Numerical Simulation Study on Optimal CO₂ Injection Well Placement for Sequestration in Old Gob: A Case Study of the Huainan Mining Area

Abstract

The old gob, as a potential CO₂ geological storage reservoir, has huge storage potential. To clarify the distribution characteristics and storage capacity of CO₂ in the old gob after different well deployment schemes, this study, based on the actual geological conditions of the old gob in the Huainan mining area, uses the COMSOL software to numerically simulate CO₂ injection into the old gob, considering the heterogeneity of permeability and the difference in coal-rock adsorption capacity within the old gob. The research indicates that the distribution characteristics of CO₂ are significantly influenced by the deployment scheme. Specifically, different deployment schemes result in varying CO₂ concentrations and distribution patterns. Particularly, when the injection well is deployed at a depth of 65 m, the distribution of CO₂ in the low-permeability upper part of the old gob will significantly increase and the horizontal distribution range will significantly decrease. Under different well deployment modes, the CO₂ storage capacity varies significantly. When the injection well is deployed at a depth of 65 m, it is more conducive to the storage of CO₂ than at other deeper depths and the total storage capacity is larger. In addition, increasing the number of monitoring wells helps the migration and diffusion of CO₂ in the old gob. Reasonably increasing the number of monitoring wells and adopting a symmetric deployment mode can significantly improve the CO₂ storage capacity in the old gob. Through a reasonable deployment scheme, the CO₂ storage capacity in the old gob can be more than 1.8 times that of the single monitoring well deployment scheme. Overall, based on the analysis of the distribution characteristics and storage capacity, the vertical positioning of the injection wells and the deployment mode of the monitoring wells that are conducive to improving the CO₂ storage capacity in the old gob are obtained, which can provide an important reference for the well deployment scheme of CO₂ storage in the old gob.

1. Introduction

China’s coal reserves rank among the highest in the world, and coal resources are the mainstay energy source for China’s industrial and agricultural production [1,2,3,4]. Coal-fired power generation accounts for a high proportion, holding an absolute dominant position in the power system. According to statistics, coal-fired power enterprises account for about 70% of the country’s total carbon dioxide emissions [5,6,7]. The Huainan mining area is located within Huainan City, Anhui Province, China, and is one of China’s important energy bases [8,9]. The coal resources in this mining area are abundant, making it a crucial coal production base for the East China region. The Huainan mining area is one of the national key coal production bases and has maintained a relatively high level of coal production for many years, providing important coal supply for the power, industry, and other sectors in the East China region. The Huaihe Energy Group is one of China’s 14-billion-ton coal bases and six large-scale coal-fired power bases [10]. While seeking to reduce carbon emissions, it is also necessary to ensure the security of coal and power supply to serve economic and social development, making the task even more challenging [11,12]. Under the “dual carbon” (carbon peaking and carbon neutrality) strategy, how to properly handle the relationship between energy demand and carbon reduction is a severe challenge facing the Huaihe Energy Group. The Huainan mining area is an important energy base in China, but it faces significant challenges due to its special geological conditions and severe natural gas problems. In mining, particularly coal mining, a goaf refers to the abandoned areas of a mine that have been mined out and left without support. Understanding the geological conditions and natural-gas-related problems in old goaf areas is critical for ensuring safety and environmental integrity in post-mining operations. These areas often suffer from serious natural gas issues such as methane accumulation, which poses a significant explosion risk, and spontaneous combustion. Despite its importance, research on old goaf areas in the Huainan mining area is currently limited.

Carbon capture, utilization, and storage (CCUS) technology is a key technology for reducing CO₂ emissions from fossil-fuel-based power generation and industrial processes [13,14,15,16,17,18,19,20]. It is also an important approach for achieving large-scale low-carbon utilization of fossil fuels such as coal at the present stage [21,22,23,24,25,26]. In recent years, during the process of deeply building a clean, low-carbon, safe, and efficient energy system, the number of closed mines in China has continued to increase, and it is expected that, by 2030, the cumulative number of closed mines will reach 15,000 [27,28,29,30]. The abandoned mining goafs left behind after mine closure are large in quantity and widely distributed, and their unique storage environment provides good sealed storage space for geological CO₂ sequestration [31,32]. Therefore, storing CO₂ in abandoned mining goafs can not only help achieve the “dual-carbon” goals but also revitalize the underground space resources of closed mines, with broad application prospects [33,34,35].

