Deposition of Inorganic Carbonate, Sulfate and Sulfide Scales on Anti-fouling Surfaces in Multiphase Flow

Impairment of flow by way of mineral scale formation is a major complication affecting production in the oil and gas industry. Soured reservoirs contain hydrogen sulfide (H2S) that can prompt the formation of exotic metal sulfide scales, leading to detrimental fouling that can negatively impact production. The contrast in the mode of precipitation (solid formation from liquid solution) and deposition of both sulfide scale and conventional inorganic carbonate and sulfate scales is herein examined. Design of an experimental rig allowing diffusion of H2S gas into the brine phase of a sealed reaction vessel, resulted in a realistic representation of scaling processes occurring within sour reservoirs. Multiphase conditions, induced by introduction of a light oil phase to scaling brine within a turbulent regime, aimed to study the effect of oil and water wetting on pipeline fouling. Performance of a range of anti-fouling surfaces was determined through measurement of scale deposition by gravimetry and microscopy techniques. Under conditions modelled to reflect a typical H2S-containing reservoir, the contrasting scaling mechanisms of conventional calcium carbonate (CaCO3) and barium sulfate (BaSO4) scales when compared to lead sulfide (PbS) scale, highlighted the critical role of the light oil phase on deposition. Whilst conventional scales showed deposition by both crystallization and adhesion onto surfaces, the thermodynamic driving force for PbS prompted rapid bulk nucleation, with adhesion acting as the overwhelmingly dominant mechanism for deposition. The results showed that the addition of a 5% v/v light oil phase had a profound effect on scale particle behavior and deposition onto anti-fouling surfaces of varying wettability as a result of two processes. Primarily, the oil wetting of hydrophobic surfaces acted as a barrier to deposition; and secondly, adsorption of scale crystals at the oil/water interface of oil droplets within a turbulent oil-in-water emulsion, resulted in adhesion to hydrophilic surfaces after impaction. It is therefore proposed that sulfide scale, typically deposited in the upper regions of production tubing, is driven by adhesion after formation of a PbS solid-stabilized Pickering emulsion. This contrasts with the commonly held view that metal sulfides precipitate and deposit similarly to conventional scales, whereby salts crystallize both directly upon surfaces and in the aqueous bulk phase as solubility decreases towards the wellhead.