A microscopic characterization of wettability in shale kerogen with varying maturity levels

Abstract

Kerogen is defined as the insoluble macromolecular organic matter in sedimentary rocks and is a complex mixture of organic chemical compounds. Typically, a single chemical formula is inadequate to represent kerogen whose composition depends on a long history of geologic events and thermal maturity of the shale. The process of kerogen maturation is accompanied by the loss of functionalized molecules, leading to a decreased H/C and O/C ratios, as well as a reduction in molecular weight. The degree of maturation is well reflected in the van Krevelen diagram with H/C and O/C ratios as indicators. Even though the widely held view for kerogen pores is generally hydrocarbon-wetting, some recent experimental work indicates the existence of water content in kerogen. The key questions arising from this observation are whether kerogen may have mixed-wet characteristics and if so, is this likely to depend on kerogen maturity.Addressing these concerns is very essential because wettability is directly related to the dynamics of fluids, and is likely to be extremely relevant to developing models for reserves estimates and multiphase flow. Additionally it may provide some answers to the common observation of low recovery of hydraulic fracture water.In this paper, pore-scale molecular dynamics simulations are used to understand the relationships between kerogen maturity and its wettability. The traditional graphene model is chosen as a proxy for organic pores, and the varying degrees of maturity is approximated by grafting varying amounts of functionalized groups onto the pore surfaces. We study a mixture of water and alkanes in organic pores with effective width of 5 nm. We find that water molecules aggregate together to form clusters due to the polarity of water, and the shapes and the locations of the water clusters are strongly linked to the density of the functionalized groups. By determining the contact angles, we are able to quantify the relationship of kerogen wettability with the maturity level.The results demonstrate that kerogen maturity governs wettability of organic kerogen pores, due to the influence of surface heterogeneity and the distribution of polar molecules on kerogen. The wettability in kerogen is very likely to be heterogeneous. Depending on the maturity level, kerogen may be hydrocarbon wetting at high maturity, neutral wetting at intermediate maturities or even hydrophilic for organic surfaces of very low maturities. Additionally, our results indicate that in kerogen there may be a critical local density of surface functionalized sites that may create optimal conditions for hydrophilic kerogen surfaces.