Methods for Treating Surface and Subsurface Waters
- A. Lee Larsen (Marathon Oil Co.)
- Document ID
- Society of Petroleum Engineers
- Journal of Petroleum Technology
- Publication Date
- December 1967
- Document Type
- Journal Paper
- 1,531 - 1,534
- 1967. Society of Petroleum Engineers
- 4.2.3 Materials and Corrosion, 4.1.2 Separation and Treating, 2.4.3 Sand/Solids Control, 5.4.10 Microbial Methods, 4.1.5 Processing Equipment
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This article considers new methods for treating surface and subsurface waters, and discusses older industrial water-treating methods seldom used in oil production. Discussed are benefits that can be derived from little-used lime and iron compounds for removing both total and dissolved solids and removing oil found emulsified as a standard or reversed emulsion. Also discussed is the use of ion exchange for the selective removal or reduction of sulfate and chloride where necessary, such as disposal of waters with sulfate and chloride contents sufficiently high to make the water unacceptable for pollution control. The following water softeners are discussed: the one-pass softener, the two-tank or polishing softener and the countercurrent softener.
Two occurrences prompted this investigation of methods now being used for water treatment in the oil industry and other industries, and a review of some of the original water-treating methods.
The first problem concerned a produced water to be used in a steam generator. It is imperative that no oil. clay, iron or hydrogen sulfide be present in steam generator or boiler feedwater. The water in this case contained 600 ppm of oil as a reversed emulsion, various amounts of iron up to 40 ppm, and suspended clay, along with hydrogen sulfide, CO2 and sulfate-reducing bacteria. The water originally was produced from three formations: one contributed the oil, hydrogen sulfide and sulfate-reducing bacteria; the second contributed the suspended clay; and the third produced water containing iron. The three formations did not produce consistent amounts of water; therefore there was variation in the amount of contaminants produced, as well as in the total solids dissolved, the pH, and hardness.
The second problem involved reducing nitrates from subsurface water to make it potable. In accomplishing this, a method was discovered for selectively removing sulfate from the water. The process was later found to be patented, but little has been done with the patent. It is a method with which the oil industry should become familiar since water conservation has become a major concern.
Problem and Theories
Because of equipment corrosion up-stream of the steam generators, it was necessary to eliminate excess carbon dioxide from the produced water to raise the pH of 6.8. Iron and clay, as well as the reverse emulsion oil, had to be removed. The iron was in solution as ferrous bicarbonate. The clay was a colloidal, bentonitic type and would not readily settle. The oil, a reverse emulsion, was stable and could not be skimmed, and adsorption-type filters would not work because they became water-wet, preventing adsorption of the oil from the water-outside phase emulsion. The clay could be removed with a diatomaceous earth filter but this equipment was considerably more expensive than other available filters. If oxygen were present the iron would oxidize and settle out. However, since oxygen was not present in the water, we did not want to increase oxygen content because of the boiler operation. Therefore, it was decided to use chlorine, an oxidizing agent that is readily available and cheap, and that controls bacteria.
The chlorine oxidized the iron, precipitating it. This would then adsorb the clay and further decrease the pH. This floc made solids in the water suitable for removal by cheaper filtration methods. The pH was increased so that the floc formed in a few minutes (retention time was 4 minutes between chemical addition and filtration). This could be accomplished with sodium carbonate that would further precipitate calcium if an excessive amount of the carbonate were used. Reduction of hardness in this case meant reduction of permanent hardness, i.e., the chloride or sulfate hardness (Fig. 1). If sodium hydroxide were used, the carbon dioxide would be eliminated, if an excess were added, a reduction would occur in temporary hardness, or that attributed to bicarbonate (Fig. 2).
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