Little research has been done to understand how chelating agents might reverse the effect of Fe(III)/Fe(II) on scale inhibitors in oilfield conditions. Iron ions are common cations existing in oil and gas production water. Most Fe(II) ions come from dissolution of siderite in reservoir and corrosion of steel pipes. Fe(III) in solution, can be formed by Fe(II) reduction of H2O, dissolution of magnetite or other Fe(III) containing minerals or when produced water is exposed to air. Both Fe(II) and Fe(III) can cause severe problems in production. Fe(II) can form FeS and FeCO3 scale. Fe(III) can precipitate as ferric hydroxide or with Fe(II) as magnitite particles and cause damage to formation permeability. Moreover, both Fe(II) and Fe(III) have significant detrimental effects on common scale inhibitors. One of the most popular methods for iron control is the application of chelating agents, such as EDTA, NTA or citric acid, to chelate Fe(II)/Fe(III)to prevent such detrimental effects.
In this work it is shown that Fe(II) and Fe(III) can significantly impair performance of common scale inhibitors such as DTPMP, PPCA and PVS. The inhibition time of DTPMP for barite can drop more than 90% in the presence of 1 mg/L Fe(II) or Fe(III)at 70°C. In this research, a mechanistic study has been done to deduce whether EDTA and citric acid can reverse the loss of scale inhibitor performance by Fe(II)/Fe(III). We find that Fe(III) impairs scale inhibitor performance by forming iron hydroxide particles which can adsorb scale inhibitors from solution. Both EDTA and citric acid are found to be able to eliminate the negative impact of Fe(III) on scale inhibitors, but through different mechanisms. EDTA works by direct chelation of Fe(III) in solution, before a solid phase is formed; while citric acid works by competitive adsorption with scale inhibitor onto ferric hydroxide particle surfaces. The impact of EDTA and citrate on reversing Fe(II) effect on scale inhibition is also discussed.