Analysis of the seepage-diffusion law and transformation mechanism in the CO₂ flow adsorption test under various laminae

Abstract

Physical simulation experiments of CO₂ flow adsorption under various stratigraphies were conducted in the coal seam CO₂ injection project for gas replacement, as CO₂ is in the flow state and the mechanism of CO₂ adsorption in the flow state in the coal seam is still unclear. In the process of CO₂ flow adsorption, the results indicate that seepage in the horizontal stratigraphic direction is predominant, while diffusion in the vertical stratigraphic direction is predominant. The ratio of CO₂ flow adsorption in the horizontal stratigraphic direction is primarily distributed below 50%, and seepage is the dominant process during the CO₂ flow adsorption test. Conversely, the ratio of adsorption in the vertical stratigraphic direction is typically greater than 50%, and diffusion is the dominant process. During the control test, when the outlet was sealed (one-way CO₂ injection without return), the seepage impact diminished while the diffusion effect intensified, resulting in an increase of the saturated CO₂ adsorption quantity to 1.3 to 4.8 times that of the flow adsorption system. In the CO₂ replacement of CH₄ project, when CO₂ penetrates a specific distance, the extraction holes are sealed, thereby prolonging the diffusion period of CO₂ to ensure complete adsorption in competition with CH₄, ultimately enhancing the utilization efficiency of CO₂.

Introduction

Anthropogenic carbon dioxide (CO₂) emissions and the effects of climate change on the environment have emerged as a significant global concern. To address this challenge, researchers are investigating diverse carbon capture and utilization (CCU) technologies^1,2,3,4. China possesses abundant coal carbon resources, presenting significant potential for exploitation and utilization^5,6,7,8. Coalbed methane (CBM) can substitute coal due to its substantial CO₂ adsorption capacity, facilitating carbon sequestration and the generation of new energy sources. However, the majority of coal seams exhibit low permeability, rendering conventional extraction methods ineffective^9,10. Coal seams are classified as low-permeability coal seams^11,12,13and the efficacy of standard extraction methods is suboptimal. Current prevalent techniques for coalbed methane extraction include hydrodynamic permeability augmentation technology¹⁴gas injection replacement, and protective layer mining, among other permeability modification methods. CO₂-enhanced coal bed methane mining (CO₂-ECBM) ensures production safety and alleviates the greenhouse effect, emerging as a novel method for the effective extraction of coal bed methane in China. It has emerged as a method for the efficient extraction of coalbed methane in China^{15,16,17,18,19}.

To enhance the efficiency of CO₂-enhanced coalbed methane extraction, researchers have investigated the fundamental mechanisms of CO₂-driven gas. In the examination of the CBM storage and transportation mechanism, utilizing the dual pore medium theory^20,21during CO₂-enhanced CBM extraction (CO₂-ECBM), CO₂ initially experiences Darcy-non-Darcy composite seepage through the macroscopic fissure system (pore size > 100 nm), subsequently diffusing into the micro- and nano-pore system (pore size < 10 nm), where it then competes for adsorption with CH₄. The efficacy of CO₂ permeation and diffusion is a critical determinant in enhancing CBM extraction. Zhang Lei et al.²² conducted a binary gas adsorption and displacement experiment to elucidate the mechanisms of CO₂ seepage and diffusion during gas injection and displacement. Their findings revealed that seepage displacement predominated in the initial phase of the experiment, while diffusion displacement became dominant in the latter phase. Li Jinghua et al.²³ categorized the gas injection displacement impact into two sub-effects: pressure potential difference and gas injection, which enhance CH₄ desorption. They discovered that the CO₂-induced enhancement of desorption is predominantly observed in the first phase of the experiment. Yang Hongmin et al.²⁴ discovered that in the N₂ gas injection test, the displacement and desorption effects of gas injection are predominant in the initial phase, the displacement effect diminishes during the intermediate phase as the replacement effect intensifies, and in the final phase, there is an equilibrium between the displacement and replacement interactions.

