In a recent work by Loahardjo et al. (2008), it was shown that a consistent trend of increase in oil recovery can be achieved by cyclic water and oil floods into an asphaltic reservoir. However, our experimental results obtained from strict one-dimensional two-phase flow at a low constant flow rate demonstrate that it is impossible to enhance the recovery under the same experimental conditions as theirs.

We find that the oil productions before and after water breakthrough consistently decrease in the subsequent injections (floods). The consistent decrease in oil recovery in the cyclic injections is confirmed both experimentally and theoretically, and comprised as follows:

  1. Less oil recovery before the water breakthrough is caused by colloidal asphaltenes that block the throats during the preceding oil injection. Consequently, the continuous large-throat-controlled domain shrinks and the pore-throat aspect ratio reaches the critical value in greater number of pores, so that less channels for the invading front and additional disconnection of oil by snap-off occurs during the subsequent water injection.

  2. Less oil recovery after the water breakthrough is caused by destruction of the asphaltene layers deposited on the solid surfaces by the preceding water injection, so that there is even more disconnection of oil by snap-off during the subsequent water injection due to the solid surfaces that have become more water-wet.

Furthermore, experiments in this work show that the original wettability can be restored only if the water-wetted channels in a rock are soaked with asphaltic crude oil under high temperature for a long time period. Therefore, under oil-field operational condition, the wettability of the oil formation will not be restored as much as it has been in the original reservoir, unless more heat is added into the recovered reservoir.

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