Competitive adsorption characteristics and diffusion process of CO2−CH4 on mineral surface: A case study of the 2nd Section of Shanxi Formationin Yan 'an Gas Field
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摘要:研究目的
CO2−EGR技术能提高天然气采收率，同时将CO2永久封存于地下，助力实现碳中和目标。CO2−CH4在纳米孔隙中竞争吸附和扩散是增采和封存的关键机理。
研究方法本文以鄂尔多斯盆地延安气田山2储层为靶区，利用分子动力学（MD）和巨正则系统蒙特卡罗（GCMC）方法建立模型，研究储层温压条件下CO2−CH4混合气体在关键矿物（石英和伊利石）纳米基质孔隙中的竞争吸附规律，分析CO2自扩散系数与温压关系。
研究结果（1）在等温（353.15 K）变压（5.9~17.7 MPa）和等压（11.8 MPa）变温（313.15~373.15 K）条件下，石英和伊利石对CO2吸附能力大于CH4，CO2−CH4在石英孔隙中的竞争吸附选择性大于伊利石孔隙；（2）等温（353.15 K）变压（5.9~17.7 MPa）和等压（11.8 MPa）变温（313.15~373.15 K）条件下, CO2−CH4在石英和伊利石孔隙中的竞争吸附选择性分别随压力、温度的增加而降低；（3）低压（5.9 MPa）高温（373.15 K）条件下，CO2在CO2−CH4−石英和CO2−CH4−伊利石系统的流动和扩散效率更高。
结论石英和伊利石对CO2吸附量更高，置换CH4能力更高，CO2封存效果更好。
创新点:阐明了纳米孔隙中CO2−CH4的竞争吸附和扩散规律；指出了延安气田山西组2段储层条件下CO2自扩散特性。
Abstract:This paper is the result of environmental geological survey engineering.
ObjectiveThe CO2−Enhanced Gas Recovery (CO2−EGR) technology significantly augments natural gas extraction efficiency while concurrently facilitating the permanent subsurface sequestration of CO2. This dual benefit substantially aids in achieving carbon neutrality goals. The mechanisms pivotal to enhanced recovery and storage include the competitive adsorption and diffusion of CO2−CH4 within nanopores.
MethodsThis study focuses on the 2ndsection of Shanxi formation in the Yan'an Gas Field located in the Ordos Basin. Using molecular dynamics (MD) and grand canonical Monte Carlo (GCMC) methods, a model was established toinvestigate the competitive adsorption behaviors of CO2−CH4 mixed gases in the nanoporous matrices of key minerals, specifically quartz and illite, under reservoir−specific temperature and pressure conditions. Additionally, the study analyzes the correlation between the self−diffusion coefficient of CO2 and the variabilities in temperature and pressure.
ResultsThe study yields several findings: (1) At an isothermal condition of 353.15 K and varying pressures from 5.9 to 17.7 MPa, both quartz and illite exhibit heightened adsorptive capacities for CO2 in comparison to CH4. Furthermore, the competitive adsorption selectivity for CO2−CH4 is found to be greater in quartz pores than in illite pores. (2) Under similar isothermal conditions and at a constant pressure of 11.8 MPa with temperatures ranging from 313.15 K to 373.15 K, the competitive adsorption selectivity for CO2−CH4 in both quartz and illite pores is observed to diminish with increasing pressure and temperature. (3) Under conditions of low pressure (5.9 MPa) and high temperature (373.15 K), there is an enhancement in the mobility and diffusion efficiency of CO2 within both CO2−CH4−quartz and CO2−CH4−illite systems.
ConclusionQuartz and illite have higher CO2 adsorption capacity, greaterCH4displacement capacity, and better CO2 storage effect.
Highlights:The competitive adsorption and diffusion behavior of CO2−CH4 in nanopores is clarified, and the self-diffusion characteristics of CO2 under the reservoir conditions of the 2nd Section of the Shanxi Formation in the Yan’an Gas Field is identified.