In the multi-physical field coupling process of CO₂-driven gas, researchers have systematically elucidated the independent functions of each mechanism by constructing an idealized single-component adsorption system and mitigating the effects of competing adsorption interactions, thereby revealing the coupled kinetic behavior of seepage, diffusion, and adsorption of CO₂ in the coal matrix. Zunguo Zhang et al.²⁵ conducted a concurrent experimental investigation of coal adsorption and deformation, analyzed the CO₂ adsorption/desorption characteristics of coal, developed a model for CO₂ adsorption/desorption in coal samples, and clarified the correlation between the adsorption/desorption deformation of coal samples and the adsorption quantity and equilibrium pressure. Zheng Yi²⁶ conducted kinetic studies on CO₂ adsorption, revealing that the highest absolute adsorption capacity of CO₂ was approximately threefold that of CH₄ under identical temperature and pressure conditions, signifying a superior CO₂ adsorption capacity in shale compared to CH₄. Zhang Mingjie et al.²⁷ conducted isothermal adsorption studies of anthracite coal on CO₂ gas utilizing the gravimetric method, examining the variations in the density of the adsorbed phase and the quantity of adsorbed molecular layers of CO₂. Zunguo Zhang et al.²⁸ examined the dynamic diffusion features of CO₂ in coal prior to adsorption equilibrium using a single pore diffusion model, discovering a significant correlation between adsorption equilibrium time and the particle size of coal samples. Wang Shuifeng et al.²⁹ conducted high-temperature and high-pressure CO₂ adsorption experiments, examined the density distribution characteristics of various pore phases under temperature and pressure variations, and categorized the CO₂ micro-sequestration modes in coal beds into three distinct adsorption types. Wang Jianmei et al.³⁰ conducted high-pressure adsorption experiments of supercritical CO₂ on anthracite, revealing a mutation point in the high-pressure adsorption process and indicating that the coal’s microstructure could be altered to improve the adsorption capacity of the anthracite surface. Several academics^31,32,33 investigated the adsorption behavior of CO₂ molecules under varying temperatures, pressures, and moisture levels, concluding that low temperatures and high pressures enhance CO₂ adsorption, whereas water molecules impede it. Certain academics^34,35,36 conducted adsorption and desorption experiments on laminated specimens with varying inclination degrees in a laboratory setting, discovering that both permeability and adsorption rate augmented as the inclination angle of the coal rock surface increased.

The aforementioned studies have conducted various investigations into the adsorption mechanisms of individual gases via seepage and diffusion; however, during the process of CO₂ displacing gas, CO₂ exists in a flowing state, and there is a paucity of research regarding the interplay between seepage and diffusion in the context of CO₂ flow adsorption. This paper enhances the conventional adsorption test method by maintaining the flow state of CO₂ during testing. It examines the influence of varying injection pressures and laminar structures on the evolution of seepage and diffusion in the CO₂ flow adsorption process, integrating factors such as CO₂ adsorption amount, injection quantity, exit flow rate, and flow adsorption ratio.

Experimental methodology

Sample collection and Preparation

Coal samples were collected from the S2207 tape transit lane of Changzhi Yuwu Coal Industry in Shanxi Province. The coal rank is bituminous, with a density of 1.39 g/cm³ and porosity ranging from 5.14 to 6.64%. Upon sampling, the specimens were promptly wrapped with vacuum cling film to prevent air exposure and thereafter transported to the laboratory for the extraction of Φ50mm×100 mm cylinders based on horizontal and vertical stratification. The test samples S1, S2, S3, S4, S5, and S6 are illustrated in Fig. 1. S1, S2, and S3 represent horizontal stratification (parallel to the axial line), whereas S4, S5, and S6 denote vertical stratification (perpendicular to the axial line). S4, S5, and S6 are vertical strata, oriented perpendicular to the axis. Table 1 presents the findings of the industrial analysis of the coal samples. The coal samples were subjected to a vacuum drying oven, heated to a continuous temperature of 100 °C to 105 °C for 24 h, then cooled to room temperature and set aside for future use. Figure 1 illustrates the sample preparation process. First of all, the retrieved raw coal will be cut out parallel surfaces by rock cutter (DJ-4 cutter) in accordance with the direction of the stratification, and the distance between the two parallel surfaces is greater than or equal to 120 mm, and then the cut and flattened coal samples will be placed on the operation platform of the core drilling machine (DJ-4 automatic core drilling machine), and the coal cores are drilled through the drilling bit with a diameter of 50 mm, and finally the cutter will be utilized to cut the 120 mm coal cores into specimens with a diameter of 100 mm. The final cutter is used to cut the 120 mm coal core into 100 mm specimen.

