For more efficient unconventional oil/gas recovery and other energy-related subsurface operations, it is important to understand the roles of abundant Fe-bearing phyllosilicates in the degradation of injected additive chemicals, such as scale inhibitor. In this study, we examined the degradation of 0.5 mM DTPMP (diethylenetriaminepenta(methylene)phosphonate; a model phosphonate-based scale inhibitor) with phyllosilicates under subsurface relevant conditions (i.e., 50–95 °C and 102 atm CO<sub>2</sub>). An Fe-poor muscovite, an Fe(II)-rich biotite, and an Fe(III)-rich nontronite were tested as Fe-bearing phyllosilicates which are often found in shales. These three phyllosilicates induced different effects on DTPMP degradation, with no distinguishable effect by muscovite, slight promotion by nontronite, and remarkable promotion by biotite. We found that Fe associated with phyllosilicates is key to the redox degradation of DTPMP. Reactive oxygen species were generated through the reduction of molecular oxygen by Fe(II) adsorbed on the mineral surface or in the mineral structure, and the hydroxyl radicals further degraded DTPMP to form phosphate, formate, and DTPMP residuals. Furthermore, DTPMP degradation was favored at higher temperatures, owing to more exposed reactive Fe(II) sites created by enhanced biotite dissolution and also owing to faster electron transfers. Dissolved Fe and Al then precipitated with phosphate or degraded DTPMP, forming secondary minerals. This study provides new information about how DTPMP degradation is affected by the presence of Fe-bearing phyllosilicates under conditions relevant to many engineered subsurface operations.

Degradation of phosphonate-based scale inhibitors by Fe-bearing phyllosilicates under  energy-related subsurface environmental conditions

2019 Spring National Meeting

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