Iron sulfide scale is a common scale in sour oil and gas wells. It can form in wells producing oil and gas, as well as water injection wells. Iron sulfide scale has different forms based on the iron-to-sulfur ratio. Iron sulfide scale can form in soft forms that are removed easily by acids such as phyrrotite. The scale can also form in hard forms such as pyrite that is difficult to remove by acids. Several attempts have been made to investigate the removal of different forms of iron sulfide. Scale can reduce production and injection rates. At the end, it may completely plug the tubing and the only remedy will be to change the tubing. In the previous work either low pH corrosive fluids were used or high pH fluids with low solubility.
In this study, we introduced a new formulation to remove the iron sulfide scale (especially the hard scale) pyrite form of the iron sulfide. The new formulation has a pH value greater than six with low corrosion rate. Solubility, stability, as well as corrosion experiments, were performed to assess the new formulation. The new formulation is based on a chelating agent and a low pH fluid mixture. Also coreflooding experiments were performed on carbonate cores to assess the performance of the new formulation if it invaded the near-wellbore during scale removal operations.
The XRD of actual scale sample from a gas producer well showed that the scale sample contains 43 % pyrite, 35% mackinawite, and 22% iron silicon compounds. The scale solubility reached 81.6% using the new formulation. The new formulation remained stable for more than 48 hours at a high temperature. The corrosion rate of actual tubing coupons was 0.03 lb/ft2 without using corrosion inhibitors at 125°C/257°F. This corrosion rate within the allowable limit for the industry standards (should be less than (0.05 lb/ft2). Coreflooding experiments showed permeability enhancement using the new formulation which eliminates the risk of using this chemical in scale removal because it will not cause formation damage to the near-wellbore if it invades the formation.