Experiment and Simulation Study on the Adsorption Interaction between a Fluorescent Tracer and a Montmorillonite Crystal in Drilling Fluid

The adsorption interaction of oil field tracer in drilling fluid plays a significant role in tracer monitoring (TM) technology in the petroleum industry. In this work, the adsorption performances of Rhodamine B (RhB⁺) and fluorescein sodium (Fln^-) tracers with montmorillonite (MMT) crystal in drilling fluid were investigated by both experimental and simulation methods. For the experimental aspect, the macroscopic results indicate thermodynamic monolayer adsorption by the Langmuir model and kinetic chemical adsorption by the pseudo-second-order (PSO) model. As a result, MMT shows a larger adsorption capacity (q_m) for RhB⁺ than for Fln^- with $q_{m\left({\mathrm{RhB}}^{+}\right)}=0.069g{g}^{-1}>q_{m\left({\mathrm{Fln}}^{-}\right)}=0.016g{g}^{-1}$ but stronger adsorption spontaneity (Δ_rG_m^θ) for Fln^- than for RhB⁺ with ${\Delta }_r{G_m^{\theta }}_{\left({\mathrm{Fln}}^{-}\right)}=-7.92$ kJ mol^-1 < ${\Delta }_r{G_m^{\theta }}_{\left({\mathrm{RhB}}^{+}\right)}=-6.90$ kJ mol^-1. Meanwhile, the interaction rate (k₂) of Fln^- was shown to be faster than that of RhB⁺ with ${k_2}_{\left({\mathrm{Fln}}^{-}\right)}=1.07{\min }^{-1}>{k_2}_{\left({\mathrm{RhB}}^{+}\right)}=0.95{\min }^{-1}$. For simulation insight, MMT shows much higher system stability (E) for Fln^- than for RhB⁺ with $E_{{\mathrm{Fln}}^{-}···\mathrm{MMT}}<E_{{\mathrm{RhB}}^{+}···\mathrm{MMT}}$ and $\Delta E_{{\mathrm{Fln}}^{-}···\mathrm{MMT}}>\Delta E_{{\mathrm{RhB}}^{+}···\mathrm{MMT}}$. Meanwhile, the microscopic simulation results reveal configuration changes and site distinctions for RhB⁺ and Fln^- interactions with the MMT crystal. The different adsorption responses were explained by proposing an interaction mechanism of force dominance and position orientation. Specifically, Fln^- was deduced to interact with metal (Al, Ca) and metalloid (Si) elements in the MMT crystal interlayer by "upright-insertion" orientation while RhB⁺ was deduced to interact with oxygen atoms on the MMT crystal surface by a "flat-lying" orientation. Hydrogen bonds, the electrostatic interaction, and the coordination effect were revealed to dominate for the interaction of tracer adsorption. This work provides both performance and mechanism investigation of fluorescent tracer adsorption interaction with the MMT crystal in drilling fluid, which is of great significance in reservoir exploitation.