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Sealing of oil and gas production wells is done to protect overlying aquifers and the land surface from vertical migration of drilling fluids, produced water (PW), and natural gas. The integrity of the contact between the well casing and cement seal is especially important as both the steel and the cement are subject to reactions caused by exposure to very high salinity PW. The objective of this study was to identify corrosion and precipitation products that form at this contact and determine their effect on the microannular space between the two materials. Steel cylinders were embedded in Type G Portland cement to simulate a sealed wellbore. They were then exposed to simulated and PW sampled from the Permian Basin in the southwestern United States. Solid phases in the cement were identified by X-ray diffraction and electron microprobe analyses and included portlandite, a calcium silicate, and brownmillerite. Gas flow measurements were used to estimate the aperture of the microannulus between the steel surface and the cement. A decrease in the aperture with increasing reaction time was detected for all experiments. The findings suggest that exposure to PW has the potential to reduce the microannular space between the casing and the cement seal as a result of precipitation of calcium- and magnesium-carbonate as dominant phases, with the co-occurrence of sulfate and silicate minerals. These results have implications related to the long-term integrity of annular seals used to seal oil wells exposed to very high salinity PW. 

Open dumping and burning of solid waste are widely practiced in underserved communities lacking access to solid waste management facilities. The generation of microplastics from these sites has been overlooked. 

The co-occurrence of uranyl and arsenate in contaminated water caused by natural processes and mining is a concern for impacted communities, including in Native American lands in the U.S. Southwest. We investigated the simultaneous removal of aqueous uranyl and arsenate after the reaction with limestone and precipitated hydroxyapatite (HAp, Ca10(PO4)6(OH)2). In benchtop experiments with an initial pH of 3.0 and initial concentrations of 1 mM U and As, uranyl and arsenate coprecipitated in the presence of 1 g L−1 limestone. However, related experiments initiated under circumneutral pH conditions showed that uranyl and arsenate remained soluble. Upon addition of 1 mM PO43− and 3 mM Ca2+ in solution (initial concentration of 0.05 mM U and As) resulted in the rapid removal of over 97% of U via Ca−U−P precipitation. In experiments with 2 mM PO4 3− and 10 mM Ca2+ at pH rising from 7.0 to 11.0, aqueous concentrations of As decreased (between 30 and 98%) circa pH 9. HAp precipitation in solids was confirmed by powder X-ray diffraction and scanning electron microscopy/energy dispersive X-ray. Electron microprobe analysis indicated U was coprecipitated with Ca and P, while As was mainly immobilized through HAp adsorption. The results indicate that natural materials, such as HAp and limestone, can effectively remove uranyl and arsenate mixtures.