The Impact of Kerogen Tortuosity on Shale Permeability

Abstract

Summary Shale-matrix-associated transport phenomena exhibit multiple mechanisms including advective-,diffusive-, and adsorptive-driven transport modes, depending on the pore type. Diffusive processes are governed by the shale organic constituents known as kerogens. Kerogens, composed of fine-scale organic microstructures, vary with respect to their petrophysical properties, depending on their origin and maturity level. The extent to which kerogens contribute to the overall transport is governed by their ability to diffuse hydrocarbons contained within. The diffusion coefficient is a crucial parameter used to quantify diffusivity based on the interactions between the host material and the diffusing molecules. Kerogen as a hosting medium allows for diffusion of natural gas at various rates based on several factors. One of these factors, kerogen porosity, is conjectured to significantly influence diffusive transport phenomena. In this paper, taking advantage of the predictive power of molecular dynamics (MD) simulation, we investigate the impact of kerogen porosity on the diffusivity coefficient of natural gas. Starting from a single type II kerogen macromolecule, several kerogen structures for a realistic range of porosity values were created and, subsequently, used for diffusivity calculations of methane molecules. Simulation results suggest a direct link between diffusion and kerogen porosity, allowing for delineation of the diffusion tortuosity factor. Furthermore, the microscale tortuosity–diffusivity relationship in kerogens was investigated at the reservoir scale by means of a shale permeability model. The results substantiate the critical impact of the diffusion process on the shale permeability.

The Impact of Pore Structure on Kerogen Geomechanics

“…The convergence in the density is consistent with estimated mechanical moduli as shown in Figure 8. Similar observations of the number of units needed to form a realistic kerogen structure are reported in the literature [79][80][81][82][83][84].…”

Petrophysics of Kerogens Based on Realistic Structures

“…For this purpose, PSD was extracted for all six nanoporous kerogen structures. PSD characterization was conducted by positioning nonoverlapping spheres at the vertices of the resulting polyhedral grid, with a minimum threshold sphere radius of 0.2 nm (a sphere was created if its radius was greater than or equal to 0.2 nm). , The minimum threshold radius was selected based on the kinetic diameter of methane. A sample visual representation of the PSD calculations is given in Figure .…”

Geomechanics of Organic Matters Contained in Shales: A Molecular-Level Investigation

“…To control the porosity, a specified number of director molecules were inserted in the initialization stage and then removed upon final convergence to the targeted pressure and temperature (i.e., heptane was used as the choice of director in our case). The same approach of using heptane to control the porosity was followed in some other articles. , The broader range of porosity can help establish a better understanding of how mechanical properties vary with porosity. The created structures, which consisted of kerogen II-A, had porosity values of 14.33, 14.96, 17.65, and 20.25%, respectively.…”

“…The same approach of using heptane to control the porosity was followed in some other articles. 29 , 32 The broader range of porosity can help establish a better understanding of how mechanical properties vary with porosity. The created structures, which consisted of kerogen II-A, had porosity values of 14.33, 14.96, 17.65, and 20.25%, respectively.…”