Abstract

Scale deposition is one of the most serious oilfield problems when two incompatible waters interact chemically and precipitate minerals. Typical examples are sea water, with high concentration of sulfate ions and formation waters with high concentrations of calcium, barium, and strontium ions. Mixing of these waters could cause precipitation of calcium sulfate, barium sulfate and/or strontium sulfate. Sea water treatment does not remove the entire sulfate ions from the injected water, at least 100 ppm sulfate will remain and even this low concentration may cause damage.

This study was conducted to investigate the damage caused by deposition of calcium sulfate precipitation, describing the damage using material balance method, and then a new technique will be proposed to prevent the damage due to calcium sulfate. The result of the experimental data showed the reduction of permeability at least by 20% from its initial value due to sea water injection. The results from field data showed a reduction of the water injection rate from 25,000 to 5000 bbl/day in one month due to calcium sulfate precipitation. The results of the new analytical model showed that in about one month the injection rate will stop or the injection pressure will exceeds the fracture pressure of the formation.

High salinity water injection caused severe formation damage and the injectivity declined faster compared to the low salinity water injection. The material balance calculations showed a good match between the experimental, field, and predicted data. The new method was very effective in preventing and removing sulfate precipitation. The cost of the new method was lower compared to the sulfate removal process and this method can be used with the injected water and no sulfate will precipitate.

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