A new approach for measuring the permeability of shale featuring adsorption and ultra-low permeability

• We made a new design that the upstream and downstream reservoir volumes can be changed for reducing the measurement error.
• A by-pass pipe was added for measuring the bidirectional permeability instantaneously, which can reduce the total test time.
• Methane was utilized to measure the shale permeability, which can reflect the real gas transport mechanisms in shale.
• We modified the conventional interpretation model by incorporating the physical mechanism of gas adsorption.

Based on the critical issues associated with shale permeability measurements, a new experimental approach by modifying the traditional pressure-pulse decay method was developed. In order to reduce the measurement error caused by the pore volumes of the traditional method, we made a new design that the upstream and downstream reservoir volumes can be changed, In addition, we added a by-pass pipe to measure the bidirectional permeability instantaneously, which can reduce the total test time significantly. Except for nitrogen or helium, methane was utilized to measure the shale permeability in this work, which can be more practical and better understand the real gas transport mechanisms in shale. Furthermore, we modified the conventional interpretation model of permeability measurements by incorporating the physical mechanism of gas adsorption. We also performed a series of experimental measurements and data analyses using different cores from pure shale, sand shale, to sandstone, which are from the Ordos basin (Chang 7 section) in China. The results show that: (1) the error caused by the pore volume errors of the traditional method is decreased by nearly half if variable reservoir volumes are used. The total test time is reduced by around 7 h by adding the by-pass pipe on the apparatus. The value of permeability measured with methane is higher than that measured with nitrogen while lower than that measured with helium. (2) The effective gas adsorption porosity increases with the increasing Langmuir pressure and decreasing pore pressure. If without considering gas adsorption, the measured permeability value will be underestimated, especially under lower pore pressure, higher adsorption capacity, and higher Langmuir pressure. (3) The total error is less than 10% using this new apparatus and the modified permeability interpretation method. The measured permeability values are reliable by comparing the measurements using the new apparatus and the standard instrument of ProPDP-200 under the same condition. (4) The influence of gas adsorption on permeability measurement in shale cannot be ignored, and the permeability is underestimated by up to 97% in pure shale while by only 7.5% in sandstone if the gas adsorption is not taken into account.