The oil, gas and water molecules in the formations are under pressure naturally. By drilling a well and inserting pipe, a producer can control the pressure in the pipe. When the producer lowers the pressure in the pipe, the molecules of oil, gas, and water flow to the point of least pressure, the pipe, and then to the surface tanks or pipelines. You are fortunate that the operator can control the change in pressure and therefore the production rate of the well otherwise they would have a blowout as occurred in the Gulf of Mexico last year and millions of barrels of oil were wasted. With oil wells, the slower the oil is produced, the longer it will last before the formation water starts coming in.