Iron Sulfide Formation in Water Supply Wells With Gas Lift

Abstract

Abstract Iron sulfide scale is present in sour gas, oil and water wells. It is present in various chemical species with different iron to sulfur ratios. Iron sulfide species with high iron content is soluble in acids, whereas those rich in sulfur are almost insoluble in acids. Iron sulfide scale was detected in several water supply wells where a sour gas is used for gas lifting operation. The objectives of this study were to determine the mechanism that led to the formation of this type of scale, to characterize this scale and determine the most effective chemical treatments to remove and prevent reoccurrence. Lab work included characterization of the scale, and examine various acids and non-acids formulae to remove various iron sulfide species. Several wells in a sandstone aquifer are used to supply water needed for the operation of gas oil separation plants (GOSPs). The reservoir pressure is low, therefore the water is produced using gas lifting. The gas used is an associated gas that is produced with the oil from a carbonate reservoir. The gas is sour with a hydrogen sulfide content of 2 mol%. In addition, the supply water contains total iron at 5–10 mg/L. Hydrogen sulfide reacts with the iron present in the aquifer water and precipitates iron sulfide on the well tubulars and gas injection nozzles. Accumulation of iron sulfide has caused many operational problems. Lab results indicated that iron sulfide scale deposited on the inside wall of the well tubulars. The scale was uniform with a thickness that increased from 0.025" above the gas injection point to 0.25" at the well head. The composition of the scale changes with the length above the gas injection point. Just above the gas injection point, the scale was identified as FeS whereas close to wellhead, the scale was identified as FeS2. Acid solubility varied across the length of tubing. In areas where there is FeS, acid solubility in 20 wt% was 85–90 wt%. On the other hand, acid solubility was only 3–5 wt% in areas where FeS2 was present. Experimental results showed that 20 wt% could be used to remove a portion of the scale, however a suitable hydrogen sulfide scavenger should be added to the acid. Several non-acid formulae were tested and some of them were effective in dissolving acid-insoluble scale. Unlike typical types of oilfield scales, iron sulfide is present in different species. The chemical structure of these species affects the method that should be used to remove this scale. This study addresses various types of iron sulfide and methods to remove and prevent the formation of this type of scale. Introduction Iron sulfide deposits have been known to cause operational problems in the oil industry. For example accumulation of iron sulfide in the critical near wellbore area can cause loss of performance of disposal wells,1 water supply wells,2 water injectors,3–5 and gas wells.6,7 Iron sulfide forms in the wellbore area as a result of the reaction of hydrogen sulfide and iron. Hydrogen sulfide originates downhole from several sources including: thermal decomposition of drilling mud additives,8 sulfate reducing bacteria,5 thermo-catalytic reduction of the sulfate ion.9 The sources of iron are the corrosion products or iron leached from minerals present in the formation. This is especially important in sandstone reservoirs that contain iron-rich minerals, e.g. chlorite, hematite, and ankerite. Unlike calcium carbonate or sulfate scales, iron sulfide is present in several forms.6,10 Mackinawite, troilite, pyrrhotite, pyrite, and marcasite are found on tubular surfaces. These species have different crystalline forms and, as a result, have different reaction rates and solubilities in acids.11