Research on Gas Reservoir Space Characteristics in the Goaf of Xinzhuangzi Closed Coal Mine, Huainan Mining Area

Abstract

China has numerous closed coal mines with abundant residual resources. The combined effect of multiple factors has driven research on gas resources in the goafs of these mines. Elucidating the characteristics of goaf gas reservoir space is crucial for analyzing gas distribution and extraction. Therefore, investigating gas reservoir space characteristics under coal seam group conditions is vital. This study uses the Xinzhuangzi closed coal mine as a case study and presents methods for calculating goaf space under both individual coal seam and coal seam group conditions. The volumes of the caving, fissure, and floor failure zones are determined. The results reveal that 14 coal seams have been mined at the Xinzhuangzi coal mine, with a total mined area of 4583.04 × 10⁴ m² across all seams. Under individual coal seam assumptions, the caving, fissure, and floor failure zones have volumes of 455.98 × 10⁶, 1648.40 × 10⁶, and 614.65 × 10⁶ m³, respectively, with a total goaf volume of 2719.03 × 10⁶ m³. Under coal seam group conditions, the caving, fissure, and floor failure zones have volumes of 438.22 × 10⁶, 871.24 × 10⁶, and 154.90 × 10⁶ m³, respectively, with a total goaf volume of 1464.36 × 10⁶ m³. The caving, fissure, floor failure zones and total volumes under coal seam group conditions are 96.11%, 52.85%, 25.20%, and 53.86% of the corresponding volumes calculated under individual coal seam assumptions. This indicates that the coal seam group goaf volumes are neither a simply the sum nor a proportional reduction in the individual coal seam goaf volumes. This study provides a concise approach for investigating goaf gas reservoir space under coal seam group conditions.

1. Introduction

The number of closed or abandoned coal mines in China has steadily increased each year. By the end of 2021, about 5700 coal mines had been shut down or abandoned [1]. Estimates further indicate that the number of abandoned coal mines in China will reach 15,000 by 2030 [2,3]. These closed or abandoned mines still contain ~420 × 10⁸ t of coal, nearly 5000 × 10⁸ m³ of unconventional natural gas, and other valuable resources [4,5]. Owing to factors such as the recoverable value of residual resources, gas explosion risks, environmental pressures from methane-related greenhouse gas emissions [6,7], and national dual-carbon targets [8], research on resource recovery from abandoned coal mines has received increasing attention. International studies on the utilization of residual gas resources from closed coal mines began earlier and provide valuable reference cases. For example, in the UK, gas-extraction projects have been implemented in 30 of 300 closed underground coal mines [9]. Durucan [10] used numerical simulations to evaluate methane recovery from abandoned mines in the Saar Coalfield, Germany, and predicted future methane production based on the model results. Karacan [11] investigated abandoned coal mines in Pennsylvania, USA. The study identified two key uncertainties affecting the economic utilization of methane: the total methane available in the gas emission zone of the abandoned mine and the recoverable fraction of this methane.

For unmined coal seams, gas resources are contained within the coal, known as in situ gas resources [12,13], and research has mainly focused on their thickness and distribution. In mined coal seams, some residual gas remains in the goaf, known as goaf gas resources [13,14]. Therefore, studies in these areas have focused on the volume and distribution of the goaf. In closed coal mines mined for decades, many coal seams have been extracted, making goaf gas a key research focus. Reservoir characteristics are crucial for underground energy extraction, and therefore research on reservoir spaces is very extensive, such as coal reservoirs for coalbed methane [15], reservoirs of deep-water turbidite [16], and depleted gas reservoirs [17]. Goaf gas reservoir space is a special type of reservoir space. The goaf gas reservoir space reflects the volume and spatial arrangement of its components, including the size and distribution of voids. This gas reservoir space is crucial for investigating gas migration and extraction in closed coal mines and estimating free gas resources based on void volume and methane concentration [6]. While gas reservoir space under individual coal seam conditions is well-studied [18,19], most coal mines, including Xinzhuangzi, operate with coal seam groups. Here, interactions between adjacent goafs create a more complex system [20,21], a scenario that remains less explored. Although some studies have examined gas reservoir space under coal seam group conditions [22], research remains limited in scope, methodology, and findings. Therefore, investigating the characteristics of the goaf gas reservoir space under coal seam group conditions is crucial.

