Water flooding of carbonate reservoirs: Effects of a model base and natural crude oil bases on chalk wettability

Polar components, especially the carboxylic material in crude oil, are known to have great impact on the wetting conditions of carbonates. The water-wetness decreases as the acid number, AN, of the crude oil increases. The success of enhanced oil recovery by water flooding in fractured carbonates is strongly dependent on the wetting conditions of the formation. The impact on wettability of basic compounds present in crude oil has received much less attention than that of carboxylic material, even though the base number, BN, is usually much higher than the AN. Furthermore, acids and bases present in crude oil will react with each other to form acid-base complexes. In this paper, we have studied the impact of basic components on the wetting properties of chalk by using an oil with a constant AN = 0.5 mg KOH/g oil and varying the AN/BN ratio in the range of 0.24- 4.6. The initial water saturation was varied from 0 to 47%. The relative impact of basic material on the wetting properties was studied by means of spontaneous imbibition. In the first series of experiments, a model base ( benzyl amine) was used, and it was observed that the water-wetness decreased as the content of base increased up to about 4 times the concentration of acid. For natural bases present in the crude oil, the effect on the wetting property was the opposite, i.e., the water-wetness increased as the amount of bases increased. The difference in the wetting behavior was discussed in relation to the molecular size of the basic material. It was also confirmed that seawater was able to modify the wetting properties and increase the oil recovery by spontaneous imbibition. Experimentally, it should be noticed that special care must be taken to avoid upconcentration of potential determining ions like SO4 (2-), when using a porous plate to drain the core to S-wr with water saturated N2-.