Application of High‐Pressure Water Jet Reaming and Permeability Increasing Technology in Outburst Coal Seam Mining

To study the shrinkage rule of borehole diameter and its effect on gas extraction, a visco-elastoplastic model for boreholes considering strain softening and the dilatancy characteristic was established to obtain the expressions of the coal stress, variation in diameter, and pressure relief range. The stress distribution and pressure relief effect of the boreholes in soft and hard coal seams were comparatively analyzed. The shrinkage rule of the borehole diameter was studied. The reasons for the rapid reduction in the extraction concentration of the borehole in soft coal seams were described. A technology of improving the gas extraction effect in soft coal seams was developed. The research results showed that the radius of the plastic softening zone is 0.405 m for a borehole in a soft coal seam and 0.224 m for that in a hard coal seam. This indicates that the borehole in a soft coal seam has a better pressure relief effect. The boreholes in both hard and soft coal seams will incur a shrinkage phenomenon; however, the soft coal seam has low strength and a weak ability to resist damage, and thus the surrounding coal will have a more intense creep deformation, leading to an instability failure during a short period of time and thus, a blocking of the extraction channel, thereby causing a rapid reduction in the gas extraction concentration. The borehole in a hard coal seam also shows a shrinkage phenomenon, but remains in a stable state without a blockage; thus, high-concentration gas can be extracted from this borehole for a long period of time. The geo-stress and coal strength are the two main factors controlling the amplitude of borehole shrinkage. From an increase in stress, the borehole in a hard coal seam shows a more intense creep deformation in a deep mine, which may lead to blockage. The key to improving the gas extraction effect in soft coal seams is to maintain a smooth extraction channel. The full screen pipe is installed through the drill pipe to retain an extraction channel, leading to an average gas extraction increase from 0.043 m3/min to 0.12 m3/min, an increase of 2.77 times. These research results are consistent with actual production, and can provide theoretical guidance for determining the gas extraction parameters.

To study the shrinkage rule of borehole diameter and its effect on gas extraction, a visco-elastoplastic model for boreholes considering strain softening and the dilatancy characteristic was established to obtain the expressions of the coal stress, variation in diameter, and pressure relief range. The stress distribution and pressure relief effect of the boreholes in soft and hard coal seams were comparatively analyzed. The shrinkage rule of the borehole diameter was studied. The reasons for the rapid reduction in the extraction concentration of the borehole in soft coal seams were described. A technology of improving the gas extraction effect in soft coal seams was developed. The research results showed that the radius of the plastic softening zone is 0.405 m for a borehole in a soft coal seam and 0.224 m for that in a hard coal seam. This indicates that the borehole in a soft coal seam has a better pressure relief effect. The boreholes in both hard and soft coal seams will incur a shrinkage phenomenon; however, the soft coal seam has low strength and a weak ability to resist damage, and thus the surrounding coal will have a more intense creep deformation, leading to an instability failure during a short period of time and thus, a blocking of the extraction channel, thereby causing a rapid reduction in the gas extraction concentration. The borehole in a hard coal seam also shows a shrinkage phenomenon, but remains in a stable state without a blockage; thus, high-concentration gas can be extracted from this borehole for a long period of time. The geo-stress and coal strength are the two main factors controlling the amplitude of borehole shrinkage. From an increase in stress, the borehole in a hard coal seam shows a more intense creep deformation in a deep mine, which may lead to blockage. The key to improving the gas extraction effect in soft coal seams is to maintain a smooth extraction channel. The full screen pipe is installed through the drill pipe to retain an extraction channel, leading to an average gas extraction increase from 0.043 m3/min to 0.12 m3/min, an increase of 2.77 times. These research results are consistent with actual production, and can provide theoretical guidance for determining the gas extraction parameters.

The solidity coefficient of the main coal seam in Xinjiang Coking Coal Group 1930 Coal Mine is below 0.3, which is a soft outburst coal seam with poor air permeability, a small effective range of gas drainage drilling, low porosity of soft coal seam, low gas drainage rate, and unsatisfactory drainage effect. Using directional drilling equipment to construct directional long drilling, combined with the hydraulic fracturing pump set independently developed by Xi’an Research Institute, directional drilling were constructed in the 1930 coal mine 1600 level 6# coal seam, and hydraulic fracturing was carried out. The results show that after the hydraulic fracturing of the 6# coal seam, the production has been stable for nearly 6 months. During the period, the average gas drainage rate of the drilling was 134.9m3/d (maximum 561.7m3/d), which was 2.10 times that before fracturing, The average drainage concentration is 1.90%, which is 1.45 times that before fracturing. At present, the total amount of gas drainage is 25,900 m3, which is 4 times the original amount. This technology has achieved enhanced permeability of broken soft coal seam, increased drainage volume, extended drainage time, and advanced gas outburst elimination at a long distance. It provides a technical guarantee for the enhanced permeability enhancement of the broken soft low-permeability outburst coal seam and the high-efficiency underground gas drainage.

