The wettability of organic-rich mudrocks has significant impacts on multiphase fluid flow and hydrocarbon recovery. Since kerogen can constitute a significant volumetric fraction of mudrocks, the wettability of kerogen can considerably affect the wettability of organic-rich mudrocks. Previously we showed using a combination of experimental methods and molecular simulation studies that kerogen type, thermal maturity, and reservoir temperature conditions affect the contact angle of the water droplet on kerogen surface, and therefore, wettability of kerogen. Wettability affects the distribution of fluids in the porous media. Moreover, the interfacial interactions between water/hydrocarbon molecules and the kerogen pore walls would significantly influence water and hydrocarbon transport. Here, we quantify the impact of kerogen wettability and geochemistry on fluid flux and apparent fluid permeability in organic-rich mudrocks.

We use molecular dynamics (MD) simulations to determine the impacts of kerogen geochemistry and wettability on the fluid flux and apparent water permeability in kerogen pores. The inputs to the simulation include molecules of kerogen packed in a cubical simulation box. The molecules are then condensed to form a flat kerogen surface. We then model kerogen pores by creating pore space between two sheets of kerogen. Water molecules are then placed at the end of modeled kerogen pores, and differential pressure is applied to the water molecules. We then calculate the water flux in the kerogen pore space at a given pressure drop (Δp). Based on the slope of the flux-time curve, we then determine the apparent water permeabilities in the kerogen pore space and quantify the impact of kerogen wettability on water/hydrocarbon permeability. These measurements are performed for types I, II, and III kerogen molecules at different thermal maturity levels.

We calculate the water and methane flux and their apparent permeabilities at various kerogen hydrophilicity levels and under pressure drops (Δp) ranging from 0 to 50 MPa. We observed that the hydrophilicity/hydrophobicity of kerogen plays a key role in governing water and hydrocarbon permeability. We also found that the apparent flux of water molecules in the kerogen pores increases with differential pressures. We show that the apparent permeability of water molecules increases with increasing hydrophobicity. The apparent permeability of water decreases by approximately 20% as the wettability of kerogen samples to water increases (i.e., from a water contact angle of 75.5° to 20°).

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