Hydraulic fracturing, or "fracking" as it's commonly called, is an oil and gas production technique that has been around since the 1940s, but has been the key enabler of the recent shale oil and shale gas boom in the United States. Until fairly recently, most oil and gas produced in the United States and around the world has been produced from sandstone reservoirs. While these are rock formations, they are derived from ancient sand deposits that were deposited at the bottom of seas and lakes, then compacted by the pressure of subsequent soil layers and cemented by minerals that percolate through the sand. Sandstone is characterized by relatively high porosity, i.e., there is sufficient space between the grains of sand to allow fluid to flow. Conversely, when the depositional material is clay, which consists of much finer particles, the result is shale which is characterized by low porosity.
The oil industry has known for decades that some deposits of shale rock contain significant quantities of oil and natural gas. But the rock was considered too impermeable (or "tight") for the oil and/or gas to flow sufficiently to allow commercial production. In recent years, advances in two technologies have overcome the challenge to produce oil and gas in low porosity shale reservoirs:
- The first was "fracking", which involves injecting water, sand and chemicals under high pressure into the oil or gas bearing formation to create cracks in the reservoir rock. Fracking has actually been used since the late 1940s, most often to stimulate wells in traditional sandstone reservoirs that get plugged over time. It is estimated that fracking is used to some extent in as much as 90% of all wells. When fracking is used in shale reservoirs, the cracks create passageways that allow the otherwise trapped oil and gas to flow to the well bore .
- The second technology is horizontal drilling. Horizontal drilling started out more modestly as directional drilling, which was developed for offshore use. Expensive offshore platforms require the ability to drill multiple wells from a single location and reach reservoir drainage points that are often quite distant laterally. Continuing research into new drilling and completion equipment and techniques developed during the 1980s and 1990s allowed directional drilling to go completely horizontally. Since oil bearing reservoirs are generally horizontal, or near horizontal layers of rock, a single well could now access more productive rock than was ever possible with a conventional vertical well.
Modern day fracking is the combination of these two technologies. Horizontal drilling allows the well to follow the reservoir for thousands of feet. Fracking then opens up channels to allow oil or gas to flow over long stretches of the well. As a result, oil companies are able to drill into shale reservoirs and produce large quantities of oil and gas.