The Xinzhuangzi coal mine, with 14 of its 16 mineable coal seams mined, exhibits typical coal seam group mining characteristics [23]. After 70 years of operation, the coal mine was closed at the end of 2017, thereby terminating a production capacity of 400 × 10⁴ t/a. Therefore, the Xinzhuangzi closed coal mine was selected as a case study to investigate the characteristics of the goaf gas reservoir space under coal seam group conditions.

2. Geological Background

The Xinzhuangzi coal mine is a closed mine located in Bagongshan District, Huainan City, Anhui Province, China. This mine is bounded to the south by Xiejiaji No. 1 coal mine and to the north by the Lizuizi coal mine. The western boundary of the mine is defined by the westernmost line of the weathering and oxidation zone of all coal seams. The eastern boundary corresponds to the easternmost line of the −1000 m elevation horizontal projection of all coal seams [23]. The mine spans about 5.40 km strike length and 3.75 km dip width (total 17.7861 km²; Figure 1).

Figure 1. Boundaries and structural divisions of the Xinzhuangzi coal mine.

The coal-bearing strata of the Xinzhuangzi coal mine include the Carboniferous Taiyuan Formation and the Permian Shanxi, Lower Shihezi, and Upper Shihezi Formations. The Carboniferous Taiyuan Formation has a total thickness of ~120 m and contains 7–10 coal seams. These seams are thin, highly unstable, and sulfur-rich, making them unmineable and unsuitable for exploration. The Shanxi, Lower Shihezi, and Upper Shihezi Formations have a total thickness of 820.69 m and contain over 40 coal seams with an average thickness of 41.52 m and a coal-bearing coefficient of 5.06% (Table 1). The Permian strata are divided from bottom to top into five coal-bearing members (A, B, C, D and E) and seven coal-bearing sections (No. 1, No. 2, No. 3, No. 4, No. 5, No. 6 and No. 7). The coal seams in members D and E are thin, unstable, and of low grade. Consequently, these seams were not mined at the Xinzhuangzi coal mine.

Table 1. Characteristics of the Permian coal-bearing strata.

Formation	Member	Section	Section Thickness (m)	Coal Seam Coverage	Total Thickness (m)	Coal-Bearing Coefficient (%)
Upper	E	No. 7	290.40	E26~E22	2.10	0.72
Shihezi	D	No. 6	72.55	D21~D18	2.80	3.86
		No. 5	103.00	D17~D16	1.00	0.97
	C	No. 4	97.80	C15~C12	7.70	7.87
Lower	B	No. 3	53.92	B11~B10	6.32	11.72
Shihezi		No. 2	157.02	B9~B4	16.00	10.19
Shanxi	A	No. 1	46.00	A3~A1	5.60	12.17
Total			820.69		41.52	5.06

The Xinzhuangzi coal mine contains 16 mineable coal seams distributed across the No. 1 to No. 4 coal-bearing sections of the Permian. From top to bottom, the coal seams are labeled C14, C13, B11b, B11a, B10, B9, B8, B7b, B7a, B7a-, B6, B5b, B5a, B4, A3, and A1, with an average total thickness of 32.47 m (Table 2). The key mineable coal seams are C13, B11b, B8, B7a, B6, B4, A3, and A1.

Table 2. Major characteristics of the 16 mineable coal seams.

Section	Coal Seams	Min. Thickness (m)	Max. Thickness (m)	Avg. Thickness (m)	Percent of Mineable Area (%)	Distance to Next Mineable Coal Seams (m)
No. 4	C14	0	1.99	0.55	41.2	18.00
	C13	1.80	12.20	6.52	100	63.10
No. 3	B11b	1.51	7.06	4.09	100	2.10
	B11a	0.35	2.57	0.93	74.2	29.50
	B10	0	2.25	0.81	78.7	32.50
No. 2	B9b	0	2.11	0.39	17.5	8.50
	B8	0	3.11	1.75	98.5	4.30
	B7b	0	1.77	0.65	54.4	3.10
	B7a	1.08	9.36	3.84	100	1.07
	B7-	0.33	1.74	1.10	48.8	12.37
	B6	0	5.56	3.41	93.4	30.50
	B5b	0	1.36	0.67	34.1	1.10
	B5a	0	1.27	0.64	38.8	4.50
	B4	0	8.21	3.34	91.2	59.60
No. 1	A3	0	5.65	1.67	96.3	7.70
	A1	1.02	4.48	2.11	100	—
Total				32.47

