Iron-sulfur redox reactions play an important role in geochemical processes starting from the early stage of sedimentation and continuing throughout burial, diagenesis, and organic matter transformation. Iron sulfide forms from clay-bearing sediments interacting with sulfur-rich organics to produce iron sulfide. In iron-poor sediments, as in carbonates, the sulfur remains associated with the organic matter. As a result, carbonate-rich oil and gas reservoirs are often the source of significant quantities of H2S gas produced along with hydrocarbons resulting from sulfate-reducing bacteria, thermal decomposition of organosulfur compounds, or acid treatment of sour wells. The end result, though, is still iron sulfide formation when the H2S reacts with iron-based steel well tubulars, though in this case the iron sulfide deposits in the well, posing a major hindrance to oil and gas operations. Iron sulfide scale is often removed mechanically but the process is very expensive and requires shutting down production for an extended time. Chemical dissolution has achieved limited success, in part because some phases of iron sulfide such as pyrite are intransigent to dissolution and yet constitute a significant portion of the scale deposits. New dissolver chemistry has been developed, utilizing knowledge of iron sulfide redox processes, that readily consumes pyrite. Dissolution experiments were performed with a series of oxidizing agents, over a temperature range of room temperature to 150 °C. The resulting solutions were characterized using several analytical techniques such as electrospray ionization mass spectroscopy, fourier transform infrared spectroscopy, and inductively coupled plasma. Most strong oxidizers, delivered in aqueous solution, demonstrated only mild reactivity toward the pyrite substrate, leading to limited solubility. In the optimized scenario, a combination of strong oxidizers demonstrating synergistic effects dissolve pyrite stoichiometrically in a time frame of minutes to hours. Solution characterization allowed identification of all reaction products as well as some key reaction intermediates. The formation of acid as a byproduct plays a contributory role, though the reaction mechanism bypasses any H2S generation. The end result of this study is a package that effectively and efficiently removes otherwise insoluble scale deposits from both downhole tubulars and surface facilities and tubes containing iron sulfide.

Rapid iron sulfide dissolution and new applications for downhole scale deposits

254th National Meeting (2017)

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