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This text is meant to provide a quick overview of some useful facts about parameters that play a role in displacement processes. The following items will be reviewed: wettability, permeability, relative permeability, capillary pressure, end-point mobility ratio and shock-front mobility ratio. In addition, capillary and Bond number, and the spreading coefficient will be shortly reviewed. For a detailed discussion, the interested reader is referred to standard textbooks like Fundamentals of Reservoir Engineering by Dake [1] and references therein.
Wettability
Wettability describes the relative preference of a rock to be covered by a certain phase. Rock is defined to be water-wet if the rock has (much) more affinity for water than for oil. In that case, a major part of the rock surface in the pores will be covered with a water layer. Clearly, wettability will be affected by the minerals present in the pores. Clean sandstone or quartz is extremely water-wet, but sandstone reservoir rock is usually found to be intermediate-wet. Intermediate wettability means that some pores are water wet and other pores are oil wet. Carbonates are believed to be more oil-wet than clastics: a major part of the rock surface in the pores is then covered with oil. It should be stressed that in practice extreme water-wetness or extreme oil-wetness is rare. Only for gas-liquid systems, one can safely assume that gas is always the non-wetting phase.
Basic reservoir properties like relative permeability, capillary pressure and resistivity depend strongly on wettability. It is therefore important that laboratory experiments in which these properties are measured are carried out on samples whose wettability is representative of the reservoir from which they are taken. Since no well-established techniques exist at present for downhole measurement of the wettability of reservoir material, it is not possible to state categorically whether a certain wettability is representative in a given case. In the laboratory, wettability is characterised usually by the so called Amott and USBM indices. These indices are derived from capillary pressure measurements, further discussed below.