Effects of energetic heterogeneity on gas adsorption and gas storage in geologic shale systems

Capturing heterogeneity in an adsorbate-adsorbent interaction is important to understand the underlying mechanisms controlling adsorption behavior; especially, for natural shale characterized by various mineral compositions and multi-scale pore sizes, its energetic heterogeneity is extremely complex. In this work, experimental investigations of methane adsorption on natural shale were conducted over a range of pressures (0.5-20 MPa) and temperatures (333-393 K), and a multi-site model was introduced to describe the measured adsorption isotherms. Specially, the adsorption energy distribution in our model is related to a pore-size and surface-property dependent function, rather than any traditionally assumed ones, such as Gaussian or exponential distributions. Besides, as an apparent indicator of adsorbent heterogeneity, isosteric heat was analyzed for our studied shales. Results showed that isosteric heat for an energetically homogeneous adsorbent is a constant, while the observed heat for our studied shales gradually decreases with an increase in surface coverage. This coverage-dependent heat is mainly controlled by the ordered occupation of gas molecules on heterogeneous adoption sites; in this respect, the most favorable sites with the highest energy are occupied first followed by secondary sites with lower energy. Furthermore, in application fields, our proposed model can be used to estimate total gas in place for a whole shale gas reservoir and gas resources contributed by individual pores. This work presents comprehensive knowledge about the effect of surface heterogeneity on methane adsorption on shale, and can thus be used as a solid foundation for accurately estimating gas storage and production in geologic systems.