Gas Drainage Boreholes Research Articles

Performance of a new coal-dust-based composite sealing material for gas-drainage borehole

Performance of a new coal-dust-based composite sealing material for gas-drainage borehole

Dynamic Secondary Borehole-Sealing Method for Gas Drainage Boreholes along the Coal Seam

Under the influence of advanced abutment pressure, the tightness of a borehole is destroyed, so secondary borehole sealing is needed; however, the opportune time for a secondary borehole sealing is unclear. In addition, current borehole-sealing devices do not have a secondary borehole-sealing functionality. In this paper, an opportunity election model of secondary borehole sealing was established, and a dynamic secondary borehole-sealing device was developed. In working face 3908 of the Kaiyuan Coal Mine, an in situ dynamic secondary borehole-sealing test was conducted. The test results show that the tightness of the boreholes sealed with high-water-expansion slurry is obviously better than that of the boreholes sealed with cement mortar and polyurethane. When a borehole is destroyed by the peak of the advanced abutment pressure, the secondary borehole-sealing measures can significantly improve borehole suction pressure, gas concentration, and gas drainage amount in the advanced abutment pressure loading stage.

Expansion properties and creep tests for a new type of solidified expansive sealing material for gas drainage boreholes in underground mines

This study was conducted to find the optimal expansion for a new type of solidified expansive sealing (SES) material. The aim was to better meet the requirements for sealing gas extraction boreholes in underground coal mines by examining the expansion and creep properties of the material. The effect of different proportions of expansion agent on expansion time, expansion mechanisms, and creep properties of the SES material was investigated using an electronic universal testing machine and other systems. Based on the results of step loading creep tests, the Kelvin–Voigt viscoelastic model was selected to further analyze the mechanical properties of the material. The analysis showed that when the proportion of expansion agent in the SES is increased to 2.0%, the final expansion percentage increases to 11.5%, and the rapid expansion time and expansion preparation time quickly decreases to 18 and 12 min, respectively. In addition, the step loading displacement and constant force load displacement increase sharply. The creep parameters E0, E1, and η1 of the Kelvin–Voigt model reflect the gradually weakening of the compressive capacity and anti-deformation ability of the SES material. Based on the analysis of the experimental results and field application on sealing boreholes using SES material, it was possible to establish the optimum amount of expansion agent that should be added to the SES material. The optimized proportions can meet the requirements of good flowability, high compressive capacity, and anti-deformation ability for borehole sealing in a mine.

Analysis on the multi-phase flow characterization in cross-measure borehole during coal hydraulic slotting

Hydraulic slotting in a gas drainage borehole is an effective method of enhancing gas drainage performance. However, it frequently occurs that a large amount of slotting products (mainly the coal slurry and gas) intensely spurt out of the borehole during the slotting, which adversely affects the slotting efficiency. Despite extensive previous investigations on the mechanism and prevention-device design of the spurt during ordinary borehole drilling, a very few studies has focused on the spurt in the slotting process. The slotting spurt is mainly caused by two reasons: the coal and gas outburst in the borehole and the borehole deslagging blockage. This paper focuses on the second reason, and investigates the hydraulic deslagging flow patterns in the annular space between the drill pipe and borehole wall. Results show that there are six deslagging flow patterns when the drill pipe is still: pure slurry flow, pure gas flow, bubble flow, intermittent flow, layering flow and annular flow. When the drill pipe rotates, each of those six flow patterns changes due to the Taylor vortex effect. Outcomes of this study could help to better understand the slotting-spurt mechanism and provide guidance on the anti-spurt strategies through eliminating the borehole deslagging blockage.

