An experimental study of polymer flooding is here presented focusing on the influence of initial core wettability and flood maturity (volume of water injected before polymer injection) on final oil recovery. Experiments were carried out using homogeneous Bentheimer sandstones of similar properties. The cores were oil flooded using mineral oil for water wet conditions, and with crude oil (after an ageing period) for intermediate wet conditions keeping constant in all experiments the viscosity ratio between oil and polymer solution. Polymer, which is a partially hydrolysed polyacrylamide (HPAM) was used at a concentration of 2500ppm in a moderate salinity brine. The polymer solution was injected in the core at different maturity times (0PV, Breakthrough, 1PV, 1.75PV, 2.5PV, 4PV and 6.5PV).
Coreflood results show that the maturity of polymer injection plays an important role on final oil recovery regardless wettability. The 0PV maturity (secondary polymer flooding) leads to the best sweep efficiency while final oil production decreases when the polymer flood maturity is high (late polymer injection after waterflooding). A difference of 15% in recovery is observed between secondary polymer flooding and late maturity (6.5 PV). Concerning the effect of wettability, the recovery factor obtained with water wet cores is always lower (from 10% to 20% depending on maturity) than the values obtained with intermediate wet cores, raising the importance of correctly restoring core wettability to obtain representative values of polymer incremental recovery. The influence of wettability can be explained by the oil phase distribution at the pore scale. Considering that the waterflooding period leads to different values of the oil saturation at which polymer flooding starts, we measured the core dispersivity using a tracer method at different states. The two-phase dispersivity decreases when water saturation increases which is favourable for polymer sweep.
This study shows that in addition to wettability, maturity of polymer flooding plays a dominant role in oil displacement efficiency. Final recovery is correlated to the dispersion value at which polymer flooding starts. The highest oil recovery is obtained when the polymer is injected early.