Transport Properties of Shale Gas in Relation to Kerogen Porosity
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2018 American Chemical Society. Kerogen is a microporous amorphous solid, which is the major component of the organic matter scattered in the potentially lucrative shale formations hosting shale gas. A deeper understanding of the way kerogen porosity characteristics affect the transport properties of hosted gas is important for the optimal design of the extraction process. In this work, we employ molecular simulation techniques to investigate the role of porosity on the adsorption and transport behavior of shale gas in overmature type II kerogen found in many currently productive shales. To account for the wide range of porosity characteristics present in the real system, a large set of 60 kerogen structures that exhibit a diverse set of void space attributes was used. Grand canonical Monte Carlo simulations were performed for the study of the adsorption of CH4, C2H6, n-C4H10, and CO2 at 298.15 and 398.15 K and a variety of pressures. The amount adsorbed is found to correlate linearly with the porosity of the kerogen. Furthermore, the adsorption of a quaternary mixture of CH4, C2H6, CO2, and N2 was investigated under the same conditions, indicating that a composition resembling that of the shale gas is achieved under higher temperature and pressure values, i.e., conditions closer to those prevailing in the hosting shale field. The diffusion of CH4, C2H6, and CO2, both as pure components and as components of the quaternary mixture, was investigated using equilibrium molecular dynamics simulations at temperatures of 298.15 and 398.15 K and pressures of 1 and 250 atm. In addition to the effect of temperature and pressure, the importance of limiting pore diameter (LPD), maximum pore diameter (MPD), accessible volume (Vacc), and accessible surface (Sacc) on the observed adsorbed amount and diffusion coefficient was revealed by qualitative relationships. The diffusion across the models was found to be anisotropic and the maximum component of the diffusion coefficient to correlate linearly with LPD, indicating that the controlling step of the transport process is the crossing of the limiting pore region. Finally, the transport behavior of the pure compounds was compared with their transport properties when in mixture and it was found that the diffusion coefficient of each compound in the mixture is similar to the corresponding one under pure conditions. This observation agrees with earlier studies in different kerogen models comprising wider pores that have revealed negligible cross-correlation Onsager coefficients.
