A Weight- and Space-Saving Seawater-lnjection System Design

Abstract

Summary This paper describes an integrated and compact seawater-injection system built around backwashable cartridge filters and catalytic deoxygenation. Suspended solids are removed by a high-efficiency cartridge filter in which the cartridges are regenerated by a proprietary acid rinse. Dissolved oxygen is removed in a single stage to a level of 20 ppb by use of dissolved hydrogen over a palladium catalyst. A combined-cycle power plant makes the unit self-contained. A unit for treating 150,000 B/D [23 plant makes the unit self-contained. A unit for treating 150,000 B/D [23 850 m3/d] of seawater is estimated to weigh 260 metric tons [260 Mg] (operating), or about 35% of a comparable "conventional" system using media filters and vacuum deaeration. Space requirements are reduced proportionally. proportionally. Introduction Waterflooding has become common to increase production rates and to maintain pressure in oil fields. In the North Sea alone, more than 30 injection plants are currently in operation. The injected water must meet certain specifications that depend in part on the characteristics of the reservoir being flooded.The water must be sufficiently free of suspended particles that plugging of the injection wells does not occur.The water must be sufficiently free of dissolved oxygen to minimize the corrosion of injection facilities and piping. Aside from the risk of damaging expensive equipment, corrosion products are a serious source of contamination of injection water. A typical specification for injected seawater is 20 of dissolved oxygen per liter.The water should be completely free of living micro-organisms that otherwise would cause fouling of facilities and injection wells.In offshore installations, floor space is at a premium, and unnecessary weight is expensive. Conventional water-injection systems with large, heavy media filters and vacuum deaeration towers are a promising area for the introduction of new technology. A complete seawater-injection system includes filtration, deoxygenation, pumping, power generation, and chemical treatment. Only filtration and deoxygenation are discussed in this paper. Calculations show that new approaches to the remaining unit processes can result in further significant reduction of space, weight, and cost. These considerations, however, are currently undemonstrated for service with seawater and are outside the scope of this paper. Filtration The choice of filter system for the present application was dictated partly by the requirements of the deaeration method. In addition to the previously noted weight penalty, media filters require the use of a previously noted weight penalty, media filters require the use of a polymeric coagulant or bed conditioner for maximum efficiency. Some danger polymeric coagulant or bed conditioner for maximum efficiency. Some danger of polymer leakage through the filter always exists. The resulting irremediable fouling of the expensive catalyst bed in the deaerator was a strong incentive to find an alternative to media filters. Among the options identified was a unique type of cartridge filter system that uses concentrated sulfuric acid for cartridge regeneration. Tests with this filter on seawater have shown that virtually all particles larger than 2 can routinely be removed. Good performance was particles larger than 2 can routinely be removed. Good performance was obtained even during the spring biotic bloom. Flow rates up to 0.3 L/sec per 10-in. [25-cm] cartridge can be maintained. This flow rate is per 10-in. [25-cm] cartridge can be maintained. This flow rate is comparable with that of a diatomaceous filter of similar size. The system gives excellent water quality immediately upon restart after the regeneration cycle and at all flow rates as long as the pressure drop across the cartridge does not exceed 2 bar [200 kpa]. A filter system of this type installed for long-term testing on a North Sea platform in July 1986 has given excellent performance. Two stages of filtration are used in the proposed system. However, these are contained in one filter housing to minimize layout space and piping requirements. piping requirements. Backflow air pulses and seawater backwashes are applied approximately twice a day to keep the pressure drop across the filters at a low level. This procedure maximizes the running time between acid regeneration cycles. The filter normally is acid-cleaned about once every 9 days. During the bloom season, more frequent backwashes and acid regenerations are required. Sulfuric acid is used to recondition the cartridges. The organic deposits are transformed to finely divided carbon by dehydration. The carbon is not adherent and is easily washed away. The acid is foamed in a foam generator and piped separately to each filter stage. About 90 minutes is required for acid regeneration of a two-stage filter. Monitoring filtered-water quality is, of course, essential to the operation of an injection-water plant. In a two-stage filtration process, routine sampling from both stages gives a substantial advantage in reliability. The failure frequency of the cartridges is very low; the chance that a cartridge failure will occur in both stages simultaneously is virtually nil. Equivalent filtration efficiencies could probably be achieved with only one stage. Fig. 1 shows a weight comparison between the sand filters used on a typical North Sea platform and an acid-backwashed cartridge filter system of equivalent capacity.