Geochemical Trapping of H2S via Iron Mineral Interactions in Carbonate Reservoirs: Experimental and Modeling Study

Summary

Hydrogen sulfide (H2S) capture and long-term retention in subsurface carbonate formations are critical for safe geological disposal and effective field development. This study examines geochemical interactions between H2S and iron-bearing minerals, focusing on permanent mineralization through FeS precipitation under reservoir conditions.

A static thermodynamic model of the H2S-FeCO3-CaCO3-FeS aqueous system was developed, and dynamic column-flooding experiments were carried out to investigate whether the geochemical reactions modeled could suitably explain the kinetic system scenario. Borosilicate columns packed with calcite-siderite mixtures (0–50 wt% siderite) were flooded with H2S-containing 1% Na+ solution under ambient conditions. Lithium tracer [analyzed by inductively coupled plasma optical emission spectroscopy (ICP-OES)] determined porosity, effluent sulfide was quantified after ZnCl2 quenching, pH was monitored, and FeS precipitates were confirmed by environmental scanning electron microscopy coupled with energy-dispersive X-ray (ESEM-EDX).

Pure calcite systems showed negligible H2S scavenging, with near-identical inlet/outlet concentrations and no FeS formation; minor breakthrough delays reflected transient adsorption. In contrast, calcite-siderite mixtures exhibited active H2S removal, driven by siderite dissolution, Fe²+ release, and FeS precipitation. Scavenging efficiency increased proportionally with siderite content. Effluent pH remained elevated (~8) due to carbonate buffering, with flow equilibrium [H2S] and [Ca²+] achieved after ~4.5 pore volumes (PVs), accompanied by secondary CaCO3 precipitation.

The custom thermodynamic model showed strong qualitative agreement with PHREEQC simulations, reproducing observed trends in pH, [Ca²+], [Fe²+], and FeS formation, despite minor differences in speciation databases, equilibrium constants, and activity treatment. Both models captured the key dissolution-precipitation dynamics.

These integrated experimental and modeling results demonstrate proof-of-concept for H2S mineral trapping as FeS in iron-bearing carbonate reservoirs, whether naturally iron-rich or supplemented with targeted iron, providing key mechanistic understanding toward improved H2S management.