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Understanding the wetting properties of shale reservoirs can benefit their development for energy-related purposes and their potential for long-term carbon dioxide injection and storage. Given its potential volumetric abundance and high surface area, the wetting behavior of kerogen in shale requires assessment. Despite their known limitations, wettability studies are commonly limited to static contact angle (θ) measurements. In this Article, the conflicting factors related to the analysis and interpretation of kerogen wetting via static contact angle measurements are discussed. Contact angle data for deionized water, brine (5% NaCl), and n-dodecane are presented for seven paleomarine type-II kerogens spanning a wide range of thermal maturities (vitrinite reflectance, Ro: 0.55 to 2.75%) and chemical composition (aromatic carbon content, H/C ratio, O/C ratio). Droplets of n-dodecane instantaneously absorbed (θ* ≈ 0°) upon contact with all kerogen pellet surfaces, showing the oleophilic nature of kerogen for all maturities tested. Apparent contact angles of water with kerogen surfaces were positively correlated with H/C ratios and inversely correlated with aromatic carbon content, while the bulk and surface oxygen concentrations did not strongly correlate with the measured data. Kerogen exhibited hydrophobic (θwater > 90°) behavior, except at the highest thermal maturities. For example, the least thermally mature and most thermally mature samples studied presented apparent contact angles for water of 123 ± 15 and 59 ± 10°, respectively. Profilometry analyses showed roughness average values ranging from 0.4 ± 0.1 to 3.9 ± 0.7 μm, with the indication that sample topology can affect measured contact angles, albeit in second order as compared to sample chemistry in this study. We recommend caution when associating contact angle data alone with wetting behavior, as data obtained through sessile droplet analysis are subject to known but not always considered, caveats.
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Is contact-line mobility a material parameter?
Abstract Dynamic wetting phenomena are typically described by a constitutive law relating the dynamic contact angle θ to contact-line velocity U C L . The so-called Davis–Hocking model is noteworthy for its simplicity and relates θ to U C L through a contact-line mobility parameter M , which has historically been used as a fitting parameter for the particular solid–liquid–gas system. The recent experimental discovery of Xia & Steen (2018) has led to the first direct measurement of M for inertial-capillary motions. This opens up exciting possibilities for anticipating rapid wetting and dewetting behaviors, as M is believed to be a material parameter that can be measured in one context and successfully applied in another. Here, we investigate the extent to which M is a material parameter through a combined experimental and numerical study of binary sessile drop coalescence. Experiments are performed using water droplets on multiple surfaces with varying wetting properties (static contact angle and hysteresis) and compared with numerical simulations that employ the Davis–Hocking condition with the mobility M a fixed parameter, as measured by the cyclically dynamic contact angle goniometer, i.e. no fitting parameter. Side-view coalescence dynamics and time traces of the projected swept areas are used as metrics to compare experiments with numerical simulation. Our results show that the Davis–Hocking model with measured mobility parameter captures the essential coalescence dynamics and outperforms the widely used Kistler dynamic contact angle model in many cases. These observations provide insights in that the mobility is indeed a material parameter.
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- Award ID(s):
- 1637960
- PAR ID:
- 10345526
- Date Published:
- Journal Name:
- npj Microgravity
- Volume:
- 8
- Issue:
- 1
- ISSN:
- 2373-8065
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
-
Accepted Manuscript
- Journal Article:
- https://doi.org/10.1038/s41526-022-00190-y
