PREDICTING SCALE FORMATION IN WASTEWATER DISPOSAL WELLS OFCRUDE-OIL DESALTING PLANTS

Scale formation and well plugging due to the incompatibility of injected wastewaters is a critical fieldproblem in wastewater disposal wells. When different wastewaters are mixed, it is necessary to evaluatetheir compatibility prior to the injection into disposal wells. The individual wastewaters may be quitestable at all system conditions and present no scale problems. However, once they are mixed, reactionbetween ions dissolved in the individual wastewaters may form insoluble products that cause permeabilitydamage in the vicinity of the wellbore.In this paper, the composition of different wastewaters, collecting from a few disposal wells of southwestIranian crude oil desalting plants, were analyzed critically. Laboratory studies as well as field experiencehave shown that formation damage in wastewater disposal wells may occur mainly due to the conceptionof Iron Sulfide in the case of mixing a wastewater, which contains Iron ions with a wastewater containingH2S. In this repect, a new correlation has been developed for estimating the critical concentration ofIron ions, Fe(2+) (ferrous ion), which will stay in solution at various pH values and a wide range of H2Sconcentration in crude oil desalting plants’ disposal wastewaters. This correlation eliminates the needfor compatibility assessment, which is usually assessed either by solubility calculations or by experimentaltesting, for water mixtures that contains Iron ions and dissolved H2S. Finally, a real case field problemwas analyzed and based on the correlation’s results, three different potential solutions were recommendedfor further field trial implementation.