Probably the most common technique for enhanced oil recovery is that of using downhole injection water. The quality of the injection water, which is pumped into the oil-bearing strata, is critical. It must be free from suspended solids and have a low oxygen content to prevent the growth of aerobic microorganisms. It is also important that the water is free from bacteria, especially sulfate reducing bacteria (SRB). These bacteria flourish in anaerobic conditions by reducing existing sulfate ions to hydrogen sulphide (H2S) gas. This is highly corrosive and reduces the market value of the oil by increasing its sulphide content, as well as causing pipeline corrosion. It can also be a potential health hazard to personnel. Disinfection of the injection water is therefore a more suitable alternative to ‘sweetening’ the oil to improve its quality, a costly process.
As well as corrosion problems, injection of poor quality seawater can lead to the obstruction of injection pipe outlets by solid matter or clumps of microorganisms. Several stages of water treatment are often needed to obtain injection water of adequate quality and a combination of physical treatment and disinfection may be used. Typical treatment techniques include screening to remove larger solids and aquatic organisms, filtration to remove smaller particles, de-aeration, which removes oxygen, and disinfection which reduces levels of bacteria and other microorganisms.
Installation of UV disinfection systems is typically recommended immediately prior to use, following any other stages in the water treatment process. This ensures that there is minimal opportunity after disinfection for re-introduction of contamination microorganisms. Granular or membrane filters, which are often used to remove fine particles from the water provide an ideal environment for microbiological growth and post-filtration UV disinfection is recommended to reduce elevated bacterial counts.
Disinfection Using UV
On-line UV disinfection, supplemented by periodic shock dosing using chemical biocides, is one of the most cost-efficient methods of ensuring microbiological contamination in the injection water, and associated pipework, is minimized.
UV disinfection has been shown to effectively deactivate planktonic SRB in the injection water. However, because UV has no residual effect, it cannot reduce levels of bacteria colonizing pipework walls. To achieve this, shock dosing with biocides is recommended at regular intervals. UV disinfection dramatically reduces the frequency of chemical dosing and therefore the cost of handling and transporting chemicals.
UV disinfection systems are very compact, modular and easy to install into existing water treatment plant. Shock dosing with chemicals does not require the complex monitoring equipment which is necessary if biocides are used as a primary disinfectant. In addition, UV does not cause the formation of chemical disinfection by-products. These can be a problem with the continued addition of chemicals, causing unwanted and often unforeseen effects in the oil composition.
UV is the part of the electromagnetic spectrum between visible light and X-rays. The specific portion of the UV spectrum between 185-400nm (also known as UV-C) has a strong germicidal effect, with peak effectiveness at 265nm. At these wavelengths UV kills microorganisms by penetrating their cell membranes and damaging the DNA, making them unable to reproduce and effectively killing them. The UV dose received by a microorganism is dependent on the energy output of the UV lamp, the flow rate of the fluid, the ability of the fluid to transmit UV and the geometry of the treatment chamber.
A typical UV disinfection system consists of a UV lamp housed in a protective quartz sleeve which is mounted within a cylindrical stainless steel chamber. The water to be treated enters at one end and passes along the entire length of the chamber before exiting at the other end.
As flow rates increase, chamber size and lamp power output can be increased as required. For larger flows, multiple chambers are used, in series or in parallel, until the required degree of disinfection is reached. The use of high intensity UV lamps is recommended to treat large flow rates without taking up valuable platform space.
Reliable disinfection requires that a constant UV dose is applied to the water. Power transformers are available to absorb fluctuations in power supply whilst maintaining constant power to the UV lamp(s). These fluctuations are common in offshore facilities, being caused by the switching on or off of pumps and other electrical equipment. Power switching options are also available, adjusting the lamp power on-line as the water flow or the quality of the water changes. The power switching option maintains a constant, pre-determined UV dose level whilst ensuring maximum energy efficiency.
The UV Monitor
In most disinfection systems an instantaneous means of monitoring effectiveness is desirable. With chemical dosing techniques, disinfection is presumed to have been effective after a certain dose has been applied. Similarly with UV, if a minimum dose of UV energy, calculated from the maximum flow rate, can be shown to have reached the outer surface of the treatment chamber (where the UV monitor is situated) then the necessary disinfection has taken place. The UV monitor informs of variations in UV transmission of the fluid, warns of falling UV intensity and constant indication of the disinfection process.
New Developments in Automated Systems
The development of effective monitoring has led to the design of reliable automated disinfection systems. To maintain the required UV dose under conditions of varying water quality and flow rate, the power output to each UV lamp is constantly adjusted and stand-by UV chambers brought on or off line as required. UV systems are also available with industry standard data-logging software which interfaces with existing process control systems, providing information on the flow rate, UV intensity and power output of the UV disinfection system.