Abstract
Injecting carbon dioxide to enhance shale gas recovery (CO2-EGR) is a useful technique that has raised great research interests. Clear understanding of the two-component gas transport mechanisms in shale nanopores is the foundation for the efficient development of shale gas reservoir (SGR) and also the long-term geological storage of CO2. Although extensive studies on single-component gas transport and corresponding models in shale nanopores have been carried out in recent years, limited studies have been conducted on two-component or even multi-component gas transport models in shale nanopores. In this work, the shale nanopores were classified into inorganic and organic nanopores. The corresponding models for two-component gas transport were constructed. Mechanisms including Knudsen diffusion, slip flow, viscous flow, and molecular diffusion are considered in the inorganic pores. In the organic pores, due to existence of adsorption gas, surface diffusion is further considered besides the aforementioned mechanisms. Effects of pressure, temperature, fraction of organic nanopores, and gas concentration were analyzed. Results show that gas apparent permeability is negatively correlated with pressure, and positively correlated with temperature and organic nanopore fraction. As the concentration of CH4 decreases, the apparent permeability of CH4 increases continuously, while the apparent permeability of CO2 decreases. The permeability ratio of CH4 in the total permeability is negatively correlated with pressure and gas concentration ratio. Additionally, the contribution of transport mechanisms to the total gas apparent permeability has been analyzed. It is found that the surface diffusion contributes up to 5.68% to gas apparent permeability under high pressure. The contribution of molecular diffusion can reach up to 88.83% in mesopores under low pressure. Under high pressure and macropores, it contributes less than 1.41%. For all situations, the contribution of viscous flow is more than 46.36%, and its contribution can reach up to 86.07%. Results of this study not only can improve the understanding of two-component gas transport in nanochannels, but also can lay the foundation for more reliable reservoir simulation of CO2-EGR.
Highlights
Unconventional resources are becoming increasingly important due to the energy demand.Different from conventional reservoirs, shale gas reservoirs (SGRs) have developed various types of nanopores and have the characteristics of low porosity and permeability
It is found that the surface diffusion contributes up to 5.68% to gas apparent permeability under high pressure
Under high pressure and macropores, it contributes less than 1.41%
Summary
Unconventional resources are becoming increasingly important due to the energy demand. The effect of water film is rarely considered in the current apparent (SGRs), which considered gas adsorption, surface diffusion on from the adsorption layer, permeability model for one-component gas. It can be learned actual logging data stress that sensitivity, and non-Darcy flow. The is formed was not considered in SGRs. the2effect of be inorganic nanopores was the surface of IONPs in CH4 and CO cannot adsorbed on the pore ignored.The. established the in apparent permeability which surface. Proposed a gas transport model for shale organic nanopores In this model, multiple gas transport mechanisms and real gas effects were considered, but effects of water film and inorganic matter pores were neglected. Results of this study can provide both theoretical and practical significance for the reservoir simulation of CO2 -EGR, which can contribute to the efficient development of SGRs and the geological storage of CO2
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