Key points: • The detrimental impact of preadsorbed water on the methane adsorption capacity and rate is more pronounced than that of water vapor • The distributions of adsorbed methane and water in shale pores were compared between the SAWM and PMMS • Water vapor preferentially enters mesopores (1.5-20 nm), and preadsorbed water mainly occupies micropores (0.3-1.5 nm) Abstract Water plays an essential role in shale gas migration and adsorption, and most studies on the influence of water on shale gas adsorption refer only to moisture-equilibrated shales. To investigate the impact of water vapor on methane adsorption in shales, three experiments were conducted and compared: (1) pure methane adsorption onto dry shale (PMD), (2) pure methane adsorption onto moisture-equilibrated shale (PMMS), and (3) simultaneous adsorption of water vapor and methane (SAWM) onto shale. Comparison of the experimental results reveals that the detrimental impact of water vapor on methane adsorption is inferior to that of preadsorbed water. Two mechanisms, i.e., water blocking and adsorption competition, are responsible for the reduction and difference in the methane adsorption capacity and adsorption rate between the PMMS and SAWM. Compared to the PMD, the methane adsorption capacity decreases by 81-96% in the PMMS, and by 20-49% in the SAWM. Methane adsorption equilibrium is achieved the fastest in the PMD. Before the equilibration degree reaches 95%, methane adsorption during the SAWM progresses more rapidly, while the reverse occurs when the equilibration degree exceeds 95%. The pore size distribution and water film thickness calculations indicate that the impacts of water vapor in the SAWM on micro-to mesopores are weaker than those of preadsorbed water. In the PMMS, adsorbed water mainly

The permeability of shale is a controlling factor in fluid migration, solute transport, and overpressure development in a sedimentary basin. However, shale permeabilities measured with different fluids can be very different. To investigate the effects of fluid type on shale liquid permeability, a series of flow experiments on three samples were conducted using deionized water, liquid CO2 and 1 mol/L NaCl solution as permeating fluids. The results indicate that the liquid CO2 flow obeys Darcy’s law, showing a constant permeability. The liquid CO2 permeabilities of samples C01, C02 and C03 are 6.90×10-19 m2, 3.80×10-20 m2 and 1.59×10-18 m2, respectively. The transport of the deionized water and NaCl solution in these samples deviates from Darcy’s law, and threshold pressure gradient is observed. The permeabilities measured with these two fluids exhibit nearly identical ranges (10-20~10-21 m2). The sample permeated with NaCl solution generally shows a lower permeability (under the same pressure gradient) but a higher threshold pressure gradient. The relationship between water permeability and pressure gradient follows a power function, with exponents ranging from 0.96~3.41 for deionized water and 0.34~3.30 for NaCl solution. The permeability reduction magnitude (ω) was defined to describe the difference between the three liquid permeabilities and the helium absolute permeability. The range of ω is 0.25~0.96 for liquid CO2, 1.44~2.32 for deionized water and 1.89~3.09 for NaCl solution. The dependence of permeability on fluid type results from the differences in the fluid properties (viscosity and polarity) and fluid-mineral interactions.