Dynamic leakage mechanism of gas drainage borehole and engineering application

Borehole leakage not only affects the gas drainage effect but also presents considerable risk to human security. For the research on the leakage mechanism of gas drainage borehole, the rheological and visco-elastic-plastic characteristics were considered to establish the mechanical model of coal mass around borehole, which is used to analyze the leakage mechanism and deduce the dynamic leakage model. On the basis of the real coal seam conditions, the variation rules of the stress, leakage ring, and air leakage amount were analyzed through numerical simulation, and the influence factors of air leakage amount were also investigated to provide the theoretical basis for the sealing technology. Results show that the air leakage amount of borehole is inversely proportional to the increase in supporting stress and sealing length, and directly correlated with the increase in borehole radius and softening modulus. Using theoretical analysis, we design a novel active supporting sealing technology that can use grouting material to seal the fractures to reduce the leakage channels and also provide supporting stress to prevent borehole deformation. The engineering test results indicate that the average gas concentration with the novel active supporting sealing technology is increased by 162.12% than that of traditional polyurethane sealing method. Therefore, this technology not only effectively resolves borehole leakage but also significantly improves the gas drainage effect.

The Location Optimum and Permeability-Enhancing Effect of a Low-Level Shield Rock Roadway

High (low)-level roadways are effective gas control measures for coal seams with high outburst potential and low permeability. The spatial location of the low-level roadway directly determines the gas drainage efficiency and can affect the success of the whole engineering project. Currently, determining the location of a low-level roadway is based mostly on empirical engineering analogies and lacks relevant theoretical support. In this work, based on the Ji-15-24080 engineering project at the No. 10 Coal Mine of Pingdingshan Tian’an Coal Industry Co., Ltd., a numerical model for the location of a low-level roadway was established using ANSYS numerical software to analyze the stress distribution in the rock surrounding the conveyor roadway for the mining face and assess the permeability-enhancing effect of the low-level roadway when it is placed at different locations. A location directly beneath the conveyor roadway was found to provide a good location for a low-level shield roadway. In addition, a quantitative distance between the conveyor roadway and the low-level roadway can also be determined. These optimization results were then directly applied in the field. Relevant industrial field tests were conducted to determine fracture development, gas pressure and stability of the surrounding rock. The field test results confirm that the proposed location for a low-level shield roadway is reasonable. We recommend that when the low-level shield roadway is placed directly beneath a conveyor roadway, it should be within 50 m of the working face, and an effective support structure should be employed to support the rock surrounding the conveyor roadway to ensure the stability of the rock. We further recommend that, in the area in front of the working face, particularly within 25 m of the working face, densely distributed gas drainage boreholes should be drilled to drain gas from the coal seam to reduce the gas pressure and lower the outburst risk. The results of this work will provide guidance for similar engineering projects to determine the location of a low-level roadway and to ensure gas control.

Effects of geomechanical properties of interburden on the damage-based permeability variation in the underlying coal seam

Interaction between multi-layer coal seams is of significant guidance for designing gas drainage boreholes and maximizing the drainage performance. Although extensive investigations on the multi-seam interaction have been conducted from perspectives of rock deformation and variations in mechanics and permeability, sensitivity analysis on the seam interaction is done at a less extent. In this study, a 2D simulation model is established based on data of a coal mine. Weibull statistical distribution is adopted to assign the geomechanical property in the model to reveal rock heterogeneity. Meanwhile, a mathematical model that incorporates coal permeability with the coal damage, mechanics change and gas desorption is developed. This permeability model is implemented into the numerical simulation to investigate the effects of geomechanical properties of interburden on the permeability change in the de-stressed underlying coal seam, as well as the gas drainage performance. Results show that: (1) when assigning the geomechanical property using Weibull statistical distribution, the property homogeneity is positively correlated with Weibull shape parameter, (2) overall, the permeability increases significantly in the de-stressed underlying coal seam, and with the increase in the distance to interburden, the increase ratio decreases rapidly before a relatively slight decline, and (3) the average permeability ratio decreases initially but then rises with the increasing Young's modulus. In contrast, the average permeability ratio and the Poisson's ratio show a positive linear relationship. The gas drainage performance in the underlying coal seam is corresponding to the permeability variation of each geomechanical parameter group and could be verified with each other. In summary, when the interburden possesses smaller Young's modulus and greater Poisson's ratio, the interaction between adjacent seams becomes more intense and the permeability in the underlying coal seam tends to increase more. Therefore, when designing the mining sequence for the coal-seam group with high interaction potential, mining the neighbouring coal seams of the targeted seam first could be an effective method to increase its permeability and gas drainage efficiency.

