Optimization and Thermal Stability of the THPS and NH4Cl Blend to Dissolve Iron Sulfide FeS Scale at HPHT Conditions
Authors: Patel Dhvanil , Ramanathan Raja , Nasr-El-Din Hisham A. 
Published: November 2019

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 NH4Cl 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 (HPHT) conditions is not available in literature. The thermal stability of THPS and NH4Cl 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 cm3 dissolver solutions prepared at concentrations of 0.1 to 1 mol/L THPS and 0.25 to 1.5 mol/L NH4Cl were added to 0.1 g FeS. An Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES) analysis of the supernatant solution at 1, 4, 8, 12, 24, 48, and 96 hours revealed the kinetics of the dissolution process. The morphology of the undissolved iron sulfide particles was studied through a Scanning Electron Microscope (SEM). Thermal degradation experiments were performed in an OFITE aging cell and analyzed using Fourier-transform Infrared Spectroscopy (FTIR).
At 150°F, a blend of 0.5 mol/L THPS and 0.5 mol/L NH4Cl proved to be the optimum combination for the dissolution of iron sulfide scale, dissolving 7,484 ppm of the iron from iron sulfide. Increasing the concentration beyond these values does not result in significant improvement in the solubility of the scale. The optimal time for treatment was found to be 48 hours for the optimum blend of THPS and NH4Cl. HPHT testing in an autoclave with hypoxic conditions showed similar characteristics to the bottle tests. Ammonium chloride was proved to be effective in a field setting. However, it was shown that adding NH4Cl to THPS is not very effective at 300°F. There was an improvement of 35% in the iron sulfide dissolution when 1 wt% NH4Cl was added to 0.2 mol/L THPS at 300°F. In contrast, the scale solubility increased by 2.7 times when the same blend was used at 150°F. At 300°F, the 0.2 mol/L THPS and 0.2 mol/L NH4Cl blend dissolved 40% more iron sulfide than at 150°F. SEM studies showed the presence of vuggy balls of iron sulfide particles after reaction with the blend instead of the original smooth surface. THPS was completely degraded to THPO and BMPA at 350 and 400°F.
This work provides an investigation of the kinetics of iron sulfide dissolution (FeS) using the THPS and NH4Cl blend. It optimizes the blend composition and treatment time, at 150°F. Autoclave experiments were conducted at hypoxic conditions to mimic field treatment. A morphology study of the iron sulfide scale after treatment with the THPS and ammonium chloride blend was not done before. Thermal stability evaluation along with the dissolution study fills the gaps in the literature and provides an optimized solution for well treatment.