Molecular Simulation Study of Gas–Water Adsorption Behavior and Mobility Evaluation in Ultra-Deep, High-Pressure Fractured Tight Sandstone Reservoirs

Abstract

Under high-temperature and high-pressure conditions, understanding the competitive adsorption and mobilization mechanisms of gas and water in fractured tight sandstone gas reservoirs is crucial for optimizing the recovery factor. This study employs molecular dynamics simulation to investigate the adsorption behavior and mobilization characteristics of H₂O and CH₄ in 10 nm quartz nanopores under the conditions of the Keshen fractured tight sandstone gas reservoir. The results indicate that H₂O exhibits strong adsorption on the quartz surface, forming two high-density adsorption layers with a thickness of approximately 0.6 nm, whereas CH₄ forms three adsorption layers with a thickness of about 1.1 nm. Under gas–water coexistence conditions, the competitive adsorption effect of the water phase significantly influences the distribution of CH₄. Due to the hydrophilicity of the quartz wall, H₂O molecules preferentially adsorb onto the wall surface, forming a stable water film that significantly inhibits CH₄ adsorption. When the water saturation reaches 35%, water molecules form liquid bridges within the pores, segmenting the gas phase into different regions. As water saturation further increases, more stable liquid bridge structures develop, and microscopic water lock effects emerge, further restricting gas flow. During depletion development, H₂O remains difficult to mobilize due to strong wall adsorption, with a recovery factor of only 7%. In contrast, CH₄ exhibits high mobility, with a recovery factor of up to 75%. However, as water saturation increases from 30% to 70%, the recovery factor of CH₄ decreases significantly from 75% to 29%, indicating that the water phase not only occupies pore space, but also exerts a blocking effect that significantly inhibits CH₄ percolation and production. This study provides important theoretical support for the development strategies of ultra-deep fractured tight sandstone gas reservoirs and offers key insights for improving the ultimate recovery factor under gas–water coexistence conditions.

1. Introduction

With the exploitation of shallow-to-middle oil and gas resources, the exploration and development of ultra-deep oil and gas have gradually become the focus and hotspot [1,2,3]. In China, significant breakthroughs have been made in deep reservoir exploration, particularly in the Keshen gas field, which has emerged as a key area for increasing natural gas reserves and production [4,5,6]. The Keshen gas field, located in the Keshen structural belt of the Kuqa foreland basin at the northern Tarim Basin, is one of the primary regions contributing to natural gas production growth. However, it is characterized as an ultra-deep, ultra-high-pressure fractured tight sandstone gas reservoir [7,8], with extreme burial depths (5500–8100 m), high formation temperatures (106–175 °C), and pressures ranging from 88.9 to 150 MPa. In addition, its strong heterogeneity and complex gas–water interactions pose significant challenges to conventional development strategies. In particular, under high-temperature, high-pressure, and confined-space conditions, gas adsorption behavior and water phase blockage can significantly impact permeability and the ultimate recovery factor [9,10]. Therefore, a comprehensive understanding of gas–water transport and competitive adsorption at the microscale is crucial for the efficient development of the Keshen gas field.

