Microscopic Dynamics of Water and Hydrocarbon in Shale-Kerogen Pores of Potentially Mixed Wettability

Abstract

Summary Distribution of alkanes and water in organic pores of shale, referred to as kerogen, is essential information required for the estimation of shale-reservoir oil and gas in place, adsorption of hydrocarbon, and fate of hydraulic-fracture water. A practical modeling approach is presented for the proper description of the kerogen pore systems with different mixed wettability, surface roughness, tortuous paths, and material disorder. Three kerogen models—activated kerogen, kerogen free of activated sites, and graphite-slit pore—with proper surface-oxidized functional groups and high-temperature and pressure maturation are constructed by simulation. Distribution of octane and water in the organic pores of these models is predicted by molecular dynamics (MD) simulation. The results from our studies underscore the need for accurate characterization of kerogen pore systems in terms of the pore morphology, level of surface activation, and pore size. The improved kerogen models constructed to structurally resemble real organic materials have the potential to enable a better understanding of the placement, distribution, and trapping mechanisms of hydraulic-fracture water in shales. We demonstrate that depending on the maturity of the kerogen within organic-rich shales, organic pore systems may have mixed-wet characteristics and may create opportunities for water entrapment. The differences in the uptake of water are shown to be a function of the existence of oxygenated functional groups. In addition, the adsorption characteristics of alkanes in pores characterized by surface roughness are shown to be significantly different from those observed in the graphite-slit pore model. Our results indicate that careful consideration of the pore morphology is merited when estimating hydrocarbons in place with the Langmuir monolayer-adsorption theory.

Impact of Maturity on Kerogen Pore Wettability: A Modeling Study

“…However, some recent experimental work challenges this notion and the observations in Chalmers and Bustin (2010) and Ruppert et al (2013) suggest that organic pores are accessible to water. In a previous study (Hu et al 2014) using more realistic models for kerogen including the presence of surface roughness, nanocracks and different O/C ratios, we demonstrate that water may have an affinity for organic pore surfaces depending on the maturity; however Hu et al (2014) was restricted in its ability to provide any quantification of wettability. This paper is an extension of the previous study where the sole focus is the quantification of wettability from hydrophobic or octanewetting to even hydrophilic because of changing degrees of surface activation.…”

Study on the Adsorption, Diffusion and Permeation Selectivity of Shale Gas in Organics

“…The thermodynamic properties of different types of organics such as maturity, density, compressibility, swelling and adsorption isotherm were studied [20]. Hu [49] studied the change of gas and water distribution in organic pore by molecular dynamics simulation. The results showed that, if the kerogen pore is water wetted, the water molecules are adsorbed near the functional groups and the methane molecules were adsorbed on the surface of the organics.…”

Interactions Between Stratigraphy and Interfacial Properties on Flow and Trapping in Geologic Carbon Storage

“…The mobile CO 2 was able to connect with more stable or larger CO 2 ganglion in regions of the sand pack with a more CO 2 ‐philic surface (Wang et al, ; Zhao et al, ). This same process would likely work in reverse, creating preferential flow paths for water through the mixed‐wet porous medium (Hu et al, ), to bypass the CO 2 ganglia and leave a high residual trapping.…”