Saturation is defined as that fraction, or percent, of the pore volume occupied by a particular fluid (oil, gas, or water). This property is expressed mathematically by the following relationship:
Applying the above mathematical concept of saturation to each of the fluid gives..
Volume of oil
So =----- ……….1
Volume of gas
Volume of water
Sw = ----……… 3
So = oil saturation
Sg = gas saturation
Sw = water saturation
Thus, all saturation values are based on pore volume and not on the gross reservoir volume. he saturation of each individual phase ranges between zero to 100 percent. By definition, the sum of the saturations is 100%, therefore
Sg + So + Sw = 1.0………………..4
The fluids in most reservoirs are believed to have reached a state of equilibrium and, therefore, will have become separated according to their density, i.e., oil overlain by gas and underlain by water. In addition to the bottom (or edge) water, there will be connate water distributed throughout the oil and gas zones. The water in these zones will have been reduced to some irreducible minimum. The forces retaining the water in the oil and gas zones are referred to as capillary forces because they are important only in pore spaces of capillary size. Connate (interstitial) water saturation Swc is important primarily because it reduces the amount of space available between oil and gas. It is generally not uniformly distributed throughout the reservoir but varies with permeability, lithology, and height above the free water table. Another particular phase saturation of interest is called the critical saturation and it is associated with each reservoir fluid. The definition and the significance of the critical saturation for each phase is described below.
Critical oil saturation, Soc
For the oil phase to flow, the saturation of the oil must exceed a certain value which is termed critical oil saturation. At this particular saturation, the oil remains in the pores and, for all practical purposes, will not flow.
Residual oil saturation, Sor
During the displacing process of the crude oil system from the porous media by water or gas injection (or encroachment) there will be some remaining oil left that is quantitatively characterized by a saturation value that is larger than the critical oil saturation. This saturation value is called the residual oil saturation, Sor. The term residual saturation is usually associated with the non wetting phase when it is being displaced by a wetting phase.
Movable oil saturation, Som
Movable oil saturation Som is another saturation of interest and is defined as the fraction of pore volume occupied by movable oil as expressed by the following equation:
Som = 1 - Swc - Soc where Swc = connate water saturation Soc = critical oil saturation
Critical gas saturation, Sgc
As the reservoir pressure declines below the bubble-point pressure, gas evolves from the oil phase and consequently the saturation of the gas increases as the reservoir pressure declines. The gas phase remains immobile until its saturation exceeds a certain saturation, called critical gas saturation, above which gas begins to move.
Critical water saturation, Swc
The critical water saturation, connate water saturation, and irreducible water saturation are extensively used interchangeably to define the maximum water saturation at which the water phase will remain immobile.