In recent years, advancements in microscale experimental techniques and computational simulation methods have highlighted the critical influence of confined spaces on gas adsorption behavior. Researchers have employed molecular dynamics simulations and other approaches to explore this phenomenon in greater detail. Zhang et al. used molecular dynamics simulation to investigate CH₄ adsorption behavior in confined spaces, focusing on nanopore adsorption layer formation. They analyzed the effects of pressure, pressure gradient, pore width, and temperature on adsorption. The results showed that as pressure increases from 1 MPa to 80 MPa, the CH₄ adsorption layer transitions from a single layer to three layers, forming a multilayer adsorption structure. Although higher pressure enhances CH₄ molecular interactions, the force exerted by the pore wall remains unchanged [11]. Ren et al. studied methane flow behavior in nanopores using molecular dynamics simulation. They developed a slit pore model to examine the effects of pore size, pressure, mineral composition, and pore water saturation on methane diffusion. Their findings indicate that methane diffusion accelerates with increasing temperature and pore size but slows as pressure rises [12]. Zhang investigated the transport behavior of natural gas and oil in a single nanopore under reservoir conditions using molecular dynamics. The study examined the behavior of C₁₀H₂₂, CO₂, and CH₄ in a 4 nm nanopore. The results showed that CH₄ and CO₂ form distinct adsorbed and free molecular groups, leading to different extraction behaviors. However, both gases follow similar diffusion behavior, with CH₄ exhibiting a higher effective diffusivity [13]. Xiong et al. explored methane adsorption in organic-rich pores through experiments and molecular simulations. Their findings indicate that kerogen exhibits the highest methane adsorption capacity, followed by clay minerals and quartz. At a fixed pore size, the volume fraction of adsorbed gas decreases as pressure increases. Similarly, at a constant pressure, larger pores contain a lower proportion of adsorbed gas [14].

In reservoirs, besides gas adsorption behavior, water also plays an important role in gas production. Zhang et al. investigated supercritical methane adsorption in nanopores under initial water saturation conditions. They visualized water distribution within reservoir pore networks and analyzed its impact on methane adsorption at the microscopic level. Their results indicate that neglecting the influence of water distribution can lead to an overestimation of natural gas reserves [15]. Passey et al. demonstrated that the thickness of the water layer adsorbed on nanopore walls in the inorganic matrix is approximately equal to the pore diameter [16]. Shi et al. observed that the impact of the adsorbed water layer on the permeability of the inorganic matrix varies with water saturation [17]. Li et al. reported that an adsorbed water layer exists on clay mineral surfaces, with its thickness primarily determined by relative humidity and pore size [18]. Jin and Firoozabadi, using grand canonical Monte Carlo simulations, showed that adsorbed water layers form on clay mineral surfaces, restricting available space for gas flow [19]. Liu et al., through molecular dynamics simulations, found that water molecules exhibit stronger adsorption energy on clay mineral surfaces than methane molecules, leading to significant water molecule aggregation in the adsorption layer [20]. Xie et al. examined how water and salinity affect CO₂ adsorption and storage capacity in organic and clay nanopores. The results indicate that the presence of water reduces available adsorption space, leading to lower CO₂ storage capacity [21]. Zhang et al. studied H₂O and CH₄ flow in calcium montmorillonite nanoslits. They found that water forms bridges and thin films, blocking CH₄ flow at high water saturation (>80.87%) [22]. Zhang developed a kinetic model showing that water reduces CH₄ diffusion and adsorption while being more affected by pore structure. Water rapidly infiltrates pores, preventing CH₄ from reaching the surface [23].

In this study, molecular dynamics simulations were conducted to investigate the adsorption behavior and recovery degree evaluation of H₂O and CH₄ under high-temperature and high-pressure conditions. First, the wetting properties of the wall model were adjusted to match real reservoir conditions. Then, pore models for pure water and methane were established to analyze the effects of gas and water and varying water saturation on the density distribution under reservoir conditions. Subsequently, we conducted depletion development to investigate the recovery factor of pure gas and water under different water saturations. The results obtained from this study are significant for evaluating the development potential of CH₄ in ultra-deep tight sandstone gas reservoirs and provide certain reference values for enhancing the recovery factor of fractured tight sandstone gas reservoirs.

2. Methods

2.1. Model

The reservoir in the Keshen area of the Tarim Basin is mainly composed of quartz. Therefore, this study used the α-quartz single crystal cell to construct the wall model. As shown in Figure 1a, the lattice unit was obtained by cleaving the (1 0 0) surface (Figure 1b) and exposing the oxygen atoms, resulting in an orthogonal cell structure. Subsequently, the SiO₂ wall model was expanded, as shown in Figure 1c.

