A comparison of different image analysis techniques for mapping spatiotemporal pH and carbon dissolution in density-driven convection of CO2 in water

TL;DR

This study compares image analysis techniques for mapping pH and carbon dissolution during CO2 density-driven convection in water, finding the Hue method most robust for dynamic, high-resolution measurements.

Contribution

It introduces a comparative analysis of three image-processing techniques for quantifying pH and carbon concentrations in convection experiments, highlighting the robustness of the Hue method.

Findings

Hue technique is most robust and invariant to environmental variations.

All methods produced similar estimates of total dissolved carbon.

Hue method enables continuous spatial distribution mapping of pH and concentration.

Abstract

Density-driven convection enhances the carbon dissolution rate, which is significant for the geological carbon storage. This process will also influence the spatiotemporal pH and carbon concentrations of the underground fluid. To illuminate the convection mechanism, it is critical to understand the evolution of those properties within the porous media. However, determining the spatiotemporal pH and concentration within porous media is always challenging. This study employed a combination of three pH indicators that can track a wide range in pH from 4 to 9.5 in a convection experiment. Furthermore, we compared three image-processing techniques: Hue, gray-difference, and angular representation of RGB color space $\mathbf{(\phi,\theta)}$ for quantifying color changes from the universal $\text{pH}$ indicator arising from the carbon convection. The characterized colors were mapped into pH by calibrating against benchmark solutions. The comparative results demonstrate that the color quantified by the Hue technique is most robust, showing invariance to fluid thickness, camera settings, and LED luminance. In the convection experiments, it produces a continuous spatial distribution of pH and concentration level in the system. In contrast, the $\mathbf{(\phi,\theta)}$ and gray-difference techniques were more sensitive to environmental variations. They also have significant limitations for $\text{pH}$ interpolation in the critical range due to their non-monotonic calibration paths. Although all methods ultimately produced similar estimates of total dissolved carbon, the Hue technique offers greater stability and universality for high-resolution, dynamic measurements of pH and carbon concentration in the convection experiments.