Permeability Of Coal Samples Research Articles

Effect of Mixed Acid Fluid on the Pore Structure of High Rank Coal and Acid Fluid Optimization.

Acidizing technology is an important means to increase production in oil-gas reservoirs. In recent years, acidizing technology has been widely used to increase the permeability of coal seams to enhance gas extraction, where acidizing fluid is the key factor to determine the permeability improvement effect by acidizing technology. In order to clarify the influence of mixed acid fluid on the pore structure of high rank coal and seek the optimal mixed acid fluid suitable for acidizing and permeability improvement of high rank coal in the Jiaozuo coal mine area. Taking the Jiulishan Mine in the Jiaozuo mining area as an example, low field nuclear magnetic resonance (LFNMR) test and static dissolution test were conducted to obtain the T2 spectrum, porosity, movable fluid saturation, pore throat distribution, nuclear magnetic permeability, and dissolution rate of coal samples before and after treatment with distilled water and three mixed acid fluids. On this basis, the influence of mixed acid fluid on the pore structure of high rank coal was analyzed and the optimal mixed acid fluid suitable for high rank coal was selected. The results showed that the pore size, number, and volume of all kinds of pore sizes of coal samples treated with distilled water all decreased, which was manifested by the decrease of effective porosity and nuclear magnetic permeability. After acidification, the proportion of micropore volume in coal decreased significantly, the number and proportion of pore volume of mesopores and macropore-microfractures increased significantly, and the connectivity between mesopores and macropore-microfractures was enhanced, which was characterized by the increase in effective porosity and nuclear magnetic permeability of coal samples. After acidification, the pore-throat ratio of adsorption pores of all coal samples decreased, while the pore-throat ratio of seepage pores increased. By comparatively analyzing the change law of pore structure of coal samples before and after acidizing with three kinds of mixed acid fluids, the optimal mixed acid fluid suitable for acidizing and permeability improvement of high rank coal in the Jiaozuo coal mine area was selected, which was 12%HCL +3%HF.

NMRI online observation of coal fracture and pore structure evolution under confining pressure and axial compressive loads: A novel approach

Quantitative characterization of the spatial distribution, content and heterogeneity of fracture and pore structure (FPS) in coal reservoirs under confining pressures and axial compressive loads is significant for the engineering of coal bed methane. A novel online observation approach that combines nuclear magnetic resonance imaging with triaxial loading techniques is employed to achieve the visualization and full-scale quantitative characterization of the evolution of FPS in coals in the laboratory. The relationship between the stress states and FPS evolution was formulated. The results show that the spatial distribution of the FPS evolution process of coal samples can be divided into four stages: initial pore and fracture compaction closure, pore and fracture stable growth, pore and fracture unstable growth, and failure stages. As the deviatoric stress increases, the content of the adsorption pores, the heterogeneity of the adsorption space, and the gas adsorption capacity of coal samples gradually increase. In contrast, the seepage pore and fracture content as well as the permeability of coal samples decrease first and then increase. The heterogeneity of the seepage space of coal samples initially increases and then decreases. The maximum compression of seepage space and increase of adsorption space are 4.742% and 14.743%, respectively.

Influence of hygroscopic expansion of clay minerals on the dynamic evolution of coal permeability

Exploring the evolution characteristics of hygroscopic expansion of clay minerals in coal on its permeability has a positive effect on improving coal seam permeability. Firstly, the dynamic evolution model of coal permeability affected by hygroscopic expansion was established. Secondly, numerical simulation is carried out after an adaptive multi-field multiphase coupling tester verifies the model. Finally, the evolution law of coal permeability under different confining pressures and different moisture absorption expansion coefficients is analyzed. Research shows that the permeability of coal samples decreases first and then increases with the stress in the shape of a deep “V”. Due to the hydration and softening of clay minerals caused by hygroscopic expansion, the peak stress and permeability decreased by 3.8% and 1.69%, respectively, and the plastic strain increased by 6%. In this process, the change of permeability of axially compressed coal samples plays a leading role. Before reaching the yield stress, the influence of hygroscopic expansion on the change of permeability of the coal sample is more obvious. The research results enrich the theory of permeability dynamic evolution and have great significance in guiding coalbed methane exploitation.