The coal mine exhibits a monocline structure (Figure 2), with well-developed faults and locally developed small folds. The strata strike is N40°W, the dip is toward NE, and dip angle varies significantly from 8° to 55° but mostly between 25° and 28°. Two fault groups, F6–F7 and F10-5–F11-9, control the overall structure of the coal mine. The coal mine is divided into three structural divisions based on these fault groups: the North Division (north of F6–F7), the Central Division (between F6–F7 and F10-5–F11-9), and the South Division (south of F10-5–F11-9). The coal seam extends from shallow to deep with significant depth variation. The mine is further subdivided into seven mining levels: −47 (Level 1), −112 (Level 2), −262 (Level 3), −412 (Level 4), −612 (Level 5), −812 (Level 6), and −1000 m (Level 7).

Figure 2. Geological profile along the No. IV exploration line.

3. Characterization Methods for the Goaf Gas Reservoir Space

3.1. Individual Coal Seam Conditions

The goaf gas reservoir space created by coal mining under individual coal seam conditions includes the caving and fissure zones in the overlying strata and the floor failure zone in the underlying strata. This concept is widely accepted by researchers [19,20,24].

First, the horizontal area of the goafs is determined. The coal seam floor contour maps of the 16 mineable coal seams [23] reveal the positions of the working faces, mining events, and residual coal resource distributions for each seam. From these maps, the horizontal distribution of goaf areas for each coal seam in the different structural divisions can be determined. Here, A_ND, A_CD, and A_SD represent the goaf areas of a coal seam in the North, Central, and South Divisions, respectively.

Second, the vertical distribution of the goafs is determined. Reference [23] summarizes long-term resource exploration, coal production, and scientific research in the coal mine. This reference provides data on the heights of the caving and fissure zones and the depth of the floor failure zone for eight key mined coal seams. For the other six mined coal seams without these data, using empirical formulas for calculation is a concise and effective method [25,26], and among many empirical formulas, the empirical formulas in national regulations and standards are widely used [27,28], so these values are calculated using national regulations [29] and standards [30] (Equations (1)–(3)). Equations (1)–(3) correspond to medium-hardness overlying strata of this coal mine, and without loss of generality, the Equations omit the uncertainty range (±root mean square error).

$${\mathrm{H}}_{\mathrm{M}}={\displaystyle \frac{100\sum \mathrm{m}}{4.7\sum \mathrm{m}+19}}$$

(1)

$${\mathrm{H}}_{\mathrm{L}}={\displaystyle \frac{100\sum \mathrm{m}}{1.6\sum \mathrm{m}+3.6}}$$

(2)

H_D = 0.303L^0.8

(3)

where H_M denotes the height of the caving zone (m), H_L represents the height of the fissure zone (m), H_D indicates the depth of the floor failure zone (m), ∑m signifies the total thickness of the mined coal seam (m), and L denotes the inclined length of the working face (m).

Third, the volume of the goafs is calculated. The data (A_ND, A_CD, A_SD, H_M, H_L, and H_D) are used to compute the volumes of the caving, fissure, and floor failure zones for each coal seam in the North, Central, and South Divisions (V_M+ND, V_M+CD, V_M+SD; V_L+ND, V_L+CD, V_L+SD; V_D+ND, V_D+CD, V_D+SD) (Equations (4)–(6)).

V_M+ND = A_ND × H_M; V_M+CD = A_CD × H_M; V_M+SD = A_SD × H_M

(4)

V_L+ND = A_ND × H_L; V_L+CD = A_CD × H_L; V_L+SD = A_SD × H_L

(5)

V_D+ND = A_ND × H_D; V_D+CD = A_CD × H_D; V_D+SD = A_SD × H_D

(6)

where V_M+ND, V_M+CD, and V_M+SD represent the volumes of the caving zones in the North, Central, and South Divisions, respectively (×10⁶ m³). V_L+ND, V_L+CD, and V_L+SD denote the volumes of the fissure zone in the North, Central, and South Divisions, respectively (×10⁶ m³). V_D+ND, V_D+CD, and V_D+SD represent the volumes of the floor failure zones in the North, Central, and South Divisions, respectively (×10⁶ m³). Other parameters are described above.

