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

• The methane adsorption/desorption and CO 2 displacement processes of two cylindrical samples of highly volatile bituminous coal from tectonic deformation areas are compared by NMR method. • An effective method for evaluating the total methane content (combining adsorbed and free gas) in the ideal saturation state of core plug samples core plug sample is provided. • The adsorption saturation of the core plug samples increases with increasing pressure at apparent saturation. • For low-permeability coal samples, approximately 65 % of the adsorbed methane remains retained in the coal under CO 2 injection pressures up to 12 MPa. Accurate simulation of CH 4 adsorption/desorption and CO 2 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). CO 2 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 CO 2. [ABSTRACT FROM AUTHOR]