Deposition of Inorganic Carbonate, Sulfate, and Sulfide Scales on Antifouling Surfaces in Multiphase Flow

Abstract

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 (H 2 S) 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 H 2 S 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 antifouling surfaces was determined through measurement of scale deposition by gravimetry and microscopy techniques. Under conditions modeled to reflect a typical H 2 S-containing reservoir, the contrasting scaling mechanisms of conventional calcium carbonate (CaCO 3 ) and barium sulfate (BaSO 4 ) scales when compared to lead sulfide (PbS) scale highlighted the critical role of the light oil phase on deposition. While 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 antifouling surfaces of varying wettability as a result of two processes. Primarily, the oil wetting of hydrophobic surfaces acted as a barrier to deposition, and second, 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 toward the wellhead.

Lead sulfide scaling in multiphase systems and co-precipitation in the presence of calcium carbonate

“…Building on previous work, 4,13 the emulsion stabilizing characteristics and depositional mechanisms of PbS precipitated in multiphase systems were scrutinized. Sessile drop measurements determined wetting behavior of galena (PbS) in order to explain particle arrangement at the o-w interface in conjunction with cryo-SEM analysis.…”

“…Whilst numerous studies have considered the fouling behavior of sulfide mineral scales in single phase systems, 11,12 the significance of the presence of a light oil phase with regards to deposition of various salts is considerable. 4,13 In a multiphase system where PbS is spontaneously nucleated within the bulk homogeneous phase, the degree of fouling deposition can be reasonably predicted based on the wettability of the substrate. 13 Heterogenous nucleation however occurs directly onto surfaces that results in the compromising of a substrate's anti-fouling attributes, ultimately changing the wettability.…”

“…4,13 In a multiphase system where PbS is spontaneously nucleated within the bulk homogeneous phase, the degree of fouling deposition can be reasonably predicted based on the wettability of the substrate. 13 Heterogenous nucleation however occurs directly onto surfaces that results in the compromising of a substrate's anti-fouling attributes, ultimately changing the wettability. Consequently, the two key elements for determining the deposition of bulk precipitated particles in turbulent multiphase systems include; i) particle or agglomerate wetting at the oil-water (o-w) interface and the formation of a Pickering emulsions; and ii) the hydrophobicity of the substrate.…”

“…Consequently, the two key elements for determining the deposition of bulk precipitated particles in turbulent multiphase systems include; i) particle or agglomerate wetting at the oil-water (o-w) interface and the formation of a Pickering emulsions; and ii) the hydrophobicity of the substrate. 13 When immiscible crude oil and water are simultaneously present within a producing system, particles or droplets of one phase will become dispersed within the continuous phase of the other, dependent on the o:w ratio. The natural evolution of a producing well dictates that there will be an inevitable increase in water-cut throughout its lifetime and as such, the potential for inversion of any solid stabilized water-in-oil (w/o) emulsion to an o/w state.…”

“…15,26,27 Mobility of particlestabilized droplets largely occurs via mechanical means, whereby particle stabilized droplets impact against surfaces within a high-energy turbulent o-w multiphase system and either rupture or adhere. 13 Rupture of an oil or water droplet (dependent on emulsion type) after impacting a substrate can result in the deposition and adhesion of particles that previously contributed to droplet stabilization. 4,28,29 The adhesive strength of particles once deposited has been shown to depend on the particle size and surface energies of the two interacting species.…”