Thickness measurements of nanoscale brine films on silica surfaces under geologic CO2 sequestration conditions using synchrotron X-ray fluorescence

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 SiO2 windows 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 degrees C (representative of conditions at about 0.78 km below the land surface), with scCO(2) as 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.