How does oil exploit? Many people think that it sprays out from the bottom of the reservoir, some think that it sprays out by beam pumping unit. Actually oil exploitation is quite complicated. When firstly exploiting, the oil would flow out because of the great formation pressure, which is called primary oil recovery; gradually, it needs water to push the oil out as the formation pressure is not so heavy, which is called secondary oil recovery, or water flooding; After a period of water flooding, big pore in sandstone underground has been got through by water, and the oil inside has been recovered. Then water injection is useless towards small pore.