Measuring CO2 Dissolution in South China Sea Brines to Improve Offshore Storage

Deep saline aquifers beneath the South China Sea have long been viewed as promising vaults for carbon dioxide captured from the dense industrial corridor along China’s southern coast. Yet a persistent problem has haunted every prediction of how such storage would actually behave: the fluid properties fed into the models were usually not measured at all, but estimated from empirical correlations or borrowed from datasets gathered under simplified laboratory conditions.

A new open-access study published in the journal Carbon Neutrality on 29 July 2026 tackles that gap directly. A team led by Ying Teng, Qiang Han and colleagues, with Heping Xie as senior author, developed an integrated experimental and numerical framework in which the key properties of the CO2–brine system were measured on actual formation brines drawn from two reservoir intervals in the South China Sea, rather than on synthetic brines or generalized correlations. Those measured properties were then fed into Darcy-scale numerical simulations to quantify how dissolved carbon dioxide migrates, destabilizes and mixes under conditions representative of the target aquifers.

The central mechanism at stake is solubility trapping. When injected CO2 dissolves into the brine that saturates a saline aquifer, it leaves the buoyant free phase behind and becomes far less prone to remigration and leakage. Crucially, dissolution also increases the density of the brine, creating a gravitationally unstable layer beneath the gas–brine interface. Once that dense boundary layer becomes unstable, it collapses into downward-migrating fingers of CO2-enriched fluid, a process known as density-driven convective mixing. Convection continuously renews the fluid at the interface, sustains the concentration gradient, and accelerates the conversion of mobile CO2 into a more secure dissolved form. How quickly this transition happens determines how long buoyant, potentially mobile CO2 persists in the reservoir.

The trouble, as the authors note, is that predictions of convective dissolution are acutely sensitive to the fluid-property data used to parameterize the system. Traditional pressure-decay experiments, the workhorse method for inferring CO2 diffusivity, can be contaminated by boundary effects and by density-driven natural convection within the sample itself, so the values they yield are apparent effective diffusivities rather than true molecular diffusion coefficients. In the new work, the team instead used Raman spectroscopy combined with capillary confinement. CO2-enriched brine was loaded into a fused-silica capillary with an inner diameter of just 300 micrometers, small enough that the confined geometry suppresses convective interference and allows transport to be treated as one-dimensional diffusion governed by Fick’s second law. Local CO2 concentrations were read from the Raman intensity ratio between the CO2 Fermi doublet and the O–H vibrational band of water, which served as an internal calibrator.

The measurements, performed at temperatures of 313.15 to 363.15 kelvin and pressures of 7.5 to 17 megapascals, corresponding to typical in-situ conditions in deep South China Sea saline aquifers, yielded molecular diffusion coefficients ranging from 2.89 × 10⁻⁹ to 4.78 × 10⁻⁹ square meters per second. Temperature emerged as the strongest control, with diffusivity increasing nearly linearly as temperature rose, a consequence of enhanced molecular mobility and reduced solution viscosity. Pressure, by contrast, had only a weak influence within the investigated range, with total variation below roughly seven percent. The authors are explicit that this weak pressure dependence was observed only between 7.5 and 17 megapascals, that the data are insufficient to determine whether the trend persists above 30 megapascals, and that direct extrapolation beyond the experimental range is not recommended. Salinity suppressed diffusivity: the higher-salinity brine S1, at 40.77 grams per liter, diffused CO2 more slowly than the lower-salinity S2, at 35.63 grams per liter, because increased ionic strength and viscosity jointly hinder diffusive transport.

Density was measured with an oscillating U-tube densitometer calibrated against NIST-traceable standards, using a method disclosed in Chinese Patent CN118392713B. The results showed that CO2-enriched brine density increases approximately linearly with pressure and decreases with temperature, and that saline brines are consistently denser than deionized water under comparable conditions. Although higher salinity reduces CO2 solubility through the salting-out effect, the measurements confirmed that CO2 dissolution still increases brine density sufficiently under all tested conditions to generate the buoyancy contrast required to trigger gravitational instability. That density contrast is the immediate physical driver of convection and a key input to the Rayleigh number, the dimensionless ratio of buoyancy destabilization to diffusive and viscous stabilization that governs the entire process.

