Coal Rock Research Articles

An Evaluation of the Coalbed Methane Mining Potential of Shoushan I Mine Based on the Subject–Object Combination Weighting Method

The parameters of coalbed methane reservoirs have large differences, and the precise values cannot represent the resource and production characteristics of the whole block. In order to address these problems, an index system for evaluating the production potential of coalbed methane blocks was constructed, the weights of evaluation parameters were determined, and a model for the preferential selection of coalbed methane blocks based on the subjective–objective combination of weights method was established. The main coal seams (No. 2-1 and No. 4-2) of the Pingdingshan-Shoushan I Mine Block were taken as the research objects to rank the development potential of CBM blocks in a preferential way. The results show that the six resource and production parameters of No. 2-1 coal are gas content, top and bottom rock properties, coal seam thickness, coal seam depth, coal body structure, and tectonic conditions, in descending order of importance, and the parameters of No. 4-2 coal are gas content, coal body structure, coal seam thickness, top and bottom rock properties, coal seam depth, and tectonic conditions, in descending order of importance. It is predicted that the favorable CBM gas development sweet spot areas of the No. 2-1 coal seam and No. 4-2 coal seam will be located along the exploration wells W15–W29 and W31, respectively. This paper aims to make a multi-dimensional and more comprehensive evaluation of coalbed methane mining potential in the Shoushan I mine, and provide a technical basis for the next step of coalbed methane mining in the study area.

Limitations of Upper Protective Layers as Pressure Relief Measures for Extra-Thick Coal Seam Mining: Insights from a Case Study

Upper protective layer (UPL) mining is extensively utilised as a pressure relief strategy to prevent outbursts and coal bursts. However, when the excavation height of the protected layer is substantial, the depressurisation efficacy of the protective layer may be diminished. This paper takes the Haishiwan coal mine in China as a case study and explores the stress evolution and influencing factors in the mining of extra-thick coal seam beneath the protective layer through theoretical analysis, numerical simulation, and field observation. The results indicate that increasing the excavation height of the coal seam will lead to the upward development of the collapse zone in the overburden of the goaf, with the “masonry beam” structure formed at a higher position by key strata blocks. The overburden above the masonry beam will be supported by the coal rock masse on both sides of the structure, leading to increased stress on the coal seam near the goaf and eliminating the depressurisation effect of the protective layer. Numerical simulation shows that factors such as faults, protective layers, interlayer spacing, and the height of coal seam excavation significantly affect the stress distribution in the protected layer. With the increase in interlayer spacing and the thickness of coal seam extraction, the stress reduction phenomenon of the UPL gradually decreases, especially with an abnormal stress concentration of the gob-side coal seam. Observations of Surface subsidence and the distribution of mining-induced seismic events corroborate the conclusions of theoretical analysis and numerical simulations. The results offer valuable guidance for the mining of extra-thick coal seams and the selection of the UPL.

3D modeling of cryogenic cracking by liquid nitrogen in coal under true triaxial stresses

Coalbed methane (CBM) plays an indispensable role in total natural gas production and consumption, and for the bottlenecks encountered in stimulating CBM reservoirs by conventional fracturing methods, studying more environment-friendly and efficient stimulation methods is imperative. Cryogenic fracturing using liquid nitrogen has been trialed for the development of coalbed methane, demonstrating great significance as more and more research corroborated. This study extended our previous coupled thermo-mechanical-damage (TMD) numerical procedure to investigate the cryogenic fracturing or damage process in 3D coal cubes. The cryogenic fractures on concrete cubes along with analytical solutions of temperature and first principle stress distributions for the borehole cooling process were first employed to verify the correctness and feasibility of this method, which incorporates the boundary conditions affected by the Leidenfrost effect at the initial stage. Then sensitivity analyses of cryogenic fracture generation including the maximum fracture cross-sectional area and the fractured volume were carried out. Results show that the coal rock with a high modulus of elasticity and coefficient of thermal expansion is more prone to be damaged in terms of the cross-sectional area and volume than other parameters within the parameter range analyzed. Damage cross-sectional area and damage volume expand with both the increasing coefficient of thermal expansion and the increasing modulus of elasticity, while they shrink with increasing heat capacity and coal density. Cryogenic fractures are relatively less sensitive to the thermal conductivity, density, and heat capacity of coal. This 3D modeling study provides a technical route and a panorama of the cryogenic fracturing process under varied conditions in opaque coal rocks.

