The term "unconventional gas" actually covers three main types of natural gas resources: shale gas, tight gas and coalbed methane (also known as coal seam gas). Conventional and unconventional gases differ not by their chemical compositions (all these resources are natural gas) but rather by the geological characteristics of their reservoir rock.
Shale Gas and Tight Gas: Permeability Challenges
Hydrocarbons (mainly natural gas, but some oil as well) are trapped in subsurface formations called "reservoir rock." Despite the word’s connotation, however, these are not huge, continuous "pools" but rather minuscule pores between the grains that make up the rock matrix.
The quality of a reservoir rock is determined by its porosity and its permeability.
Porosity is the void space between the grains, and thus represents the rock’s capacity to contain fluids (liquid or gaseous hydrocarbons). A highly porous reservoir rock therefore can contain a large volume of oil or gas. But porosity alone will not suffice: the fluids must be able to flow, meaning that the pores must be interconnected. This characteristic, called permeability, is the measurement of the rock’s ability to transmit or flow the oil or gas.
A common feature of shale gas and tight gas is that both are trapped in very low-permeability rock – ultra-compact structures that prevent or sharply limit the migration of the gas.
The unit of permeability measurement is the Darcy. Permeability is one of the parameters by which conventional gas reservoirs can be distinguished from unconventional formations. A good-quality hydrocarbon trap will have permeability of 1 Darcy or more, while tight gas reservoirs, more compact than brick, may have permeability of only a few dozen microDarcy. The permeability values of gas shales are even lower – as little as one one-thousandth of the permeability of tight gas formations. The unit here is the nanoDarcy.
- Tight gas is trapped in ultra-compact reservoirs characterized by very low porosity and permeability. The rock pores that contain the gas are minuscule, and the interconnections between them are so limited that the gas can only migrate through them with great difficulty.
- Shale gas is extracted from a geological layer known as the "source rock" rather than from a conventional petroleum reservoir structure. This clay-rich sedimentary rock has naturally low permeability. The gas it contains is either adsorbed (i.e., closely "inserted" into the organic matter) or in a free state in the void spaces (pores) of the rock.
What is a source rock?
Source rock is the geological layer in which oil and gas are generated. It formed when organic-richsediments were deposited on the bottom of oceans or lakes, then gradually covered over by additional sediment layers. As they became more deeply buried, the sediments were consolidated into rock, and the organic matter was transformed into hydrocarbons (oil and natural gas). The changes are the result of the combined effects of subsurface pressure and temperature. These hydrocarbons tended to migrate upward through the pores and cracks of the surrounding rock, sometimes reaching the surface. However, some hydrocarbons remained trapped under an impermeable rock barrier, and collected beneath this "cap." With time, the accumulation developed into a conventional reservoir, the target of conventional oil and gas exploration.
In the case of gas shale, some or all of the gas released during the transformation of the biomatter stayed in place. To be a candidate for gas extraction, source rocks must have reached sufficient maturity to generate the gas, without yet having expelled it.
Coalbed methane, as its name suggests, is trapped in coal deposits. It is also known as coal seam gas. Most of the gas is adsorbed on the surface of the coal, which is an excellent "storage vessel": it can contain two to three times more gas per unit of rock volume than conventional gas deposits.