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Molecular Simulation of Hydrocarbon Occurrence and Phase Behavior in Nano Pores


Molecular simulation (MS) can mimic physical movements of interacting atoms and molecules in a complex system. The trajectories of atoms and molecules are determined by numerically solving the Newton's equations of motion where forces between atoms and molecules are defined by fields of molecular mechanics force. MS is able to deal with a vast number of particles and associated properties in complex systems numerically. In studying unconventional petroleum systems such as tight oil and shale gas, the occurrence and phase behavior of hydrocarbons in nano pores remain enigmatic. There are quite a few hypotheses on how hydrocarbons are stored in or adsorbed on the reservoir matrix but they cannot be adequately verified in the laboratory due to physical constraints. MS provides a valuable tool to numerically test various models from a molecular perspective. We applied MS to simulate light hydrocarbons and natural gas adsorption in zeolites, montmorillonite and quartz and have obtained the following understandings: (1) MS of a typical natural gas in SiO2 at layer spacings of 1-2 nm and under 310 K and 0-15 MPa shows that the adsorption capacity on SiO2 is affected by layer spacings and pressure; Propane and C2H6 have stronger adsorption capacity than CH4. (2) MS of natural gas in Na-Otay montmorillonite in a layer spacing of 2 nm, at 353.5 K and 5-25 MPa shows that the mole fraction of CH4 in the adsorbed phase is lower than in the bulk phase, but C2H6 and C3H8 in the adsorbed phase are higher; In Na-Otay Na+ is closest to CH4 (3.2Å) followed by O (3.9Å) and Si (4.6Å); Water (7.15% wt) in Na-Otay reduces the adsorption capacity of CH4 by 40%. (3) MS of CH4 and CO2 in FAU zeolites indicates that temperature has little effect on the adsorption at 1 MPa for CO2 and 10 MPa for CH4 with both having the same adsorption capacity. (4) MS of light hydrocarbons and quartz in a layer spacing of 20 nm at 350 K and 20 MPa shows that benzene is preferentially adsorbed onto the quartz by an order of magnitude over n-hexane. In nano pores natural gas is adsorbed on the mineral surface as single layers. The aromatic fraction appears to be preferentially adsorbed on the mineral surfaces over the n-alkanes. Water has an adverse effect on the gas adsorption capacity. Those findings provide useful insights for understanding the occurrence of hydrocarbons in tight reservoirs and shales.