Experimental study on isothermal adsorption of methane gas on three shale samples from Upper Paleozoic strata of the Ordos Basin

The organic-rich shale samples of the Upper Paleozoic strata of the Ordos Basin were chosen to conduct isothermal adsorption experiments at different temperatures (46, 49, 60, 75, 80, and 90 °C), and each temperature isothermal adsorption curve was obtained at different pressure points. The study revealed that the absorbed gas content of shale rises with increasing pressure under isothermal condition and exhibits a linear decline with increasing temperature under isobaric condition. When both temperature and pressure are allowed to vary, the impact of pressure is greater than that of temperature on shale adsorption, and the absorbed gas content of shale rises with increasing pressure in zones with low temperature and pressure, whereas in the zones with high temperature and pressure, the impact of temperature is greater than that of pressure on shale adsorption capacity and the absorbed gas content of shale falls. Under the isothermal and isobaric conditions, the absorbed gas content of shale rises with increasing total organic carbon content. At a stratigraphic depth where the highest experimental pressure is not exceeded, an isothermal adsorption experiment can be performed at the temperature corresponding to the sampling-point depth so as to directly read the adsorbed gas content (AGC) at this stratigraphic pressure from the isothermal adsorption curve. At a stratigraphic depth where the highest experimental pressure is exceeded but the highest experimental temperature is not, the AGC of shale can be obtained by solving a relational expression between pressure and AGC established under stratigraphic temperature conditions. At a stratigraphic depth where the highest experimental temperature is exceeded, the AGC of shale can be acquired by solving a relational expression established between gas content and depth.