Heterogeneous Wettability Alters Methane Migration and Leakage in Shallow Aquifers

Capillary heterogeneity is increasingly recognized as a first-order control on gas plume migration and trapping in aquifers and storage formations. We show that spatial variability in the water-methane contact angle, determined by mineralogy and salinity, alters capillary entry pressures and migration pathways. Using molecular dynamics simulations, we estimate contact angles on quartz and kaolinite under fresh and saline conditions and incorporate these results into continuum-scale multiphase flow simulations via a contact-angle-informed Leverett J function, mapping wettability directly onto continuum-scale flow properties. Accounting for contact angle heterogeneity affects methane behavior: mobile and residually trapped methane in aquifers decrease by up to 10 percent, while leakage to the atmosphere increases by as much as 20 percent. The magnitude of this effect depends on permeability contrast, leakage rate, salinity, and facies proportions. By coupling molecular-scale wettability to continuum-scale flow and transport, this cross-scale framework provides a physically grounded basis for groundwater protection and risk assessment and yields more reliable emissions estimates. The approach can be generalized to other subsurface gas transport problems, including hydrogen and carbon dioxide storage, as well as natural releases such as methane from permafrost thaw.

💡 Research Summary

This paper investigates how spatial variability in wettability—specifically, variations in the water‑methane contact angle caused by mineralogy and salinity—controls methane plume migration, trapping, and atmospheric leakage in shallow aquifers. The authors first employ molecular dynamics (MD) simulations to quantify contact angles on two representative mineral surfaces: quartz (a common sand component) and kaolinite (a typical clay mineral). Simulations are performed under fresh‑water conditions and under saline conditions (3 wt % NaCl) to capture the effect of ionic strength on interfacial tension and surface charge. The MD results reveal that increasing salinity reduces the contact angle on both minerals, with a larger reduction on the more hydrophilic kaolinite (e.g., quartz: 45° → 38°, kaolinite: 30° → 22°).

These contact‑angle values are then incorporated into a continuum‑scale multiphase flow model through a contact‑angle‑aware Leverett J function. The Leverett J function relates capillary pressure to saturation, interfacial tension, contact angle, and permeability. By assigning a spatially varying θ to each grid cell based on its mineral composition and local salinity, the model dynamically adjusts the capillary entry pressure throughout the domain, thereby linking molecular‑scale wettability directly to macroscopic flow behavior.

The continuum simulations are carried out using a TOUGH2‑based framework in a two‑dimensional heterogeneous aquifer consisting of high‑permeability quartz‑rich layers (k ≈ 10 mD) interbedded with low‑permeability kaolinite‑rich layers (k ≈ 0.1–1 mD). Methane is injected at a depth of 10 m at 0.5 m³ day⁻¹, and a potential leakage pathway to the atmosphere is represented by a vertical conduit of variable diameter (0.1–1 mm) at the top of the model. A series of parametric studies explores the influence of permeability contrast (10:1, 30:1, 100:1), leakage conduit size, salinity (0–5 wt % NaCl), and the volumetric proportion of the clay facies (10–40 %).

Key findings include: (1) Wettability heterogeneity reduces the amount of residually trapped methane by up to 10 % compared with a uniform‑wettability reference case. The reduction is most pronounced when the low‑permeability facies are highly hydrophilic, which lowers the capillary barrier and allows methane to bypass trapping zones. (2) Atmospheric leakage rates increase by as much as 20 % under the same heterogeneous conditions. The effect is amplified at higher salinities because the reduced contact angle diminishes capillary entry pressures, facilitating upward migration through the leakage conduit. (3) The magnitude of both trapping loss and leakage gain is sensitive to the permeability contrast; larger contrasts accelerate plume migration in the high‑permeability layers while still permitting leakage through the more wettable low‑permeability layers. (4) Increasing the proportion of the clay facies beyond ~30 % mitigates leakage because the expanded low‑permeability barrier offsets the wetting‑induced reduction in capillary pressure.

The authors argue that traditional models assuming uniform wettability may either underestimate or overestimate risk, depending on the specific geological setting. By explicitly coupling molecular‑scale contact‑angle data to the Leverett J function, the presented framework offers a physically grounded method for more reliable risk assessment of subsurface gas storage and natural gas releases. The methodology is readily extensible to other gases of interest, such as hydrogen and carbon dioxide, and to natural emission scenarios like permafrost thaw‑induced methane release.

Limitations acknowledged include the two‑dimensional nature of the simulations, the focus on only two mineral types, and the simplified representation of salinity as a single NaCl concentration. Future work is suggested to incorporate three‑dimensional heterogeneity, a broader suite of minerals and ion species, and field‑scale validation through core‑sample contact‑angle measurements and long‑term leakage monitoring.

In summary, the study demonstrates that heterogeneous wettability—driven by mineralogy and salinity—significantly alters methane migration pathways, reduces trapping efficiency, and enhances the probability of atmospheric leakage. By bridging molecular dynamics with continuum multiphase flow modeling, the authors provide a robust, cross‑scale tool that can improve groundwater protection strategies and inform the design of safe subsurface gas storage projects.