Poroelasticity of Methane-Loaded Mature and Immature Kerogen from Molecular Simulations

Abstract

While hydrocarbon expulsion from kerogen is certainly the key step in shale oil/gas recovery, the poromechanical couplings governing this desorption process, taking place under a significant pressure gradient, are still poorly understood. Especially, most molecular simulation investigations of hydrocarbon adsorption and transport in kerogen have so far been performed under the rigid matrix approximation, implying that the pore space is independent of pressure, temperature, and fluid loading, or in other words, neglecting poromechanics. Here, using two hydrogenated porous carbon models as proxies for immature and overmature kerogen, that is, highly aliphatic hydrogen-rich vs highly aromatic hydrogen-poor models, we perform an extensive molecular-dynamics-based investigation of the evolution of the poroelastic properties of those matrices with respect to temperature, external pressure, and methane loading as a prototype alkane molecule. The rigid matrix approximation is shown to hold reasonably well for overmature kerogen even though accounting for flexibility has allowed us to observe the well-known small volume contraction at low fluid loading and temperature. Our results demonstrate that immature kerogen is highly deformable. Within the ranges of conditions considered in this work, its density can double and its accessible porosity (to a methane molecule) can increase from 0 to ∼30%. We also show that these deformations are significantly nonaffine (i.e., nonhomogeneous), especially upon fluid adsorption or desorption.

Mesoscale structure, mechanics, and transport properties of source rocks’ organic pore networks

“…We also report the same quantities in the LEF case but with diffusion in the nanoporous phase described by a more general free-volume model that accounts for adsorption-induced swelling ( SI Appendix ), which leads to an exponential increase as a function of pressure (46, 47). Due to swelling, the adsorption isotherm evolves linearly with P (until P > 50 MPa) (48), thus lowering the impact of sorption on transport at the mesoscale. As a result, mesopores do not induce a qualitative change on the transport behavior at the mesoscale.…”

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

“…26 However, flexible kerogen models have been considered, where they exhibited an increase in gas diffusivity with respect to rigid kerogens due to fluctuations in the pore connectivity. [27][28][29][30] Due to their robust properties, the representative molecular models have become a preferential approach for modeling shale matrices with a broad diversity of physical properties such as TOC, density, porosity, and permeability, as demonstrated by the growing number of molecular simulation studies found in the literature. 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.…”

Coupling between adsorption and mechanics (and vice versa)

“…The PSD can evolve in a complex manner upon adsorption, due to the inhomogeneity of the microscopic deformations, as was shown numerically for a microporous carbon in presence of methane [40]. The PSD being in fact stress-dependent, Siderius et al [41] showed that how the PSD evolves over the adsorption process theoretically contains information related to the flexibility and deformation characteristics of the adsorbent.…”