Coal Seam Permeability Research Articles

Modelling and simulation of earthquake‐induced changes in methane emission from the working face in an underground coal mine

Evidence demonstrates that unusual methane emissions in coal mines may happen after earthquakes or mine tremors. Here, a seismicity-induced permeability change model is constructed and validated using field data from the Laohutai coalmine. The validated model is employed to explore the extent to which different peak ground velocities (PGVs), seismic acceleration coefficients, remnant coal seam methane pressures, and initial coal seam permeabilities affect earthquake-induced methane emissions. Detailed simulation results suggest that the permeability of a coal seam subjected to earthquakes is both controlled by the dual competing mechanism of earthquake-imposed strain and excess pore pressure. A higher PGV leads to a rapid increase in the coal seam permeability and the maximum of methane concentration in the airflow, while higher seismic acceleration coefficients could somewhat reduce them. Meanwhile, increasing the remnant methane pressure and initial permeability in a coal seam subjected to an earthquake increases the maximum of methane concentration in the airflow at working faces. These results have provided reliable guidance for how to adopt emergent measures to prevent earthquake-triggered methane accidents in underground coal mines.

Crack Propagation Characteristics of Coal Samples Utilizing High-Voltage Electrical Pulses.

The technique of high-voltage electrical pulses (HVEP) is a new method to enhance the permeability of coal seams and improve the efficiency of coalbed methane (CBM) exploitation. This paper is aimed at investigating the crack propagation characteristics of samples of different strengths, proposing the improved procedure of HVEP in field application, and proving that the electrohydraulic effect has a wide use in field application of CBM extraction. In this paper, an experimental system utilizing HVEP in water condition is established, coal samples with different strengths are crushed, and the extended processes of cracks are analyzed. According to the research results, the electrohydraulic effect has a good breakage on the coal; the number of main cracks is 2–3 and the length of the main cracks is about 30 cm in the vertical direction of the hard samples; and the formation of cracks is relevant to the discharge voltage, discharge times, and mechanical parameters of the samples. The results of scanning electron microscopy (SEM) demonstrate that the cracks and pore connectivity of the coal samples are improved obviously, and the permeability results show that the permeability of crushed coal samples is 20% greater than that of the raw coal sample. Meanwhile, the generation process of cracks can be divided into four periods: namely, fatigue damage accumulation, slow development, rapid development, and failure; the rapid development stage is the optimal phase in field application. Moreover, the shock wave produced by HVEP via electrohydraulic effect can crush the samples mainly; furthermore, the energy produced by bubble rupture also has a great influence on the formation of cracks. This study can provide a foundation for the HVEP to improve CBM exploitation.

Coupling Model of Stress-Damage-Seepage and Its Application to Static Blasting Technology in Coal Mine.

Most coal mine field application processes are carried out using empirical formulas because of the insufficient understanding of the fracture development law of the static blasting technology. This lack of understanding results in poor coal seam gas extraction. In this study, a stress–damage coupling model was established to investigate the construction parameters of the static blasting technology using COMSOL simulation software. Then, a stress–damage–seepage coupling model was designed to study the evolution of the fracture field (seepage field) during the static blasting process using realistic failure process analysis simulation software. Finally, the influencing factors and fracturing effects were analyzed comprehensively. The research results show the following. (1) Comparing the simulation results with previous field tests reveals that the seepage law of the numerical simulation of the static blasting technology is consistent with the field test results, verifying the rationality of the stress–damage–seepage coupling model. (2) The development of coal seam fractures is affected by the expansion pressure, elastic modulus, and guide hole arrangement; the guide hole arrangement can play a role in guiding the development direction of fractures and enhancing the effect of fracturing. (3) The coal body mainly experienced the following five stages of fracturing: coal body compaction, microdamage formation, microfracture formation, large fracture formation, and fracture propagation. In addition, because of the rapid release of soundless cracking agents during the large fracture formation stage, the gas flow decreased in a short time. (4) The static blasting technology causes the coal seam permeability coefficient to increase. Compared with conventional extraction, the effective influence radius in the horizontal direction increases by 5.1 times, and the effective influence radius in the vertical direction increases by approximately 3 times. The static blasting technology can increase the number of coal seam fractures and significantly reduce the coal seam gas pressure, thereby enhancing coal seam permeability and realizing safe coal mining.