Current research on coal mine goaf both domestically and internationally remains limited. The existing research findings are primarily concentrated on saline aquifers, depleted oil and gas reservoirs, and deep unmineable coal seams. Regarding well placement, previous studies have often utilized numerical simulation software such as TOUGH2 to investigate the migration and sequestration processes of supercritical CO₂ injected into saline aquifers. These studies have revealed the distribution patterns of CO₂ plumes after injection and found that employing different well configurations and increasing the number of injection wells can, to some extent, improve the sequestration efficiency [36,37].

There has also been extensive research on CO₂ sequestration in depleted oil and gas reservoirs. Scholars have used various software to study the sequestration process of supercritical CO₂ in oil and gas reservoir formations. It was discovered that, within the supercritical temperature and pressure range of CO₂, the displacement front can form a relatively stable flow zone, allowing the injected CO₂ to establish a stable high-density cap layer in the reservoir, enabling CO₂ storage for over 100 years. Overall, numerous researchers have utilized numerical simulation software to investigate the sequestration processes of CO₂ in different geological structures, demonstrating the capability of these tools in capturing the coupled multi-physics phenomena involved in geological CO₂ sequestration [38,39].

In contrast to the previous research on supercritical CO₂ sequestration in geological formations such as saline aquifers and deep unmineable coal seams, the coal mine goafs have undergone prolonged geological evolution, resulting in a potentially more complex distribution of pores and fractures, as well as more significant permeability heterogeneity within the storage formation. Additionally, adsorption plays a crucial role in CO₂ sequestration in old gob, while the adsorption capacity of the coal and rock materials within the old gob may vary. These heterogeneity factors make the distribution characteristics and sequestration capacity of injected CO₂ in coal mine goafs less clear and the suitability of well placement strategies more difficult to accurately assess.

Therefore, it is necessary to investigate the distribution characteristics and sequestration capacity of CO₂ under different vertical injection positions of the injection wells and various monitoring well deployment schemes. This study is based on the actual geological conditions and data of the old gob in the Huainan mining area, fully considering the heterogeneity of permeability and adsorption capacity within the depleted storage formation. Using the COMSOL software, a coupled fluid–structure interaction analysis was performed based on COMSOL 6.2’s built-in interfaces, incorporating solid mechanics, the Brinkman equation, and dilute species transport in porous media. The research will examine the distribution characteristics, sequestration capacity, and sequestration composition of CO₂ in the old gob under different vertical injection positions of the injection wells and various monitoring well deployment schemes. This will provide theoretical support for the key technical aspects of well placement for CO₂ sequestration in old gob.

2. Study Area

The Huainan mining area is located in the central and northern part of Anhui Province in the hinterland of East China [9,10]. It administratively belongs to Bengbu City, Huainan City, and Fuyang City, with the main part located within Huainan City, on the north bank of the middle reaches of the Huaihe River. It extends eastward to the line of Changfen-Luohe Town and westward to the line of Zhumaodian-Tangdian, connecting with the Xin’ji mining area. It is bounded by the Bagong Mountain and Shuneng Mountain ranges in the south and the line of Changxing Town-Tangji Town in the north. The area stretches about 70 km from east to west and about 40 km from south to north, covering an area of approximately 1600 km². Its geographic co-ordinates are between east longitude 116°20′ and 117°03′ and north latitude 32°34′ and 32°55′ (Figure 1).

The Huainan mining area is rich in mineral resources and has a long history of mining, which has resulted in the formation of a large number of abandoned mining goafs. During the field investigation, the author conducted geological surveys, sampling, and gas detection. Specifically, the geological surveys involved measuring geological structures and fault characteristics and creating detailed geological maps. The sampling process collected soil and rock samples from various depths and locations, which were then analyzed in the laboratory for their physical and chemical properties. Gas detection involved using specialized equipment to monitor methane and other gas concentrations (Figure 2).

3. Materials and Methods

3.1. Field Investigation and Sampling

The author conducted a relatively detailed field investigation of some abandoned mining goafs in the Huainan mining area and obtained coal samples from the main coal seams in the Huainan mining area. The coal samples were then prepared into cylindrical samples of 50–100 mm, 25–50 mm, 30–60 mesh particles, and 200 mesh particles as required.

