Investigation of CH4/CO2 competitive adsorption-desorption mechanisms for enhanced shale gas production and carbon sequestration using nuclear magnetic resonance

Understanding the competitive adsorption-desorption mechanisms in shale is of fundamental significance for enhancing CH4 recovery and CO2 sequestration. This study adopted nuclear magnetic resonance to reveal the influence of CO2 on adsorption-desorption behaviors of CH4 in plug-sized samples. Three distinctive peaks were observed in the transverse relaxation time (T2) spectrum of a CH4-saturated sample, which indicated the adsorbed CH4 (0.1 ms < T2 < 1 ms), free state CH4 in pores (2 ms < T2 < 30 ms) and free state CH4 in fractures (100 ms < T2 < 1000 ms). When CH4 reached adsorption equilibration under 20 MPa, the total T2 signals of adsorbed CH4, free state CH4 in pores and free state CH4 in fractures were 565.3, 591.5 and 306.6 p. u., respectively. Subsequently, CO2 was pumped into the CH4-saturated sample under 22 MPa. When CO2-CH4 completed the competitive adsorption process, T2 signals decreased from 565.3 to 396.6 p. u. for adsorbed CH4, increased from 591.5 to 707.3 p. u. for free state CH4 in pores, and increased from 306.6 to 359.5 p. u. for free state CH4 in fractures. Afterwards, the desorption of shale sample began. CH4 concentration decreased from 79% to 55% while CO2 concentration increased from 21% to 45%. Finally, the total desorption rate of adsorbed CH4 (65%) was much higher than that without introducing CO2 (25%-40%).

Automated summary

Understanding the competitive adsorption-desorption mechanisms in shale is crucial for improving CH4 recovery and CO2 sequestration. This study used nuclear magnetic resonance to investigate the impact of CO2 on the adsorption-desorption behaviors of CH4 in plug-sized samples. The results revealed distinct peaks in the transverse relaxation time (T2) spectrum, indicating the different states of adsorbed CH4 and free state CH4 in pores and fractures. The introduction of CO2 led to changes in the T2 signals, resulting in a higher desorption rate of adsorbed CH4.