A Gas–Solid–Liquid Coupling Model of Coal Seams and the Optimization of Gas Drainage Boreholes

For a gas–solid–liquid coupling model of coal seams, previous permeability models basically supposed uniaxial strains as the boundary condition for coal reservoirs without considering the deformation caused by changes in humidity. The permeability model varies under different boundary conditions. According to the true triaxial stress state of coal reservoirs, a permeability model considering the effective stress, sorption and desorption, and wet strain was established. Based on the permeability model, the continuity equation of gas and water and the stress field equation were coupled. Then, the model was incorporated in the COMSOL suite to simulate gas drainage from boreholes in floor roadways passing through seams in a coal mine. By comparing with the measured gas flow on site, the model was verified as being reliable. Moreover, the spacing and layout shape of boreholes in floor roadways were simulated. To achieve the aim of eliminating regional outburst within 180 days and decreasing the number of boreholes so as to reduce the cost, the spacing and shape of boreholes were optimized. When the superimposed effect of the boreholes was not considered, the optimal spacing of boreholes was 3 r; if the superimposed effect was taken into account, the spacing could be set to within 3 r ≤ L ≤ R, where r and R represent the effective gas drainage radius and the influence radius of gas drainage, respectively. The borehole spacing could be appropriately increased when the boreholes were arranged in rhomboidal form. To achieve the same range of outburst elimination, the rhomboidal layout can decrease the number of boreholes to reduce cost, thus realizing the objective of this optimization process.

Similarity simulation of mining-crack-evolution characteristics of overburden strata in deep coal mining with large dip

Figuring out the mining-crack-evolution characteristics of overburden strata is very important to roof control, gas drainage, disaster prevention and high efficient mining. In order to obtain the mining-crack-evolution characteristics of overburden strata in deep coal mining with large dip, in this study, No.1221 coal mining face of Zhao coal mine is chosen as the research object to establish an in-house experiment system of similarity simulation. The experiments could measure the stress and displacement of overburden strata, which can provide useful information to investigate the mining-crack-evolution characteristics, displacement variations and movement characteristics of overburden strata associated the coal mining face. Experimental results show that with the advancing of coal mining face, the scope of gob increases gradually, and formation of the false roof of overburden strata basically reflects the evolution process of collapsing. The overburden strata weight is constantly transferred to the front and rear of coal mining face, which forms the supporting pressures on both sides of coal pillars, and causes the gangue's collapsing in gob. The large dip of coal seam results in pressure disequilibrium in stress-increasing zone. Thus, the stress on the underside of gob becomes larger and larger, while the stress on upper side of gob decreases. The strata separation appears in the overburden strata of roof. Pressure-relief zone is mainly concentrated in the side of outlet roadway. Along the inclination direction of coal mining face, the crack development and strata separation are obvious, which create the passages for gas flow and migration. The similarity simulation results provide fundamental information for better understanding those mining-crack-evolution characteristics of overburden strata, which has become more and more important to control the roadway stability in coal mining and optimize the layout of gas drainage boreholes for improving mining safety.

Influence of polycrystalline diamond compact bit geometric parameters on drilling performance during gas drainage borehole in soft coal seam

Polycrystalline diamond compact bit is the main rock-breaking tool of gas drainage borehole. In order to investigate the influence of geometric parameters of polycrystalline diamond bit on its drilling performance, the integrated force model of polycrystalline diamond bit pressed into rock was established. The stress analysis results show that the tooth rake angle, pressing depth, and horizontal tangential displacement are the main factors to influence the normal load and the axial load of polycrystalline diamond bit. There is an optimal tooth rake angle value of polycrystalline diamond bit cutter when it is used in the soft coal seam. The drilling experiment system was built to verify the drilling performance of drill bits with different geometric structure and different tooth rake angle through drilling the soft coal wall. The experiment results show that the drilling torque and feeding resistance of polycrystalline diamond bit are higher than that of the wing bit at the beginning of drilling the soft coal wall. With the increase in the drilling depth, the feeding resistance of wing bit will increase to exceed that of the polycrystalline diamond bit. So the polycrystalline diamond bits should be selected to drill the gas drainage borehole in soft coal seam. The polycrystalline diamond bit with bigger tooth rake angle cannot cut and break the coal rock effectively, which will reduce the drilling efficiency and the feeding speed. When the tooth rake angle is smaller, polycrystalline diamond bit cannot cut in coal wall effectively, which will increase the feeding resistance. So the polycrystalline diamond bit with tooth rake angle of 15°–20° is suitable for the gas drainage borehole in soft coal seam. With the increase in the rotary speed of polycrystalline diamond bit, the cutting thickness of coal rock decreases and the horizontal tangential displacement increases. When the rotary speed of polycrystalline diamond bit is 290 r/min, the mean drilling torque and mean feeding resistance of polycrystalline diamond bit reach the minimum. Therefore, in order to reach the optimum drilling performance, the working parameters of low drilling pressure and high rotary speed are usually selected during the actual gas drainage borehole in soft coal seam.

