Deep Coal Research Articles

Real-time pore structure evolution difference between deep and shallow coal during gas adsorption: A study based on synchrotron radiation small-angle X-ray scattering

Coal seam CO2 sequestration is an important option to address global warming. A better knowledge on coal pore structure evolution during gas adsorption can provide guidance for coal seams CO2 sequestration. However, few investigations on the pore structure evolution differences between the deep and shallow coal were conducted during gas adsorption. In this study, based on the real-time synchrotron radiation small-angle X-ray scattering (SAXS) observation, the average pore diameter and pore surface fractal dimension evolution differences between deep and shallow coal were investigated from the aspects of coal compositions and stress history. Two types of coal deformation (inner-swelling and outer-swelling) coexist during gas adsorption. Coal compositions have significant impact on the dominance of deformation type. The dominance of inner-swelling in deep coal is induced by the higher ash contents, and there is the decrease of average pore diameter during gas adsorption. The impact of stress-history (burial depth) on adsorption-induced deformation is more prominent than that of gas adsorption capacity. In deep coal, the surface fractal dimension evolution presents a negative correlation with the evolution of pore diameters. In shallow coal, the surface fractal dimension evolution presents a Langmuir-type correlation with the adsorption time.

Influence of microstructure characteristics of deep mine coal on oxidation heat effect

Influence of microstructure characteristics of deep mine coal on oxidation heat effect

Case Study of Rock Bolting in a Deep Coal Mine in Poland

Case Study of Rock Bolting in a Deep Coal Mine in Poland

Thickness and Strength Analysis of Prestressed Anchor (Cable) Compression Arch Based on Safe Co-Mining of Deep Coal and Gas

The stability of the gas extraction roadway is very important for the safe mining of coal and gas. The compression arch formed by the combined action of the prestressed bolt (cable) support and surrounding rock has been widely used in the engineering practice of the gas extraction roadway. It is of great engineering application value to analyze the influence of prestressed bolt (cable) parameters on the compression arch. In this paper, combined with the engineering practice of the deep roadway in Huainan and Huaibei mining area of Anhui Province, the mechanical parameters of surrounding rock are measured via field coring and the laboratory. The numerical simulation software FLAC3D is used to analyze the typical position of fractured mudstone, mudstone, sandy mudstone and muddy sandstone under the bolt pre-tightening force of F = 50 kN, 70 kN and 100 kN; the bolt spacing of a × b = 400 mm × 400 mm, 500 mm × 500 mm and 600 mm × 600 mm; the bolt length of L = 1500 mm, 2000 mm, 2600 mm and 3000 mm; and the distribution characteristics of additional compressive stress on the surface of the side. The influence of the different lithology and bolt parameters on the thickness and strength of the compression arch was analyzed, and on this basis, prestressed anchor cables with a pre-tightening force of F = 80 kN, 100 kN and 120 kN and length of L = 3000 mm, 4000 mm and 6000 mm were applied, and their influence on the thickness and strength of the compression arch was analyzed. The results show that the bolt pre-tightening force (F) and the bolt length (L) have a significant effect on the thickness of the compression arch, while the surrounding rock lithology, the bolt spacing (a × b), the anchor cable pre-tightening force (F) and the anchor cable length (L) have no obvious effect on the thickness of the compression arch. The surrounding rock lithology, the bolt pre-tightening force (F), the bolt length (L), the bolt spacing (a × b), the anchor cable pre-tightening force (F) and the anchor cable length (L) have a significant effect on the strength of the compression arch.

Application research on the prediction of tar yield of deep coal seam mining areas based on PSO-BPNN machine learning algorithm

There are abundant deep coal resources in northern Shaanxi, but the fragile natural environment in this area hinders the large-scale exploitation of oil-rich coal. In-situ thermal conversion of deep coal to oil and gas will become an environmentally friendly technology for oil-rich coal mining. Accurate prediction of oil-rich coal tar yield in various regions is a prerequisite. Based on a particle swarm optimization algorithm and two machine learning algorithms, BP neural network and random forest, a prediction model of tar yield from oil-rich coal is constructed in this paper. Combined with the particle swarm optimization method, the problem of slow convergence speed and possibly falling into local minimum value of BP neural network is solved and optimized. The results showed that the PSO-BP had a convergence speed about five times faster than that of the BP neural network. Furthermore, the predicted value of the PSO-BP was consistent with the measured value, and the average relative error was 4.56% lower than that of the random forest model. The advantages of fast convergence and high accuracy of the prediction model are obviously apparent. Accurate prediction of tar yield would facilitate the research process of in-situ fluidized mining of deep coal seams.

