Lattice Boltzmann simulation of CO₂ transport in kerogen nanopores—An evaluation of CO₂ sequestration in organic-rich shales

Abstract

Organic-rich shale resources remain an important source of hydrocarbons considering their substantial contribution to crude oil and natural gas production around the world. Moreover, as part of mitigating the greenhouse gas effects due to the emissions of carbon dioxide (CO₂) gas, organic-rich shales are considered a possible alternate geologic storage. This is due to the adsorptive properties of organic kerogen and clay minerals within the shale matrix. Therefore, this research looks at evaluating the sequestration potential of carbon dioxide (CO₂) gas in kerogen nanopores with the use of the lattice Boltzmann method under varying experimental pressures and different pore sizes. Gas flow in micro/nano pores differ in hydrodynamics due to the dominant pore wall effects, as the mean free path (λ) of the gas molecules become comparable to the characteristic length (H) of the pores. In so doing, the traditional computational methods break down beyond the continuum region, and the lattice Boltzmann method (LBM) is employed. The lattice Boltzmann method is a mesoscopic numerical method for fluid system, where a unit of gas particles is assigned a discrete distribution function (f). The particles stream along defined lattice links and collide locally at the lattice sites to conserve mass and momentum. The effects of gas-wall collisions (Knudsen layer effects) is incorporated into the LBM through an effective-relaxation-time model, and the discontinuous velocity at the pore walls is resolved with a slip boundary condition. Above all, the time lag (slip effect) created by CO₂ gas molecules due to adsorption and desorption over a time period, and the surface diffusion as a result of the adsorption-gradient are captured by an adsorption isotherm and included in our LBM. Implementing the Langmuir adsorption isotherm at the pore walls for both CO₂ gas revealed the underlying flow mechanism for CO₂ gas in a typical kerogen nano-pore is dominated by the slip flow regime. Increasing the equilibrium pressure, increases the mass flux due to adsorption. On the other hand, an increase in the nano-pore size caused further increase in the mass flux due to free gas and that due to adsorbed gas. Thus, in the kerogen nano-pores, CO₂ gas molecules are more adsorptive indicating a possible multi-layer adsorption. Therefore, this study not only provides a clear understanding of the underlying flow mechanism of CO₂ in kerogen nano-pores, but also provides a potential alternative means to mitigate the greenhouse gas effect (GHG) by sequestering CO₂ in organic-rich shales.

Keywords

kerogen / nanopores / lattice Boltzmann method / CO₂ sequestration / slip flow / Langmuir isotherm

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