Research on the evolution of pore and fracture structures during spontaneous combustion of coal based on CT 3D reconstruction

To study the evolution law of the pore and fracture structures during the spontaneous combustion of coal, a self-built high-temperature tube furnace was used in the experiment to heat the coal samples at different temperatures and gas atmospheres. The coal samples were scanned by Xray-CT technology, and the three-dimensional (3D) pore structure and equivalent pore network model of coal samples were extracted by CT 3D reconstruction technology. In the process of heating at 25–200 °C, the porosity and fractal dimension were more significantly developed for coal samples heat-treated in an air atmosphere than those for the coal samples heat-treated in nitrogen atmosphere. The connectivity and permeability of coal samples were investigated, and it was found that the connectivity of pore and fracture structures of coal samples heated in the air atmosphere was better. However, the coal samples heated after nitrogen injection had weak connectivity and permeability. At this temperature stage, the nitrogen injection into the goaf will have a stronger inhibitory ability to develop coal sample pore and fracture structures and a more obvious weakening of the permeability. Therefore, oxygen circulation inside the coal sample may be suppressed, which is more conducive to preventing and controlling coal spontaneous combustion.

Comparative experimental study on the effects of water- and foam-based fracturing fluids on multiscale flow in coalbed methane

To quantitatively evaluate their influence on coalbed methane (CBM) flow, water- and foam-based fracturing fluids were prepared. The effects of these two fracturing fluids on the flow of methane in coal were compared and analyzed using isothermal adsorption, diffusion capacity, and seepage tests. The results showed that an increase in the concentration of the fracturing fluid can exacerbate damage to methane flow. After immersed with water-based gel, gas diffusivity in the coal sample dropped to less than one fifth of the original level. However, when immersed with foam-based fracturing fluid, the diffusion coefficient of coal samples exceeded 3 × 10−13 m2/s. The residual permeability of the coal samples measured at a high pressure was significantly greater than that measured at a low pressure. Compare with the foam, the gel will cause a more serious adverse impact on the permeability of coal samples. Foam fracturing fluids with low viscosity and higher volume fraction of the gas phase will cause lesser damage to methane adsorption, desorption, or percolation. These results can help in the development of a coal seam clean fracturing technology.

Development of a methodology to research the influence of liquid nitrogen exposure on carbon material

Recently, studies, which are dedicated to the effect of cryogenic substances on the structure of coal, are gaining popularity. The impact of liquid nitrogen on coal leads to micro- and macrostructural disturbances in the continuity of coal due to drastic cooling of the moisture in the microcracks and pore space of the coal, while the volume of ice microcrystals increases. This approach provides the opportunity to treat the massif without additional injection of liquid agents into seams to create pressure to disintegrate it, unlike traditional methods of increasing the filtration surface in coal. Research work, which are presented in this field, are aimed at studying the multiplicity of increase in porosity and fracturing of coal in the process of single or multiple exposure to portions of liquid nitrogen. A distinctive feature of the ongoing research is the evaluation of the results of cryotreatment of samples by non-destructive methods of measuring the pore space. Thus, the conclusion about the change in permeability is made without direct measurements of the gas filtration rates in the samples. Changes shown by CT scans show an increase in the total number of open pores and channels, however they do not illustrate the increase in gas permeability in general. As a part of this work, we carried out a number of studies on axial gas filtration in dense coal samples, which were under controlled all-round loading. Preparatory work was carried out to dry the samples and further saturate them with moisture to identical values in the entire batch. A scheme for conducting experiments to study the effect of cryotreatment on the internal structure of coal using the methods of stationary axial gas filtration and NMR relaxometry is proposed. The development of this technique contributes to obtaining reliable data on the increase in the gas permeability of coal samples. The data can be useful in designing methods and approaches to increase the degree of degassing by treating degassing wells at coal mining enterprises.

