AbstractIn fractured reservoirs, the efficiency of water flood is governed by spontaneous imbibition of water into oil-containing matrix blocks. When the matrix is oil-wet or mixed-wet, little oil can be recovered by imbibition. The objective of this work is to identify chemicals that can be added to the injection water that can induce imbibition into an originally mixed-wet, tight, fractured sandstone reservoir. Several surfactants were evaluated for their aqueous stability at the reservoir temperature and salinity. Contact angles were measured on a clay-rich sandstone. Spontaneous imbibition tests were conducted on the reservoir rocks. It is shown that the use of dilute (0.1 wt %) surfactant solution can alter the wettability from oil-wet towards more water-wet condition on the mineral plates. Incremental oil recovery as high as 68% OOIP is obtained through spontaneous imbibition experiments performed on tight (∼10 µD) oil-wet/mixed-wet sandstone reservoir cores. Parametric studies performed using numerical simulation show that the rate of oil recovery increases with increasing wettability alteration, increasing fracture density, and decreasing oil viscosity.