2:15 PM - 2:30 PM
[HSC06-15] CO2 Adsorption-Induced Swelling of Shale Kerogen: Sorption Isotherms, Micropore Structures, and Swelling Mechanisms
Keywords:Shale Gas, CO2 Sequestration, Swelling, Pore Connectivity, Hybrid GCMC/MD Simulation
CO2 sequestration in shale gas reservoirs has recently gained recognition due to the massive available pore space in shale formations, offering dual benefits of mitigating CO2 emissions and enhancing CH4 production. Understanding CO2/CH4 competitive adsorption in shale formations is essential for effective CO2 storage. Shale formations feature a complex pore structure, with micropores (<2 nm) and mesopores (2-50 nm), which significantly impact gas sorption behaviors. Moreover, gas adsorption induces shale deformation, which in turn affects further gas adsorption. This study employs hybrid grand canonical Monte Carlo (GCMC)/Molecular Dynamics (MD) simulation to investigate the adsorption behaviors of pure CH4 and CO2, and their 5:5 binary mixture in kerogen nanopore systems with both micropores and mesopores, where MD captures kerogen deformation and GCMC addresses selective adsorption. The impact of adsorption-induced kerogen swelling on sorption isotherms, pore structure, and connectivity is analyzed, with pressure profile analysis elucidating the underlying deformation mechanism. These findings contribute to a deeper understanding of gas transport and storage mechanisms in shale formations, offering valuable implications for CO2 sequestration and enhanced CH4 recovery strategies.
The swelling factor generally increases with pressure, although it fluctuates due to the combined effects of adsorption-induced swelling and pressure-induced compaction. Notably, CO2 induces a greater swelling effect compared to CH4. The excess sorption isotherms derived from both rigid and flexible kerogen models align well with experimental data. In terms of absolute sorption, CH4 and the mixture exhibit almost identical sorption behavior in both models. For pure CO2, there is a small yet detectable difference in absolute sorption, indicating specific interactions between kerogen and CO2 molecules.
The influence of adsorption on the kerogen matrix was further analyzed. For all cases, the surface area-to-volume ratio, which is inversely proportional to pore size, increases with pressure, suggesting the formation of new and smaller pores. Furthermore, pure CO2 can create more new and smaller pores than pure CH4. The accessible pore volume fraction ranks as CO2 > mixture > CH4, underscoring a greater capacity of CO2 to generate accessible pore volume in the kerogen matrix compared to CH4. Then Persistent Homology (PH) analysis was conducted to quantify the topological features of micropores in the kerogen matrix, such as the number of pores (PH0) and ring-shaped structures (PH1). This analysis will help us understand the connectivity changes during CO2 injection. A lower PH0 value for CO2 compared to CH4 suggests that CO2 injection is more effective at reducing the number of isolated pores, while higher PH1 values for CO2 compared to CH4 indicate that CO2 injection enhances the formation of ring-shaped pathways. The combined analysis of the surface area-to-volume ratio, PH0, and PH1 values suggests that CO2 injection significantly improves pore connectivity by forming new pores that serve as bridges, thereby linking previously isolated pores within the kerogen matrix.
Finally, the pressure profile was analyzed to understand the kerogen deformation mechanism. The matrix pressure exceeds the bulk pressure initially, but gradually decreases until equilibrium is nearly reached, suggesting that the elevated pressure within kerogen micropores after gas adsorption serves as the driving force for kerogen swelling. This analysis highlights how gas-induced swelling affects sorption behavior, enhances kerogen matrix connectivity, and reveals the relationship between pressure and deformation. These findings provide key insights into gas transport and storage in shale formations. The method that we developed can also be applied to understanding sorption-related behavior in other porous materials, such as activated carbon.
The swelling factor generally increases with pressure, although it fluctuates due to the combined effects of adsorption-induced swelling and pressure-induced compaction. Notably, CO2 induces a greater swelling effect compared to CH4. The excess sorption isotherms derived from both rigid and flexible kerogen models align well with experimental data. In terms of absolute sorption, CH4 and the mixture exhibit almost identical sorption behavior in both models. For pure CO2, there is a small yet detectable difference in absolute sorption, indicating specific interactions between kerogen and CO2 molecules.
The influence of adsorption on the kerogen matrix was further analyzed. For all cases, the surface area-to-volume ratio, which is inversely proportional to pore size, increases with pressure, suggesting the formation of new and smaller pores. Furthermore, pure CO2 can create more new and smaller pores than pure CH4. The accessible pore volume fraction ranks as CO2 > mixture > CH4, underscoring a greater capacity of CO2 to generate accessible pore volume in the kerogen matrix compared to CH4. Then Persistent Homology (PH) analysis was conducted to quantify the topological features of micropores in the kerogen matrix, such as the number of pores (PH0) and ring-shaped structures (PH1). This analysis will help us understand the connectivity changes during CO2 injection. A lower PH0 value for CO2 compared to CH4 suggests that CO2 injection is more effective at reducing the number of isolated pores, while higher PH1 values for CO2 compared to CH4 indicate that CO2 injection enhances the formation of ring-shaped pathways. The combined analysis of the surface area-to-volume ratio, PH0, and PH1 values suggests that CO2 injection significantly improves pore connectivity by forming new pores that serve as bridges, thereby linking previously isolated pores within the kerogen matrix.
Finally, the pressure profile was analyzed to understand the kerogen deformation mechanism. The matrix pressure exceeds the bulk pressure initially, but gradually decreases until equilibrium is nearly reached, suggesting that the elevated pressure within kerogen micropores after gas adsorption serves as the driving force for kerogen swelling. This analysis highlights how gas-induced swelling affects sorption behavior, enhances kerogen matrix connectivity, and reveals the relationship between pressure and deformation. These findings provide key insights into gas transport and storage in shale formations. The method that we developed can also be applied to understanding sorption-related behavior in other porous materials, such as activated carbon.