ABSTRACT
The discussion in Chapter 4 on permeability referred to absolute permeability of a porous medium that was fully saturated with a single fluid phase. The experiments that Darcy conducted included 100% water-saturated sands, and later developments of Darcy’s law that is commonly used in the petroleum industry extended it to include a generalized case of the flow of any single-phase fluid, for example, oil or water. However, petroleum reservoirs having such simple single-phase fluid systems seldom exist because reservoir rocks are saturated with at least two immiscible fluid phases, for example, the pore space is shared by gas and oil or oil and water or by gas, oil, and water. Therefore, it becomes necessary to further extend or modify Darcy’s law to include the simultaneous fluid flow of two or more fluid phases present in a porous medium. This is achieved by including the concept of effective permeability of each fluid phase instead of absolute permeability. The concept of effective permeability plays an important role in the reservoir flow processes when petroleum reservoirs are produced by primary recovery mechanism or immiscible displacement methods involving the injection of gas or water. It is under these circumstances that more than one fluid phase is flowing or is mobile through a porous medium; thus, the flow of one fluid phase interferes with the other.