Finally, the volume of the goafs is calculated. The volumes of each coal seam in the caving, fissure, and floor failure zones (∑V_M, ∑V_L, ∑V_D) are determined using Equation (7). The total volumes in each structural division (∑V_ND, ∑V_CD, ∑V_SD) are calculated using Equation (8), and the overall goaf volume (∑V) is determined using Equation (9).

∑V_M =V_M+ND + V_M+CD + V_M+SD; ∑V_L = V_L+ND + V_L+CD + V_L+SD; ∑V_D = V_D+ND + V_D+CD + V_D+SD

(7)

∑V_ND = V_M+ND + V_L+ND + V_D+ND; ∑V_CD = V_M+CD + V_L+CD + V_D+CD; ∑V_SD = V_M+SD + V_L+SD + V_D+SD

(8)

∑V = ∑V_M + ∑V_L + ∑V_D

(9)

Here, ∑V_M, ∑V_L, and ∑V_D denote the total volumes of the caving, fissure, and floor failure zones, respectively (×10⁶ m³). ∑V_ND, ∑V_CD, and ∑V_SD represent the total volumes in the North, Central, and South Divisions, respectively (×10⁶ m³). ∑V indicates the total volume of the goaf (×10⁶ m³). Other parameters are described above.

3.2. Coal Seam Group Conditions

The Xinzhuangzi coal mine operates under coal seam group conditions, with 14 mined coal seams. The goafs of the upper and lower coal seams often intersect. The caving, fissure, and floor failure zones of a coal seam may overlap with those of adjacent coal seams. Therefore, the goaf gas reservoir space under coal seam group conditions cannot be calculated as a simple sum. Instead, the reservoir space must be analyzed based on the specific characteristics of these interactions.

When the floor failure zone of the upper coal seam does not intersect the caving or fissure zones of the lower seam, the gas reservoir spaces of both seams can be calculated independently (Figure 3a). Under coal seam group conditions, coal mining is usually carried out from top to bottom, that is, the upper coal seam is mined first, the strata movement of upper coal seam occurs first, while the lower coal seam is mined later, the strata movement of lower coal seam occurs later. And then, the floor failure zone of the upper coal seam and the fissure zone of the lower coal seam are both characterized by fissure development. So, when the floor failure zone of the upper coal seam intersects the fissure zone of the lower coal seam, the overlapping volume is assigned to the fissure zone of the lower coal seam and not counted twice (Figure 3b). The caving zone, with larger and more fissures, and also with strong connectivity, is completely different from floor failure zone and fissure zone. So, similarly, when the floor failure zone of the upper coal seam intersects the caving zone of the lower coal seam, the overlapping volume is assigned to the caving zone of the lower seam and not counted twice (Figure 3c). Analogously with above, if the overlap involves a caving zone, the overlapping volume is assigned to the caving zone. If the caving zones of both coal seams are involved, the overlapping volume is assigned to the caving zone of the lower coal seam. If no caving zone is involved, the overlapping volume is assigned to the fissure zone of the lower coal seam.

Figure 3. Characterization methods for the goaf gas reservoir space under coal seam group conditions. (a) the floor failure zone of the upper coal seam does not intersect the caving or fissure zones of the lower seam. (b) the floor failure zone of the upper coal seam intersects the fissure zone of the lower coal seam. (c) the floor failure zone of the upper coal seam intersects the caving zone of the lower coal seam.

According to the above rules for handling overlaps between upper and lower coal seam goafs, the goaf volume can be calculated using Equations (4)–(9).

