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Thermodynamic Modeling of the Equilibrium Partitioning of Hydrocarbons in Nanoporous Kerogen Particles
journal contribution
posted on 2019-01-11, 00:00 authored by Jinlu Liu, Walter G. ChapmanIn unconventional shale, petroleum
compounds are generated during
kerogen maturation and chemical fractionation occurs between fluids
that are expelled out of kerogen and retained within kerogen because
of different solubilities of hydrocarbons in kerogen. The compositional
variation of petroleum compounds from the organic matter source (kerogen)
to nanoscale pores in the organic matter and to large fractures makes
the estimation of fluid storage and the multicomponent compositional
distributions in shale a challenging problem. A thermodynamic model
that considers the different interactions between hydrocarbons and
kerogen is needed to quantify the partitioning of fluids among different
phases. In this work, a new thermodynamic model is proposed that predicts
both the adsorption of hydrocarbons in nanoscale pores and the dissolution
of fluids in organic matter in equilibrium with the bulk fluid phase.
First, by taking into account the chemical and structural similarities
of kerogen with asphaltene, we leverage the perturbed chain-statistical
associating fluid theory (PC-SAFT) model of asphaltene to create a
cross-linked kerogen matrix model. The swelling ratios of five kerogens
of varying maturities and types in different solvents are well quantified
by phase equilibrium calculation using the new model. Second, the
new kerogen matrix model is incorporated in interfacial statistical
associating fluid theory (iSAFT) to form a nanoporous kerogen composite
model. The inhomogeneous fluid distribution at the fluid/kerogen interface
is then able to be solved, and the equilibrium partitioning of mixtures
of fluids from the bulk phase to the nanoporous kerogen phase can
be determined. The results show the enrichment of aromatic and cyclic
molecules in kerogen, the preferential adsorption/absorption of high
molecular weight molecules and carbon dioxide, and the significant
amount of hydrocarbon storage both in pore space and kerogen matrix.
Examples that compare with fluid at the same condition in a graphite
pore indicate that graphite as the postmatured kerogen provides the
largest fractionation effect for fluid mixtures.