A calculation method of gas emission zone in a coal mine considering main controlling factors

The gas emission zone is an important parameter for the space–time effect of coal excavation and gas emission. In this paper, according to the effect of roadway excavation, a numerical model of gas emission zone based on the evolution of stress and permeability was established to obtain the width of gas emission zone with different pressure and permeability coefficient. Then the numerical simulation results were verified by measuring the gas content at different depths. Through numerical simulation and field measured data, the theoretical calculation formula is established on the basis of comprehensive consideration of the influencing factors of gas emission zone. The results showed that the gas emission zone increases with the increase of coal seam gas pressure and permeability coefficient when the roadway section and exposure time are the same. The measured gas emission zone, when taking gas content as the index with the same logistic function growth curve, matches the measured results with a relative error of 1.3 to 6%. The validity of the model is also verified by field experiments. The results can provide guidance for mine gas emission and gas drainage design.

An experimental and simulation study of the flow pattern characteristics of water jet impingements in boreholes

Hydraulic slotting has become one of the most common technologies adopted to increase permeability in low permeability in coal field seams. There are many factors affecting the rock breaking effects of water jets, among which the impact force cannot be ignored. To study the influencing effects of contact surface shapes on jet flow patterns and impact force, this study carried out experiments involving water jet impingement planes and boreholes under different pressure conditions. The investigations included numerical simulations under solid boundary based on gas–liquid coupling models and indoor experiments under high-speed camera observations. The results indicated that when the water jets impinged on different contact surfaces, obvious reflection flow occurred, and the axial velocity had changed through three stages during the development process. Moreover, the shapes of the contact surfaces, along with the outlet pressure, were found to have impacts on the angles and velocities of the reflected flow. The relevant empirical formulas were summarized according to this study's simulation results. In addition, the flow patterns and shapes of the contact surfaces were observed to have influencing effects on the impact force. An impact force model was established in this study based on the empirical formula, and the model was verified using both the simulation and experimental results. It was confirmed that the proposed model could provide important references for the optimization of the technical parameters water jet systems, which could provide theoretical support for the further intelligent and efficient transformation of coal mine drilling water jet technology.

Response Characteristics of Coal Measure Strata Subjected to Hydraulic Fracturing: Insights from a Field Test

Hydraulic fracturing (HF) is an effective method to improve the permeability of coal seams and enhance coalbed methane recovery in underground coal reservoirs. However, the induced stress and strai...

Effect of the water injection pressure on coal permeability based on the pore‐fracture fractal characteristics: An experimental study

AbstractCoal seam water injection is a commonly used method by which to improve coal seam permeability. To reveal how this method can enhance coal permeability from the perspective of the pore‐fracture structure, the permeability and porosity of raw coal samples under different water injection pressures were tested. Low‐temperature nitrogen adsorption (N2GA) and high‐pressure mercury intrusion porosimetry (MIP) were employed to characterize the changes in the pore‐fracture structures of water‐injected coal samples. Moreover, the relationship between the fractal dimension and coal permeability was studied using fractal theory. The results revealed the following. The internal pore‐fracture structures of the coal samples were rich in variety, and the pore span was large. With the increase of the water injection pressure, the volume ratios of macropores and mesopores gradually increased. Moreover, in the test interval, the fractal dimension of the water‐injected samples did not fluctuate substantially. With the increase of the water injection pressure, the integral dimension of seepage pores became positively correlated with permeability, while the fractal dimension of diffusion pores became negatively correlated with permeability. The results indicate that the increase in the water injection pressure not only increases the proportion of seepage pores, but also densifies the diffusion pores. Thus, the uniformity of diffusion pores can well reflect the permeability level of coal samples. © 2021 Society of Chemical Industry and John Wiley & Sons, Ltd.

Study on the Influence of Liquid Nitrogen Cold Soaking on the Temperature Variations and Seepage Characteristics of Coal Samples with Different Moisture Contents