Field pilot testing and reservoir simulation to evaluate processes controlling CO2 injection and associated in-situ fluid migration in deep coal

A key strategy for reducing global CO2 emissions is geologic sequestration of CO2. Deep, unmineable coal seams have been proposed as possible targets for CO2 injection and storage because of the global distribution of coal resources, and relative security of CO2 storage associated with CO2 adsorption. However, the current paradigm is that near-injector permeability loss caused by pure CO2 adsorption-induced coal swelling limits the effectiveness of this process making coal a less than an ideal target for CO2 injection/sequestration.In order to test the viability of CO2 injection and storage in deep coal seams, and evaluate processes controlling CO2 injection, migration and storage therein, a field pilot was implemented in the Mannville Coal (∼1500 m depth) of Alberta, Canada. An additional objective of the field pilot was to quantify the migration of water and natural gas away from the CO2 injection well, an important mechanism used to improve the performance of the pilot operator's enhanced hydrogen recovery (EHR™) process. To achieve these multiple objectives, a vertical injection well and closely-spaced offsetting vertical observation well were drilled, cored, and completed in the Mannville coal seams, and instrumented with pressure sensors to monitor both the coal and bounding zones during injection testing. Periodic fluid sampling at the observation well allowed for quantification of CO2 plume and fluid migration. Pre-CO2 injection/falloff testing was performed with water at variable injection rates/pressures to assess coal permeability/porosity changes without the complication of CO2-coal interaction. CO2 injection/falloff tests were then performed at increasing CO2 injection rates to assess the combined impact of injection pressure/CO2 rate and CO2-coal interaction (e.g., adsorption) on injectivity. All tests were history-matched with a numerical reservoir simulator; the calibrated model was used to study the key controls on CO2 storage, CO2 plume migration, in-situ fluid displacement, and injectivity.A key finding from simulation of the pilot results is that, for the Mannville coals in the study area, permeability increases caused by coal natural fracture dilation during injection outweigh permeability decreases caused by CO2 adsorption-induced coal swelling, resulting in an injectivity index increase with CO2 injection rate. In addition, the coals exhibit strong anisotropy in permeability and geomechanical properties; ignoring coal anisotropy in the simulation model causes coal swelling effects to be overestimated, resulting in an underestimate in CO2 plume extent.From the pilot results and analysis, it can be concluded that the deep Mannville coal could be a viable target for CO2 storage.

Coal microstructural and mechanical alterations induced by supercritical CO2 exposure: Role of water

Injection of CO2 into deep unminable coal reservoirs is regarded as an effective method to reduce greenhouse gas emissions. Coal seams with burial depth > 800 m are the potential reservoirs for CO2 sequestration, and CO2 is stored in the form of supercritical state. The interactions between supercritical CO2 (ScCO2) and coal are a complex process, which has influence on coal structures and strength. Recent studies are rarely reported regarding the impact of ScCO2-coal-water reactions on coal microstructures and mechanical strength. In this paper, comparative analysis was conducted on the microstructural changes and mechanical degradation of coal induced by ScCO2 treatment and ScCO2-water saturation. The results show that water has great influence on coal pore structure, functional groups, mineral compositions and mechanical properties during the reaction. ScCO2 saturation only triggers a small decline of 11.27–17.65% in aliphatic groups in coal, while approximately 62.75–85.92% of aliphatic groups are extracted by ScCO2-water. The largest hysteresis loops are found in samples saturated by ScCO2-water, followed by the specimens treated by ScCO2, illustrating that the complexity of coal pore structure is enhanced by ScCO2 interaction, and the presence of water can further increase the pore roughness. The UCS is reduced by < 13% after ScCO2 saturation on dry coal, but it is greatly decreased by 26.58–44.21% in the presence of water. In the process of interaction, water accelerates the hydrocarbon extraction and mineral dissolution to aggravate the degradation of coal strength. This work is conducive to understanding the process of CO2 geological sequestration.

