Effect of Pore Shape on Nanoconfined Gas Flow Behavior: Implication for Characterizing Permeability of Realistic Shale Matrix

Abstract

Good knowledge of nanoconfined gas flow behavior will significantly contribute to advance ultimate gas recovery from shale gas reservoirs. However, up to now, it has still been extremely challenging to have a clear understanding of the key issue. In particular, little attention is drawn to the effect of pore geometry on nanoconfined gas flow behavior. By means of developed experimental technology, realistic images reflecting pore characteristics can be obtained, which demonstrate that various pore shapes are exhibited in the shale matrix. To my knowledge, current related research is focused on gas transport through several conventional pore shapes, including circle, rectangle, and slit. As a result, the urgent issue, i.e., research on gas flow behavior through unconventional pore shapes, is highlighted. In this work, a model for gas transport through elliptical nanopores is established, which possesses an excellent varying-shape nature with changing aspect ratio (AR) and therefore covers lots of pore shapes and shares great application value. Both effects of gas slipperiness and surface diffusion are incorporated in this model. Moreover, the Langmuir isotherm is utilized to quantify the thickness of the adsorption layer of methane. When AR is equal to 1 and an infinite number, the proposed model can achieve excellent agreement compared with gas transport data through circular and slit-like nanopores. Results show that (a) the gas transport capacity will decrease with increasing AR value at a specific pressure point; (b) contribution of surface diffusion will become more prominent with higher AR value; (c) under different gas adsorption/desorption characteristics, there is no difference in gas transport capacity through nanopores with different ARs at high pressure and relatively large difference at low-pressure atmosphere; (d) current production prediction models or commercial software require consideration of the effect of various pore shapes to enhance application accuracy. In sum, the findings of this study can help in better understanding the methane flow feature through nanopores with various cross-sections, which further serve as the necessary theoretical attempt with regard to accurate characterization for flow capacity of a realistic shale matrix.