Effects of Moisture Contents on Shale Gas Recovery and CO2 Sequestration.

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.

Introduction

Shale gas, which predominantly consists of methane, has become an increasingly important energy source owing to its low emissions, high energy efficiency, and abundant reserves in the world.[1,2] In contrast to the conventional reservoirs, shale rocks have the characteristics of extremely low permeability in the nanodarcy range and low porosity.[3] The development of horizontal and hydraulic fracturing methods has greatly advanced shale gas exploitation in the United States.[4,5] Unfortunately, the widely used depressurization method becomes inefficient and significant amounts remain unrecoverable.[6] Recently, enhanced gas recovery (EGR) with CO2 injection is considered to be a promising alternative that can not only enhance shale gas productivity but also mitigate the climate change via geological CO2 sequestration,[7] in which CO2 captured from power plants is injected into the shale gas reservoirs and sequestrated in a safe and permanent manner. The idea of swapping CO2 for CH4 can also be applied to other replacement reactions, such as the recovery of CH4 from hydrate reservoirs.[8]

Kerogen makes up the major organic part of the shale rocks[9] and pores, mainly at the nanoscale, where a large amount of shale gas can be stored.[10] Therefore, it is crucial to understand the adsorption properties of CH4 and CO2 in kerogen in an attempt to enhance the efficiency of shale gas recovery and CO2 sequestration. It is also well known that shale is under moist conditions.[11−14] For example, kerogen from the Kuonamka Formation was reported to have moisture contents ranging from 0.6 to 5%.[11] Plenty of experimental work studied the effects of moisture on pure methane adsorption and suggested that the presence of water occupies the pore volume and thus leads to a sharp decrease in the methane adsorption capacity in shale.[14−17] On the other hand, relatively few experiments focused on the influence of moisture on the competitive adsorption of CH4 and CO2. Gensterblum et al.[18] investigated the effect of preadsorbed water on the CH4 and CO2 adsorption in coals and observed that the CO2/CH4 adsorption ratios at low surface coverage are generally higher in moist coals than in dry conditions, and with the presence of water, the mobility of CO2 and CH4 is reduced. Few laboratory studies have been reported on the competitive adsorption of CO2 and CH4 in the moist kerogen. Isolation of the kerogen from shale samples with the morphology of the kerogen intact remains challenging for experiments.[9] Moreover, shale rocks contain a large amount of nanoscaled pores, further increasing the difficulties of experimental approaches in probing the adsorption behaviors in shales.

Molecular simulations have been successfully applied to interfacial and colloid science,[19] including adsorption,[20−22] wetting,[23,24] and surfactants.[25] Much research in recent years has made significant progress in understanding the competitive adsorption of CH4 and CO2 in kerogens using grand canonical Monte Carlo (GCMC) simulations.[26−30] Huang et al.[29] investigated the effect of moisture on the CO2/CH4 competitive adsorption in kerogen matrix and found that the increased moisture content leads to a decrease in adsorption capacity, and the CO2/CH4 adsorption selectivity first decreases and then increases. However, most of the work focused on the adsorption inside the kerogen matrix, in which the pores are isolated and not well connected, and the pore size is only a few angstroms. Recently, Wang et al.[31] studied the adsorption of CH4/CO2 mixtures in moist kerogen using a 2 nm wide slit-pore model and stated that the effect of moisture content on adsorption selectivity is not obvious for shale kerogen slit nanopores. A more realistic kerogen pore structure is used in their study, but the water molecules are placed inside the kerogen matrix rather than within the slit pores. The interaction between water and the adsorbate gas, which is mainly located in the slit pores, is weakened due to the separation. Therefore, the moisture effects on the adsorption of CH4 and CO2 are still not fully understood. In addition, the dependence of adsorption selectivity on pressure and pore size is extensively used in the previous research.[29−32] To some extent, it can imply the relative adsorption affinity of CH4 and CO2 to the adsorbent but fails to reflect the shale gas recovery process with CO2 injection, namely, CO2 huff-n-puff process, consisting of three stages: CO2 injection, soaking, and production.[6] CO2 is first injected into the production well (huff) and then the well is shut in and soaks for a period; finally comes the production (puff).[6]