The efficiency of gas extraction from the soft coal seam with ultralow permeability is low. Gas extraction with large-diameter borehole is proposed to deplete gas content for preventing gas outburst disaster in this study. The fractures around the large borehole will enhance the permeability in the damage area to promote gas extraction. We established a damage-stress-seepage coupling model for large-diameter borehole gas extraction in soft coal seam. This mathematical model contains governing equations of gases sorption and transport, coal deformation, and damage, reflecting the coupling responses between gas and coal seam. The model is solved by the finite element method to simulate the gas drainage large-diameter borehole through roadway. Distributions of elastic modulus, damage area, and maximum principal stress in soft coal seam with different borehole diameters including 94 mm, 133 mm, 200 mm, and 300 mm are analyzed. The gas pressure, gas content, and effective extraction area in soft coal seam are discussed. Results show that the shear failure zone appears around the large-diameter borehole, and its permeability rises sharply. This opens up the gas transport channel and is conducive to the rapid extraction. It is confirmed that gas extraction using large-diameter borehole (300 mm) can greatly improve the efficiency of the gas preextraction in soft coal seam by increasing gas extraction rate. These provide a foundation for guiding the operation of gas extraction with large borehole from the soft coal seam in the field.

Coalbed methane, which is a significant potential unconventional source of energy, exhibits considerable challenges in the implementation of the exploration and production process, especially the degasification in low permeability of coal seams with high gas concentrations. Due to the difficulties in gas extraction, long mining period and low extraction concentration in soft and outburst coal seams, the permeability improvement method using presplitting and blasting technology with multiple boreholes has been proposed and applied to improve the permeability of soft coal seam, which is achieved by optimizing the inseam distribution of generated fractures through multiple control boreholes. Research results showed that the permeability improved by presplitting and blasting with deep boreholes is 2.5 times higher than that of the original coal seam. Within 6 days of extraction, the average extraction concentration and pure volume of the extraction borehole were 40.13% and 0.113 m3/min, respectively, which are 2.9 times and 4.1 times higher than the original data, respectively. Likewise, the average extraction concentration and purity of the extraction hole were 21.75% and 0.063 m3/min after being extracted within 14 days, which is twice and three times higher than that of the original average extraction concentration and purity volume, respectively. Besides, proposed technology has a little influence on the methane concentration of the coal seam 70 m ahead of the heading face. The research results promote the prevention and control of coal‐gas outburst in low permeability and high gas concentration coal seams.

An integrated drilling, protection and sealing technology for improving the gas drainage effect in soft coal seams

Horizontal wells within the roof are an effective method to develop gas in broken and soft coal seams, and layer-penetrating fracturing is a key engineering method for the stimulating of horizontal wells within the roof of a coal seam. To understand the propagation law of fracture in the composite roof of coal seams, this study conducted research using numerical simulation and physical similarity simulation methods. Furthermore, engineering experiments were carried out at the Panxie coal mine in the Huainan Mining Area and the Luling coal mine in Huaibei Mining Area, to further validate this technology. The numerical simulation results indicated that fracture within the coal seam roof can propagate from the roof to the target coal seam, effectively fracturing the coal seam. Due to the coal seam’s plasticity being greater than that of the roof mudstone, the coal seam forms a broader fracture than the roof. With the increase in pseudo roof mudstone thickness and being under constant fracturing displacement, the energy consumed by the pseudo roof mudstone during fracturing causes a decrease in pore pressure when fracture extends to the coal seam, resulting in a reduction in fracture width. Therefore, the pseudo roof mudstone is an adverse factor for the expansion of hydraulic fracturing. Physical similarity simulation results demonstrated that when horizontal boreholes were arranged within the siltstone of the coal seam roof, were under reasonable vertical distance and high flow rate fracturing via fluid injection conditions, and if the coal seam had a thin pseudo roof mudstone, the fracture could propagate through the direct roof-pseudo roof interface and the pseudo roof-coal seam interface, extending to the lower coal seam. The fracture form was curved and had irregular vertical fractures, indicating that hydraulic fracturing can achieve production enhancement and the transformation of soft and hard coal seams. However, when the coal seam had a thick pseudo roof mudstone, the mudstone posed strong resistance to hydraulic fracturing, making it difficult for the fracture to propagate to the lower coal seam. Therefore, the pseudo roof mudstone plays a detrimental role in hydraulic fracturing and the production enhancement of coal seams. The engineering verification conducted at Panxie coal mine and Luling coal mine showed that by utilizing a construction drainage rate of 7.5 cubic meters per minute at Panxie coal mine, the maximum fracture length reached 218.3 m, with a maximum fracture height of 36.8 m. The maximum daily gas production of a single well reached 1450 cubic meters per day, with a total gas extraction volume of 43.62 × 104 cubic meters across 671 days. At Luling coal mine, utilizing a construction drainage rate of 10 cubic meters per minute, the maximum fracture length reached 169.1 m, with a maximum fracture height of 20.5 m. The maximum daily gas production of a single well reached 10,775 cubic meters per day, with a total gas extraction volume of 590 × 104 cubic meters for 1090 days. This indicated that the fracture within the roof of coal seams can penetrate the composite roof of coal seams and extend to the interior of the coal seams, achieving the purpose of transforming fractured and low-permeability coal seams and providing an effective mode of gas extraction.

