Delineation and Application of Gas Geological Units for Optimized Large-Scale Gas Drainage in the Baode Mine

Abstract

Addressing the challenge of efficient gas control in high-gas coal mines with ultra-long panels, this study focuses on the No. 8 coal seam in the Baode Mine. A multi-parameter integrated methodology was developed to establish a hierarchical classification system of Gas Geological Units (GGUs), aiming to identify regions suitable for large-scale gas extraction. The results indicate that the overall structure of the No. 8 coal seam is a simple monocline. Both gas content (ranging from 2.0 to 7.0 m³/t) and gas pressure (ranging from 0.2 to 0.65 MPa) generally increase with burial depth. However, local anomalies in these parameters, caused by geological structures and hydrogeological conditions, significantly limit the effectiveness of large-scale drainage using ultra-long boreholes. Based on key criteria, the seam was classified into three Grade I and ten Grade II GGUs, distinguishing anomalous zones from homogeneous units. Among the Grade II units, eight (II-i to II-viii) were identified as anomalous zones with distinct geological constraints, while two (II-ix and II-x) exhibited homogeneous gas geological parameters. Practical implementation of large-scale gas extraction strategies—including underground ultra-long boreholes and a U-shaped surface well—within the homogeneous Unit II-x demonstrated significantly improved gas drainage performance, characterized by higher methane concentration, greater flow rate, enhanced temporal stability, and more favorable decay characteristics compared to conventional boreholes. These findings confirm the critical role of GGU delineation in guiding efficient regional gas control and ensuring safe production in similar high-gas coal mines.

1. Introduction

Coal mine safety is persistently threatened by gas-related disasters [1,2,3], underscoring the critical need to develop efficient and controllable gas control technologies. Moreover, the rational extraction and utilization of gas are essential not only for ensuring mining safety but also for enhancing energy supply and contributing to emission reduction efforts [4,5]. In high-gas coal mines, implementing large-scale, proactive gas control strategies has become a pivotal approach to achieving safe and highly efficient production [6], particularly in response to increasing mining depths and intensities. This strategy reflects both industry best practices and regulatory mandates, such as those outlined in China’s Coal Mine Safety Regulations, which emphasize a “regional precedence over local measures” approach to outburst prevention.

Recent advances in gas drainage techniques, supporting equipment, and long-hole drilling technology have provided a solid technical foundation for the implementation of such large-scale preemptive gas control measures [1,6]. A representative case is the Baode Coal Mine, a typical high-gas mine in China, where the absolute methane emission rate reached 119.3 m³/min in 2023. Panels in its main mining seam (Seam No. 8) often exceed 3000 m in strike length. To significantly improve gas drainage efficiency, the mine has shifted its gas control philosophy from localized strategies targeting individual panels with boreholes shorter than 1000 m, to a proactive regional strategy that covers entire ultra-long panels and broader areas. This transition is exemplified by the successful implementation and validation of in-seam gas drainage boreholes exceeding 2000 m in length.

The effectiveness of gas drainage through long in-seam boreholes is highly dependent on prevailing geological and hydrogeological conditions [7,8,9,10]. Therefore, the successful implementation of large-scale gas control strategies requires a thorough and predictive understanding of the gas-geological characteristics of the target region. The occurrence of gas in coal seams is ultimately the result of long-term and complex geological evolution within the coal-bearing basin, governed by a combination of synergistic geological factors that influence gas generation, migration, and accumulation [11,12,13,14].

Fault zones and individual faults not only influence the occurrence and distribution of coal seams but also cause structural damage to the coal and create anomalous stress fields in adjacent areas [3,15]. These geological disturbances significantly reduce drilling success rates and compromise the long-term stability of underground boreholes [16]. Moreover, coal seams near such structures often exhibit abnormal gas pressure and permeability, which further diminish the efficiency of gas drainage operations. However, it is important to note the dual role of these structures. Depending on their architecture and the in situ stress field, faults can occasionally create localized channels of enhanced permeability. This can lead to increased gas emissions and, if properly managed, has the potential to promote gas drainage from specific sectors of the coal seam, presenting both a hazard and an opportunity for gas control strategies.

