Occurrence and mobility of trace elements during oxidant stimulation of shales in Yichang, Hubei province of China

Abstract

Oxidant stimulation is a promising technology for shale permeability enhancement, but it is still faced with the problem of produced water with high total dissolved solids. For the consideration of environmental protection and water reuse, the mobility of trace elements (TEs) should be evaluated before the in-situ application of this technology. In this study, carbonate-rich shale and silicate-rich shale, collected in Yichang, Hubei province of China, were used to react with sodium persulfate (Na2S2O8) at different experimental conditions. The sequential chemical extraction was used to analyze the occurrence of TEs in shales and their mobilization mechanism. Influence factors including oxidant concentration, solid:liquid ratio, initial pH and temperature were systematically investigated to explore their effect on the mobility of TEs. Results showed that a similar affinity between TEs and fractionation phases was observed in both two shales. Tough the TEs associated with residual fraction was most obvious in shales, various associations between TEs and other extractable fractions were also apparent, such as Co (61.7% on average) and Cu (59.2% on average) in organic matter-bound fraction, and Sr (53.2% on average) in carbonates-bound fraction. The occurrence of TEs determined their extent of mobilization during oxidant stimulation, and the complex interactions including acidization, oxidation, adsorption and precipitation influence the mobility of TEs as well. The carbonate minerals and pyrite were both critical minerals for decreasing the mobility of TEs. Carbonate minerals in shale could effectively buffer the pH of the system to mitigate the acidization reaction. The near-neutral environment was also beneficial for the Fe(OH)3 precipitation then resulted in the fixation of TEs through adsorption. Besides, the generated gypsum from carbonate dissolution also contributed to the incorporation of TEs. Meanwhile, sulfate generated from pyrite oxidation and persulfate hydrolysis also directly precipitated with TEs such as Ba and Sr to reduce their dissolved content in the reaction system. Influence factors analysis showed that the mobility of TEs was strongly dependent on the oxidant concentration and solid: liquid ratio and to a lesser extent on the temperature and initial pH. Considering the environmental risk and water management cost, systematic investigation on shale composition and associated TEs, and optimization of engineering parameters should be carried out before the in-situ application of this technology.