Analysis of the Thermal Behavior Characteristics and Dynamics of Coal Under High Primary Temperature Conditions in Deep Mines

ABSTRACT The exposure to spontaneous combustion of coal subjected to high ground temperatures increases as the depth of mine excavation increases. To research the oxidative heat release characteristics of deep coal, a study was carried out to characterize the oxidation kinetics and thermal effects on coal in deep thermal environments; deep coals (heat-treated coals) in 30°C (T30), 45°C (T45) and 60°C (T60) environments were prepared using a simultaneous thermal analysis, and the thermal weight characteristics (TG) and thermal release characteristics (DSC) of heat-treated coals were tested by simulating coal samples in different deep thermal environments; the thermal effects of heat-treated coals were analyzed by oxidation kinetic theory. The findings indicated that the characteristic point temperature of the coal gradually reduces (T60 has the lowest oxidation characteristic temperature) as the heat treatment temperature increases, and the maximum mass loss (63.76%) during thermal treatment coal oxidation occurs in the combustion stage. The oxidation reaction mechanism function for the low-temperature oxidation phases of the deep coal conforms to the Z-L-T equation with a three-dimensional diffusion reaction mechanism. As the temperature of the thermal environment is increased, the apparent activation energy of the coal decreases and the exothermic heat increases. This increases the tendency for oxidation of the deep coal, making it more amenable to spontaneous combustion as the ambient temperature at depth rises. The results contribute to a theoretical framework for monitoring and preventing coal spontaneous combustion fires in deep mine excavations.

Assessment of CO2 geological storage capacity based on adsorption isothermal experiments at various temperatures: A case study of No. 3 coal in the Qinshui Basin

Carbon dioxide (CO 2 ) capture, utilization, and storage (CCUS) is an important pathway for China to achieve its “2060 carbon neutrality” strategy. Geological sequestration of CO 2 in deep coals is one of the methods of CCUS. Here, the No. 3 anthracite in the Qinshui Basin was studied using the superposition of each CO 2 geological storage category to construct models for theoretical CO 2 geological storage capacity (TCGSC) assessment, and CO 2 adsorption capacity variation with depth. CO 2 geological storage potential of No. 3 anthracite coal was assessed by integrating the adsorption capacity with the static storage and dissolution capacities. The results show that (1) CO 2 adsorption capacities of XJ and SH coals initially increased with depth, peaked at 47.7 cm 3 /g and 41.5 cm 3 /g around 1000 m, and later decreased with depth. (2) four assessment areas and their geological model parameters were established based on CO 2 phase variation and spatial distribution of coal thickness, (3) the abundance of CO 2 geological storage capacity (ACGSC), which averages 40 cm 3 /g, shows an analogous circularity-sharp distribution, with the high abundance area influenced by depth and coal rank, and (4) the TCGSC and the effective CO 2 geological storage capacity (ECGSC) are 9.72 Gt and 6.54 Gt; the gas subcritical area accounted for 76.41% of the total TCGSC. Although adsorption-related storage capacity accounted for more than 90% of total TCGSC, its proportion, however, decreased with depth. Future CO 2 -ECBM project should focus on high-rank coals in gas subcritical and gas-like supercritical areas. Such research will provide significant reference for assessment of CO 2 geological storage capacity in deep coals.

Experimental Investigation on the Influence of Temperature on Coal and Gas Outbursts

With the increasing mining depth, the dynamic disaster of coal and gas outbursts in coal mines has become increasingly prominent, and the bursting liability of coal and rock mass in deep coal seam mining is a necessary condition for the occurrence of rock burst and an important index to measure the failure of coal and rock mass. Thermal damage leads to rock instability and failure, which seriously influences the safe and efficient operation of coal mines. To investigate the effect of thermal damage on the bursting liability of deep coals, the burst tendency index of standard coal was measured after subjecting it to thermal damage at different temperatures. The effects of different thermal damage temperatures on the uniaxial compressive strength index, dynamic failure duration, stiffness ratio index, effective impact energy index, residual energy index change rate, and impact energy velocity of the coal and the influence of the post-peak failure mode of the coal were evaluated. The results revealed that the uniaxial compressive strength of the coal generally decreased with increasing thermal damage temperature. At temperatures above 200 °C, the strength significantly decreased. The comprehensive impact property index indicated that, with increasing thermal impact temperature, the burst tendency first increased up to the peak value at 200 °C and then gradually decreased. With the increase in the thermal damage temperature, the burst tendency decreased and disappeared in the temperature range of 250–300 °C, and the failure mode of the coal changed from brittle failure to brittle plastic failure, and finally ductile failure. The influence of thermal damage on coal bursting liability is studied, which provides a theoretical basis for preventing and controlling coal impact ground pressure hazards.