In areas with water-rich anomalies, boreholes are prone to failure due to hydraulic sealing or water-blocking effects, which severely limits the performance of long-hole gas drainage. Similarly, anomalies in gas abundance and pressure are major constraints on drainage efficiency. Hence, a fundamental strategy for enabling large-scale proactive gas control is to apply principles of gas geology. This involves systematically integrating and analyzing all relevant geological parameters that affect gas drainage, followed by the scientific zoning of gas geological units, to clarify how different geological factors control gas occurrence [17].

However, existing studies often focus on qualitative descriptions of geological impacts or single-parameter anomalies, lacking a quantitative and integrated methodology to hierarchically classify mining areas into zones with distinct gas geological characteristics and, consequently, differentiated gas control suitability. This gap limits the ability to move from a one-size-fits-all approach to a precision-guided strategy for large-scale gas extraction.

This study develops a multi-parameter fusion methodology to delineate hierarchical GGUs for the No. 8 coal seam in the Baode mining area. The methodology systematically integrates key geological criteria, including the gas weathering zone, gas content thresholds, structural complexity, hydrogeological anomalies, and gas abundance. The primary objective is to assess the geological suitability for implementing super-long boreholes and the feasibility of large-scale preemptive gas control strategies. Consequently, a practical GGU classification system is established that distinguishes anomalous zones requiring tailored solutions from homogeneous units amenable to standardized large-scale technologies.

2. Geological Background

The Baode Block, covering an area of 55.91 km², is located at the northern termination of the Baode–Xingxian Anticline Belt within the Jinxi Flexural-Slip Fold Zone on the eastern margin of the Ordos Basin in China [18] (Figure 1A,B). The Ordos Basin, which originated on the Mesozoic North China Cratonic platform, is a large intracontinental superimposed basin. Its evolution, shaped by Yanshanian to Himalayan tectonism, produced a structure defined by a stable central craton flanked by active peripheral belts. Structurally, the block features a simple monoclinal structure, with strata dipping approximately 260° at angles between 2° and 15°, resulting in a general topographic high in the east and low in the west. Influenced by the regional tectonic setting, large-scale faults and folds are poorly developed within the block. Observed faults are mainly small-scale normal faults with throws less than 5 m. No large-scale folds, collapse columns, or evidence of magmatic activity have been detected during exploration. In contrast, fractures are relatively well-developed in the coalfield, typically occurring in sets and forming banded patterns in plan view, with generally low dip angles.

The coal-bearing strata in this area belong to the Carboniferous–Permian system and are generally shallowly buried. The main minable seam, Seam 8, occurs at depths ranging from 60 to 600 m. It is situated on the S3 sandstone at the base of the Shanxi Formation (P₁s). The coal seam demonstrates good overall continuity, though its thickness varies significantly (2.15–10.50 m) due to paleo-channel erosion. The thickness increases from east to west: extra-thick coal is distributed in the northwestern and southeastern parts of the mining area, while medium-thick coal occurs only along the northern boundary. The average thickness is 6.02 m.

In terms of coal quality, Seam 8 consists predominantly of Long-Flame Coal (CYM) and Gas Coal (QM) across the entire area, with maximum vitrinite reflectance (R_o,max) values ranging from 0.55 to 0.88%. The immediate roof is composed mainly of sandy mudstone and mudstone, intercalated with lenses of coarse-grained sandstone. Petrographic analysis indicates that these sandstones, as well as the underlying strata which include coarse-, medium-, and fine-grained sandstones, are predominantly arkosic arenites. The floor consists primarily of mudstone, followed by siltstone.

Hydrogeological conditions in the area are relatively simple. The aquifers within the coalfield comprise a Cenozoic unconsolidated porous aquifer, Carboniferous–Permian bedrock fissure aquifers, and an Ordovician limestone karst aquifer. Mine water inflow is primarily derived from karst water, with fissure water as a secondary source. The water infusion mode is classified as indirect through the roof and floor.

