Halite Thin Section

[Timothee Cazares]

The use of caverns in rock salt (halite) for Underground Gas Storage (UGS) and Compressed Air Energy Storage (CAES) has been identified as a strategic option to meet seasonal energy demand fluctuations in the electricity grid (Ozarslan 2012; Evans and Chadwick 2009; British Geological Survey 2016; Peach 1991). UGS, used to meet the seasonal fluctuations in energy demand, operates with moderate gas production rate and just one or few cycles per year. UGS has been also assessed as a more feasible option for short-term peak shaving operations compared to surface gas tanks. Indeed, salt caverns present lower specific construction costs, smaller surface footprint, and very high level of protection from external influences. The combination of big volumes and large range of operating pressures allows a typical large salt cavern to hold more than 60 times the volume of traditional gas tank (Michalski et al. 2017). CAES is a promising solution for energy storage with a significant and growing scientific and economic interest to support intermittent sources of renewable energy such as wind energy (Brest Pierre et al. 2013; Bullough Chris et al. 2004; Lund and Salgi 2009). It also implies that CAES present daily cycles of injection-withdrawal operations compared to UGS (Brest Pierre et al. 2013).

Salt caverns have also been identified as a key element in the UK energy system decarbonization strategy for storing hydrogen (Sadler 2016), for power generation purposes to meet the UK government target to reduce greenhouse emissions (Parliament 2008). Hydrogen is presented as the only gas decarbonization option suitable to replace natural gas, by doing some modifications to the current gas networks, ensuring energy supply in the long term, from a cost-optimal perspective (Dodds and McDowall 2013).

Halite has low (i) creep strength, (ii) porosity, (iii) permeability, and iv) density, making it a very good seal rock (Brest Pierre et al. 2012) capable of trapping hydrocarbons on a geological time scale for instance. The creation of caverns for UGS in halite formations and the operation activities of gas injection and withdrawal occurring under lithostatic pressure lead to local deviatoric stresses resulting in rock salt creep deformation (Carter et al. 1982; Van Sambeek et al. 1993). Additionally, periodic injection-production activities in response to seasonal temperature changes and associated gas consumption imply regular fluctuation of both mechanical (due to internal operational pressure changes) and thermal (due to adiabatic processes) stresses in salt caverns (Jiang et al. 2016; Fan et al. 2016). Therefore, understanding the fatigue of rock salt under these mechanical cyclic conditions is paramount to ensure safe and sustainable UGS (Fan et al. 2019).

Schematic map of the Triassic and Permian salt fields in the United Kingdom and Triassic basins. Winsford Mine is identified in the map as part of Cheshire Basin and Boulby Mine as part of the Zechstein Basin

Winsford Mine is stratigraphically located at the base of the Northwich Halite Formation, which contains 92% NaCl, and the extraction is made by room and pillar mining (Norton et al. 2005). Within the salt mine, two different economically workable levels of rock salt have been differentiated on the halite purity, namely Zone B and Zone F. Zone B and Zone F are located between 130 and 200 m depth, respectively. Layer B has a higher concentration of halite, where the purest material is at the basal part of the salt bed. Layer F has a higher content of second phase mineralogy, mainly anhydrite and clay, and a lower concentration in halite. The geological formation, of approximately 150 m in thickness, was deposited during the Triassic under arid climatic conditions with terrigenous reddish-brown sediments containing red clay and other silicates. The mine extension and structure are limited by two main faults crossing the geological formation on the east and the west, and the mining activity in the site is limited to the zone in between the fault lines. During mining extension works in layer B, horizontal and vertical boreholes were drilled to delimit the location of the working horizon within the salt bed and the limits dictated by the structural settings of the site. The site engineers kindly provided core samples from the boreholes for testing in the laboratory. Additional rock salt samples were collected on site from layers B and F.

Cylindrical samples of 51 mm diameter were cut from same diameter borehole cores using a diamond-tipped rock saw, so that the length-to-diameter ratio was approximately 2:1 (International Society for Rock Mechanics 1979). Off-cuts from each cylinder were used to produce thin sections previous to the cyclic mechanical loading test to provide information of the undamaged microstructure .

For the mechanical testing, each cylindrical sample was mounted between two hardened steel platens prepared with a layer of heat-resistant silicone tape for sealing purpose. The assembly was then encased in two 5 mm heat-shrink Polytetrafluoroethylene (PTFE) membranes to prevent ingress of confining fluid into the sample. Locking wires were used to complete the seal between the PTFE jacket and the platens. The sample was then instrumented with two axial extensometers (MTS 632.90F-12, accurate to \(\pm \, 0.01\%\)), positioned diametrically opposite each other over the central 50 mm of the sample, and a circumferential chain extensometer (MTS 632.92H-03, accurate to \(\pm \, 0.01\%\)) positioned at mid-length. A third platen, not part of the aforementioned sample assembly, was spherically seated to prevent eccentric loading. This spherically seated platen was in turn fixed to a 2.6 MN capacity force transducer (MTS 661.98B.01, accurate to \(\pm \, 0.32\%\) of load) to measure the load applied to the sample.