Due to its advantages such as environmental friendliness and remarkable permeability enhancement effect, the technology of liquid nitrogen cold soaking (LNCS) cracking coal has become a hot spot in the research on coal seam permeability enhancement in recent years. The frost heave force generated by water-ice phase transformation and the temperature stress are the main mechanisms of LNCS cracking water-containing coal. This paper focuses on the effect of LNCS on the temperature variations and seepage characteristics of coal. To further this purpose, the temperature measurement test and the permeability test were conducted on coal samples with different moisture contents under LNCS, respectively. In addition, by comparing the computer tomography test results of coal samples before and after LNCS, the internal pore structure changes of coal samples were further analyzed from a three-dimensional perspective. The test results show that the coal sample with a higher moisture content consumes a shorter time to reach internal temperature equilibrium and experiences faster temperature changes. LNCS can enhance coal permeability, and the growth rate of permeability increases exponentially with the increase of moisture content. After the LNCS treatment, the dried coal sample is mainly sprouting new pores on the basis of primary pores; in contrast, for water-containing coal samples, new pores are sprouted while primary pores will penetrate each other spatially to form a fracture network. In the process of LNCS, moisture has a significant effect on the seepage characteristics of coal, so appropriately increasing the moisture content of the coal seam conduces to achieving a better permeability enhancement effect.

Study on the technology of enhancing permeability by deep hole presplitting blasting in Sanyuan coal mine

To reduce gas disasters in low permeability and high gas coal seams and improve gas predrainage efficiency, conventional deep hole presplitting blasting permeability increasing technology was refined and perfected. The damage degree of coal and rock blasting was quantitatively evaluated by using the value range of the damage variable D. According to the actual field test parameters of coal seam #3 in the Sanyuan coal mine, Dlim = 0.81 ~ 1.0 was the coal rock crushing area, Dlim = 0.19 ~ 0.81 was the coal rock crack area, and Dlim = 0 ~ 0.19 was the coal rock disturbance area. The blasting models under different blasting parameters were established by ANSYS/LS-DYNA software. The influence radius of single-hole blasting was 3.1 m, the hole diameter of double-hole blasting was 113 mm, the hole spacing was 5.5 m, and the delayed blasting time was 25 ms. According to the numerical simulation results, the determined parameters were tested on the working face of the 1312 transportation roadway in coal seam #3 of the Sanyuan coal mine. The results show that after blasting, the permeability of the original coal seam was increased by more than 30 times, the gas concentration was increased by 2.16 times, and the single hole purity and mixing volume were increased by 4.73 and 4.27 times, respectively. The positive effects of deep hole presplitting blasting permeability enhancement technology on the pressure relief and permeability enhancement of a low pressure and high gas coal seam were determined.

N2 injection to enhance coal seam gas drainage (N2-ECGD): Insights from underground field trial investigation

Pre-gas drainage plays a significant role of preventing gas disasters in underground coal mines. However, it is difficult to reduce the gas content to below the threshold values, especially in low permeability coal seams in China. Previous field trials on the enhanced coalbed methane recovery (ECBM) by nitrogen injection mostly focused on the ground. Until now, few attempts on N2 injection to enhance coal seam gas drainage (N2-ECGD) and no sufficient evidences can indicate the good effect. In this study, exploratory tests of N2-ECGD were carried out in underground low-gas coal seams (with gas content less than 5 m3/t). An experiment to displace the CH4 in coal by N2 injection under constant pressure conditions was first carried out. The results show that continuous N2 injection can promote the desorption of CH4 in the coal pores, which cannot be achieved under normal pressure. N2-ECGD field trials were conducted in the working face of a coal mine in Hejin City, Shanxi Province, China. The effective pressure of N2 injection (EPI) was investigated. Intermittent and continuous N2 injection trials were carried out to study the response characteristics of the gas flow rate, CH4 concentration, and flow rate. The results show an EPI of 0.45 MPa and gas injection radius of 5 m. The gas flow rate increased significantly after N2 injection. Although the CH4 concentration decreased, the CH4 flow rate remained high. On the basis of the change characteristics of gas drainage parameters, the physical process of N2-ECGD was analyzed. The research results obtained in this study can serve as a reference for the application of N2-ECGD in low permeability coal seams. The study also provides a new technical approach for local gas prevention and control in underground coal mines.

