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Downhole scale deposition in the Khuff sour gas wells in Saudi Arabia has been a persistent problem, which negatively affects operation and production. Scale deposits are composed of predominantly iron sulfides with other types of minerals also present. Mechanical descaling treatment, although expensive and time-consuming, is often required. Effective scale dissolver is highly desirable to enhance descaling efficiency and to reduce treatment cost. An ideal dissolver is required to have high scale dissolving power, no damage to downhole completion and well productivity, and minimal H2S liberation. This paper presents the laboratory studies on the new scale dissolvers developed by service companies. These products have pH values ranging from strong acidic (pH < 2) to high alkaline (pH > 12). Dissolvers were evaluated for thermal stability, corrosivity to mild steel, and compatibility with formation water at downhole temperatures. The potentials of iron sulfide re-precipitation in spent solutions and free H2S generation were also examined. The qualified chemicals were then evaluated for their dissolving capacity using authigenic pyrrhotite and field scales at elevated temperatures. The obtained results show that most effective acidic dissolvers evaluated in this study were very aggressive to low alloy carbon steel at downhole temperatures. For these with acceptable corrosivity, formation of iron sulfide reprecipitation in spent dissolvers and the generation of a large quantity of free H2S gas also prevented them from field application. Some dissolver products were disqualified due to incompatibility with formation water. Dissolvers with near neutral and alkaline pH values, in general, were inefficient to dissolve the heterogeneous iron sulfide scales. The performance of tested dissolvers varied with scales from different wells, attributed by differences in composition, microstructure, and the presence of hydrocarbon. Results also suggested that pyrite and marcasite were possibly formed during the dissolution process. This paper presents an objective assessment on the currently available iron sulfide scale dissolvers, highlights the challenges on downhole scale dissolution in high temperature sour wells, and provides new insights on the scale dissolution process. The results suggest that further R&D efforts are required to develop more effective chemical solutions to mitigate the iron sulfide scale problem.
Downhole scale deposition in the Khuff sour gas wells in Saudi Arabia has been a persistent problem, which negatively affects operation and production. Scale deposits are composed of predominantly iron sulfides with other types of minerals also present. Mechanical descaling treatment, although expensive and time-consuming, is often required. Effective scale dissolver is highly desirable to enhance descaling efficiency and to reduce treatment cost. An ideal dissolver is required to have high scale dissolving power, no damage to downhole completion and well productivity, and minimal H2S liberation. This paper presents the laboratory studies on the new scale dissolvers developed by service companies. These products have pH values ranging from strong acidic (pH < 2) to high alkaline (pH > 12). Dissolvers were evaluated for thermal stability, corrosivity to mild steel, and compatibility with formation water at downhole temperatures. The potentials of iron sulfide re-precipitation in spent solutions and free H2S generation were also examined. The qualified chemicals were then evaluated for their dissolving capacity using authigenic pyrrhotite and field scales at elevated temperatures. The obtained results show that most effective acidic dissolvers evaluated in this study were very aggressive to low alloy carbon steel at downhole temperatures. For these with acceptable corrosivity, formation of iron sulfide reprecipitation in spent dissolvers and the generation of a large quantity of free H2S gas also prevented them from field application. Some dissolver products were disqualified due to incompatibility with formation water. Dissolvers with near neutral and alkaline pH values, in general, were inefficient to dissolve the heterogeneous iron sulfide scales. The performance of tested dissolvers varied with scales from different wells, attributed by differences in composition, microstructure, and the presence of hydrocarbon. Results also suggested that pyrite and marcasite were possibly formed during the dissolution process. This paper presents an objective assessment on the currently available iron sulfide scale dissolvers, highlights the challenges on downhole scale dissolution in high temperature sour wells, and provides new insights on the scale dissolution process. The results suggest that further R&D efforts are required to develop more effective chemical solutions to mitigate the iron sulfide scale problem.
Iron sulphide (FeS), zinc sulphide (ZnS) and lead sulphide (PbS) are considered to be among the most challenging scales in terms of inhibition and removal. They can form by direct reaction of aqueous sulphide species with dissolved Fe, Zn and/or Pb and by the exchange between aqueous sulphide species with preformed iron compounds, such as iron oxide hydroxide. These existing iron compounds may have formed during production and/or intervention, such as an acid treatment. Similarly, PbS and ZnS can form by extracting sulphide from a more soluble sulphide scale i.e. Zn exchanging with Fe in FeS. The objective of this work was to investigate FeS formation and inhibition under a range of conditions including pH, temperature, salinity and proposed mode of formation. In addition, the interaction between iron, zinc and lead within solutions containing sulphide species was investigated The majority of this study was conducted under anaerobic conditions, with the scale formation and/or inhibition experiments being monitored by inductively coupled plasma (ICP) analysis, pH and particle size measurements. Among the tested scale inhibitors, two showed high efficiency against iron sulphide, however high pH and salinity had a detrimental impact on the performance of one of these products. Interestingly, these scale inhibitors prevented iron sulphide deposition even under aerobic conditions i.e. iron hydroxide partially preformed. Moreover, at sufficiently high concentrations of scale inhibitor, the deposition of zinc sulphide and lead sulphide was prevented even when these scales were formed via cation displacement i.e. zinc and lead displaced sulphide ions from pre-formed iron sulphide. The route of formation for FeS, ZnS and PbS was seen to have a significant impact on the inhibition process. The particle sizes of inhibited (suspended) FeS were significantly lower than the blank FeS samples, with this effect increasing with increased scale inhibitor concentration. This difference in particle size may have an important influence on in-line filter blocking tests and produced water quality issues.
Summary Iron sulfide (FeS), zinc sulfide (ZnS), and lead sulfide (PbS) are considered to be among the most challenging scales in terms of inhibition and removal. They can form by the direct reaction of aqueous sulfide species with dissolved Fe, Zn, and/or Pb and by the exchange between aqueous sulfide species with preformed Fe compounds, such as Fe oxide hydroxide. These existing Fe compounds might have formed during production or intervention, such as an acid treatment. Similarly, PbS and ZnS can form by extracting sulfide from a more soluble sulfide scale (i.e., Zn exchanging with Fe in FeS). The objective of this work was to investigate FeS formation and inhibition under a range of conditions, including pH, temperature, and salinity, and to propose a mode of formation. In addition, the interaction between Fe, Zn, and Pb within solutions containing sulfide species was investigated. The majority of this study was conducted under anaerobic conditions, with the scale-formation/inhibition experiments being monitored by inductively coupled plasma (ICP) analysis and pH and particle-size measurements. Among the tested scale inhibitors (SIs), two showed high efficiency against FeS, but high pH and salinity had detrimental effects on the performance of one of these products. Interestingly, these SIs prevented FeS deposition even under aerobic conditions (i.e., Fe hydroxide partially preformed). Moreover, at sufficiently high concentrations of SI, the deposition of ZnS and PbS was prevented even when these scales were formed by means of cation displacement (i.e., displaced ZnS and PbS ions from preformed FeS). The route of formation for FeS, ZnS, and PbS was seen to have a significant effect on the inhibition process. The particle sizes of inhibited (suspended) FeS were significantly lower than those of the blank FeS samples, with this effect increasing with increased SI concentration. This difference in particle size might have an important influence on in-line filter-blocking tests and produced-water quality issues.
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