| dc.description.abstract | Iron sulfide scales create well deliverability and integrity problems such as reduced production rates and damage to well tubulars. Problems associated with the use of HCl to remove these scales such as high corrosion rate, H2S generation, and scale reprecipitation, have required the use of alternative dissolvers such as tetrakis (hydroxymethyl) phosphonium sulfate (THPS)-ammonium chloride blend and chelating agents to dissolve iron sulfide scales. This work investigates Ethylenediaminetetraacetic acid (EDTA), Diethylenetriaminepentaacteic acid (DTPA), N-(2-Hydroxyethyl) ethylenediamine-N, N’, N’-triacetic acid (HEDTA), and THPS for their iron-sulfide (FeS) dissolution capacities and kinetics at 150 and 300°F.
To displace HCl as the standard field treatment for iron sulfide scales, the application of the alternative dissolver in well tubulars requires laboratory testing to determine the optimum conditions such as dissolver concentration, treatment time, and dissolver-scale ratio (cm3/g). The dissolution must be evaluated in oilfield-like conditions as well such as crude oil-wetted scale samples, presence of salts, mixed scales, and additives. The potential to remove the iron sulfide scale must be investigated using several potential synergists.
The behavior of the chelating agents was significantly different at 150 and 300℉. The dissolution depended on the pH, dissolver concentration, treatment time, and dissolver/scale ratio. DTPA removed the most amount of scale amongst the aminopolycarboxylic acids. The order of the chelating agents in terms of dissolution capacity was DTPA > HEDTA > EDTA at all pH conditions. 100% of the iron from iron sulfide was complexed by 0.3 mol/L K2-DTPA after 20 hours of soaking. For pH < 5 dissolvers, 16-20 hours was sufficient to obtain the maximum dissolution capacity. At 150°F, the mechanism of dissolution at pH < 5 was determined to be H+ attack with surface complexation. At 300°F, the dissolution of the scale was significantly improved in alkaline dissolvers. There was an improvement in the effectiveness of the ligands due to the lowering of Fe-S bond strength and increased activity of the chelating agent. THPS-ammonium chloride blend was also optimized for its maximum iron sulfide scale removal. The role of corrosion inhibitor and H2S scavenger did not decrease the dissolution characteristics of the alternative dissolvers. Mixed scales containing calcium carbonate impacted the dissolution of iron sulfide due to the dissolver’s preference to remove the calcium deposit. Overall, the dissolution of the total deposit was unaffected. Synergists such as potassium iodide, potassium citrate, and sodium fluoride helped enhance the dissolution capacity of EDTA and DTPA at 150 and 300°F.
The role of THPS and chelating agents in iron-sulfide dissolution has not been thoroughly investigated. No study reports the optimum treatment parameters. The role of the pH of the dissolver also needs more attention. Oilfield-like conditions are rarely studied in the laboratory for scale removal research. New synergists are also introduced that could help improve the dissolution rate. The current work provides an in-depth investigation of alternative dissolvers so that chemical operators could design field treatments for the removal of iron sulfide scale. | en |
| dc.subject | Iron sulfide, chelating agents, mineral scale, oilfield chemistry, high pressure, high temperature, pipelines, well tubulars | en |
