Usage
Any liquid-producing reservoir will have a 'reservoir pressure': some level of energy or potential that will force fluid (liquid, gas or both) to areas of lower energy or potential. The concept is similar to that of water pressure in a municipal water system. As soon as the pressure inside a production well is decreased below the reservoir pressure, the reservoir will act to fill the well back up, just like opening a valve on a water system. Depending on the depth of the reservoir and density of the fluid, the reservoir may or may not have enough potential to push the fluid to the surface - a deeper well or a heavier mixture results in a higher pressure requirement.
Most oil production reservoirs have sufficient potential to naturally produce oil and gas - which are light - during the early phases of production. Water - which is heavier than oil and much heavier than gas - often will eventually encroach into production, possibly causing the well to stop flowing entirely. Also, reservoir pressure will decrease as many reservoirs deplete, reducing the natural flow to below a profitable rate. At some point, economics can justify the cost of an artificial lift plan to continue or increase production. Most water-producing wells, by contrast, will need artificial lift from the very beginning of production because they do not benefit from the lighter density of oil and gas.
Read more about this topic: Artificial Lift
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