Gas Seepage Model and Experiment Based on Bedding Effect of Fractured Coal Body

Gas drainage is of great significance for the efficient and safe mining in coal mine, in which the coal seam layer bedding has a great influence on it. For obtaining gas permeability characteristics of coal body with the parallel and vertical bedding in fractured coal under the action of stress loading and unloading, experimental research was carried out employing a three-stress-axis simulation device. Experimental results showed that in the stress loading process, the permeability decreased with increasing effective stress; the decrement was initially rapid albeit it slowed later. With the increase of effective stress, the coal sample underwent three stages, namely, crack compaction, elastic deformation, and plastic deformation. In the stress unloading process, the permeability of coal samples increased with decreasing of effective stress, and the increasing trend of permeability was consistent. The degree of fracture compaction of the parallel bedding coal samples after compression was much higher than that of vertical bedding. In the stress-relieved coal seam, gas drainage boreholes should be arranged vertically to the bedding fissure to maximise the gas drainage effect. A group of parallel and vertical bedding gas drainage holes were arranged in the test mine to investigate the drainage effect. Field engineering application also showed that the drilling direction should be perpendicular to the bedding direction as far as possible, so as to improve the gas drainage effect. The research results can provide a reference for the gas drainage borehole layout, thus maximising the gas extraction efficiency and ensuring the sustainability of mine safety production.

Experimental Investigation on the Effect of Loading and Unloading on Coal Permeability with Different Sediment Beddings

Abstract The sediment bedding direction and loading-unloading have significant effect on permeability of coal sample. The coal samples taken from Zhangcun Coal Mine in Shanxi were used to measure coal permeability with different sediment beddings under the effect of loading and unloading by the triaxial coal-rock seepage experimental apparatus. The influence of gas pressure, loading-unloading, and sediment bedding direction on the permeability of coal samples was analyzed, and the functional relationship among these parameters was recovered by fitting. The results show that the permeability of all vertical, dip, and horizontal bedding coal samples decreases exponentially with the increase of effective stress during stress loading, while the permeability of three kinds of bedding coal samples increases exponentially with the decrease of effective stress during stress unloading. Under the same gas pressure, the fracture space of coal samples with vertical and dip bedding is more likely to be compressed and closed at the initial stage of loading, resulting in great decrease of the permeability. In the initial stage of unloading, the microcracks and natural beddings in coals gradually expand and connect. Due to the well development of fractures, the permeability of vertical bedding coal samples increases greatly, while the permeability of dip and horizontal bedding coal samples increases slightly. In the loading process, the permeability of coal sample is vertical bedding > dip bedding > horizontal bedding in order. For the tested samples, the permeability of vertical bedding coal samples is 1.3 to 2.8 times that of dip or horizontal bedding coal samples. In the unloading process, the permeability of the vertical bedding coal samples was 2.8 to 3.3 times that of the dip or horizontal bedding coal samples.

Experimental Investigation of Pore Pressure Effect on Coal Sample Permeability Under Different Temperatures

Experimental Investigation of Pore Pressure Effect on Coal Sample Permeability Under Different Temperatures

Impact of coal damage on permeability evolution based on an improved permeability model

Gas-induced deformation during matrix-fracture interaction can cause damage to the microstructure of coal and thus impact the permeability. In this study, A full damage-flow coupling permeability model with geometric equations for the effective matrix-fracture strain was improved. The model was solved by the COMSOL livelink MATLAB program. Then, the impact of the damage effect on the permeability of coal samples with different Young's modulus ratios under uniaxial strain and constant stress conditions was analysed. Damage that occurred first during coal fracture instantaneously accelerated the increase in permeability, while damage that occurred in the matrix slowed the increase in coal permeability. Because the damage ratio increased with increasing Young's modulus ratio, the permeability decreased with increasing Young's modulus ratio. Correspondence between damage and permeability for different Young's modulus ratios was analysed. The improved model was advantageous in solving the problem of mismatch between the theoretical and experimental permeability results. The new model considering the impact of the damage effect was more suitable for the production prediction of CBM production.

