Gas Permeability of Shale

Permeability is one of the most fundamental properties of any reservoir rock required for modeling hydrocarbon production. However, shale permeability has not yet been understood fully because of the complexities involved in modeling flow through nanoscale throats. In this paper, we analyze the effects of adsorbed layers of methane (CH4) and of gas slippage at pore walls on the flow behavior in individual conduits of simple geometry and in networks of such conduits. The network is based on scanning elecron microscope (SEM) images and a drainage experiment in shale. To represent the effect of adsorbed gas, the effective size of each throat in the network depends on the pressure. The hydraulic conductance of each throat is determined on the basis of the Knudsen-number (K-n) criterion (Knudsen 1909). The combined effects of adsorption and slip depend strongly on pressure and on conduit diameter. The results indicate that laboratory measurements made with N-2 at ambient temperature and 5-MPa pressure, which is typical for a transient pulse-decay (TPD) method, overestimate the gas permeability at early life of production by a factor of four. This ratio increases if the measurement is run at ambient condition because the low pressure enhances the slippage and reduces the thickness of the adsorbed layer. Moreover, the permeability increases nonlinearly as the in-situ pressure decreases during production. This effect contributes to mitigating the decline in the production rate of shale-gas wells. Laboratory data available in the literature for CH4 permeability at pressures below 7 MPa agree with model predictions of the effect of pressure.