Table 1 Industrial analysis parameters of coal samples.

Fig. 1

Sample preparation process.

Experimental apparatus

The two variables, laminar structure and gas injection pressure, were designated as orthogonal factors to establish an orthogonal control test system, comprising a total of six test groups, as illustrated in Table 2. Subsequent to the flow adsorption test, the specimens were positioned in a drying oven and subjected to a continuous temperature of 100 °C to 105 °C for 24 h, following which they were removed and allowed to cool to room temperature for a control test backup. The entire testing procedure was conducted on a specimen measuring Φ50mm×100 mm.

Fig. 2

Schematic diagram of experimental apparatus.

Figure 2 illustrates the schematic diagram of the test setup, which primarily comprises a high-pressure gas supply system, adsorption system, pressurization system, thermostatic control system, data collecting system, and evacuation system. The high-pressure gas supply system comprises 99.99% CO₂, a pressure reduction valve, a mass flow meter (model: MF4701), among other components. The diameter of the adsorption chamber in the adsorption system is 72 mm, the length is 213 mm, the maximum loading value of axial pressure is 300 MPa, and the maximum peripheral pressure is 80 MPa. Coal samples are placed in the adsorption system, and the axial and peripheral pressures are exerted for the samples by the pressure pumps in the pressurization system. The range of the pressure sensor model is 0 ~ 50 MPa.The mass flow meter monitors the real-time CO₂ inlet and output flow rates, as well as the cumulative flow rate. A constant temperature control system maintains a stable and adjustable temperature for both the coal body and the injected gas. The data collection system comprises a paperless recorder and a computer for the gathering of temperature and flow rate data, while the vacuum system is employed to eliminate residual gas in the coal samples to reduce experimental interference.

Table 2 Experimental plan.

The test procedure is illustrated in Fig. 3 Upon completion of the preparatory work prior to the test, CO₂ flow adsorption is conducted as follows: (1) Connect the experimental apparatus according to the schematic diagram and verify the system’s airtightness; (2) apply a circumferential pressure of 5.0 MPa to the shaft, then evacuate the system; (3) open the cylinder and adjust the injection pressure to 0.6 MPa per the test protocol, simultaneously opening the inlet and outlet valves as well as the mass flow meter. The flow adsorption was deemed to achieve dynamic equilibrium when the flow meters at both the inlet and outlet indicated identical readings, signifying the conclusion of the flow adsorption test; (4) Substitute the coal samples, reiterate the procedures outlined in (1)-(2), modify the gas injection pressure to 0.8 MPa and 1.0 MPa, and replicate the aforementioned operation. A control test was established to determine whether the air exit is open in the two test groups. During the control test, the outlet valve remained closed while the intake maintained the programmed pressure to continuously inject CO₂, and the test concluded when the input flow meter indicated zero flow.

In order to minimize the test error, each group of tests was repeated three times under the same conditions, and the inlet and outlet flow rate patterns were the same in the three tests, with the error of flow rate less than 5% in the horizontal lamination direction, the error of flow rate less than 10% in the vertical lamination direction, and the error of flow adsorption amount of the coal samples was less than 5%, which illustrated the reproducibility and stability of the test, and the article chose to repeat the test once due to the limitation of the article’s length.

Fig. 3

Schematic diagram of the experimental process.

The flow adsorption test primarily examined the variations in flow velocity at both the inlet and outlet, the pattern of adsorption quantity throughout the flow adsorption process, and the CO₂ flow adsorption ratio. Additionally, it investigated the seepage and diffusion effects along with their transformation mechanisms based on the alterations of these parameters during the CO₂ flow adsorption process. This study’s control experimental system inhibited the pressure gradient-driven seepage mass transfer impact by implementing a closed boundary condition at the gas outlet, subsequently analyzing the diffusion-dominated mechanism effect.