Effect of pore structure on competitive sorption and diffusion of mixed methane and carbon dioxide in anthracite, South Qinshui Basin, China

A deep understanding of the competitive adsorption and diffusion of methane (CH4) and carbon dioxide (CO2) in coal is significant for implementing the CO2-ECBM project. For three high-rank coal samples, industrial analysis, coal maceral, and vitrinite reflectance (RO) determination, mercury injection test, and low-temperature nitrogen/carbon dioxide specific surface test were carried out, and the composition and pore structure of coal samples were studied systematically. Meanwhile, through the three mixed gases (75%CH4 + 25%CO2, 50%CH4 + 50%CO2, 25%CH4 + 75%CO2) adsorption and desorption test, the absolute adsorption amount, free phase and adsorption phase of CH4 and CO2 under different equilibrium pressures, and the equivalent diffusion coefficients of gas migration in the process of adsorption/desorption equilibrium were calculated. Finally, the relationship among the coal composition, pore structure, and the competitive adsorption and diffusion characteristics of methane and carbon dioxide mixture in coal were studied. The results show that: (1) the composition of coal has an important influence on the development of coal pore structure and the adsorption and transport of gas in coal. Supermicropore (<2 nm in size) is the prominent adsorption place. Furthermore, its development, which essentially determines the adsorption capacity of coal samples, is positively correlated with fixed carbon content, vitrinite content, and vitrinite reflectance. (2) When the pores >100 nm in size relatively developed, the coal samples have higher porosity, lower micropore diffusion, and a smaller desorption hysteresis index. Because the pore's connectivity is limit, its content has no apparent correlation with the permeability of coal samples and most of the adsorption and diffusion parameters. (3) The desorption hysteresis is mainly caused by the opposite migration direction of gas in the process of adsorption and desorption and the resulting dislocation of coal matrix swelling and shrinkage. The competitive adsorption of CH4 or CO2 and their separation index (SCO2/CH4) in coal samples decreases with the increase of adsorption equilibrium pressure. Also, the desorption process often shows a higher SCO2/CH4 value than that of the adsorption process. (4) During the adsorption process, the equivalent macro-diffusivities of the coal sample decrease as a power function with increasing pressure and increase with the carbon dioxide content increasing in the mixed gas. However, the equivalent micro-diffusivities change little with the different CH4 and CO2 mixtures migration in different coal samples. The equivalent diffusion coefficient of coal is positively correlated with macropore, mesopore, transitional and micropore content in coal samples.

Experimental Study of the Influence of Moisture Content on the Pore Structure and Permeability of Anthracite Treated by Liquid Nitrogen Freeze-Thaw.

The liquid nitrogen freeze–thaw (LN2-FT) method has been widely used to improve the coal permeability in the coalbed methane (CBM) production. However, the influence of moisture content on the permeability of coal treated by LN2-FT remains unclear, limiting the broad application of this technique. A novel seepage system was proposed to analyze the permeability evolution of anthracite coal samples treated by LN2-FT. Moreover, variations of the pore structure were analyzed using scanning electron microscopy (SEM), mercury intrusion porosimetry (MIP), and low-field NMR. The results showed that pores and fractures appeared on the coal surface after the LN2-FT treatment. As the moisture content of the coal increased, more pores and fractures tended to be formed during the LN2-FT treatment. The total pore volume, porosity, and average pore diameter of the anthracite coal after the treatment were 1.77, 2.44, and 5.58 times higher, respectively, than that of the raw coal. The change in the specific surface area exhibited three trends as the moisture content of the coal samples increased: a slow descent, a steady increase, and a rapid descent. Moreover, it was found that the LN2-FT treatment increased the connections between pores and fractures, improving gas migration in the coal. Furthermore, the LN2-FT treatment significantly increased the permeability of the anthracite coal samples. The higher the coal moisture, the higher the permeability of the coal samples after the LN2-FT treatment. Hence, the LN2-FT technique can substantially improve the permeability of coal reservoirs, providing essential information for the efficient utilization of CBM.

