Shale gas refers to natural gas (mainly methane) found in fine-grained, organic-rich rocks (gas shales).
When talking about shale gas, the word shale does not refer to a specific type of rock. Instead, it describes rocks with more fine-grained particles (smaller than sand) than coarse-grained particles, such as
Gas-prone shale may be associated with other resources, such as tight gas and coalbed methane (CBM), in areas where shale is interbedded with coal.
Gas shales are source rocks that have not released all of their generated hydrocarbons. In fact, source rocks that are "tight" or "inefficient" at expelling hydrocarbons may be the best prospects for shale gas potential.
In gas shale, shale is a reservoir, source rock and trap for natural gas. The natural gas found in these rocks is considered unconventional, similar to coalbed methane. Current technology makes this type of gas more accessible and economical today than it was 30 years ago.
A drop in pressure is required to get the gas to desorb (detach) from the clays and organic matter.
A drop in pressure is required to make the gas separate from the bitumen.
Shale gas is generated by any combination of
Thermogenic and biogenic gases may both exist in the reservoir.
Gas is stored in shales in three different ways:
The amount of adsorbed methane usually increases with an increase in organic matter or surface area of organic matter and/or clays.
Higher free-gas content in unconventional shale gas wells generally results in higher initial rates of production. This is because the free gas resides in fractures and pores and is easier to get out relative to adsorbed gas. The high, initial flow rates decline rapidly to a low, steady rate within about one year as adsorbed gas is slowly released from the shale.
Compared to most conventional reservoirs like sandstone, limestone or dolostone, gas shales have extremely low permeability.