The mining levels of the Coal Seam 8 in Baode Coal Mine are between elevations of +940 m and +420 m. The coal structure of Seam 8 is intact. Gas content ranges mainly from 2.0 to 7.0 m³/t, and gas pressure is between 0.2 and 0.65 MPa. The spatial distribution of these gas occurrence characteristics is shown in Figure 1C. In terms of permeability, Seam 8 has an average permeability of 2.31 mD and an average gas permeability coefficient of 0.57 m²/(MPa²·d). These parameters indicate that the seam is suitable for gas drainage.

The in situ stress field is a key factor controlling gas occurrence and outburst potential in coal seams [19,20,21]. This is clearly demonstrated by the well-established correlation between increasing burial depth and elevated gas content and pressure, which is fundamentally governed by stress conditions. However, the in situ stress distribution in underground mines is highly complex and dynamic, resulting from the superposition of the regional tectonic stress field onto local perturbations caused by geological structures (e.g., faults, folds), variations in rock mass mechanical properties, and mining-induced activities (e.g., excavation layout, goaf formation). Therefore, accurate characterization of the in situ stress field is essential for reliable gas geological assessment and prediction.

Given the directional nature of stress, equivalent stress is commonly used to represent the overall stress magnitude in a geological body. In Seam 8, the equivalent stress decreases gradually from the southwest toward the east and north (Figure 2), with values ranging from 0.11 MPa to 23.93 MPa. The minimum stress occurs in the northeastern part of the study area, while the maximum is concentrated in the southwest. This distribution is largely controlled by burial depth. Although the regional structural dip of the seam is generally westward, stronger surface denudation in the north has resulted shallower burial depths and consequently lower stress levels in that region. Overall, the in situ stress distribution is predominantly influenced by burial depth.

3. Gas Geological Unit Delineation

3.1. Key Delineation Criterion for Gas Geological Units

Due to the influence of various geological factors, coal seam gas distribution exhibits significant heterogeneity. To systematically investigate its underlying patterns, a comprehensive methodology is employed: the division of the mining area into zones of different ranks based on established criteria—a process referred to as gas geological unit delineation. This approach provides a critical scientific foundation for developing and implementing large-scale gas control strategies in mining operations [22,23,24].

In the Baode Coal Mine, the simple monoclinal structure and the absence of major faults result in a primary depth dependence of gas content, emission rate, gas pressure, and in situ stress. However, localized anomalies occur where these parameters significantly deviate from regional depth-related trends. Such deviations exhibit strong spatial correlation with specific geological structures (e.g., synclines and faults) and hydrogeological anomalies (Figure 1C). Consequently, while burial depth remains the dominant regional controlling factor, gas occurrence in these anomalous areas is attributed to the combined effects of depth, local geological structures, and hydrogeological conditions.

Gas Geological Units delineation process in the Baode Coal Mine is shown as Figure 3. A two-tier system (Tier I and Tier II) was used for gas geological unit delineation. Tier I units were defined based on the gas weathering zone boundary and a pre-drainage gas content threshold of 4 m³/t. This classification reflects the depth-dependent gradient variation in gas controlling factors observed in the Baode Mining Area. Tier II units were further subdivided within the Tier I framework by incorporating local heterogeneities, including complex geological structures, water-rich anomalies, and abnormal gas accumulation zones.

The pre-drainage gas content threshold of 4 m³/t in the Baode Mine is established based on the following analysis. During the extraction of Coal Seam 8 in the Baode Mine, gas emissions originate mainly from the working seam itself and adjacent layers. According to the Basic Requirements for Coal Mine Gas Drainage (GB 41022-2021) [25] the desorbable gas content prior to mining must not exceed 4 m³/t for a coal mining face with a daily output exceeding 10,000 tonnes. When the gas content of Seam 8 reaches 4 m³/t, calculations using Equations (1)–(3) from the Prediction Method of Mine Gas Emission Rate (AQ 1018-2006) [26] indicate a relative gas emission rate of 2.56 m³/t from the working seam and 5.09 m³/t from adjacent seams, resulting in a total relative gas emission rate of 7.65 m³/t for the panel. Using Equation (4) from the Calculation Method of Mine Air Volume in Coal Mine (MT/T 634-2019) [27], the required air volume for the panel was calculated as 1205.4 m³/min. Given that the ventilation cross-section of roadways at Baode Mine typically exceeds 17 m², the achievable air volume for a mining face can exceed 3000 m³/min, which is sufficient to meet ventilation demand.

