A molecular dynamics explanation for fast imbibition of oil in organic tight rocks

Recent imbibition experiments show that the oil uptake of organic-rich tight rocks is significantly higher than their water uptake. The ratio, m, between oil and water imbibition rate from these experiments, mexp, is significantly higher than that predicted by classical imbibition models which are based on Young-Laplace equation such as Handy model, mHD; i.e. mexp > mHD. We hypothesize that this discrepancy is due to fast imbibition of oil into organic nanopores of these tight rocks that are visualized by scanning electron microscope (SEM) images. We test this hypothesis by simulating the dynamics of octane and water molecules imbibition into graphite and quartz slits with the width, w, of 1, 1.5, and 2 nm. For quartz slits, the simulated ratio msim-Q is fairly close to mHD. However, in graphite silts‚ msim-G > mHD and this observation is consistent with the results of the imbibition experiments on organic-rich tight rocks: mexp > mHD. The comparative analyses of simulated relative density profiles and calculated intermolecular potentials of octane and water in graphite and quartz slits indicate that this behavior is primarily due to the strong intermolecular interactions of octane and graphite at the nanopore wall compared with that of water molecules. These results are in favor of the proposed hypothesis and suggest that organic nanopores constitute a large fraction of the pore volume of the target tight rocks.