Improving safety by further increasing the permeability of coal seams using air cannons after hydraulic punching

Improving safety by further increasing the permeability of coal seams using air cannons after hydraulic punching

A numerical method of hydraulic fracturing in coal seam for enhancing gas drainage

Hydraulic fracturing technology, as one of the measures to enhance the coal seam permeability artificially and increase the coalbed-methane gas extraction rate, has its remarkable advantages. Although the interaction between fluid and coal has been studied comprehensively in the process of fracturing and gas drainage, fewer researchers focus on the combination of solid-gas-water three-phase flow, which brings great deviations to the design of hydraulic fracturing enhanced underground gas drainage. In this paper, a fracture-pore equivalent model is proposed, which establishes the equivalent relationship between the fracture aperture under hydraulic fracturing and the permeability coefficient of coal in the process of gas extraction, thus realizing the weak coupling between gas-solid-water. The hydraulic fracturing takes into account the deformation and fracture of solids driven by fluids. And the gas extraction considering simultaneously adsorption and desorption effect and the permeability coefficient caused by hydraulic fracturing is realized by the coupling of solid stress field and pore seepage field. The accuracy of this method is verified by comparing with the theoretical solution of the KGD model. Taking the fracturing boreholes in a mine’s field test as the background, the change of the cracks distribution with the injection pressure is analysed quantitatively. And the relationship between gas extraction rate and time is verified by the field monitoring data. The results show that the model has positive significance for the optimal design of gas extraction scheme under hydraulic fracturing.

Study on the Technology of Enhancing Permeability by Millisecond Blasting in Sanyuan Coal Mine

To reduce gas disasters in low permeability and high-gas coal seams and improve gas predrainage efficiency, conventional deep-hole presplitting blasting permeability increasing technology was refined and perfected. The numerical calculation model of presplitting blasting was established by using ANSYS/LS-DYNA numerical simulation software. The damage degree of coal and rock blasting was quantitatively evaluated by using the value range of the damage variable D. According to the actual field test parameters of coal seam #3 in the Sanyuan coal mine, Dlim = 0.81–1.0 was the coal rock crushing area, Dlim = 0.19–0.81 was the coal rock crack area, and Dlim = 0–0.19 was the coal rock disturbance area. By comparing and analysing the damage distribution nephogram of coal and rock mass under the influence of different millisecond blasting time interval and the blasting effect of simulation model, the optimal layout parameters of multilayer through cracks were obtained theoretically. And, the determined parameters were tested on the working face of the 1312 transportation roadway in coal seam #3 of the Sanyuan coal mine. The permeability effect was compared and analysed through the analysis of the gas concentration, gas purity, and mixing volume before and after the implementation of deep-hole presplitting blasting antireflection technology, as well as the change of gas pressure, attenuation coefficient, permeability coefficient, and other parameters between blasting coal seams. The positive role of millisecond blasting in reducing pressure and increasing permeability in low permeability and high-gas coal seam were determined.

High-pressure water and gas alternating sequestration technology for low permeability coal seams with high adsorption capacity

Traditional ECBM techniques, which cannot extract methane adsorbed in coal pores effectively, is still difficult to greatly improve coalbed methane recovery rate until now. Waterflooding or CO2-ECBM can displace methane adsorbed in nanopores but increases the risk of coal and gas outbursts. We proposed a novel coalbed methane recovery technique, e.g. high-pressure water and gas alternating sequestration technique (H–P-WGAS), which can displace adsorbed methane in coal pores without introducing other gases with high adsorption capacity. Additionally, the injected H–P gas can eliminate the water lock and be easily extracted during methane drainage. Numerical simulation and field test were conducted to verify effectiveness of the H–P-WGAS technique. The numerical results show that the H–P-WGAS technique can enhance methane migration in coal seam by changing pore pressure distribution and increase influence radius to 70 m at 20 MPa in 48 h. For water and air co-injection, methane content can decrease by up to 37.6%, which is much more effective comparing with that under single water injection or gas injection. Meanwhile, field tests show that the reduction of methane content by percentage lies between 25.14% and 80.62% with an average of 53.17%. Meanwhile, the influence radius of field test reached ∼48 m. The average methane drainage flow rate of single borehole increased by 2.65 times. Average methane drainage concentration increased from 31.3% to 47.6%. The methane drainage flow rate and concentration of the whole coal mine increased by 50.2% and 32.4%, respectively. The above results indicate that the proposed H–P-WGAS has best effectiveness comparing with single water injection or air injection.

