Abstract

Little attention has been paid to the reaction of mixed calcium-iron carbonate in solutions in comparison with the reaction of other solids such as calcium carbonate, calcium sulfate and barium sulfate. As a result, little information is presently available about the interaction and precipitation of iron carbonate; and mixed calcium/iron. It is known that iron carbonate precipitation kinetics is sensitive to temperature. At high temperature (65 oC), a sharp drop in iron concentration was observed during the first minutes in a solution devoid of calcium. And the drop was gradual at 55 OC. However, adding calcium to the solution under the same conditions had a great influence in reducing the drop in iron concentration. The apperant increases in the solubility of ferrous iron was proportional to the concentration of calcium present in the solution. In addition, presence of calcium in solution also reduced the effect of temperature on siderite precipitation. On the other hand, the presence of iron did not show significant impact in the solubility of calcium. The cumulative molar ratio of ferrous iron to calcium in the precipitated solid was found to be varied based on the amount of siderite that initially precipitated. The X-ray diffraction analysis showed that the presence of ferrous iron inhibited the growth of calcite but did not prevent the formation of aragonite. The crystal shape of siderite, showed by scanning electron microscope, was completely different from that of calcite and the crystal shape of the solid collected in the co-existence of ferrous iron and calcium varied based on the molar ratio of iron to calcium in the solution.

Introduction

Calcium carbonate and iron carbonate minerals are widely observed in oilfield industry. They are also common carbonate salts typically found in the municipal water and wastewater industries. Calcium is commonly present in the formation brine while iron could either be inherently present in the formation brine or released from the formation rock and production strings due to intervention treatments and/or corrosion process.

There is a close relation between the formation of scale and the corrosion. Ferrous carbonate deposit has been recognized as protective layer that effectively inhibits or reduces the corrosion rate [Foss, 2006 and Dugstad et al., 2001]. Also positive scaling of calcium carbonate in the pipe wall of municipal water is often advocated as a mean of corrosion mitigation [Marril et al, 1977]. However, recent evidence suggests that ferrous carbonate may be more important to forming an adherent protective layer. Both iron carbonate and calcium carbonate formation can be useful for corrosion control. On the other hand, excessive formation of either mineral is found to have an adverse effect on tube plugging. They may accumulate in the pumps; valves and production strings, which eventually cause a sever decline in the fluid flow. Also their suspended particles in water, especially in the water injection, may cause formation damage. Therefore, understanding of their chemistry is an important element in the water and oil industries. Most of previous studies have reported results about the kinetics of calcium carbonate solubility and deposition. Few reports have been found about ferrous iron carbonate precipitation. This is probably due to the difficulties of establishing an anoxic system to precipitate ferrous carbonate in the laboratory. In spite of the fact that formation of mixed scale of both calcium and iron carbonate, FexCa1-xCO3, has been observed in several oil wells and water treatment plants, not enough results have been presented in the literature showing the interactions of both simultaneously.

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