Regional Characteristics of Porosity and Permeability of Dahebian Syncline Coal and Its Application

In order to study the basic properties of coal seam thus to provide reference for coalbed methane (CBM) production, the porosity and permeability of coal from DH and WJZ regions of Dahebian syncline in Liupanshui Coalfield were systematically studied. The results shows that the porosity and permeability of coal samples in DH region are between 7.4–10.1% and 0.01 mD∼0.04 mD respectively. The permeability is obviously anisotropic, and it is distributed in a U-shape along bedding angle. The porosity and permeability in the WJZ region are between 12.4–24.85% and 0.3 mD–4.8 mD, but the correlation of permeability and bedding angle is not obvious. There are many primary cracks in coal matrix of WJZ region, while the coal of DH region is relatively complete. As a result, the porosity and permeability of WJZ coal samples are much higher than that of DH coal samples. With the change of confining pressure, obvious change has taken place in the permeability of coal in the two regions. When the confining pressure decreases from 2 to 8 MPa, the permeability loss rate of coal samples is close to 80%. The internal mechanism of the permeability loss is related to crack closure and plastic deformation caused by confining pressure. In contrast, the porosity and permeability of coal in WJZ region are much higher than those in DH region, which indicates that the permeability of the same coal seam in the same structural will have great differences and inhomogeneities. Therefore, the WJZ region is the preferred area, which can be included in the preferential exploitation plan of CBM. In addition, for the typical syncline gas reservoir in Liupanshui Coalfield, it is necessary to consider the block division according to the porosity and permeability indexes, and select the optimal block and give priority to development of CBM. Furthermore, we should pay attention to the difference and adaptability of CBM development in different regions.

An Experimental Investigation of the Gas Permeability of Tectonic Coal Mineral under Triaxial Loading Conditions

Underground coal mining of CH4 gas-rich tectonic coal seams often induces methane outburst disasters. Investigating gas permeability evolution in pores of the tectonic coal is vital to understanding the mechanism of gas outburst disasters. In this study, the triaxial loading–unloading stresses induced gas permeability evolutions in the briquette tectonic coal samples, which were studied by employing the triaxial-loading–gas-seepage test system. Specifically, effects of loading paths and initial gas pressures on the gas permeability of coal samples were analyzed. The results showed the following: (1) The gas permeability evolution of coal samples was correlated with the volumetric strain change during triaxial compression scenarios. In the initial compaction and elastic deformation stages, pores and cracks in the coal were compacted, resulting in a reduction in gas permeability in the coal body. However, after the yield stage, the gas permeability could be enhanced due to sample failure. (2) The gas permeability of the tectonic coal decreased as a negative exponential function with the increase in initial gas pressure, in which the permeability was decreased by 67.32% as the initial gas pressure increased from 0.3 MPa to 1.5 MPa. (3) Coal samples underwent a period of strain development before they began to fail during confining pressure releasing. After the stress releasing-induced yield stage, the coal sample was deformed and cracked, resulting in a quickly increase in gas permeability. With a further releasing process, failure of the sample occurred, and thus induced rapidly increasing gas permeability. These obtained results could provide foundations for gas outburst prevention in mining gas-rich tectonic coal seams.

Coal permeability prediction method based on the microscopic pore-fracture dual-porosity structure

To better understand the effect of the pore-fracture dual-porosity structure on the coal permeability, the methods for calculating the pore size distribution as well as the fracture width distribution based on the Euclidean distance map function were given first. On this basis, a permeability estimation model considering pore-fracture structure was derived. Then, the pore-fracture structure and permeability of coal samples were quantitatively analyzed by scanning electron microscopy (SEM). By considering the pore size distribution and fracture width distribution characteristics separately, the complex pore structure of the coal sample can be more clearly defined. We found that both pores and fractures have their own distribution characteristics, but due to the effect of resolution, there is some influence in quantifying the minimum size distribution, especially in quantifying the pore size distribution. In predicting the permeability, the direct calculation using the pore and fracture size distribution is significantly larger than the experimental results (1.18✕10−14 m2 vs. 6.9✕10−16 m2 (measured) for the A1 and 1.22✕10−14 m2 vs. 8.1 ✕10−16 m2 (measured) for the A2 at ✕1000 magnification). However, by further considering the effect of tortuosity, more accurate prediction results can be obtained (1.14✕10−15 m2 for the A1 and 1.22✕10−15 m2 for the A2 at ✕1000 magnification). Fractures occupy less space, but they would cause higher permeability. The method proposed in the current study can be further applied to quantify the pore-fracture structure and permeability characteristics in microgeology.

