Injection water sources in Kuwait range from brackish water (TDS=4000 mg/1) through seawater (TDS = 30,000 mg/1) to high saline brines (TDS = 200,000 mg/1). Some of these water sources are highly sulphide sour and may require treatment to prevent scaling, corrosion or iron sulphide precipitation. Another particular problem in water injection systems is the uncontrolled growth of sulphate-reducing bacteria (SRB) which leads to increased corrosion of the process plant. Therefore, evaluation schemes were undertaken to study the relative risks of MIC in a range of Kuwait?s water sources, using a combination of field sampling and laboratory biofouling trials. Recirculating biofouling loops were set up with the appropriate site water, and inoculated with the bacteria from the system so that an active biofilm was set up on small studs. These befouled studs were treated with proprietary biocide inhibitors under various dose rates in order to select the most appropriate control regime for particular water chemistries and process options.
In aerobic conditions, SRB corrosion invariably occurs beneath deposits of inorganic or organic detritus, microbial slimes or tubercules caused by the action of the iron-oxidizing bacterial?2. Unique features of SRB corrosion are that it occurs at neutral pH and in anaerobic conditions. Oxygen is not involved, and the corrosion products include iron sulphides. There is a range of possible mechanisms proposed3?4 for the corrosion reactions of such a process.
Typical electrochemical corrosion would not be expected to occur under anaerobic conditions, because the cathode becomes polarized by the build-up of a layer of atomic hydrogen. SRB are potentially able to stimulate the electrochemical corrosion mechanism by a number of possible means. The most widely accepted ones are : (1) the enzymic removal of polarizing cathodic hydrogen; (2) the formation of iron sulphides which are themselves cathodic to steel; (3) the formation of elemental sulphur upon re-oxidation of the sulphides; and (4) the formation of aggressive iron phosphides by SRB. It is generally accepted that, although all of these mechanisms may operate in nature, the production of iron sulphides and cathodic depolarization are the two most important corrosion mechanisms.
A combination of chlorination, ozonization and organic biocides is typically used to control and minimize MIC and reservoir souring in oilfield water injection and reservoir systems. In seawater systems, chlorine injection at the seawater lift pumps is the most common primary method of contro15. Because, chlorine is generally lost at the deaeration stage, and biological control is therefore maintained by the use of organic biocides. The most common strategy is a regular batch treatment for 3- 6h. The dose rate and contact time are both critical in order to optimize the killing effect of the biocide; in most cases laboratory trials are used to determine the best treatment.
Care must be taken in selecting and applying biocides for water injection systems to ensure that they are compatible with the system, can be handled safely, and have no environmental impact. Both batch and low-level continuous treatments of organic biocides will lead to the build-up of resistant and tolerant strains of the bacteria. Therefore, biocide effectiveness will decline with time, and monitoring must take place to identify when a change in biocide type is required.