With these experimentally constrained properties in hand, the team built a two-dimensional Darcy-scale model in COMSOL Multiphysics, using a 2-meter by 2-meter porous domain with a porosity of 0.2 and a permeability of 1 × 10⁻⁹ square meters, discretized into 400 by 400 interior elements with specialized boundary-layer meshes. The simulations revealed the interplay vividly. At a fixed density contrast, higher diffusivity smoothed the concentration boundary layer and delayed early finger development; at a fixed diffusivity, increasing the density contrast from 0.1 to 26.6 kilograms per cubic meter transformed the system from almost purely diffusive behavior to fingers penetrating more than 90 percent of the domain within ten days. Density contrast proved the more powerful lever: maximum flow velocities more than doubled across the density-contrast cases, while peak Sherwood numbers, a measure of convective mass-transfer enhancement over pure diffusion, climbed from about 90 to about 1300, whereas varying diffusivity shifted mainly the timing of convection rather than its intensity.

From these simulations the researchers extracted quantitative scaling laws. Over the investigated range, the Rayleigh number spanned roughly 3024 to 13,608, and the onset time of convection fell from about 0.145 to 0.020 days, following the relation t-onset equals 1.45 × 10⁷ times Ra to the power of −1.98, an approximately inverse-square dependence. The peak Sherwood number followed Sh-peak equals 0.045 times Ra to the power of 1.12, with a coefficient of determination of 0.965, rising from about 257 at Ra of 2295 to about 1317 at Ra of 10,055. The team also showed that the pre-instability boundary layer grows in a diffusion-controlled manner, with the critical equivalent thickness scaling linearly with the diffusive length scale. They caution that the fitted exponents are empirical results of their finite-domain, homogeneous-permeability configuration rather than universal stability exponents, and that permeability heterogeneity, three-dimensional geometry and field-scale plume dimensions could shift the relationships, so direct extrapolation to full reservoir scale should be made with caution.

Applied to four representative South China Sea storage scenarios, the framework delivered a practically important conclusion. By day five, cumulative dissolved CO2 in the 2-meter model reached 514.46 to 659.96 moles depending on the case, roughly 18 to 28 percent higher in the warmer, higher-pressure scenarios, yet the final dissolved amounts converged to about 800 moles in all four. Reservoir conditions, in other words, have limited influence on the ultimate dissolved CO2 mass but a strong influence on dissolution kinetics: the time to reach 80 percent of final dissolved mass ranged from 4.65 to 6.99 days, and the time to reach 95 percent ranged from about 20 to 41.36 days. Because faster dissolution shortens the persistence of buoyant free-phase CO2 and lowers leakage risk, the authors argue that reservoir screening and injection design should prioritize conditions that enhance early-stage convective dissolution rather than focusing solely on equilibrium storage capacity, and should rest on experimentally constrained in-situ fluid properties rather than generalized thermodynamic correlations. The calibrated scaling relations are offered as a baseline for site-specific assessments in the South China Sea and, with appropriate caution, in other offshore basins where dissolution trapping governs storage security.

Subject of Research: CO2 convective dissolution in offshore South China Sea saline aquifers

Article Title: In-situ parameterization and scaling analysis of CO2 convective dissolution for offshore saline aquifer sequestration in the South China Sea

Article References: Teng, Y., Han, Q., Chen, Y., Wang, P., Wang, J., Sun, H., An, S., Shan, X., Chen, X., Han, S., Zhu, J., & Xie, H. (2026). In-situ parameterization and scaling analysis of CO2 convective dissolution for offshore saline aquifer sequestration in the South China Sea. Carbon Neutrality, 5(1), Article 24. https://doi.org/10.1007/s43979-026-00178-7

Image Credits: AI Generated

DOI: 10.1007/s43979-026-00178-7

Keywords: CO2 sequestration, South China Sea, saline aquifer, convective dissolution, Rayleigh number, Raman spectroscopy, density-driven convection, solubility trapping, formation brine, Darcy-scale simulation, carbon capture and storage, offshore storage