Abstract
The presence and content of water will certainly affect the gas adsorption capacity of shale and the evaluation of shale gas content. In order to reasonably evaluate the gas adsorption capacity of shale under actual reservoir conditions, the effect of water on methane adsorption capacity needs to be investigated. Taking the Da’anzhai Member of the Lower Jurassic Ziliujing Formation in the northeastern Sichuan Basin, China as an example, this study attempts to reveal the effect of pre-adsorbed water on methane adsorption capacity in shale-gas systems by conducting methane adsorption experiments in two sequences, firstly at different temperatures under dry condition and secondly at different relative humidity levels under the same temperature. The results show that temperature and relative humidity (i.e., water saturation) are the main factors affecting the methane adsorption capacity of shale for a single sample. The key findings of this study include: 1) Methane adsorption capacity of shale first increases then decreases with depth, reaching a peak at about 1,600–2,400 m. 2) Lower relative humidity correlates to greater maximum methane adsorption capacity and greater depth to reach the maximum methane adsorption capacity. 3) 20% increase of relative humidity results in roughly 10% reduction of maximum methane adsorption capacity. As a conclusion, methane adsorption capacity of shale is predominately affected by water saturation, pore type and pore size of shale. This study could provide a theoretical basis for the establishment of a reasonable evaluation method for shale adsorbed gas content.
Highlights
Shales have large accumulations of hydrocarbons and are not merely regarded nowadays as source rocks and seals anymore (Krooss et al, 1995; Hill et al, 2007; Tian et al, 2013; Chen et al, 2017a; Gao et al, 2020)
It can be seen that the absolute adsorption amount of methane shows a gradually increasing trend as pressure increases, and the absolute adsorption isotherms are similar to the type I adsorption isotherm recommended by the International Union of Pure and Applied Chemistry (IUPAC)
At high water saturation, the micropores of both organic matter and clay minerals are blocked predominantly by water molecules and barely capable of adsorbing methane molecules, while for mesopores and macropores, water molecules occupy the majority of adsorption sites forming water film, and yet clay minerals pores have higher water molecules concentration compared to organic matter counterparts
Summary
Shales have large accumulations of hydrocarbons and are not merely regarded nowadays as source rocks and seals anymore (Krooss et al, 1995; Hill et al, 2007; Tian et al, 2013; Chen et al, 2017a; Gao et al, 2020). As nano-scale pores are most developed in shales (Zou et al, 2017; Chen et al, 2019a; Liu et al, 2020), the adsorbed gas content generally accounts for 20–85% of the total gas content (Curtis, 2002; Chen et al, 2019b; Qiao et al, 2020). The average water saturation of shale gas reservoirs in Barnett, Marcellus and Woodford Formations in the US ranges from 15 to 35% (Ambrose et al, 2012), compared to 10–95% for the marine shale plays found in South China (Liu and Wang, 2013; Fang et al, 2014). It is of great significance to study the methane adsorption capacity of water-bearing shales
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