Predicting scale formation in wastewater disposal wells of crude-oil desalting plants

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 into 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, collecting from a few disposal wells of southwest Iranian crude oil desalting plants, were analyzed critically. Laboratory studies as well as field experience have 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. In this repect, a new correlation has been developed for 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.