Fundamentals of Fluid Flow in Porous Media
Multi-phase Saturated Rock Properties:
Wettability: Spreading Coefficients
When two interfaces are present in a pore and are approaching each other, such as a water/oil interface and a water/gas interface, there is a point where the fluids are spontaneously redistributed in the pore. In that, oil spreads between water and gas thus eliminating the water/gas interface and creating an oil/gas interface. This condition where oil spreads over water in the presence of gas is best treated in terms of what is referred to as the spreading coefficient, defined by the equation:
Where, So/w is the final spreading coefficient of oil over water, σwg′, σog′, and σow′ are the interfacial tensions when all three fluids are in thermodynamic equilibrium. The numerical value of the spreading coefficient, as computed from the above equation, can be a positive or a negative value. The spreading parameter for non-equilibrium situation is the energy gained when covering one unit area of the water with a flat oil film of macroscopic thickness. When the spreading coefficient is positive, oil is expected to spread over water, whereas a negative spreading coefficient indicated that oil will not spread over water and the situation where the water is covered by the oil film is not favourable. The saturation of a water-gas interface with a monolayer of oil results in an interfacial value, σwg′ which is smaller than the interfacial tension of pure water against gas, σwg, measured at the same temperature and pressure. The value of the spreading coefficient is an important parameter in some enhanced oil recovery methods. The spreading coefficient also defined for the gas-liquid-solid system. Positive spreading coefficient means that the solid is covered by the liquid film and the system gains some energy because of that. For positive S, in the G-L-S system, the liquid wet the solid completely, in which case a liquid drop will spread with time after its deposition on the surface, with its initially nonzero contact angle moving towards its limiting equilibrium, zero value.