3.2. Geometric Model and Boundary Conditions

3.2.1. Geometric Model

The sealing and integrity of the cap layer play a crucial role in the effectiveness of CO₂ geological sequestration [19]. The overlying sandstone layer in the study area has good compactness, low permeability, and high hardness, which is difficult to break, and the thickness of the formed rock layer is large enough to meet the integrity requirements of the cap layer. In the CO₂ sequestration in abandoned mining areas, it can achieve the effect of sealing and isolation and avoid the leakage of the sequestered CO₂ [27]. For the modeling of the area below the cap layer, the scope of the abandoned mining area for CO₂ sequestration is simplified and set as a geometric model of 3000 m × 2250 m × 100 m. A 0.5 m high coal residue layer is set at the bottom and an injection well with a radius of 0.2 m is set at the position of (0, 0, z) with different distances from the bottom (5 m, 20 m, and 35 m, respectively). A monitoring well with a radius of 0.2 m is set at a position 40 m above the bottom of the model, and a CO₂ concentration probe is set at the outlet of this well to realize the monitoring function. This well also serves as the gas production well for the residual CH₄ in the abandoned mining area (the focus of the numerical simulation research is on the CO₂ concentration at this well). The specific co-ordinates of this well are (10,10,40). Figure 3a shows the specific geometric model formed. Figure 3b shows the cross-section and planar grid division of the computational domain, and the grid around the coal residue layer, injection well, and monitoring well is encrypted using free tetrahedral grids.

3.2.2. Boundary Conditions

There is a constant velocity inlet, and the flow is a normal flow inlet, the injected gas is pure CO₂, with a concentration of 44.6 mol/m³, and the injection flow rate is 600 m³/h. Outlet boundary: the monitoring well is set as the pressure outlet, and the pressure at the outlet is set to atmospheric pressure, with natural outflow. Boundary conditions: the velocity on the roadway wall is 0, with no slip, and convection of the fluid at the boundary is considered. Considering that the gas in the old goaf still contains a small amount of gas, based on the field data, the residual CH₄ concentration in the old goaf is set to 0.3 mol/m³.

3.3. Basic Assumptions and Equations

3.3.1. Basic Assumptions

1. Gas is a continuous medium that adheres to the ideal gas equation of state. 2. The gas flow process is characterized as incompressible flow. 3. This analysis of gas flow neglects heat transfer and chemical reactions. 4. The component transport model in the old goaf area exclusively considers CH₄ and CO₂. 5. The old goaf area is comprised of a porous media structure. 6 Water was not considered in this multi-field coupling system.

3.3.2. Basic Equations

Based on the built-in interface of the COMSOL software, under the consideration of porous media, the Brinkman equation and N-S equation are coupled to model the flow of the injected CO₂ in the well, as well as to better describe the characteristics of the non-Darcy fast seepage in the fractures of the old goaf.

$$\begin{array}{c}𝛻\cdot [-pI+\mu (𝛻u+{(𝛻u)}^T)]=\rho 𝛻u\\ 𝛻\cdot [-pI+K]-(\mu k^{-1}+\beta \varphi \rho |u|+{\displaystyle \frac{\rho 𝛻u}{{\varphi }^2}})u=0\\ K=\mu {\displaystyle \frac{1}{\varphi }}(𝛻u+{(𝛻u)}^T)-{\displaystyle \frac{2}{3}}\mu {\displaystyle \frac{1}{\varphi }}(𝛻\cdot u)I\end{array}$$

(1)

where p is the pressure, in Pa; i is the imaginary unit; u is the velocity vector, in m/s; μ is the dynamic viscosity, in Pa·s; ρ is the fluid density, in kg/m³; k is the permeability, in m²; K is the stress tensor; β is the inertial resistance coefficient; and $\varphi$ is the porosity.

The motion of CO₂ in the porous medium is mainly divided into the diffusion motion of CO₂ molecules and the convective motion of CO₂ [23]. The convection–diffusion equation is as follows:

$$\varphi {\displaystyle \frac{\partial c}{\partial t}}+u𝛻c=D{𝛻}^{2}c$$

(2)

where t is the time variable, in seconds; c is the concentration, in mol/m³; and D is the diffusion coefficient, in m²/s.