Simulation Study on the Influence of Chimney Structure on the Efficiency of Gravity Heat Pipes for Controlling Spontaneous Combustion in Coal Waste Rock Piles

ABSTRACT Heat pipe is an efficient heat transfer element. Using heat pipes to control spontaneous combustion of coal gangue piles is a simple and effective method. However, the heat conducted by the heat pipe is usually directly released into the atmosphere, causing waste of resources. In order to utilize this part of the heat, this paper studied the method of enhancing the heat dissipation of the heat pipe. Based on the chimney effect, a chimney that utilizes the heat conducted by the heat pipe to enhance its heat dissipation efficiency was designed. The effects of five different parameters, including the distance between the chimney and the ground, the length of the chimney expansion section, the radius of the chimney inlet, the distance between the heat pipe and the inner wall of the chimney, and the height of the chimney, on the enhancement of heat dissipation of the chimney effect by the heat pipe were mainly studied. A large number of numerical simulation studies were carried out for chimneys with different structural sizes. The results show that all of the above five parameters have certain effects on the enhancement of heat dissipation of the chimney by the heat pipe, and each parameter has an optimal value to achieve the best enhancement effect of the chimney on the heat dissipation of the heat pipe. The response surface analysis was used to optimize the parameter values. The final designed chimney can increase the heat dissipation efficiency of the heat pipe by three times, providing a new direction for heat dissipation of heat pipes and the utilization of waste energy.

Internal stress transfer characteristics of coal–rock medium under concentrated force based on particle flow method

To solve the problem that the macroscopic deformation and failure of coal–rock medium under external loads are easy to be observed while the internal stress transfer mode and path are unclear. Based on the discrete element idea, the numerical models for pure coal or rock samples and coal–rock combination samples with different lithologies and combination methods under concentrated force are established by PFC2D software. Then the influence of coal or rock strength and combination methods on the internal stress transfer law and distribution evolution characteristics of coal–rock medium are discussed from the perspectives of macroscopic stress and mesoscopic force chain, respectively. The results showed that under concentrated load, the macroscopic stress transfer paths within pure coal or rock samples and coal–rock combination samples are primarily in the form of ‘point source radiation’. However, when transferring between coal–rock interfaces, there is a certain interface effect. For pure coal or rock samples, differences in lithology does not change the transfer rules and macro distribution patterns of internal stress, but it can cause changes in internal unit transfer stress value and local area transfer direction. For coal–rock combination samples, the greater the difference in lithology between the two sides of the interface, the more likely the interface effect will occur. In addition, the internal stress transfer is also influenced by the relative stratigraphic relationships of coal and rock. When the stress is transferred from a higher-strength rock to a lower-strength coal mass, the interface effect will be more significant. However, regardless of the combination pattern, the locations where significant stress surges occur are always within the higher strength rock mass near the interface. The findings are helpful to understand the mechanical properties and failure mechanism of mining coal and rock mass, and provide a theoretical basis for the study of the mining-induced mechanical behavior of the floor under the action of the coal pillar.

Study on stress distribution law of surrounding rock of roadway under the goaf and mechanism of pressure relief and impact reduction

To safely extract residual coal from a mine affected by rock bursts, the concept of “roadways under goafs” has been proposed as an effective layout. The pressure relief and shock reduction mechanism of roadways under goafs is elucidated through theoretical analysis, numerical simulation, and engineering verification. The stress model for goafs is established based on elastoplastic mechanics, followed by the construction of a force model for coal beneath the goafs. The stress composition of roof, floor and two sides of roadway under a goaf are analyzed. Using the FLAC-PFC coupling simulation method, the excavation process from top coal mining to bottom coal mining roadways of a No. 17 coal mine in Heilongjiang Province was simulated. The simulation results validate the spatiotemporal evolution of the stress and displacement in the surrounding rock mass of the roadway situated beneath goafs and determine the precise location of the roadway and length of its bottom coal face. A large diameter borehole of the roadway and presplitting blasting of the coal face roof are used to solve the local high stress danger area. Moreover, the roadway support mode of the “steel shed + spray arch support + monomer + π steel beam” is proposed. Finally, through the data of the field micro-seismic and stress monitoring system, the effectiveness of the pressure relief and shock reduction of roadways under goafs, the high efficiency of local danger relief of the roadways and working faces, and the practicability of the “steel shed + spray arch support + monomer + π steel beam” support of the roadways broken by surrounding rock are verified. A good basis for choosing a reasonable location for roadways and surrounding rock control methods for residual bottom coal mining and rock burst mining can be found in the research results.

