Molecular Simulation of Displacement of Methane by Injection Gases in Shale

Abstract

Displacement methane (CH₄) by injection gases is regarded as an effective way to exploit shale gas and sequestrate carbon dioxide (CO₂). In our work, the displacement of CH₄ by injection gases is studied by using the grand canonical Monte Carlo (GCMC) simulation. Then, we use molecular dynamics (MD) simulation to study the adsorption occurrence behavior of CH₄ in different pore size. This shale model is composed of organic and inorganic material, which is an original and comprehensive simplification for the real shale composition. The results show that both the displacement amount of CH₄ and sequestration amount of CO₂ see an upward trend with the increase of pore size. The CO₂ molecules can replace the adsorbed CH₄ from the adsorption sites directly. On the contrary, when N₂ molecules are injected into the slit pores, the partial pressure of CH₄ would decrease. With the increase of the pores width, the adsorption occurrence transfers from single adsorption layer to four adsorption layers. It is expected that our work can reveal the mechanisms of adsorption and displacement of shale gas, which could provide a guidance and reference for displacement exploitation of shale gas and sequestration of CO₂.

1 Introduction

Recently, the exploration and development of shale gas have received extensive attention because of the demand for resources and pollution problems [1,2,3]. Shale gas has gained tremendous attention as an unconventional gas resource [4, 5]. The main component of shale gas is CH₄, which has three states in shale, i.e., adsorbed state, free state and dissolved state [6, 7]. The volume percentage of the adsorbed CH₄ in the shale reservoirs could even account for 20%–85% [8, 9]. Therefore, it is significant to investigate the adsorbed CH₄ in shale reservoirs for shale gas resource evaluation.

Exploration shale gas become very difficult due to the ultralow porosity and permeability shale. Right now, many methods are proposed to boost production of shale gas, for example, re-fracturing, hydro-fracturing, supercritical CO₂ fracturing, injection gases, etc. Hydro-fracturing, as a method of widely application, is used to enhance permeability of unconventional reservoirs [10,11,12]. Moreover, this method could waste large amount of water and cause severe environment problems [13,14,15]. Alternatively, the new method of injection gases is regarded as a good way to improve the recovery efficiency [16,17,18]. Meanwhile, shale gases are usually stored as adsorption state in silt pore and carbon nanotube. Therefore, investigations about the displacement and diffusion of methane in silt pores are of great significance for estimating and exploiting the shale gas.

Extensive computational studies and experiments on the displacement of CH₄ in shale matrix reported at present. Yang et al. [18] investigated competitive adsorption between CO₂ and CH₄ in Na-montmorillonites by Monto Carlo simulations, and found that the Na-montmorillonite clay shows obviously high adsorption capacity for CO₂, as compared with CH₄. Wu et al. [16] explored the displacement of CH₄ in carbon nanochannels by using molecular dynamics simulations, and found that CO₂ can displace the adsorbed CH₄ directly. Akbarzadeh et al. [19] also performed MD simulations on the mixture of shale gas (methane, ethane and propone) in a nanoscale pore graphite model, and found that the most selectivity (and also recovery) of methane obtains at the methane mole fraction of 0.95. Huo et al. [20] conducted experiments to study the displacement behaviors of CH₄ adsorbed on shales by CO₂ injection, and found that the amount of recovered CH₄ and stored CO₂ increase with CO₂ injection pressure. Huang et al. [21] investigated the adsorption capacities of CH₄, CO₂ and their mixtures on four kerogen models with different maturities by GCMC simulations. And they found that the adsorption capacity of gas molecules is related to the maturity of kerogen.

From the studies mentioned above, most of the shale models are nanosized and simplified. Because some free gases would occupy large pores and adsorbed gases would exist in organic matter and inorganic minerals. For the shale matrix, it is indispensable to simplify the complicated structure of shale matrix to deal with the complex situation. Someone argued that the structure of montmorillonite with some ions could represent the shale. However, this model still did not include the organic matter. At last, it is reasonable and appropriate to construct an all-atom shale model including inorganic silica and organic matter to investigate the displacement and diffusion of CH₄ in gas shale matrix.

