This paper discusses various mechanisms that can lead to the formation of iron sulfide scale downhole, techniques that can be used to prevent its formation and methods to remove it. Iron sulfide scale is present in oil and gas producing wells, water injection and supply wells. There are various mechanisms that can lead to the formation of iron sulfide. However, all of these mechanisms require sources of hydrogen sulfide and iron. Hydrogen sulfide can result from sulfate reducing bacteria, thermal decomposition of sulfate, or being introduced into the well as in gas lift operations. Iron can be produced from the formation, especially sandstone reservoirs and is also present downhole as a result of various corrosion processes. Combination of hydrogen sulfide and iron will cause formation of various iron sulfide species. The ratio of iron to sulfide in these species depends on temperature, pressure, pH, and hydrogen sulfide concentration. This ratio plays a key role in determining the best method to remove iron sulfide scales. Hydrochloric acid can be used to dissolve iron sulfide species that contain iron and sulfur at a molar ratio close to unity. Non-acid formulae can be used to remove iron sulfide scale, however, their ability to dissolve iron sulfide depends on the molar ratio of iron to sulfide. To prevent the formation of iron sulfide, squeeze treatments to the formation were found to be very effective. This paper discusses various mechanisms that can lead to the formation of iron sulfide, chemical and mechanical methods to remove it and chemical squeeze treatments to prevent its formation and/or deposition.
Extensive field work was conducted to identify the type of iron sulfide scale present, and the mechanisms that lead to its formation. Iron sulfide species were present in gas, oil and water supply wells. The chemical and physical characteristics of iron sulfide scale were found to be a function of temperature, pressure, pH and the age of the scale. Other properties of the scale, density and thickness, were found to vary with the scale depth and age. Various mechanical and chemical treatments to remove iron sulfide scale were examined in detail. Advantages and disadvantages of each method were identified. The best method to deal with iron sulfide scale is to avoid its formation in the first place. Chemical squeeze treatments were found to be effective in this regard. Once iron sulfide scale is formed, then it is recommended to remove the scale using acid washes with appropriate additives. Mechanical means are recommended for old iron sulfide scale, which has low acid solubility.
Area of Interest
Iron sulfide scale is present in sour oil and gas wells and injectors that are contaminated with sulfate reducing bacteria (SRB). It enhances the corrosion rate of the downhole tubulars, and adversely affects the performance of various wells. It reduces the efficiency of oil-water separation in various GOSPs. Removing iron sulfide scale is a complex process, especially at downhole conditions. Optimizing this process will require full understanding of various chemical interactions.
Iron sulfide species have been known to cause operational problems in the oil industry. The presence of iron sulfide particles in the injected water can cause loss of injectivity in power water injectors1–3 and water disposal wells.4 Accumulation of iron sulfide and biomass around downhole screens and perforations can cause loss of productivity of water supply wells.5 Build-up of iron sulfide scale in the tubing can create problems during wireline work and can reduce well deliverability. 6–7 The presence of fine particles of iron sulfide in the produced crude oil can cause many operational problems in oil-gas separation plants (GOSPs). 8 Recently, Nasr-El-Din et al.9 found that iron sulfide scale deposited in the nozzles of gas mandrels reduced the productivity of water supply wells with gas lift.