Experimental tests and modeling of CO2 and H2S co-sequestration in saline aquifers

A simulation study and a series of autoclave experiments were performed, reproducing gas-rock-water systems under reservoir conditions, after injection of CO₂ and the mixture of CO₂ with H₂S into rocks representing the Upper Silesian Coal Basin and the adjacent Małopolska Block (Central Europe). The water-rock-gas interactions were modeled in two stages: the first–aimed at simulating the short-term changes in system impacted by the gas injection, and the second–long-term effects of sequestration.

On the basis of the simulations, the reactions behind mineral transformations were identified. These phenomena are different for the injection of CO₂ alone. and CO₂+H₂S mixtures, resulting in the formation of secondary minerals responsible for mineral sequestration. Depending on the original mineral composition of the rock, in the case of pure CO₂, these are mainly carbonate minerals siderite, dawsonite, magnesite, dolomite and calcite, while in the case of mixture injection: elemental sulfur, sulfur sulfides–pyrite and pyrrhotite.

In experiments with the H₂S+CO₂ mixture, dissolution of skeletal grains was observed, which was most visible in the case of carbonates, feldspars, and chlorites. The analysis of rocks containing hematite revealed the formation of elemental sulfur surrounded by FeS₂ crystals, which had not been previously reported.

The experiments generally confirmed the interactions in gas-rock-water systems identified by numerical simulation. This allowed to estimate the amount of mineral phases precipitated or dissolved in the analyzed reactions, and consequently the impact on changes in porosity and the amount of sequestered carbon dioxide and sulfur.

In samples abundant in carbonate minerals (the Dębowiec Formation psephites), the decomposition of ankerite, due to the injection of CO₂+H₂S, is not compensated for by the precipitation of sufficient amounts of other carbonates, which leads to the desequestration process–CO₂ release.

Based on the calculations, it was found that the potentially most favorable conditions for the sequestration occur in the Paralic Series mudstones, rich in chlorites–a maximum of 22.36 kg CO₂/m³ and 12.50 kg S/m³, trapping capacity after 10,000 years of storage.