The Performance of Scale Inhibitors in the Inhibition of Iron Carbonate Scale

Abstract

Abstract A potential mechanism for part of the production decline observed in the Prudhoe Bay field is siderite (iron carbonate) scale deposition. This laboratory investigation focused on evaluating the effect of the existing downhole scale inhibitor program on the proposed siderite formation damage mechanism. A tube block test was employed to generate iron carbonate and evaluate commercial products for its inhibition. This study indicates that the current inhibition program for barium sulfate and calcium carbonate is not protecting against damage by iron carbonate deposition. The other products tested were also ineffective. Furthermore, precipitation of the scale inhibitors by iron(II) is suspected. There is no field evidence of damage from this precipitation with the current inhibition program. Introduction The deposition of inorganic scales is a common operational problem. Scale inhibitors are effective at preventing scale deposition at concentrations significantly below the levels required to sequester or chelate the divalent cations. The molar ratio of precipitate kept in solution to the inhibitor used is typically of the order of 10,000:11. Scale inhibitors prevent, slow, or distort crystal growth by blocking growth sites2. They are a viable solution for many types of scale due to the low cost of protection. Iron (II) carbonate is not a common scale problem and its inhibition hasn ot been studied to any extent. Thin layers of iron carbonate are frequently observed on tubulars recently pulled from service; this is usually attributed to corrosion and causes no operational problems. Under certain conditions iron carbonate deposition serves to passivate metal surfaces against further corrosion. However, in formations with a high iron carbonate mineralogy, it is possible for significant deposits of siderite to precipitate from the produced water. Iron (II) is a common component of formation waters, and produced waters commonly contain a few mg/L of iron3. However, in rare instances the natural iron (II) content may reach 100 mg/L. Iron (II) is a stable species in anaerobic environments, however; when exposed to air iron (II) quickly oxidizes to iron (III)4. Iron (III) forms insoluble oxides and hydroxides which precipiate from solution. Because of the rare occurrence of iron carbonate scale due to precipitation from production water and because of the need for anaerobic experimental procedures, inhibitory products specific for iron carbonate scale have not been developed. The existing products, developed for calcium/barium, carbonate/sulfate scales have not been evaluated for iron carbonate inhibition. In this laboratory study a method was developed for generating iron carbonate in a tube block testing device and several products were evaluated for iron carbonate inhibition.