In this work, we proposed a modified and generalized shale matrix model including inorganic silica and organic matter. Then, the mechanism of the displacement of CH₄ by injection gases in shale matrix model was investigated through molecular simulations. Finally, some discussion was also addressed. In Sect. 3.1, the occurrence behaviors of CH₄ in different pore size are found to become from one adsorption peak to four adsorption peaks. In Sect. 3.2, CO₂ is injected into shale model to displace the adsorbed CH₄. The displacement efficiency and sequestration amount of CO₂ are investigated. In Sect. 3.3, the displacement CH₄ by CO₂ and N₂ are compared and analyzed.

2 Simulation Models and Methods

2.1 Shale Models

In order to construct the shale model containing inorganic minerals and organic matter, two silica sheets are used to stand for the inorganic mineral. Because the silica’s brittleness is favorable for fracture propagation. We can get the initial silica lattice from the structure database of Material Studio software [22]. Along the (1 1 0) crystallographic orientation, we can cleave a repeat unit with the thickness 3.0 nm. Generally, the polycyclic aromatic hydrocarbon is regarded as the major organic component of organic matters, especially for shale gas reservoirs. Therefore, we used methylnaphthalene molecules to stand for the organic matter in the shale matrix here.

A simulation box was constructed to (32.43 × 39.30 × c ų), which contains two inorganic layers and two organic layers (see Fig. 1). As mentioned above, the perfect silica sheets were used to represent the inorganic layers. First, two perfect silica sheets were stacked each other in such a way as shown in Fig. 1. Then, methylnaphthalene molecules were absorbed into the interlayer space. The adsorbed methylnaphthalene molecules in slit pores were fixed [23].

Fig. 1.

The model of shale matrix. Color scheme: yellow, silicon; red, oxygen; white, hydrogen; black, carbon. (Color figure online)

2.2 Methods

GCMC simulations are carried out by SORPTION code in the MATERIAL STUDIO (MS) software developed by Accelrys Inc. The interatomic interactions are described by the force field of condensed-phase optimized molecular potential for atomistic simulation studies (COMPASS), which is a general all-atom force field. First, we took the GCMC method to investigate the displacement of CH₄ by CO₂. The temperature and the pressure of CH₄ were 313 K and 15 MPa respectively. The acceptance or rejection of trial move is set as the Metropolis algorithm. Each equilibration procedure for CO₂ and CH₄ is 2 × 10⁶. Next, CO₂ was put into the slit pores with injection pressure from 0 to 110 MPa. We could get the equalized structure until the process of simulation was ended.

In order to adjust the atomic coordinates to reach a stable initial configuration, the equalized structure was minimized by using the conjugate gradient algorithm. Then, we took MD method to study the density profile of adsorbed CH₄. First, the model was relaxed for 2 ns in a NVT ensemble with a time step of 1 fs. The Nose-Hoover thermostat method was used to maintain temperature. When we found that total energy of this model became time-independent, the equilibrium was arrived. In the last stage, this system experienced a simulation process of 2 ns in a NVE ensemble (constant number of atoms, isovolumetric, and constant energy conditions) with a time step of 1 fs, and the data were recorded for analysis. During these simulations, all the atoms of the shale matrix model were fixed as a rigid material.

3 Results and Discussion

3.1 Occurrence Behavior of Methane in Different Pores

The occurrence behaviors of methane in different pores are absolutely different, which is a significant aspect to reveal the adsorption behaviors of methane in shale. In this section, a series of shale model of different size pores from 10 Å to 100 Å are built. All of these shale models have undergone GCMC process to achieve the equilibrium state of adsorption, before they are subjected to MD simulations. Then, we can get some data about the density profile of CH₄ in pores. Figure 2 shows the different density profiles.

Fig. 2.