Results and analysis

Changes in gas flow rate

Figure 4 illustrates that the import and export flow rates during the CO₂ flow adsorption test, oriented along the horizontal laminae, exhibited a two-stage pattern over time: the entrance flow rate shown a sudden decline followed by a gradual decrease, while the outlet flow rate displayed a rapid increase succeeded by a gradual decline. In the CO₂ flow adsorption test conducted in the vertical stratification direction, the entrance flow rate exhibited a similar trend to that observed in the horizontal stratification, but the output flow rate shown a rapid initial increase followed by a gradual rise over time. The disparities in the two stratigraphic structures resulted in divergent outlet flow trends, with seepage prevailing in the horizontal stratigraphic orientation, while the injected phase was preferentially conveyed along the predominant fracture network, exhibiting a pulsed flow response at the outlet (the outlet flow rate peaked within a brief timeframe), accompanied by diffusive adsorption-induced swelling of the coal matrix and a reduction in seepage rate; diffusion predominated in the vertical stratigraphic direction, with transport impeded by adsorption potential well trapping and the retention effects of the pore network, leading to a diminished transport rate. The vertical laminar flow is primarily governed by diffusion, with CO₂ being absorbed through adsorption potential traps and pore retention effects. The transport process exhibits typical unsteady-state mass transfer, and the output flow rate commences after an approximate diffusion time lag of 80 min.

Upon comparison with the magnified map of outlet flow rate in Fig. 4, it is evident that the gas injection pressure in the horizontal laminar direction exerts a substantial modulation effect on the gas breakthrough characteristics: as the gas injection pressure increases from 0.6 MPa to 1.0 MPa, the response time of the peak outlet flow rate exhibits a notable reduction (1400 s→502 s→436 s), concurrently, the peak flow rate demonstrates a nonlinear growth pattern (41.8 ml/min→56.5 ml/min→59.8 ml/min). 56.5 ml/min to 59.8 ml/min. The flow rate increased by 35.2% with an injection pressure of 0.8 MPa and diminished to 5.8% at 1.0 MPa. A substantial negative correlation exists between gas injection pressure and seepage rate following flow adsorption equilibrium in the vertical laminae direction: the equilibrium flow rate exhibits a consistently declining trend (6.06 ml/min → 5.68 ml/min → 5.37 ml/min) as gas injection pressure is incrementally raised from 0.6 to 1.0 MPa, resulting in an overall reduction of 11.4% in flow rate with a pressure increase of 66.7%. The literature³⁷ indicates that the parallel and vertical laminae orientations exhibit comparable pore size distribution characteristics; however, the connectivity between macropores and microfractures in the parallel laminae direction, along with the connectivity between mesopores and macropores with pore sizes under 500 nm, is more pronounced. This leads to enhanced effective porosity and improved pore-fracture connectivity, resulting in accelerated seepage velocity in the horizontal laminae direction under identical pressure differentials.

Fig. 4

Changes in inlet and outlet flow rates over time under different injection pressures.

CO₂ flow adsorption and permeability changes

The difference in cumulative flow rates obtained from coal samples at both the inlet and outlet of the flowmeter allows for the derivation of the adsorption amount over time curve during the test, as illustrated in Fig. 5. The adsorption amount of CO₂ in both laminar flow directions increases with rising injection pressure. Furthermore, as the injection pressure gradually increases, its influence on the adsorption amount diminishes, particularly in the horizontal laminar direction, where the injection pressure ranges from 0.6 MPa to 0.8 MPa. The saturated adsorption amount rose from 0.50 mol to 2.13 mol when the injection pressure increased to 1.0 MPa, with a saturated adsorption amount of 2.25 mol at this pressure level. The saturated adsorption quantity rose from 1.72 mol to 2.35 mol, and subsequently to 2.36 mol, when the injection pressure along the vertical laminations escalated from 0.6 MPa to 0.8 MPa, and then to 1.0 MPa. With the exception of the 0.6 MPa injection pressure, the saturated CO₂ flow adsorption quantities of the two-layer structures were comparable at the other injection pressures. However, the duration required to achieve the saturated adsorption amount for the horizontal layer was consistently shorter than that for the vertical layer, with the time difference between the two layers being an order of magnitude. The configuration of the layer significantly influenced the adsorption rate.

