Laboratory Experiments

Gelation kinetics and the rheological properties of PVA solutions of different concentrations (from 0.5 to 10 wt%) have been studied in the temperature range from -30 to +10 0C. We have also studied PVA solutions containing additives, which affect gelation kinetics and rheological properties of gels and cryogels, as well as their adhesion to rocks, in order to create flow barriers within ground constructions. Viscosity and Young's modulus were chosen as rheological characteristics.

Viscosity was measured using a "Reokinetika" vibration viscosimeter equipped with a tuning-fork gauge. Distilled water was used as a calibration liquid.

Modulus of elasticity for gels was determined based on "tension -deformation" diagrams, obtained during quasi-elastic cylindrical-sample compression using in-house developed equipment based on a micrometer and electronic scales. The modulus of elasticity was calculated as the angle of slope of the initial linear segment, where Hooke's law is observed, relative to the compression - deformation curve.

The kinetics of cryotropic gelation of 5 and 10 % PVA solutions was studied at - 32 and - 18 0C by using the modulus of elasticity as the principle monitoring parameter. Solutions were maintained at sub-zero temperatures for a pre-determined time and then they were subjected to thawing at 2 and 18 0C. The mode of thawing is a factor which significantly affects visco-elastic properties of cryogels. Slow thawing promotes generation of cryogels exhibiting high viscosity and modulus of elasticity, and thus the lower the freezing temperature the stronger the cryogels (Figs. 1 and 2). At higher thawing rates the effect of freezing temperature on the visco-elastic properties of cryogels decreases.

The effect of the number of "freeze-thaw" cycles on the properties of cryogels, prepared from aqueous PVA solutions, has been studied. The cryogels were frozen at - 18 and - 32 0C and then thawed at 18 and 6 0C. The results obtained in the study indicate that the modulus of elasticity increases with the number of "freeze-thaw" cycles and it does not depend on the thawing rate (Fig. 3). The greatest increase in elasticity was observed after 2 - 4 freeze-thaw cycles.

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Figure 1. Viscosity change of 5 % PVA solution after thermostatting at -32 °C and thawing at different rates.

Figure 2. Elasticity changes of 5 % PVA solution after thermostatting at -32 0 C and thawing at different rates.
Figure 3. Dependence of the modulus of elasticity (for cryogels obtained from 10% PVA solution and which were frozen at -32 and -18 °C), on the number of freeze-thaw cycles at 18 and 6 °C.

We studied gelation kinetics of a 5 % PVA solution at +2 °C and compared the visco-elastic properties of the prepared gel with those of cryogels obtained from the same solution during freezing at -18 and -32 °C, followed by thawing at +18 °C. The study showed a significant increase in gel viscosity and modulus of elasticity at the lower freezing temperature (Fig. 4).

The viscoelastic properties of gels prepared from 5 and 7 % PVA solutions at -5, 0, +2 and +22 °C have also been studied. The gels contained electrolytes as additives, which affected gelation kinetics and the rheological properties of gels and cryogels, as well as their adhesion to rocks. Gelation kinetic studies of PVA solutions containing electrolytes as additives already showed that the gels were formed in several hours at 0 - 18 °C.

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Figure 4. Dynamics of modulus of elasticity for 5 % PVA solution thermostated at different.

temperatures.

Moreover subsequent thermostatting or freezing - thawing of the gels improved their strength. It has been determined that viscosity and elasticity of the solutions increased 3-5 times when a cross-linking agent was added to the PVA aqueous solutions, depending on agent concentration and thermostatting time (1 to 8 days) at +2 °C

Strength changes occurring in cryogels with different fine-filling agents were also studied, such as crushed carbonate rock (fraction 0.16 - 0.5 mm), sand and crushed core material having sandstone / argillite ratios of 2:1; 1:1 and 1:2. The highest modulus of elasticity was observed with the filling agents in a 1:1 ratio and cryogel prepared from a PVA solution with a cross-linking agent and filled with crushed carbonate rock and core material. Moreover the modulus of elasticity of the cryogel also increased with an increase in PVA and cross-linking agent concentrations. The addition of electrolyte and a cross-linking agent to the PVA solution significantly increased the modulus of elasticity in all PVA solutions - crushed carbonate rock ratios (Fig. 5).

The filtration characteristics of gel-forming systems were studied under thermobaric conditions simulating water filtration through anti-filtration barriers in constructions in the northern climatic zone. Experiments consisted of water filtration at a constant discharge rate through a laboratory model of a filtering medium.

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Figure 5. Increase in modulus of elasticity for PVA solutions with added electrolyte and cross-linking agents after thermostatting with crushed carbonate rock at a ratio of 1:2 at +2 °C.

The model was packed with crushed carbonate rock (fraction 0.16-0.5 mm), with permeability varying from 9.78 to 87 ^m2. A gel-forming solution containing 3 % PVA and a cross-linking agent (Fig. 6) was injected into a model consisting of crushed carbonate rock with a permeability of 87 ^m 2 at 6 °C at lower pressure values (from 0.3 to 2.25 atm/m). The model was subsequently held at 6°C for 5 hours to form the gel, and then once again water infiltration experiments were continued. At pressure increases up to 50 atm/m no water infiltration was observed (Fig. 6).

Figure 6. Creation of a cutoff wall via injection of a gel-forming solution (3 % PVA and a cross-linking agent) into a crushed carbonate rock model with permeability of 87 ^m2 at 6 oC.

Figure 7 presents the results of the creation of a cutoff barrier in a model consisting of crushed carbonate rock having a permeability of 10.254 ^m 2 at 20 oC. Injection of a gel-forming solution containing 10 % PVA and electrolyte was performed at a pressure gradient of 3.5-4 atm/m. The model was subsequently held for 12 hours at 20 °C to form the gel, after which water infiltration was once again initiated.. No water infiltration was observed at pressure increases up to 140-160 atm/m (Fig. 7).

Figure 7 presents the results of the creation of a cutoff barrier in a model consisting of crushed carbonate rock having a permeability of 10.254 ^m 2 at 20 oC. Injection of a gel-forming solution containing 10 % PVA and electrolyte was performed at a pressure gradient of 3.5-4 atm/m. The model was subsequently held for 12 hours at 20 °C to form the gel, after which water infiltration was once again initiated.. No water infiltration was observed at pressure increases up to 140-160 atm/m (Fig. 7).

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Figure 7. Creation of cutoff screen via injection of gel-forming system: 10 % PVA and electrolyte into a model of carbonate rock with permeability of 10.254 ^m2 at 20 oC.

Thus, PVA-based gel-forming solutions with a cross-linking agent and electrolyte as additives are capable of creating reliable cutoff barriers in carbonate rocks. The study of infiltration characteristics of soils with PVA-based gel-forming solutions under thermobaric conditions demonstrated that after gelation one could observe a sharp decrease in water mobility, even up to pressure differences higher than 100 atm/m.

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