Dull Coal Research Articles

Pore structure and adsorption capability of marcolithotypes in the Weizhou mining area, western margin of Ordos Basin

Differences in the pore and fracture systems of four marcolithotypes (bright，semi‐bright, semi‐dull and dull) in coal reservoirs affect the quality of coalbed methane (CBM) and constrain exploration and development of CBM. In this paper, vitrinite reflectance (RO), maceral composition, proximate analysis, scanning electron microscopy (SEM), mercury intrusion porosimetry (MIP), low‐field nuclear magnetic resonance (NMR) and high‐pressure isothermal adsorption experiments and the mathematical method of fractal dimension were carried out on high‐rank coal samples of different marcolithotypes from the Weizhou mining area in the western margin of Ordos Basin. The results show that from bright coal to dull coal, the RO and vitrinite content gradually decrease, while the mineral content and Aad gradually increase. SEM images show that the bright and semi‐bright coals develop open fractures, whereas semi‐dull and dull coals develop closed fractures. According to pore size distribution established by MIP and NMR, the proportion of seepage pores gradually increased from bright coal to dull coal, and MIP fractal dimension of seepage pores (DM2; 2.3352, 2.3532, 2.3755 and 2.4727, respectively) and NMR fractal dimension of seepage pores (DN2; 2.8767, 2.9142, 2.9297 and 2.9981, respectively) show that the structure of the seepage pore is progressively more complex from bright coal to dull coal. In addition, percentage of NMR adsorption pores progressively increases from bright coal to dull coal (64.96%, 76.64%, 82.04% and 89.67%, respectively), but total porosity of bright coal (6.74%) is much greater than that of dull coal (1.34%), which results in that bright coal also has the strongest CH4 adsorption capability (Langmuir volume is 22.898 cm3/g).

Pore-fracture and permeability heterogeneity of different marcolithotypes of medium-rank coals in Jixi Basin, China

The pore-fracture system of different marcolithotypes (bright, semi-bright, semi-dull and dull coals) is quite different, which has great influences on coalbed methane (CBM) exploitation. Here the optical microscope (OM), scanning electron microscope (SEM), mercury intrusion porosimetry (MIP), low-temperature nitrogen adsorption (LTND) and X-ray computed tomography (X-CT) are performed on different marcolithotype coals from Jixi Basin. From bright coals to dull coals, “ink bottle” shaped pores account for a gradually increasing proportion of the transition pores, but contribute fewer mesopores. The results of LTND and MIP indicate that bright coals have the poorest mercury removal efficiency and lowest BET specific surface area, while the opposite appears to be the case for dull coals. According to the pore size distribution based on LTND and MIP, from bright coal, semi-bright coal, semi-dull coal to dull coal, the total pore volume gradually decreases, with the proportion of macropore volume occupying 76.2%, 5.1%, 3.7% and 1.7%, respectively. On the basis of the 3D reconstruction of the pore-fracture system obtained from X-CT, the bright coal is characterized by high connected porosity and total porosity, presenting the best pore-fracture system. The pore network of bright coal is three-way interconnected with the largest number of pores and the shortest and densest throats, which facilitates the communication of fluids between the pores. In contrast, dull coals with a unidirectionally connected pore network model, fewer pores and sparse throats, have a poor seepage capacity. In addition, the X-CT seepage simulation results and the gas permeability results confirm the superiority of the pore-fracture system in bright coals.

Experimental study on the effect of high-temperature nitrogen immersion on the nanoscale pore structure of different lithotypes of coal