Methane Adsorption in Anthracite Coal under Different Pressures and Temperatures—A Study Combining Isothermal Adsorption and Molecular Simulation

In situ gas content is an important parameter associating coalbed methane, while the influence of pressure and temperature on methane adsorption and desorption still needs to be revealed. In this study, the molecular structure and methane adsorption capacity of anthracite coal collected from Diandong Coalfield (China) were studied based on 13C nuclear magnetic resonance (13C NMR), Fourier transform infrared spectroscopy (FT-IR), and methane isothermal adsorption experiment. The results show that the carbon skeleton of coal sample is mainly composed by aromatic carbon (72%), followed by aliphatic carbon structure (14.2%). Carbons connected to the oxygen atoms contribute 13.7% of the total carbons in coal molecule, and the oxygen atoms are mainly in the form of carbonyl. The 2-dimension structure and 3-dimension molecular structure of coal sample was also reconstructed. The average chemical formula of the coal molecule is C200H133O21N3. The experimental methane adsorption isothermal data of the coal sample under different temperatures shows that with increasing the temperature, the methane adsorption amount at each pressure decreases obviously. At 7 MPa and 20°C, the methane adsorption amount of the coal sample is 28.5 cm3/g. Comparably, at 100°C and 7 MPa, the methane adsorption amount is only 15.9 cm3/g, decreasing by 44%. In mesopores, temperature has stronger influence on methane adsorption under higher pressure than that of lower pressure. On the contrary, in micropores, temperature has weaker effects on methane adsorption at higher pressure than that at lower pressure. The results can be beneficial for understanding methane adsorption characteristics of deep coal.

Mechanism of the coal bursts in the working face during mining of steeply inclined and extra thick coal seam

Coal bursts caused by mining-induced seismicity occur in the working face in steeply inclined and extra thick coal seams (SIETCSs). This article presents a case study in SIETCS through discrete element simulation as well as field investigation. The evolution of the coal burst was studied from the perspective of stress distribution, crack propagation and coal fragment ejection. The results showed that dynamic failure mainly occurred in the coal near the roof. The deep bottom coal and the top coal near the roof are under high static stress due to the clamping effect of the roof and floor and the rotation of the deep roof. The coal burst in the working face was triggered by mining-induced seismicity. The coal burst occurred when the sum of the static stress and the dynamic stress caused by the mining-induced seismicity exceeds the sum of the ultimate strength of the coal and the support strength, which was called the ‘clamping-rotation-dynamic loads’ mechanism. During the coal burst, the cracks developed rapidly into the deep bottom coal and the top coal on the roof side. The results can serve as an important reference for the coal burst failure mechanism in the working face in SIETCSs.

Difference in Surface Damage between Deep and Shallow Mining of Underground Coal Resources in China

The mining of underground coal resources often results in extensive damage to the ground surface, particularly in China, which has a large amount of coal resources. However, the laws of surface damage caused by the mining of deep and shallow underground coal resources are relatively different. This study analyzes the difference in surface damage induced by deep and shallow mining and its mechanism by field measurement and similar material simulation experiments. Surface damage is mainly manifested in the form of cracks, which can be categorized as permanent and dynamic cracks. Permanent cracks occur above the mining boundary of shallow and deep coal mines. Dynamic cracks (including dynamic stretching cracks and stepped cracks) only appear above the goaf in shallow mining. This disparity is due to the fact that strata movement in deep mining occurs in a “three zones” mode, with the failure height of strata increasing with the mining degree in an “S” shape. However, rock strata movement in shallow mining follows a “two zones” mode, with the failure height of rock strata increasing exponentially with the mining degree. Thus, the rock strata are prone to slide and become unstable in the form of benched rock beams, producing dynamic cracks, such as stepped cracks on the surface. This research improves the existing mining subsidence theory and provides technical support for relevant mines to take targeted treatment measures.

