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Positions of initial fluid contacts are critical for field reserve estimates and for field development. Typically, the position of fluid contacts are first determined within control wells and then extrapolated to other parts of the field.

Positions of initial fluid contacts are critical for field reserve estimates and for field development. Typically, the position of fluid contacts are first determined within control wells and then extrapolated to other parts of the field.

Definitions of fluid contacts are based on comparison to [[capillary pressure]] curves ([[:file:fluid-contacts_fig1.png|Figure 1]]) . The ''[[free water surface]]'' is the highest elevation at which the pressure of the hydrocarbon phase is the same as that of water. The ''hydrocarbon-water'' (''[[oil-water contact|oil-water]]'' or ''[[gas-water contact|gas-water]]'') ''contact'' is the lowest elevation at which mobile hydrocarbons occur. The ''[[fluid transition zone|transition zone]]'' is the elevation range in which water is coproduced with hydrocarbons. The ''[[gas-oil contact]]'' is the elevation above which gas is the produced hydrocarbon phase.

Definitions of fluid contacts are based on comparison to [[capillary pressure]] curves ([[:file:fluid-contacts_fig1.png|Figure 1]]) . The ''[[free water surface]]'' is the highest elevation at which the pressure of the hydrocarbon phase is the same as that of water. The ''hydrocarbon-water'' (''[http://www.glossary.oilfield.slb.com/en/Terms.aspx?LookIn=term%20name&filter=oil-water%20contact oil-water]'' or ''[http://www.glossary.oilfield.slb.com/en/Terms.aspx?LookIn=term%20name&filter=gas-water+contact gas-water]'') ''contact'' is the lowest elevation at which mobile hydrocarbons occur. The ''[[fluid transition zone|transition zone]]'' is the elevation range in which water is coproduced with hydrocarbons. The ''[http://www.glossary.oilfield.slb.com/en/Terms.aspx?LookIn=term%20name&filter=gas-oil%20contact gas-oil contact]'' is the elevation above which gas is the produced hydrocarbon phase.

[[file:fluid-contacts_fig1.png|thumb|{{figure number|1}}Contact definitions and relationship of contacts in a pool (right) to reservoir capillary pressure and fluid production curves (left). The free water surface is the highest elevation with the same oil and water pressure (zero capillary pressure). The oil-water contact corresponds to the displacement pressure (DP) on the capillary pressure curve. The transition zone is the interval with co-production of water and hydrocarbons. The fraction of co-produced water is shown by the dashed line on the left. The gas-oil contact is controlled by the volume of gas in the trap, not the capillary properties.]]

[[file:fluid-contacts_fig1.png|thumb|{{figure number|1}}Contact definitions and relationship of contacts in a pool (right) to reservoir capillary pressure and fluid production curves (left). The free water surface is the highest elevation with the same oil and water pressure (zero capillary pressure). The oil-water contact corresponds to the displacement pressure (DP) on the capillary pressure curve. The transition zone is the interval with co-production of water and hydrocarbons. The fraction of co-produced water is shown by the dashed line on the left. The gas-oil contact is controlled by the volume of gas in the trap, not the capillary properties.]]