A NEW FRACTAL MODEL OF COAL PERMEABILITY BASED ON THE INCREASING FRACTAL CONSTRUCTION METHOD OF THE MENGER SPONGE

The permeability of coal seams directly determines the disaster prevention effect of water injection. It is of great significance to realize the accurate fractal expression of coal structure and construct a fractal permeability model for elucidating the dynamic evolution law of the permeability of coal during and after water injection. Therefore, a rough Menger sponge is used as the physical model to characterize the structure of the coal in this paper. Combined with the structure fractal dimension calculated by the increasing fractal construction method, a new fractal permeability model is established to describe the seepage process of water injection. The influence of the tortuosity and roughness of the pore surfaces on the seepage characteristics is discussed, and the theoretical permeability is compared with the results of NMR experiments and existing models. The volume fractal dimension obtained by calculation is between 2.74 and 2.77, which decreases with increasing confining pressure and increases with increasing pore water pressure. The research shows that the fractal dimensions of the pore structure and volume can quantitatively describe the specific surface area of porous medium and are directly proportional to the specific surface area. With the increase in the structure fractal dimension, the fluid flow resistance increases, and the theoretical permeability decreases. The results of the permeability comparison show that the variation law of the theoretical permeability is consistent with that of the liquid permeability and that the theoretical result is closer to the measured value because the tortuosity and roughness of the seepage channel surfaces is further considered in the model. The theoretical model provides reliable theoretical support for further perfecting seepage theory of porous media and improving the effect of coal seam water injection.

Applied Research of Deep-hole Presplitting Blasting Technology in Coal

Deep-hole presplitting blasting is a method to solve the problems of gas outburst and low recovery rate in thick and hard coal seam. After explosion, the coal seam permeability is higher than before, and the mass of coal-dropping is increased in result of the local stress change caused by loose blasting. Blasting parameters and cracking zone is calculated based on the theory of explosion. The application indicated that the gas content of the coal seam decreased and the recovery rate increased. The experiment result is in concordance with the result of calculation by theory .

Application on removing outburst with dig-hole drillings in coal seam

In order to resolve the problem that low gas drainage concentration, poor results of the gas drainage through crossing hole which cause removing the outburst inadequate, we adopt dig-hole drillings technology on the base of designing crossing hole which is used to drainage gas. It can increase disturbance around the coal, increase the expansion deformation degree of the coal around the drilling, make pressure relief of the coal seam more fully, strengthen the permeability of coal seam, increase the effect of drainaging gas and removing outburst and reduce the possibility of coal and gas outburst.

Effect of CO2 injection on CH4 desorption rate in poor permeability coal seams: An experimental study

CO2 injection into coal seams is an effective way for reducing greenhouse gas emissions and increasing gas recovery in coal seams. To study the influence rule of CO2 injection on the CH4 desorption rate, an experimental system of CO2 displacing CH4 was used to analyse the changes in gas composition, outlet flow rate, coal seam temperature, and displacement efficiency parameters during CO2 injection. From the perspective of enhancing CH4 recovery, the CO2 critical injection amount (Vinc) is 297.4 L and critical injection time is 188.75 min. When the CO2 injection amount V < Vinc, the amount of CH4 desorption, CO2 storage, and CH4/CO2 gas content all increased. When V > Vinc, except CH4 desorption amount remained almost unchanged, the other parameters increased. In the process of CO2 displacing CH4, the coal seam temperature rises (exothermic process), showing a law of “hierarchical displacement”. With the injection of CO2, the pore pressure first decreased and then increased, the CH4 partial pressure decreased, and the effective diffusion coefficient first increased and then decreased. After the injection of CO2, the desorption time of CH4 was 269.45min earlier and the desorption rate of CH4 was increased by 12.43 %.

Research on Application of Directional Long Drilling Fracturing Technology in 1930 Coal Mine

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.

A MULTI-FIELD COUPLED SEEPAGE MODEL FOR COAL SEAM WITH FRACTURES OF POWER LAW LENGTH DISTRIBUTIONS

In the process of gas mining, the fracture distribution with power law length and the pore structure with adsorption effect have an important influence on the coal seam permeability. In recent years, the research on the internal structure of coal seam and the fluid flow mechanism has attracted a large number of researchers. In this paper, by considering the coal matrix deformation caused by adsorption, a pore-fracture model coupled with the multi-field effects and with power law length distribution of fractures in coal seam is established based on the fractal theory for porous media. In this work, we study the influences of the power law exponent [Formula: see text] of fracture length and the ratio [Formula: see text] of the minimum to maximum fracture lengths on the permeability of coal seam and the evolution mechanism of permeability with the structural and mechanical parameters of coal seam. It is found that the permeability of coal seam is inversely proportional to [Formula: see text], directly proportional to [Formula: see text], and to Langmuir volume constant and Langmuir volume strain constant. Compared with other factors, the power law component [Formula: see text] of fractures has the most significant effect on the coal seam permeability.