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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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Characterizing surface wetting and interfacial properties using enhanced sampling (SWIPES)
We introduce an accurate and efficient method for characterizing surface wetting and interfacial properties, such as the contact angle made by a liquid droplet on a solid surface, and the vapor–liquid surface tension of a fluid. The method makes use of molecular simulations in conjunction with the indirect umbrella sampling technique to systematically wet the surface and estimate the corresponding free energy. To illustrate the method, we study the wetting of a family of Lennard-Jones surfaces by water. For surfaces with a wide range of attractions for water, we estimate contact angles using our method, and compare them with contact angles obtained using droplet shapes. Notably, our method is able to capture the transition from partial to complete wetting as surface–water attractions are increased. Moreover, the method is straightforward to implement and is computationally efficient, providing accurate contact angle estimates in roughly 5 nanoseconds of simulation time.
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- Award ID(s):
- 1652646
- PAR ID:
- 10128345
- Date Published:
- Journal Name:
- Soft Matter
- Volume:
- 15
- Issue:
- 5
- ISSN:
- 1744-683X
- Page Range / eLocation ID:
- 860 to 869
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1039/C8SM02317D
