Experimental and Theoretical Study of Double Diffusive Instabilities during the Superposition of Two Saline Layers of Sodium Chloride and Sodium Sulphate

Overview & Synopsis

To address climate change and achieve net-zero greenhouse gas emissions, reducing anthropogenic CO₂ emissions is essential. However, due to persistent global energy demands, fully eliminating emissions within the required timeframe is highly challenging. Therefore, enhancing CO₂ removal through innovative carbon capture and storage (CCS) techniques is critical. Among various CCS options, deep saline aquifers are recognized as promising due to their storage potential. However, optimizing CO₂ injectivity and ensuring long-term stability require a deeper understanding of transport mechanisms and interactions within these formations.This thesis contributes to this understanding by integrating experimental measurements, modeling, and numerical simulations to study the stability of two superimposed salt layers—characteristic of deep saline aquifers—initially separated by a sharp interface. Three types of diffusion cells were used, all enabling the superposition of two saline layers under stable initial density gradients. One cell, previously developed, allows vertical observation parallel to the gradient and is analyzed using shadowgraphy. Two new cells developed for this work allow horizontal observation, analyzed with a Mach-Zehnder interferometer.The optical techniques were first calibrated through free-diffusion experiments using NaCl, Na₂SO₄, and CaCl₂. For shadowgraphy, the effect of sample thickness was incorporated into the analysis of structure functions. For interferometry, vertical concentration profiles were used to determine diffusion coefficients as a function of concentration, offering a broader and more detailed dataset than traditional methods that typically use single concentration values.These techniques were then applied to study double-diffusion processes in NaCl and Na₂SO₄ mixtures with initially stable configurations. Shadowgraphy allowed visualizing convection patterns, while interferometry also measured refractive index profiles. The experiments revealed density inversions and the development of two types of double-diffusive instabilities: diffusion layer convection (DLC) and double-diffusion convection (DDC), depending on the concentration.To interpret the results, ionic species diffusion coefficients were computed using PHREEQC with Pitzer equations for activity coefficients and interaction parameters for Na⁺, SO₄²⁻, and Cl⁻. Main and cross-diffusion coefficients of the NaCl + Na₂SO₄ + H₂O system were derived from Nernst-Planck equations and used in a numerical model describing ternary isothermal diffusion. At the macroscopic level, system dynamics were described by the Navier-Stokes and mass transfer equations under the Boussinesq approximation, incorporating both diffusion and cross-diffusion effects.Good agreement between experiments and simulations validated the proposed method for calculating multicomponent diffusion coefficients and demonstrated the effectiveness of interferometry for studying double-diffusive instabilities both qualitatively and quantitatively.