What is Shale Gas?
This Colorado Group shale outcrop occurs along the Smoky River, south of Peace River, Alberta.
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
- shale (fissile) and mudstone (non-fissile)
- siltstone
- fine-grained sandstone interlaminated with shale or mudstone
- carbonate rocks.
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 Creation
Shale gas is generated by any combination of
- primary thermogenic degradation of organic matter
- secondary thermogenic cracking of oil
- biogenic degradation of organic matter.
Thermogenic and biogenic gases may both exist in the reservoir.
Shale Gas Storage
Gas is stored in shales in three different ways:
- Adsorbed gas is gas attached to organic matter or to clays.
- Free gas is gas held within the tiny spaces in the rock (pores, porosity or micro-porosity) or in spaces created by the rock cracking (fractures or microfractures).
- Solution gas is gas held within other liquids, such as bitumen and oil.
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.
Shale Gas Permeability and Porosity
Compared to most conventional reservoirs like sandstone, limestone or dolostone, gas shales have extremely low permeability.
- Effective bulk permeability in gas shale is typically much less than 0.1 millidarcies (md), although exceptions exist where the rock is naturally fractured; for example, the well-fractured Antrim shale in the Michigan Basin in the U.S.
- In most cases, it is normal for the well to need artificial stimulation, such as fracturing, to increase permeability to the well. This helps the well to produce gas in economical quantities.
- The role of natural microfractures in reservoir production or in assisting artificial fracturing is not well understood.