Three-Dimensional Heterogeneity of the Pore and Fracture Development and Acoustic Emission Response Characteristics of Coal Rocks in the Yunnan Laochang Block

Studying the heterogeneity of coal reservoirs is significant to coal bed methane (CBM) exploitation. To investigate the development of the pore–fracture and acoustic emission response characteristics of the coal rock in the Yunnan Laochang block, four cores were extracted from the same coal rock in different directions. Through a comprehensive analysis using CT scanning and three-axis compression tests combined with synchronous acoustic emission experiments, a three-dimensional visualization of the pore–fracture structure and an analysis of the acoustic emission process during the elastic phase were conducted. Additionally, the impact of the heterogeneous development of pore–fractures on the acoustic emission characteristics was discussed. The results show that: there is strong heterogeneity in pore and fracture development within the coal rock, with the most significant development occurring along the direction of vertical stratification; the acoustic emission process in the elastic phase can be divided into three stages: strong–weak–strong; the development of pores and fractures affects the acoustic emission characteristics, with both counts and signal strength increasing as the percentage of voids rises; and the inferred in situ stress aligns with strike-slip faulting stress using acoustic emission. These results can provide a reference for the actual project.

Features of rare earth elements geochemistry in coals of Central Kazakhstan

AbstractThis research presents the results of a comprehensive study of mineralogical and geochemical features of REE distribution in coals of Central Kazakhstan deposits—Karaganda coal basin and Shubarkol deposit, which have large hard coal reserves and are industrially important for the coal industry of Kazakhstan; the research is based on 205 samples of clayey interlayers and coal seams. It shows basic patterns of distribution and features of concentration for impurity elements, gives an estimate of the impurity elements concentration, including REE, defines conditions and factors of their accumulation, and studies features of their forms in coal and coal-bearing rocks, which allows estimating the mechanisms of their migration and conditions of accumulation. According to the results of geochemical indicators, the article establishes the factors of REE dislocation, reveals the composition of margin rocks that have influenced REE concentration in coal seams, and the presented latest data on mineralogy allowed to establish the ways of their transportation to the paleobasin during the syn- and epigenetic periods of formation of the coal deposits of Central Kazakhstan being researched. It was found that the coals are insignificantly enriched with heavy lanthanides from Ho to Lu. The distribution curves of UCC normalized REE values in the coals are similar and coincide, but they are less than the average value for world coal, and amount to only one-third of the UCC. It was found that the highest concentrations of all REE are characteristic of clayey interlayers and oxidized coals. The La/Yb ratio in this case increases upwards along the section, indicating mainly clastogenic mechanism of REE delivery to the coals. In coal and clay samples, the predominant mineral form of REE is light lanthanide phosphates. Identified particles of REE from minerals and their composition peculiarities suppose autigene nature of their formation. The formation of the bulk of autigene minerals occurred during the maturation of brown coals and their transformation into hard ones.

Mechanical-permeability characteristics of composite coal rock under different gas pressures and damage prediction model

Determining the influence of gas pressure on the mechanics, permeability, and energy evolution of gas-bearing composite coal is helpful to better understand the formation process and prevention measures of gasdynamic disasters. In this paper, true triaxial mechanical-permeability tests are carried out on the gas-bearing composite coal rock under different gas pressures, focusing on the influence of gas pressure on the mechanics, permeability, and energy response characteristics of the composite coal rock, and a damage constitutive model based on energy dissipation is established. The results show that increasing the gas pressure decreases the load bearing capacity, strain, pre-peak relative permeability, and deformation capacity of the sample. The greater the gas pressure is, the greater the relative permeability decreases and the greater the post-peak relative permeability increases. The gas pressure has a great influence on the energy of the sample. The elastic strain energy ratio (Ue/U) increases with the increase in gas pressure, and the dissipative energy ratio (Ud/U) decreases with the increase in gas pressure. The coal-rock composite constitutive model based on energy dissipation is in good agreement with the experimental curves.

