Kinetics and Inhibition of Ferrous Sulfide Nucleation and Precipitation

Abstract

This article, written by Dennis Denney, contains highlights of paper SPE 169748, ’Kinetics and Inhibition of Ferrous Sulfide Nucleation and Precipitation,’ by Qiliang Wang, Zhang Zhang, Amy Kan, SPE, and Mason Tomson, SPE, Rice University, prepared for the 2014 SPE International Oilfield Scale Conference and Exhibition, Aberdeen, 14-15 May. The paper has not been peer reviewed. Ferrous sulfide (FeS) precipitation increases during shale-gas and -oil production because of increased biologically and thermally induced sulfide production. Although FeS scale is abundant, little is understood about its precipitation and inhibition properties. A plug-flow apparatus adds reliable data to the limited database of scaling kinetics in flowing pipes, and it provides a new method to study the effect of inhibitors in oil-production systems. Introduction During oil production, mineral-salt precipitation (scale) builds up inside wellbores. It is very difficult to study the formation and inhibition of FeS because the experiments must be performed under rigorous anoxic conditions and the induction time is very short. Compared with common oilfield scales, FeS scale has distinctive characteristics. FeS can cause galvanic corrosion in the presence of water. FeS has several crystalline forms with different sulfur/iron ratios that result in different solubilities of FeS. Additionally, FeS particles are oil-wet and can be coated easily with oil or hydrocarbons; these coatings can act as a diffusion barrier that retards acid reaction with the scale. To evaluate the risk of scaling in pipes, a plug-flow apparatus was used to investigate CaCO3-scale precipitation on stainless-steel tubes. In the study of CaCO3, a precoated plug-flow reactor provided a constant composition at the tubing exit, including constant pH, saturation index (SI), ionic strength, and surface area. The plug-flow system provided a feasible approach to study the kinetics of initial attachment and crystal growth. FeS had not been studied with a plug-flow system; therefore, it was important to develop a plug-flow setup to investigate FeS formation and precipitation, enhancing the limited database of scaling kinetics in flowing pipes. To eliminate the influence of scale on oil production, scale inhibitors often are needed to minimize the scale buildup. The controlling method of scale inhibitors is to delay, reduce, or prevent scale formation. Scale inhibitors can prevent further growth of scale crystals by adsorbing onto the surface of the crystals. In addition to scale inhibitors, dispersants and chelating agents are applied to sequester metal cations, preventing them from reacting with anions. The objectives of this research were to investigate the effects related to the driving force on FeS precipitation in batch and plug-flow systems, and to conduct a series of laboratory studies on scale-inhibition approaches for FeS by use of commercial scale inhibitors and chelating agents.