Molecular Dynamics Study on CO₂ Storage in Water-Filled Kerogen Nanopores in Shale Reservoirs: Effects of Kerogen Maturity and Pore Size

Abstract

CO2 sequestration in shale reservoirs is an economically viable option to alleviate carbon emission. Kerogen, a major component in the organic matter in shale, is associated with a large number of nanopores, which might be filled with water. However, the CO2 storage mechanism and capacity in water-filled kerogen nanopores are poorly understood. Therefore, in this work, we use molecular dynamics simulation to study the effects of kerogen maturity and pore size on CO2 storage mechanism and capacity in water-filled kerogen nanopores. Type II kerogen with different degrees of maturity (II-A, II-B, II-C, and II-D) is chosen, and three pore sizes (1, 2, and 4 nm) are designed. The results show that CO2 storage mechanisms are different in the 1 nm pore and the larger ones. In 1 nm kerogen pores, water is completely displaced by CO2 due to the strong interactions between kerogen and CO2 as well as among CO2. CO2 storage capacity in 1 nm pores can be up to 1.5 times its bulk phase in a given volume. On the other hand, in 2 and 4 nm pores, while CO2 is dissolved in the middle of the pore (away from the kerogen surface), in the vicinity of the kerogen surface, CO2 can form nano-sized clusters. These CO2 clusters would enhance the overall CO2 storage capacity in the nanopores, while the enhancement becomes less significant as pore size increases. Kerogen maturity has minor influences on CO2 storage capacity. Type II-A (immature) kerogen has the lowest storage capacity because of its high heteroatom surface density, which can form hydrogen bonds with water and reduce the available CO2 storage space. The other three kerogens are comparable in terms of CO2 storage capacity. This work should shed some light on CO2 storage evaluation in shale reservoirs.

Diffusion Behavior of Methane in 3D Kerogen Models

“…PCFF+ was originally used in the development of kerogen model units by Ungerer et al However, the short-range non-bonded interactions in PCFF+ are described by a repulsive–attractive 9–6 Lennard-Jones (LJ) potential, which is incompatible with many other force fields that include a 12–6 LJ potential, such that it is difficult to simulate kerogens with other compounds. CVFF is one of the most commonly used force fields in kerogen simulations, − ,,, and the 12–6 LJ potential in its functional form is compatible with other force fields. CVFF correctly reproduces the experimental density of the Type II kerogen series and provides a reasonable description of kerogen interactions with its constituents, for example, carbon dioxide, methane (TraPPE-UA), and water .…”

“…CVFF is one of the most commonly used force fields in kerogen simulations, − ,,, and the 12–6 LJ potential in its functional form is compatible with other force fields. CVFF correctly reproduces the experimental density of the Type II kerogen series and provides a reasonable description of kerogen interactions with its constituents, for example, carbon dioxide, methane (TraPPE-UA), and water . The partial atomic charges of CVFF atoms were assigned using a bond increment scheme.…”

“…Both studies observed that the CH 4 adsorption isotherm in the kerogen follows type I Langmuir adsorption behavior. Li et al . conducted MD simulations to study CO 2 storage in water-filled slit-shaped nanopores of different sizes through a kerogen matrix of the Type II series.…”

A comprehensive review on the flow behaviour in shale gas reservoirs: Multi‐scale, multi‐phase, and multi‐physics

“…(a) Fluid transport behaviour in shale nanopores with realistic surfaces: although the elementary compositions and molecular structures of OM and IOM can be obtained with chemical analysis, nanopores with realistic surface properties still cannot be fully structured in MD simulations, including the maturity of OM, surface charge of IOM, surface roughness, and the wettability heterogeneity of pore surfaces, which can have significant influences on gas adsorption and water transport. [265][266][267] (b) The properties of fluids in nanopores: for gas transport, multicomponent gas transport in nanopores should be paid more attention; for water transport, pure water can be extended to a saline solution or non-Newtonian fluid. [35,268,269] (c) Compared with single-phase transport, simulation of multiphase transport in shale nanopores is insufficient: the flow pattern and interfacial interactions of multiple fluids at the nanoscale are still unclear.…”

Ethanol Blending to Improve Reverse Micelle Dispersity in Supercritical CO2: A Molecular Dynamics Study

“…In addition, the scCO 2 flooding in tight reservoirs benefits from the generally excellent miscibility between CO 2 and oils − and promotes oil swelling and viscosity reduction, which are favorable for EOR. On the other hand, as one of the several geological carbon sequestration (GCS) and CO 2 utilization schemes, − CO 2 injection into tight formations can help mitigate carbon emissions. However, CO 2 flooding in tight formations is subject to gas channeling, which can adversely impact the efficacy of EOR .…”