Rate dependency of permeability in tight rocks

Abstract

Matrix permeability of tight rocks is a challenging parameter to measure. The gas permeability measurements of low-permeability rocks are sensitive to pressure, effective stress, temperature, and testing technique. However, the influence of gas flow rate on permeability measurements of tight rocks is unknown.In this work, steady-state gas permeability experiments are conducted on four shale and siltstone samples using methane and nitrogen as flowing fluid. The gas permeability is measured at several flow rates, while mean pressure and mean effective stress are held constant. The measurements are repeated at several mean pressure values. The tests are designed to study the influence of mean pressure as well as flow rate on gas permeability. After the gas permeability measurements, the samples are saturated with water to measure water permeability. Subsequently, the samples are sheared in a triaxial cell and similar gas and water permeability tests are repeated to compare the permeability behavior before and after failure. The results indicate strong rate sensitivity in permeability of tight rocks. Measured permeability is observed to rise as flow rate increases and reaches a constant value at higher rates. Permeability of the failed samples shows a similar behavior, although with higher permeability values. The rate dependency of gas permeability seems to follow a trend, similar to what is reported in the literature as the transition between pre-laminar and laminar flow regimes. Based on the experimental results, a discussion is provided on the rate at which the transition between pre-laminar and laminar flow occurs. Finally, by combining the theories on rate dependent flow regimes and pressure dependent (Knudsen number) flow regimes, a more complete picture of gas permeability in porous media is proposed.