Optimizing Borehole Diameter for Maximum Gas Extraction Efficiency in Coal Seams

China's abundant coal resources, characterized by a widespread distribution, confront a significant challenge with a substantial number of mines susceptible to gas outbursts, posing frequent and high-risk incidents [1], [2]. The technique of borehole gas extraction has emerged as the foremost strategy for mitigating such hazards [3]. Critical parameters in the drilling process for gas extraction include the borehole diameter, the negative pressure applied during extraction, and the length of the coal hole [4]. Investigating the influence of coal hole length and negative pressure on the efficacy of gas extraction across boreholes of varied diameters, alongside the determination of an optimal borehole diameter, holds paramount importance for the enhancement of gas extraction results and the minimization of drilling expenses. The specialization in the deployment of a novel outburst prediction position sampler, featuring a borehole diameter of Ф42mm, underscores the innovative approaches being explored in this domain [5].

Recent years have witnessed a proliferation of research by a myriad of experts and scholars focusing on variables such as borehole diameter, gas extraction concentration, and the effective radius of extraction. Sharma et al. [6] delved into the present and future prospects of gas extraction within Indian coal mines. Bressan and Deshaies [7] accentuated the pivotal role of gas extraction within the context of the energy transition. Aziz et al. [8] elaborated on the strategies implemented for the secure mining of outburst-prone mines in Australia. Viney et al. [9], [10] offered modeling illustrations depicting the repercussions of coal mining and coal seam gas extraction on runoff within five Australian research catchment areas. Taheri et al. [11] engaged in the examination of gas flow within coal masses to ascertain gas pressure and molecular velocity amid gas emission operations. Aghighi et al. [12] elucidated the analytical principles governing both surface vertical and subterranean gas extraction methodologies. Frank et al. [13] acknowledged the establishment of directional drilling techniques as the industry benchmark for efficacious gas emission drilling. Deng [14], through the utilization of COMSOL numerical simulations and field trials, deduced that an augmentation in borehole diameter favorably impacts coal seam gas extraction efficiency. Fan et al. [15] pursued an examination of the impacts exerted by large diameter boreholes on the efficiency of gas extraction from coal seams.

In the investigation into gas extraction from the No. 3 coal seam at Yicheng Coal Industry, Wei et al. [16] analyzed the relationship between borehole diameters of 94mm and 75mm, and the corresponding gas concentration and pressure. It was observed that a correlation exists between extraction concentration and pressure, yet the borehole diameter exerts a minimal impact on both gas extraction pressure and concentration. Zhang et al. [17], employing similar simulations and numerical modeling techniques, explored the effect of negative pressure on gas permeation within the borehole. Their findings suggest that an extraction negative pressure within the range of 25-35Kpa is conducive to efficient gas extraction, albeit with a diminishing effectiveness over time. Further, Cao et al. [18] undertook a statistical evaluation of layout parameters and extraction data for 53 high-level boreholes, thereby optimizing both borehole diameter and extraction negative pressure for the gas extraction efforts at Wangjialing Coal Mine. Through numerical simulation, Xue [19] established an optimum diameter for extraction boreholes. Similarly, Cheng et al. [20] utilized COMSOL software to delve into the mechanisms through which negative pressure influences gas extraction. Zhao et al. [21] amalgamated engineering practices with FLUENT software simulations to ascertain the ideal negative pressure for gas extraction at the Zhaozhuang mine’s 1309 working face, pinpointing it at 20Kpa. Gao et al. [22] posited that the plastic zone and effective influence zone of a borehole expand in tandem with an increase in borehole diameter, thereby enhancing the interaction among adjacent boreholes. Specifically, when the borehole diameter reaches 1.0m, the radius of the effective influence zone is augmented to 4m, which is 2.67 times greater than that of standard boreholes.

Yao et al. [23] delved into the complexities of borehole gas extraction in soft coal. Li et al. [24] embarked on a study to evaluate the outcomes of gas extraction processes. Through the lens of numerical simulations, Zhang et al. [25] scrutinized the stress alterations and the likelihood of damage failure, attributing these phenomena to the scouring of coal and the geometric configurations of boreholes. Utilizing Fluent software, Xu et al. [26] undertook an analysis and simulation of disparate gas extraction effects under varied stratifications and negative pressure conditions across different roadway levels, subsequently optimizing the technological parameters pertinent to these environments. Saber et al. [27] investigated the partial differential equations governing gas flow within extraction mechanisms. Ahamed et al. [28] explored the ramifications of hydraulic fracturing on the efficacy of gas extraction. Danesh et al. [29] highlighted the significance of coal creep within the context of evaluating gas extraction performance. While a considerable volume of research has been dedicated to exploring the effective radius of gas extraction, the impact of large diameter boreholes, and the refinement of gas extraction methodologies—culminating in a spectrum of findings—a paucity of inquiry has been directed towards determining the optimal diameter for gas extraction boreholes. This analysis, predicated on empirical data derived from borehole gas extraction at the Puxi Well operated by Jiahe Mining Co., Ltd., assesses the influence of coal hole length and negative pressure on the efficiency of gas extraction across boreholes of varying diameters. The determination of an appropriate borehole diameter for gas extraction emerges as a critical consideration.