This paper presents the heat treatment study on different lithotypes of coal using a custom designed high-temperature thermal nitrogen immersion experimental setup. The results revealed that the heat treatment significantly increased the surface roughness of coal particles and the structural complexity. The pores with diameters >200 nm of coal increased after heat treatment. Some of the new fissures still retain the pore morphology, exhibiting the pore connections. With increasing heat treatment temperature, the total pore volume (TPV) and specific surface area (SSA) of all four selected coal types tended to decrease. Thermal action also significantly increased the average and maximum pore diameters, and the most obvious effect was observed after heat treatment at 120 °C and 210 °C. After heat shock at 210 °C, the average diameters of bright and semibright coal samples were enlarged by 3.5 and 2.7 times, respectively. The semidull coal enlarged 1.6 times, and the dull coal enlarged 1.2 times. In addition, the fractal dimension, which represents the surface roughness of pores, continued decreasing as the heat treatment temperature increases. The pore structure fractal dimension first tends to increase, then decrease, and finally increase again. Therefore, the heat treatment enhanced connectivity and increased fracture, facilitating the transport of methane.

Effect of interlayer mechanical properties on initiation and propagation of hydraulic fracturing in laminated coal reservoirs

Coal macrolithotypes control the heterogeneity of coal physical properties in coalbed methane (CBM) reservoirs and have a significant effect on the success of hydraulic fracturing stimulation. However, only a few studies have focused on this heterogeneity, and the propagation mechanism of hydraulic fractures related to the coal macrolithotype is not well understood. In this study, we considered the Hancheng area, Ordos Basin, China, as an example to understand macrolithotype differences, establish finite element numerical models of the cohesive zone, and evaluated the hydraulic fracture initiation and vertical propagation behavior of laminated coal reservoirs. We used physical experiments, such as direct shear test and Digital Image Correlation (DIC), along with finite element simulation and a finite element model; the results show that the tensile strength of bright coal is the lowest and that of dull coal is the greatest; the average cohesion and shear strength of the bright-dull coal interface are 0.418 MPa and 1.3778 MPa (σ = 3 MPa), respectively. Notably, behavioral differences are likely to impact the geometric evolution of hydraulic fractures. In the numerical models of hydraulic fracturing for a laminated coal reservoir, the hydraulic fracture propagates predominantly vertically as the interlayer has a lower elastic modulus and higher tensile strength. Because the interfacial shear strength is weak, the fractures can easily penetrate and propagate into the bedded interface between layers, and often, the distance of lateral slip increases into the interlayer interface. Thus, when dull coal is fractured, the geometry of the hydraulic fracture is often characterized as an isolated fracture distribution. The fracture then, rapidly propagates into the interlayer, activating the natural fractures in the bright coal reservoir, thus improving the fracture scale in the interlayers (model 2). However, as bright coal is a productive strata, the geometry of the hydraulic fracture is also dominated by crisscross network structures, and the fractures preferentially propagate along the interface (model 1). • The differences of mechanical properties that linked to the macrolithotype contribution were investigated systematically. •The impact of these sensitivity factors on hydraulic fracture geometry evolution was evaluated. •The influences of coal macrolithotype on hydraulic fracture geometry evolution were investigated.

Coal Macrolithotype Distribution and Its Genetic Analyses in the Deep Jiaozuo Coalfield Using Geophysical Logging Data

Coal macrolithotypes are closely correlated with coal macerals and pore–fracture structures, which greatly influence the changes in gas content and the coal structure. Traditional macrolithotype identification in coalbed methane (CBM) wells mostly depends on core drilling observation, which is expensive, time-consuming, and difficult for broken core extraction. Geophysical logging is a quick and effective method to address this issue. We obtained coal cores from 75 wells in the deep regions of the Jiaozuo Coalfield, northern China, quantitatively analyzed the logging cutoff number corresponding to various macrolithotypes, and established natural γ (GR), deep lateral resistivity (LLD), and γ–γ log (GGL) response rules for each coal macrolithotype. The formation mechanisms of different coal macrolithotypes are discussed from the perspective of coal facies and pore structures. The results show that GGL decreased but GR and LLD increased from bright coal to dull coal. Most coal macrolithotypes can be distinguished based on the established thresholds of various logging curves. However, excessively high or low ash yields significantly affect the validity of identification. The vertical coal macrolithotypes attributed to the peat marsh environment in Shanxi Formation mostly comprise three to six sublayers; dull or semi-dull coals are predominant close to the 21 coal seam, and the bright or semi-bright types usually appear in the middle part. The semi-bright and bright coals are usually vitrinite rich, whereas the semi-dull and dull coals are primarily inertinite rich. For pore structure arguments, the highest average specific surface area (SBET) and the total pore volume (VBJH) are found in bright coals, followed by dull and semi-bright coals; those of semi-dull coals are the lowest. However, SBET and VBJH change significantly for different samples, even though the coal macrolithotype is the same. Therefore, the macrolithotype is not the key factor determining the coal parameters of pore structures. Rapid and effective identification of coal macrolithotypes can help determine the CBM enrichment area, the CBM well location, and the exploration horizon.

