Enhances the recovery of oil and gas from wells by fracturing formation rocks to release the hydrocarbons, allowing them to flow more easily through the rocks to the wellbore. Not all formations require such supplementary well completion techniques to permit extraction of hydrocarbons. Some rocks naturally contain abundant fractures and connected pore space that, although often only a millimeter or less across, allow fluids to move freely through them. Other rock formations, such as many shale gas reservoirs, are not permeable (i.e., the pores are not connected) and have few natural fractures and visible pore space. Gas or oil trapped within such impermeable rock can only be extracted by fracturing the rocks.

Hydraulic fracturing is performed soon after a well has been drilled and the metal well casing has been cemented into place by filling the annular space around the casing with cement. Selected segments of the wellbore are isolated, and specialized equipment is used to perforate holes through the production casing and cement of each segment. Water containing sand and chemical agents is then pumped at very high pressures, typically thousands of pounds per square inch, through the perforations into the surrounding rock. The intense pressure exerted by the water cracks the rock, creating minute fractures that propagate sometimes hundreds of feet away from the wellbore. Fracturing jobs are normally engineered to restrict the fractures to the target formation. The sand in the fracturing fluid, usually silica sand, is added as a “proppant”; that is, the fractures are propped open by the sand grains after the pressure is released. Although the fractures are held open only the width of a sand grain, it is enough to allow hydrocarbons trapped in the rocks to flow to the wellbore. Some wells are hydrofractured in more than one producing horizon, depending on where oil and gas occur in the subsurface.

Fracturing Fluids:
Fluids used for hydraulic fracturing consist primarily of water and a proppant (usually sand), with various additives that serve different purposes. Water and sand make up 98% or more of the fluid, while the additives constitute 2% or less. The additives used vary according to site-specific characteristics of the well, the target formation(s), the water source, and individual company practices. Some companies keep the compositions of their frack fluids confidential for proprietary reasons, or simply list all the ingredients and keep their relative percentages confidential, whereas other companies disclose the exact composition of their frack fluids. Companies can voluntarily disclose the chemical additives they use for hydraulic fracturing on the web-based registry.