FLUID SATURATION AND CAPILLARY PRESSURE

۴٫۱ Fluid Saturations
We have seen that the viability of a reservoir depends upon three critical parameters. The first two of
these are the porosity of the reservoir rock, which defines the total volume available for hydrocarbon
saturation, and the permeability, which defines how easy it is to extract any hydrocarbons that are
present. The final critical parameter is the hydrocarbon saturation, or how much of the porosity is
occupied by hydrocarbons. This, and the related gas and water saturations are controlled by capillary
pressure.
The pore space in a rock is occupied by fluids. In hydrocarbon reservoirs these fluids are hydrocarbon
gasses, oil and an aqueous brine. We define the pore fraction of each of these as Sg, So and Sw,
respectively. Hence, Sg + So + Sw = 1.
The amount of each of these fluids present at a given level in the reservoir depends upon gravity
(buoyancy) forces, which tend to stratify the reservoir fluids according to their density, external
hydrodynamic forces such as flow from a remote aquifer, and interfacial forces that act between the
various reservoir fluids and between the fluids and the rock matrix.