In the
hydrocarbon
In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons are examples of group 14 hydrides. Hydrocarbons are generally colourless and hydrophobic, and their odors are usually weak or ...
industry water contact is the elevation above which fluids other than water can be found in the
pore
Pore may refer to:
Biology Animal biology and microbiology
* Sweat pore, an anatomical structure of the skin of humans (and other mammals) used for secretion of sweat
* Hair follicle, an anatomical structure of the skin of humans (and other m ...
s of a rock.
For example, in a traditional hand-excavated
water well
A well is an excavation or structure created in the ground by digging, driving, or drilling to access liquid resources, usually water. The oldest and most common kind of well is a water well, to access groundwater in underground aquifers. T ...
, the level at which the water stabilizes represents the water table, or the elevation in the rock where air starts to occupy the rock pores.
In most situations in the hydrocarbon industry the term is qualified as being an oil-water contact (abbreviated to "OWC") or a gas-water contact ("GWC"). Often there is also a gas-oil contact ("GOC").
In an oil or gas
field
Field may refer to:
Expanses of open ground
* Field (agriculture), an area of land used for agricultural purposes
* Airfield, an aerodrome that lacks the infrastructure of an airport
* Battlefield
* Lawn, an area of mowed grass
* Meadow, a grass ...
, hydrocarbons migrate into rocks and can be trapped if there is a
permeability barrier to prevent upward escape. Gas and oil are lighter than water, so they will form a bubble at the high end of the "trap" formed by the impermeable barriers. A simple physical model of this would be a coffee cup held upside down underwater with an air bubble occupying the highest portion of the cup's interior. The base of the bubble is the water contact.
Capillary action
Capillary action (sometimes called capillarity, capillary motion, capillary rise, capillary effect, or wicking) is the process of a liquid flowing in a narrow space without the assistance of, or even in opposition to, any external forces li ...
can obscure the true water contact in permeable media like
sandstone
Sandstone is a clastic sedimentary rock composed mainly of sand-sized (0.0625 to 2 mm) silicate grains. Sandstones comprise about 20–25% of all sedimentary rocks.
Most sandstone is composed of quartz or feldspar (both silicates ...
.
Capillary pressure prevents the hydrocarbons from expelling all of the water in the pores, which creates a transition zone between the fully saturated hydrocarbon levels and the fully saturated water levels.
In poorly porous intervals, the oil-water, gas-water or gas-oil contacts can similarly be obscured, which makes estimation of
hydrocarbon reserves
Oil and gas reserves denote ''discovered'' quantities of crude oil and natural gas (oil or gas fields) that can be profitably produced/recovered from an approved development. Oil and gas reserves tied to approved operational plans filed on the ...
difficult. Descriptions of the well's petrophysics will then often further qualify to delineate a gas-down-to, oil-up-to, oil-down-to and water-up-to line, clearly showing the uncertainties involved.
References
Petroleum geology
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