Modeling the Kerogen 3D Molecular Structure

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/cm 3 which falls in the range of experimental results from 0.95 to 1.45 g/cm 3 . 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.

Modeling the Effect of Maturity on the Elastic Moduli of Kerogen Using Atomistic Simulations

“…The ν for kerogen ranges between 0.2 and 0.35 with increasing density and is insensitive to changes in its composition. This result is consistent with other studies that have calculated a Poisson’s ratio of 0.25 for dense kerogen from molecular dynamics simulations. , This estimate of the Poisson’s ratio can be used to inform experimental measurements made using AFM and nanoindentation. In both methods, the Young’s modulus is not measured directly but rather indirectly through the measurement of the reduced modulus.…”

“…The predictions from simulations are in good agreement with previous estimates of the Young’s modulus computed from molecular models of kerogen at similar densities. Zhang and Jamili calculated the Young’s modulus and Poisson’s ratio of Green River shale with a density of 0.968 g/cm 3 to be 3.50 GPa and 0.25, respectively, using the COMPASS force field. Using the OREBO potential, Bousige et al reported an increase in the Young’s modulus of kerogen from 1–5 to 25 GPa as the density increases from 1.0 to 1.5 g/cm 3 , respectively.…”

“…Instead, we use molecular modeling techniques to compute the mechanical properties of pure kerogen and to understand the relationship between the changes in the structure of kerogen as a result of burial and its elastic properties. Experimental observations from a variety of measurements such as elemental analysis, 13 C nuclear magnetic resonance (NMR) and X-ray photoelectron spectroscopy (XPS), and infrared and Raman spectroscopy are integrated to build the molecular models of kerogen. ,− The atomic models have been used to study the thermodynamic and mechanical properties of kerogen such as density, enthalpy of formation, heat capacity, elastic moduli, and adsorption on kerogen surfaces ,, as well as to model the thermal cracking of kerogen into petroleum products. , …”

Study of the Mechanical Behavior of Organic Matters Contained in Source Rocks: New Insights into the Role of Bitumen

“…Studies of mechanical properties based on the available models revealed a high degree of consistency with the reported values experimentally. For instance, the outcomes of Green River shale’s mechanical properties investigated by Zhang & Jamili 29 were in alignment with the experimental findings. 30 Moreover, observations made by Kashinath et al ( 31 ) on Poisson’s coefficients matched decently the experimental ones.…”

Effect of Diagenesis on Geomechanical Properties of Organic‐Rich Calcareous Shale: A Multiscale Investigation

“…Thermal maturation also affects the geomechanics of organic matter, and combined geochemical/geomechanical studies (Khatibi et al, 2018;Jakob et al, 2019) and molecular simulations (Z. Zhang & Jamili, 2015;Bousige et al, 2016;Kashinath et al, 2020) are just beginning to reveal the detailed link between chemical composition, thermal maturity and mechanical response.…”