Loading density profile and adsorption state of methane in different pores. (a) H = 10 Å, single adsorption layer; (b) H = 15 Å, two adsorption layers; (c) H = 25 Å, two adsorption layers and central single layer; (d) H = 40 Å, two adsorption layers and two secondary layers; (e) H = 60 Å, two adsorption layers and middle bulk phase; (f) H = 100 Å, two adsorption layers and middle bulk phase.

A single adsorption peak is founded in Fig. 2a. The distance between two solid walls of 10 Å is very close, the attractive potentials of two walls are strengthened by each other. A large amount of CH₄ would accumulate in the central of the pore. The peak of adsorption layer is the second highest at 0.65 g$\cdot$cm⁻³, compare with 0.75 g$\cdot$cm⁻³ at 15 Å. Since the pore size is the narrowest in all cases, the amount of adsorption methane is limited. A small amount of methane molecules absorbed in the pore, which results in the peak of adsorption layer is not the highest. As the size of pore increases, the single adsorption layer would become two adsorption layers. The attractive potentials of two walls become weak or even disappear. Meanwhile, the peaks of adsorption layer are the highest. It can be seen from the Fig. 2b. When the distance of two walls increases to 25 Å, apart from two peaks, a central single layer appears. Two peaks near the walls are lower, around half the figure for H = 15 Å. Four peaks of adsorption layers, including two primary adsorption layers and two secondary, will appear at 40 Å [17]. Figure 2e and f show that the density of bulk phase is all keep in at 0.2 g$\cdot$cm⁻³ at the pore width of 60–100 Å. This situation illustrates that the central bulk phase will not change with the silt width increases.

In order to show the state of molecular occurrence more intuitively, we give snapshots of the adsorption model in two pore sizes. The pore sizes are set as 25 Å and 60 Å, representing mesopores and macropores respectively. Figure 3a shows that the amount of adsorbed CH₄ next to the walls is much more than the bulk’s. So near the walls, two symmetrical peaks appeared. However, the center of the pore is still adsorbed with a certain amount of CH₄. Correspondingly, a small peak appears in the center of the density profile graph. In contrast to, when the pores become macropores, no more peaks appear in the center of the pores. It can be seen from Fig. 3b. Because in the case of large pore, the density of methane in the central area basically does not change.

Fig. 3.

Adsorption occurrence of CH4 in different pores.

3.2 Methane Displacement by Carbon Dioxide in Different Pores

Injection gases has become a high efficient way to exploit the shale gas. CO₂ and N₂ are usually considered ideal gases to displace methane. Some studies find that the adsorption capacity of gases is related to the attractive potentials between gases and shale matrix atoms. Figure 4a shows the changes of loading amount of CH₄ at different CO₂ injection pressure in different pores. The size of pore is set 15–100 Å. Obviously, the downward trend in loading amount of CH₄ is significant at different pores. As the injection pressure of CO₂ increases, the loading amount of CH₄ in the shale model diminishes. For injection pressure of 0–30 MPa, the loading amount of CH₄ decreases quickly. When CO₂ is injected, the molecules can adsorb on the walls to replace the adsorbed methane. At high pressure, there are no more adsorption sites for CO₂ molecules to adsorb. The curve becomes smooth at high CO₂ injection pressure. The loading amount of CH₄ starts to stay stable. With the increase of the pores width, the loading amount of CH₄ at different CO₂ injection pressure increases obviously. Similarly, the sequestration amount of CO₂ rises significantly. For the same width of pore, the figure for sequestration CO₂ grows dramatically, which is shown in the Fig. 4b. Furthermore, the displacement amount of CH₄ at CO₂ injection pressure of 90 MPa is studied. Much more CO₂ is absorbed into the pores with the increase of pore width, causing more methane to be driven out. So Fig. 4c shows that the displacement amount of CH₄ is the highest in the pore size of 100 Å. Correspondingly, the sequestration amount of CO₂ is also the most. It can be seen from Fig. 4d.

