Transport Properties of Shale Gas in Relation to Kerogen Porosity

Abstract

Kerogen is a micro-porous amorphous solid, which consist the major component of the organic matter scattered in the potentially lucrative shale formations hosting shale gas. 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 in order to investigate the role of porosity on the adsorption and transport behavior of shale gas in overmature type II kerogen found at 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 (GCMC) simulations were performed for the study of the adsorption of CH4, C2H6, n-C4H10 and CO2 at 298.15 K and 398.15 K and a variety of 2 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 in the same conditions, indicating that the composition resembling that of the shale gas is achieved under higher temperature and pressure values, i.e. conditions closer to these 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 (MD) 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 in pure. This observation agrees with earlier studies in different kerogen models comprising wider pores that have revealed negligible cross-correlation Onsager coefficients.

Deformation and Swelling of Kerogen Matrix in Light Hydrocarbons and Carbon Dioxide

“…The temperature is 393.15 K. The total uptake of carbon dioxide (6.37 × 10 –3 mmol m –2 at 400 atm) and methane (4.15 × 10 –3 mmol m –2 at 400 atm) have increasing trend between 1 and 400 atm. The trends are consistent with absolute adsorption results based on measurement of excess adsorptions. ,, The trends are also consistent with molecular simulation results by Collell et al (methane and ethane), Sui et al (methane and carbon dioxide), Vasileiadis et al (methane, ethane, n -butane, and carbon dioxide) in rigid kerogen matrices. Ho et al (methane, carbon dioxide, and helium), Tesson and Firoozabadi (methane), and Wu and Firoozabadi (methane) show also the same trends in flexible kerogen matrices.…”

Effect of Kerogen Maturity, Water Content for Carbon Dioxide, Methane, and Their Mixture Adsorption and Diffusion in Kerogen: A Computational Investigation

“…The general trend of pure CH 4 and pure CO 2 self-diffusion coefficients is similar. Namely, both of them will decrease with the pressure increase, and similar results have been reported in previously published studies. ,, This result is significant for CO 2 storage, enhancing shale gas recovery if CO 2 has a similar behavior. We will do a further study in the next research.…”

“…For two gases, whose self-diffusion coefficients increase with lower gas concentration or/and temperature increase, and at high temperatures, the correlation between CO 2 and CH 4 can be neglected, and the diffusion of gas mixture can be simplified to that of pure gas. Vasileiadis et al 53 used equilibrium MD (EMD) simulations to investigate the diffusion behavior of CH 4 , C 2 H 6 , and CO 2 , and the results show that CH 4 diffused ∼2 times faster than C 2 H 6 , and the CO 2 diffusion coefficient is slightly smaller than that of C 2 H 6 at the same conditions.…”

“…The diffusion of hydrocarbons within kerogen pores is essential for shale gas transport. − Collell et al studied the light hydrocarbon diffusion behavior in type II kerogen, and the results indicated that as the molecular weight increases, the diffusion coefficient decreases. The light hydrocarbon diffusion coefficient in kerogen is one order of magnitude lower than that in bulk.…”

Methane and carbon dioxide in dual‐porosity organic matter: Molecular simulations of adsorption and diffusion

“…For example, Collell et al 31 performed MD simulations of hydrocarbons permeating through oil‐prone type II kerogen, and found the permeation mechanism is purely diffusive. Michalec and Lísal, 3 and Vasileiadis et al 32 simulated adsorption and diffusion of various proxies of shale gas along with CO 2 in overmature type II kerogens with control microporosity. The microporosity was introduced into the kerogen structures by dummy particles of various size to mimic neglected compounds in real OM.…”