An Experimental and Modelling Study of H₂S Scavenging in Calcite/Iron-Bearing Formations

H₂S capture is an important area of research maximizing the level of H₂S trapping and subsequently permanently retaining in the subsurface. One of the objectives of the current H₂S field development program is to investigate the geochemical interaction of H₂S with iron minerals in carbonate and how to mineralize H₂S permanently and efficiently in the reservoir. The main objective of this research is to develop a static geochemistry model and the experiments of these ideas.

Dynamic pack floods testing was conducted by packing a mixture of calcite and iron minerals (siderite or hematite) in a borosilicate chromatography column, and the subsequent flooding of the H₂S-containing 1% Na⁺ (10000ppm Na⁺ in Distilled Water) through the column at ambient conditions. An unreactive lithium tracer was used to determine pore volume and porosity of the packs, which was analyzed by inductively coupled plasma-optical emission spectroscopy (ICP-OES). In addition, the outlet samples of the packs were collected through a zinc quenching solution enabling the total sulphide concentrations to be back-calculated and measured [H₂S] at each stage. FeS precipitated in the column was analyzed by Environmental Scanning Electron Microscopy/Energy Dispersive X-Ray (ESEM-EDX). Moreover, the dynamic pH of effluent solution was measured by pH-meter as well as the scavenging capacity of the minerals was quantified. A static geochemistry model was also developed governing H₂S/FeCO₃/CaCO₃/FeS system.

The results showed that in the pure calcite pack, the H₂S is not scavenged due to absence of scavenging agent (Iron minerals) and the inlet [H₂S] is the same as the outlet [H₂S]. ESEM/EDX results showed no FeS detected, just some precipitate of NaCl due to the precipitation of NaCl from 1% Na⁺ solution. However, with the calcite/iron mineral packs, the outlet [H₂S] samples were less than the injected H₂S because of the reaction of H₂S and siderite/hematite minerals resulting in H₂S mineralisation in the packs. The injected H₂S dissolves some iron from the siderite/hematite (rock dissolution), then the free Fe²⁺ in the solution reacts with H₂S-containing 1% Na⁺ solution and FeS precipitated. Again, ESEM/EDX detected some tracers of Sulphur due to FeS formation in the packs. Furthermore, it is shown in this work that the scavenging capacity of hematite (Fe₂O₃) is higher than siderite (FeCO₃). Also, the higher mixing ratio of calcite/iron minerals, the greater H₂S scavenged. Moreover, final pH of effluent was higher (pH_f~8) than the pH of injected H₂S due to calcite and siderite/hematite dissolution leading to liberation of Ca²⁺ and a few Fe²⁺. In the second round of flooding with 1%Na⁺, [Ca²⁺]/[Fe²⁺] generated in-situ is less than that of generated in the first round of flooding. However, the final [Ca²⁺]/[Fe²⁺] reaches to the amount of Ca²⁺/Fe²⁺ in the first round of flooding. In addition, while flooding the packs with H₂S solution, more Ca²⁺ and Fe²⁺ are generated.