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Where two immiscible liquids, or a liquid and a solid, are in contact with each other, the surface molecules of each substance are also attracted to each other across the interface by weak intermolecular forces.

Where two immiscible liquids, or a liquid and a solid, are in contact with each other, the surface molecules of each substance are also attracted to each other across the interface by weak intermolecular forces.

Therefore, at a solid-liquid boundary interface, the molecules of the liquid are subjected to opposing forces of attraction; in the first instance, the liquid is attracted by its own molecules and secondly by the molecules of the solid across the boundary. The degree to which force is dominant controls what is termed the wettability (Vavra et al., 1992). For instance, glass is water wet, in that water will spread across the surface of a glass plate as a thin sheet. The adhesive attraction of the water for the glass is greater than the cohesive attraction of the water molecules for each other. A liquid such as mercury will form globules on a glass surface and is nonwetting. The cohesive attraction of the mercury molecules for each other is greater than the adhesive attraction of glass and mercury ([[:File:M91FG24.JPG|Figure 1]]).

Therefore, at a solid-liquid boundary interface, the molecules of the liquid are subjected to opposing forces of attraction; in the first instance, the liquid is attracted by its own molecules and secondly by the molecules of the solid across the boundary. The degree to which force is dominant controls what is termed the wettability.<ref name=Vavra>Vavra, C. L., J. G. Kaldi, and R. M. Sneider, 1992, [http://archives.datapages.com/data/bulletns/1992-93/data/pg/0076/0006/0000/0840.htm Geological applications of capillary pressure: A review]: AAPG Bulletin, v. 76, no. 6, p. 840–850.</ref> For instance, glass is water wet, in that water will spread across the surface of a glass plate as a thin sheet. The adhesive attraction of the water for the glass is greater than the cohesive attraction of the water molecules for each other. A liquid such as mercury will form globules on a glass surface and is nonwetting. The cohesive attraction of the mercury molecules for each other is greater than the adhesive attraction of glass and mercury ([[:File:M91FG24.JPG|Figure 1]]).

Where a reservoir rock is water wet, the water forms a thin film over most of the grain surfaces and will also fill the smaller pores. The oil or gas will occupy the remaining, more central volume of the pore system. Conversely, in a reservoir that is oil wet, it is the oil that covers the grain surface and occupies the smaller pores; the water is located centrally within the pore structure (Anderson, 1986).

Where a reservoir rock is water wet, the water forms a thin film over most of the grain surfaces and will also fill the smaller pores. The oil or gas will occupy the remaining, more central volume of the pore system. Conversely, in a reservoir that is oil wet, it is the oil that covers the grain surface and occupies the smaller pores; the water is located centrally within the pore structure (Anderson, 1986).

The equation for capillary forces is given by

The equation for capillary forces is given by

:<math>P_c = \frac{2 \sigma \cos \theta}{r}</math><br>

:<math>P_c = \frac{2 \sigma \cos \theta}{r}</math><br>

where Pc is the capillary pressure, sigma is the interfacial tension, thetas is the contact angle between the wetting fluid and the solid surface, and r is the capillary (pore throat) radius (Vavra et al., 1992).

where Pc is the capillary pressure, sigma is the interfacial tension, thetas is the contact angle between the wetting fluid and the solid surface, and r is the capillary (pore throat) radius.<ref name=Vavra />

The volume of water remaining at a given height in a reservoir is a function of the balance of capillary forces pulling the water up from the hydrocarbon-water interface and the force of gravity acting together with the density contrast between the reservoir fluids, acting to pull the water down (Arps, 1964). Thus, a given part of the pore space within the hydrocarbon leg can contain both hydrocarbons and water. The fraction (percentage) of water to total fluid volume is termed the water saturation.

The volume of water remaining at a given height in a reservoir is a function of the balance of capillary forces pulling the water up from the hydrocarbon-water interface and the force of gravity acting together with the density contrast between the reservoir fluids, acting to pull the water down (Arps, 1964). Thus, a given part of the pore space within the hydrocarbon leg can contain both hydrocarbons and water. The fraction (percentage) of water to total fluid volume is termed the water saturation.

Capillary pressure is typically measured in the laboratory by injecting mercury under pressure into a core plug. The mercury is a nonwetting phase, which replicates the behavior of hydrocarbons in reservoir rocks. The procedure simulates the entry of hydrocarbons into a water-wet rock and the way in which buoyancy pressure increases with height in the hydrocarbon column.

Capillary pressure is typically measured in the laboratory by injecting mercury under pressure into a core plug. The mercury is a nonwetting phase, which replicates the behavior of hydrocarbons in reservoir rocks. The procedure simulates the entry of hydrocarbons into a water-wet rock and the way in which buoyancy pressure increases with height in the hydrocarbon column.

Mercury will not enter the rock immediately. The pressure required to do this will depend on the radius of the pore throats, the contact angle, and the mercury-air interfacial tension. The pressure at which the mercury effectively enters the pore network is termed the displacement or entry pressure (Vavra et al., 1992). Lower entry pressures are found in the better quality reservoir rocks, that is, those with larger pore throat diameters. A cap rock with tiny capillaries, shale for instance, has a very high displacement pressure. The displacement pressure for a cap rock can be so high that the tightly bound water in the pore space of the shale will prevent the oil from entering and the oil remains trapped in the underlying reservoir rock (Berg, 1975; Schowalter, 1979).

Mercury will not enter the rock immediately. The pressure required to do this will depend on the radius of the pore throats, the contact angle, and the mercury-air interfacial tension. The pressure at which the mercury effectively enters the pore network is termed the displacement or entry pressure.<ref name=Vavra /> Lower entry pressures are found in the better quality reservoir rocks, that is, those with larger pore throat diameters. A cap rock with tiny capillaries, shale for instance, has a very high displacement pressure. The displacement pressure for a cap rock can be so high that the tightly bound water in the pore space of the shale will prevent the oil from entering and the oil remains trapped in the underlying reservoir rock (Berg, 1975; Schowalter, 1979).

With increasing injection pressure, more and more mercury is forced into the rock. The shape of the curves on a capillary pressure plot reflects the grain sorting and the connection of pores and pore throats. The longer the plateau shown by the capillary curve, the better the reservoir quality. Poorly sorted, fine-grained sediment with narrow pore throats will retain water to higher pressures than coarser grained, better sorted sediments. A homogenous reservoir rock can be represented by a single capillary pressure curve. By contrast, a heterogenous reservoir will have a family of rock types, each with its own capillary pressure curve ([[:File:Mem91BuoyanceForcesFig27.jpg|Figure 4]]).

With increasing injection pressure, more and more mercury is forced into the rock. The shape of the curves on a capillary pressure plot reflects the grain sorting and the connection of pores and pore throats. The longer the plateau shown by the capillary curve, the better the reservoir quality. Poorly sorted, fine-grained sediment with narrow pore throats will retain water to higher pressures than coarser grained, better sorted sediments. A homogenous reservoir rock can be represented by a single capillary pressure curve. By contrast, a heterogenous reservoir will have a family of rock types, each with its own capillary pressure curve ([[:File:Mem91BuoyanceForcesFig27.jpg|Figure 4]]).

Petrophysicists will use capillary pressure curves as the basis for deriving a water saturation versus height relationship for a reservoir (Vavra et al., 1992).

Petrophysicists will use capillary pressure curves as the basis for deriving a water saturation versus height relationship for a reservoir.<ref name=Vavra />

==Relative permeability==

==Relative permeability==