<P> Water (usually salty) may be present below the hydrocarbons . Water, as with all liquids, is compressible to a small degree . As the hydrocarbons are depleted, the reduction in pressure in the reservoir allows the water to expand slightly . Although this unit expansion is minute, if the aquifer is large enough this will translate into a large increase in volume, which will push up on the hydrocarbons, maintaining pressure . </P> <P> With a water - drive reservoir the decline in reservoir pressure is very slight; in some cases the reservoir pressure may remain unchanged . The gas / oil ratio also remains stable . The oil rate will remain fairly stable until the water reaches the well . In time, the water cut will increase and the well will be watered out . </P> <P> The water may be present in an aquifer (but rarely one replenished with surface water). This water gradually replaces the volume of oil and gas that is produced out of the well, given that the production rate is equivalent to the aquifer activity . That is, the aquifer is being replenished from some natural water influx . If the water begins to be produced along with the oil, the recovery rate may become uneconomical owing to the higher lifting and water disposal costs . </P> <P> If the natural drives are insufficient, as they very often are, then the pressure can be artificially maintained by injecting water into the aquifer or gas into the gas cap . </P>

If a reservoir rock contains water the oil and natural gas will
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