SECTION TITLEPRODUCTION • PROCESSING • HANDLING
LIGHTING THE WAY
Paul Hennessey discusses how ultraviolet disinfection is emerging as a best available technique for SRB reduction
re-injection is used to enhance oil production (enhanced oil recovery (EOR)), the need to control microbiological activity is high on the agenda for operators worldwide. In particular, sulphate reducing bacteria (SRB) that consume dissolved sulphates in the sea water and produce hydrogen sulphide (H2
A
s seawater and more recently, produced water
The WF range offers a considerable footprint reduction compared to alternative equipment
Found naturally, SRBs are present in seawater used for well injection, but are typically only activated when introduced to anaerobic conditions such as a piping network or oil reservoir. Process stages such as vacuum de-aeration can lead to problems increasing or occurring earlier than expected. In addition to H2
S) is of major concern due to
the associated risks of microbial induced corrosion (MIC), well souring, reservoir plugging with iron sulphide (FeS) and damage to process equipment and infrastructure.
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formation, bacteria can proliferate and excrete extracellular polysaccharides that stick the cells together to form adherent slimes or biofilms, damaging equipment and causing blockages of porous rock strata, reducing yield and defeating the object of injection.
S
Traditionally the approach was to inject high doses of chemical biocides, e.g. hypochlorite or glutaraldehyde, at both continuous dosing and batch dosing intervals – ‘shock dosing’ – to kill SRBs and other bacteria present in injection water. In particular, SRBs can multiply at an alarming rate – given the right conditions they will double in number every 20 minutes. With over 220 strains of SRB, this reproductive cycle and natural genetic variation that occurs has led to SRB species becoming naturally immune to certain biocides. Changing to an alternative biocide provides a temporary solution in this war of attrition, until the bacteria become resistant and an alternative biocide is required. In addition to the above, rising costs, changes in regulations such as OSPAR and HOCNF, and operational concerns with the delivery, storage and handling of chemicals has seen operators look to alternative disinfection solutions. Use widely in drinking water and
wastewater, the chemical-free, physical treatment process of UV disinfection has emerged as a new ‘best available technique’ for injection water and, following successful pilot trials, is now being used to treat both seawater and produced water for re-injection in EOR applications worldwide. UV radiation in the UV-C band has
a wavelength of 254nm, which is very close to the absorbance wavelength of the amino acid bases that form the ‘rungs’ of the DNA double helix. UV radiation fuses adjacent amino acid groups, making it impossible for the molecule to replicate and permanently damages the thymine strand of the DNA helix. Bacteria exposed to UV radiation then die at their next natural reproductive cycle.
A major benefit is that unlike chemical biocides, no microorganism has shown any immunity to UV-C light. Te UV intensity (or ‘fluence rate’) produced per unit area by a UV lamp is normally measured in mW/cm2
. Multiplying this
by the hydraulic retention time in the UV reaction chamber in seconds gives the effective UV dose (or ‘fluence’) in mJ/ cm2
providing a >4 log . In particular, SRBs have proved to be
very sensitive to UV-C, with standard UV doses used for drinking water disinfection of 20-40mJ/cm2
reduction (99.99%) reduction of SRBs in a single pass (0.5 seconds exposure to UV-C light).
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