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==Buoyancy forces in reservoir fluids==
==Buoyancy forces in reservoir fluids==
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<gallery mode=packed heights=500px widths=500px>
Mem91BuoyancyForcesFig26.jpg|{{figure number|3}}Water saturation decreases with height in an oil column. The volume of water is a function of the balance of capillary forces pulling the water up from the oil-water interface and the force of [[gravity]] acting together with the density contrast between the reservoir fluids, tending to pull the water down.
Mem91BuoyancyForcesFig26.jpg|{{figure number|3}}Water saturation decreases with height in an oil column. The volume of water is a function of the balance of capillary forces pulling the water up from the oil-water interface and the force of gravity acting together with the density contrast between the reservoir fluids, tending to pull the water down.
Mem91BuoyanceForcesFig27.jpg|{{figure number|4}}The shape of the curves on a capillary pressure plot reflects the grain sorting and the connection of pores and pore throats within the various rock types. The longer the plateau shown by the capillary curve, the better is the reservoir quality of the rock (from Sneider et al).<ref>Sneider, R. M., F. H. Richardson, D. D. Paynter, R. E. Eddy, and I. A. Wyant, 1977, Predicting reservoir rock geometry and continuity in Pennsylvanian reservoirs, Elk City, Oklahoma: Journal of Petroleum Technology, v. 29, no. 7, SPE Paper 6138, p. 851–866.</ref> Reprinted with permission from SPE.
Mem91BuoyanceForcesFig27.jpg|{{figure number|4}}The shape of the curves on a capillary pressure plot reflects the grain sorting and the connection of pores and pore throats within the various rock types. The longer the plateau shown by the capillary curve, the better is the reservoir quality of the rock (from Sneider et al).<ref>Sneider, R. M., F. H. Richardson, D. D. Paynter, R. E. Eddy, and I. A. Wyant, 1977, Predicting reservoir rock geometry and continuity in Pennsylvanian reservoirs, Elk City, Oklahoma: Journal of Petroleum Technology, v. 29, no. 7, SPE Paper 6138, p. 851–866.</ref> Reprinted with permission from SPE.
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The two forces acting on the fluids in the pore space are controlled by physical laws. The equation for the [[buoyancy pressure]] is given by
The two forces acting on the fluids in the pore space are controlled by physical laws. The equation for the [[buoyancy pressure]] is given by
:<math>P_b = (\rho_w - \rho_{nw})gh</math><br>
:<math>P_b = (\rho_w - \rho_{nw})gh</math><br>
where P<sub>b</sub> is the buoyancy pressure; ρ<sub>w</sub> and ρ<sub>nw</sub> are the specific gravities of the wetting and nonwetting phases respectively; g is the acceleration of gravity; and h is the height above the free-water level.
where P<sub>b</sub> is the buoyancy pressure; ρ<sub>w</sub> and ρ<sub>nw</sub> are the specific gravities of the wetting and nonwetting phases respectively; g is the acceleration of [[gravity]]; and h is the height above the free-water level.
The equation for capillary forces is given by
The equation for capillary forces is given by