American Institute of Mining, Metallurgical, and Petroleum Engineers, Inc.

Abstract

Conoco uses vacuum deaeration in treating seawater for injection in California and Dubai. With multistage towers, oxygen concentration in the effluent water is reduced to less than 0.05 mg/L, precluding need for corrosion inhibitor treatment. Improvements in the Dubai towers are noted, as are problems, such as foaming, which have been encountered. Data on throughput and oxygen concentrations are included.

Vacuum deaeration of water is a method known and used for many years. That use, however, has been limited by cost and available technology. Several writers ascribe a practical lower limit of 0.3 mg/L to 0.5 mg/L practical lower limit of 0.3 mg/L to 0.5 mg/L oxygen in the effluent water when using vacuum deaeration. Gas stripping and chemical scavenging have been considered more practical answers when truly low dissolved oxygen concentrations are required for waterflood operations. These methods have proven satisfactory in numerous applications proven satisfactory in numerous applications where availability of gas for stripping or the economics of chemical scavenging did not preclude their use. The limited solubility of preclude their use. The limited solubility of calcium sulfite has also interfered with sulfite ion scavengers in some systems.

In those systems which require deaeration, the need is based on those reactions involving oxygen; e.g., corrosion, oxidation of ferrous iron in solution, growth of aerobic bacteria, etc. Of these, corrosion control is probably the most important. Corrosion control by removal of dissolved oxygen is particularly important in waterflood operations. For large waterflood systems, the economics of a capital expense for vacuum deaeration are often more favorable than the continuing operating expense of chemical scavenging.

Recent cost estimates were $450,000 for a vacuum tower, vacuum pump, and booster pump to deaerate 100,000 b/d from 6.5 mg/L oxygen to less than 0.02 mg/L. Chemical scavenging with catalyzed sodium sulfite (32 cents/lb) would run $212,500 per year for chemicals alone.

Improvements in vacuum stripping technology have made this a practical and economically acceptable unit operation for use in waterflood treating systems.

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