In this work, we aim to reveal the effects of moisture on the competitive adsorption of CH4/CO2 binary mixtures in kerogen using GCMC simulations. Kerogen slit-pore models of different pore widths are constructed to represent the pore structure in shale.[33,34] First, we analyze the adsorption of pure methane in kerogen slit pores with and without the presence of moisture under a wide range of pressures from 10 to 60 MPa. Then, we discuss the moisture effects on the adsorption of CH4/CO2 binary mixtures in detail. Finally, we simulate the enhanced shale gas recovery by CO2 huff-and-puff and compare the mechanisms of the pressure drawdown and CO2 injection processes on shale gas recovery. The effects of moisture on the shale gas recovery and CO2 sequestration are also revealed.

Methods

Molecular Models

Kerogen can be classified into four types according to the depositional origin.[35,36] Type II kerogen, typically derived from marine sediments, is selected in this work for its abundance in shale deposits, as well as good potential for generating oil/gas.[37−39] The kerogen molecular unit (type II-C) used in the simulations was proposed by Ungerer et al.[40] on the basis of experimental data from the Duvernay shale formation.[41] Besides, type II-D kerogen has also been used to study the shale gas sorption behavior.[42]

Molecular dynamics (MD) simulations are conducted in the canonical ensemble (NVT) and isobaric–isothermal ensemble (NPT) using the large-scale atomic/molecular massively parallel simulator (LAMMPS) package[43] to generate kerogen matrixes and slit pores. The Dreiding force field[44] is employed to describe the kerogen properties. First, the initial structure of the kerogen macromolecule is relaxed individually. Then, 12 relaxed kerogen units are randomly placed in a simulation box of 100 × 100 × 100 Å3. The final configuration of the kerogen matrix was created through a series of annealing procedures, as reported in Collell et al.[45] The density of the simulated kerogen matrix is 1.22 ± 0.02 g/cm3, which is within the range of the experimental value of mature shales (1.18–1.35 g/cm3).[46] The pore size distribution of the kerogen matrix is presented in Figure S1, calculated by the method of sphere insertion proposed by Bhattacharya and Gubbins.[47] Finally, we extend the simulation box in the Z-direction to create a kerogen slit nanopore, as shown in Figure . Note that we use the slit-shaped pore model as slit pores are very common in shale formations.[33,34] The pore width W is defined as the distance between the rightmost atom in the left slab and the leftmost atom in the right slab in the Z-direction. Two pore widths of 2 and 4 nm are constructed to investigate the pore size effects.

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.

Methane molecules are simulated using TraPPE force field, and the united-atom model is applied.[48] The carbon dioxide molecule is treated as a rigid and linear structure with TraPPE-EH force field, where the C–O bond length and O–C–O bond angle are fixed as 1.16 Å and 180°, respectively.[49] The SPC/E model[50] is chosen for water with the O–H bond length of 1 Å and the H–O–H angle of 109.47°. Interaction between two atoms is calculated by the sum of Lennard-Jones (LJ) and electrostatic potential energywhere r is the distance between atoms i and j; ε and σ represent the LJ potential well depth and the zero-potential distance, respectively; q is the charge of atoms; and k = 8.988 × 109 N·m2·C–2 is the electrostatic constant. The force field parameters are listed in Table . Lorentz–Bertherlot mixing rules[51] are adopted to calculate interactions between unlike atoms. A cutoff distance of 14 Å is employed for short-range Lennard-Jones interactions, and analytical tail corrections are applied.[52] As our system has a finite length along the Z-direction, the conventional three-dimensional Ewald summation technique is not valid for the calculation of the long-range electrostatic interactions. Here, we use the same approach as for the three-dimensional Ewald summation, but an empty space is inserted between periodic replicas to avoid the artificial influence from the periodic images in the Z-direction.[52,53] Tests are carried out to ensure the length of the empty space is long enough that the artificial effects could be eliminated. The entire simulation box including the empty space is shown in Figure S2. During the simulations, the molecules are only allowed to move within the kerogen slit pore, but not into the vacuum.

