Changes

==Relative permeability==

==Relative permeability==

[[File:M91FG30.JPG|thumb|300px|{{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, 1992). 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, 1992). Reprinted with permission from the AAPG.

Permeability is the measure of the ease of movement of fluid through the pore space in a rock. Where more than one fluid phase is present (e.g., oil and water), the permeability of one phase is reduced by the presence of the other phase within the pore system. In this instance, the permeability to a particular fluid is called the relative permeability (Hawkins, 1992).

Permeability is the measure of the ease of movement of fluid through the pore space in a rock. Where more than one fluid phase is present (e.g., oil and water), the permeability of one phase is reduced by the presence of the other phase within the pore system. In this instance, the permeability to a particular fluid is called the relative permeability (Hawkins, 1992).

For water-wet reservoirs, Craig (1971) gave some general water-oil relative permeability end points. Water will start flowing along with oil once the water saturation is greater than roughly 20–25%. This value is the irreducible water saturation; the volume of water bound and immobilized by adhesive attraction to the surface of the pores. Oil will stop flowing where the water saturation in the rock is about 70–80%. When this happens, there will not be enough oil to provide a continuous volume throughout the rock. Interfacial tension will cause the oil stream to snap off and fragment into immobilized globules and strands of residual oil.

For water-wet reservoirs, Craig (1971) gave some general water-oil relative permeability end points. Water will start flowing along with oil once the water saturation is greater than roughly 20–25%. This value is the irreducible water saturation; the volume of water bound and immobilized by adhesive attraction to the surface of the pores. Oil will stop flowing where the water saturation in the rock is about 70–80%. When this happens, there will not be enough oil to provide a continuous volume throughout the rock. Interfacial tension will cause the oil stream to snap off and fragment into immobilized globules and strands of residual oil.

The relative permeability end points may vary significantly between reservoirs; the quoted values can be considered as approximate. The relative permeability of water and oil as a function of water saturation is illustrated by relative permeability curves ([[:File:M91FG30.JPG|Figure 5]]).

The relative permeability end points may vary significantly between reservoirs; the quoted values can be considered as approximate. The relative permeability of water and oil as a function of water saturation is illustrated by relative permeability curves ([[:File:M91FG28.JPG|Figure 5]]).

==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 (Jennings, 1987). Several zones can be defined (Figure 29):

# ''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.

==Tilted oil-water contacts==

==Tilted oil-water contacts==

[[File:M91FG30.JPG|thumb|300px|{{figure number|30}}Different configurations of oil-water contact are shown. A hydrodynamically tilted oil-water contact results from the movement of water under the oil column. A perched oil-water contact forms where water is locally trapped when the oil migrated into the reservoir.]]

[[File:M91FG30.JPG|thumb|300px|{{figure number|7}}Different configurations of oil-water contact are shown. A hydrodynamically tilted oil-water contact results from the movement of water under the oil column. A perched oil-water contact forms where water is locally trapped when the oil migrated into the reservoir.]]

Some fields have a hydrodynamically tilted oil-water contact (Figure 30). This results from variations in aquifer pressure associated with the movement of water in the subsurface, mainly as a result of a mobile artesian aquifer or basin dewatering (Hubbert, 1953). The tilts are toward the direction of reducing pressure and are generally less than 2deg in gradient (Dennis et al., 2000, 2005).

Some fields have a hydrodynamically tilted oil-water contact ([[:File:M91FG30.JPG|Figure 7]]). This results from variations in aquifer pressure associated with the movement of water in the subsurface, mainly as a result of a mobile artesian aquifer or basin dewatering (Hubbert, 1953). The tilts are toward the direction of reducing pressure and are generally less than 2deg in gradient (Dennis et al., 2000, 2005).

==Perched oil-water contacts==

==Perched oil-water contacts==