Effect of water-soluble organic acids on wettability of sandstone formations using streaming zeta potential and NMR techniques: Implications for CO₂ geo-sequestration

Rock wettability is influenced by several conditions, and one such factor is the potential at the rock/fluid interface. While most studies have reported zeta potential in this context, very few investigations have carried out streaming potential measurements. Further to this, most previous studies have only evaluated zeta potential/streaming potential for NaCl brine-clean sandstone systems, while it is clear that organic matter exists in subsurface rock. However, the effect of these forms of organic matter on streaming potential has not yet been probed. Accordingly, in this study streaming potential measurements were firstly conducted on pristine San Saba sandstone samples with 0.3 mol.dm3 NaCl brine-saturated at (6.895 MPa overburden and 3.447 MPa back-pressure). Secondly, the streaming potential of the aged samples was measured under identical conditions (the cores were saturated with aqueous HA solutions of different concentration; 1–100 mg/L) in 0.3 mol.dm⁻³ NaCl. Thirdly, a comparative analysis of zeta potentials was conducted via electrophoretic and streaming potential. Lastly, the T₂ spectrum for the initial water saturation of pristine and aged cores, along with the T₂ spectrum of residual water saturation after CO₂ flooding for pristine and aged cores, were measured with NMR core flooding measurements. Accordingly, this work analyses the effects of organic acids on wettability alteration in sandstone formation in accordance with these procedures. A strong correlation exists between surface adsorption of organic acid and streaming potential coefficient, where the amount of residual water saturation decreases in humic-acid aged cores – suggesting the presence of organic acid that changes wettability towards CO₂ wet in pores, where the CO₂ displaces more brine in aged cores compared to pristine cores.