Abstract
When oil is produced by water injection, sulphide formation (souring) can be
stimulated. Souring is often caused by sulphate-reducing bacteria (SRB) which
oxidize organic carbon (oil organics) with sulphate in the injection water to
CO2 and sulphide. As a result, H2S concentrations in the
produced water, oil and gas gradually increase. A consequence can be that the
piping infrastructure must be redesigned from sweet to sour service. A
relatively novel biotechnology aimed to remedy souring is to add nitrate to the
injection water. Nitrate tracks the injection water effectively and its cost
allows continuous and field-wide treatment. In a field-wide nitrate injection,
the injection water (approximately 3,500 m3/day) was amended
continuously with 2.4 mM (150 ppm) nitrate. Three points in the injection water
system and 12 production wells were monitored by sampling every 2 to 3 weeks.
The concentrations of sulphide, sulphate, nitrate, nitrite and ammonia in
injection and produced waters were determined, as well as the activities of
nitrate-reducing bacteria (NRB). Field-wide nitrate injection gave a 70% drop
of aqueous sulphide within the first 5 weeks, after which the concentration
recovered somewhat for the next 20 weeks. The activity of NRB increased
throughout this period, indicating the possibility of further decreases in
souring in the future.
Introduction
Microbial production of sulphide by sulphate-reducing bacteria (SRB) in oil
reservoirs (i.e. souring) often occurs during secondary oil recovery processes
when water is injected to maintain reservoir pressure. Souring is largely
perceived to have negative effects because dissolved sulphide (HS-)
and precipitated metal sulphides (e.g. FeS) are corrosive towards metal pipes
and equipment. Injection of suspended metal sulphides may decrease reservoir
injectivity by plugging zones near the injection wellbore, decreasing oil
production. Suspended metal sulphides also stabilize oil-water emulsions
preventing effective separation of produced water and oil. Hence, souring is
highly undesirable from a business and operating point of view, especially when
the original facilities were not designed to handle sour production. Souring
gives rise to safety concerns regarding the exposure of workers in the field to
released hydrogen sulphide and to complaints about odours from surface rights
owners and, in urban settings, over the threat to real estate values of a sour
service operation on or near residential property.
© 2009. Petroleum Society of Canada (now Society of Petroleum Engineers)
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History
- Meeting paper published:
17 June 2009
- Revised manuscript received:
20 February 2009
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