$$q_3=q_1+q_2$$

(1)

where q₃: The relative gas emission rate of the coal mining panel (m³/t); q₁: The relative gas emission rate from the working seam (or extracted seam) (m³/t); and q₂: The relative gas emission rate from adjacent seams (m³/t).

$$q_1={\mathrm{k}}_1\times {\mathrm{k}}_2\times {\mathrm{k}}_3\times {\displaystyle \frac{{\mathrm{m}}_1}{{\mathrm{m}}_0}}\times \left({\mathrm{W}}_0-{\mathrm{W}}_{\mathrm{c}}\right)$$

(2)

where q₁: The relative gas emission rate from the working seam (m³/t); k₁: The gas emission coefficient from surrounding strata; k₂: The gas emission coefficient from the goaf, the value of which is the reciprocal of the panel’s recovery ratio; and k₃: The influence coefficient accounting for the pre-drainage effect of gate roads on gas emission from the coal in the panel. For longwall retreat mining, it is calculated as k₃ = (L − 2h)/L; L: The length of the working face (m); h: The equivalent width of gas pre-drainage along the roadways (m); m₁: The thickness of the coal seam (m); m₀: The extraction height of the panel (m); W₀: The original gas content of the coal seam (m³/t); W_c: The residual gas content of the coal after being transported out of the mine (m³/t).

$$q_2={\sum }_{\mathrm{i}=1}^{\mathrm{n}}\left({\mathrm{W}}_{0\mathrm{i}}-{\mathrm{W}}_{\mathrm{c}\mathrm{i}}\right)\cdot {\displaystyle \frac{{\mathrm{m}}_{\mathrm{i}}}{\mathrm{M}}}\cdot {\mathsf{\eta }}_{\mathrm{i}},$$

(3)

where q₂: The relative gas emission rate from adjacent seams (m³/t); m_i: The thickness of the i-th adjacent seam (m); M: The mining height of the working panel (m); W_0i: The original gas content of the i-th adjacent seam (m³/t); and W_ci: The residual gas content of the i-th adjacent seam (m³/t).

$${\mathrm{Q}}_0=100·\mathrm{Q}·\mathrm{K},$$

(4)

wherein Q₀: The air volume supplied to the mining panel, (m³/min); Q: The gas emission rate from the mining panel (m³/min); and K: The gas emission irregularity coefficient.

3.2. Delineation of Gas Geological Units

Based on zonal boundaries, the coal seam 8 occurrence area is divided into three Grade I gas geological units (I-i, I-ii, and I-iii), as shown in Figure 4. Unit I-i primarily represents the gas weathering zone, bounded by the lower boundary of this zone. It exhibits low gas content and low gas emission rates, and ventilation is the primary method for gas control. Unit I-i extends from the lower boundary of the gas weathering zone to areas with a gas content of 4 m³/t. While ventilation remains the main gas control method, localized areas with abnormal gas emissions require integrated measures combining gas drainage and ventilation. Unit I-iii comprises areas with gas content ≥ 4 m³/t, where gas drainage or regional control measures must be implemented. Coal mining may proceed only after gas content is reduced to a safe threshold.

Grade II gas geological units are further delineated within Units I-ii and I-iii. The delineation is based on the comprehensive coefficient of structural complexity (K_G) (Equations (5)–(8)) [28], water-enriched anomalies (low-resistivity anomalies in Coal Seam 8 identified by transient electromagnetic detection), and anomalies in the curvature of gas resource abundance contour lines. The comprehensive coefficient of structural complexity quantitatively expresses the degree of geological structural complexity within an assessment unit by integrating three fundamental parameters: fault density, fold intensity, and dip angle variability. Its calculation is provided in Equations (5)–(8).