4. Goaf Gas Reservoir Space Results

4.1. Goaf Volumes Under Individual Coal Seam Conditions

First, the horizontal distribution of the B11b goaf is analyzed. B11b, a key mined coal seam at the Xinzhuangzi coal mine, is used to illustrate the horizontal distribution of the goaf under individual coal seam conditions (Figure 4). In the South Division, Part 1 covers 115.59 × 10⁴ m², and Part 2 covers 54.28 × 10⁴ m², yielding a total A_SD of 169.87 × 10⁴ m². In the Central Division, Part 1 covers 173.64 × 10⁴ m², and Part 2 covers 65.25 × 10⁴ m², resulting in a total A_CD of 238.89 × 10⁴ m². The North Division (A_ND) covers 106.37 × 10⁴ m². Therefore, the total goaf area of B11b is 515.13 × 10⁴ m².

Figure 4. Horizontal distribution of the B11b goaf.

Second, the horizontal distribution of all mined coal seams is determined using the same method as for B11b, with goaf areas calculated for each seam in each structural division (Table 3).

Table 3. Goaf areas in each structural division.

Coal Seams	South Division (×104 m2)	Central Division (×104 m2)	North Division (×104 m2)	Total (×104 m2)
C14	96.48	11.86	0	108.34
C13	152.85	303.11	116.51	572.47
B11b	169.87	238.89	106.37	515.13
B11a	172.48	173.01	79.57	425.06
B10	123.89	88.60	42.33	254.82
B9b	60.06	0	17.29	77.35
B8	230.32	173.66	117.08	521.06
B7b	205.44	10.35	23.81	239.60
B7a	161.71	138.00	104.36	404.07
B7-	187.87	0	0	187.87
B6	188.46	75.49	111.75	375.70
B4	212.63	10.23	97.11	319.97
A3	191.33	0	4.64	195.97
A1	247.71	59.88	78.04	385.63
Total	2401.10	1283.08	898.86	4583.04

Among the 16 mineable coal seams, B5a and B5b have not been mined. The total goaf area of the remaining 14 coal seams is 4583.04 × 10⁴ m². Regarding the coal seams, C13, B8, B11b, B11a, B7a, A1, and B6 have relatively large mined areas, accounting for 12.49%, 11.37%, 11.24%, 9.27%, 8.82%, 8.41%, and 8.20% of the total, respectively. These seven coal seams represent 69.80% of the mined area. In contrast, B4, B10, B7b, A3, B7-, C14, and B9b have smaller mined areas, accounting for the remaining 30.20%. Regarding structural divisions, the South, Central, and North Divisions have goaf areas of 2401.10 × 10⁴, 1283.08 × 10⁴, and 898.96 × 10⁴ m², respectively, corresponding to 52.39%, 28.00%, and 19.61% of the total goaf area.

Table 4 presents the vertical goaf distribution for each coal seam, including the caving zone height (H_M), fissure zone height (H_L), and floor failure depth (H_D). In this table, the coal seam thickness represents the average thickness within the mined area, rather than the overall average thickness for the entire mine. The H_M and H_L values for eight coal seams—C13, B11b, B8, B7a, B6, B4, A3, and A1—are obtained from the reference [23]. The H_M and H_L values for the other six coal seams—C14, B11a, B10, B9, B7b, and B7a—are calculated using Equations (1) and (2). The H_D value is calculated using Equation (3). According to this equation, a longer inclined length of the working face (L) results in a greater depth of the floor failure zone (H_D). The working faces were relatively small before 2004, but with the advancement of mining technology, the working faces became larger after 2004. Owing to variations in the sizes and the inclined lengths of working faces at different stages of the mine production history, H_D varies. Therefore, two H_D values in Table 4 reflect these differences, L₁ and L₂ represent the average inclined length of the working face before and after 2004, while H_D1 and H_D2 represent the depth of the floor failure zone corresponding to the inclined length L₁ and L₂.

Table 4. H_M, H_L, and H_D for each mined coal seam.

