To ensure the efficiency of subsurface operations, it is key to understand the fate and transport of the fluids injected during the process. Phosphonates are a group of organic chemicals injected to inhibit scale formation in the operations. Phyllosilicates are ubiquitous in shales, the main reservoirs for unconventional oil/gas recovery, and they are important players in redox processes of organic substances. However, their effects on the stability of phosphonates remain poorly understood. In the present study, diethylenetriaminepenta(methylene)phosphonate (DTPMP, a phosphonate-based scale inhibitor) was reacted with three phyllosilicates: an Fe-poor muscovite, an Fe(II)-rich biotite, and an Fe(III)-rich nontronite, under subsurface relevant conditions (50-95 °C, and 102 atm CO<sub>2</sub>). During the reactions, DTPMP degraded and formed phosphate and formate products. Compared with control experiments without any minerals, muscovite and nontronite showed slight promotion effects on DTPMP degradation, but, biotite significantly promoted the degradation of DTPMP. Interestingly, we found that the Fe(II) sites in the mineral structure play important roles in the redox degradation of DTPMP. Specifically, reactive oxygen species were generated through reduction of molecular oxygen by Fe(II) in biotite. Furthermore, with increasing temperature, both biotite dissolution and DTPMP degradation were favored. Because of biotite dissolution and DTPMP degradation, dissolved Fe and Al precipitated with phosphate or degraded DTPMP to form secondary minerals. This study broadens our understanding of the roles of Fe-bearing phyllosilicates in the geochemical processes of redox-active substances, and provides new insights on unexplored but important feature of the fate and transport of phosphonate additives in subsurface systems.<br/> <br/>

Redox degradation of DTPMP phosphonate with Fe-bearing phyllosilicates under subsurface conditions

255th National Meeting (2018)

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