Iron chemistry in petroleum production

The aqueous chemistry of iron in souring petroleum reservoirs was modeled in the laboratory. The salt FeSO4·7H2O was allowed to react with Na2S·9H2O in synthetic seawater solutions for three days at 55–85°C. Elemental sulfur and crude oil were added to some of the reactions. The major iron precipitates formed were carbonates and sulfides, e.g. FeCO3, FeS, Fe3S4 and FeS2. These were also found in production solids from the Alaska North Slope. In the laboratory modeling experiments, the amount of precipitate increased with increasing Na2S·9H2O, and the identity of the precipitates depended on the amount of Na2S·9H2O. Elemental sulfur had little effect on what phases formed, but it lowered yields. For a series of reactions with two equivalents of added Na2S·9H2O for each equivalent of FeSO4·7H2O, the product mass increased at basic pH. A pathway was proposed for iron sulfide formation, in which HS− combines with aqueous iron(II) to give FeS and, subsequently, FeS2 and Fe3S4, which are produced from FeS when S2− is oxidized to S22−, and when Fe(II) is partly oxidized to Fe(III), respectively.