Changes

==Buoyancy==

==Buoyancy==

A free hydrocarbon phase rises in a water column because its density is less than that of water. This buoyancy force is proportional to the density difference and the height of the hydrocarbon column. It acts vertically. When the rising hydrocarbons encounter a low-[[permeability]] (high capillary entry pressure) sloping surface, they are deflected updip and the resultant force is decreased by an amount proportional to the slope of that surface.

A free hydrocarbon phase rises in a water column because its density is less than that of water. This buoyancy force is proportional to the density difference and the height of the hydrocarbon column. It acts vertically. When the rising hydrocarbons encounter a low-[[permeability]] (high capillary entry pressure) sloping surface, they are deflected updip and the resultant force is decreased by an amount proportional to the slope of that surface.

==Temperature and pressure effects on buoyancy==

==Temperature and pressure effects on buoyancy==

Rising temperature (T) increases the buoyancy force as the hydrocarbon is buried. The density of hydrocarbons decreases more rapidly than that of water as temperature increases. If the temperature is high enough, liquid hydrocarbons may alter to a gaseous phase. Rising pressure (P) decreases the buoyancy force as the hydrocarbons are buried. The density of hydrocarbons increases more rapidly than that of water as pressure increases. If the pressure is high enough, gaseous hydrocarbons may alter to a liquid phase.

Rising temperature (T) increases the buoyancy force as the hydrocarbon is buried. The density of hydrocarbons decreases more rapidly than that of water as temperature increases. If the temperature is high enough, liquid hydrocarbons may alter to a gaseous phase. Rising pressure (P) decreases the buoyancy force as the hydrocarbons are buried. The density of hydrocarbons increases more rapidly than that of water as pressure increases. If the pressure is high enough, gaseous hydrocarbons may alter to a liquid phase.

The phase diagram below summarizes the competing effects of pressure and temperature changes for a typical volatile oil.

[[:file:migration-of-petroleum_fig7-1.png|Figure 1]] summarizes the competing effects of pressure and temperature changes for a typical volatile oil.

[[file:migration-of-petroleum_fig7-1.png|thumb|{{figure number|7-1}}After McCain.<ref name=ch07r7>McCain, W., D., Jr., 1990, The Properties of Petroleum Fluids: Tulsa, PennWell Books, 548 p.</ref> Copyright: PennWell Publishing Co.]]

[[file:migration-of-petroleum_fig7-2.png|thumb|{{figure number|2}}After Klimenko.<ref name=ch07r3>Klimenko, A., P., 1983, Diffusion of gasses from hydrocarbon deposits, in Petroleum Geochemistry, Genesis, and Migration: AGI Reprint Series, vol. 1, p. 117–122.</ref> Copyright: AGI.]]

==Chemical potential==

==Chemical potential==

Diffusive forces spontaneously transfer hydrocarbons dissolved in water from areas of higher concentration to adjacent areas of lower concentration. The kilometers-long diffusion gradients of benzene and toluene within reservoirs demonstrate the effectiveness of this process. However, the lack of significant transport of benzene and toluene through barriers or seals indicates diffusion and active aqueous solution transport are minor mechanisms of the accumulation process. The figure below shows the diffusion of gas in water as a function of concentration at origin and distance from origin (free hydrocarbons).

Diffusive forces spontaneously transfer hydrocarbons dissolved in water from areas of higher concentration to adjacent areas of lower concentration. The kilometers-long diffusion gradients of benzene and toluene within reservoirs demonstrate the effectiveness of this process. However, the lack of significant transport of benzene and toluene through barriers or seals indicates diffusion and active aqueous solution transport are minor mechanisms of the accumulation process. [[:file:migration-of-petroleum_fig7-2.png|Figure 2]] shows the diffusion of gas in water as a function of concentration at origin and distance from origin (free hydrocarbons).

 

==Capillary imbibition==

==Capillary imbibition==