Gas Adsorption Capacity of Type-II Kerogen at a Varying Burial Depth

Abstract

Kerogen is the main host of methane in most shale formations. Although many studies focused on its isothermal adsorption, the relationship between its methane adsorption capacity and structural characteristics has yet been well addressed. The pore structures and methane adsorption of four kinds of type-II kerogens with different maturities and at a varying burial depth are studied using a combined approach of molecular dynamics and grand canonical Monte Carlo simulations. Attributed to the combined effect of temperature, pressure, and the surface structures of kerogen, the methane adsorption capacity varies with the kerogen maturity and the burial depth. A maximum methane adsorption is predicted at the depth of 1–3 km, which agrees with the observations. Meanwhile, a mismatch between the porosity change and the adsorption change of kerogens is noted and then interpreted by the surface structure evolution of kerogens. Analysis on the amorphous kerogens reveals that the surface and bulk compositions of kerogens are different, and the difference varies with the kerogen maturity. Methane adsorption depends not only on the porosity but also on the proportion of aromatic and aliphatic carbon atoms on the surface. The simulations are expected to be useful for the reserve prediction and interpretation of shale gas formations.

Development of Atomistic Kerogen Models and Their Applications for Gas Adsorption and Diffusion: A Mini-Review

“…As shown in Figure 4, Wu et al studied methane adsorption in type II kerogens at different maturities while treating both the temperature and pressure as functions of the burial depth. 92 They found a maximum capacity at a depth of 1−3 km, consistent with practical observations. In agreement with other observations on different sample types and maturities, it was also found that the porosity change does not correlate directly with an adsorption change.…”

“…In an attempt to move toward shale gas recovery, molecular models were also considered to investigate the gas content in kerogen in practical conditions. As shown in Figure , Wu et al studied methane adsorption in type II kerogens at different maturities while treating both the temperature and pressure as functions of the burial depth . They found a maximum capacity at a depth of 1–3 km, consistent with practical observations.…”

“…In the case of molecular models, the density of the final 3D model is both an input and an outcome of the relaxation, because it depends on the number of building block molecules and fluid molecules (or dummy particles ,, ) used, and the applied temperature/pressure conditions . Note that despite the introduction of large (up to 40 Å) dummy particles to force a mesoporosity, upon their removal the 3D structure always relaxes and partly fills the created voids, allowing a maximum pore diameter of ∼20 Å to be reached independently of the number or size of spacer particles introduced. , This maximum pore size would be ∼8 Å at 1 bar and 298 K without any fluid or dummy particles, with decreasing values for larger pressures. ,, For IG, QMD, and HRMC models, density is imposed at the beginning of the simulation by the number of atoms placed in the simulation box of a fixed size.…”

“…The color code, which corresponds to kerogens of increasing maturity as obtained using HRMC, is as follows: MEK (yellow), EFK (red), MarK (blue), and PY02 (black). (b) Methane adsorption isotherm as a function of burial depth for 4 different molecular models of type II kerogen . Snapshots of the models (reproducedand adaptedwith permission from ref .…”

“…(b) Methane adsorption isotherm as a function of burial depth for 4 different molecular models of type II kerogen . Snapshots of the models (reproducedand adaptedwith permission from ref . Copyright 2022 American Chemical Society) are shown on the right for the sake of clarity.…”