Laboratory Evaluation of Iron-Control Chemicals for High-Temperature Sour-Gas Wells

Abstract

Abstract The objective of this work was to evaluate iron control chemicals used in acidizing systems in high-temperature sour-gas wells. This work is required because iron compounds that precipitate during acidizing can reduce reservoir permeability in the critical near-wellbore area. Formation damage from fines and precipitated material can be difficult to remove in subsequent remediation treatments. A high temperature see-through cell was used to investigate the behavior of 28 wt% HCl acid formulae provided by three different service companies. Temperatures up to 300 °F and pressures up to 500 psi were used in this work. This is the first time in the literature that these high acid concentrations have been studied at 300 °F. Previous studies in the industry are available for acid concentrations of up to 15 wt% HCl, temperatures of up to 250 °F, and pressures of 500 psi. The following conclusions are made: Nitrilotriacetic acid (NTA) in 28 wt% HCl at 300 °F degrades completely after 5 hours. Therefore, NTA is not effective as an iron control chemical under these conditions. At 250 °F, 77% of the initial NTA remains after 24 hours. It was estimated that the degradation rate of NTA doubles for each 14 °F increase in temperature in the range 250 to 300 °F. Service company acid formulae degrade to produce black precipitates in 28 wt% HCl at 300 °F. These materials contain significant amounts of polyaromatic hydrocarbons and chlorinated aromatics. The acid formulae of two companies oxidize iron (II) to iron (III) at 300 °F in live acids. In spent acid at 300 °F, iron (III) is reduced to iron (II) by all three acid formulae. Both reactions may lead to degradation of acid additives. The acid formula from Company C shows an increase in iron sulfide precipitation in spent acid compared to a control. The acid formulae from Companies A and B show no significant effect on iron sulfide precipitation, when compared to a control. In coreflood tests, iron sulfide precipitates immediately when acid containing dissolved iron (II) and hydrogen sulfide spends on a carbonate rock at high temperature. Iron sulfide accumulates on the injection face, not within the rock itself. The following recommendations are made: Reduce total iron concentration in the injected acid to the lowest practical level. Pickle tubing, mixing tanks and lines before the acid treatment. Eliminate the use of NTA at temperatures above 250 °F. Investigate other chelating chemicals and hydrogen sulfide scavengers for control of iron sulfide precipitation at temperatures above 250 °F.