Dependence of gas emission from the sources undermined by stope mine workings on the rocks displacement parameters

The parameters are established in the work of the stope mine workings and rocks displacement, which correspond to the maxima of gas emission into the gas drainage boreholes. The angular and linear parameters of the crustal movement and rocks displacement have been calculated with the use of empirical dependences. These parameters correspond to the maxima of gas emission into the gas drainage boreholes. The obtained positive results for determining the possible areas of maximum gas emission allow us to recommend the empirical dependences for calculating the angles of maximum subsidence and complete displacements at the stage of stope works development, as well as the dimensions of the stope mine workings, at which the processes of rocks displacement reach the earth surface. The established relations between the gas drainage boreholes location and the rocks displacement zones with discontinuity make it possible to design the optimal schemes of draining-out of gases from the gas-and-coal deposits. For the first time, three independent methods have been used to determine the changes in dimensions of rocks displacement zones with discontinuity after the primary main roof squeezing until the formation of stable upper boundaries of these zones.

Study on Permeability Improvement Technology by Injecting Air into the Gas Drainage Borehole in Low‐Permeability Coal Seam

Gas extraction cycle is too long in low‐permeability coal seam. In order to solve the problem, the basic principle about gas drainage drilling for gas injection technology is studied to increase permeability. And the mathematical model is established. Gas is injected into the low‐permeability coal seam by numerical simulation. The results indicate that the best condition is a negative pressure drainage at 26 kPa and a gas injection pressure at 0.6 MPa in the vertical direction and in the horizontal direction of the injection hole. In Shanxi Daping Coal Mine 3113 working face, the field test is implemented. As a result, the test is successful. During the 14 d gas injection constantly, gas content of coal seam is reduced from 12.33 m3/t to 7.12 m3/t, greatly reducing the risk of coal and gas outburst elimination time required.

Swelling Pressure of Double‐Expansive Material and Its Active Support Effect for Coal Seam Gas Drainage Borehole

To improve coal seam gas drainage performance, we developed a double‐expansive (DE) material to seal the borehole. The swelling process of this material includes an initial swelling stage and a secondary swelling stage. We studied the swelling pressure properties of the DE material under four constraint conditions using a self‐made swelling test device. Further, the active support effect of the DE material on the borehole was analyzed by simulating borehole stability with COMSOL Multiphysics software. The results exhibit the following: (1) The swelling pressure of the DE material exhibits time‐dependent behavior, and the mathematical relationship between the swelling pressure and time can be obtained by nonlinear fitting. (2) The radial swelling potential is principally formed during the secondary swelling stage, providing the main active support on the radial constraint. (3) The active support imposed on the hole wall can prevent the extension of plastic and damage regions around the borehole, for improved stability of the gas drainage borehole. Finally, field tests demonstrate improved gas drainage performance of the borehole sealed by the DE material compared to a conventional sealing material.