Based on established delineation criteria for gas geological units, the occurrence area of Coal Seam 8 is classified into three Grade I gas geological units (I-i, I-ii, and I-iii), as illustrated in Figure 4.

Unit I-i corresponds to the gas weathering zone, delineated by its lower boundary. This unit is characterized by low gas content and low emission rates; ventilation alone is generally sufficient for gas control.

Unit I-ii extends from the lower boundary of the gas weathering zone to the contour where gas content reaches 4 m³/t. Although ventilation remains the primary control measure, localized areas with anomalous gas emissions require an integrated approach combining drainage and ventilation.

Unit I-iii encompasses areas with gas content ≥ 4 m³/t, where targeted gas drainage or regional pre-drainage measures are mandatory. Mining operations may proceed only after gas content is reduced to a safe threshold.

Within Units I-ii and I-iii, Grade II gas geological units are further delineated based on the following criteria: the comprehensive coefficient of structural complexity (K_G), computed using Equations (5)–(8) [28]; water-enriched anomalies, identified as low-resistivity zones within Coal Seam 8 via transient electromagnetic detection; and anomalies in gas resource abundance contour curvature.

The coefficient K_G > quantitatively assesses structural complexity by integrating fault density, fold intensity, and variability in dip angle.

$$K_G=\sqrt{K_{\mathrm{Q}}{(K}_{\mathrm{D}}+K_{\mathrm{Z}})},$$

(5)

where K_G: The comprehensive coefficient of structural complexity; K_D: The fault complexity coefficient; K_Z: The fold complexity coefficient; and K_Q: The dip angle complexity coefficient.

$$K_{\mathrm{D}}=\sum HL/S,$$

(6)

where H: The throw of the fault; L: The length of the fault; and S: The area of the calculation unit.

$$K_{\mathrm{Z}}=\left({\mathrm{L}}_{\mathrm{m}\mathrm{a}\mathrm{x}}-{\mathrm{L}}_{\mathrm{m}\mathrm{a}\mathrm{x}0}\right)/{\mathrm{L}}_{\mathrm{m}\mathrm{a}\mathrm{x}},$$

(7)

where L_max: The length of the contour line with the maximum curvature within the calculation unit; and L_max0: The shortest straight-line distance between the two endpoints of the aforementioned contour line (i.e., the one represented by L_max).

$$K_{\mathrm{Q}}=({\alpha }_{\mathrm{m}\mathrm{a}\mathrm{x}}-{\alpha }_{\mathrm{m}\mathrm{i}\mathrm{n}})/90°,$$

(8)

where α_min: The minimum dip angle of the rock strata within the calculation unit; and α_max: The maximum dip angle of the rock strata within the calculation unit.

To quantify the structural complexity of the No. 8 coal seam, a 1000 m × 1000 m grid system was established, comprising 58 cells. The comprehensive coefficient of structural complexity (K_G) was calculated for each cell using a set of equations (Equations (5)–(8)). For clarity in visualization, all K_G values were multiplied by 100. The resulting values range from 0.05 to 3.24 (Figure 5).

Analysis of Figure 4 reveals that, approximately bounded by the 4 m³/t gas content contour, the K_G values are generally higher in the region above the contour and lower in the region below it. The mean K_G value of all evaluation cells traversed by this contour was calculated to be 0.59. Cells with K_G values exceeding this mean were selected for further subdivision.

The final threshold for delineating structurally complex zones was set at K_G ≥ 0.76, which represents the mean K_G value calculated from secondary subdivided units within the study area (Figure 5). It is important to note that the critical K_G value varies significantly across different coal basins, as it reflects local tectonic intensity, fold density, and seam floor undulation. For example, reported thresholds are K_G > 0.3 in the Pingdingshan Coalfield (Henan Province) [28] and K_G > 0.5 in the Shaliang Mine (Hagu Mining Area, Northern Shanxi) [29]. In our study area, the K_G > 0.76 zone accurately captures the axes of the largest and a secondary syncline. Thus, the selected threshold of 0.76 is well-justified and appropriate for the specific geological conditions of this site.

