Imagine 300 million years ago, just about the end of the Carboniferous period. Pangaea is coming together nicely and you are a simple phytoplankton floating in the upper sunlit layer of the ocean. You are not alone; this is a multicultural pelagic zone. Floating or swimming along with you are an assortment of zooplankton, cephalopods, the earliest squid and other Paleozoic marine plants and animals.
Life is good, but it is also short. Soon you and so many of your other planktonic kin, as well as a whole menagerie of marine animals perish and sink to the sea floor. To add insult to injury, lots of other dead plants and animals continue to rain down on top of your decaying carcass. Then the deposition of silt, sand and mud leaves you buried thousands of feet below the sea floor. Forced further and further underground, you are entombed in newly formed sedimentary rock.
As the millennia pass, tectonics cause the organics left over from your short life to shift deeper under the earth’s crust. With this plunge, the heat and the pressure increases, causing chemical reactions that change organic molecules first into a waxy material found in oil shales (kerogen) and ultimately into liquid and gaseous hydrocarbons (oil and natural gas).
Pressure beneath the earth’s surface force oil and gas to move through porous (lots of cracks and holes) and permeable (interconnected cracks and holes) sedimentary rock. Eventually the migrating oil and gas encounter overlying or adjacent non-porous rock causing the fossil fuels to pool in vast and easily accessible reserves. Some fossil fuels however, are not as easy to get.
In layers of rock with finer and poorly connected pore spaces, fossil fuels don’t flow as freely. Fine grained, low permeability rocks such as shale are considered ‘tight’ formations. Oil and gas cannot be pumped from shale beds without fracturing the rock and holding the new spaces open. Hydraulic fracturing, (a.k.a. fracking) increases the porosity and permeability of tight formations, allowing natural gas and oil to be economically removed.
The process can be seen in the animation above.
Fracking injects fluid (fracking fluid) under high pressure into existing wells. New technologies have allowed for horizontal drilling. Wells can now run laterally for miles along layers of oil and gas rich shale. The pressure of the injected fracking fluid fractures the impermeable rock, creating cracks and causing pore spaces to open, and more importantly, become well connected.
To maintain spaces once the pressurization injection stops, proppants are included in the fracking fluid. A proppant is a solid material, such as sand and ceramics. Fracking fluid carries the proppants into the fractured rocks keeping the new spaces open, allowing for the easy removal of the fossil fuels.
Fracking fluid is a mixture of water, proppants and chemical additives and one of the more controversial aspects of the process. While fracking fluid is mainly water, thousands of gallons of potentially harmful chemicals including acids, biocides and detergents are used in each fracturing. Hydraulic fracturing companies are not required to (and generally do not) disclose the chemicals used in the fracking fluid, adding fuel to the controversy.
After fracturing, some fracking fluid remains underground and some is returned to the surface (flowback water) along with the oil and natural gas removed from the previously impermeable sedimentary rock formation. In addition to flowback water, the naturally occurring water in the rock formation known as produced water containing large amounts of dissolved salts returns to the surface. The natural gas and oil is separated from the flowback and produced water at the well site and returned fluid is stored on-site in tanks or lined pits.