Interfacial tension (IFT) between crude oil and surfactant solution at low salinity is normally higher than IFT at optimal ionic strength. However, it is possible to further reduce IFT by 1-2 orders of magnitude by changing the molar ratio of calcium to sodium cations (Ca2+/Na+) at a constant low ionic strength in a low-salinity surfactant (LSS) solution. IFTs of 0.03 and 0.1 mN/m were measured at a Ca2+/Na+ ratio of 0.005 and in the absence of calcium in LSS solutions, respectively. Two flooding experiments using these fluid systems were performed on a strongly water-wet Berea sandstone. The in situ core flooding in micro-computed tomography showed that more oil was trapped in the presence of calcium and that the residual oil clusters were larger after LSS injection. The results showed that there was a small wettability shift from being strongly water-wet toward being (weakly) water-wet by LSS flooding both with and without calcium. The results also showed that this wettability shift was stronger in the absence of calcium in the injected LSS, causing a reduction in the oil trapping. However, in the secondary low-salinity water flooding, the residual oil saturation was smaller in the presence of calcium. The cause of these behaviors is discussed in relation to the oil surfactant and oil water interfacial elastic modulus with initial rock wettability.