Table 1

Force Field Parameters for Methane, Carbon Dioxide, and Water

atom	ε (K)	σ (Å)	q (e)
methane
CH4	148	3.73	0
carbon dioxide
C	27	2.80	0.70
O	79	3.05	–0.35
water
H	0	0	0.4238
O	78.18	3.166	–0.8476

Simulation Details

GCMC simulations are carried out in the grand canonical ensemble (μVT) to investigate the adsorption of CH4 and CO2 in kerogen slit pores. In the GCMC simulations, gases inside the kerogen slit pores are assumed to be in equilibrium with an external bulk reservoir under the same temperature and chemical potentials. The equilibration process is achieved by performing insertion, deletion, and translation moves for the gas molecules. For CO2 and H2O molecules, rotational moves are also applied. The chemical potentials are obtained by Widom’s insertion method[54,55] using Monte Carlo simulations in the NVT ensemble, where the fluids are simulated in bulk phase without confinement. The calculated chemical potential values are verified by μVT simulations compared to those of NIST database[56] (Figure S3). The bulk densities of CO2 and CH4 mixtures at a given pressure and temperature are calculated from the Peng–Robinson equation of state (PR-EOS).[57,58] Bulk densities calculated by PR-EOS have been verified by comparing μVT simulations, as shown in Figure S4. MCCCS Towhee, a Monte Carlo molecular simulation code, is utilized in all of the GCMC simulations.[59]

During the GCMC simulations, the kerogen slit is kept rigid, but the water molecules are allowed to move. The number of water molecules is kept constant, based on the assumption that the water molecules remain in the pore during the shale gas recovery process. Besides, the water content is reported as the volumetric mass density of water, ρHave, in the kerogen slit pore with the unit of g/cm3. The pressure in the simulations denotes the pressure of the external bulk reservoir, which is in equilibrium with the confined system.

Results and Discussion

In this section, we first investigate the adsorption of pure CH4 and CH4/CO2 binary mixtures in kerogen slit pores, and the moisture effects are analyzed in detail. Then, we discuss the practical implications for the shale gas recovery and CO2 sequestration during the CO2 huff-and-puff process.

Adsorption of Pure CH4 in Kerogen

Dry Condition

We present methane density distributions in various nanopores under dry conditions at 338.15 K in Figure . For all of the pressures, two strong methane adsorption layers are formed near the kerogen surfaces, and the methane density within the adsorption layer increases with pressure. Under higher pressures (over 10 MPa), the CH4–CH4 interaction becomes stronger and a weak second adsorption layer can be observed. In the middle of kerogen slit pores, the methane density is higher in the smaller pore (W = 2 nm) due to the strong fluid–surface interactions. As the pore width increases (W = 4 nm), the fluid–surface interaction becomes weaker. As a result, the methane density in the middle of the kerogen slit pore reaches bulk.

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.[56]

Moist Condition

To investigate the moisture effects on methane adsorption in kerogen, the methane adsorption under different moisture contents is simulated. The snapshots of CH4 and H2O molecules in the moist kerogen slit pores (ρHave 0.186 g/cm3) under different pressures are shown in Figure . For all of the pressure conditions ranging from 10 to 60 MPa, water molecules form clusters in the middle of the kerogen slit pore, unlike in clay nanopores.[60] This is because the kerogen is hydrophobic, while the clay is hydrophilic. Similar phenomena were also reported on graphene and montmorillonite.[61]

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.

The effects of water on the methane density distribution in 2 nm kerogen slit pores under different pressures are presented in Figure . In general, the CH4 density in the middle of the kerogen slit pore decreases significantly as water molecules are predominantly distributed in the middle of the pore in the form of clusters. The methane density distribution in the presence of water is not symmetric because the water cluster is not located exactly at the center of the slit pore. For methane within the adsorption layer, the density also slightly decreases. From the water density distribution (as shown in Figure S5), some water molecules are distributed near the kerogen surface, occupying the adsorption sites on the surface and resulting in the decrease of methane in the adsorption layer.

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 (ρH2Oave = 0 g/cm3) and moist condition (ρH2Oave = 0.186 g/cm3), respectively.

