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M91FG28.JPG|{{figure number|5}}When more than one fluid phase is present, the permeability of one phase is reduced by the presence of the other phases within the pore system. Relative permeability curves display these relationships. The plots show a water- displacing-oil relative permeability curve for a water-wet rock and a water-displacing-oil relative permeability curve for an oil-wet rock (modified from Hawkins).<ref name=Hawkins>Hawkins, J. T., 1992, [[Relative permeability]], in D. Morton-Thompson and A. M. Woods, eds., [http://store.aapg.org/detail.aspx?id=612 Development geology reference manual]: AAPG Methods in Exploration Series 10, p. 226–228.</ref> Reprinted with permission from the AAPG.

M91FG28.JPG|{{figure number|5}}When more than one fluid phase is present, the permeability of one phase is reduced by the presence of the other phases within the pore system. Relative permeability curves display these relationships. The plots show a water- displacing-oil relative permeability curve for a water-wet rock and a water-displacing-oil relative permeability curve for an oil-wet rock (modified from Hawkins).<ref name=Hawkins>Hawkins, J. T., 1992, [[Relative permeability]], in D. Morton-Thompson and A. M. Woods, eds., [http://store.aapg.org/detail.aspx?id=612 Development geology reference manual]: AAPG Methods in Exploration Series 10, p. 226–228.</ref> Reprinted with permission from the AAPG.

M91FG29.JPG|{{figure number|6}}The decrease in water saturation with height controls the producing behavior of an oil column. Redrawn from Jennings, 1987, with permission from the AAPG.

M91FG29.JPG|{{figure number|6}}The decrease in water saturation with height controls the producing behavior of an oil column. Redrawn from Jennings,<ref name=Jennings>Jennings, J. B., 1987, [http://archives.datapages.com/data/bulletns/1986-87/data/pg/0071/0010/1150/1196.htm Capillary pressure techniques: Application to exploration and development geology]: AAPG Bulletin, v. 71, no. 10, p. 1196–1209.</ref> with permission from the AAPG.

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==The static distribution of fluids in unproduced reservoirs==

==The static distribution of fluids in unproduced reservoirs==

The producing behavior in an oil column will vary according to the fluid saturations (Jennings, 1987). Several zones can be defined ([[:File:M91FG29.JPG|Figure 6]]):

The producing behavior in an oil column will vary according to the fluid saturations.<ref name=Jennings /> Several zones can be defined ([[:File:M91FG29.JPG|Figure 6]]):

# ''The zone of 100% oil production.'' This is located above the height where the water saturation is less than the relative permeability end point to water, e.g., less than 20% water saturation. The water is immobile and only oil will flow.

# ''The zone of 100% oil production.'' This is located above the height where the water saturation is less than the relative permeability end point to water, e.g., less than 20% water saturation. The water is immobile and only oil will flow.

# ''The oil-water transition zone.'' Both water and oil are produced in this interval. The water saturations here lie between the end points at which the relative permeability to water is zero and the relative permeability to oil is zero. In coarse-grained sediments, the transition zone may be less than a meter thick; in very fine-grained sediments, it may be many tens of meters thick or more. In some reservoirs, the entire oil column may be within the transition zone (Fanchi et al., 2002).  

# ''The oil-water transition zone.'' Both water and oil are produced in this interval. The water saturations here lie between the end points at which the relative permeability to water is zero and the relative permeability to oil is zero. In coarse-grained sediments, the transition zone may be less than a meter thick; in very fine-grained sediments, it may be many tens of meters thick or more. In some reservoirs, the entire oil column may be within the transition zone (Fanchi et al., 2002).  

# ''The zone of 100% water production.'' This is that part of the oil column where there is still a small volume of oil present. However, its relative permeability is zero, so the oil will not flow, whereas the water will.

# ''The zone of 100% water production.'' This is that part of the oil column where there is still a small volume of oil present. However, its relative permeability is zero, so the oil will not flow, whereas the water will.

# ''The 100% water level.'' This is the level at which the buoyancy pressure of the oil equals the capillary displacement pressure of the reservoir rock. The 100% water level is the level above which the reservoir rock has a water saturation less than 100% (Schowalter, 1979). It is effectively the base of the oil column, although some authorities state that a small volume of residual oil may be present below this level (e.g., Jennings, 1987).

# ''The 100% water level.'' This is the level at which the buoyancy pressure of the oil equals the capillary displacement pressure of the reservoir rock. The 100% water level is the level above which the reservoir rock has a water saturation less than 100% (Schowalter, 1979). It is effectively the base of the oil column, although some authorities state that a small volume of residual oil may be present below this level (e.g., Jennings<ref name=Jennings />).

# ''The free-water level.'' At some point below the base of the oil column in a water-wet reservoir is the free-water level. The free-water level is a horizontal surface where theoretically the water would stand in a large open hole unconstrained by the effects of capillary forces. At this point, the buoyancy pressure is zero.

# ''The free-water level.'' At some point below the base of the oil column in a water-wet reservoir is the free-water level. The free-water level is a horizontal surface where theoretically the water would stand in a large open hole unconstrained by the effects of capillary forces. At this point, the buoyancy pressure is zero.

The definition of an oil-water contact varies somewhat within the literature (Jennings, 1987). The commonly used definitions are:

The definition of an oil-water contact varies somewhat within the literature.<ref name=Jennings /> The commonly used definitions are:

# The base of the oil column corresponding to the 100% water level. This is the most frequently used definition for estimating the oil in place volume.

# The base of the oil column corresponding to the 100% water level. This is the most frequently used definition for estimating the oil in place volume.