| dc.description.abstract | Iron sulfide scales of different forms exist in sour oil/gas-producing wells as well as seawater injection wells. Traditionally, they are dissolved using HCl and other inorganic acids. In the past decade, a tetrakis (hydroxymethyl) phosphonium sulfate (THPS) and ammonium chloride blend have shown potential to dissolve FeS scales. The objective of this study is to optimize the dissolver composition and treatment time for the dissolution of FeS using different concentrations of THPS and ammonium chloride at 150 and 300°F. This work also evaluates the thermal stability of the blend at 350 and 400°F using aging cells. The optimum blend composition and treatment time at high pressure-high temperature (HP/HT) conditions is not available in the literature. The thermal stability of THPS and ammonium chloride is unknown at a temperature greater than 300°F. Bottle tests at 150°F helped optimize the THPS and ammonium chloride blend composition and treatment time. 10 cm^3 dissolver solutions prepared at concentrations of 0.1 to 1 mol/L THPS and 0.25 to 1.5 mol/L NHv4Cl were added to 0.1 g FeS.
An Inductively Coupled Plasma (ICP) analysis of the supernatant solution at 1, 4, 8, 12, 24, 48, and 96 hours revealed the kinetics of the dissolution process. Thermal degradation experiments were performed in an OFITE aging cell and analyzed using Fourier-transform Infrared Spectroscopy (FTIR). At 150°F, the blend with 0.1 mol/L THPS showed maximum FeS dissolution at 1.5 mol/L NHv4Cl. But for THPS concentration greater than 0.1 mol/L, maximum dissolution was observed at 0.5 mol/L NHv4Cl. For a 0.25 mol/L NHv4Cl, 1.0 mol/L THPS showed the maximum dissolution capacity, but for NHv4Cl concentration greater than 0.25 mol/L, maximum dissolution was observed at 0.5 mol/L THPS. Based on these results, 0.5 mol/L THPS and 0.5 mol/L NHv4Cl is the best dissolver composition. The optimal time for treatment was lower than 48 hours when THPS concentration was higher than 0.8 mol/L or NHv4Cl concentration was higher than 1.0 mol/L. The spent dissolver pH values were lower than the initial pH values, indicating the generation of acid. At 300°F, the 0.2 mol/L THPS and 0.2 mol/L NHv4Cl blend dissolved 40% more iron sulfide than at 150°F. This work provides an investigation of the kinetics of iron sulfide dissolution using the THPS and NHv4Cl blend. This study optimizes the blend composition and treatment time, at 150 and 300°F. Thermal stability evaluation along with the dissolution study fills the gaps in the literature and provides an optimized solution for well treatment. | |
