Gassy Rocks – Coals and Shales

Rocks that contain gas are important sources of commercial gas. They may also be a danger to mining or tunnelling. This danger comes from the potential release of the gas which may be in the form of an outburst, which is a failure of rock or coal which is then driven by the energy of expanding gas. The gas itself may be a danger either because it is toxic, such as carbon dioxide or hydrogen sulphide, because it may lead to an asphyxiating atmosphere or because it may produce an explosive atmosphere within the workings.

Gas may be stored in rock within the pore or fracture space. Here the key parameters are the pressure of the gas and the volume of that space. It may also be stored as a liquid. For example carbon dioxide becomes a liquid if the pressure exceeds 7.4 MPa and the temperature is below 31oC. Gas may also exist in solution. In petroleum reservoirs there are frequently changes of phase and solution during production. In carbonaceous rock such as coal and shales the gas is also stored by a process described by the all encompassing term,  sorption. Sorption principally involves bonding of the gas by Van der Waal’s forces to the hydrocarbons in the rock. It may also involve chemisorption.

In dealing with gas in rock the key parameters that need to be considered are the amount of gas, the pressure at which it is stored and the type of gas. The way in which gas is transported through the rock is also important – is it by Darcy flow or by some form of diffusion?

In summary the key information that is needed in dealing with the gassy rocks are:

  • the quantity of gas,
  • the pressure at which it is stored,
  • the gas type, and
  • its means of release from the rock.

The manner in which the gas is stored is also important as it will affect the process of release.