Influence of porosity on the reactivity of inertinite and vitrinite toward sodium hypochlorite: Implications for enhancing coal seam gas development

Sodium Hypochlorite (NaClO) oxidation has been reported as a promising method for coal seam permeability enhancement and extracting valuable hydrocarbons from coal. Different coal macerals, particularly vitrinite and inertinite behave differently in NaClO with varying reaction extent exhibited. The reasons for variability are not completely understood. This study aims to reveal the controlling factors of different coal maceral oxidation by comparing their molecular structures and pore characterization. A Permian coal from Bowen Basin, Australia, was collected and separated into vitrinite-rich concentrate (bright coal) and inertinite-rich concentrate (dull coal). Their reaction phenomena with NaClO are examined and their molecular structures and pore structures are compared with multiple techniques. The vitrinite-rich and inertinite-rich concentrates share similar organic molecular structures although there is some marginal difference with the bright coal being slightly more aliphatic with higher oxygen content. By contrast, their pore structures are significantly different such that the dull coal is more porous with higher content of mesopores and better connectivity. For coal powders vitrinite is more reactive than inertinite and yet for coal particles inertinite is more reactive. Given that inertinite is more porous, these results suggest that the porosity of the inertinite is the key property making the inertinite more reactive. Therefore, it is proposed that the different extent of oxidation for different macerals from isorank coal is controlled primarily by the difference in their porosity rather than molecular structures.

Analysis of hydraulic fracture behavior and well pattern optimization in anisotropic coal reservoirs

Macrolithotypes control the pore-fracture distribution heterogeneity in coal, which impacts stimulation via hydrofracturing and coalbed methane (CBM) production in the reservoir. Here, the hydraulic fracture was evaluated using the microseismic signal behavior for each macrolithotype with microfracture imaging technology, and the impact of the macrolithotype on hydraulic fracture initiation and propagation was investigated systematically. The result showed that the propagation types of hydraulic fractures are controlled by the macrolithotype. Due to the well-developed natural fracture network, the fracture in the bright coal is more likely to form the “complex fracture network”, and the “simple” case often happens in the dull coal. The hydraulic fracture differences are likely to impact the permeability pathways and the well productivity appears to vary when developing different coal macrolithtypes. Thus, considering the difference of hydraulic fracture and permeability, the CBM productivity characteristics controlled by coal petrology were simulated by numerical simulation software, and the rationality of well pattern optimization factors for each coal macrolithotype was demonstrated. The results showed the square well pattern is more suitable for dull coal and semi-dull coal with undeveloped natural fractures, while diamond and rectangular well pattern is more suitable for semi-bright coal and bright coal with more developed natural fractures and more complex fracturing fracture network; the optimum wells spacing of bright coal and semi-bright coal is 300 m and 250 m, while that of semi-dull coal and dull coal is just 200 m.

