Water Treatment for Once-Through Steam Generators
Authors: Burns W.C. 
Published: December 2007

Emphasis on steam flooding has led to a new steam generator, the once-through type. This new generator, in turn, has changed requirements for water treatment for the generator. This paper discusses the water sources, requirements for treating source water for steam generators, equipment necessary for treatment, and a new ion-exchange unit. The new unit features a series service (double pass) set-up and a special upflow-downflow countercurrent regeneration technique. Oil filters for produced waters and deaerators are also discussed in this review of water treatment.

Introduction

The majority of oilfield steam generation equipment used for steam flooding is the water-tube single-pass or "once-through" type. These units are being built for very high pressures (up to 3,000 psig). These two features alone make this type of unit vastly different from the conventional, industrial drum-type boiler, and consequently dictate a change in thinking regarding feedwater treatment. To further confuse the situation the product being produced by these generators is wet steam, and not being produced by these generators is wet steam, and not the dry, saturated variety which is a necessity for most industrial processes. Yet it is this wetness of the steam which allows this equipment to be used on water supplies that were formerly too expensive to treat. This means that the water treatment equipment can be simplified and the initial and subsequent operating costs greatly reduced over that required to treat a given water for a conventional steam generator. However, there is little or no margin for error, because mistakes in treatment, equipment misoperation, lack of reserve capacity, etc., cannot usually be tolerated. This presentation does not review the endless combinations and varieties of water treatment equipment needed to treat all possible water supplies, but rather discusses equipment and design philosophy for the majority of situations.

Water Supplies

The most important step in the design of a water treatment system is the chemical analysis of the water to be used. Unfortunately there is a tendency to minimize this on the false assumption that a workable system can be developed from fragmentary or incomplete analysis data. This can be a costly assumption. There is no substitute for a complete, accurate and representative series of water analyses.

Our natural water supply results from the unending cycle of evaporation from oceans, lakes and rivers with subsequent precipitation in the form of snow or rain. Upon reaching the earth, water either remains on the surface or seeps into the ground. Such a division results in distinguishing characteristics as follows.

Surface Supplies

Moving supplies (rivers, creeks) - generally contain mud, silt, other suspended matter, dissolved gases and dissolved minerals. In some cases color, waste products and sewage are present. Static supplies (lakes, reservoirs) - generally contain dissolved gases and dissolved minerals. Also bacteria, algae, and other organic matter may be present. Amount of suspended matter is usually low, due to natural settling, but shallow lake bottoms are easily disturbed by storms.

Ground Supplies

Water wells - filtration effect of earth reduces suspended matter, organic matter, and algae, but increases dissolved mineral content. In general, the deeper a well the more highly mineralized the water will be. Produced water - this is a special type of ground water, in that it is usually contaminated with oil, sometimes contains hydrogen sulfide, and may be extremely mineralized (often in excess of 20,000 ppm). Occasionally such a water will contain considerable organic matter and might even have a high redox potential.

Table 1 lists the more common impurities in water and shows an approximate comparison between a ground and surface supply. The differences are substantial and treatment methods to produce the same purity will vary considerably. In addition to the basic differences in the two natural water sources, it is extremely rare to find two surface or two well supplies with identical analyses. Therefore, it is important that specific water analyses data be developed before undertaking design of even moderately sized water treatment plants.

Surface Supplies

Moving supplies (rivers, creeks) - generally contain mud, silt, other suspended matter, dissolved gases and dissolved minerals. In some cases color, waste products and sewage are present. Static supplies (lakes, reservoirs) - generally contain dissolved gases and dissolved minerals. Also bacteria, algae, and other organic matter may be present. Amount of suspended matter is usually low, due to natural settling, but shallow lake bottoms are easily disturbed by storms.

Ground Supplies

Water wells - filtration effect of earth reduces suspended matter, organic matter, and algae, but increases dissolved mineral content. In general, the deeper a well the more highly mineralized the water will be. Produced water - this is a special type of ground water, in that it is usually contaminated with oil, sometimes contains hydrogen sulfide, and may be extremely mineralized (often in excess of 20,000 ppm). Occasionally such a water will contain considerable organic matter and might even have a high redox potential.

Table 1 lists the more common impurities in water and shows an approximate comparison between a ground and surface supply. The differences are substantial and treatment methods to produce the same purity will vary considerably. In addition to the basic differences in the two natural water sources, it is extremely rare to find two surface or two well supplies with identical analyses. Therefore, it is important that specific water analyses data be developed before undertaking design of even moderately sized water treatment plants.