Within the context of Jiahe Coal Mine, a systematic drilling of boreholes was undertaken across the 2254 and 2454 floor roadways. This endeavor resulted in the establishment of seven sets of boreholes, encompassing three sets with diameters of 75mm and one set each for diameters of 87mm, 94mm, 105mm, and 113mm. Each set comprised five boreholes, uniformly matching in diameter. In a detailed arrangement, the 2254 floor roadway was designated for the construction of four experimental borehole sets, characterized as follows: the initial set was defined by a 75mm diameter, utilizing conventional sealing technology; a subsequent set, also of 75mm diameter, was distinguished by the integration of a two-plug-one-injection sealing process augmented by lower screen pipe protection technology; a third set, marked by an 87mm diameter, adhered to the aforementioned sealing methodology coupled with lower screen pipe protection; and a fourth set, with a 94mm diameter, similarly employed the two-plug-one-injection sealing approach alongside lower screen pipe protection. Additionally, the 2454 floor roadway witnessed the construction of three borehole sets: a fourth set of 94mm diameter, following the two-plug-one-injection sealing protocol with lower screen pipe protection; a fifth set of 105mm diameter, adhering to the same sealing method; and a sixth set of 113mm diameter, also utilizing the two-plug-one-injection sealing process with lower screen pipe protection. The fundamental parameters of these boreholes are systematically cataloged in Table 1.

In the investigation of borehole gas extraction, a detailed analysis was conducted on the fundamental aspects of gas extraction across seven distinct sets of boreholes, designated as 75, 75-2, 75-3, 87, 94, 105, and 113. The parameters scrutinized included mixed flow rate, concentration, negative pressure, and cumulative pure flow rate, with the findings systematically represented in Table 2 and illustrated through Figure 1, Figure 2, Figure 3, Figure 4.

The analysis revealed that borehole groups with diameters of φ94mm, φ105mm, and φ113mm experienced relatively uniform changes in negative pressure, which were observed to be higher than those of the φ75mm and φ87mm borehole groups. It was noted that the extended sealing lengths associated with the φ75mm and φ87mm borehole groups resulted in higher average concentrations when compared to the larger diameter groups of φ94mm, φ105mm, and φ113mm. Furthermore, the average mixed flow rates and pure flow rates for gas extraction were more pronounced within the φ94mm and φ113mm borehole groups. Prior to the 20th day of extraction, significant fluctuations in mixed flow rate were recorded for both; however, the pure flow rate of the φ113mm boreholes demonstrated a rapid decline while still retaining a commendable level of extraction efficiency. Among the groups, the φ87mm boreholes exhibited the lowest average mixed flow rate, with the φ75mm borehole group recording the lowest average pure flow rate, thereby indicating the efficacy of larger borehole diameters in augmenting gas extraction efficiency.

In the comparative analysis of gas extraction efficiency across a range of borehole diameters within the Jiahe Coal Mine, the φ75mm borehole group served as a baseline for examining the interplay between borehole diameter and two pivotal metrics: gas extraction concentration and cumulative pure flow rate.

Figure 5 and Figure 6, derived from the core data on gas extraction, serve to visually articulate the differential outcomes in gas extraction efficiency as influenced by borehole diameter. These graphical representations provide insight into the variance in gas extraction concentration and pure flow rates among the studied borehole groups. The comparative results are presented in Table 3.

Under homogeneous gas occurrence conditions, a pattern of similarity in average gas concentration trends was discerned across boreholes of varying diameters. Notably, the φ87mm borehole group's concentration exceeded that of the φ94mm and larger diameter groups, as evidenced by a comparative analysis between Figure 3 and Figure 5. Conversely, under identical gas occurrence conditions, the gas concentration attributed to the φ87mm borehole group was observed to be lower than that of the φ94mm and larger diameter groups. This discrepancy underscores the propensity of larger borehole diameters to facilitate an enhancement in gas extraction concentration.

