Effects of Moisture Contents on Shale Gas Recovery and CO2 Sequestration

Abstract

Enhanced recovery of shale gas with CO₂ injection has attracted extensive attention as it combines the advantages of improved efficiency of shale gas recovery and reduced greenhouse gas emissions via CO₂ geological sequestration. On the other hand, the microscopic mechanism of enhanced shale gas recovery with CO₂ injection and the influence of the subsurface water confined in the shale nanopores remain ambiguous. Here, we use grand canonical Monte Carlo (GCMC) simulations to investigate the effect of moisture on the shale gas recovery and CO₂ sequestration by calculating the adsorption of CH₄ and CO₂ in dry and moist kerogen slit pores. Simulation results indicate that water accumulates in the form of clusters in the middle of the kerogen slit pore. Formation of water clusters in kerogen slit pores reduces pore filling by methane molecules, resulting in a decrease in the methane sorption capacity. For the sorption of CH₄/CO₂ binary mixtures in kerogen slit pores, the CH₄ sorption capacity decreases as the moisture content increases, whereas the effect of moisture on CO₂ sorption capacity is related to its mole fraction in the CH₄/CO₂ binary mixture. Furthermore, we propose a reference route for shale gas recovery and find that the pressure drawdown and CO₂ injection exhibit different mechanisms for gas recovery. Pressure drawdown mainly extracts the CH₄ molecules distributed in the middle of kerogen slit pores, while CO₂ injection recovers CH₄ molecules from the adsorption layer. When the water content increases, the recovery ratio of the pressure drawdown declines, while that of CO₂ injection increases, especially in the first stage of CO₂ injection. The CO₂ sequestration efficiency is higher under higher water content. These findings provide the theoretical foundation for optimization of the shale gas recovery process, as well as effective CO₂ sequestration in depleted gas reservoirs.

Figure 1

Atomistic model of kerogen slit nanopore. The pore width is 2 nm. Carbon atoms are depicted by gray balls, hydrogen by white, oxygen by red, nitrogen by blue, and sulfur by purple.

Figure 2

Methane density distributions in (a) 2 nm and (b) 4 nm kerogen slit pores under various bulk pressures and T = 338.15 K. Dashed lines represent the CH₄ bulk density obtained from the NIST Chemistry Webbook.

Figure 3

Snapshots of CH₄ and H₂O molecules in a 2 nm kerogen slit nanopore at 338.15 K under different pressures: (a) 10, (b) 20, (c) 30, (d) 40, (e) 50, and (f) 60 MPa with an average water density of 0.186 g/cm³.

Figure 4

Methane density distributions in a 2 nm kerogen slit nanopore at different pressures: P = 10 MPa (blue) and 60 MPa (red), T = 338.15 K. The solid and dashed lines represent the CH₄ density distribution under dry condition (ρ_H2O^ave = 0 g/cm³) and moist condition (ρ_H2O^ave = 0.186 g/cm³), respectively.

Figure 5

Average density of methane confined in kerogen slit pores with pore widths (a) 2 nm and (b) 4 nm at 338.15 K. The black, red, and blue lines represent the CH₄ adsorption at the dry condition and moist condition of different contents.

Figure 6

Density distributions of CH₄ and CO₂ molecules at 10 MPa and 338.15 K, respectively, in kerogen slit pores of widths (a) 2 nm and (b) 4 nm. The blue dashed lines represent the CH₄ density distribution in single-component adsorption and the solid lines represent the CH₄ and CO₂ density distributions in a binary mixture with a mole fraction of 0.5. The black dashed lines represent the bulk density of CH₄/CO₂.

Figure 7

Average density of CH₄ (left) and CO₂ (right) in mixtures of different compositions confined in kerogen slit pores with pore widths of 2 nm (top) and 4 nm (bottom) under different pressures at 338.15 K.

Figure 8

Snapshots of CH₄/H₂O mixtures (top) and CH₄/CO₂/H₂O mixtures (bottom) in 2 nm kerogen slit nanopores at 338.15 K under different bulk pressures: 10, 30, and 60 MPa from left to right with an average water density of 0.186 g/cm³. The mole fraction of CH₄ in the CH₄/CO₂ binary mixtures is 0.5.

Figure 9

Total uptake of (a) CH₄ and (b) CO₂ molecules in mixtures of different compositions at 338.15 K in 2 nm kerogen slit nanopores with different moisture contents. The solid lines represent the mixtures with mole fractions of CH₄/CO₂ = 3:1, dashed lines CH₄/CO₂ = 1:1, and dotted lines CH₄/CO₂ = 1:3. The different colors represent different water contents: black for ρ_H2O^ave = 0 g/cm³, red for ρ_H2O^ave = 0.186 g/cm³, blue for ρ_H2O^ave = 0.372 g/cm³.

Figure 10

Schematic representation of the shale gas recovery process. More information about the recovery process is provided in the Supporting Information.

Figure 11

Composition of fluids in the 2 nm kerogen slit pores during the gas recovery process with different moisture contents: (a, b) 0 g/cm³, (c, d) 0.186 g/cm³, and (e, f) 0.372 g/cm³. The arrows in the figure indicate the direction of the recovery process.

Figure 12

CH₄ recovery ratio (a) and CO₂ sequestration ratio (b) with respect to water content during the shale gas recovery process in 2 nm kerogen slit pores at 338.15 K.

Figure 13

Evolution of the CH₄ density distributions inside the 2 nm kerogen slit pores during the gas recovery process with varying moisture contents: (a) 0 g/cm³, dry condition (b) 0.186 g/cm³, and (c) 0.372 g/cm³. The CO₂ density distribution during the CO₂ injection process is presented in Figure S10.