Developed in the 1940s, the process of hydraulic fracturing has become increasingly important in oil drilling. It comes in handy with "tight" reservoirs -- where the rocks containing the oil don’t have large pores. This means that the oil flow from the rocks is weak, and drilling a simple well into the rock won’t get much of the oil out.
To help stimulate the well and drive out the trapped oil, drillers employ hydraulic fracturing. In this process, they inject water combined with chemicals into the well with enough pressure to create fractures in the rock formations -- fractures that can extend hundreds of feet long. To keep the fractures from closing again, drillers send down a proppant, which is a mixture of fluids, sand and pellets. These fractures allow oil to flow more freely from the rock.
According to the American Petroleum Institute, in the United States alone, hydraulic fracturing has helped pump an extra 7 billion barrels of oil from the ground.