INTRODUCTION
The formation of iron carbonate films, or by converting carbonate to magnetite at higher temperatures, are important means used to control the rate of corrosion in the oil and gas industry. The FeCO3 film can slow the corrosion process by covering up the steel surface and preventing the diffusion of corrosive species to the pipe surface. On the other hand, the excessive build up of iron carbonate salt in the oilfield production line can be a major issue, particularly, in the absence of effective inhibitors. The growth and thickness of the film depend upon the kinetics of precipitation and crystal growth of iron carbonate. In such kinetic studies, it is important to keep these parameters constant. This work was intended to study the kinetics of crystal growth of mixed calcium-ferrous carbonate salts at constant experimental conditions. In this study all the parameters are well controlled. A special experimental design is used to keep the initial concentrations, pH (7.1), temperature (31 and 48oC), pressure (1 atm), and ionic strength (0.5 M) constant during the course of the experiment. The composition of calcium-ferrous carbonate salt that formed was determined. The mass flux of crystal growth was also investigated and found to be constant.
Oilfield brines have a lot of impurities. When the brines become supersaturated due to co-mixing and changes in conditions, they usually form mixed salts with cation-substitution. Barium-strontium sulfate is a common example, it is often found in suspended matters in ocean water and in marine sediments.1,2 Similarly, calcium-magnesium carbonate is common in fresh water interfaces and in sedimentary rock.3 Ankerite is a mixed calcium-iron carbonate salt which forms in iron rich hydrothermal veins.4 Calcium-ferrous carbonate is often found in ground water, oilfields and in corrosive environments containing calcium-bearing water.5-7 It has been suggested that the cation content of water is not controlled by equilibrium with pure salts.8 In case of iron-calcium carbonate, the iron/calcium content in the water is often not controlled by pure siderite or calcite, but by equilibrium with Fe/Ca-substituted siderite or calcite.
The precipitation kinetics of co-precipitated calcium-iron carbonate is important because these mixed salts or a defined crystal lattice with substituted ion often co-exist in oilfield production lines.9 Constant composition research has been conducted to study the dissolution kinetics of pure calcium phosphate and calcium carbonate salts.10-12 Dual constant composition studies of multi-cation carbonate salts is more practical and may have more applications, as will be shown below. These studies may help to design the proper treatment and to select the right inhibitor that might work better. This is more critical when the scale contains ferrous carbonate, because most of the inhibitors tested in the literature show either weak inhibition or adverse effects toward iron-containing salts.13,14To improve our knowledge of oilfield mineral thermodynamics, it is important to know the actual composition of the salt formed. This enables comparison of the precipitation and dissolution kinetics as either individual pure salts and as components of a mixture.
