Abstract
Recently, sulfide scale related issues have been significantly increased, probably due to implementation of more aggressive technologies and exploration of unconventional fields, such as high temperature, high pressure, and high salinity. Firstly, sulfide scales (i.e., FeS, PbS, and ZnS) are one of the most or significantly unsolved deposition problem in oil and gas production. Secondly, sulfide scales have oleophilic nature so that they can be difficult to separate sulfide scales from oil phase during production processes. Polymeric dispersants have exhibited their feasibility to prevent the deposition of sulfide scales, but dispersants have not been widely validated to control sulfide scale problems and limited numbers of trials and reports have been available. The objectives of this study are: (1) to evaluate dispersion efficiency of sulfide scale dispersants in a wide range of conditions of brine ionic strength, calcium concentrations, solution pH, and temperature; (2) to examine the effect of dispersants on sulfide scale wettability; (3) to understand the mechanism of a dispersion process; and (4) to apply the newly developed dispersion model for a prediction of a minimum dispersant concentration (MDC). Among tested natural and synthetic water-soluble polymers, carboxymethyl cellulose (CMC) showed the best dispersion efficiency for sulfide scales with an individual particle size of around 4 nm. The second-best dispersants identified in our study were polyvinyl pyrrolidone (PVP) and polyacrylamide (PAM). Despite some degrees of aggregation of dispersed sulfide particles, their size was still in the nanometer ranges of 100 to 500 nm. Dispersed sulfide particles remained in the water phase, while settled ones were transferred to the oil phase. Transmission electron microscope (TEM) and Fourier-transform infrared (FT-IR) results showed that CMC was adsorbed on the surface of FeS particles through H-bond and complexation between Fe(II) and carboxylate groups, controlling particles growth and preventing them from settling. CMC was effective to disperse sulfide scales in conditions of brine ionic strength (58.5 – 234 g/L NaCl), Ca2+ concentrations (1,000 – 8,000 mg/L), pH (4.3 – 6.7), and temperature (70 – 120 °C). In these reaction conditions, MDC of CMC ranged from 5 to 200 mg/L. The combination of CMC and diethylenetriamine penta(methylene phosphonic) acid (DTPMP) enhanced CMC dispersion efficiency in some conditions. MDC of CMC for PbS and ZnS scales were 2 and 5 mg/L, respectively. MDC prediction model predicted MDC quite reasonably in wide range of NaCl concentrations (58.5 – 234 g/L) and SI of FeSm (0.13 – 2.03).