Local anomalies in gas occurrence conditions in Coal Seam 8 are primarily attributed to variations in hydrogeological and structural conditions. The hydrogeologically complex or water-rich anomalous zones in Seam 8 have been identified through geophysical exploration methods, as shown in Figure 4. In contrast, structurally complex zones within the seam can be delineated using the structural complexity coefficient (K_G). Furthermore, a methodology needs to be established to identify anomalous zones of gas resource abundance in Seam 8.

Coalbed methane resource abundance (N) refers to the quantity of gas stored per unit area within a coal seam, typically expressed in units such as m³/m² or 10⁶ m³/km². It represents the volume of gas accumulated within a specific planar extent of the coal reservoir and serves as a key parameter for evaluating the gas enrichment potential and distribution heterogeneity of the seam. Physically, it integrates both the thickness of the coal seam and its gas-bearing capacity, thereby reflecting the total in situ gas resources available within a given geological unit. The product of coal seam thickness (in meters) and gas content (in m³/t), denoted as N, serves as a practical proxy for estimating gas resource abundance (Equation (9)). Elevated values of N are directly correlated with increased gas emission rates during coal extraction. High N not only signifies elevated gas emission risks but also indicates greater potential for methane extraction. Consequently, the identification and delineation of anomalous N zones are critical for effective gas control and utilization strategies.

$$N=\mathrm{h}\times \mathrm{c},$$

(9)

where N: Gas resource abundance (m⁴/t); h: Coal seam thickness (m); and c: Gas content (m³/t).

Figure 6 clearly shows a disparity between the spatial variations in the gas resource abundance contours and the gas content contours. This characterization indicates that anomalous zones in resource abundance can serve as a criterion for delineating Grade II gas geological units.

The area within the Baode Mining Area with a gas content below 4 m³/t was partitioned into 22 units at a spacing of 1000 m. The curvature index (K_N) of each resource abundance contour line within every unit was then calculated using Equation (10). Areas with a K_N-value greater than 1.3, i.e., the mean curvature value, were subsequently delineated as resource abundance anomaly zones, which is shown in Figure 6. Since the calculation principles of K_N and K_G are fundamentally consistent, the average K_N value within the study area was similarly adopted as the delineation threshold. The zones delineated based on this K_N threshold correspond to areas with significant gradients in gas resource abundance, confirming that the selected value is appropriate for the actual geological conditions of the study area.

$$K_N=(L_{\mathrm{m}\mathrm{a}\mathrm{x}}-L_{\mathrm{m}\mathrm{a}\mathrm{x}0})/L_{\mathrm{m}\mathrm{a}\mathrm{x}}$$

(10)

wherein, L_max: The length of the contour line with the maximum curvature within the calculation unit; L_max0: The shortest straight-line distance between the two endpoints of the aforementioned contour line (i.e., the one represented by L_max).

Integrating with identified water-rich anomalies, structurally complex zones, and gas resource abundance anomaly zones, the secondary Gas Geological Units were delineated within the region below the gas weathering zone (i.e., within Units I-ii and I-iii), as shown in Figure 7.

A total of ten Grade II gas geological units were delineated. Among these, eight units (II-i to II-viii) were characterized as anomalous zones, each influenced by distinct geological constraints. Specifically, Unit II-i is situated within a structurally complex zone. Units II-iv, II-v, II-vi, and II-viii are located within hydrogeologically anomalous (water-rich) zones. Unit II-iii was classified as a gas-abundance anomaly. Furthermore, Unit II-ii occurs at the confluence of a structurally complex zone and a water-rich zone, while Unit II-vii is affected by the overlap of a gas-abundance anomaly and a water-rich zone.

In contrast to these heterogeneous and geologically constrained units, Units II-ix and II-x exhibit notably homogeneous gas geological parameters. This homogeneity is characterized by minimal structural disruption, absence of hydrogeological anomalies, and a predictable relationship between gas content and depth. Consequently, these units are considered highly suitable for the implementation of regional-scale gas extraction technologies (e.g., ultra-long boreholes). This division between anomalous and homogeneous units provides a critical geological basis for optimizing gas control strategies across the mine field.

