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AAPG Foundation 2019 Grants-in-Aid Projects

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Molecular dynamics simulations of oil occurrence state in nanopores in shale

Abstract

In recent years, shale oil has become a hot spot for unconventional hydrocarbon exploration and development all around the world. Dongpu sag, located in the southwest Bohai Bay Basin, East China, is a typical hydrocarbon-rich region. The organic-rich shale in the third member of Shahejie Formation (Es3) has great shale oil resource potential, the kerogen type of shale is mainly type I and II1 and the average TOC content is 2.04%, indicating a good hydrocarbon generation potential. The molecular dynamics simulations (MD) of oil occurrence state are significant for evaluating the mobility of shale oil in tight reservoir, which is benefit to the study on recovery efficiency and exploration of shale oil. The shale oil mainly stores in the organic pores. Compared with minerals, there is a stronger interaction between organic matter and liquid hydrocarbons, which has a great influence on the occurrence state of shale oil. Molecular dynamics provides a powerful method to calculate motion and equilibrium of each individual atom or molecule in nanopores system. By studying the laws of motion of microscopic molecules, the macroscopic characteristics and basic laws of the shale oil in nanopores system can be obtained. In this project, Es3 shale samples are collected to analyze mineral composition and pore structure of shale by SEM analysis, MICP test and X-ray diffraction. Combined with oil component analysis and physical property measurement, a molecular model of pores and liquid hydrocarbons can be established. Comparing the parameters, such as density of alkane, calculated by the simulation and that from experiments under the same conditions, the correct force and potential field between oil and solid can be chosen, which is important for the accuracy of MD. To perform MD simulations, the molecular models of oil are inserted into nanopore models. In order to calculate oil density distribution and absorption layers across the pore, data are collected at regular intervals as the system reaching equilibrium. The impact of various factors on the occurrence of oil would be studied by changing the pore size, temperature and pressure in the model. Finally, considering different occurrence of oil, a mathematical formulation can be set up to estimate recoverable oil-in-place according to petrophysical model of a typical organic-rich shale. Prospectively, a new mathematical formulation would be developed to estimate recoverable oil-in-place. Meanwhile, it can be used to assess the shale reservoir quality and the movability of shale oil in Es3, Dongpu Depression, which can guide the exploration of shale oil.