Wettability modification by surfactants and temperature in shale oil reservoir conditions

Shale wettability affects pore-scale oil/water distribution and reservoir-scale oil extraction efficiency. For enhanced oil recovery, a common method to modify rocks' wettability is adding surfactant. However, previous experiments mainly focused on room temperature conditions, which are not representative of the high-temperature conditions in shale oil reservoirs. Meanwhile, shale consists of various compositions (e.g., clay, carbonate, organic matter), and each composition is expected to have a different influence on shale wettability in a complex way. So far, a comprehensive investigation of shale wettability modification by surfactants and temperature in oil reservoir conditions is still lacking.Therefore, in this work, the effects of temperature (25 ℃ vs 50 ℃), surfactant type (Cetyltrimethylammonium bromide [CTAB] vs Sodium dodecyl sulfate [SDS]) and concentration (0 ∼ 0.2 wt%), and shale compositions (carbonate-, clay- and organic-rich) on shale wettability were investigated systematically via contact angle (θ) measurements in the water–oil-shale system.It is demonstrated that 1) as the temperature increased, the carbonate-rich shale became more oil-wet, no matter the presence or absence of surfactants, while clay- and organic-rich shale became either more oil-wet or more water-wet; 2) At high temperature (i.e., 50 ℃), higher CTAB concentration made all the shales more hydrophobic; higher SDS concentration modified clay- and organic-rich shale more hydrophilic, while had a non-monotonous influence on carbonate-rich shale; 3) At 50 ℃, clay- and organic-rich shale shifted from strongly water-wet to strongly oil-wet with the addition of CTAB, while it became more water-wet with the addition of SDS; carbonate-rich shale became more oil-wet with the addition of CTAB, while it fluctuated erratically with the addition of SDS.This research provides fundamental guidance on wettability modification via surfactant, which has promising applications in EOR in shale oil reservoirs.