Dependency of Remaining Oil Saturation on Wettability and Capillary Number

Wettability is a major factor that affects the flow behavior and recovery efficiency in oil reservoirs. In this study, the effects of wettability on water-oil relative permeability (k_r), capillary pressure (P_c) curve and capillary desaturation curve (CDC) have been investigated in laboratory on core scale.

Water floods and surfactant floods at different capillary numbers (N_c) were carried out in sandstone rock at different wettability conditions. These floods were followed by oil floods. The k_r and P_c curves were estimated by history matching the experimental production and pressure drop.

The recovery after water flood was higher at non water-wet conditions but a higher throughput of water was needed. Estimated k_r and P_c curves vary with the wettability of the rock. The relative permeability of the wetting phase is more curved than that of the non wetting phase. The measured CDC for oil in mixed-wet condition and the CDC for water in water-wet condition deviate from the typical CDC shape. In these cases, it is found that the measured remaining oil or water saturation is a function of the number of pore volumes injected and also it is largely affected by capillary end effects. Since the residual saturation is difficult to be obtained in core floods at non water-wet conditions, the measured CDC in laboratory experiments does not represent the true CDC. More focus should be directed at relative permeability (corrected for capillary end effects) than the residual oil saturation.

The results presented in this paper demonstrate the importance of the wettability of rock on recovery efficiency and also the importance of correcting the laboratory data especially predicting the flow behaviour of non water-wet conditions. Remaining oil saturation based on relative permeability should be used instead of residual oil saturation in evaluation of the potential for tertiary recovery, e.g. surfactant.