Geological exploration of coal mine burnt rock and waterlogged area boundary based on transient electromagnetic and high-density electrical resistivity

The water-rich burnt rock may threaten the safe production of coal mines. Identifying the boundaries of burnt rock and the water-rich area is of great practical significance for the ensuring the safety of mining operations. Transient electromagnetic and high-density resistivity methods are commonly employed in geophysical exploration, such as for investigating the presence of groundwater or delineating boundaries of altered rocks. These methods are non-invasive and provide detailed information about subsurface conditions without the need for drilling or excavation. The Jiangjun Gobi No. 1 open-pit coal mine is situated in the Kalamaili fault zone and is characterized by a high groundwater content. In certain mining sites within the study area, the inflow of water reaches tens of thousands of cubic meters per day, which significantly impacts production and presents major risks. To accurately determine the boundaries of burnt rock and water accumulation areas in the Jiangjun Gobi No. 1 open-pit coal mine, this paper uses different geophysical prospecting methods based on the depth of the strata. The middle and deep parts are investigated using the high-resolution transient electromagnetic method, while the shallow parts are examined using the high-accuracy high-density electric method. Through analyzing the electrical characteristics of the study area, it is inferred that the low-resistivity area in the northwest represents a shallow surface water-rich region. This area extends continuously towards the northwest, is not trapped, and is supplied by surface water. The deep low-resistivity zone primarily consists of sandstone and coarse sandstone. It is inferred that the low-resistance area in the southern part of the study area is also a shallow surface water-rich region, extending towards the east and west sides, not trapped, and supplied by surface water. The deep low-resistivity zone mainly comprises a combination of sandstone, coarse sandstone, and burnt rock, with intermittent layers of mudstone and argillaceous sandstone. The boundary line of burnt rock (coal-bearing strata) is located in the south of the study area. The resistivity of burnt rock (coal-bearing strata) is higher than that of the surrounding rocks, and the resistivity of coal seams is slightly higher than that of sandstone with larger porosity. Estimating the boundaries of groundwater and altered rocks serves to prevent geological disasters and provides valuable information for mineral development and ecological protection.

Investigation of the Effect of Changing Water Heights on the Performance of a Solar Distillation System Using Crushed Porous Coal Rocks

Investigation of the Effect of Changing Water Heights on the Performance of a Solar Distillation System Using Crushed Porous Coal Rocks

Study on Pressure Relief, Pre-Splitting, Softening and Gas Extraction in Highly Tough and Extra-Thick Outburst Coal Seams with Hard Roofs: A Case Study

ABSTRACT With Yushutian Coal Mine as the engineering background, the feasibility of pressure relief, pre-splitting and softening (PPS) by adjacent seam mining and its impact on top coal caving and gas extraction were studied, and the technology was applied on site. Through theoretical analysis, numerical simulation and onsite investigation, the feasibility of using gob-side entry retaining technology to achieve continuous mining of adjacent seam (Lower 4# coal seam) for PPS of underlying coal rock was demonstrated. The onsite application result shows that after the mining of the Lower 4# coal seam, the periodic weighting step of the 110,504 working face is shortened by half; the goaf basically caves along with mining; and the maximum monthly mining rate rises to 97.3%. After high-level directional long boreholes gas extraction, the gas concentration in return air of the 110,504 working face declines to 0.3–0.35%. Through the application of PPS by adjacent seam mining, the mining rate of the working face jumps significantly, and the gas concentration drops notably. Meanwhile, the coal and gas outburst coal seam is recovered safely and efficiently. The above research serves as a valuable reference for the safe and efficient mining of coal mines and enhance gas energy utilization with similar geological conditions.

Research on the Multiscale Microscopic Pore Structure of a Coalbed Methane Reservoir

Coal rock pores are the space in which coalbed gas is stored and flows. Accurately characterizing the pore structure of coalbed gas is the foundation of coalbed gas reserve assessment and production forecasting. Traditional experimental methods are unable to characterize the multi-scale pore structure characteristics of coal rock. In this paper, a multi-scale pore structure characterization method is proposed by coupling various experimental methods, including low-pressure nitrogen gas adsorption experiments, X-ray computed tomography (XCT) imaging technology, and scanning electron microscopy (SEM). Using Zhengzhuang coalbed gas as an example, the micro-pore structure of coalbed gas reservoirs is characterized and depicted from a multi-scale perspective. The results indicate that a single experimental approach can only partially reveal the microstructure of coal rock pores. The combined use of multiple methods can accurately reveal the full-scale microstructure of coal rock pores. The pore structure of the experimental coal rock samples exhibits multi-scale characteristics, with a complex variety of pore types, including inorganic pores, organic pores, and fractures. Organic pores are predominant, with a small number of inorganic pores, and their sizes range from 2 nm to 50 μm. Mineral particles and fractures are observed at both the nanoscale and microscale, exhibiting typical multi-scale characteristics, with quartz being the predominant mineral.

