that occurred along pre-existing cracks.Figures 4d and 4e shows cracks crossing the artificial channel and dissolution growing upward only into the cracks.

Figure 4.

(a–e) Core cross section (X-Z plane) in CT images of salt cores with predrilled 5 mm hole reveals massive dissolution at the channel entrance, dissolution development upward (all cross sections), and dissolution expansion along pre-existing cracks upward (Figures 4c and 4e). Yellow arrows show flow direction and yellow rectangles show initial channel location. (f–h) The 3D CT images show that dissolution grows preferentially wider on the Y axis compared to the Z axis. Upward dissolution into small cracks can clearly be seen in Figure 4h.

[19] The above observations imply that gravity plays a key role in the dissolution patterns formed in the salt bulk. As the diluted solutions dissolve salt, they become denser and heavier. Based on density differences, lighter solutions with higher dissolution potential flow above the denser solutions and dissolution proceeds upward (Figure 4). Dissolution expansion upward along a pre-existing crack (Figure 4d) exemplifies both gravity's impact on the channel pattern and the existence of an area with minimum resistance to flow (i.e. crack). Our results are comparable with those of Dijk et al. [2002], who suggested that dissolution patterns in salt fractures develop preferentially upward, dominated by flow structure and by the orientation of the fracture walls.

3.4. Implications

[20] The development of dissolution channels could have very important implications for the overall permeability of the salt layer. Here, we suggest that dissolution channels develop along preferential pathways in areas with minimum resistance to flow, such as large pores or cracks. If an unsaturated solution is flowing through salt, there is most likely an area of minimum resistance to flow, and dissolution channels might develop along this area. Our experimental results are in agreement with previous simulations showing that dissolution that develops along preferential pathways increases permeability and flow through the salt, which in turn enhances dissolution [Békri et al., 1995].

[21] The development of dissolution pathways in salt rocks is also of importance in areas where hazardous waste is buried in salt formations. Where dissolution channels are developing, they may dramatically decrease the transport time for waste out of the repository. It should be noted that no hazard is expected during the salt-dissolution process until a full-sized channel has developed, allowing water to flow from one layer to another and cross a previously known low-permeability layer.

[22] The rate of salt dissolution is extremely important in cases where such a process can create cavities in the subsurface and subsequent sinkholes at the surface, such as in the DS area. This rate is related to the dissolution structure. If dissolution creates preferential pathways it will be much faster than if it is moving as a dissolution front. When the entire DS coastal area was covered with DS solution, which was nearly saturated with respect to halite, no significant dissolution occurred and groundwater flow was very slow, according to the permeability of the salt layer. However, when the DS level began to drop, together with the fresh-saline water interface, dissolution started to occur, probably in a channel system, as also simulated byShalev et al. [2006]. Once there is a connection through the salt layer (from bottom to top), the flow will increase by many orders of magnitude, allowing rapid sinkhole formation, as has indeed been found in the DS area. This is similar to the laboratory process in which we saw that the flow velocity increases very little until a connecting channel forms and then the permeability increases abruptly by an order of magnitude (at which point the experimental procedure was terminated). Once the dissolution channel develops, the enlargement process is extremely rapid, as observed in the experiments with the pre-existing channel. The channel system thus allows the saturated solution to flow out of the salt layer to the regional base level (the DS), enhancing salt dissolution and the creation of sinkholes.

[23] The sinkholes at the DS are formed due to dissolution of subsurface salt layer by ascending relatively fresh groundwater from an aquifer below which has higher pressure than the salt layer. Thus, density can enhance dissolution since the less dense water will remain in contact with the salt layer and continue the dissolution while the denser water will sink to the bottom of the aquifer.

4. Summary and Conclusions

[24] The data presented herein describe the pattern and evolution of dissolution channels in a natural salt layer during flow. It was observed that even in salt rock that seems very homogeneous, if the flow rate increases above a threshold value, dissolution channels may develop. If the flow rate is below this value, the dissolution front pattern will dominate the process and in some cases, flow through the core can completely cease due to re-precipitation of salt inside the pores. Solution density and micro-cracks were also shown to play an important role in the structure of the developed dissolution channels. Once dissolution channels develop, the dissolution process through salt layers will be much faster than through bulk salt and will control the transport of solution across the salt layer. At this stage, salt dissolution can create an extremely hazardous situation due to cavities in the subsurface. Such cavities can result in the movement of waste-disposal material to unexpected locations and in the formation of sinkholes in areas of hydrological changes.

Acknowledgments

[25] We thank Uri Nachshon for his assistance with the CT image processing, and Jiwchar Ganor and Eyal Shalev for their useful comments. This work was funded by the Binational Science Foundation (BSF), contract 2006018, and by a grant from the Israeli Ministry of Infrastructure.

[26] Paolo D'Odorico would like to thank Eyal Shalev and an anonymous reviewer for their assistance with this manuscript.

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