The Impact of Coal’s Petrographic Composition on Its Suitability for the Gasification Process: The Example of Polish Deposits

In this paper, we discuss the impact of the rank of coal, petrographic composition, and physico-chemical coal properties on the release and composition of syngas during coal gasification in a CO2 atmosphere. This study used humic coals (parabituminous to anthracite) and lithotypes (bright coal and dull coal). Gasification was performed at temperatures between 600 and 1100 °C. It was found that the gas release depends on the temperature and rank of coal, and the reactivity increases with the increasing rank of coal. It was shown that the coal lithotype does not affect the gas composition or the process. Until 900 °C, the most intense processes were observed for higher rank coals. Above 1000 °C, the most reactive coals had a vitrinite reflectance of 0.5–0.6%. It was confirmed that the gasification of low-rank coal should be performed at temperatures above 1000 °C, and the reactivity of coal depends on the petrographic composition and physico-chemical features. It was shown that inertinite has a negative impact on the H2 content; at 950 °C, the increase in H2 depends on the rank of coal and vitrinite content. The physicochemical properties of coal rely on the content of maceral groups and the rank of coal. An improved understanding these relationships will allow the optimal selection of coal for gasification.

Coalbed methane production of a heterogeneous reservoir in the Ordos Basin, China

Macrolithotypes control the pore-fracture distribution heterogeneity in coal impacting gas adsorption, diffusion coefficient, and the gas flow. However, the behaviors that are impacted by coal macrolithotype are traditionally ignored and the drainage radius that linked to the macrolithotype contributions are lack of systematic research all the way. Thus, to evaluate the gas production relationship for macrolithotype, four blocks (bright, semi-bright, semi-dull, and dull) were obtained from the same seam. The methane absorption, diffusion, and permeability data were obtained and used in mathematical modeling to identify well production, drainage radius, and well-interference locations. Production data for 104 wells and 11 test wells were also obtained and with COMET3 simulation software to validate the mathematical model. From the bright to dull coal: there was a lower micropore contribution, a lower gas capacity, and lower gas diffusion coefficient. The cleat frequency and aperture differences impacted the reservoir permeability with bright coal having 195% higher than the dull coal. As these macrolithotype differences impact the coalbed methane (CBM) development and cause heterogeneous gas flow systems within the resource, a coupled mathematical model for comprehensive consideration of the adsorbent-diffusion-seepage was established capturing the macrolithotype contribution to the pressure propagation, gas flow, and gas drainage radius estimation. The bright coal had the higher Langmuir volume, CH4 diffusion coefficient, and permeability thus, producing a higher gas production volume, and a larger gas drainage area than the dull macrolithotype. Thus, the macrolithotype diversity and its stratification cause partitioning of the gas flow with multi-stage pressure drop effecting exploration and development. In addition, the well interference was also observed and show that the wells of bright and semi-bright coal have a continuous pressure drop, which achieving the purpose of overall pressure reduction.

Properties of lignite and key factors determining the methane adsorption capacity of lignite: New insights into the effects of interlayer spacing on adsorption capacity

The lignite reservoir and related parameters that determine the adsorption capacity remain largely unexplored and not understood. The coal composition, pore characteristics, mineral matter, coal rank, and molecular structure of lignite were systematically studied, and the influence of various factors on the adsorption capacity was analyzed. Lignite samples were separated into bright and dull types by handpicking. The adsorption capacity of dull coal is always higher than that of bright coal. Based on this fact, the differences between types of lignite were further clarified to determine the effects of various factors on the adsorption capacity of lignite. The differences in the lithotypes of lignite are related to the maceral composition and mineral composition. The adsorption capacity of lignite was significantly affected by the maceral composition. The adsorption capacity increased with an increase in the huminite content. The difference in the adsorption capacities of huminite and inertinite may be attributed to differences in the chemical structures. The interlayer spacing of huminite is larger than that of inertinite at lower rank levels. The large interlayer spacing in lignite may provide considerable storage space for methane molecules. Moreover, the effect of the dilution of mineral matter on the adsorption capacity of lignite is less significant than that of medium-high-rank coal. Clay minerals in lignite can also contribute a nonnegligible adsorption capacity, especially for smectite, whose adsorption capacity is even higher than that of the organic matter in lignite.

