Hydraulic fracturing (AKA “fracking”) is the process of extracting liquid petroleum oil or gas from an underground reservoir that would otherwise be inaccessible for extraction. Hydraulic fracturing (HF) is a type of injection mining, an extraction processes where fluids and/or other materials are injected into the ground, and has been carried out in practice for over 50 years in the United States. HF is a specific type of injection mining that involves the injection of liquids (eg. water, or a “slurry” of water and other compounds) under hydraulic pressure into wellbores (the entry point to a well) where those fluids increase the pressure of deep, but permeable rock like sandstone or shale. As the pressure inside the permeable rock increases, the forces pushing out on the rock to separate it begin to outweigh the forces pushing down on the rock keeping it together, and cracks (fractures) in the rock begin to form.
As the fractures increase in size, liquid petroleum and natural gases begin to seep out of the surround rock, travel through the fractures, and into the borehole drilled down through the wellbore. The wellbore is a specially designed structure at the entrance to the well which has been drilled out and reinforced with cement and steel to prevent leakage of fluids into the external environment and ensure as much injected water/slurry is recovered as possible. Once the well has been fractured, oil and gas will be collected at the wellbore where it is later transported for further refining. Complications can occur at the wellbore that result in the leakage of methane and other gases, which makes the process of hydraulic fracturing itself (not only the refined oil and gas that is burned later) a known minor contributor towards global greenhouse gas emissions.
Recovered injection water/slurry must also be recovered and treated, depending on its contents, before the water can be re-released into the ecosystem. The time required to properly treat and recover usable water from the injection well makes large quantities unusable for human consumption, which places a major encumbrance on water availability in areas where water resources are limited. Furthermore, recovered water/slurry can contain mildly radioactive components called radionuclides that might bioaccumalate in the environment if directly released, making water treatment processes even more complicated. In addition to its chemical waste problems, hydraulic fracturing has also been noted to cause minor earthquakes in proximity to the injection well, but the magnitude and extent of these earthquakes is an active area of research.
In spite of its complications, hydraulic fracturing is still commonly used today globally because of its propensity to extend the life of aging wells, or make previously unextractable wells extractable. In the United States, there were over 150,000 Class II injection wells as of 2016. Class II oil and gas related injection wells include disposal wells, which are wells where already contaminated (“slurry”) is re-injected back into the ground for permanent disposal; enhanced extraction wells, which are new or old wells where extraction of oil and gas is enhanced by the injection/fracturing process; and strategic reserve wells (e.g., US strategic oil reserve), where already recovered oil and gas are injected into an underground system for later recovery.