Review of Competitive Adsorption of CO2/CH4 in Shale: Implications for CO2 Sequestration and Enhancing Shale Gas Recovery

Abstract

The injection of CO₂ into shale gas reservoirs can not only enhance shale gas recovery (ESGR), but also realize CO₂ geological storage (CGS). In this study, the competitive adsorption behaviors of CO₂ and CH₄ in shale were systematically reviewed, and the implication for shale gas recovery efficiency and CO₂ storage potential were discussed. The adsorption advantage of shale for CO₂ compared to CH₄ provides a guarantee of the feasibility of supercritical CO₂ (ScCO₂) enhanced shale gas exploitation technology. The selective adsorption coefficient of CO₂ and CH₄ by shale (S _CO2/CH4 ) is an important parameter in evaluating the competitive adsorption behavior of CO₂/CH₄ in shale gas reservoirs, which is closely related to the mineral composition, reservoir temperature, pressure conditions, water content, and mixed gas composition ratio. In addition, the injection type, injection mode, and injection rate of gases also exhibit different effects on CO₂/CH₄ competitive adsorption. Furthermore, the interaction between ScCO₂ and the water-rock system will change the mineral composition and microstructure of shale, which will lead to changes in the adsorption behavior of shale on CO₂ and CH₄, so its influence on the competitive adsorption of CO₂/CH₄ cannot be ignored. Future research should integrate different research methods and combine with practical engineering to reveal the competitive adsorption mechanism of CO₂/CH₄ in shale reservoirs from both micro and macro aspects. This study can provide support for the integration technology of ScCO₂ enhanced shale gas exploitation and its geological storage.

Figure 1

Actual production amount of shale gas wells. (Photograph courtesy of Yang, R., copyright 2022, and the images in the figure are free domain.)

Figure 2

Schematic diagram of the integration of shale gas recovery and CO₂ geological storage.

Figure 3

Mixed gas adsorption system. (Photograph courtesy of Qin, C., copyright 2021, and the images in the figure are free domain.)

Figure 4

Construction method of a composite heterogeneous nanopore model containing organic matter and inorganic matter. (Photograph courtesy of Ma, J., copyright 2022, and the images in the figure are free domain.)

Figure 5

Schematic diagram of the double-porosity and double-permeability homogeneous model.

Figure 6

Calculation procedure of S_CO2/CH4.

Figure 7

Effect of the types of clay minerals and organic matter on S_CO2/CH4. (a) Clay minerals; (b) organic matter. (Photograph courtesy of Hu, X., copyright 2019, and Sui, H., copyright 2020, and the images in the figure are free domain.).

Figure 8

Relationship of S_CO2/CH4 versus TOC and clay content. (Photograph courtesy of Xie, W., copyright 2022, and the images in the figure are free domain.)

Figure 9

Effect of pore structure on S_CO2/CH4. (a) Shale samples, (b) kaolinite, and (c) organic matter. (Photograph courtesy of Gu, M., copyright 2017, Zhou, W., copyright 2019, and Zhou, W., copyright 2018. The images in the figure are free domain.)

Figure 10

Selectivities in different adsorption layers. (Photograph courtesy of Zhou, W., copyright 2019, and the image in the figure is free domain.)

Figure 11

Effect of pressure (MPa) and temperature (K for (a) and (b),°C for (c) and (d)) on S_CO2/CH4. (a) Shale organic nanopores, (b) shale of the Longmaxi Formation, (c) shale of the Longmaxi Formation, and (d) shale of the Longmaxi Formation. (Photograph courtesy of Zhou, W., copyright 2018, Lu, T., copyright 2022, and Xie, W., copyright 2021. The images in the figure are free domain.)

Figure 12

Change in S_CO2/CH4 with adsorption pressure in shale. (Photograph courtesy of Liao, Q., copyright 2023, and the image in the figure is free domain.)

Figure 13

Effect of the water content on S_CO2/CH4. (a) Different kerogen models; (b) type II-D kerogens (10 water, 50 water, and 100 water mean 10, 50, and 100 water molecules). (Photograph courtesy of Huang, L., copyright 2018, and Sui, H., copyright 2020, and the images in the figure are free domain.).

Figure 14

Effect of the mixing ratio of CO₂/CH₄ on S_CO2/CH4.^,, (Photograph courtesy of Liao, Q., copyright 2023, Qin, C., copyright 2021, and Xie, W., copyright 2022, and the images in the figure are free domain.)

Figure 15

Effects of different CO₂/N₂ injection ratio on recovery efficiency of shale gas (solid lines mean <4 MPa overpressure injection and dashed lines mean <8 MPa overpressure). (Photograph courtesy of Li, Z., copyright 2019, and the image in the figure is free domain.)

Figure 16

Effects of changes in CO₂ and N₂ injection rates on gas recovery and CO₂ storage. (Photograph courtesy of Ma, H., copyright 2022, and the image in the figure is free domain.)

Figure 17

Effect of ScCO₂ immersion on S_CO2/CH4 and pore structure of shale.^, (Photograph courtesy of Qin, C., copyright 2021, and Zhou, J., copyright 2021, and the images in the figure are free domain.)

Figure 18

Variations in shale mineral composition before and after ScCO₂ exposure.

Figure 19

CO₂-ESGR project in the Chachattooga shale reservoir. (Copyright 2017 and the image in the figure is free domain.)

Figure 20

Schematic diagram of ScCO₂ enhanced shale gas exploitation and its geological storage integration.

Figure 21

Pyramid model of the CO₂ geological storage capacity.