Influence of CO_2 on the adsorption of CH_4 on shale using low-field nuclear magnetic resonance technique

摘要

Due to the higher adsorption capacity, CO2 can efficiently replace the adsorbed CH4 from shale surface. To understand the effect of CO2 on the adsorption of CH4 on shale is significant for comprehending the mechanisms of enhanced shale methane recovery using CO2 method in shale gas reservoirs. In this work, the low-field nuclear magnetic resonance (NMR) technique is employed to quantitatively investigate the influence of CO2 on the CH4 adsorption on typical shale samples. CH4 is first introduced to "saturate" the shale samples at given pressures; based on the measured T-2 spectrum for the "CH4-saturated" shale samples, the states that CH4 exists in shale samples are identified. CO2 is then introduced into the "CH4 -saturated" shale samples at higher pressures. By comparing the measured T-2 spectrum before and after CO2 introduction, the change of CH4 adsorption of due to the presence of CO2 is comprehensively analyzed. According to the measured T-2 spectrum, CH4 exists on shale samples in three different states, i.e., the adsorbed CH4 on pore surface, the free-state CH4 in pore center, and the free-state CH4 among shale particles. Compared to the free-state CH4, the "CH4-saturated" shale samples are dominated by the adsorbed CH4. As pressure increases, the adsorbed amount of CH4 first increases and then tends to level off. After introducing CO2 into the "CH4-saturated" shale samples, the adsorbed CH4 is firstly reduced, suggesting the more affinity of CO2 to the organic shale surface, and then tends to level off, achieving the adsorption/desorption equilibrium. CO2 can replace the adsorbed CH4 from pore surface, decreasing the adsorbed molar amount of CH4. However, the replaced CH4 seems to only become free-state CH4 in pore center and hardly escape from the organic pores. Thereby, other stimulating methods, such as secondary hydraulic fracturing, should be supplemented with the CO2 injection for further development of the shale gas reservoirs.