The Role of the Reverse Fault on the Permeability Evolution in Mining Coal

To prevent and control the coal seam gas disaster affected by the reverse fault, we performed gas seepage tests, which consider stress-loading and unloading schemes, to investigate the stress change and coal permeability of the mining coal with reverse fault. The experimental results show that the mechanical behavior and permeability change of the mining coal are related to the distance between the coal and the reverse fault. The stress concentration coefficient of the coal body gradually increases. The closer is the distance between the coal and the reverse fault, the larger are the deviatoric stress peak and strain. In comparison with the coal sample M1 that is 5 m away from the reverse fault, the deviatoric stress peak and axial strain of the coal sample M3, 35 m away from the reverse fault, increase by 40.74% and 26.73%, respectively. In this stage, the permeability of M1, M2, and M3 coal samples increases by 22.1%, 28.0%, and 36.7%, respectively. In another stage, the stress concentration coefficient of coal increases to the peak and then decreases, causing the deviatoric stress peak and strain of coal to rise first and then fall. In comparison with the coal sample M4 that is 65 m away from the reverse fault, the deviatoric stress peak and axial strain of coal sample M6, 5 m away from the reverse fault, decrease by 29.48% and 5.55%, respectively. The permeability of coal samples M4, M5, and M6 increases by 23.6%, 37.2%, and 20.8%, respectively. Based on the gas seepage test results, we established the permeability model of mining-induced coal under the influence of a reverse fault, with consideration of the volume changes of coal fractures induced by adsorption and desorption. In the model, the variations of permeability in both stages of the prepeak and postpeak were deduced, which was verified with the experimental data. The verification results demonstrate that the proposed model has the capacity to predict the permeability evolution of mining coal under the influence of a reverse fault.

Experimental Study of the Impact of Chlorine Dioxide on the Permeability of High-Rank Contaminated Coal Reservoirs.

The permeability of high-rank coal reservoirs is generally low, and high-viscosity working fluid can also contaminate the reservoirs and reduce permeability during drilling and fracturing engineering. These two reasons lead to a lot of low-yield CBM wells in the southern Qinshui Basin, China. The impact of chlorine dioxide on the permeability of high-rank coal has been studied in detail. The coal samples with changed characteristics before and after treatment were compared using coal–ash, displacement, immersion, and plug removal experiments. The ash experiment results show that the ash content of the coal samples decreased by 16.35%. The displacement and immersion experiments using chlorine dioxide solution showed that displacement with chlorine dioxide could increase permeability. The permeability of coal samples increased by 3005.77% after 80 h of immersion. The plug removal experiment results show that the permeability of contaminated coal samples was recovered by 11.10–38.90%, with an average recovery of 27.90%. The experimental results show that chlorine dioxide is effective in improving the permeability of high-rank contaminated coal reservoirs. This research result can be applied to low-yield CBM wells polluted by high-viscous working fluid to increase gas production.