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图 3 CO2−CH4在石英纳米孔隙的竞争吸附模拟图（绿色为CO2分子，紫色为CH4分子，采用联合原子力场）
Figure 3. Simulation of competitive adsorption of CO2−CH4 in quartz nanopores (green is CO2 molecule, purple is CH4 molecule)
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图 4 不同目标压力Pt下的CO2（a）和CH4（b）在石英孔隙中的密度分布（红色虚线为±1 nm）
Figure 4. Density distribution of CO2 (a) and CH4 (b) in quartz pores under different target pressures Pt (red dashed line is ±1nm)
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图 5 CO2−CH4 在石英和伊利石表面的吸附选择性与压力关系图
Figure 5. Relationship between adsorption selectivity and pressure of CO2−CH4 on quartz and illitesurface
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图 6 不同温度下CO2(a)和CH4 (b)在石英表面的密度分布（红色虚线为±1 nm）
Figure 6. Density distribution of CO2 (a) and CH4 (b) on quartz surface at different temperatures (red dashed line is ±1 nm)
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图 7 CO2−CH4在石英和伊利石表面的吸附选择性与温度的关系图
Figure 7. Relation between adsorption selectivity and temperature of CO2−CH4 on quartz and illite surface
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图 8 CO2−CH4在伊利石孔隙的竞争吸附模拟（绿色为CO2分子，紫色为CH4分子，目标压力为11.8 MPa, 温度为353.15 K)
Figure 8. Simulation of competitive adsorption of CO2−CH4 in illite pores (green is CO2 molecule, purple is CH4 molecule, target pressure 11.8 MPa, temperature 353.15 K)
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图 9 不同目标压力Pt 下CO2(a)和CH4 (b)在伊利表面的密度分布（红色虚线为±1 nm）
Figure 9. Density distribution of CO2 (a) and CH4 (b) on Erie surface under different target pressures Pt (red dashed line is ±1 nm)
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图 10 在不同温度下的CO2 (a)和CH4 (b)在伊利石表面的密度分布（红色虚线为±1 nm）
Figure 10. Density distribution of CO2 (a) and CO2 (b) on illite surface at different temperatures (red dashed line is ±1 nm)
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图 11 CO2在CO2−CH4−石英系统和CO2−CH4−伊利石系统中的自扩散系数与压力(a)和温度(b)的关系图
Figure 11. The relationship of self−diffusion coefficient of CO2 with pressure (a) and temperature (b) in CO2−CH4−quartz system and CO2−CH4−illite system
表 1 不同压力下CO2−CH4等摩尔化学势及计算压力和摩尔比（温度T = 353.15 K）
Table 1 Equimolar chemical potential, calculated pressure, and molar ratio of CO2−CH4 at different pressures (temperature T = 353.15 K)
目标压力/
MPaCO2化学势/
(Kcal/mol)CH4化学势/
(Kcal/mol)计算混合气体压力/
MPaCO2−CH4
摩尔比5.90 −9.4468 −8.2991 5.93 1.01 9.44 −9.1792 −8.0029 9.27 1.01 11.80 −9.0491 −7.8357 11.82 1.00 14.16 −8.9538 −7.716 14.13 1.02 17.70 −8.8396 −7.5482 18.07 1.01 
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表 2 不同温度下CO2−CH4等摩尔化学势及计算压力和摩尔比（压力P=11.8 MPa）
Table 2 Equimolar chemical potential, calculated pressure, and molar ratio of CO2−CH4 at different temperatures (pressure P = 11.8 MPa)
温度/
KCO2化学势/
(Kcal/mol)CH4化学势/
(Kcal/mol)计算混合
气体压力/MPaCO2−CH4
摩尔比313.15 −6.7985 −7.9549 11.54 1.01 333.15 −7.3047 −8.4817 11.93 1.00 353.15 −9.0491 −7.8357 11.82 1.00 363.15 −8.0965 −9.3324 11.85 0.99 373.15 −8.3638 −9.6213 11.84 0.99
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表 3 不同目标压力下混合气体压力在无孔隙（仅CO2和CH4）和孔隙中的对比
Table 3 Comparison of mixed gas pressures in no pores (only CO2 and CH4) and pores under different target pressures
目标压力/
MPa混合气体压力−
无孔隙/MPa混合气体压力−
石英孔隙/MPa混合气体压力−
伊利石孔隙/MPa5.90 5.93 7.22 7.78 9.44 9.27 11.88 9.48 11.80 11.82 15.06 14.68 14.16 14.13 18.39 16.61 17.70 18.07 23.86 21.78
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表 4 不同温度下混合气体压力在无孔隙（仅CO2和CH4）和孔隙中的对比（目标压力为11.8 MPa）
Table 4 Comparison of mixed gas pressure in no pores (only CO2 and CO2) and pores at different temperatures (target pressure is 11.8 MPa)
温度
（K）混合气体压力−
无孔隙 (MPa)混合气体压力−
石英孔隙 (MPa)混合气体压力−
伊利石孔隙 (MPa)313.15 11.54 16.04 15.72 333.15 11.93 15.68 15.24 353.15 11.82 15.11 14.78 363.15 11.85 15.69 15.17 373.15 11.84 15.00 13.62
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