Methane adsorption/desorption and carbon dioxide replacement in low permeable coal using LF-NMR-experiments on cylindrical sample under apparent adsorption equilibrium condition

Accurate simulation of CH4 adsorption/desorption and CO2 displacement in coal reservoirs is essential for understanding methane recovery and carbon dioxide sequestration. Difficulty achieving gas adsorption equilibrium challenges understanding adsorption-desorption and displacement mechanisms. Low field magnetci resonance (LF-NMR) and isothermal adsorption are used to create a novel correction method for methane content in cylindrical coal samples. This method involves calculating theoretical adsorption amount using NMR data, adjusting adsorption amount using isothermal adsorption, and measuring free methane content using NMR results. Corrected adsorption capacity of HL13 and HQ7 samples is 3.19–11.83 times and 3.31–5.12 times of NMR calibration, respectively. Maximum methane adsorption capacity and saturation of the samples increase from 0.47 m3/t and 8.45 % at 2 MPa to 3.74 m3/t and 31.36 % at 10 MPa respectively. With the correction method, adsorbed methane and total gas content of the coal sample reach 11.93 m3/t and 14.26 m3/t respectively under a pressure of 10 MPa. Natural depression-desorption improves desorption efficiency from 4.16 % to 17.75 % (pressure from 10 MPa to 5 MPa). CO2 injection increases methane desorption efficiency from 16.77 % to 35.64 % at 12 MPa, but 65 % of methane remains adsorbed, suggesting that the low permeability of the coal seam could potentially act as a barrier to successful sequestration of CO2.