4. Discussion

4.1. Gas Occurrence Difference Among Gas Geological Units

To further elucidate the heterogeneity of gas occurrence across different geological units, a series of survey lines (a–e) were strategically deployed along an east–west transect within the sub-weathering zone strata of the Baode Coal Mine (Figure 8). Specifically, survey line a–a’ was positioned to traverse the normal zone of a Grade II Gas Geological Unit. A significant gradient increase in gas abundance was observed within the middle segment of this line, which strongly supports the current delineation of the boundary between Sub-units I-ii and I-iii.

As shown in Figure 9A–F, both gas content and gas pressure across the studied region exhibit a strong, approximately linear positive correlation with increasing burial depth. However, further analysis indicates that this trend is significantly influenced by higher-order geological structures. For example, data obtained from within the Grade II unit show considerably greater scatter in the depth–gas parameter relationship (Figure 9F), indicating a deviation from the regional trend and implying the presence of localized geological controls. In terms of structural control, the structurally complex zones (K_G ≥ 0.76) in the Baode Mine are primarily associated with synclinal axes. Synclines can promote gas enrichment by creating zones of stress concentration and potentially acting as seals due to reduced permeability. This pattern is consistent with observations from the Changping Mine in the North China Basin [30], where synclinal structures were also identified as a key control on gas enrichment. Regarding hydrogeological control, groundwater dynamics constitute another critical variable. Anomalous zones identified in this study show a strong spatial correlation between hydrogeological conditions and abnormal gas parameters. This correlation can be interpreted through hydrodynamics: gas tends to be preserved in stagnant groundwater zones (typically indicative of a hydraulic sealing environment), leading to high gas content, whereas active groundwater flow zones (hydraulic escape environments) facilitate gas dissipation. This hydrogeological control mechanism is strongly supported by findings from the southern Junggar Basin [31], which explicitly links high-gas-content areas to groundwater stagnant zones.

Moreover, survey lines crossing anomalous zones associated with Grade II units recorded pronounced variations in these parameters. This discrepancy highlights the essential role of Grade II units in compartmentalizing the gas reservoir and promoting heterogeneity within gas-bearing domains.

These findings suggest that areas unaffected by Grade II units anomalies exhibit more homogeneous and predictable gas distribution patterns. As a result, such regions are highly suitable for the implementation of efficient gas drainage systems—particularly those employing ultra-long boreholes, which require consistent gas conditions for optimal performance [6,32,33]. In contrast, the presence of Grade II units introduces structural and gas heterogeneity that complicates gas drainage design. In these areas, adaptive strategies such as targeted drilling, customized borehole layouts, and real-time monitoring are recommended to manage local variability effectively [9,34,35].

4.2. Analysis of Gas Control Effectiveness in Preferred Gas Management Units

To validate the effectiveness of large-scale gas drainage technologies in regions exhibiting homogeneous changes in gas parameters (such as Units II-ix and II-x), both ultra-long boreholes and U-shaped surface well have been implemented in Unit II-x. As illustrated in Figure 10A–C, borehole CCK1 and CCK2, with trajectory lengths of 2311 m and 2570 m, respectively, were completed in this unit. The location of the Shenhua Ping-01U surface well is provided in Figure 10A.

Borehole CCK1 achieved a total gas drainage volume exceeding 2.23 million cubic meters, with an average daily drainage rate of 3284 m³. Both methane concentration and pure gas flow rate exhibited a slow decay trend throughout the extraction period, contributing to prolonged, stable, and highly efficient gas production.

As shown in Figure 10D, the gas flow rate of CCK1 demonstrated a characteristic multi-phase pattern: an initial increase to a peak, followed by a slight decline, a subsequent secondary rise, and ultimately stabilization. This borehole maintained stable production over an extended duration without significant. In contrast, conventional directional boreholes typically exhibit maximum flow rates immediately upon commissioning, followed by a pronounced decay.

Borehole CCK2 extracted a total of over 970,000 m³ of gas, with an average daily rate of 2981 m³. Following the start of extraction, both gas concentration and methane flow rate increased steadily, reaching a peak around 20 days. Even after 100 days of production, although some decline was observed, output remained stable without evidence of rapid decline.

