Assessment of Kerogen Wettability from Contact Angle Goniometry

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.

深入理解页岩储层的水润湿特性，对于其能源开发及其长期二氧化碳注入与储存的潜力具有重要意义。鉴于其潜在的体积丰富性和高比表面积，需对页岩中干酪根的润湿行为进行评估。尽管润湿性研究已知存在局限性，但通常仅限于静态接触角（θ）的测量。在本文中，我们讨论了通过静态接触角测量分析及解释干酪根润湿的相互矛盾因素。本文展示了七种不同热成熟度（镜质体反射率，Ro：0.55至2.75%）和化学组成（芳碳含量，H/C 比率，O/C 比率）的古海洋型-II干酪根的接触角数据，涉及去离子水、盐水（5% NaCl）和正十二烷。正十二烷滴在与所有干酪根颗粒表面的接触瞬间被吸收（θ* ≈ 0°），表明在所有测试成熟度下干酪根均具有亲油性质。水与干酪根表面的表观接触角与 H/C 比率呈正相关，与芳碳含量呈负相关，而总体和表面氧浓度与测得数据的相关性不强。干酪根表现出疏水性（θwater > 90°）行为，除在最高热成熟度外。例如，最不成熟和最成熟的样品分别呈现了水与干酪根表面的表观接触角为 123 ± 15° 和 59 ± 10°。轮廓分析显示粗糙度平均值介于 0.4 ± 0.1 至 3.9 ± 0.7 μm 之间，表明样品拓扑结构可能影响测量的接触角，尽管在本研究中，相较于样品化学性质，其影响较小。我们建议在仅将接触角数据与润湿行为相关联时保持谨慎，因为通过静置滴液分析获得的数据存在已知但并非总是被考虑到的限制。