Environmentally friendly techniques for high gas content thick coal seam stimulation─multi-discharge CO2 fracturing system

To solve the problems that gas outburst accidents are prone to occur in the mining process of broken and soft outburst coal seam, and the existing research on the directional roof hydraulic fracturing of broken soft outburst coal seam lacks the analysis of roof bedding and perforation design, this paper uses the method of combining experiment with particle flow code (PFC), investigates the influence of perforation on the directional hydraulic fracturing of roof, and defines the optimal perforation layout scheme under the joint action of bedding and perforation. The results reveal that hydraulic fracture propagation is significantly influenced by the bedding angle and in situ stress conditions of the rock mass. During roof fracturing, once fractures propagate to the coal-rock interface, they induce the formation of a complex fracture network within the coal seam, effectively enhancing its permeability. PFC has accuracy and engineering adaptability in hydraulic fracturing numerical simulation. Under the conditions of different bedding angles, the directional hydraulic fracturing perforation should be arranged in different ways, a vertical perforation configuration is recommended for bedding angles of 0°–15°, while for angles of 30°–60°, perforations should be oriented perpendicular to the bedding direction. At 45°–60°, perforations should avoid alignment with the bedding plane and instead adopt a large-angle orientation. Under the parameter combination of “three perforations-optimized angle-suitable bedding,” the effect of directional hydraulic fracturing of bedding rock broken soft coal seam roof is the best. This perforation scheme offers an effective strategy for gas control in outburst-prone soft coal seams.

Technology and application of pressure relief and permeability increase by jointly drilling and slotting coal

Gas disasters are a major factor influencing safe production in mines: Gas extraction can reduce the gas content in coal seams, providing a guarantee of safer production. The parameters for gas extraction are the primary factors influencing the effectiveness thereof. Aiming at the creep properties of soft coal, a fluid‐solid coupling mathematical model considering creep properties of coal was established based on dynamic evolution equation for permeability considering the effects of matrix shrinkage and effective stress. Additionally, by utilizing COMSOL Multiphysics software, the gas extractions from a single borehole and multiple boreholes were calculated. Moreover, the parameters for gas extraction were optimized and applied and tested in field conditions. The result showed the reduction in gas pressure around the boreholes was larger than that from a single borehole when conducting gas extraction from multiple boreholes. The borehole spacing when extracting gas in coal seams by drilling multiple boreholes should be more than twice that of the effective drainage radius. The optimal borehole spacing ranged from 3.2 to 4.2 m for gas extraction lasting 180 d. Numerical simulation was carried out to ascertain the distribution of stress on coal around a roadway. The result revealed that the damage radius of the roadway was 11.8 m, and a reasonable hole‐sealing depth was 12 m. On condition that the borehole spacing during gas extraction from multiple boreholes was 4 m, the reasonable pre‐extraction time was 180 days taking the gas pressure being reduced to <0.74 MPa as a critical point. Furthermore, the gas content, the amount of extracted gas, etc, in a working face after the parameters for gas extraction were optimized were measured. The result suggested that the effect of gas extraction after optimizing parameters conformed to industry standards.