Since the real reservoir conditions consist of dense sandstone with a minimum pore size of 10 nm, this study selected 10 nm as the representative pore size to construct a quartz wall model. This model was designed to investigate the effects of confinement on the development process. Based on this, a quartz wall with a 10 nm pore diameter was constructed using a modified wall structure. Additionally, graphene carbon plates were installed on both sides of the pore wall model, with one plate fixed and the other movable. By applying a specific acceleration to the movable carbon plate, different pressure conditions were simulated, providing the necessary experimental setup for subsequent research. As shown in Figure 2, the size of the entire model was 20.00 nm × 4.25 nm × 10.00 nm. In addition, the overall simulation system remained electrically neutral.

2.2. Simulation Details

During the simulation, the quartz model was implemented using the CLAYFF force field [24]. To reduce the computational cost, a rigid wall was employed, meaning that wall atoms were fixed, and interactions between them were ignored. The water molecules were modeled using the SPC/E force field [25]. The CH₄ molecules were modeled using the TraPPE-UA force field [26]. The carbon atoms were modeled using the OPLS-AA force field [27]. The simulation was conducted using GROMACS software (version 2020.6) [28], with periodic boundary conditions applied in all three spatial directions (X, Y, Z). Molecular assembly was carried out using Packmol software (version 18.169) [29]. The NVT ensemble was used, maintaining a simulation temperature of 100 °C for 20 ns. After the simulation, VMD [30] was utilized to visualize molecular configurations and trajectories.

3. Result and Discussion

3.1. Wettability Adjustment

The reservoirs in the Keshen area of the Tarim Basin have a wettability angle of approximately 30°. To replicate this condition, the wall model was modified by introducing hydroxyl groups (–OH), which were generated by directly bonding hydrogen atoms to the exposed oxygen atoms. Quartz surface models with 0% (see Figure 3a) and 100% (see Figure 3b) hydroxylation were constructed. For the water droplet, a spherical water droplet with a density of 1.0 g/cm³ was established and placed on the surface of the wall model to construct initial configurations. During the equilibration stage, water molecules from the final 5 ns of the simulation were selected to generate a 2D density cloud map.

Figure 4a,c depicts the final configurations of the 0% and 100% hydroxylated models, respectively, while Figure 4b,d shows the corresponding 2D density cloud maps. The color gradient from red to blue represents a decrease in density. The wettability outline was extracted, and a tangent was drawn to determine the wettability angle. On the surface without hydroxyl modification, the water droplet is hemispherical after equilibrium, yielding a calculated wettability angle of 61.38°. As hydroxylation coverage increases, the droplet spreads more on the surface, enhancing wettability. When the surface is 100% hydroxylated, the wettability angle decreases to 29.77°, aligning with real reservoir conditions and consistent with literature reports [31].

3.2. Density Distribution

To better understand the impact of adsorption on gas reservoir development, density distribution curves for each substance were recorded at 0.02 nm intervals after the simulation reached equilibrium. A detailed analysis of these curves allows for an intuitive observation of the adsorption behavior of gas and water on the wall and their variation patterns, providing insights into the role of adsorption in gas reservoir development assessment.

3.2.1. The Density Distribution of Pure H₂O

Figure 5 presents the density distribution curve of pure water under reservoir conditions (100 MPa, 100 °C). It can be seen from the figure that there is an adsorption zone of high density and a free zone of gentle density curve distribution in the quartz pores. The density of the adsorption layer is significantly higher than that of the free zone, indicating strong water molecule adsorption on the quartz pore wall under reservoir conditions. Specifically, water molecules form two high-density adsorption layers within 0.6 nm near the wall surface, with density peaks decreasing as the distance from the wall decreases. Each peak represents an adsorption layer, with the first adsorption layer closest to the wall reaching a density of 1.7 g/cm³, approximately 1.7 times the bulk water density under the same temperature and pressure conditions. The second adsorption layer exhibits a lower density peak of 1.1 g/cm³, showing that adsorption strength weakens as the distance from the wall increases. Beyond these adsorption layers, water density gradually decreases, and in the center of the pore, the density stabilizes at 0.99 g/cm³, matching the bulk density. This suggests that the free zone behaves as a bulk phase. Xu et al. employed molecular dynamics simulations to investigate the flow behavior of alkane–water systems in quartz nanopores and found that under conditions of 50 °C and 20 MPa, water molecules formed four adsorption layers with a total thickness of 0.78 nm near the quartz surface. This result is in good agreement with the findings of the present study [32].

