(640e) Kerogen Wettability: Correlations to Surface Chemistry and Topology

Wettability has a key role in many aspects of unconventional reservoirs of oil and gas, ultimately controlling fluid dynamics during hydrocarbon recovery. This study elucidates correlations between kerogen wettability and physico-chemical characteristics in a series of nine isolated kerogen samples.

In-field exploration primarily relies on the use of hydraulic fracture (fracking), where large amounts of frac-fluid are injected into a wellbore, creating fissures and displacing petroleum products from their source rocks. The process efficiency fundamentally relies on solid-fluid interactions, governed by the intrinsic characteristics of both phases. Hence, a deeper understanding of the factors that determine the wetting characteristic of shale reserves may help to address several issues. For instance, in addition to economic-related challenges, environmental concerns arise due to significant losses of fracking fluid during the exploration of oil and gas. The unpredictable wetting mechanism of source rocks is associated with the complex mixture of inorganic and organic constituents. Kerogen, a major element in shale reserves, is the insoluble portion of sedimentary organic matter, being not only responsible for hydrocarbon formation but also its storage. Throughout geological times, kerogen undergoes a maturation process that dramatically changes its chemical and physical structure. As hydrocarbons and other light molecules, e.g. CO₂, are formed, kerogen’s H/C and O/C ratios decrease while the porosity simultaneously grows. The maturity stage of kerogen determines its petroleum production window (oil, mixed oil-gas, gas) and is an important factor for designing the extraction process. Acknowledging that both the chemical and physical properties of kerogen may be directly related to hydrocarbon recovery efficiency, it is important to understand how they correlate with wettability.

The ability of a liquid to wet a solid surface is often characterized by the contact angle (θ) formed at the air-liquid-solid interface. Upon stabilization, the solid surface may be characterized as hydro/oleo- philic (θ <90°) or hydro/oleo- phobic (θ> 90°) based on contact angles for water and oil. Under atmospheric conditions, this analysis is conducted on the assumption that the system components, i.e., air–liquid-solid, are in thermodynamic equilibrium. The surface chemistry and topography can affect interfacial interactions. On a non-textured (i.e., smooth) surface, the equilibrium contact angle θ for a contacting liquid is given by Young’s relation. On a rough surface, the apparent contact angle (θ*) can be described by either the Wenzel or the Cassie-Baxter relations. In this study, the wettability of isolated type-II kerogen samples at different maturation stages was evaluated by conducting contact angle measurements. Flat cylindrical pellets (diameter = 24.0 mm, thickness = 2.0 mm) were fabricated using a pneumatic pellet press. Their wettability was characterized by measuring contact angles for solvents with various chemical polarity, as a way of mimicking fracking fluids and assessing the hydro/oleo- philicity of the tested kerogens. In this study, water (γ_lv = 72.1 mN m^-1) and brine (5 wt% NaCl, γ_lv ~ 78.0 mN m^-1) as polar, and n-dodecane (γ_lv = 25.4 mN m^-1) as a non-polar hydrocarbon were used.

Prior to the contact angle measurements, physical and chemical characterization tests were conducted on the samples. The surface elemental composition was assessed via X-Ray Photoelectron Spectroscopy (XPS), where an inverse trend was observed between thermal maturity and heteroatom-related peaks, especially for oxygen-bearing groups. Similarly, Fourier Transform Infrared Spectroscopy (FT-IR) suggests that the composition trends observed in XPS spectra may also be observed in the bulk of kerogen, where the intensity of characteristic bands related to oxygen-bearing functional groups, e.g., carbonyl, decreased as vitrinite reflectance increased. Furthermore, the transition to a dominant aromatic carbon chain in mature samples was observed based on the intensity of characteristic bands correlated with C-H bonds in aromatic and aliphatic chains. Using nitrogen adsorption-desorption isotherms, a positive correlation between thermal maturity and specific surface areas was observed, with values ranging from 54 – 280 m² g^-1. Alongside, the porosity of the samples was also found to positively correlate with thermal maturity, where the volume of pores with an approximate diameter of 20 Ã… was found to consistently increase. Lastly, optical profilometry data indicates slightly heterogeneous surface morphology, with mean surface roughness (Ra) values ranging from 390 to 663 nm. These results indicate that the pore structure of kerogen may be accessible during interfacial interactions with fluids.

The contact angle measurements imply that regardless of thermal maturity, n-dodecane was readily imbibed by kerogen (i.e., θ* ~0°). This indicates that despite its maturation stage, the tested kerogen samples are predominantly oleophilic in air. In the case of polar liquids, the surface wettability seems to be heavily influenced by kerogen specific surface area, as the contact angles of water (56 – 95°) and brine (55 - 106°) decreased as the surface areas increased. These results suggest that kerogen can possess both hydrophilic and hydrophobic wettability, strongly controlled by its porosity. From all the pellets tested, the most mature sample, characterized by a vitrinite reflectance (VR_o) of 2.75%, displayed the lowest water/brine contact angles at time ~0 s (t₀). Furthermore, the increased porosity resulted in a higher imbibition rate for this specific sample in comparison to the other systems tested with aqueous solvents. No strong correlations were observed with the heteroatom content, even though the tested samples were in a completely different thermal maturity stage.

In conclusion, the wetting properties of kerogen may have a predominant role in hydrocarbon recovery from unconventional reservoirs, and the contact angle measurements can be a convenient technique to assess solid surface-fluid interactions. Demineralized type-II kerogens were tested at distinct thermal maturity stages with a variety of solvents that would mimic the fluids employed during the fracking. An inverse relation between water contact angle and maturity was observed, where the specific surface area and porosity of kerogen seem to greatly impact the wettability of kerogen. These results indicate that despite its predominant hydrocarbon structure, kerogen may also contribute to the hydrophilic nature of shale reservoirs. Furthermore, the instant imbibition of the n-dodecane as a hydrocarbon, suggests that kerogen may be oleophilic at most of its production window. These findings may help the understanding of reservoir wetting characteristics, and further studies may greatly benefit the findings reported here.