Coal Seams	Thickness (m)	HM (m)	HL (m)	L1 (m)	HD1 (m)	L2 (m)	HD2 (m)	Distance to the Next Mined Coal Seam (m)
C14	1.18	4.81	21.5	80.69	10.16	146.47	16.37	18.00
C13	6.63	15.18	45.95	94.41	11.52	127.81	14.68	63.10
B11b	4.03	12.54	44.64	74.87	9.57	174.54	18.83	2.10
B11a	1.34	5.30	23.33	80.49	10.14	169.01	18.36	29.50
B10	1.25	5.03	22.32	81.68	10.26	165.07	18.01	32.50
B9	1.21	4.90	21.86	66.18	8.67	176.19	18.98	8.50
B8	1.97	9.09	34.43	97.70	11.84	172.28	18.64	4.30
B7b	1.11	4.58	20.65	99.35	12.00	155.84	17.20	3.10
B7a	4.16	12.60	44.78	107.80	12.81	173.36	18.73	1.07
B7a-	1.10	4.55	20.52	90.02	11.09	106.50	12.69	12.37
B6	3.79	12.75	43.67	105.70	12.61	174.95	18.87	36.10
B4	3.70	12.12	43.59	110.86	13.10	157.83	17.38	59.60
A3	2.72	10.16	37.15	107.91	12.82	—	—	7.70
A1	2.10	9.27	34.86	113.19	13.32	148.15	16.52	—

Finally, the calculated goaf volumes for each coal seam and each structural division under individual coal seam assumptions are presented in Table 5 and Table 6.

Table 5. Statistics of goaf volumes under individual coal seam conditions.

Coal Seams	Caving Zone Volume (1 × 106 m3)	Fissure Zone Volume (1 × 106 m3)	Floor Failure Zone Volume (1 × 106 m3)	Total Volume (1 × 106 m3)
C14	5.21	23.29	11.75	40.25
C13	86.90	263.05	79.21	429.16
B11b	64.60	229.95	65.19	359.74
B11a	22.53	99.17	51.21	172.91
B10	12.82	56.88	36.29	105.99
B9b	3.79	16.91	8.49	29.19
B8	47.36	179.41	71.55	298.32
B7b	10.97	49.48	29.99	90.44
B7a	50.91	180.94	59.41	291.26
B7-	8.55	38.55	20.83	67.93
B6	47.90	164.07	55.67	267.64
B4	38.78	139.47	46.07	224.32
A3	19.91	72.80	25.13	117.84
A1	35.75	134.43	53.86	224.04
Total	455.98	1648.40	614.65	2719.03

Table 6. Goaf volumes by structural division under individual coal seam conditions.

Division	Caving Zone Volume (1 × 106 m3)	Fissure Zone Volume (1 × 106 m3)	Floor Failure Zone Volume (1 × 106 m3)	Total Volume (1 × 106 m3)
South	218.93	812.93	281.69	1313.55
Central	140.23	490.64	173.39	804.26
North	96.82	344.83	159.57	601.22
Total	455.98	1648.40	614.65	2719.03

4.2. Goaf Volumes Under Coal Seam Group Conditions

Under coal seam group conditions, 14 coal seams have been mined. Thus, the goafs of upper and lower coal seams may overlap both horizontally and vertically. Owing to the significant thickness of the coal-bearing strata and the large number of mined coal seams, the goaf volume is analyzed in sections. The 14 mined seams are assigned to Sections No. 1–4 of the coal-bearing strata (Table 1). Section No. 1 includes two mined coal seams (A3 and A1). Because the vertical distance between A3 and A1 is 7.70 m and the height of the A1 caving zone is 9.27 m, A3 falls within the A1 caving zone. Therefore, the overlapping portion of the A1 and A3 goafs is assigned to the A1 caving zone, rather than the A3 floor failure zone. Section No. 2 of the coal-bearing strata includes seven mined coal seams (B9, B8, B7b, B7a, B7a-, B6, and B4). The vertical distance between B4 and A3 is 59.60 m; thus, the overlapping portion of the B4 floor failure zone and the A3 fissure zone is assigned to the A3 fissure zone, rather than the B4 floor failure zone. Similarly, as the distance between B6 and B4 is 36.10 m, the B6 floor failure zone falls within the B4 fissure zone. Accordingly, the overlapping fraction of B6 is attributed to the B4 fissure zone. For B9, B8, B7b, B7a, B7a-, and B6, the distances between adjacent seams are 8.50, 4.30, 3.10, 1.07, and 12.37 m, respectively. Consequently, the upper coal seams fall within the caving zones of the lower seams. Additionally, B9, B7b, and B7a- have significantly smaller mined areas than their adjacent seams. Thus, the overlapping portions of these coal seams are assigned to the caving zones of the lower coal seams. Section No. 3 of the coal-bearing strata includes three mined coal seams (B11b, B11a, and B10). The distance between B10 and B9 is 32.50 m. Consequently, the overlapping portion of the B10 floor failure zone with the B9 and B8 fissure zones is assigned to the fissure zones of B9 and B8, rather than the B10 floor failure zone. The distances between B11b, B11a, and B10 are 2.10 and 29.50 m, respectively. Thus, the upper coal seams fall within the caving zones of the lower coal seams, and the overlapping fractions are assigned to the caving zone of the lower coal seams. Section No. 4 of the coal-bearing strata includes two mined coal seams (C14 and C13). The distance between C13 and B11b is 63.10 m. Consequently, the overlapping portion of the C13 floor failure zone and B11b fissure zone is assigned to the B11b fissure zone. However, the remaining portion of the C13 floor failure zone remains unchanged. The distance between C14 and C13 is 18.00 m, and C14 has a significantly smaller mined area than C13. Therefore, the overlapping portion of the C14 floor failure zone and C13 fissure zone is assigned to the C13 caving zone. The remaining portion of the C14 floor failure zone remains unchanged.

