Iron sulfide scale is a significant problem in the oil and gas industry and it is a common scale in sour oil and gas wells. Iron sulfide depositions have an adverse impact to production and injection operations. The precipitation of iron sulfide scale in the near-wellbore area will diminish the productivity of the production wells and loss of the injectivity of 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.
The objectives of this paper are to: (1) evaluate the possibility of using different organic acids (maleic, glutamic, succinic, and gluconic), chelating agents (ethylenediaminetetraacetic acid (EDTA), and diethylenetriaminepentaacetic acid (DTPA)), or a mixture of these fluids to remove iron sulfide scale, (2) develop a new formulation of acids mixture to remove iron sulfide scale, and (3) perform a corrosion test for the developed formulation to determine the operating conditions for the developed formulation. Actual field iron scale sample was used in this study. X-ray diffraction (XRD) was used to obtain the composition of the field sample. Standard corrosion test was used to determine the corrosion rate for the developed formulation.
XRD results showed that the field sample contained pyrrhotite (55 wt%), calcite (21 wt%), pyrite (8 wt%), and troilite (6 wt%). The solubility results showed that organic acids (maleic, glutamic, succinic, and gluconic) are very weak to remove iron sulfide scale at different conditions of pH and concentrations. Chelating agents (EDTA, DTPA) at a concentration of 25 wt% and pH of 6 had a solubility of 40 g/liter of iron sulfide. The solubility results illustrated that mixing chelating agents with organic acids reduce the efficiency of removing iron scale. The developed formulation, which contains a mixture or organic and mineral acid, was able to remove 78 g/liter of iron sulfide scale at 100°C. The corrosion test results confirmed that the recommended temperature range for the developed formulation is up to 100°C with a corrosion rate of 0.014 lbm/ft2 without adding corrosion inhibitors