Hydromechanical Impacts of CO2 Storage in Coal Seams of the Upper Silesian Coal Basin (Poland)

Deep un-mineable coal deposits are viable reservoirs for permanent and safe storage of carbon dioxide (CO2) due to their ability to adsorb large amounts of CO2 in the microporous coal structure. A reduced amount of CO2 released into the atmosphere contributes in turn to the mitigation of climate change. However, there are a number of geomechanical risks associated with the commercial-scale storage of CO2, such as potential fault or fracture reactivation, microseismic events, cap rock integrity or ground surface uplift. The present study assesses potential site-specific hydromechanical impacts for a coal deposit of the Upper Silesian Coal Basin by means of numerical simulations. For that purpose, a near-field model is developed to simulate the injection and migration of CO2, as well as the coal-CO2 interactions in the vicinity of horizontal wells along with the corresponding changes in permeability and stresses. The resulting effective stress changes are then integrated as boundary condition into a far-field numerical model to study the geomechanical response at site-scale. An extensive scenario analysis is carried out, consisting of 52 simulation runs, whereby the impacts of injection pressures, well arrangement within two target coal seams as well as the effect of different geological uncertainties (e.g., regional stress regime and rock properties) is examined for operational and post-operational scenarios. The injection-induced vertical displacements amount in maximum to 3.59 cm and 1.07 cm directly above the coal seam and at the ground surface, respectively. The results further demonstrate that neither fault slip nor dilation, as a potential consequence of slip, are to be expected during the investigated scenarios. Nevertheless, even if fault integrity is not compromised, dilation tendencies indicate that faults may be hydraulically conductive and could represent local pathways for upward fluid migration. Therefore, the site-specific stress regime has to be determined as accurately as possible by in-situ stress measurements, and also fault properties need to be accounted for an extensive risk assessment. The present study obtained a quantitative understanding of the geomechanical processes taking place at the operational and post-operational states, supporting the assessment and mitigation of environmental risks associated with CO2 storage in coal seams.

Influencing Factors and Adaptability of Working Face Length in Deep Thick Coal Seam

Influencing Factors and Adaptability of Working Face Length in Deep Thick Coal Seam

Stability Analysis of Roof and Rib and Source Control Countermeasures of Ultrahigh Roadway in Deep Coal Mine

Driven by the high original rock stress and engineering disturbance, the ultrahigh roadways of about 7∼8 m appear in the deep coal mine. Based on the field measured data, the stability in different zones of roof and rib is divided. From shallow to deep, the roof forms potential leakage and falling zone, metastable zone, and stable zone, and the coal rib forms potential spalling zone, crack developed zone, and transition zone. Based on the theory of transverse isotropy, the mechanical analysis model of layered roof is established, the real stress state at any point of layered roof is deduced, the roof failure criterion is constructed, and the theoretical solution of the failure depth of layered roof is obtained. The mechanical model of pillar bar of the splitting coal body in the potential spalling zone is established, and the deflection deformation expression of the pillar bar is derived. In view of the deformation and failure characteristics of the roof and rib, this paper puts forward the cooperative integrated support technology based on high-strength prestressed cables and deep-shallow hole grouting, which effectively reduce the hidden danger of roof falling and eliminate the formation of ultrahigh roadway from the source.