Further examination of Figure 4 and Figure 6 revealed a marked enhancement in the extraction efficiency of the φ94mm borehole group. Despite the presence of shorter coal hole lengths in borehole diameters of d≥94mm relative to those of d<94mm, an elevated pure flow rate was recorded, suggesting an absence of correlation between the gas's pure flow rate and the coal hole length at the extraction site.

Consistent with the conditions of gas occurrence, an augmentation in gas extraction efficiency was associated with the application of high negative pressure extraction and the utilization of larger borehole diameters. Specifically, the φ94mm borehole group was distinguished by its superior extraction efficiency.

To ascertain the relative contribution of various borehole diameters to gas extraction under uniform gas occurrence conditions, an analysis was conducted on the efficiency of gas extraction per millimeter of borehole diameter within a specified coal hole length. This analysis focused on two pivotal metrics: the concentration of extracted gas and the pure flow rate of gas extraction. The outcomes of this investigation are depicted in Figure 7 and Figure 8, offering a visual representation of the concentration variation and pure flow rate variation per unit diameter, respectively. The synthesized data, providing a quantitative measure of gas extraction per unit diameter, are systematically presented in Table 4.

Analysis of Figure 7 reveals that the highest concentration of gas extraction per unit diameter was attained by the φ105mm borehole group. Over the course of the study, the disparity in gas extraction concentration per unit diameter between the φ105mm and φ94mm borehole groups was observed to diminish after the initial 16 days.

Furthermore, Figure 8 demonstrates that the φ94mm borehole group secured the maximum pure flow rate of gas extraction per unit diameter, effectively doubling the rate observed in the φ105mm borehole group. Despite witnessing a marked reduction in the pure flow rate of gas per unit diameter over time, the φ94mm borehole group maintained a position of superiority in comparison to other borehole groups. Consequently, under equivalent conditions of gas occurrence, the extraction efficiency of the φ94mm borehole group was markedly superior, showcasing the highest utilization rate per borehole diameter among the groups analyzed.

Accounting for the variations in extraction pressure observed across each borehole, an investigation was conducted to assess the efficiency of gas extraction among different borehole diameters under a range of negative pressures. The efficiency of gas extraction was quantified based on the negative pressure applied, with an analysis of the variations in gas concentration and pure flow rate per unit of negative pressure (1 Kpa) for the various borehole groups being undertaken. The outcomes of this investigation are depicted in Figure 9 and Figure 10, which illustrate the comparative analysis of average concentration and average pure flow rate, respectively. The comparative results of this study are succinctly tabulated in Table 5, presenting the gas extraction effects under identical extraction negative pressures across different borehole groups.

Under the application of a unit of negative pressure, it was discerned from Figure 9 that the φ87mm borehole group exhibited the highest average concentration effect among the groups studied. Following closely, the φ113mm group demonstrated a commendable average concentration effect, while the φ94mm group's performance was noted to be lower, achieving only 90% of the extraction effect observed in the φ75mm group.

Figure 10 delineated a comparison where the φ113mm group's pure flow rate extraction effect was significantly superior to that of other groups, especially when compared to the φ75mm group. The pure flow rate extraction effect of the φ94mm group was also noteworthy, surpassing that of the φ87mm group in efficiency.

Through the analysis of variation curves pertaining to each borehole group's extraction effect, coupled with the comparative results under a single unit of negative pressure, several conclusions have been drawn. It was found that the gas flow rate for borehole groups with diameters d≥94mm surpassed those of groups with diameters d<94mm. Conversely, the gas concentration for borehole groups with diameters d≥94mm was observed to be lower than that of groups with diameters d<94mm. This phenomenon can be attributed to the fact that borehole groups with diameters d≥94mm were subjected to higher extraction negative pressures compared to the φ75mm and φ87mm groups, leading to increased leakage and subsequently lower gas concentrations in the larger diameter boreholes. Nonetheless, employing a suitably high extraction negative pressure has been recognized as conducive to enhancing gas extraction efficiency.

To explore the utilization rates of borehole diameters among different groups under uniform negative pressure conditions, with the objective of optimizing gas extraction efficiency while minimizing operational costs, an analysis was undertaken. This analysis evaluated the volume of gas concentration and pure flow rate extractable per millimeter of borehole diameter for each 1 Kpa of extraction negative pressure applied. The results of this study are depicted in Figure 11 and Figure 12, presenting the variation in gas concentration and pure flow rate per unit diameter, respectively. The compiled data, quantifying gas extraction per unit diameter of borehole, are methodically summarized in Table 6.