Preparation and Sealing Performance of a New Coal Dust Polymer Composite Sealing Material

In this study, a new coal‐dust polymer composite material (CP) was fabricated to improve the borehole gas drainage effect and address the limitations of traditional sealing materials. This material used coal dust generated during underground drilling construction as filler, amino resin as binder, and also some other additives. Based on the orthogonal test scheme, 16 groups of experiments were conducted to investigate the effects of coal‐dust content, water‐material ratio, and cross‐linking agent content on viscosity, 3‐day (3‐d) compressive strength, and gel time of the CP. The influences of the various factors were studied using a range of analytical methods to obtain the optimal proportion of the CP. Moreover, the mechanical property and microstructure of the CP were also analyzed to evaluate the sealing performance of this material. Results showed that this material exhibited obvious plastic characteristic and could provide an effective support for the borehole; it can adapt to the deformation of the borehole to some extent and achieve a long‐term sealing effect. The scanning electron microscopy (SEM) results show a good correlation between the resin molecules and coal dust inside the material, which were conducive to improve the sealing performance of the CP material.

Gas emission quantity prediction and drainage technology of steeply inclined and extremely thick coal seams

Gas emissions of workfaces in steeply inclined and extremely thick coal seams differ from those under normal geological conditions, which usually feature a high gas concentration and a large emission quantity. This study took the Wudong coal mine in Xinjiang province of China as a typical case. The gas occurrence of the coal seam and the pressure-relief range of the surrounding rock (coal) were studied by experiments and numerical simulations. Then, a new method to calculate the gas emission quantity for this special geological condition was provided. Based on the calculated quantity, a further gas drainage plan, as well as the evaluation of it with field drainage data, was finally given. The results are important for engineers to reasonably plan the gas drainage boreholes of steeply inclined and extremely thick coal seams.

Study on determination of borehole drainage radius by gas content method

In order to determine the radius of borehole gas drainage, using five kinds of gas content under different conditions corresponding to the extraction rate were compared according to different drainage objectives, determine the need for gas drainage amount, according to a certain period of time the average single hole gas drainage quantity, calculate the number of drilling in the region need to be arranged, and then get the borehole gas drainage radius. Based on the actual investigation and comparison analysis, the effective radius of borehole gas drainage in certain extraction time is obtained. The results show that the method of gas content determination of drilling drainage radius is feasible.

IndianOil & LanzaTech to build “first” refinery off gas-to-bioethanol plant

Coal seam degasification by underground drilling and its effect are directly related to mining safety and coalbed methane (CBM) recovery efficiency. The coal seam permeability is a critical parameter affecting the gas drainage effect and the evolution of gas flow field around the borehole. However, the distribution and evolution of coal permeability during gas drainage remains unclear and has resulted in failure of gas drainage radius determination and borehole arrangement in field applications. In this work, we propose an improved coal permeability model, which incorporates the coal deformation, gas diffusion in matrix, and gas flow in fracture. For the dual-porosity attribute of CBM reservoir, two gas pressures, namely, fracture and matrix gas pressures, and the swelling stress caused by gas adsorption have been introduced to improve the effective stress principle. Furthermore, because the classical unipore diffusion model fails to calculate the diffusion coefficient of coal masses, we propose a multistage pore model, which considers a widely distributed pore structure. Based on the multistage pore model, a dynamic diffusion coefficient is introduced in the permeability model. The improved permeability model is then compared with an analytical solution and field data. With the verified model, we analyzed the sensitivity of coal permeability to the key parameters in the model. The results show that the permeability increases with the increase of Langmuir adsorption constants a and b, initial permeability k0, and diffusion coefficient D0, and the decrease of the attenuation coefficient λ. From the analyses, we find that when a relatively large value is used for a, b, k0, and D0, and a relatively small value is given to λ, a phenomenon of permeability rebound, which is thought to be related to the matrix shrinkage, is observed during gas drainage. This phenomenon contributes to the improvement of gas drainage effect. Therefore, certain measures, such as protective seam mining and hydraulic flushing, can be adopted to accelerate the appearance of the permeability rebound. In addition, the determination of the effective drainage radius (EDR) is discussed from the perspective of dual-porosity medium. Both the fracture and matrix gas pressures are used to determine the EDR, and the results show that the EDR determined by matrix gas pressure matches well with the field test result. Therefore, it is concluded that the matrix gas pressure is the reasonable index for the determination of the EDR.