Coal seam porosity and fracture heterogeneity of marcolithotypes in the Fanzhuang Block, southern Qinshui Basin, China

Marcolithotypes play a significant role in coal seam porosity and fracture heterogeneity. The optical microscopy, CO2 and N2 adsorption, and X-ray CT were used for reconstructing the pore-fracture system of different marcolithotypes. The results show that the microfractures are well developed in the bright coal with the characteristics of high fracture density, good connectivity, and strongest fractal dimension, corresponding to the highest permeability and the best seepage capacity. The adsorption capacity measured with N2, CO2, and CH4 indicates that the specific surface area of micropores (<2 nm) increases linearly from the bright to the dull coal, and the bright coal is favorable for gas adsorption by providing more available surface sites. The difference in physical properties of various macrolithotypes causes different logging response characteristics, including the compensated density (DEN), the gamma ray (GR), and the acoustic time (AC). Based on that, the index (N = AC/(GR × DEN)) was put forward to forecast the distribution of coal macrolithotypes quantitatively (N ≤ 1.3, Partings; 1.3 < N ≤ 3, Dull coal; 3 < N ≤ 5, Semi-dull coal; 5 < N ≤ 8, Semi-bright coal; N > 8, Bright coal), and its accuracy was verified by coal cores from different areas, i.e. Fanzhuang, Zhengzhuang, and Hancheng blocks.

Fractal characterization of pore structure for coal macrolithotypes in the Hancheng area, southeastern Ordos Basin, China

Coalbed methane (CBM) reservoir pore structure is extremely complex and heterogeneous. With the fractal theory, the geometric and structural characteristics of the pore structure could be quantified. However, few studies have aimed to understand the fractal features of compositional-based macrolithotypes (bright, semi-bright, semi-dull, and dull coals). In this work, samples from the Hancheng area, in the southeastern Ordos Basin, China, were collected to investigate the fractal features of pore structures for macrolithotypes and calculate fractal dimensions (Da1 and Da2 for adsorption pores; Ds1 and Ds2 for seepage pores) based on low temperature N2 adsorption and mercury intrusion data, respectively. Meanwhile, the impact of the fractal dimensions on the gas adsorption capacity and the seepage ability were discussed. The results demonstrate that when bright coal turns into dull coal, the vitrinite content, helium permeability, and gas adsorption capacity gradually decrease, while the coal density, mineral content, and ash yield increase. Since the Da1 and Ds1 either deviate from the natural fractal dimension (2 < D < 3) or have a poor correlation with macrolithotypes, Da2 and Ds2 are regarded as the adsorption and seepage pore fractal dimensions, respectively. Of all the coal macrolithotypes, bright coal has the smallest BET specific surface area and micropore proportion, which results in the smallest fractal dimension Da2, while for the dull coal, this is just the contrary; due to the higher mineral content, a higher Da2 of the dull coal indicates a more complicated adsorption pore structure, and the coal with a larger Langmuir volume has a smaller Da2. For the seepage pores, when the bright coal turns into dull coal, the macropore proportion gradually decreases, but the pore proportion tends toward equilibrium, and the mercury withdrawal efficiency increases; the dull coal has a larger Ds2, indicating a more complex seepage pore structure than that in the bright coal. Due to the decreasing macropore proportion and mineral filling, Ds2 demonstrates an obvious negative correlation with the helium permeability and macropore proportion.

Experimental investigation of hydraulic sand fracturing on fracture propagation under the influence of coal macrolithotypes in Hancheng block, China

The mechanical properties, brittleness and micro-fracture development of different coal macrolithotypes (bright coal, semi-bright coal, semi-dull coal and dull coal) are different, and these differences have an impact on the propagation of hydraulic fractures. To understand the influence of coal macrolithotypes on fractures propagation, several hydraulic sand fracturing experiments considering the classification of coal macrolithotypes were carried out by taking No. 11 coal seam of the Taiyuan formation in Hancheng block, China. We mainly investigated the influence of coal macrolithotypes and perforation location on the propagation of hydraulic fractures. Combined with observation results of mining excavation, the experimental results are verified. The research finds that fractures morphology and proppants distribution in bright-type coal is better than dull-type coal. Thus, bright-type coal is the fracturing sweet-spot section in coal seam. We also find that hydraulic fractures are prone to propagate along the stratigraphic interfaces or lithologic interfaces to form horizontal major-fractures, and the overall stimulation effect of coal seam is dependent on the fractures morphology and proppants placement of these horizontal major-fractures. The stimulation effect of horizontal major-fractures for coal seam is better when perforation location is selected near the weak planes instead of the middle of coal seam. The research results provide theoretical guidance for efficient fracturing of coal seam.

