Ann: Operations Update, page-242

WikiMarcellus -- Marcellus Shale and Other Appalachian Plays

Flowback water (aka backflow water) is the murky, salty water from fracing natural gas wells. It consists of frac fluid which returns to the surface (aka the frac load recovery) as well as produced water. This water contains clay, dirt, metals, chemicals and even diesel that may have been added.
The frac load recovery can be anywhere from 15-40% percent of the volume of fluid that is injected, that is forced down a well, and it flows back over a period of 3-4 weeks after fracing--most of it within 7 to 10 days. More than half of the frac fluid remains in the formation. At a certain point there is a transition between primarily recovering frac fluid to that of produced water. Usually this point is difficult to distinguish, yet may be discerned from the different chemical signatures of the frac water versus the naturally occurring water produced by the formation. A typical flowback of frac fluids might run 40,000 bbl. After the initial 3-4 week post-fracing recovery of fluids, an additional 10,000 to 30,000 bbl of produced water may flow for up to two years.
Flowback water may be characterized as having high salinity and total dissolved solids (TDS). It is laden with the same fracing chemicals that were pumped into the well, in addition to any unique contaminants that are present in the rock formation water deep below. In addition to natural salinity of water in the formation, any fresh water that is forced down a well, when it is fraced, will tend to dissolve salts in the formation thus giving the recovered water very high salinity.