3.2.2. The Density Distribution of Pure CH₄

Figure 6 shows that the density distribution curve of pure CH₄ follows a similar trend to that of pure water, with regions of a high-density adsorption zone and regions where the density curve is gently distributed in the free zone, and the density of the adsorption layer is much higher than that of the fluid in the free zone. However, methane molecules form three adsorption layers on the quartz wall, with a greater adsorption layer thickness than that of water molecules, measuring 1.1 nm. This is because hydrogen bonding interactions cause water molecules to form a more compact molecular layer on the wall. Additionally, methane molecules have a larger molecular volume than water molecules, further contributing to the increased layer thickness. The first adsorption layer of methane, closest to the wall, has a density of approximately 0.42 g/cm³, which is lower than that of water molecules (1.7 g/cm³), confirming that water molecules near the wall surface are more compactly arranged. The second adsorption layer exhibits a lower density peak of 0.32 g/cm³, while the density in the pore center stabilizes at 0.3 g/cm³, consistent with the bulk density value. Ren et al. investigated methane adsorption in 10 nm quartz pores under conditions of 110 °C and 30 MPa and reported an adsorption layer thickness of approximately 1 nm, which is in good agreement with the present findings [12].

3.2.3. The Density Distribution of Gas-Water Under Different Water Saturation Conditions

Under gas–water coexistence conditions, due to the hydrophilicity of the quartz wall, H₂O and CH₄ compete for adsorption, with water molecules preferentially occupying specific positions on the wall surface. As shown in the final molecular configuration of Figure 7a, at 30% water saturation, water molecules form a thin water film along the two sides of the wall surface. Consequently, no CH₄ adsorption layer forms near the wall, and CH₄ exists solely in the pore center as a free phase. The gas phase exists as a continuous phase in the pore center. At this point, the CH₄ density in the pore center is approximately 0.3 g/cm³.

As water saturation increases, water molecules aggregate under the action of hydrogen bonds, and at 35% water saturation, liquid bridges are formed in the pores, at which time the gas phase is divided into different regions, forming discrete bubbles and trapped gases (see Figure 7b). The gas–water density distribution curve in Figure 8b shows that the water density in the pore center increases to 0.13 g/cm³, while the gas density decreases to 0.23 g/cm³. With further increases in water saturation, the thickness of the liquid bridge thickens (see Figure 7c,d), intensifying the gas-trapping effect. When water saturation reaches 70%, the final molecular configuration reveals a significant increase in water volume within the pore, drastically reducing the available space for CH₄. Additionally, the water density in the free zone also increases (see Figure 8d). At this stage, the water density in the pore center reaches 0.50 g/cm³, while the CH₄ density decreases to 0.14 g/cm³.

3.3. Mobility Evaluation

To better understand the adsorption effect of the wall and the influence of water on the recovery factor, this study evaluated the recovery degree of the adsorption layer and the free zone during the depletion development and discussed the impact of different water saturations on the recovery factor of the gas and water. In the simulation, the depletion development process was achieved in the following way. The final molecular configuration of the gas–water distribution state under reservoir pressure was taken as the initial configuration for the depletion development simulation. Then, according to the requirements of depletion to different pressures, the acceleration applied to the carbon plate was changed, thereby realizing the depletion process from 100 MPa to 10 MPa. During each depletion development, after the system was completely balanced, the last 5 ns were taken to analyze the distribution state of the gas and water and the corresponding recovery factor.

