Assessing Impact of Shale Gas Adsorption on Free-Gas Permeability via a Pore Network Flow Model

Shale gas has become an increasingly important unconventional energy resource in the last decade. However, commercial gas production is fraught with difficulties due to lack of understanding of the physicochemical behaviors of the gas in confined pores at sub-micron to nanometer scales. Gas transport behaviors are complex. Some of the shale gas molecules are thought to be adsorbed on the surface of pores in organic matter, at in-situ conditions, while the remaining molecules occur as free gas flowing in the pores in multiple flow regimes. We developed a pore network flow model to simulate complex flow behaviors of non-ideal gas and showed that a large discrepancy in calculated gas permeability, of more than an order of magnitude, can occur depending on whether the gas is treated as ideal- or non-ideal gas and whether the nano-scale flow effects are explicitly addressed or not. Since the adsorbed molecules can reduce the pore space for free gas flow, gas adsorption can have an adverse impact on the gas flow in nanometer pores in gas shales. To examine this, we developed an effective multi-layer gas adsorption model and integrated it with our pore network flow model. That extended model allows us to estimate the effect of the gas adsorption on apparent gas permeability of realistic node-and-bond pore networks of shale samples. Our results show that if the gas adsorption is not being considered, the calculated gas permeability may be overestimated by as much as 50% for the pore system modeled in this study. To assess the real impact of gas adsorption on free gas permeability, there is a need to obtain realistic estimates of the distribution and concentration of adsorbed gas molecules on pore surfaces for a set of characteristic pore types. We carried out molecular simulations of methane adsorption in carbon pores of either slit or triangular shape at a range of pressures and temperatures using the Grand Canonical Monte Carlo (GCMC) algorithm. We report here some preliminary results on adsorption data to highlight the potential impact of the gas adsorption on the free gas flow.