Coal and gas protrusion risk evaluation based on cloud model and improved combination of assignment

The proposed study presents an enhanced combination weighting cloud model for accurate assessment of coal and gas outburst risks. Firstly, a comprehensive evaluation index system for coal and gas outburst risks is established, consisting of primary indicators such as coal rock properties and secondary indicators including 13 factors. Secondly, the improved Analytic Hierarchy Process (IAHP) based on the 3-scale method and the improved CRITIC based on indicator correlation weight determination method are employed to determine subjective and objective weights of evaluation indicators respectively. Additionally, the Lagrange multiplier method is introduced to fuse these weights in order to obtain optimal weights. Subsequently, a prominent danger assessment model is developed based on cloud theory. Finally, using a mine in Hebei Province as an example, the results obtained from IAHP combined with improved CRITIC weighting method are compared with those from traditional AHP method and AHP-CRITIC combination weighting method. The findings demonstrate that among all methods considered, IAHP combined with improved CRITIC exhibits superior performance in terms of distribution expectation Ex, entropy value En, and super entropy He within cloud digital features; thus indicating that the risk level of coal and gas outbursts in this particular mine can be classified as general risk. These evaluation results align well with actual observations thereby validating the effectiveness of this approach. Consequently, this constructed model enables rapid yet accurate determination of coal and gas outburst risks within mines.

Feasibility Study of Material Deformation and Similarity of Spatial Characteristics of Standard Coal Rocks

The comparison between similar materials and original coal rock is the basis for similar simulation experiments in coal mines. The differences in mechanical properties, acoustic characteristics, and damage laws between similar materials and the original coal rock are of great significance for similar simulation research, to reveal objective laws. First, materials similar to coal rock with similar theoretical ratios were taken as the object of research, and the sand–cement ratio, the carbon paste ratio, and the water content were determined by multivariate linear regression to accurately match the ratios. Second, by using acoustic emission and digital scattering technology to explore the acoustic law, deformation characteristics, and spatial feature similarities of the materials similar to coal rock, the acoustic emission evolution law of the original rock was found to be the same as that of the similar materials. Digital scattering was able to describe the localization of strain in the similar materials, and the correlation between the overall deformation and the local deformation was explored. This indicates that materials similar to coal rock can effectively simulate the deformation of actual coal rocks. Lastly, these materials were found to allow effectively simulating the deformation characteristics and spatial properties of actual coal rock, which provides an important experimental means and method for similar research in the field of coal rock engineering.

Characterization of the Time–Space Evolution of Acoustic Emissions from a Coal-like Material Composite Model and an Analysis of the Effect of the Dip Angle on the Bursting Tendency

Rock bursts pose a grievous risk to the health and lives of miners and to the industry. One factor that affects rock bursts is the dip angle of the coal seam. Because of the uniquely high gas content of the coal in a mine in Shanxi Province, China, coal specimens were obtained from this mine to produce coal–rock combination specimens and test the effects of various seam inclinations. Using a DYD-10 uniaxial compression system and a PCI-8 acoustic emission (AE) signal acquisition system, we investigated the spatial and temporal evolution characteristics of the burst tendency of specimens with different coal seam inclination angles (0°, 10°, 20°, 30°, 35°, 40°, and 45°). Uniaxial pressure was applied to the specimens, and we found that, as the inclination angle increased, the coal–rock combination specimens exhibited structural damage and destabilization, which was attributed to the generation of an interface slip phenomenon. In all tests, the coal exhibited greater damage than the rock. There was an energy convergence at the coal–rock interlayer interface, which was the main carrier for the accumulated energy. The impact energy dissipation index is defined according to the energy dissipation properties of the loading process of coal–rock composites. As the inclination angle increased, the impact energy dissipation index, energy storage limit, compressive strength, elastic modulus, and other indexes gradually decreased. This effect was strongest where the angles were 40° and 45°. The indexes used to assess the impact propensity decreased to a notable degree at these angles, revealing that the burst tendency of coal–rock is curtailed as the inclination angle increases. The results of this research are of great importance to the early evaluation of mine burst risks and the sustainable development of coal utilization.

