An experimental determination of hydrogen sulfide scavenging capacities and mechanisms in iron-bearing minerals

The injection of sea water for the pressure support of oil fields is commonly associated with the biogeneration of hydrogen sulfide (H₂S) by sulfate reducing bacteria and/or archaea (SRB/SRA). H₂S is extremely toxic and corrosive, as well as providing a source of sulfide ions for the formation of iron, zinc and/or lead sulfide scale. However, H₂S production is rarely, if ever, associated with seawater breakthrough and its retardation can be linked to a number of mechanisms. Certain minerals (e.g. siderite, FeCO₃, and/or iron oxides, Fe_xO_y) may react with produced H₂S and retard its progress towards the producer wells. This is a generally beneficial effect but it is difficult to quantify and it is generally estimated from direct matching to the field appearance of H₂S. Alternatively, H₂S retardation factors are based on correlations with the mineralogy of the field and these are rather unreliable, since few experimental results have been published. In essence, this quantity (the H₂S retardation factor) is more of a "matching parameter" rather than being truly predictive. Candidate mechanisms for sulfide scavenging by iron-bearing minerals have been experimentally identified and scavenging capacities have been determined for siderite FeCO₃ in modified static adsorption tests and in dynamic pack floods, for aqueous-only systems. The effects of changing several conditions were studied, including temperature, initial pH and grain size. A combination of dissolution/precipitation and surface displacement mechanisms were identified in the static bottle tests and further confirmed during the dynamic sand/siderite and crushed-core pack floods. ESEM-EDX and particle size analyses established the presence of mobile FeS (<100 µm) in the column after the flood had reached completion, confirming the bulk precipitation of FeS from dissolved Fe²⁺. Bringing together these two mechanisms allowed for the rationalisation of the observed scavenging profile, with reference to the K_sp of siderite. By further understanding the mechanisms of H₂S scavenging experimentally, it will be possible to incorporate these into field-prediction models. The absolute values obtained for the 8 wt% siderite packs were 1.03 and 1.74 mg/g at 25 and 96°C, respectively. Crushed core packs yielded significantly higher values of 5.76 and 5.80 mg/g at 25 and 96°C, respectively, which have been hypothetically attributed to the presence of iron-bearing clays in the core samples.