Adsorption behavior, including the thermodynamic characteristics of wet shales under different temperatures and pressures

Density Analysis of Adsorption Phase in the Thermodynamic Study of Shale Gas Adsorption

Understanding the adsorption mechanism and precisely predicting the thermodynamic adsorption properties of methane at high pressure are crucial while very challenging for shale gas development. In this study, we demonstrated that the Langmuir adsorption model combining with different empirical methods to determine the adsorption phase density makes the calculated isothermal adsorption heat violate Henry's law at low pressure. For instance, the isothermal adsorption heat calculated by the Langmuir-Freundlich model contradicts Henry's law when the absolute adsorption quantity is zero. Given the current challenge in accurately calculating the adsorption phase density, it is necessary to impose constraints on the parameters of the adsorption model by adhering to Henry's law to maintain thermodynamic consistency. We found that the adsorption phase volume of methane molecules lies between the micropore volume and the total pore volume when shale adsorption reaches saturation. The adsorption mechanism involves not only filling micropores but also monolayer adsorption in meso-macro pores. The high-energy adsorption sites for methane are primarily concentrated in organic matter, while within these methane adsorption areas in shale, the high-energy adsorption sites for water are mainly located in kaolinite within clay minerals. The zero-pressure heat of adsorption is a temperature-independent thermodynamic index, yet it is influenced by the water content. It can therefore be selected as a quantitative measure to evaluate the impact of methane adsorption on water.

Clarifying the Effect of Clay Minerals on Methane Adsorption Capacity of Marine Shales in Sichuan Basin, China

The effect of clay minerals on the methane adsorption capacity of shales is a basic issue that needs to be clarified and is of great significance for understanding the adsorption characteristics and mechanisms of shale gas. In this study, a variety of experimental methods, including XRD, LTNA, HPMA experiments, were conducted on 82 marine shale samples from the Wufeng–Longmaxi Formation of 10 evaluation wells in the southern Sichuan Basin of China. The controlling factors of adsorption capacities were determined through a correlation analysis with pore characteristics and mineral composition. In terms of mineral composition, organic matter (OM) is the most key methane adsorbent in marine shale, and clay minerals have little effect on methane adsorption. The ultra-low adsorption capacity of illite and chlorite and the hydrophilicity and water absorption ability of clay minerals are the main reasons for their limited effect on gas adsorption in marine shales. From the perspective of the pore structure, the micropore and mesopore specific surface areas (SSAs) control the methane adsorption capacity of marine shales, which are mainly provided by OM. Clay minerals have no relationship with SSAs, regardless of mesopores or micropores. In the competitive adsorption process of OM and clay minerals, OM has an absolute advantage. Clay minerals become carriers for water absorption, due to their interlayer polarity and water wettability. Based on the analysis of a large number of experimental datasets, this study clarified the key problem of whether clay minerals in marine shales control methane adsorption.