Gas permeability measurement of shales using the quasi steady-state technique

The decline in reserves from conventional reservoirs, paired with the technological advances made in the drilling, stimulation and production areas over the last two decades have placed unconventional reservoirs in the limelight. Shale plays, in particular, have become increasingly attractive prospects and production from these reservoirs has increased significantly during this period. Most of the petrophysical characterisation techniques routinely used in the laboratory were originally developed for rocks with relatively high porosity and permeability, making some of them unsuitable for tight rocks. Gas permeability measurements in shales can be particularly challenging due to their small pore and pore throat sizes, and even the validity of Darcy’s law under these conditions needs to be evaluated. The steady-state technique is generally unsuitable for measuring gas permeability in shales due to technical limitations of the instruments required, so the quasi steady-state method is proposed as an alternative. This paper presents the results of gas permeability measurements conducted on two shale core plugs using the quasi steady-state technique. Although the effect of variations in ambient conditions is not usually significant for tests performed on cores from conventional reservoirs, our results indicate that it should not be overlooked when experiments are conducted on shale samples. Furthermore, the length of the core plugs should be minimised to reduce the time required to measure gas permeability.