Paul Hennessey discusses how ultraviolet disinfection is emerging as a best available technique for SRB reduction
As seawater and more recently, produced water 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 (H2S) 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.
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 H2S 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.
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. The 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. 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 providing a >4 log reduction (99.99%) reduction of SRBs in a single pass (0.5 seconds exposure to UV-C light).
As a chemical-free, physical process UV provides a range of process benefits and operational advantages. As regulations such as HOCNF become more stringent, operators have made the holistic link between injection water and returning produced water and its impact on environmental impact factors (EIF). Because UV does not introduce any residual compounds into the water, challenges regarding residual toxic chemicals in returning produced water or the formation of disinfection by-products such as halogenated hydrocarbons (due to the chlorination of seawater) can be reduced or in some cases eliminated – driving down operational costs whilst improving environmental performance.
Operationally, UV disinfection can reduce the need to ship, store and handle chemicals offshore, helping to improve safety and drive down operational costs. In recent research (2016) conducted by Shell, the typical payback for using UV versus biocides for small flow application, e.g. 300m3/hr, could be as little as one to two years (onshore EOR injection well, the Netherlands).
For larger flow rates >2000m3/hr OPEX costs can be significant. Another major oil & gas operator compared the costs of supply, transportation, handling, storage and injection of acrolein into 300,00 bwpd against those of a chemical-free, UV disinfection package. Based on a five-year operation calculation the biocide would cost £3,500,000 against only £130,000 in lamps, maintenance and power for UV.
UV disinfection plants are typically skid-mounted or containerised and when compared to alternative disinfection technologies such as electro-chlorination, are typically 50% smaller in footprint and weight. Models such as the WF or Wafer by atg UV Technology use modern ‘in-line’ designs that see UV reactor designs mimic butterfly valves, allowing for installation or retrofits into the tightest of pipe galleries.
Developments in power supply technology and UV lamp design have significantly increased both UV system capacity and operational life, with certain UV designs being able to operate continuously for over two years before requiring maintenance, making UV systems suited to future developments such as unmanned platforms and subsea installations.
In addition to seawater injection for well injection and EOR applications, UV disinfection is now being adopted for pipeline hydrotest water, produced water reinjection and more
recently, for the treatment of flowback water before reinjection in hydraulic fracturing (shale gas and oil) applications.