Based on the above, the results about goaf volumes of four sections are shown in Table 7, and the total goaf volumes are summarized in Table 8.

Table 7. Goaf volumes of four sections under coal seam group conditions.

Section	Coal Seams	Division	Caving Zone Volume (1 × 106 m3)	Fissure Zone Volume (1 × 106 m3)	Floor Failure Zone Volume (1 × 106 m3)
No. 4	C14, C13	South	27.88	52.87	11.77
		Central	46.59	137.15	18.27
		North	17.69	53.54	6.90
No. 3	B11b, B11a, B10	South	31.15	103.48	3.71
		Central	38.05	126.41	14.30
		North	17.14	56.93	6.76
No. 2	B9, B8, B7b	South	100.71	107.43	26.13
	B7a, B7a-, B6	Central	52.14	43.10	1.26
	B4	North	54.29	51.42	11.93
No. 1	A3, A1	South	39.40	90.73	33.00
		Central	5.55	20.87	7.98
		North	7.63	27.31	12.89

Table 8. Statistics of goaf volumes under coal seam group conditions.

Division	Caving Zone Volume (1 × 106 m3)	Fissure Zone Volume (1 × 106 m3)	Floor Failure Zone Volume (1 × 106 m3)	Total Volume (1 × 106 m3)
South	199.14	354.51	74.61	628.26
Central	142.33	327.53	41.81	511.67
North	96.75	189.20	38.48	324.43
Total	438.22	871.24	154.90	1464.36

5. Discussion

Regarding the scope of goaf gas reservoir space: According to the theory of strata movement caused by coal mining, goaf gas reservoir space forms as the overlying strata fracture after a coal seam is mined. This fracturing creates three zones: the caving zone, the fissure zone, and the bending zone. The combined height of the caving and fissure zones, known as the height of the “two zones”, is a key factor in preventing roof water disasters and designing waterproof coal–rock pillars; however, the bending zone usually does not conduct water due to the continuity and integrity of the rock layers. Additionally, the underlying strata are disrupted after coal seam mining and form a floor failure zone, which is crucial for preventing floor water inrush and floor grouting reinforcement. The gas reservoir space represents the volume available for gas storage and transport. After coal seam mining, the caving, fissure, and floor failure zones contribute to the gas reservoir space and are not limited to the caving and fissure zones or the height of the “two zones”. The floor failure zone exhibits water conductivity, indicating its capacity to store and transport gas. Under coal seam group mining, the floor failure zone of the upper coal seam often overlaps with the caving or fissure zones of the lower coal seam, making the gas reservoir space more complex. This highlights the need to include the floor failure zone in research and consider the interactions between the floor failure zone of the upper coal seam and the caving and fissure zones of the lower coal seam.