Fig. 4.

Displacement of methane by carbon dioxide in different pores. (T_m = 313 K, P_m = 15 MPa) (a) Loading amount of CH₄ at different CO₂ injection pressure; (b) Sequestration amount of CO₂ at different CO₂ injection pressure; (c) Displacement amount of CH₄ at CO₂ injection pressure of 90 MPa; (d) Sequestration amount of CO₂ at CO₂ injection pressure of 90 MPa.

3.3 Comparison of Methane Displacement by Nitrogen and Carbon Dioxide

Both N₂ and CO₂ can be used to displace gases. However, our studies find that the displacement mechanisms of these gases is different. The pore width is set 25 Å. Figure 5 shows the difference between these two gases. Figure 5a shows the loading amount of CH₄ at different injection pressure. In the case of CO₂ displacing CH₄, the loading amount of methane decreases significantly as the partial pressure of CO₂ increases, compared with the case of N₂ displacing CH₄. Both kinds of gases displacement have led to a sharp decline in loading amount of CH₄. Correspondingly, the sequestration amount of CO₂ has also increased rapidly in the cases of the displacement of methane by CO₂ and N₂. More CO₂ will displace the methane to concentrate in the adsorbed layer, and N₂ will displace less. When CO₂ is added into the pores, the full of this space is filled with CO₂ molecules. Then, CO₂ molecules begin to occupy adsorption sites of CH₄, replacing the adsorbed CH₄ molecules directly. The displaced CH₄ molecules return to free phase. The adsorption capacity of these three gases is sorted as follows: CO₂ > CH₄ > N₂. In contrast to, when N₂ is injected, N₂ molecules cannot occupy adsorption sites of CH₄. These N₂ molecules can only adsorb on the vacancies due to the fact that the adsorption capacity of N₂ is weaker than CH₄. N₂ molecules are able to displace CH₄ because they can reduce the partial pressure of CH₄. Once the partial pressure of CH₄ decreases, CH₄ molecules would be desorbed and displaced. Therefore, as CO₂ and N₂ are injected, the loading amount of methane all experiences a downward trend. The screenshots of different displacement processes is shown in Fig. 6.

Fig. 5.

Comparison of methane displacement by CO₂ and N₂. (T_m = 313 K, P_m = 15 MPa, H = 25 Å) (a) Loading amount of CH₄ at different injection pressure; (b) Sequestration amount of injection gases at different injection pressure.

Fig. 6.

The adsorption sites snapshots of displacement of CH₄. (P_I = 90 MPa) (a) Displacement of CH₄ by CO₂: red-CH₄; green-CO₂. (b) Displacement of CH₄ by N₂: red-CH₄; green-N₂ (Color figure online)

4 Conclusion

On the basis of adsorption characteristics of CH₄ on the shale model, we built a new shale model by using organic-inorganic composites. Then we used GCMC simulation to study the displacement of shale gas by CO₂ and N₂. The displacement mechanisms of the injection gases are investigated. Next, the occurrence behavior of methane in different pores is investigated by using the MD method. Our conclusions are listed as follows:

• 1. With the pores width increase, the adsorption occurrence transfers from single adsorption layer to four adsorption layers. In the case of a much wide pore width, the density of the central bulk phase approaches to the same value at 0.2 g$\cdot$cm⁻³.

• 2. To displace the adsorbed CH₄, CO₂ and N₂ are injected and investigated. The results indicate that both of the CO₂ and N₂ molecules can displace the adsorbed shale gas and CO₂ is sequestrated into the shale simultaneously.

• 3. However, the displacement mechanisms of the injection gases are different. The adsorption capacity of CO₂ is much stronger than that of CH₄. The CO₂ molecules can replace the adsorbed CH₄ from the adsorption sites directly. On the contrary, when the pores are occupied by N₂ molecules, these molecules can decrease the partial pressure of CH₄.

It is expected these results and findings are of great importance for displacement exploitation of shale gas and sequestration of CO₂.