Influence of reservoir wettability changes on oil-bearing features during tight oil accumulation: A case study of Jurassic tight oils in Sichuan Basin

A physical simulation of tight oil microcosmic filling was carried out with core samples from the Jurassic tight oil reservoirs in the central Sichuan Basin. A sandstone thin section model (2.5 cm×2.5 cm×0.6 mm) was adopted. Oil for the experiment was filled through a chute, and the pressure gradually increased until oil instead of water flowed out of each outlet end. Percolation characteristics were observed and oil saturation was calculated. Experiment results showed that the sum of irreducible water saturation before filling plus oil saturation after filling was more than 100%. Reservoir wettability changed from water wet to oil wet after filling, so the final oil saturation degree is higher than the displaced moveable water saturation according to magnetic resonance analyses. The change of reservoir wettability during tight oil migration and accumulation counteracted the inhibition function of irreducible water saturation of a tight oil reservoir on the saturation of tight oil, and was favorable for tight oil migration and accumulation, which explained the high oil saturation of tight oil reservoirs.