Abstract

Formation of iron sulfide scale has been a serious flow assurance problem in sour-gas wells and other similar production scenarios. Compared to carbonate and sulfate scales, iron sulfide (FeS) is difficult to inhibit and various risks are associated with chemical removal. Hydrochloric acid and acetic acid are most common chemicals for sulfides dissolution but severe damage to the completion metallurgy, generation of H2S byproduct and its potential toxicity and possibility of re-precipitation of FeS limit its applicability. Other commercial dissolvers, iron chelators, biocides as well as dispersants have been reported, however, efficacy of these chemicals preventing iron sulfide scaling is poor and often uneconomically high concentrations are required. Despite its huge industrial importance and interest, an efficient chemical control of iron sulfide scale formation in oil and gas production is still an elusive goal.  This study seeks to identify chemical compound(s) to inhibit iron sulfide scale formation.

In order to identify the effective chemical treatment of iron sulfide scale, a strictly anoxic static experimental setup was designed and various chemicals of different functional groups as well as a range of molecular weights were evaluated. Our laboratory studies indicate that a polymeric compound containing amide functionality showed a promising effect by preventing of iron sulfide scale deposition at threshold quantities. Specifically, the polymer did not prevent the formation of the iron sulfide particles; however, it significantly influenced the growth and aggregation of the iron sulfide particles. In addition, the polymer kept the particles well dispersed for an extended period and prevented the formation of iron sulfide pads at the water-oil interface. The effect of temperature (up to 90 °C), pH, TDS on polymer efficiency on iron sulfide scale control as well as calcium tolerance were investigated and results indicate that the performance is not significantly influenced. Using various analytical techniques, underlying principle of growth inhibition and dispersion properties has been identified. It is believed that this polymer will provide a novel chemistry of controlling the metal-sulfide scale.

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