Unconventional energy resources, including coal beds, tight gas sands, and shales, have become an ever-increasing factor in the North American gas supply. In these formations, the basic transport properties of the fluids change as they typically travel through sub 100-nm pores, which falls into the field of nanofluidics. As production from shales became economically feasible, this field gained more interest in petroleum engineering.

The purpose of the current study is to determine the effective gas viscosity of shale based on pore scale simulation. We consider methane (CH4), the main constituent of natural gas, as the only component of fluid system. We use an acyclic pore model to characterize the pore structure of a shale. The acyclic model represents the effective connectivity of pore space because it can capture drainage behavior from mercury injection measurements. We calculate the effective gas viscosity of the shale at different pore pressures, and present the results with respect to the nominal value, under unconfined conditions. Our analysis indicates that the reported permeability from pressure-driven flow measurement has to be considered an effective value, if nominal values of viscosity and density are used for interpretation. That is, we have to modify viscosity and permeability simultaneously in our reservoir model. The current study has a major impact on reservoir characterization based on standard lab measurements in shales.