In the old goaf, adsorption plays an important role in the sequestration of CO₂ (only considering CO₂ adsorption, with CH₄ residuals not taken into account). The coal and rocks in the old goaf have a large surface area, providing sufficient sites for the adsorption of CO₂ molecules. The adsorbed gas component content at the hypothetical equilibrium pressure P follows the generalized Langmuir equation:

$$V_{si}={\displaystyle \frac{V_i{b}_i{P}_i}{1+b_i{P}_i}}$$

(3)

where V_i is the Langmuir volume constant of gas component i, m³/kg; P_i is the gas pressure of gas component i, Pa; and b_i is the Langmuir pressure constant of gas component i, Pa.

The void ratio distribution of old goaf areas is determined by a mathematical model based on the three-dimensional dilation factor [24]. The distribution function of the three-dimensional dilation factor for rock collapse is given by the following equation:

$$K_p=K_{p,min}+[K_{p,max1}+{\epsilon }_1+(K_{p,max2}-K_{p,max1})e^{-a_2{d}_2} -K_{\mathfrak{p},\min }]e^{-a_1(d_1+b_1)\left(1-e^{-{\epsilon }_0{a}_0(d_0+b_0)}\right)}$$

(4)

where K_p is the bulking factor distribution function between the rock blocks in the old gob; K_p,min is the bulking factor between the compacted broken rock blocks, taken as 1.1; K_p,max1 is the bulking factor of the rock near the bottom of the working face; K_p,max2 is the bulking factor of the initial collapsed rock, with the relationship K_p,min < K_p,max1 < K_p,max2; d₀, d₁, and d₂ represent the distances from the point (x, y, z) to the working face, the surrounding walls of the old gob, and the floor, respectively, in m; b₀ is the control parameter for adjusting the depth of the old gob, used together with the decay rate a₀ to fit the porosity variation at the working face and cut-out area, with units in m and m⁻¹, respectively; b₁ is the control parameter for adjustments along the direction of the working face, used together with a₁ to fit the porosity variation in the “triangle zones” on both sides of the gob, with units in m and m⁻¹, respectively; a₂ is the decay rate for the distance to the roof of the old gob, in m⁻¹; ϕ₀ represents the smoothing parameter for adjusting porosity distribution; and ϕ₁ is determined based on field conditions. Based on the relevant literature and field conditions, the following values are obtained: a₀ = 0.0386 m⁻¹, a₁ = 0.286 m⁻¹, a₂ = 0.16 m⁻¹, ϕ₀ = 0.33, and ϕ₁ = 0.85.

$$\varphi =(1-{\displaystyle \frac{1}{K_p}})$$

(5)

$$k={\displaystyle \frac{{D_p}^2{\varphi }^3}{150{\left(1-\varphi \right)}^2}}$$

(6)

where D_p represents the average harmonic particle diameter, m.

Figure 4 and Figure 5 show the porosity distribution in the old mined-out area, calculated using the COMSOL software. Horizontally, most of the region is fully compacted with low porosity, except for a small “O”-shaped zone near the coal wall. The porosity gradually increases from the center of the old gob towards both ends. Vertically, the porosity increases progressively from top to bottom.

3.4. Numerical Simulation

When selecting horizontal positions, it is necessary to consider the permeability differences in the well area. Theoretically, placing the injection well in the old mined-out region, characterized by lower permeability, and positioning the monitoring well in an area with higher permeability can effectively leverage the fluid’s tendency to flow from low- to high-permeability regions. This strategy enhances the distribution range of CO₂ within the old mined-out area following injection. Consequently, the injection well is situated in the center of the compacted area, while the monitoring well is located near the “O”-shaped zone. The methods for deploying vertical injection points and monitoring wells are examined through numerical simulation. The vertical injection points are treated as variables and, in conjunction with actual engineering applications, the injection well is positioned within the collapse and fracture zones. Specifically, it is placed at depths of 35 m (injection well depth: 65 m), 20 m (injection well depth: 80 m), and 5 m (injection well depth: 95 m) from the floor. The storage capacities at these different vertical injection points are analyzed to identify the optimal vertical injection position. Subsequently, the vertical injection position is maintained constant while the number of monitoring wells is incrementally increased to further investigate the deployment method of monitoring wells in the old mined-out area through numerical simulation. The specific simulation scheme is detailed in

Table 1. Simulation scheme.

