Improving the Dissolution of Iron Sulfide by Blending Chelating Agents and its Synergists
Authors: Ramanathan Raja , Nasr-El-Din Hisham 
Published: March 2019

Iron sulfide is a $1.4 billion/year problem in the oil and gas industry receiving little R&D attention. The low success rate of organic acids and polyaminocarboxylic acids (PACA) prompts a more focused investigation and development of new dissolvers for the treatment of iron sulfide scales. This study evaluates the solubility of the iron sulfide scale by commonly used simple organic acids and describes two new blends that outperform the aforementioned standalone dissolvers at 1,000 psi and 150°F.

Bottle and autoclave tests evaluated the efficacy of various dissolvers to dissolve the iron sulfide scale. Bottle tests helped in evaluating the dissolvers’ potential to dissolve iron sulfide. A Hastelloy-B autoclave with a maximum operating pressure and temperature of 1,800 psi and 350°F, respectively, contained the iron sulfide and the dissolver for the anoxic dissolution tests. Formic acid, maleic acid, lactic acid, citric acid, oxalic acid, ethylenediaminetetraacetic acid disodium salt (Na2EDTA), and pentapotassium diethyltriaminepentaacetic acid (K5DTPA) were used. The simple organic acids added to Na2EDTA helped in improving the solubility of the scale. Two final experiments with the most successful blends were conducted for 24 hours. Concentration of the dissolver varied from 1-10 wt%. The experiments were conducted for 4 hours at 150°F, and a pressure of 1,000 psi. Elemental analysis using the Inductively Coupled Plasma (ICP) determined the efficiency of scale removal. Dräger tubes measured the H2S concentration inside the autoclave at the end of the experiment. The degree of saturation of the dissolvers calculated from the ICP measurements helped in evaluating its utilization.

An XRD study showed the initial iron sulfide scale was mainly pyrrhotite (67%), mackinawite (23%), troilite (5%), and remaining wuestite (5%). Bottle tests showed that maleic acid is the best reactant for iron sulfide in terms of the speed of the reaction. However, citric acid can react with the iron sulfide at lower concentrations and is more effective. Similar to the bottle test, maleic acid yielded the maximum solubility among standalone treatments. An inductively coupled plasma analysis of iron concentration showed a solubility of 10.6 g/L iron in maleic acid. The next best treatment was with formic acid, dissolving a maximum of 9.7 g/L iron. Oxalic acid converted the iron sulfide to iron (II) oxalate, which is insoluble in water. K5DTPA was a poor dissolver of iron sulfide with less than 1 g/L iron solubility. Blends of Na2EDTA and a synergist helped in improving the dissolution. Adding 5 wt% potassium oxalate to 15 wt% Na2EDTA helped in dissolving 70.1% of the initial iron at 1,000 psi, 150°F, and 24 hours soaking time. A blend of 15 wt% Na2EDTA and 5 wt% potassium citrate dissolved 87% of iron at the same conditions.

Development of novel dissolvers that are less corrosive and safer than traditional dissolvers is a necessary step to improve the dissolution of iron sulfide scales. The combination of polyaminocarboxylic acids with their synergists is unexplored in dissolving iron sulfide. This study provides an evaluation of various dissolvers in addition to developing two new synergistic blends for iron sulfide scale treatment. These dissolvers are good alternatives to traditional treatments and can reduce operational risk and mitigate flow assurance problems.