This research employs real-time monitoring of upstream and downstream pressures and outlet flow rates of coal samples to use the Darcy equation for deriving changes in the permeability of the coal body during CO₂ flow adsorption.

$$k=\frac{{2{{\text{P}}_{\text{0}}}{\text{L}}{{{{\upmu}}}_{\text{g}}}Q}}{{{\text{A}}(P_{1}^{2} - P_{2}^{2})}}$$

(1)

Where k is the permeability of the coal sample (in m²); L is the length of the coal sample, L = 100 mm; P₀ is the atmospheric pressure, P₀ = 0.1 MPa; ${{{{\upmu}}}_\text{g}}$is the gas viscosity, ${{{{\upmu}}}_\text{g}}$= 0.0179 Pa·s; Q is the flow rate at the outlet end(in mL/min); A is the cross-sectional area of the coal sample, A = 1963.5 mm²; P₁ and P₂ are the absolute pressures at the inlet and outlet ends of the specimen(in Pa).

Figure 6 illustrates that the permeability of horizontal and vertical coal bodies exhibits distinct variations with respect to adsorption time. The permeability in the horizontal lamination direction rapidly attains its peak value before gradually declining, whereas the permeability in the vertical lamination direction is initially zero, subsequently increasing monotonically after a certain duration, with the rate of increase initially rapid and then decelerating. The alteration in permeability indicates that horizontal laminations possess greater effective porosity and enhanced pore-crack connectivity. Consequently, CO₂ rapidly penetrates the coal matrix under differential pressure initially, followed by a gradual decrease in permeability due to the swelling effect induced by adsorption over time. Under identical pressure differentials, CO₂ did not penetrate the coal matrix in the vertical lamination direction. After a duration, the seepage pathway within the coal matrix was established, resulting in a rapid increase in flow rate. Concurrently, as adsorption intensified, the rate of permeability enhancement diminished. The disparity between horizontal and vertical permeability is one order of magnitude. CO₂ attains adsorption saturation rapidly in horizontal laminations due to seepage followed by diffusion, whereas in vertical laminations, it takes significantly longer to reach adsorption saturation due to diffusion and adsorption followed by seepage.

The test results indicate that, owing to the analogous pore size distribution characteristics of parallel and vertical laminations, the adsorption quantities of horizontal and vertical laminations are comparable at identical gas injection pressures, with a higher adsorption rate observed when seepage precedes diffusion. Initially, when seepage commences, the fissures and substantial pores within the coal matrix are preferentially occupied by adsorbed gas, subsequently diffusing to the substrate, ultimately resulting in a reduction of the coal body’s adsorption and expansion permeability. Conversely, when diffusion initiates first, the permeability of the coal at the injection end diminishes due to adsorption and expansion, thereby increasing the resistance to gas seepage and decelerating the adsorption rate in regions of the coal body distant from the injection point.

Fig. 5

Changes in CO₂ flow adsorption capacity over time under different injection pressures.

Fig. 6

Variation of coal permeability with time under different gas injection pressures.

CO₂ flow adsorption ratio changes

The adsorption ratio is defined as the quantity of CO₂ adsorbed relative to the total amount of CO₂ injected, as represented in Eq. (2). Diffusion serves as the intermediary process between seepage and adsorption; thus, the quantity of CO₂ adsorbed can be likened to diffusion, while the total CO₂ exiting the outlet can be compared to seepage. It is established that the adsorption ratio exhibits a positive correlation with diffusion and an inverse correlation with seepage, as delineated in Eq. (2). To examine the transformation mechanism between seepage and diffusion in the flow adsorption process, the adsorption ratio was employed as an index parameter to delineate the transition between the two.

$$\eta =\frac{{{Q_{ad}}}}{{{Q_{in}}}} \times 100\% =1 - \frac{{{Q_{out}}}}{{{Q_{in}}}} \times 100\%$$

(2)

Where: η is the adsorption ratio of the coal sample to the flowing CO₂(in %); ${Q_{ad}}$ is the total amount of CO₂ adsorbed by the coal sample༈in ml༉; ${Q_{in}}$ is the total amount of CO₂ streaming through the inlet༈in ml༉; and ${Q_{out}}$ is the total amount of CO₂ flowing through the outlet༈in ml༉.