Effects of coal damage on permeability and gas drainage performance

Coal permeability is a measure of the ability for fluids to flow through coal structures. It is one of the most important parameters affecting the gas drainage performance in underground coal mines. Despite the extensive research conducted on coal permeability, few studies have considered the effect of coal damage on permeability. This has resulted in unreliable permeability evaluation and prediction. The aim of this study is to investigate the effect of coal damage on permeability and gas drainage performance. The Cui-Bustin permeability model was improved by taking into account the impact of coal damage on permeability. The key damage coefficient of the improved permeability model is determined based on the published permeability data. A finite-element numerical simulation was then developed based on the improved permeability model to investigate the damage areas and the permeability distribution around roadway. Results showed that the tensile failure occurs mainly on the upper and lower sides of the roadway while the shear failure symmetrically occurs on the left and right sides. With the increase in the friction angle value, the damage area becomes small. A good agreement was obtained between the results of the improved permeability model (γ=3) and the published permeability data. This indicated a more accurate permeability prediction by the improved permeability model. It is expected that the findings of this study could provide guidance for in-seam gas drainage borehole design and sealing, in order to enhance the gas drainage performance and reduce gas emissions into underground roadways.

Effects of coal properties on ventilation air leakage into methane gas drainage boreholes: Application of the orthogonal design

Ventilation air leakage into underground to inseam drainage boreholes is a common issue observed during gas drainage in underground coal mines. This air leakage adversely affects the gas drainage efficiency. Despite the extensive research conducted on air-leakage prevention strategies, the effects of coal properties on the air leakage has not yet been properly investigated. This paper presents a fully-coupled numerical study on the effects of coal properties such as Young's modulus, Poisson's ratio and porosity on the air leakage into drainage boreholes. This was achieved through an integrated approach of the orthogonal design method and the F-test theory. The results show that there is a direct correlation between the Young's modulus and the amount of air leakage. Furthermore, the F-test theory was used to examine the significance of the above-mentioned properties' effects. It is found that the impact of Young's modulus on the air leakage is the most significant among those three properties. The Poisson's ratio is the next significant factor with an inversely proportional relationship. Porosity is found to have the least effect on the air leakage. This study provides a systematic approach to examine the air leakage performance for various site conditions and coal properties. Research outcomes offer the mining researchers and engineers a guidance to develop appropriate leakage-prevention strategies and enhance gas drainage performance.

Dynamic diffusion-based multifield coupling model for gas drainage

Coal seam degasification by underground drilling and its effect are directly related to mining safety and coalbed methane (CBM) recovery efficiency. The coal seam permeability is a critical parameter affecting the gas drainage effect and the evolution of gas flow field around the borehole. However, the distribution and evolution of coal permeability during gas drainage remains unclear and has resulted in failure of gas drainage radius determination and borehole arrangement in field applications. In this work, we propose an improved coal permeability model, which incorporates the coal deformation, gas diffusion in matrix, and gas flow in fracture. For the dual-porosity attribute of CBM reservoir, two gas pressures, namely, fracture and matrix gas pressures, and the swelling stress caused by gas adsorption have been introduced to improve the effective stress principle. Furthermore, because the classical unipore diffusion model fails to calculate the diffusion coefficient of coal masses, we propose a multistage pore model, which considers a widely distributed pore structure. Based on the multistage pore model, a dynamic diffusion coefficient is introduced in the permeability model. The improved permeability model is then compared with an analytical solution and field data. With the verified model, we analyzed the sensitivity of coal permeability to the key parameters in the model. The results show that the permeability increases with the increase of Langmuir adsorption constants a and b, initial permeability k0, and diffusion coefficient D0, and the decrease of the attenuation coefficient λ. From the analyses, we find that when a relatively large value is used for a, b, k0, and D0, and a relatively small value is given to λ, a phenomenon of permeability rebound, which is thought to be related to the matrix shrinkage, is observed during gas drainage. This phenomenon contributes to the improvement of gas drainage effect. Therefore, certain measures, such as protective seam mining and hydraulic flushing, can be adopted to accelerate the appearance of the permeability rebound. In addition, the determination of the effective drainage radius (EDR) is discussed from the perspective of dual-porosity medium. Both the fracture and matrix gas pressures are used to determine the EDR, and the results show that the EDR determined by matrix gas pressure matches well with the field test result. Therefore, it is concluded that the matrix gas pressure is the reasonable index for the determination of the EDR.