The average density of methane stored in the kerogen slit pore can be given aswhere ⟨N⟩ is the ensemble averaged number of methane molecules in kerogen slit pores, V denotes the volume of the slit pore, and NA is the Avogadro constant. We only consider the methane molecules in slit pores, excluding those inside the kerogen matrix. Figure shows the total uptake of CH4 at 338.15 K in kerogen slit pores of different pore widths with and without moisture. Statistical uncertainties on the sorption isotherms are examined, and the errors bars are less than the symbols. For both pore widths, the amount of adsorbed methane decreases monotonically with increasing water content. As shown in Figure , the water molecules are distributed in the middle of the slit pore in the form of clusters and occupy a fraction of the pore volume. Thus, they impede the pore filling of methane molecules in the middle of the pore. By comparing the CH4 total uptakes in different pores (Figure ), the reduction in the average density of methane is similar for 2 and 4 nm kerogen slit pores, when they have the same water volumetric density (ρHave 0.186 g/cm3) rather than the same surface density. It demonstrates that the volumetric density of water may be more appropriate to define the water content in kerogen to quantify the effect of moisture on gas adsorption. CH4 excess sorption isotherms in 2 and 4 nm dry kerogen slit pores are also calculated using effective pore volume from helium adsorption,[62] and the results are shown in Figure S6. It first reaches a maximum at around 15 MPa and then decreases with increasing pressure. Excess sorption in 2 nm kerogen slit pores is larger than that in 4 nm kerogen slit pores due to the stronger fluid–wall interaction.

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.

Adsorption of CH4/CO2 Mixtures in Kerogen

The density distributions of CH4 and CO2 in the equimolar mixtures at 10 MPa and 338.15 K in kerogen slit pores of different pore widths are presented in Figure . The density distributions of both CO2 and CH4 have two peaks near the kerogen walls, but the density of CO2 is much higher than that of CH4, as CO2 has a stronger affinity to kerogen than CH4.[63] In the middle of 2 nm kerogen pores, both the densities of CO2 and CH4 are higher than the bulk density due to the strong fluid–surface interactions. When W = 4 nm, the fluid–surface interaction becomes weaker. Therefore, the densities in the middle of pores approach the bulk density. The density distributions of pure methane in kerogen slit pores under the same pressure are also shown in Figure for comparison. With the competitive adsorption of CO2, the density of methane decreases significantly, especially on the adsorption layer.

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 displays the total uptake of CH4–CO2 binary mixtures of different compositions in the dry kerogen slit pores of different pore widths at 338.15 K and over a wide range of pressures from 10 to 60 MPa. Similar trends can be observed for CH4 in the kerogen slit pores of different pore widths that the average density increases quickly with pressure at the beginning and gradually reaches a plateau. The average density of CH4 in kerogen slit pores increases as its mole fraction in the mixtures increases. For CO2 molecules, sorption increases gradually with the increasing pressure. Similar to CH4, as the mole fraction of CO2 in the binary mixtures increases, an increase in the corresponding CO2 adsorption can be observed. In the equimolar mixtures, the CO2 average density in kerogen slit pores is much higher than that of CH4.

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.

Sorption of CH4/CO2 binary mixtures in the moist kerogen slit pores is also investigated using GCMC simulations. As shown in Figure , water clusters are formed in the kerogen slit pores with the sorption of CH4/CO2 binary mixtures under pressures ranging from 10 to 60 MPa as pure CH4 sorption with moisture. When there is only methane sorption in the moist kerogen slit pore, the pore surface is occupied by methane molecules. With the CO2 cosorption, the adsorption sites on the kerogen surface are mainly covered by CO2 molecules. Besides, the solubility of methane in water is much lower than that of CO2.[64] As a result, CH4 sorption in the moist condition is significantly reduced due to the cosorption of CO2.

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.