Figure 7 illustrates the temporal variation of the CO₂ flow adsorption ratio across various laminar directions. The variation in the CO₂ flow adsorption ratio along the horizontal laminae occurs in three phases: a quick increase, a rapid reduction, and a gradual decline. Conversely, the CO₂ flow adsorption ratio in the vertical direction did not experience a swift escalation. Upon comparing the flow adsorption ratios in the two stratigraphic orientations, it was determined that the adsorption ratio in the vertical stratigraphic direction significantly exceeded that in the horizontal stratigraphic direction. This disparity was ascribed to the enhanced permeability of the coal, which could precipitate the premature breakthrough of injected gases into the coal matrix, consequently diminishing the adsorption ratio³⁸. The CO₂ flow adsorption ratios in the horizontal laminar direction were predominantly below 50%, whereas those in the vertical laminar direction were markedly higher, typically surpassing 50%. This suggests that seepage primarily influenced the flow adsorption test in the horizontal laminar direction, while diffusion was the dominant factor in the vertical laminar direction. Simultaneously, over time, the ratio of diffusion in both types of laminae progressively diminishes, while the ratio of seepage escalates.

Fig. 7

Changes trend of CO₂ flow adsorption ratio during experiment.

Table 3 Adsorption ratio of CO₂ flow after saturation.

Table 3 indicates that an increase in injection pressure results in a pronounced negative pressure-sensitive feature in the horizontal laminar direction, whereas the vertical laminar direction exhibits a positive pressure-sensitive response. The positive connection between injection pressure and flow adsorption indicates that an increase in the injection pressure gradient in the horizontal laminar direction results in a synergistic amplification of seepage and diffusion, with the seepage effect prevailing. In the vertical stratigraphic orientation, gas injection pressure exhibits a negative correlation with seepage and a positive correlation with diffusion.

Controlled trial results and analysis

The control test is a standard adsorption test conducted at constant temperature and pressure, wherein the open system (simultaneous CO₂ injection and exhaustion) is converted into a closed system (unidirectional CO₂ injection without exhaustion). In contrast to the flow adsorption test, the control adsorption test mitigates the impact of seepage effects, resulting in an adsorption process characterized solely by diffusive adsorption.

Figure 8 illustrates the temporal variation of CO₂ adsorption quantity during the control test. The saturation adsorption quantity remains consistent across different laminae at identical injection pressures, with horizontal laminae exhibiting a higher adsorption rate than vertical laminae. This suggests that the coal body laminae do not influence the saturation adsorption quantity but solely affect the CO₂ diffusion adsorption rate. Figure 9 compares the CO₂ saturation adsorption amounts in the two tests. The control test shows that when the air outlet is closed, the seepage effect decreases, CO₂ accumulates in the cracks of the coal seam, the concentration gradient increases, and the diffusion effect is enhanced, which contributes to the increase of CO₂ saturation adsorption amount to 1.3 ~ 4.8 times of that in the open system. At the same time, the increase of saturation adsorption with the increase of gas injection pressure in the control adsorption test was larger than that in the flow adsorption test, which was due to the fact that CO₂ could not be discharged out of the coal body after the air outlet was closed, and the CO₂ concentration in the fissure system increased rapidly with the increase of the gas injection pressure, which formed a potential difference in the concentration with the pore system, and diffusive adsorption behaviors occurred.

Alongside the examination of the exit seepage rate, two factors account for the disparity in adsorption: (1) within micropores or constricted pores, CO₂ molecules must diffuse to access the adsorption sites. In the open system, the seepage rate is augmented, causing gas to preferentially traverse the larger pores or fissures, while the diffusion rate within the micropores is insufficient, leading to a reduction in the effective adsorption surface area; (2) the coal matrix possesses a complex multiscale pore architecture comprising micropores, mesopores, and macropores. In the open system, the seepage rate is enhanced, and CO₂ mostly traverses the macropores or fissures, while the adsorption sites within the micropores remain underutilized due to inadequate gas residence time.

Fig. 8

Changes in CO₂ adsorption over time in the control experiment.

Fig. 9

Comparison of CO₂ saturation adsorption in open system and closed system.

Discussions

In the process of CO₂ driving CH₄, CO₂ exists in a flowing state. This research examines the influence of the laminar structure of coal bodies on the adsorption efficiency of CO₂ flow, aiming to enhance the CO₂ adsorption rate and ratio via the lens of laminar structure.