Coupled Effects of Stress, Moisture Content and Gas Pressure on the Permeability Evolution of Coal Samples: A Case Study of the Coking Coal Resourced from Tunlan Coalmine

Deep coalbed methane (CBM) is widely distributed in China and is mainly commercially exploited in the Qinshui basin. The in situ stress and moisture content are key factors affecting the permeability of CH4-containing coal samples. Therefore, considering the coupled effects of compressing and infiltrating on the gas permeability of coal could be more accurate to reveal the CH4 gas seepage characteristics in CBM reservoirs. In this study, coal samples sourced from Tunlan coalmine were employed to conduct the triaxial loading and gas seepage tests. Several findings were concluded: (1) In this triaxial test, the effect of confining stress on the permeability of gas-containing coal samples is greater than that of axial stress. (2) The permeability versus gas pressure curve of coal presents a ‘V’ shape evolution trend, in which the minimum gas permeability was obtained at a gas pressure of 1.1MPa. (3) The gas permeability of coal samples decreased exponentially with increasing moisture content. Specifically, as the moisture content increasing from 0.18% to 3.15%, the gas permeability decreased by about 70%. These results are expected to provide a foundation for the efficient exploitation of CBM in Qinshui basin.

An experimental study on coal damage caused by two-phase displacement of CO2- alkaline solution

Two-phase displacement of CO2- alkaline solution technology was proposed to address the obvious problems existing in the current coal mine gas control technologies, such as the gas enrichment and outburst risk caused by tradition CO2 injection technology, and the poor effect of conventional coal seam water injection technology. In order to investigate the damage characteristics of coal pore structure and permeability caused by two-phase displacement of CO2- alkaline solution, the composition and microstructure of coal after two-phase displacement of CO2- alkaline solution were tested experimentally, and the permeability experiment of two-phase displacement of CO2- alkaline solution was carried out. The results show that the reaction degree and dissolution of CO2-containing coal with alkaline solution are stronger than those with distilled water. The reaming effect of alkaline solution on CO2-containing coal is stronger than that of distilled water, and the two-phase displacement of CO2- alkaline solution has the greatest influence on mesopore and macropore; After two-phase displacement of CO2- alkaline solution, the permeability of coal sample increased by 47.6%–108.6%, and the water content of coal sample increased by 109%. The two-phase displacement of CO2- alkaline solution leads to the secondary development of coal pores, which significantly changes the coal pore structure, increases the permeability and water content of coal body, and thus obviously increases the gas extraction effect to reduce coal seam outburst risk and ensure coal mine safety production.

Grouting Fractured Coal Permeability Evolution Based on Industrial CT Scanning

Gas extraction from coal seams in China faces various middle-term and long-term problems, such as the poor sealing quality and low extraction rate. The mean gas extraction concentration is only 30%. Studying the flowing laws of the grout and fracture plugging mechanism is of important significance to improve the sealing quality and increase the gas extraction efficiency. For this reason, a new coal-based grouting material was prepared in this study, and its parameters such as viscosity were tested. Moreover, a grouting theoretical model with considerations of the flowing of the grout and coal fracture plugging by migration and deposition of slurry particles was constructed. The crack distribution before and after the grouting of fractured coal samples was scanned with an industrial CT, and the fracture distribution of coal samples was reconstructed using an independently compiled MATLAB program. Meanwhile, the variations of the coal permeability before and after the grouting were tested. On this basis, this study built a numerical calculation model of the grouting in fractured coal samples to simulate the leakage stoppage and permeability reduction mechanism of the coal-based sealing material grouting. Results demonstrate that (1) according to the experimental test results, the new coal-based grouting materials achieve a good fracture plugging effect, and the fractures in coal samples after the grouting are filled densely by the grouting particles. (2) According to the simulation results, the permeability of grouting coal samples declines quickly and then tends to be stable. The overall permeability of coal samples and the fracture permeability are decreased by 93.5% and more than 99.9% in average, respectively. (3) Influences of the grouting pressure on the permeability variation of coal samples were investigated through a numerical simulation. It was found that a reasonable grouting pressure for coal samples is about 0.3 MPa. (4) The numerical simulation reproduces the whole process of the grout flowing and the fracture filling. The variation laws of the diffusion and the permeability of the grout which are calculated through a numerical simulation agree well with the experimental results, which verifies the reasonability of the model. Research conclusions can provide important significance in theory and practice to disclose the leakage stoppage and the permeability reduction mechanism of the borehole grouting during the gas extraction and strengthen the sealing effects of extraction boreholes.