Experimental Measurement and Mathematical Model for Intrinsic Permeability of Tight Rocks and Shale at Low Pressure

Abstract

Gas flow in porous media differs from liquid flow due to the large gas compressibility and the klinkenberg effect. Klinkenberg effect is not negligible when the pressure is low or the permeability of the reservoir is very small. Enhancing the backpressure could eliminate the klinkenberg effect. However, high pressure is not always accessible under the lab or field production condition. As intrinsic permeability is important not only for the reservoir simulation but also for the theoretical study of gas flow in unconventional reservoir, a new method for determining the intrinsic permeability of tight and shale rocks was developed, excluding the effect of klinkenberg via both experimental measurements and numerical calculation. The steady state measurements were conducted with high purity helium. A mathematical model was then derived to determine the intrinsic permeability based on the measured gas fluxes and pressure profile. To test the presented method, extra measurements were conducted for methane flooding on three shale cores at low outlet pressure (lower than the minimum backpressure), showing good agreement with the data obtained from our model. The advantage of the method lies in two points: one is that the results are much more accurate than that from traditional PPD method when error exists in the existence of klinkenberg effect. This advantage is especially significant for tight and shale cores because the extra low permeability makes the effect of klinkenberg effect non-neglected. The other is that this method could be applied to any outlet pressure excluding the effect of klinkenberg effect.