Temperature Limits and Synergist Selection for THPS-Based Iron Sulfide Scale Dissolvers in HPHT Wells: A Critical Analysis and Test Protocol

Iron sulfide (FeS) scale in sour producers and seawater injectors is conventionally removed with hydrochloric acid, which liberates toxic H2S and corrodes tubulars. Tetrakis(hydroxymethyl)phosphonium sulfate (THPS), blended with ammonium chloride (NH4Cl), has emerged as a lower-hazard alternative, but its performance degrades as bottomhole temperature rises. This paper consolidates published bottle-test, autoclave, and thermal-aging data on THPS-based dissolvers and analyzes them to explain three observations: the plateau in dissolution above 0.5 mol/L THPS and 0.5 mol/L NH4Cl; the collapse of the NH4Cl synergy from 2.7-fold at 150 °F to 1.35-fold at 300 °F; and the complete degradation of THPS at 350–400 °F. We propose a formaldehyde–ammonium acidification mechanism that accounts for the synergy and its loss with temperature, quantify sulfide speciation across the operating pH window of competing dissolvers, and compare THPS blends with aminopolycarboxylic chelating agents. Based on this analysis, a temperature-banded dissolver-selection framework and a standardized HPHT test protocol are proposed. THPS/NH4Cl remains the preferred low-hazard option up to about 300 °F, while acidic DTPA-based chelant systems are better suited above 350 °F.