Impact of liquid nitrogen freeze–thaw on multistage gas flow: differences between shallow and deep coal in Qinshui Basin

Liquid nitrogen freeze–thaw (LNFT) is an environmentally-friendly permeability enhancement technology. There are significant differences between shallow and deep coal in their composition and stress history. Therefore, it is necessary to reveal the difference in the impact of LNFT on the multistage gas flow between shallow and deep coal. In this paper, the differences in multistage gas flow were revealed from the perspective of a multiscale structure evolution, and the differences in the multiscale structure evolution were analyzed from the coal compositions and stress history. Nuclear magnetic resonance method was used in the multiscale structure evolution characterization. As for the multistage gas flow evolution, gas ad/de-sorption, diffusion, and seepage evolution characterization were investigated. There is a quadratic relationship between the total porosity and LNFT cycles in both shallow and deep coal. The fracture expansion is dominant in deep coal, while pores connectivity enhancement is dominant in shallow coal. The permeability of both shallow and deep coal increases during the LNFT cycling, presenting a logarithmic correlation between permeability and the number of cycles. With the same porosity, shallow coal has a higher permeability. LNFT cycling can improve the Langmuir volume. Langmuir volume and gas diffusion coefficients of deep coal is smaller both pre- and post-LNFT cycling. Except for CH4 in deep coal, the Langmuir pressure and gas diffusion coefficients in both coals are improved by LNFT cycling.

Modified Method for Calculating Saturation Gas Content in Deep Coal and the Pore Size Effect of Methane Adsorption on Guizhou Middle- and High-Rank Coals

Modified Method for Calculating Saturation Gas Content in Deep Coal and the Pore Size Effect of Methane Adsorption on Guizhou Middle- and High-Rank Coals

The shaped blasting experimental study on damage and crack evolution of high stress coal seam

In the deep high stress coal and rock mass, the efficiency of the ordinary blasting method is constantly affected by the high pressure. This study investigates the shaped blasting technology with increased blasting efficiency, develops a mechanical model for coal crack expansion during shaped blasting, and examines the force-energy action of energy-gathering jets during coal splitting. The established model was combined with experiments and simulations to reveal the damage and evolution of cracks in coal and rock masses. Experiments demonstrate that shaped blasting can create an energy-gathering jet that causes a directional fracture in the test block. The peak of compressive strain in the energy-gathering direction is 1.12 times greater than that of ordinary blasting, and the static action period of the explosive gas is prolonged by 1.3 μs according to the stress curve. Moreover, the peak pressure strain increased 3.2 times, and the pull strain peak increased 3.92 times. The damage amount was represented by the resistivity and ultrasonic wave speed, and shaped blasting causes 1.2 and 1.5 times greater damage than ordinary blasting. The research results provide technical methods for safe and rapid penetration blasting in deep high-ground stressed coal mines.

The Characteristic Development of Micropores in Deep Coal and Its Relationship with Adsorption Capacity on the Eastern Margin of the Ordos Basin, China

The accurate description of micro-/nanopores in deep coal reservoirs plays an important role in evaluating the reservoir properties and gas production capacity of coalbed methane (CBM). This work studies nine continuous samples of high–rank coal from the Daning–Jixian area of the Ordos Basin. Maceral analysis, proximate analysis, field emission scanning electron microscopy (FE-SEM), low-pressure CO2 adsorption (LPA), low-temperature N2 adsorption (LTA) and high-pressure methane adsorption (HPMA) experiments were conducted for each sample. The fractal dimension (D) of the LPA data was calculated by using the micropore fractal model. The characteristics of the deep coal reservoir pore structure, proximate analysis, relationship between maceral and fractal dimensions, and gas adsorption capacity of the micropores are discussed. The results showed that the combination of LPA with nonlocalized density functional theory (NLDFT) models and LTA with NLDFT models can more accurately determine the pore size distribution of the micropores. The pore volume (PV) and specific surface area (SSA) of the coals were distributed in the ranges of 0.059~0.086 cm3/g and 204.38~282.42 m2/g, respectively. Although the degree of micropore development varies greatly among different coal samples, the pore distribution characteristics are basically the same, and the PV and SSA are the most developed in the pore size range of 0.4–0.7 nm. Ash content (Ad) and mineral composition are two major factors affecting micropore structure, but they have different impacts on the fractal dimension. The higher the vitrinite content, moisture content (Mad) and Ad are, the larger the micropore fractal dimension (D) and the stronger the heterogeneity of the pore structure. Micropores account for 99% of the total SSA in coal, and most methane can be adsorbed in micropores. The fractal dimension of micropores can be used to evaluate the pore structure characteristics. The larger the fractal dimension, the smaller the micro-SSA and micro-PV of the coal sample. Fractal analysis is helpful to better understand the pore structure and adsorption capacity of CBM reservoirs.