Shining A Light on Sulphate Reducing Bacteria
Chemical free UV disinfection for both seawater and produced water re-injection is a highly effective method to prevent SRB growth.
As seawater and more recently, produced water re-injection is used to enhance oil production (Enhanced Oil Recovery) the need to control microbiological activity is becoming high on the agenda for operators worldwide. In particular, sulphate reducing bacteria (SRBs) which consume dissolved sulphates in the seawater 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, SRB’s are present in seawater that is used for many applications offshore such as cooling water, firewater systems and well injection water. SRB’s are typically dormant in aerobic environments and only activated when introduced to anaerobic conditions such as a piping network or oil reservoir. Process stages such as vacuum deaeration can lead to problems increasing or occurring earlier than expected. In addition to H2S formation, bacteria can proliferate and excrete extracellular polysaccharides which can ‘stick’ the cells together to form adherent slimes or biofilms, risking further damage to equipment and / or causing blockages of porous rock strata, reducing yield and defeating the object of injection and enhanced oil recovery.
Traditionally the typically approach of operators was to inject chemical biocides e.g. hypochlorite or glutaraldehyde at both continuous dosing and batch dosing intervals – “shock dosing” – to target SRB’s and other bacteria present in injection water.
Like many microorganisms, SRB’s can multiply at a significant rate. Under the correct conditions, SRB’s typically will double in number every 20 minutes. With over 220 strains of SRB, this fast reproductive cycle and natural genetic variation that occurs in all microorganisms has led to SRB species developing naturally tolerance or immunity to many of the approved biocides used offshore. Often, changing to an alternative biocide will only provide a temporary solution until the bacteria again, becomes resistant or immune and an alternative biocide method is once again required (war of attrition).
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 applications and following successful pilot trials is now being used to treat both seawater and produced water for re-injection in EOR applications.
Ultraviolet radiation in the UV-C band has a wavelength of 254nm, is very close to the absorbance wavelength of the amino acid bases which form the “rungs” of the DNA double helix. UV-C radiation at 254nm fuses adjacent amino acid groups making it impossible for the molecule to replicate by permanently damaging the thymine strand of the DNA helix. Bacteria exposed to the correct level of UV-C are render harmless and then effectively ‘die’ at their next natural, reproductive cycle (approx. 20 – 30 minutes later for SRB’s).
As the damaged microorganisms can no longer reproduce, genetic mutation is significantly less likely to occur, thus no microorganism has yet shown any immunity to UV-C light, including 17 known strains of chlorine resistant microorganisms such as Mycobacterium Intracellular, Cryptosporidium and Giardia.
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, SRB’s have proved to be very sensitive to UV-C, with standard UV doses used in the drinking water industry of 40 mJ/cm2 providing a >4 log reduction (>99.99%) reduction of SRB’s 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, the holistic link between injection water and returning produced water and its impact on Environmental Impact Factors (EIF) is becoming more prominent. As UV disinfection does not introduce any residual compounds into the water, challenges regarding residual toxic compounds e.g. BTEX and Phenols in returning produced water or the formation of disinfection by-products e.g. Halogenated Hydrocarbons can be reduced or in some cases eliminated – driving down operational costs whilst improving environmental performance.
In addition, no residual compounds in the water also can alleviate issues regarding additive performance. For example, the most commonly used biocide, glutaraldehyde is not compatible with the most commonly used oxygen scavenger, Sodium bisulphate. By switching to the physical process of UV treatment as the primary disinfection technique, operators can avoid negative chemical / additive interactions.
UV Disinfection plants are typically skid mounted or containerised and when compared to alternative disinfection technology’s such as electro-chlorination are typically 50% smaller in footprint and weight. UV Chambers use modern ‘in-line’ designs that see UV reactor designs mimic ‘butterfly valves’ allowing for installation direct into the pipe (similar to a valve or flow meter) making the technology suitable for retrofits into pipe galleries. Flow capacities typically range from 100 m3/hr to over 6,000 m3/hr in a single UV Chamber.
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 2 years before requiring maintenance, making UV Systems suited to future developments such as unmanned platforms and subsea installations.