The depressurization of water-flooded reservoirs; wettability and critical gas saturation

The influence of wettability on the formation, growth and mobilization of bubbles formed when solution gas is released from water-flood residual oil have been visually observed in a series of experiments. The experiments were performed using a mixture of low boiling point hydrocarbons, which had a positive spreading coefficient for the oil phase in contact with water and its own vapour. In an oil-wet matrix, nucleation of gas bubbles occurred on the surface of the matrix. While it was impossible to determine where nucleation occurred in the water-wet condition, the results suggest that wettability influences the gas nucleation behaviour. In a water-wet system, the oil-water interfacial tension controlled the growth pattern for the gas bubbles, but in an oil-wet system the gas-oil interfacial tension was the controlling parameter. As a result, it seems likely that wettability influences the critical saturation for mobilization of the gas phase. The behaviour of the oil phase during depressurization is influenced by wettability. In a water-wet system the physical form of the oil is changed from immobile droplets into films which may be transported with the gas phase, and in which the oil is mobile. In contrast in an oil-wet system, the oil phase can move as films when the gas is released from solution but generally will not initially move over many pores.