A new correlation predicting the formation of iron sulfide precipitation in wastewater disposal wells

Scale formation and well plugging due to the incompatibility of injected wastewaters is a critical field problem in wastewater disposal wells. When different wastewaters are mixed it is necessary to evaluate their compatibility prior to the injection in disposal wells. The individual wastewaters may be quite stable at all system conditions and present no scale problems. However, once they are mixed, reaction between ions dissolved in the individual wastewaters may form insoluble products that cause permeability damage in the vicinity of the wellbore. In this paper, the composition of different wastewaters that were collected from southwest Iranian desalting plants disposal wells were analyzed critically. Laboratory studies as well as field experience has shown that formation damage in wastewater disposal wells may occur mainly due to the conception of Iron Sulfide in the case of mixing a wastewater which contains Iron ions with a wastewater containing H2S. A new correlation is developed estimating the critical concentration of Iron ions, Fe(2+) (ferrous ion), which will stay in solution at various pH values and a wide range of H2S concentration in crude oil desalting plants disposal wastewaters. This correlation eliminates the need for compatibility assessment, which is usually assessed either by solubility calculations or by experimental testing, for water mixtures that contains Iron ions and dissolved H2S. Finally, a real case field problem was analyzed and based on the correlation's results three different potential solutions were recommended for further field trial implementation.