Large quantities of seawater are injected in oil and gas fields for pressure support and sweeping efficiency of the reservoir. This injection enhances the hydrocarbon recovery. Many difficulties are induced by sea water injection, such as the risk of sulfate based scale formation like barium sulfate precipitation. Seawater contains around 2800 mg/L of sulfate and some reservoir water may contain large quantities of barium and strontium. When those two waters are mixed into the reservoir, precipitation will occur and reduce the efficiency of water injection. In the producer wells, scale deposits may significantly reduce oil production.
This is why technical solutions are more and more implemented to remove sulfate from the sea water before injection. Filtration processes using nanofiltration membranes are able to reduce the sulfate content of seawater, and this technique has been successfully operated for more than 12 years on several TOTAL offshore sites.
The use of nanofiltration membranes requires an efficient pretreatment of seawater in order to control the fouling that will reduce the capacity of the sulfate removal unit over time. The only way to remove this fouling and recover the capacity of the unit is to stop a part of the unit and operate a chemical cleaning of the membranes. Biological fouling appears to be predominant on nanofiltration membranes for this application. As a consequence, preventing biofilm growth is a key aspect to increase availability of those units and can also be of interest for corrosion management.
This paper presents the main results of experiments carried out by TOTAL on a sea water filtration pilot. Biofilm measuring probes have been tested in order to detect as soon as possible a change in the growth rate of the biofilm. Results demonstrated that such tools could be implemented in order to detect a default in the biofilm prevention strategy. This early detection tool will enable to react before the full system (all trains) requires a chemical cleaning in place.
The eventual objectives of this tool are to maximize reliability of desulfated water injection, to optimize use of chemicals, to increase nanofiltration membrane lifetime, and facilitate field operation of such units.