The impact of the coal macrolithotype on reservoir productivity, hydraulic fracture initiation and propagation

Macrolithotypes control the pore-fracture distribution heterogeneity in coal, which impacts stimulation via hydrofracturing and coalbed methane production in the reservoir. Here, the aspects of coalbed methane production that are impacted by macrolithotype were determined from production data from 56 wells in the Hancheng Block, China. The macrolithotype contribution had a significant influence on the well productivity, with dull lithotype-rich (dull and semi-dull coal) coal having significantly lower productivity than wells with bright lithotype-rich (bright and semi-bright coal) coal. The main reason for this difference is that macrolithotypes impart a fracture distribution, which further impacts the hydrofracture stimulation and subsequent coalbed methane production. Here, the hydraulic fracture was evaluated using the appropriate mechanical behavior for each macrolithotype with PFC2D software, and the impact of the macrolithotype on hydraulic fracture initiation and propagation, which are controlled by stress and mechanical properties, was investigated systematically. The result showed that the hydraulic fracture propagation types are controlled by the pre-existing natural fractures. Furthermore, due to the well-developed natural fracture network, the hydraulic fractures in bright coal were mainly composed of the opening, steering propagation, and compound types, forming a lattice-like fracture network. Dull coal is denser, and its hydraulic fractures are characterized as an isolated fracture distribution. The conclusions were verified through monitoring of hydraulic fracture by the microseism wells which show that the fracture in the bright coal is more likely to match the “complex fracture network” description, and the “simple” case when the dull coal is fractured. Meanwhile, the hydraulic fractures mainly open at a shallow depth, but with the increase in depth, the hydraulic fractures are dominated by the crossing type. If the hydraulic fractures are the opening type, only at a higher liquid pressure can the natural fractures in dull coal be initiated. Thus, the difference between hydraulic fractures in bright and dull coal lead to different flow paths and ultimately result in a variation of coalbed methane production. Therefore, during coalbed methane output, a bright coal reservoir will have more potential than a dull coal reservoir, which provides a scientific base for coalbed methane exploration and development.

Quantitative characterization of middle-high ranked coal reservoirs in the Hancheng Block, eastern margin, Ordos Basin, China: implications for permeability evolution with the coal macrolithotypes

ABSTRACTMacrolithotypes control the pore-fracture distribution heterogeneity in coal impacting the coalbed methane (CBM) production from the reservoir. However, few studies have focused on the characteristics of macrolithotypes. Here, the macrolithotype samples from the three continuous seams (No. 3, 5 and 11) in the Hancheng Block, Ordos Basin, China were quantitatively characterized by multiple length scales. The results indicate that the bright coal has the best development of micro-fracture than those of the other macrolithotypes, which with good openness and connectivity under the optical microscopy; N2 adsorption/desorption results reflect that the adsorption pores of the semi-dull and dull coal are more developed than the bright coal and semi-bright coal, and are dominated by plate-like and ink-bottle pores. The characteristics of the mercury intrusion/extrusion curves were determined by the mercury injection (MIP). From bright to dull coal the pore connectivity being worse, and the uniformity of pore and throats distribution becomes lower. The pore type that divided by NMR are recovered to be seepage type (the bright), comprehensive type (the semi-bright and semi-dull), and the type of adsorption (the dull). The behavioral differences would impact the porosity and permeability in coal. Thus, by the well logging evaluation approach, the relationship between coal macrolithotypes and select logging parameters were determined for coal macrolithotype identification, and with the petrel software included, the porosity-permeability models were built to investigate the permeability evolution with the coal macrolithotypes. Macrolithotypes are closely related to the permeability of coal reservoirs in this area. The greater contribution of permeability is typically from of the bright and semi-bright coal, and followed by semi-dull and dull coal, the better development of bright coal, the better permeability of the reservoir and vice versa.