Effects of moisture contents on the total uptake of CH4/CO2 binary mixtures are illustrated in Figure . Similar to the trends in dry conditions, the average densities of both CH4 and CO2 in moist kerogen slit pores increase when their mole fraction increases. As the moisture content increases, the methane average density decreases significantly, independent of its mole fraction in the mixtures (Figure a). However, the effect of moisture content on CO2 adsorption is associated with the mole fraction of CO2 in the binary mixtures. For the case of low CO2 bulk mole fraction in mixtures (yCO = 0.25), a slight increase in CO2 adsorption can be found in the moist condition, while the opposite is true at high CO2 mole fraction (yCO = 0.75). As stated in the previous section, the presence of moisture fills up the pore volume in kerogen slit pores, which results in the reduction in CH4 sorption. With the CO2 cosorption, the water–CH4 binary interaction turns into the water–CH4–CO2 ternary interaction. Since the water–CO2 interaction is much stronger than the water–CH4 interaction, the decrease in accessible pore volumes for CH4 can partially be occupied by the more favorable CO2 sorption. Besides, within the region near the kerogen surface, CO2 exhibits a higher affinity to kerogen compared to CH4. Therefore, the CO2 has a competitive advantage over CH4 both in the middle of the kerogen slit pore and near the surface, which are shown in the snapshots (Figure ) and density profiles (Figure S7). When the bulk mole fraction of CH4 is relatively high, the decrease in methane sorption caused by the presence of water is pronounced. When the mole fraction of CO2 is higher, both CO2 and CH4 need to compete with water for adsorption sites or space. The occupation of water in the kerogen slit pores limits the sorption of CO2 and CH4, thereby diminishing their sorption capacities. The sorption selectivity of CO2 over CH4 with different moisture contents is presented in Figure S8 to characterize the preferential adsorption. For the moisture contents considered in this work, the CO2/CH4 adsorption selectivity increases with increasing moisture content.

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 ρH2Oave = 0 g/cm3, red for ρH2Oave = 0.186 g/cm3, blue for ρH2Oave = 0.372 g/cm3.

Implications for Shale Gas Recovery and CO2 Storage

As outlined in the Introduction, this work aims to unravel the CH4 recovery mechanisms under moist condition during the pressure drawdown process and CO2 injection to investigate the recovery and sequestration efficiency. The simplified recovery process consists of two pressure drawdowns and two CO2 injections, as illustrated in Figure . The initial pressure of the target shale gas reservoir is assumed to be 30 MPa, which is within the typical pressure range of realistic reservoir conditions. The CH4 sorption amount in the kerogen slit pores is obtained by GCMC simulations. Then, the shale gas recovery is initiated by the primary pressure drawdown process, and the reservoir pressure is reduced to 20 MPa, while the amount of CH4 residing in the kerogen slit pores is calculated via μVT simulation. Subsequently, CO2 is injected into the shale gas reservoirs. During this process, we assume that the pore volume in the fractures (external bulk reservoir) remains the same; therefore, the CH4 density in the bulk phase of CH4/CO2 mixtures is the same as that in pure CH4. The resulting CH4/CO2 mixtures in the kerogen slit pores are determined by the chemical potentials of CH4/CO2 mixtures, which are obtained by NVT simulations. After the system has reached equilibrium, a pressure depletion process is applied again, while the bulk CH4/CO2 composition stays the same. The dynamics and transport processes are not considered.[65] This process achieves one cycle of CO2 huff-n-puff in enhanced gas recovery.[6,66] Consequently, a second CO2 huff-n-puff process is operated.

Figure 10

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

Evolution in the composition of fluids in the kerogen slit pores during the shale gas recovery process is shown in Figure . During the pressure drawdown, the average CH4 density in the kerogen slit pores decreases continuously, and with the injection of CO2, the CH4 molecules are further released. The CO2 molecules are sequestrated in the kerogen slit pores after injection. The effects of moisture content are also shown in Figure ; as the moisture content increases, the average density of methane at the initial stage is reduced from 13.2 mmol/cm3 in the dry condition to 7.68 mmol/cm3 with an average water density of 0.372 g/cm3, but the recovery ratio of CH4 after the whole recovery process is increased from 58.5 to 70.1%.

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.

