Kerogen Swelling in Light Hydrocarbon Gases and Liquids and Validity of Schroeder's Paradox

Z Li and J Yao and A Firoozabadi, JOURNAL OF PHYSICAL CHEMISTRY C, 125, 8137-8147 (2021).

DOI: 10.1021/acs.jpcc.0c10362

Kerogen is often the organic part of the shale and provides the pore space for a large part of the hydrocarbons in place. Kerogen molecules are flexible like polymer molecules, and may swell when in contact with specific solvents. The difference in swelling of a polymer when exposed to a pure liquid versus its saturated vapor is known as Schroeder's paradox, which has a long history of controversy. To the best of our knowledge, kerogen swelling induced by a hydrocarbon in both gas and liquid states has not been investigated. We investigate the sorption of gaseous and liquid propane, normal butane, and normal pentane in kerogen and kerogen swelling at various pressures. A flexible kerogen matrix is created using molecular dynamics simulations. The large pores are created and maintained with a dummy particle and nailed atoms, respectively. The hydrocarbon sorption and kerogen swelling induced by the sorption of hydrocarbon gases and liquids are investigated by the hybrid molecular dynamics-grand canonical Monte Carlo simulations. Results show that smaller hydrocarbon molecules in both gas and liquid states have higher sorption and induce higher swelling and structural changes in the kerogen matrix, Hydrocarbon liquids have higher sorption and induce higher kerogen swelling than the corresponding gases at saturation pressure in agreement with Schroeder's paradox. Hydrocarbon gases lead to the moderate kerogen swelling from expansion of large pores; hydrocarbon liquids swell the kerogen more significantly not only by expanding the large pore but also by creating new pores and throats. Liquid-inducing kerogen swelling is accompanied by hydrocarbon dissolution in the kerogen matrix altering significantly the pore surface area, porosity, and pore size distribution. The work sets the stage for further studies considering the effect of production on shale volumetric strain.

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