After each test, the damaged sample was recovered, halved along the axial length (parallel to the applied axial stress), and impregnated in blue dye epoxy. A one-side polished thin section was then produced from the center of the sample, thicker (200 microns) than standard thin section due to the nature of halite minerals.

The cyclic mechanical loading tests were undertaken on a triaxial servo-controlled stiff frame (MTS 815 rock test system with a maximum axial load up to 4600 kN) with a confining pressure vessel rated to 140 MPa, at the Rock Mechanics and Physics Laboratory, British Geological Survey (BGS). A thermocouple was placed as close as possible to the sample to monitor the temperature. The confining pressure vessel was then closed and an initial axial load of \(\sigma _1=1\) kN was applied, that corresponds to an axial stress of 0.5 MPa, to ensure a stable contact and alignment of the platens whilst the vessel was filled with mineral oil confining fluid. The confining pressure (\(P_\text c=\sigma _2=\sigma _3\)) was then applied hydrostatically at 1 MPa/min to 25 MPa, and then kept constant at that value. The samples were all tested at room temperature between \(22^\circ -25^\underline\mathrmo\) and moisture conditions, in a single orientation.

The microstructural analysis was performed on thin sections from Boulby mine samples only, before and after testing, using the same transmitted light microscope as for the mineralogical anaylsis, and a scanning electron microscope (SEM) to identify structures and deformation mechanisms resulting from the cyclic mechanical loading test. The same thin sections used in the transmitted light microscope were treated with pulverized coated carbon to be used in the SEM.

The distribution of second-phase content is mainly concentrated around the halite crystal boundaries, surrounding the anhedral and equant halite crystals (Fig. 3a, b). Occasionally, second-phase minerals are also included within some halite grains. Anhydrite tends to show spherical shapes, whereas polyhalite occurs as prismatic crystals. Kieserite is identified in the thin sections by a vitreous luster, very high birefringence and may have also twinning structures. Prismatic polyhalite can be observed poikilotopically enclosing finer crystals of anhydrite. Polyhalite crystals can be recognized under polarized light for the low interference colors and the chadacrysts of anhydrite enclosed in the oikocryst polyhalite are recognized by the third-order interference colors. Polyhalite also shows parallel to inclined extinction.

Rock samples from series C do not show any layering. The sample shows clear white-greyish to light-grey salt crystals (Fig. 2c) with sizes from 1 mm to 1.5 cm. The PXRD analysis from series C identifies halite as the main mineralogy with the lowest variation in content in comparison to the other series tested. Series C also shows a small content of anhydrite (around 6%) and carnallite (around 4%) and very low content of polyhalite (3%) and kieserite (0.7%) (Table 1). The thin sections from series C show a structure formed by relatively bigger, in comparison with the previously discussed samples, halite crystals (from 500 \(\upmu\)m to 1.5 cm) with lower second-phase content (Fig. 3g, h). Similar to the observations in the previous samples, the few second-phase minerals are generally located at the halite crystal boundaries, although small (about 10 \(\upmu\)m) anhedral anhydrite crystals also occur enclosed in more central parts of the halite crystals.

A slightly strain recovery can be observed in some samples. According to Berest (Berest 2011), even small thermal variations can generate thermo-elastic strains in pure halite samples (thermal expansion coefficient for rock salt is of \(\alpha\) = 4 \(\times\) \(10-5/^\circ \hbox C\)). This suggests that slightly changes in rheological behavior, like the jumps observed in sample F3 (Fig. 7), could be linked to involuntary small variations in room temperature. Hence, temperature changes during the cyclic mechanical loading experiment were recorded to account for any significant variation. Figure 8 shows the temperature evolution versus lateral strain. All tests show a decrease in temperature from the beginning of the test until the end with variations of temperature from \(\pm \, 3^\circ \hbox C\) to maximum \(\pm \, 5^\circ \hbox C\). Sample F3 is the sample showing the greatest temperature variation with a temperature drop of \(4.5^\circ \hbox C\) from the beginning (\(25.5^\circ \hbox C\)) until the end (\(21^\circ \hbox C\)) of the test. Despite F3 being one of the samples showing the least variation in lateral strain, the significant drop in temperature could be (between other factors like second-phase content) one of the reasons to explain why F3 show a repeated recovery in lateral strain around every 500 cycles. Even though thermo-elastic strains can play a significant role over creep rate in long-term creep tests in pure halite (Berest 2011), our data do not point to a clear relationship between the room temperature variations and the recorded strains during these short-term cyclic mechanical loading tests. The second-phase mineral content may also have allowed to accommodate this minor temperature effect differently.

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