Gas flow measurements in shales: laboratory, field and numerical investigations

A multi-disciplinary research project including experimental and modelling studies was carried out on shale samples to characterise their porosity and permeability. Pressure expansion techniques were used, including current industry-standard methods as well as new methods developed and modified throughout this research. The derived porosity and permeability values were cross-checked with the results from commercial laboratories. Finally, the results obtained were applied to a shale resource play currently being appraised to understand its commercial viability.
Precise grain density results were achieved using the crushed shale method as helium is able to rapidly intrude small sample pores and is not significantly adsorbed onto the constituents of the shale. Precise bulk volume measurements were obtained using mercury immersion but these are ambient stress measurements and need correcting for in-situ conditions. Mercury probably does not enter the pore-space of shale at low pressures during MICP tests and instead closes artificial microfractures. So the results may provide a method to estimate bulk density at the reservoir stresses.
The porosity measured using the crushed shale method is more accurate compared to core plug methods. It is important to dry crushed samples to standardise porosity measurements. Other laboratories produced comparable results except for one laboratory which most likely did not conduct sample cleaning procedures properly.
Permeability values obtained using the crushed shale method were orders of magnitudes different between the measurements conducted during this study and commercial laboratories. Overall, this test appears to provide no useful information regarding the flow properties of shales. Measurements made on core plugs are often dominated by the presence of microfractures but it is possible to obtain reasonably reliable permeability estimates by inverting the experimental data using a dual porosity-permeability model.
To assess the applicability of porosity and permeability methods on commercial shale play, a significant amount of in-situ field data (i.e. well tests, core data etc.) were gathered and tested during the collaborative project in Europe with a local gas exploration company. Gas-In-Place (GIP) and Estimated Ultimate Recovery (EUR) values were produced and based on these the project was approved by the company for the next stage of development. However the model constructed lacked the ability to reproduce the well flow production rates.