Influence of combustion temperature and coal types on alumina crystal phase formation of high-alumina coal ash

The alumina content (more than 40%) of high-alumina coal ash is comparative to the middle content bauxite ores in China. So far, in order to meet the high demand of alumina and the rise of circular economy industrial chain, extracting alumina from coal ash has become a way to comprehensively utilize high-alumina coal ash. However, this process has high requirements on the crystal phase and stability of alumina. Different from most studies, this paper focuses on how to produce coal ash more beneficial to the later refining of aluminum. Therefore, the effects of combustion temperature and coal types by classifying high-alumina coal into dull coal and bright coal on alumina crystal phase formation were studied. Through proximate analysis, ultimate analysis, calorific value analysis, X-ray fluorescence spectroscopy, X-ray diffraction (XRD) and scanning electron microscope (SEM) and other methods, it is found that γ-Al2O3 in high-alumina coal ash translated into more stable θ-Al2O3 and finally α-Al2O3 when combustion temperature is higher than 1000°C. Thus compared with pulverized coal boilers, circulating fluidized bed (CFB) boilers with lower combustion temperature can produce higher quality coal ash. Moreover, at the same combustion temperature, alumina crystal phase in dull coal ash is relatively less stable than that in bright coal ash, which is more suitable to the later refining and electrolysis of aluminum.

Study of microstructure, maceral and mineral matter in two different coals of the Nepal Himalaya

The Eocene coals from the western (Tosh coalfield) and central regions (Jhadewa coalfield) of the Nepal Himalaya have been studied under the petrological and the Scanning Electron Microscopes to explain the mode of occurrence of mineral matter and their microstructural relations with organic constituents. As shown by the results of the coal samples, three kinds of coals in diminishing abundance are moderately dull coal, moderately bright coal and bright coal. Distribution of mineral matter in the Eocene coals shows that the bright coal is rare in western Nepal and the moderately bright coal is abundant in central Nepal. Mineral matter in moderately dull coals contains superficial impregnating and cavity filling. Mineral matter in bright coals contains superficial mounting and pore fillings, but in moderately bright coals contains superficial impregnating, cavity filling, and intimate intergrowth. Mineral matters in&#x0D; moderately dull coals, moderately bright coals and bright coals respectively range from 38.27 to 47.12 vol%, 12.31 and 15.73 vol%, and 12.31 and 15.73 vol%. The Jhadewa coalfield (central Nepal) contains more macerels than the Tosh coalfield (western Nepal), whereas the Tosh coalfield contains more mineral matter than the Jhadewa coalfield. The results indicate two different origins for coal deposits from two different regions.

The impact of coal macrolithotype on hydraulic fracture initiation and propagation in coal seams

Macrolithotypes control the pore-fracture distribution heterogeneity in coal impacting stimulation via hydrofracturing and the coalbed methane (CBM) production. Given that it is affected by the discontinuities, hydraulic fracture geometry is complex in the vertical plane and is different from a simple fracture in a homogeneous reservoir. However, the initiation and propagation mechanism in the vertical plane is unclear. To clarify this, the cohesive zone finite element approach, with macrolithotype contributions included, was used to simulate and analyze the hydraulic fracture propagation. The experimental tests showed that, the bright and semi-bright coal usually have higher microfracture (cleat) density accompanied by the lower mechanical properties than that of the semi-dull and dull coals. The behavioral differences are likely to impact the geometry evolution of hydraulic fractures and which appears to vary when fracturing the different coal macrolithtypes. Thus, the cohesive zone finite element approach was used with two models to capture macrolithotype impacts. The result show that, when fracturing the dull coal (model 2), the overall propagation region rapidly displayed a simple plane in shape because of the less development of natural fractures. With the influence of the larger elastic modulus, the high-stress zone would be easy formed and suddenly release to generate pressure pulse when the hydraulic fracture penetrated the interface. As the hydraulic fracture initiates from the bright coal (model 1), the presence of the existing diverse cleat network contribute greatly to the increase of cracks number to form complex fractures. However, the opening of natural fractures will lead to the diversion of fracturing fluid, and the larger elastic modulus of the interlayer also plays a limiting role in the height of the hydraulic fracture. In addition, the monitoring of hydraulic fracture was carried out and shown that the height of the major fracture in model 1 was restricted and limited by the bright coal; and the height in model 2 is usually larger than the dull coal thickness, indicating that the hydraulic fracture has cut through the fracturing section (dull coal) and embedded into the upper and lower layers.

