Iron Sulfide Scale Inhibition Through Adsorption of Chelating Agents

An important oilfield issue is the formation of a wide range of scales during oil and gas well operations. Oilfield scales hinder assessing an optimum hydrocarbon production as their precipitation on formation, various surface, and downhole equipment leads to many problems, including pressure decrement, formation damage, and operational failure of subsurface equipment. One type of these scales is the iron sulfide scale and based on studies in the Khuff reservoir, iron sulfide scales are likely to deposit on production tubing and rock formation. Therefore, it becomes essential to restrain the occurrence of iron sulfide scale using environmentally friendly chemicals in production tubing, water injection wells, and near-wellbore formation. The primary focus of this work is the prevention of iron sulfide scale deposition in carbonate formations during water injection applications. Iron sulfide scale inhibition was studied through dynamic inhibition adsorption experiments. In contrast to conventional experiments, for scale inhibition and adsorption of chelating agents (static bottle, dynamic filter tube tests) and simulation studies, a novel experimental setup (coreflooding experiments) was proposed to study the inhibitor adsorption. Broad concentrations of high-pH aminocarboxylic acids (such as ethylenediaminetetraacetic acid (ETDA) and diethylenetriamine pentaacetate acid (DTPA)) were examined (10 wt%, 15 wt%, and 20 wt%), at temperatures of 120°F and 200°F. Results of the study revealed that iron (III) precipitation is an obvious threat causing severe formation damage in carbonate rocks by significantly decreasing the rock permeability. Adsorption of chelating agents on limestone rocks highly depends on their concentrations. Specifically, an increase in the concentration of EDTA and DTPA at elevated temperature conditions resulted in higher adsorption. The inhibition experiments revealed that 20 wt% EDTA could significantly decrease the iron sulfide scale precipitation. Unlike the conventional testing methods for scale formation and prevention, a novel experimental setup - coreflooding during the inhibitor adsorption, formation, and inhibition of iron sulfide scale in carbonate formation was used. The main advantage of the method is the consideration of permeability alteration happening due to the scale formation. Another point is that in previous studies, various scale control chemicals and experimental approaches have been suggested for iron sulfide scale inhibition, and polymeric, phosphonate, and sulfonated co-polymeric inhibitors were used. However, the subgroup of chelating agents - aminocarboxylic acids, was used in this study.