As elucidated by Figure 11, under a negative pressure of 1Kpa, the highest gas concentration per unit diameter was achieved by the φ87mm borehole group, exhibiting a reduction rate of 48%. Conversely, the φ94mm group manifested the minimal concentration extraction effect, registering at 0.01%, with a negligible variance from the φ105mm and φ113mm groups, which displayed a reduction rate of 31%. The φ75mm group maintained a concentration around 0.02%, with a decrease rate of 24%; the φ105mm group at 0.012%, with a decrease rate of 30%; and the φ113mm group at 0.013%, with a decrease rate of 23%.

Figure 12 reveals that, under the same negative pressure of 1Kpa, the φ113mm group extracted the highest pure flow rate of gas per unit diameter, averaging 1.07×10-5m³/min, and witnessed a decrease rate of 62%. The φ94mm group's extraction rate stood at 6.83×10-6m3/min, with a decline rate of 74.5%, and the φ105mm group's rate was 4.41×10-6m3/min, with a decrease rate of 57.2%. In comparison, the φ75mm and φ87mm groups attained lower pure flow rates per unit diameter under the same negative pressure, marked by a significantly steeper decline rate of 86%, indicative of less efficient extraction performance.

In conclusion, although the φ113mm borehole group demonstrated superior performance in terms of pure flow rate, analysis of gas concentration revealed lower concentrations for both the φ113mm and φ94mm groups compared to other diameters. This phenomenon is attributed to two principal factors: the observed gas extraction concentration parameters for the φ94mm, φ105mm, and φ113mm groups typically indicated lower concentrations relative to the smaller diameter groups, corroborating the premise that elevated extraction negative pressures engender diminished gas concentrations. Moreover, considering the construction locations of boreholes, the coal hole and sealing lengths for the φ113mm, φ105mm, and φ94mm groups were found to be shorter than those for the smaller diameter boreholes of φ75mm and φ87mm, potentially impacting the gas occurrence conditions. Furthermore, the abbreviated sealing lengths associated with these larger diameters may compromise gas tightness relative to the φ75mm and φ87mm groups, thus contributing to the reduced gas concentrations observed in larger diameter boreholes.

In the evaluation of gas extraction efficiency across varying borehole diameters under identical environmental conditions and standardized units of negative pressure, it has been observed that borehole groups with diameters equal to or greater than 94mm demonstrated superior performance. A focused examination was carried out on the efficiency of gas extraction for borehole diameters φ94mm, φ105mm, and φ113mm to ascertain the optimal diameter for gas extraction.

Observations from Figure 1 indicated that the extraction negative pressures for borehole groups of diameters φ94mm, φ105mm, and φ113mm were substantially consistent, allowing for the exclusion of extraction negative pressure as a variable influencing extraction performance. To maintain uniform coal seam conditions across all seven borehole groups, an analysis centered on the variations in gas concentration and pure flow rate per unit of coal hole length was performed, as depicted in Figure 13 and Figure 14. The sequence of φ105mm > φ94mm > φ113mm was established for gas concentration, whereas for pure flow rate, the order was determined as φ94mm > φ113mm > φ105mm. Consequently, under equivalent negative pressure and gas distribution conditions, the φ94mm borehole group was identified as exhibiting the most efficacious extraction performance. Although the φ105mm borehole group was characterized by a reduced presence of non-gas constituents and enhanced sealing properties, the φ94mm borehole group's pure flow rate markedly surpassed those of the φ105mm and φ113mm groups, establishing the φ94mm group as the most effective in terms of extraction efficiency.

The examination of empirical data from seven borehole groups concerning gas extraction concentration and pure flow rate elucidates that an escalation in borehole diameter significantly enhances gas extraction efficiency. It has been observed that under uniform conditions of gas occurrence, the efficacy of gas extraction is augmented by employing high negative pressure techniques alongside larger borehole diameters. Notably, the borehole group with a diameter of φ94mm was distinguished as exhibiting superior performance, with its pure flow rate of gas extraction being 37-fold higher than that of the φ75mm group, and its extraction concentration exceeding by a factor of 2.4.

In scenarios presenting identical gas distribution conditions, the φ94mm borehole group was found to secure the highest average pure flow rate of gas extraction per unit diameter, achieving a rate of 4.67×10-4m³/min per 1 KPa of negative pressure per unit length of coal hole. Consequently, boreholes measuring φ94mm in diameter have been identified as the most conducive for gas extraction, demonstrating optimal outcomes and efficiency in extraction processes. This study, however, did not incorporate the potential impacts of borehole wall collapse during drilling on gas extraction efficiency, which represents a limitation of the research presented.