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When hydrocarbons migrate into a trap, the buoyancy force exerted by the lighter oil (or gas) will push the water that was previously in the pore space sideways and downward. However, not all of the water is displaced; some of it will be held by capillary forces within the pores. Narrower capillaries, pores with smaller pore throats, hold onto water the strongest.
When hydrocarbons migrate into a trap, the buoyancy force exerted by the lighter oil (or gas) will push the water that was previously in the pore space sideways and downward. However, not all of the water is displaced; some of it will be held by capillary forces within the pores. Narrower capillaries, pores with smaller pore throats, hold onto water the strongest.
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.