Abstract

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, FexOy) 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 Ksp 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.

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