Experimental study on structural models of coal macrolithotypes and its well logging responses in the Hancheng area, Ordos Basin, China

Coal macrolithotype (bright, semi-bright, semi-dull, and dull coals) controls the heterogeneity of coal physical properties in the coalbed methane (CBM) reservoir. However, few scholars carried out the work aiming at the compositional-based macrolithotype analysis of coal reservoirs. In this work, the Hancheng area, Ordos Basin, China, was taken as an example to understand the macrolithotype difference, establish the structural models of macrolithotypes, and then evaluate the responses of advanced well loggings (the natural gamma-ray spectroscopy (NGS), formation microresistivity image (FMI), and cross-dipole array acoustic (XMAC) loggings). Results show that, from the bright to dull coal, the coal density, ash yield, and mineral content increase while the vitrinite content and pore-fracture development degree decrease gradually. The dull and semi-dull coals usually are found in touch with the mudstone (roof, floor, and parting) while the bright or semi-bright coal occurs as an interlayer between them. In the structural models of macrolithotypes for the CBM reservoir, the fracture model couples the artificial and macrofractures controlled by the tectonic stress field as well as the microfracture mainly restricted by the coal macrolithotypes; the pore model shows multilevel pores with gradually decreasing pore size, namely the protogenetic (plant tissue and intergranular pores) and secondary (gas and intermolecular pores) pores; and the pore-fracture relation model is characterized by the multilevel communication between the pores and fractures. Of all the three advanced logging responses of macrolithotypes, the NGS and XMAC show a good consistency with the natural gamma (GR) and interval transit time (AC) loggings since the former was derived from the latter essentially, respectively. But using FMI to identify macrolithotypes is error-prone due to the strong coal heterogeneity around the wellhole. From the bright to dull coal and mudstone, the values of thorium, kalium, uranium elements and GR (or GR without uranium) increase while those of AC including the longitudinal, transverse, and Stoneley waves decrease gradually.

Interrelation Between Mechanical and Petrographic Characteristics of Coals of Argada ‘B’ Seam: Implication To Comminution and Utilization

ABSTRACT The mechanical, as well as petrographic characteristics of coal, have their own significance. Coal is crushed to fine size for its usage in pulverized coal-fired plants, pulverized coal injection in blast furnaces, coal beneficiation and other industrial applications. Coal dusts severely affect the environment and ecology. To identify the effect of mechanical and petrographic characteristics on coal comminution and utilization, the pillar coal samples of Argada ‘B’ Seam of South Karanpura Coalfield were subjected to different analyses. The megascopic petrographic analysis reveals the dominance of banded dull coal while microscopically these are vitrinite rich followed by inertinite and liptinite. Mineral matter is mainly characterized by argillaceous minerals, disseminated carbonate minerals, and low concentration of pyrite. Vitrite is the most dominant microlithotype. These coals are sub-bituminous B to high-volatile B bituminous in rank. The mechanical properties and petrographic characteristics reveal that the ‘dull coal’ has produced more undersize 100 mesh fines than banded dull coal and banded coal. The friability index of dull coal is found higher than in ‘banded dull coal’ and ‘banded coal’. The study suggests that these coals are best suited as blend coking coals.