Evaluation of Seawater Filtration Systems for North Sea Application
Authors: Matthews Robert R. , Tunaal Terje , Mehdizadeh Parviz 
Published: December 2007

Summary
Trials of four types of filters for application in North Sea pressure maintenance projects were carried out in the Byfjord on the pressure maintenance projects were carried out in the Byfjord on the west coast of Norway. A precoat filter, a dualmedia filter, a backwashable cartridge filter, and a cartridge filter that uses sulfuric acid for regeneration were tested. Core flow-through tests showed that all four filters give a product water of high enough quality to prevent plugging of the highly permeable Brent formation of the Statfjord field. However, it was shown that some filtration was required. Filter response to operational fluctuations and contaminants differed greatly. These characteristics will affect their maintenance and efficiency in service and are therefore a part of the selection process. Considerable displacement and entrainment of formation fines occur during the flow of seawater through Statfjord cores. This phenomenon did not appear to contribute significantly to loss of phenomenon did not appear to contribute significantly to loss of permeability in this case. permeability in this case.

Introduction
Many of the major oil fields in the northern sector of the North Sea will require seawater injection to maintain reservoir pressure. The oil industry has been gaining much experience in the art of preparing seawater for injection. However, it remains an art preparing seawater for injection. However, it remains an art rather than a science. In general, experience gained in one part of the world cannot be transferred wholesale to another part. This study concentrates on the filtration of seawater. The intention was to compare the performance of four quite different filtration systems on a pilot plant scale. Another objective was to relate the acquired data to plugging mechanisms in injection wells.

Site Description
The test was carried out onshore at the Norwegian Underwater Technology Center (NUTEC). NUTEC is located about 5 km [3 miles] outside of Bergen on the Byfjord. Measurements by Bergen U. (1973-74) showed that a frequent exchange occurs between the waters of coastal fjords and seawater. Measurements from the Norwegian Oceanographic Data Center (Bergen) show essentially uncontaminated coastal seawater below about 25 m [82 ft] in the fjords. Three times during the course of the filter trials, water was collected from 10 depth intervals near the seawater intake. The intention was to investigate changes in the water quality during the entire test period. The water samples were analyzed for salinity, temperature, dissolved oxygen, chlorophyll a, total suspended matter (TSM), particulate aluminum, particulate phosphorus, particulate iron, and particulate magnesium. The phosphorus, particulate iron, and particulate magnesium. The concentration profiles of the listed components are shown in Table 1. The amounts of TSM are in reasonable accordance with Hagmeier's measurements (0.2 to 0.9 mg/dm [0.2 to 0.9 mg/L]) of surface waters in the North Sea. Temperature and dissolved oxygen are also in the desired range, but salinity shows the fjord to be slightly fresher than the North Sea (35.2% NACl). Table 1 shows that in the Byfjord, as in the North Sea, more than 90% of the suspended matter is organic. Scanning electron microscopy (SEM) showed that a large proportion of the particles present were phytoplankton produced in the upper euphotic zone of present were phytoplankton produced in the upper euphotic zone of the fjord. Diatoms, coccolithophores, and dinoflagellates were identified. Diatoms were definitely in the majority. Average counts of particles larger than 1 m [1 micron] were 34 000/cm [34,000/mL] at 25 m [82 ft] and 26 000/cm [26,000/mL) at 60 m [197 ft] during the period of the trials. period of the trials.

Description of the Filter Units
The four filter units tested represent different approaches to the technical problem of filtering large quantities of water with low concentrations of suspended solids and particle sizes between 1 and 40 m (1 and 40 microns]. Filter A (Fig. 1) was a backwashable cartridge filter. It had two stages of filtration. The first stage had a 30.5-cm [12-in.]-diameter stainless steel disk with pore size of 80 um [80 microns]. The second stage was a cylindrical filter mat of needled polypropylene felt (nominal pore size 3 um [3 microns]) 1.3 cm (0.5 polypropylene felt (nominal pore size 3 um [3 microns]) 1.3 cm (0.5 in.] thick and fitted to a perforated basket. Both stages were regenerated by backwashing with filtered water through a rotating arm passing segments of the filter elements. The filters were regenerated at preset 30-minute intervals unless the pressure drop passed 24 kPa (3.5 psi] before that time. When regeneration became continuous, the filter cartridges had to be replaced. When coagulant was added to the inlet water, the dosage was 1.0 ppm. ppm. In Filter B (Fig. 2), the water first passed through a bank of six hydrocyclones in parallel and then a two-stage filtration through fine-meshed membranes of synthetic fibers.
JPT
P. 843