Abstract
Based on the results of molecular dynamics simulation, in a gas-water miscible zone, the velocity profiles of the flowing water film do not increase monotonously but increase first and then decrease, which is due to the interaction between water and gas molecules. This exhibits a new physical mechanism. In this paper, we firstly propose a gas-water flow model that takes into account the new physical phenomena and describes the distribution of gas-water velocity in the whole pore more accurately. In this model, a decreasing factor for water film in the gas-water miscible zone is used to describe the decrease of water velocity in the gas-water miscible zone, which leads to the gas velocity decrease correspondingly. The new flow model considers the interaction among gas and water molecules in the miscible zone and can provide more accurate velocity profiles compared with the flow models not considering the miscible region. Comparison calculation shows that the previous model overestimates the flow velocity, and the overestimation increases with the decrease of the pore radius. Based on the new gas-water flow model, a new permeability correction factor is deduced to consider the interaction among gas and water molecules.
Highlights
Shale formation is a great energy source
The ratio of KAg to KAgi which is defined as the apparent permeability enhancement factor Kc, is used to evaluate the enhanced gas flow capacity by considering the miscible zone and high-viscosity flowing water film compared with the single gas model
Inspired by the results of molecular dynamics simulation, the miscible zone is considered into the gas flow model in this paper
Summary
Shale formation is a great energy source. As a result of rising energy demand and prices, research into unconventional oil and gas is deepening, and successful development of shale formation in the United States has led to a boom in research and development [1]. The pore radius varies widely and is mainly on the nanometer scale, which is quite different from the transport mechanism in conventional formation and has a significant effect on the gas flow performance [1, 7]. Based on a unified diffusion coefficient, Cai et al [19] proposed an apparent permeability model of shale which investigated the gas transport mechanism in a shale nanopore by considering convective flow, gas diffusion, and surface diffusion. Molecular dynamics simulation plays an important role in studying the mechanism of gas-water flow in pores. Xu et al [23] performed a comprehensive study on the two-phase transport characteristics of shale gas and water through hydrophilic and hydrophobic nanopores combined with molecular dynamics (MD) simulation and analysis model. The flow model takes a decreasing factor into account to describe the effect of the miscible zone on flow behaviors
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