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. Introduction 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. Acids can become contaminated with iron by reacting with iron-containing corrosion products in the surface equipment, coiled tubing, well casing, or wellbore as well as iron-containing minerals in the formation.1,2 The economically important gas-producing zones in Khuff wells in Saudi Arabia are the Khuff B and C reservoirs. Permeability of these carbonate formations ranges from 0.01 to 10 mD while average temperature ranges from 250°F to 280°F (121.1 to 137°C).3 Bottom hole pressures of up to 7,000 psi are found. At these high temperatures and pressures, the chemical compatibility and effectiveness of iron control chemicals is of great importance.