Mechanisms and Prevention of Lead Sulphide (PbS) Scale Deposition in Multiphase Systems

The deposition of mineral scale poses a major challenge in the oil and gas industry, where flow assurance issues can result in significant operating costs and well downtime. This study aimed to analyse the mechanisms of precipitation and deposition for a range of scale types (calcium carbonate, barium sulphate and lead sulphide) in simple and complex systems. Whilst traditionally, studies looking to mitigate mineral scaling have investigated precipitation and deposition in single phase brines, the introduction of a light oil phase to induce multiphase conditions in this study was representative of processes occurring within oilfield production systems. A rig that enabled the propagation of turbulent, multiphase, emulsion-forming flow within a H2S environment was designed and constructed in order to assess the interaction of bulk colloids at the oil-water interface and consequently the mechanism by which they deposited upon surfaces. Characterisation of solid stabilised Pickering emulsions took place through optical microscopy and cryo-scanning electron microscopy (cryo-SEM). As a secondary objective, the mitigation efficacy of a number of anti-fouling coatings was assessed under a range of scaling conditions, with physiochemical characteristics inherent to the surfaces that prevented initial deposition identified. Fluoropolymers, diamond-like-carbon (DLC) and sol-gel coatings, were found to be the most promising anti-fouling substrates and as such were assessed for potential application into oilfield systems and equipment. Scaling severity was measured through mass gain analysis and SEM of surface crystals to establish morphology and coverage. The relationship between coating wettability and scale adhesion was explored using atomic force microscopy (AFM), where the interfacial forces acting upon hydrophilic and hydrophobic tips when in contact with a cleaved [001] galena face. Results showed that whilst attractive hydrophobic force had a bearing on PbS deposition on hydrophobic surfaces in single phase, the presence of a light oil phase in multiphase systems was pre-dominant in determining scaling likelihood upon surfaces. The oil wetting of hydrophobic surfaces was seen to largely prevent the deposition of bulk precipitated scale, with a relationship found between the thermodynamics of precipitation of a species and the degree of accuracy to which scaling could be predicted upon surfaces of varying wettability.