Influencing Factors	Vertical Injection Points for Injection Wells	Number of Monitoring Wells
	Injection Well Depth (m)
Well location deployment	65m	1	2
	80m		3
	96m		4
Parameter settings	Inject traffic: 600 m3/h, outlet pressure: 0.1 MPa ambient temperature: 311.15 K, injection time: 365 days

, with key simulation parameters, derived from both experiments and the literature, also listed in

Table 2. Simulated key parameters.

Simulated Key Parameters	Parameter Magnitude
Simulation Parameters (m3/s)	600
CO2 Injection Concentration (mol/m3)	44.6
CO2 Langmuir Adsorption Constant of Coal (m3/kg)	0.2246
CO2 Langmuir Pressure Constant of Coal (MPa)	1.734
CO2 Langmuir Adsorption Constant of Rock Mass (m3/kg)	0.0002
CO2 Langmuir Pressure Constant of Rock Mass (MPa)	2.754
Density of Coal (g/cm3)	1.46
Rock mass density (g/cm3)	2.40
Dynamic viscosity of CH4 (Pa·s)	1.03 × 10−5
Dynamic viscosity of CO2 (Pa·s)	1.38 × 10−5
Diffusion coefficient of CH4 (m2/s)	1.084 × 10−8
Diffusion coefficient of CO2 (m2/s)	2.356 × 10−8
Temperature of the mined-out area (K)	311.15
Initial Pressure of the Mined-out Area (MPa)	0.1
Molar Volume of Gas at Standard Conditions (mL/mol)	22.4

.

4. Results and Discussions

4.1. Vertical Injection Point Distribution of CO₂ in Different Injection Wells

As shown in Figure 6. At a well depth of 65 m, CO₂ accumulates in the upper part of the old mined-out area. Due to the higher concentration of stress in the upper section, the resistance to CO₂ flow and diffusion is significant. On the right side of the injection well (without monitoring wells), the diffusion range is smaller, extending up to 150 m (165 m) from the injection well. On the left side of the injection well (with monitoring wells), the high concentration area of CO₂ gradually decreases with increasing distance from the injection well, and CO₂ moves and accumulates towards the bottom of the old mined-out area. At distances of 300 m and 700 m from the injection well, there is a pattern of lower concentration at the bottom and higher concentration at the top, due to the concentrated stress in the upper section, which increases resistance to CO₂ flow and diffusion. Around 1100 m from the injection well, the resistance to CO₂ flow and diffusion is even greater, resulting in a distribution with higher concentration in the middle and lower concentration at both ends within the “O”-shaped ring. This is because the permeability in the “O”-shaped ring area is higher, allowing CO₂ to flow faster and thus accumulate more within this range. Additionally, at the bottom of the sectional cloud diagram, a small area of “blue stripe” appears, indicating that the adsorption capacity of coal for CO₂ is much higher than that of the overlying rock, causing most of the CO₂ in the residual coal area to be adsorbed.

As shown in Figure 7. At a well depth of 80 m, CO₂ accumulates in the upper part of the old mined-out area, covering a larger range than at 60 m. This is because the injection well is located in the collapse zone, where the stress concentration effect is significantly reduced, resulting in less resistance to CO₂ flow and diffusion. Therefore, the diffusion range increases significantly. On the right side of the injection well (without monitoring wells), the diffusion range is smaller, extending up to about 150 m (1650 m) from the injection well. On the left side of the injection well (with monitoring wells), the high concentration area of CO₂ gradually decreases with increasing distance from the injection well, and CO₂ moves and accumulates towards the bottom of the old mined-out area. Compared to the 60 m depth, the concentration is higher and the diffusion range is noticeably increased.

At distances of 300 m and 700 m from the injection well, there is a pattern of lower concentration at the bottom and higher concentration at the top due to the concentrated stress in the upper section, which increases resistance to CO₂ flow and diffusion. Around 1100 m from the injection well, the resistance to CO₂ flow and diffusion is even greater, resulting in a distribution with higher concentration in the middle and lower concentration at both ends within the “O”-shaped ring. Compared to the 60 m depth, the concentration at both ends is higher. This is because the bottom is located in the collapse zone and, the closer to the bottom, the higher the permeability in the “O”-shaped ring area, allowing CO₂ to flow faster and thus accumulate more within this range. Similar to the 60 m depth, a small area of “blue stripe” appears at the bottom of the sectional cloud diagram, indicating that the adsorption capacity of coal for CO₂ is much higher than that of the overlying rock, causing most of the CO₂ in the residual coal area to be adsorbed.

