Thickness measurements of nanoscale brine films on silica surfaces under geologic CO₂sequestration conditions using synchrotron X-ray fluorescence

[1] In reservoirs used for geologic CO₂ sequestration, brine films remaining on mineral surfaces can influence flow, diffusion, and reactions. We have investigated how the capillary (disjoining) potential influences the thickness of a KCsI₂ brine film on both smooth and rough SiO₂ surfaces [root mean square roughness (Rrms), 1.6 and 330 nm, respectively], under confinement with supercritical (sc) CO₂. The thicknesses of brine films coating interior surfaces of SiO₂windows in a high-pressure cell were determined through synchrotron X-ray fluorescence of two tracer ions (I⁻ and Cs⁺) at 7.8 MPa and 40°C (representative of conditions at about 0.78 km below the land surface), with scCO₂as the immiscible confining fluid. The measured area-averaged film thicknesses on the 330 nm Rrms silica surface ranged from 265 to 249 nm for capillary potentials measured within a narrow range from 0.18 to 3.7 kPa. Over this same range of potentials, film thicknesses measured on the smooth (1.6 nm Rrms) silica surface were about 2 nm, although equilibrium does not appear to have been achieved. The measured average brine film thicknesses were strongly controlled by surface roughness, with very weak variation in response to the fairly narrow range of tested capillary potentials.