Insights into the Pore-Scale Mechanism for the Low-Salinity Effect: Implications for Enhanced Oil Recovery

摘要

The properties and behavior of the interface between mineral surfaces, adsorbed organic compounds, and water are important for oil recovery. Low-salinity (LS) water flooding releases more oil from sandstone reservoirs than conventional flooding with seawater or formation water. However, the role of strongly adsorbed organic material, as an anchor for oil molecules, is not yet completely understood. Here, we mimic reservoir pore surfaces using graphene oxide sheets deposited on flat silicon wafers. The LS response was quantified using atomic force microscopy (AFM) in chemical force mapping mode to directly measure the adhesion force. AFM tips were functionalized to serve as models for hydrophobic and polar oil molecules, i.e., with alkyl, -CH3, and carboxyl, -COO(H). Adhesion force, measured with -CH3 tips, was 18% lower in LS (similar to 1500 ppm) than high-salinity (HS, similar to 35 600 ppm) solutions, while for -COO(H) tips, adhesion force was 13% lower in LS than HS solutions. The Dejarguin-Landau-Verwey-Overbeek theory predicts that the difference in response to the salinity-dependent force with the -CH3 tips results from electric double layer (EDL) repulsion. The response to -COO(H) tips can be explained by combined EDL repulsion and cation bridging, which is consistent with density functional theory calculations. The absolute adhesion and the level of response agree with observations on sand grains from oil reservoirs, where other studies have demonstrated strongly bound organic compounds. Important implications of our study are that (i) oxidized graphene provides a convincing model for reservoir pore surfaces that is robust and reproducible and can be used for systematic testing for developing more effective enhanced oil recovery strategies and (ii) the new fundamental understanding about pore surfaces can also be applied over a range of disciplines, including improved remediation strategies for contaminated soil and groundwater.