Regarding the data on goaf gas reservoir space: Various methods can be used to determine H_M, H_L, and H_D, and the accuracy of these data is crucial for reliable analysis of the gas reservoir space. In Table 4, many values are derived from years of mining operations at the coal mine, making them specific and reflective of actual production conditions. Additionally, some data are supplemented by calculations based on national regulations and standards in coal industry, ensuring broad applicability and adaptability. Overall, combining these two approaches balances ease of data acquisition with the scientific reliability of the parameters. Other methods, such as numerical simulation [31], physical modeling, microseismic monitoring [32], and drilling detection, can provide further detailed insights into the characteristics of the caving, fissure, and floor failure zones. The combination and cross-validation of various methods are helpful to accurately characterize the goaf gas reservoir space. However, these methods are also subject to some technical and economic limitations [33]. Future research will strive to combine numerical simulation, physical modeling, or drilling detection and other methods to obtain more reliable parameters (H_M, H_L, and H_D), thereby serving the resource evaluation of favorable blocks and the design of development plans.

Regarding the goaf gas reservoir space under coal seam group conditions: The caving, fissure, floor failure zones and total volumes under coal seam group conditions are 96.11%, 52.85%, 25.20%, and 53.86% of the corresponding volumes calculated under individual coal seam assumptions (Table 9). This indicates that the coal seam group goaf volumes are neither a simply the sum nor a proportional reduction in the individual seam volumes. Therefore, developing accurate methods for calculating goaf volume under coal seam group conditions is crucial. This study provides a preliminary reference, and future research should aim to establish more scientific, practical, and concise approaches.

Table 9. Comparison of goaf volumes under two different conditions.

Condition	Caving Zone Volume	Fissure Zone Volume	Floor Failure Zone Volume	Total Volume
Individual coal seam (1 × 106 m3)	455.98	1648.40	614.65	2719.03
Coal seam group (1 × 106 m3)	438.22	871.24	154.90	1464.36
Percentage (%)	96.11	52.85	25.20	53.86

Regarding the application of goaf gas reservoir space: The void volume of the goaf can be estimated based on the goaf volume (Table 8), combined with the coefficient of void volume. The determination of void volume coefficient is also very important because it has a significant impact on the estimation of void volume of the goaf, and the void volume coefficient often varies in different cases [34,35,36]. The coefficients of 0.3 for the caving zone and 0.2 for the fissure zone are used in the research of Panji No.1 Coal Mine in Huainan Coalfield [22]. Xinzhuangzi Coal Mine is also in Huainan Coalfield, and similar to Panji No. 1 Coal Mine in gas geology. So, for the time being, with a coefficient of 0.3 for the caving zone and 0.2 for the fissure and floor failure zones, the total void volume is calculated as 336.694 × 10⁶ m³ (Table 10); Certainly, more precise and targeted coefficients of void volume need to be further studied in the future. The calculated void volume provides a basis for estimating free gas resources. The methane concentration of goaf is 35% [37], thus the estimated free gas resources are 336.694 × 10⁶ m³ × 0.35 = 117.8429 ×10⁶ m³. The void volume is not only a key parameter for investigating gas migration and extraction in goafs but also is a vital focus in multiple research fields. With ongoing research on coal mine underground reservoirs [35,36], CO₂ storage in coal mines [38,39], and underground space resources [40,41], the void volume can serve as a reference and provide a basis for cross-verification.

Table 10. Calculation of void volume in goaf.

Zone	Goaf Volume (1 × 106 m3)	Coefficient	Void Volume (1 × 106 m3)
Caving	438.22	0.3	131.466
Fissure	871.24	0.2	174.248
Floor failure	154.90	0.2	30.980
Total	1464.36		336.694

6. Conclusions

(1) A calculation method for goaf space under coal seam group conditions has been established, providing a concise approach for investigating goaf gas reservoir space.

(2) Fourteen coal seams have been mined at the Xinzhuangzi coal mine, with a total mined area of 4583.04 × 10⁴ m² across all seams.

(3) For the 14 mined coal seams at the Xinzhuangzi coal mine under coal seam group conditions, the caving, fissure, and floor failure zones have volumes of 438.22 × 10⁶, 871.24 × 10⁶, and 154.90 × 10⁶ m³, respectively. The total goaf volume is 1464.36 ×10⁶ m³.

(4) The caving, fissure, floor failure zones and total volumes under coal seam group conditions are 96.11%, 52.85%, 25.20%, and 53.86% of the corresponding volumes calculated under individual coal seam assumptions. This indicates that the coal seam group goaf volumes are neither a simply the sum nor a proportional reduction in the individual coal seam goaf volumes.