Pore-scale modelling of shale gas permeability considering shale gas adsorption

Shale gas permeability needs to be estimated in order to predict the quality of shale gas reservoirs and to develop shale gas production strategies. With advances in high-resolution imaging technology, one can characterise the pore space of a gas shale sample, which typically contains pores ranging from micrometers to nanometers, and to construct a pore-space model to simulate the gas flow numerically and to calculate the permeability. Gas flow has long been known to behave differently in such a confined space, and the smaller the pores the larger discrepancy is generally expected between gas and liquid (e.g. water) permeability. Since shale gas molecules stored mainly in nano-metre pores in kerogens by gas adsorption, adsorbed gas molecules, of half-nanometres in diameter, could reduce the pore size for free gas flow substantially and so alter the gas permeability significantly. In this work, we extended a model for modelling shale gas flow to account for the gas adsorption effect. We adopted the Langmuir single-layer adsorption model to the multiple layers. We analysed the gas adsorption impact on the permeability on a cylindrical pore analytically, and on a shale sample whose pore space are represented as a node-and–bond pore network, using our network flow model (Ma et al., 2014). The results revealed that the adsorption effect depends strongly on the gas pressure and the radii of pores. Given that low gas pressure increases gas slippage at pore surfaces and decreases the thickness of the adsorption layers then, consequently, enhances the permeability, undesirable operation conditions could lead to an earlier decline of gas production.