3.3.1. Mobility Evaluation of Pure H₂O

As illustrated in Figure 9a, the water molecular system exhibits low elastic potential energy, indicating minimal volumetric change even as pressure decreases from 100 MPa to 10 MPa. This suggests that pure water remains largely immobile during depletion development due to its limited compressibility. At the microscopic level, strong hydrogen bonding and intermolecular interactions stabilize the water structure, restricting large-scale molecular rearrangement and migration. Consequently, water in the adsorbed zone remains strongly bound to the pore walls, exhibiting extremely low mobility, whereas water in the free zone, though not directly influenced by surface adsorption, is still constrained by intermolecular forces, leading to similarly limited displacement. Figure 9b further reveals that the recovery factors in both zones remain low (<10%), with 4% in the adsorbed zone and 8% in the free zone, indicating that free-phase water demonstrates relatively higher sensitivity to pressure depletion. This can be attributed to the absence of direct solid-phase interactions, allowing for greater molecular displacement compared to the adsorbed phase. These findings highlight a fundamental challenge in ultra-deep fractured tight reservoirs, where water mobility is highly restricted under the depletion process, potentially hindering gas flow and reducing the ultimate recovery factor.

3.3.2. Mobility Evaluation of Pure CH₄

Figure 10a illustrates the molecular configurations of pure CH₄ during the depletion development process. It is visually evident that as pressure decreases, the gas phase undergoes significant expansion, further confirming the high sensitivity of CH₄ to pressure changes, which is notably different from the flow behavior of pure water. Specifically, when pressure decreases from 100 MPa to 80 MPa, the recovery factors of the adsorption layer and the free zone are 9% and 10%, respectively, both significantly higher than those of water, indicating that gas desorption and diffusion begin even at high pressures. As the pressure further decreases to 10 MPa, the recovery factors of the adsorption layer and the free zone increase substantially. This result suggests that during the depletion development process, pure CH₄ exhibits strong mobility, with gas in the free zone being more readily mobilized than that in the adsorption layer. Figure 10b further presents the recovery factor trends of CH₄ in the adsorption and free zones under different pressures. As pressure decreases, gas in all regions of the pore is progressively mobilized, and the recovery factor of the free zone consistently exceeds that of the adsorption layer. When the pressure ultimately drops to 10 MPa, the recovery factor of the adsorption layer reaches 56%, whereas the recovery factor of the free zone reaches 79%, indicating that free-phase gas has a stronger release capacity during pressure depletion. This phenomenon can be attributed to the fact that free-zone gas is not constrained by solid surface adsorption, allowing its molecules to diffuse and migrate more easily. In contrast, gas in the adsorption layer remains influenced by van der Waals forces and adsorption energy from the solid surface, leading to a lower desorption rate compared to the free zone. Nevertheless, the final recovery factor of the adsorption zone remains relatively high, demonstrating that gas desorption plays a significant role in the depletion-driven development process, highlighting the importance of pressure management strategies in enhancing ultimate gas recovery in ultra-deep fractured tight reservoirs.

Figure 11 illustrates the overall recovery factors of pure H₂O and pure CH₄ under depletion development. The results indicate that while the recovery factor of pure water increases slightly with decreasing pressure, the overall increase remains minimal, with a final recovery factor of only 7%. This suggests that water exhibits low mobility under pressure depletion, likely due to its strong intermolecular hydrogen bonding and interaction with the pore walls, which restrict its movement even under significant pressure reduction. In contrast, the recovery factor of pure CH₄ shows a dramatic increase as pressure decreases, ultimately reaching 75%. This significant difference highlights the much higher compressibility and mobility of CH₄ compared to water, allowing gas molecules to expand and migrate more effectively during the depletion process. The stark contrast between H₂O and CH₄ mobility further confirms that gas production in ultra-deep reservoirs is highly pressure-sensitive, whereas water remains largely immobile, leading to potential challenges in managing residual water saturation and optimizing overall recovery efficiency.