Upon conclusion of the CO₂ flow adsorption test, the gas injection pressures were 0.6 MPa, 0.8 MPa, and 1.0 MPa. The saturated adsorption amounts in the horizontal laminae direction were 0.5 mol, 2.13 mol, and 2.25 mol, while in the vertical laminae direction, they were 1.72 mol, 2.35 mol, and 2.36 mol. The saturated adsorption amounts in both laminae directions were comparable. The saturated adsorption capacity of CO2 for both types of laminae is comparable; the saturated adsorption durations for horizontal laminae are 20,600s, 51,040s, and 66,300s, while for vertical laminae, they are 120,000s, 93,860s, and 81,600s, indicating that the saturated adsorption time for horizontal laminae is significantly less than that for vertical laminae. The permeability change curve indicates that the sequence of seepage and diffusion initiation significantly influences the CO₂ flow adsorption rate. Seepage commences prior to diffusion in the horizontal laminae direction, whereas diffusion precedes seepage in the vertical laminae direction. Consequently, the CO₂ flow adsorption rate is higher in the horizontal laminae direction than in the vertical laminae direction, resulting in substantial time and cost savings during the injection process. At injection pressures of 0.6 MPa, 0.8 MPa, and 1.0 MPa, the adsorption ratios following CO₂ saturation in the horizontal laminar direction were 25.2%, 8.9%, and 5.9%, respectively. In contrast, the adsorption ratios in the vertical laminar direction were 42.2%, 46.0%, and 47.4%, respectively. The CO₂ flow adsorption ratios in the horizontal laminar direction were significantly lower than those in the vertical laminar direction. A significant quantity of CO₂ was squandered throughout the CO₂ flow adsorption procedure in the horizontal laminae, hence elevating the injection cost. Ultimately, the control test results indicate that, under identical conditions, transitioning from an open system to a closed system resulted in an increase in saturated adsorption capacity of 388%, 69%, and 81% in the horizontal laminar direction, and an increase of 28%, 58%, and 76% in the vertical laminar direction, demonstrating a significant enhancement in CO₂ adsorption capacity following the closure of the open system. The closure of the open system significantly enhanced the CO₂ adsorption capability.

In the engineering application of CO₂ substituting CH₄, it is essential to consider both the temporal expenses and the costs associated with CO₂ injection to optimize benefits and enhance CO₂ adsorption, thereby increasing the ratio of CO₂ replacement for CH₄. Consequently, during the CO₂ injection into coal beds, it is advisable to maintain the pumping hole switch, located farther from the injection hole, in an open position to facilitate the horizontal flow of CO₂ along the laminae and improve the CO₂ breakthrough time. Upon CO₂ breakthrough, promptly seal all extraction points, allow for CO₂ and CH₄ replacement, enhance CO₂ use efficiency, and thereafter reopen the extraction points after a designated interval to optimize advantages.

Conclusion

(1) In the CO₂ flow adsorption test, CO₂ rapidly penetrates the coal matrix along horizontal laminations. When compared to vertical laminations, the flow-saturated adsorption quantity remains comparable at identical injection pressures; however, the duration required is reduced, with the extent of reduction diminishing as injection pressure increases. Taking into account the temporal expenditure, the CO₂ flow adsorption efficiency in horizontal laminations surpasses that of vertical laminations.

(2) In the CO₂ flow adsorption process, the adsorption ratio in the horizontal laminar direction is considerably lower than in the vertical laminar direction, with the horizontal adsorption ratio predominantly below 50%, whereas the vertical adsorption ratio typically exceeds 50%. The CO₂ adsorption capacity of the vertical layer surpasses that of the horizontal layer when factoring in the injection cost.

(3) Upon closure of the system outlet, the saturation adsorption of CO₂ within the closed system markedly increased, with the saturation adsorption in the control test being 1.3 to 4.8 times greater than that of the flow saturation adsorption, while the laminate structure exhibited negligible influence on the saturation adsorption.

(4) Based on the impact of various laminations on CO₂ flow adsorption duration, quantity, and ratio, along with the control test outcomes, the system may be sealed once CO₂ permeates the coal matrix during the injection process, thereby minimizing both time and injection costs, ultimately optimizing benefits.