To compare the efficiency of the shale gas recovery and CO2 sequestration during every intermediate process and quantify the effects of moisture contents, we introduce two parameters: CH4 recovery ratio and CO2 sequestration ratio, taking the initial pressure (30 MPa) of the reservoir as the reference. The CH4 recovery ratio is defined as the number of CH4 molecules released during a single process relative to the initial amount in the kerogen slit pore under 30 MPa with/without moist. The CO2 sequestration ratio is defined as the number of CO2 molecules sequestrated during the CO2 injection process relative to its adsorption amount in the kerogen slit pore under 30 MPa, namely, the maximum amount of CO2 sequestration in a 30 MPa reservoir with/without moist. The CO2 sequestration ratio can serve as a parameter for indicating the extent of CO2 sequestration. The two parameters are plotted as a percentage in Figure under different moisture contents. As can be seen from Figure a, as the moisture content increases, the CH4 recovery ratio of the pressure drawdown processes decreases, while that of CO2 injection processes increases, which means in the moist conditions, the performance of the CO2 injection method can be even better, while the efficiency of the pressure drop method is lowered. In the dry conditions (0 g/cm3), the first pressure drawdown process accounts for the largest proportion of the total amount of shale gas recovered, while in the moist conditions, the CH4 recovery ratio of the first CO2 injection process is the largest over the whole recovery process. To clarify the role of CO2 injection in the recovery process, we calculate the recovery ratio of the process with the pressure drawdown only from 30 to 20 MPa. The recovery ratios between the two processes are compared (Figure S9). With the combination of CO2 injection, recovery ratios are enhanced by 2.4, 3.1, and 5.1 times for water densities of 0, 0.186, and 0.372 g/cm3, respectively. For the CO2 sequestration during the CO2 injection process (Figure b), the CO2 sequestration ratio rises with increasing moisture content, especially in the first stage. A considerable CO2 sequestration ratio of 63.4% in total can be achieved in the moist kerogen slit pores at an average water density of 0.372 g/cm3.

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.

To explain the different effects of moisture contents on the pressure drop method and CO2 injection method, we further investigate the CH4 recovery mechanisms of the two methods. The evolution of the CH4 density distributions during the gas recovery process with different moisture contents is plotted in Figure . After the pressure drawdown, the CH4 density in the middle of the kerogen slit pore is reduced, while after the CO2 injection, the decrease in CH4 density mainly occurs in the adsorption layer near the kerogen surface, indicating different mechanisms in shale gas recovery. In other words, the pressure drawdown releases the free gas in the middle of pores, while the CO2 injection recovers CH4 from the adsorption layers. In the presence of water in kerogen slit pores (Figure b,c), since the water molecules accumulate as clusters in the middle of kerogen slit pores, the moisture content negatively affects the performance of the pressure drawdown. Meanwhile, the adverse effect of moisture on the CO2 injection method is negligible, as the water–CO2 interaction is much stronger than the water–CH4 interaction. Hence, the presence of water could further improve the efficiency of the CO2 injection method. Generally, in the moist conditions, the CO2 injection behaves better, while the efficiency of the pressure drawdown is lower.

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.

Conclusions

In this work, the effects of moisture on the adsorption of pure CH4 and CH4/CO2 binary mixtures in kerogen slit pores are investigated using GCMC simulations. The shale gas recovery mechanisms of the pressure drawdown and CO2 injection are studied, and the moisture effects on the shale gas recovery and CO2 sequestration efficiency are further explored.

Our simulation results show that for the sorption of pure CH4 in the moist conditions, water clusters are formed in the middle of a kerogen slit pore and the clusters occupy the volume in the slit pore, thereby impeding the pore filling of CH4. Regarding the sorption of CH4/CO2 binary mixture, the CH4 sorption capacity in the mixture decreases with increasing moisture content, while the effect of moisture on CO2 adsorption capacity depends on the mole fraction of CO2 in the mixture due to the ternary interactions among CH4, CO2, and H2O. During the shale gas recovery process, the pressure drawdown and CO2 injection present different mechanisms. The pressure drawdown releases CH4 molecules in the middle of kerogen slit pores, while CO2 injection can release CH4 molecules in the adsorption layer. As 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. Finally, the CO2 sequestration efficiency is higher under moist conditions.

This work provides important insights into the effects of moisture content on gas adsorption in kerogen. As the subsurface water in the shale formations is saline, further work is planned to examine the effects of saline water on the recovery process. Besides, the recovery process introduced in this work can be extended to the optimization of the shale gas recovery process. For example, adjusting the CO2 injection pressure and depletion pressure may lead to optimal shale gas recovery efficiency or CO2 sequestration efficiency. The recovery process can also be applied to other displacement processes in confined systems.