Modeling the Kerogen 3D Molecular Structure

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Abstract

Abstract During the past decade, there has been a surge in the production of shale oil and this trend is expected to continue in future due to the abundant shale oil reserve. Currently, one main issue with shale oil production is the low primary recovery factors still less than 10%. Therefore, efficient enhanced oil recovery methods are highly desired to improve shale oil recovery. As an essential component of shale, kerogen plays a key role in generation, adsorption, storage and migration of hydrocarbons in shale as well as the geomechanical properties of shale. On the other hand, during the shale oil extraction process, shale oil can be produced from kerogen by pyrolysis, hydrogenation, or thermal dissolution reactions. The basis to improve the efficiency of both recovery and extraction processes is to establish and clarify the molecular structure of kerogen. Previously, some 2D molecular structures of kerogen have been proposed based on some characterization techniques, such as nuclear magnetic resonance (NMR) and mass spectrospcoy. However, the physical and chemical properties of kerogen are essentially determined by its 3D stereochemical molecular configuration. In this work, according to the 2D molecular structure of Green River oil shale kerogen from literature, a 3D solid state model of kerogen was built based on molecular dynamics(MD) simulation method. The kerogen of Green River oil shale is a mixture of seven molecules that are held together by physical adsorption mechanism. This kerogen belongs to type I based on O/C (0.026) and H/C (1.57) ratios. An adsorption method was applied to combine the seven molecules together. By changing the adsorption sequences of the seven molecules, a total of 24 possible 3D nanocluster models of kerogen were obtained. Then the 3D nanocluster structure with the lowest energy was further applied to build the solid state model of kerogen at 298k under 1 atm based on a MD simulation method. The final 3D solid state model of kerogen was used to calculate the density, mechanical properties, NMR, IR and Raman spectra of kerogen. The theoretical results agree well with the experimental data. The density from the 3D structure was calculated as 0.968 g/cm3 which falls in the range of experimental results from 0.95 to 1.45 g/cm3. The calculated Young's modulus, Poisson ratio, compressibility, bulk modulus and shear modulus are 3.50 GPa, 0.25, 370.64 TPa-1, 2.63 GPa and 1.37 GPa respectively that matches the experimental results very well. The calculated spectral data are also in accord with the experimental results. These results will help us better understand the physical and chemical properties of Green River oil shale kerogen. Moreover, the calculated properties of kerogen can be applied to implement new techniques or improve existing methods for enhancing oil recovery from shale oil resources.

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