Fracking – properly known as hydraulic fracturing – is a so-called “unconventional” method of extracting oil and gas.
CONVENTIONAL vs UNCONVENTIONAL
CONVENTIONAL GAS RESERVOIR
In conventional gas extraction, the hydrocarbons accumulate in a porous layer of rock, such as sandstone or limestone, which is overlain by an impermeable layer – shown in the diagram as “impermeable shale clay”. Once a well is drilled, the gas will typically flow up the well under its own pressure
Shale gas on the other hand is generally found at much deeper levels. Shale is thousands of times less permeable than sandstone: the hydrocarbons it contains can only be released by first of all fracturing the rock, and then by pumping down ‘frack fluid’ at tremendous pressure – 700 atmospheres or 10,000 pounds per square inch. Frack fluid is typically 95% water (hence “hydraulic” fracturing), around 4% silica sand to act as a “proppant” to keep the fractures open, with around 1% of other chemical additives (scale inhibitor, biocide, friction reducer, surfactant, acid)
Unlike conventional gas extraction, multiple bore holes and horizontal shafts are needed, because gas is only liberated from where the rock has been fractured