3.3.3. Mobility Evaluation of Gas-Water Under Different Water Saturation Conditions

In real reservoirs, water is commonly present. To investigate the recovery degree of gas and water in the pore, the density distribution and the recovery factor under different pressures were studied at water saturation of 30%, 35%, 50%, and 70%. The following is a specific analysis.

At 30% water saturation, there is a significant difference in the adsorption behavior of the gas and water phases within the pore. As shown in Figure 12a, the final molecular configuration indicates that due to the hydrophilicity of the pore wall, H₂O and CH₄ undergo competitive adsorption, and water molecules primarily adsorb onto the pore wall, forming a stable water film that completely covers the wall surface, causing the gas phase to exist only as a continuous phase in the pore center. Due to the strong wall–fluid interactions, the water film remains firmly adsorbed and exhibits minimal mobility, resulting in an extremely low final recovery factor of only 1%, indicating that the water film is hardly mobilized during pressure depletion. Water molecules form a continuous thin water film along the pore walls due to the hydrophilicity of quartz. As a result, CH₄ is excluded from the adsorption zone and exists entirely as a free phase in the pore center. Compared to the dry condition (pure CH₄), where CH₄ is partially adsorbed on the wall and more strongly confined, the CH₄ in the free zone under 30% water saturation is more mobile and responds more sensitively to pressure depletion. This structural change leads to a slightly higher recovery factor. It leads to a final recovery factor of 82% (Figure 12b), even exceeding the overall recovery factor of pure CH₄ (75%). Furthermore, these findings suggest that in the presence of water, the existence of the water phase may alter the gas distribution state, causing CH₄ to exist primarily as a free phase, thereby further enhancing its recoverability.

As the water saturation increases to 35%, water molecules within the nanopores aggregate through hydrogen bonding, forming liquid bridge structures (see Figure 13a, 0 ns). At this stage, the gas phase becomes segmented into different isolated regions, leading to the formation of trapped gas and gas bubbles. The presence of liquid bridges enhances the connectivity of the water phase within the pore while reducing the continuity of the gas phase, thereby restricting gas flow. With the decrease in pressure, the gas expands and flows out, gradually breaking through the liquid bridges and causing the water phase structure to rearrange. The liquid bridges progressively rupture and redistribute near the pore walls, forming thin liquid films (see Figure 13a, 20 ns). Under this water saturation condition, the gas-driven migration of liquid bridges during pressure depletion enhances water mobilization, leading to an increased final recovery factor of 8%, which represents an improvement compared to lower water saturation conditions. However, despite the increased water mobilization, gas mobility is adversely affected. As gas flows through the pore, it must overcome the blocking effect of the liquid film on the flow pathways, resulting in a decrease in overall gas recovery, which ultimately drops to 78% (see Figure 13b), lower than the gas recovery observed at 30% water saturation. These findings indicate that under moderate water saturation conditions, the formation and rupture of liquid bridges play a crucial role in the gas–water two-phase flow. While water mobilization is enhanced due to liquid bridge migration driven by pressure depletion, the obstruction effect of liquid bridges on gas flow pathways reduces the effective gas recovery. Therefore, in the development of high water saturation reservoirs, optimizing the pressure depletion rate to enhance water mobilization while minimizing gas flow resistance is a key strategy for improving recovery efficiency.