Production data from the Shenhua Ping-01U surface horizontal well in the No. 8 coal seam, recorded over 375 days, showed a peak daily gas production of 628 m³/d and a cumulative production of 143,619 m³. The Shenhua Ping-01U well, employing an innovative U-shaped design, offers distinct advantages over traditional vertical surface wells, which are particularly beneficial for drainage in homogeneous geological units like II-x. Unlike a vertical well that intersects the coal seam at a single point, the extensive horizontal leg of the U-shaped well provides thousands of square meters of contact area with the coal seam. This allows it to simultaneously initiate gas desorption across a broad volume, significantly boosting single-well gas recovery rate and overall drainage efficiency. Within geologically homogeneous units identified by the GGU framework, the U-shaped well’s horizontal section can traverse long distances through stable coal, minimizing exposure to localized structural complexities and stress disturbances. This results in more predictable and sustained production.

In stark contrast, conventional boreholes yielded significantly lower total extraction volumes compared to both ultra-long boreholes and surface horizontal wells. The flow rate from conventional boreholes declined rapidly over time, decreasing by 50% after 150 days and by 74% after 250 days of extraction.

A comparative analysis clearly demonstrates the superior performance of ultra-long boreholes and surface horizontal wells over conventional boreholes in terms of total extraction volume, temporal stability, and decay characteristics. The high and stable production achieved by CCK1, CCK2, and the Shenhua Ping-01U well confirms the efficacy of large-scale gas drainage technologies—specifically ultra-long boreholes and surface horizontal wells—in geological units characterized by homogeneous gas parameters such as Units II-ix and II-x [1,32]. The pronounced decline observed in conventional boreholes underscores their limitations for regional gas control. The enhanced performance of advanced drainage methods can be attributed to their greater reach within continuous coal seams, improved contact with gas-rich zones in areas with homogeneous gas parameters. These findings strongly support the adoption of large-scale drainage strategies as a more efficient and reliable approach for gas control in similar geological settings [1,6,35,36,37].

5. Conclusions

To address the challenges of regional gas control in high-gas mines utilizing ultra-long panels, this study focused on Coal Seam No. 8 in the Baode Mine. A multi-parameter fusion methodology was developed to establish a hierarchical classification system of Gas Geological Units (GGUs), specifically designed to guide and optimize the implementation of large-scale pre-drainage strategies. The main conclusions are as follows:

(1)

Coal Seam No. 8 is characterized by an overall monoclinal structure. While both gas content (ranging from 2.0 to 7.0 m³/t) and gas pressure (ranging from 0.2 to 0.65 MPa) generally increase with burial depth, local geological structures and hydrogeological conditions create significant heterogeneities in these parameters. These anomalies pose a primary constraint on the efficiency and predictability of large-scale gas drainage operations.

(2)

A systematic methodology for delineating GGUs was implemented, forming a scientific basis for targeted gas control strategies. Using the base of the gas weathering zone and a pre-drainage target gas content threshold of 4 m³/t, the seam was first subdivided into three Grade I units. Within the I-ii and I-iii units, ten Grade II units were further delineated based on the structural complexity coefficient, the presence of anomalous water-rich zones, and gas abundance anomalies. Among these, eight units (II-i to II-viii) were classified as anomalous zones with distinct geological constraints, while Units II-ix and II-x exhibited homogeneous gas geological parameters.

(3)

Engineering validation within the homogeneous Unit II-x, employing both ultra-long in-seam boreholes and a U-shaped surface well, confirmed the superior efficacy of large-scale extraction technologies. A comparative performance analysis demonstrated that ultra-long boreholes and the surface well significantly outperformed conventional boreholes, achieving higher total gas production volumes, enhanced temporal stability, and more favorable flow decay characteristics. These results substantiate that large-scale drainage strategies represent a more efficient and reliable gas control solution for homogeneous units such as II-ix and II-x in the Baode Mine. This structured, unit-based approach provides a replicable framework for improving gas drainage efficiency and ensuring safe mining conditions in similar geologically setting.