Iron Precipitation in Calcite, Dolomite and Sandstone Cores

Abstract

AbstractFormation damage due to iron precipitation has been a major problem in the oil field. It has been established that as the pH of the injected acid increases, iron (III) ions start to precipitate and block the pores. However, where exactly this iron precipitates and how iron precipitation changes with different lithologies has not yet been studied.Coreflood experiments were conducted to assess the effects of temperature and iron concentrations on the degree of damage caused by iron precipitation during an acid job. The temperature values tested were 200 and 300°F. Iron concentrations ranging from 5,000 to 10,000 ppm was used. The core effluent samples were analyzed by ICP to measure the concentrations of key cations.After conducting core flood experiments, it was found that iron precipitation occurs across the entire core. Iron precipitation was severely detrimental in the sandstone cores compared to those of calcite and dolomite. While the limestone cores showed the least formation damage from iron precipitation, core flood tests indicated that as the iron concentration increases, the damage was more evident, especially in the sandstone cores. On the other hand, increasing the temperature adversely affected sandstone and dolomite cores but improved the final permeability of limestone cores.In this study, the location of the iron precipitation is determined for three different lithologies. The effects of different parameters are studied to determine the best conditions that would lead to a decrease in iron precipitation and hence prevent formation damage. Iron control agents are not always needed, as previously thought.