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

Abstract 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 (pHf~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.