Gas transport model and numerical solution in roof rock based on the theory of free gas diffusion

Accurately identifying the source of coalbed methane is of great practical significance in preventing gas concentration overruns and improving extraction efficiency. At present, it is widely believed that underground gas mainly comes from coal desorption, and the mechanism of gas flow inside the coal seam is explored in depth, while the gas released inside the rock is often neglected, and there is insufficient understanding of its internal transportation law. In this study, the density gradient theory (FGDGD) in the coal matrix was used as a guide to characterize the adsorption and transport properties of gases within the rock. The anthracite and mudstone from the same roadway in Yuxi mine were selected for the experiments, and coal rock samples with particle sizes of 0.425 mm–0.25 mm were ground. The adsorption constant test and adsorption process experiments were carried out under the constant volume condition, and the transport behavior of gas within the rock was simulated and numerically solved, and the content and flow behavior of gas in the rock were discussed in detail. The results are as follows: (i) The simulation curves based on the density gradient model fit well with the experimental data for both coal and rock downholes, indicating that the gas transport law inside the rock downhole is also in accordance with the density gradient theory, which broadens the scope of application of the theory. (ii) The free gas diffusion coefficients Dks (rock) are smaller than Dks (coal) for the gases in the rock matrix and coal matrix, which may be due to the difference in the pore structure of the rock matrix, resulting in lower adsorption sites than those of the coal. (iii) Although the gas content inside the rock is relatively low compared with that of coal, it is significant for reducing the prediction error of gas influx in engineering practice. Therefore, it is necessary to consider the storage of gas in rocks and its migration behavior when studying the sources of gas emissions in engineering fields.

Quantitative palaeogeographical reconstruction of the North China Block during the Carboniferous and Permian transition: Implications for coal accumulation and source rock development

The Carboniferous-Permian strata of the North China Block (NCB) contain significant economic reserves of coal and gas. In this paper, a comprehensive dataset comprising 401 stratigraphic sections encompassing the Carboniferous-Permian transition in the NCB is compiled from the OneStratigraphy and Geobiodiversity databases. We conducted quantitative analyses of this dataset using ArcGIS and GPlates software, and utilized an up to date plate motion model, to perform a dynamic palaeogeographical reconstruction. The spatio-temporal distribution of depositional facies were also analysed, including basement terranes, fluvio-lacustrine settings, swamps, shorelines, tidal flats, and carbonate platforms. Our reconstructions reveal that the NCB was flanked by basement areas of the Yinshan-Yanshan landmass in the north and the Qinling-Dabie landmass in the south. Peat (coal) deposition occurred across a range of terrestrial to coastal environments, including fluvio-lacustrine settings, deltas, shorelines and tidal flats. Carbonaceous and organic-rich clastic rocks also developed in these areas while organic-rich carbonate rocks were primarily deposited in carbonate platform. The conditions conducive to peat (coal) formation were intricately connected to eustatic fluctuations, which resulted in frequent transgressions and regressions. The sea-level fluctuations during the Pennsylvanian might be, in part, a response to both the waxing and waning of continental ice sheets and tectonic process in the northern margin of the NCB. In contrast, sea-level changes during the early Permian were probably controlled by the tectonic process only. This paper provides a state-of-the-art review of palaeogeographical evolution of the NCB during the Carboniferous and Permian transition, in response to eustatic and palaeoclimate change, and sheds light on resource accumulation based on quantitative analyses.

Study on Numerical Simulation of Gas–Water Two-Phase Micro-Seepage Considering Fluid–Solid Coupling in the Cleats of Coal Rocks

The increasing demand for natural gas energy will promote unconventional natural gas, such as coal seam gas and shale gas, to play a key role in future energy development. The mechanical properties of coal seams are weaker compared with conventional natural gas reservoirs. The fluid–solid coupling phenomenon exists widely at the pore scale and macro scale of coal seams, and runs through the whole process of coalbed gas exploitation. The objective of this study is to establish a microscale gas–water flow model for coalbed methane considering fluid structure coupling. Frist, this study used scanning electron microscopy (SEM) to obtain microscopic pore images of coal rocks. Then, we constructed a numerical model to simulate the movement of coalbed methane and water within the scale of coal cleats based on the Navier–Stokes equation, phase field method, and solid mechanics theory. Finally, we analyzed the effects of injection pressure and wettability on the microscopic two-phase seepage characteristics and displacement efficiency of coal. Our research shows that when the injection pressure is increased from 60 kPa to 120 kPa, the displacement completion time is shortened from 1.3 × 10−4 s to 7 × 10−5 s, and the time is doubled, resulting in a final gas saturation of 98%. The contact angle increases from 45° to 120°, and the final gas saturation increases from 0.871 to 0.992, an increase of 12.2%.

Cracks Propagation and Permeability Enhancement Caused by Supercritical CO2 and Water Fracturing in Natural Coal–Rock Combination

Cracks Propagation and Permeability Enhancement Caused by Supercritical CO₂ and Water Fracturing in Natural Coal–Rock Combination