As shown in Figure 8. At a well depth of 96 m, CO₂ accumulates in the upper part of the old mined-out area, covering a smaller range than at 80 m, while the diffusion range at the bottom increases significantly. This is because the injection well is closer to the floor, reducing the resistance to CO₂ lateral movement and increasing its diffusion range. However, as the vertical migration distance increases, the resistance also increases, resulting in a reduced diffusion range in the upper area.

On the right side of the injection well (without monitoring wells), the diffusion range is smaller, extending up to about 150 m (1650 m) from the injection well. On the left side of the injection well (with monitoring wells), the high concentration area of CO₂ gradually decreases with increasing distance from the injection well, and CO₂ moves and accumulates towards the bottom of the old mined-out area. Compared to the 80 m depth, the concentration in the upper area decreases, while the diffusion range at the bottom increases. At distances of 300 m and 700 m from the injection well, there is a pattern of lower concentration at the bottom and higher concentration at the top due to the concentrated stress in the upper section, which increases resistance to CO₂ flow and diffusion. Around 1100 m from the injection well, the resistance to CO₂ flow and diffusion is even greater, resulting in a distribution with higher concentration in the middle and lower concentration at both ends within the “O”-shaped ring. Compared to the 80 m depth, the concentration in the upper area decreases. This is because the bottom is located in the collapse zone and, the closer to the bottom, the higher the permeability in the “O”-shaped ring area, allowing CO₂ to flow faster and thus accumulate more within this range. Similar to the depths of 60 m and 80 m, a small area of “blue stripe” appears at the bottom of the sectional cloud diagram, indicating that the adsorption capacity of the residual coal for CO₂ is much higher than that of the overlying rock, causing most of the CO₂ in the residual coal area to be adsorbed.

In summary, under all three injection well deployment methods, the distribution characteristics of CO₂ in the old mined-out area are similar. Influenced by gravity, CO₂ generally exhibits a greater distribution range and higher concentration closer to the floor. Due to the coal’s much higher adsorption capacity for CO₂ compared to the overlying rock, most of the CO₂ in the residual coal area is adsorbed, resulting in a small “blue stripe” at the bottom of the sectional cloud diagram.

4.2. CO₂ Sequestration Amount at Different Vertical Injection Points of Injection Wells

Figure 9 illustrates the variation in CO₂ concentration within the monitoring well at different vertical injection positions of the injection well. As the depth of the injection well increases, the time required for the CO₂ concentration in the monitoring well to begin rising gradually decreases. From the analysis above, when the injection well depth is 65 m, CO₂ shows a significant increase in distribution within the low-permeability upper area of the old mined-out zone, causing CO₂ flow and diffusion within this area to be slower than at the other two injection points. Since the 95 m injection depth is closer to the floor, the resistance to CO₂ diffusion at the bottom is reduced, resulting in a delayed increase in CO₂ concentration within the monitoring well.

As shown in

Table 3. Calculation of sequestration amounts at different injection well depths.

Injection Well Deployment Depth (m)	Injection Time (Day)	CO2 Sequestration Amount (kg)	Adsorbed State Sequestration Amount (kg)	Free State CO2 Sequestration Amount (kg)
65	222	6,273,400	3,261,000	3,012,400
80	200	5,651,712	3,496,440	2,155,272
95	192	5,425,643	3,383,238	2,042,405

, the CO₂ concentration in the monitoring well reaches 0.0138 mol/m³ at approximately 192, 200, and 222 days for injection well depths of 95 m, 80 m, and 65 m, respectively. The calculated total sequestration amounts are 5,425,643 kg, 5,651,712 kg, and 6,273,400 kg, respectively. The total sequestration amount is highest at the 65 m injection depth, approximately 1.10 to 1.15 times that of the other two deployment schemes. Considering all factors, the sequestration effect is optimal when the injection well is deployed at a depth of 65 m.

4.3. CO₂ Distribution Characteristics with Different Numbers of Monitoring Wells

Based on the injection well depth of 65 m, the injection time was increased to 600 days while keeping other engineering parameters unchanged. The number of monitoring wells was then sequentially increased. The specific deployment of monitoring wells is shown in Figure 10.