At 50% water saturation, it can be observed that as water saturation increases, the distribution of the water phase within the pore undergoes significant changes, with the thickness of liquid bridges increasing, leading to the occurrence of microscopic water blocking (Figure 14a, 5 ns). During the depletion process, although the expansion of gas drives some movement of the water phase, the liquid bridges remain intact and do not rupture. Meanwhile, the trapped gas phase enclosed by the liquid bridges fails to break through the water phase barriers, restricting gas flow pathways (Figure 14a, 15 ns). Figure 14b further reveals the impact of water saturation on the gas–water two-phase flow and mobilization. Due to the increased thickness of the liquid bridges, the expansion of gas enhances water mobilization, resulting in a final water recovery factor of 57%, which represents a 49% increase compared to the 35% water saturation condition. However, despite the improvement in water mobilization, the mobility of CH₄ is significantly restricted, leading to a final CH₄ recovery factor of only 39%. This indicates that as water saturation further increases, the blocking effect of the water phase within the pore intensifies, causing a substantial decline in the gas recovery factor.

When the water saturation increases to 70%, water molecules occupy the vast majority of the pore space, leaving only a small amount of gas trapped in the remaining pores, as shown in Figure 15a. At this point, the liquid bridges thicken, forming a more stable blocking structure that further restricts gas flow pathways. In the subsequent decompression process, the influence of gas on liquid bridge flooding was significantly weakened due to the increase in liquid bridge thickness and the decrease in gas content. As a result, the recovery factor of the water phase decreases, ultimately reaching only 46%. Furthermore, in a high water saturation environment, the strong blocking effect of liquid bridges further limits the mobilization of gas, leading to a significant reduction in the final CH₄ recovery factor, which drops to only 29% (Figure 15b). Compared to the pure gas condition, the gas recovery factor is reduced by 46%. This phenomenon indicates that under high water saturation conditions, the water phase not only occupies a large portion of the pore space, but also severely inhibits CH₄ percolation and production through enhanced blocking effects. The stability of liquid bridges and their obstructive influence on gas flow pathways have become critical factors affecting the efficiency of gas reservoir development.

4. Conclusions

This study employed molecular simulation to systematically analyze the density distribution, adsorption behavior, and mobilization characteristics of gas and water in 10 nm pores of the Keshen tight sandstone gas reservoir in the Tarim Basin. Additionally, it evaluated the evolution of gas and water mobilization during depletion development. The results indicate that H₂O and CH₄ exhibit distinct adsorption behaviors. Near the quartz wall, H₂O forms two adsorption layers with a total thickness of 0.6 nm, whereas CH₄ forms three adsorption layers with a total thickness of 1.1 nm. During depletion, H₂O is constrained by strong wall adsorption, leading to low mobilization, with recovery factors of only 4% and 8% in the adsorption and free zones, respectively. In contrast, CH₄ is highly sensitive to pressure changes, achieving recovery factors of 56% and 79% in the adsorption and free zones, respectively. These findings indicate that CH₄ exhibits greater mobility under pressure depletion, while H₂O remains largely immobile due to strong wall adsorption.

Under water-bearing conditions, the presence of water significantly affects the distribution and mobilization of CH₄. Due to the hydrophilic nature of the quartz wall, H₂O preferentially adsorbs onto the wall surface, dominating the adsorption process and significantly inhibiting CH₄ adsorption. At 30% water saturation, CH₄ mainly exists in the pore center as a free phase, maintaining relatively high mobility. However, as water saturation increases, CH₄ mobilization becomes significantly restricted. When water saturation increases from 30% to 70%, the water phase forms stable liquid bridges within the pores, segregating the gas phase into isolated regions. As water saturation further increases, microscopic water lock effects emerge, further inhibiting CH₄ flow, and the recovery factor of CH₄ decreases significantly from 75% to 29%.

This study provides a microscale perspective on gas–water transport mechanisms in ultra-deep tight sandstone gas reservoirs and reveals the impact of water phase blocking on CH₄ mobility, which is crucial for optimizing gas extraction strategies in high-water-saturation reservoirs. However, this study is based on molecular simulations of 10 nm pores, without considering complex pore networks or multiscale effects. Future studies should incorporate larger-scale pore network modeling, high-pressure and high-temperature experimental validation, and coupled flow simulations to further improve the understanding of gas–water transport mechanisms in ultra-deep tight sandstone reservoirs.