Study on key grouting blocking parameters of gas drainage boreholes in soft coal seams

The support of roadways in soft coal seams is a challenge in deep coal mines. For gas-rich coal seams, many methods are developed to drain the gas before mining. The penetrating hydraulic reaming (PHR) is one of the most efficient methods to reduce the gas content and pressure in the coal seams. However, the PHR method will cause many large holes in the coal seam, which brings difficulty to in-seam roadways control. The strength parameters of the coal are essential for roadway stability analysis. However, in soft coal seams, the sampling of the coal is difficult, and the direct evaluation of the coal strength parameters is unfeasible. In the present research, the surrounding rock damage characteristics of the holes induced by PHR are first evaluated by ground-penetrating radar (GPR). Then, the strength parameters of the coal are determined via the back-analysis method. After that, the deformation and failure of the roadway are analyzed, and an optimized support scheme is proposed. According to the monitored displacement and measured damage zone of the roadway, the roadway is well controlled by the proposed support scheme.

Addressing the practical challenges of difficult drilling for blasting-induced permeability enhancement in deep, soft, and high-gas coal seams, where fractures remain underdeveloped and prone to re-compaction, this study proposes blasting operations within the floor strata. This approach aims to enhance the permeability of soft coal seams, thereby extending the duration of effective gas extraction. A bidirectional loading gas–solid coupling blasting simulation system was established in the laboratory, enabling multi-faceted analysis of experimental models through macroscopic crack patterns, internal damage mechanisms, and strain data of coal and rock masses. Comparative experiments were conducted, contrasting various control hole spacings with conventional blasting techniques. The findings reveal that as the blasting stress wave traverses the control hole walls, tensile stress waves are reflected, facilitating crack propagation. The guiding effect of the control holes and the spatial compensation they provide significantly increase the extension distance of explosion-induced cracks, resulting in directional failure of the test specimens and heightened damage in the far field of the blast. After the blasting process, the arrangement of control holes can result in an increase of up to 133% in damage to the coal seam and a reduction of up to 167% in damage to the floorboard compared to the model without control holes. Notably, when the control holes are proximal to the coal-rock interface, the near-end coal body experiences the most pronounced effects, with peak damage and tensile strain in the d = 20 mm model being 1.93 and 1.79 times higher, respectively, than those in models without control holes. Conversely, for control holes located further from the interface, the distal coal body experiences the greatest influence, exhibiting 1.53 and 1.55 times higher peak damage and tensile strain, respectively, in the d = 80 mm model compared to uncontrolled counterparts. Field observations at the C13 coal seam of a mine within the Huainan mining area corroborate these findings, where the volume of gas extraction and its concentration experienced a rapid increase following blasting and penetration enhancement. Optimum permeability enhancement occurs when the blasting hole is situated 4 m from the extraction point, resulting in a 131% increase in gas extraction purity from 0.15 × 10–3 m3/min to 1.97 × 10–3 m3/min. Furthermore, gas concentration soars by 373%, from 5.86% to 21.86%. These research outcomes offer valuable insights and hold considerable reference significance for blasting-induced permeability enhancement in deep, soft, and high gas coal seams.

Efficient gas extraction technology is an important topic for low permeability and high gas outburst coal seam. Based on the engineering background of Hudi Coal Mine with the soft and hard coal seams and the existing hydraulic permeability enhancement technology, a new construction process to improve coal seam permeability was proposed to effectively reduce coal seam gas content and the risk of coal and gas outburst. In this measure, the roadway in floor was replaced with a directional main borehole, directional branch boreholes were used to replace crossing holes, and soft coal was mined along soft sub layers via the directional drilling machine and directional hydraulic jet. Main boreholes are drilled parallel to the seam in the coal seam floor, and branch boreholes are drilled through the floor and coal seam. The numerical simulation was used to study the permeability improvement effect of different mining diameters by the proposed measure. The result showed that, as the mining diameter increased from 2 m to 4 m, the average influence diameter of coal seam porosity increased from 15.44 m to 19.65 m, and the average influence diameter of the permeability increased from 15.75 m to 20.07 m, which is three times the influence range of the ordinary borehole. The application of the proposed measure and its supporting equipment was carried out under the special coal seam and gas conditions of Hudi Coal Mine. Results show that the soft coal was mined efficiently along the soft sub layer using the main borehole, branch boreholes, and directional hydraulic jet. Compared the traditional hydraulic flushing in the borehole with the ordinary drilling machine, the average speed of mining soft sub layers increased from 0.5 t/h to 3.6 t/h, the equivalent mining diameter of soft sub layers increased from 1.2 m to 7.6 m, and the average flow of gas extraction increased from 0.41 m3/d to 6.25 m3/d. The conclusions obtained in this study can provide a reference for coal mine gas extraction with similar coal seam conditions.