Figure 11 illustrates the variations in CO₂ concentration within monitoring wells under different deployment schemes. With three monitoring wells, the CO₂ concentration in the wells begins to increase the earliest. When four monitoring wells are deployed, the CO₂ concentration starts to rise slightly later than with two monitoring wells. The initiation of CO₂ concentration increase for the three deployment schemes occurs much later than that for a single monitoring well deployment.

Since CO₂ tends to migrate towards low-pressure and low-concentration areas, with three monitoring wells, there are more pressure outlets on the multi-well side, causing the CO₂ to primarily flow and diffuse towards the multi-well side after injection. The increase in the number of monitoring wells introduces new CO₂ flow paths in the old goaf, expanding the diffusion range of CO₂ along the direction of the old goaf. As the flow and diffusion range of CO₂ in the old goaf increases, the concentration of CO₂ in the old goaf at the same time significantly decreases, reducing the concentration gradient between regions. Therefore, with the deployment of multiple monitoring wells, the migration speed of CO₂ in the old goaf significantly decreases. In the case of three monitoring wells, the injected CO₂ mainly flows towards the multi-well side, where the concentration gradient of CO₂ is larger, resulting in faster flow and diffusion of CO₂, and it reaches the monitoring well locations earlier. Consequently, the CO₂ concentration in the three monitoring wells starts to increase much earlier than in the two and four monitoring wells.

With the increasing number of monitoring wells and since the injection points in the injection well remain consistent, the variations in permeability and adsorption capacity of the injection well deployment area no longer affect the results. Therefore, the CO₂ sequestration in the old goaf is calculated based on the CO₂ concentration in the monitoring wells reaching 0.0138 mol/m³ under different deployment schemes. The calculation results are shown in the table below.

As seen in Figure 11 and

Table 4. CO₂ sequestration with different numbers of monitoring wells.

Number of Monitoring Wells	Injection Time (Day)	CO2 Sequestration Amount (kg)	Sequestration Amount of Adsorbed CO2 (kg)	Free State CO2 Sequestration Amount (kg)
1	222	6,273,400	3,261,000	3,012,400
2	474	13,394,557	11,059,200	2,335,357
3	417	11,783,586	9,736,950	2,046,636
4	487	13,761,918	11,361,300	2,400,618

, under the multi-well deployment scheme, the CO₂ concentration in the monitoring wells reaches 0.0138 mol/m³ at approximately 474, 417, and 487 days, respectively. The total sequestration amounts calculated are 13,394,557 kg, 11,783,586 kg, and 13,761,918 kg, which are 2.13, 1.87, and 2.19 times that of the single monitoring well deployment. The four-well deployment scheme has the highest total CO₂ sequestration.

Based on the previous analysis, for the well deployment in the old goaf in this case, the injection well is positioned at the center of the fully compacted area, 35 m from the floor, with multiple monitoring wells symmetrically deployed. This configuration results in the maximum total CO₂ sequestration after injection.

5. Conclusions

This study thoroughly investigates the CO₂ sequestration well deployment strategies in the old goaf of the Huainan mining area, leading to the following conclusions:

(1) The distribution characteristics of CO₂ are significantly influenced by the deployment scheme. Under different deployment schemes, CO₂ shows a broader distribution range and higher concentration as it gets closer to the floor of the old goaf. Particularly, when the injection well is deployed at a depth of 65 m, the distribution of CO₂ in the upper low-permeability area of the goaf increases significantly, while the distribution range in the strike direction decreases notably. This phenomenon is mainly due to the lower permeability of the shallower deployment depth, causing greater resistance to CO₂ flow and leading to the accumulation of CO₂ towards the floor of the goaf.

(2) The CO₂ sequestration amount varies significantly with different well deployment methods. When the injection well is deployed at a depth of 65 m, the total CO₂ sequestration amount is the highest. Increasing the number of monitoring wells facilitates the migration and diffusion of CO₂ in the old goaf. After CO₂ injection, it tends to migrate towards areas of lower pressure and concentration. Increasing the number of monitoring wells provides more paths for CO₂ flow, expanding its distribution range within the goaf.

(3) Increasing the number of monitoring wells and adopting a symmetrical deployment significantly enhances the CO₂ sequestration in the old goaf. The increased number of monitoring wells expands the diffusion range of CO₂ within the goaf and increases the coal and rock mass involved in adsorption, significantly improving CO₂ sequestration, reaching more than 1.8 times that of the single monitoring well deployment method.