Under the conditions of 3000 m water depth (approximately 30 MPa high pressure), the rheological behavior of calcium bromide (CaBr₂) solution is influenced by high pressure, low temperature, and salt concentration, exhibiting the following trends:
1. Viscosity Changes
- High-Pressure Effect:
High pressure compresses intermolecular distances and enhances ion hydration, leading to increased viscosity. However, in highly concentrated CaBr₂ solutions, electrostatic interactions between ions may initially strengthen and then saturate, resulting in nonlinear viscosity changes. - Concentration Dependence:
- Low concentration (e.g., <30%): Viscosity increases gradually with pressure, consistent with typical electrolyte solutions.
- High concentration (e.g., >40%): Ion aggregates or localized ordered structures (e.g., [CaBrₙ]²⁻ⁿ complexes) may form, causing a significant viscosity rise under pressure, possibly even shear-thinning (pseudoplastic) behavior.
2. Transition in Rheological Model
- Newtonian → Non-Newtonian Fluid:
At low concentrations under ambient pressure, CaBr₂ behaves as a Newtonian fluid. However, high pressure combined with high concentration may introduce shear-dependent behavior:- Shear-thinning: High pressure disrupts ion clusters, reducing viscosity at higher shear rates.
- Yield stress: Near saturation concentrations, a critical stress may be required to initiate flow (Bingham plastic behavior).
3. Coupled Effects of Temperature and Pressure
- Synergistic Low-Temperature & High-Pressure Effects:
At 3000 m depth, water temperature is ~1–4°C. Low temperature increases viscosity, while high pressure suppresses ice formation, extending the supercooled liquid range. Together, they amplify viscosity more than either factor alone.
4. Ion Hydration and Microstructure
- Compressed Hydration Shells:
High pressure reduces the hydration layers around Ca²⁺ and Br⁻, decreasing ion mobility and increasing viscosity. - Enhanced Ion Pairing:
High pressure promotes contact ion pairs (Ca²⁺–Br⁻), reducing free water molecules and increasing structural order.
5. Experimental & Simulation Recommendations
- High-Pressure Rheometry: Use piston-cylinder or diamond anvil cell (DAC) setups to simulate 30 MPa conditions.
- Molecular Dynamics (MD) Simulations: Can reveal ion cluster dynamics under pressure (e.g., via radial distribution function *g*(*r*) analysis).
Practical Implications
- Oilfield Drilling Fluids: If used as high-density completion fluids, test shear recovery at 3000 m to avoid gelation-induced pumping issues.
- Deep-Sea Chemical Transport: High-concentration CaBr₂ may experience abrupt flow resistance in pipelines; optimize concentration (e.g., 25–35%).
Summary: At 3000 m depth, CaBr₂ solutions generally exhibit higher viscosity, with non-Newtonian tendencies at high concentrations. Rheological behavior must be evaluated based on specific concentration and shear conditions, with strict control of temperature and pressure in experiments.
