Supercritical methane adsorption on overmature shale: effect of pore structure and fractal characteristics

To investigate supercritical methane adsorption on shale and its controlling factors, high-pressure (up to 20 MPa) methane adsorption experiments were performed on overmature Niutitang shales from the Upper Yangtze area in China. Combining field emission scanning electron microscopy, low-pressure N-2 adsorption (LP-N-2-GA), and CO2 adsorption (LP-CO2-GA), the pore structure and fractal characteristics were studied. According to the LP-N-2-GA and Frenkel-Halsey-Hill (FHH) model, pore surface and spatial structure are characterized by the fractal heterogeneity with corresponding fractal dimensions D-1 (2.32-2.69) and D-2 (2.49-2.82). The measured supercritical methane excess adsorption isotherms show three stages: (i) a sharp increase under 6 MPa, (ii) a slow increase until reaching the maximum (V-ex(m) = 1.06-4.60 cm(3)/g) at the pressure of P-m (7.53-10.41 MPa), and (iii) a decline at various rates over the P-m. The rates of decline in excess adsorption at high pressures vary (0.031-0.074 cm(3)/g/MPa) and positively correlate with the total organic carbon (TOC) content, pore volume, and specific surface area (SSA) of micropores and fractal dimension D-1, whereas the P-m possesses weakly negative relationships with these factors. The excess adsorption data can be accurately fitted by the supercritical Langmuir-based adsorption model with the maximum absolute adsorption capacities (V-L) ranging from 2.88 to 6.57 cm(3)/g. Misinterpreting the low-pressure (0-10 MPa) experimental excess adsorption data as the absolute adsorption values to fit the adsorption isotherms, the actual adsorption capacity will be underestimated with the errors ranging from 18.44 to 45.34% for the calculated V-L, and an underestimation will exist in extrapolated in situ adsorbed gas content. TOC still plays an important role in promoting methane adsorption capacity even for the overmature shales. Meanwhile, the methane adsorption capacity is positively correlated with the SSA, micropore volume, and fractal dimension D-1, and microporosity is the governing factor on adsorbed gas occurrence