Coal Core Research Articles

Insights into matrix compressibility of coals by mercury intrusion porosimetry and N2 adsorption

Matrix compressibility and pore properties (pore size distribution) of a rank range of coals was investigated using mercury intrusion porosimetry (MIP) on coal cores with the pore size distribution also being determined using low temperature at 77 K nitrogen adsorption/desorption isotherms for crushed samples. The coal matrix compressibility is significant when the pressure of MIP is from 0.0074–35 MPa. Mathematical models were developed (based on MIP and nitrogen adsorption/desorption isotherms) to establish the porosity/pore size distribution relationships with matrix compressibility. For coal ranks, the matrix compressibility was between 0.24 × 10−4 to 13.56 × 10−4 MPa−1, and had a negative exponential relationship with the vitrinite reflectance (Ro,m%). Lignites have the maximum matrix compressibility due to their structural open structure having limitied compaction during coalification. In addition to the pore structure relationship the composition, moisture, and ash yields impacts on compressibility were also examined. Inertinite-rich coals however had a low matrix compressibility across the rank range, which may be due to the interinhibitive relationships between the mesopores, macropores and minerals. The wetting action of high moisture (water molecules) weakens the link between the coal particles of the lignites and the subbituminous coals, which causes abnormally high compressibility. Observations here relate to hydrofracturing or CO2 injection behaviors during enhancing coalbed methane (CBM) recovery.

Experimental Investigation of the Impact of Coal Fines Migration on Coal Core Water Flooding

Coalbed Methane (CBM) has become an important gas resource in recent decades. The brittle property of coal matrix and overactive operation make the migration of coal fines inevitable. Blockage by coal fines that plugs flow paths is a non-negligible issue that results in a significant decline in gas production. By setting different experimental conditions with the following factors—coal fines concentration of the mixture displacing fluids, constant flow pump rate, inlet pressure, outlet pressure and confining pressure—six experimental schemes were designed to investigate the two-phase water and coal fines flow in natural core samples. When the differential pressure and flooding flow reach a pseudo-steady status, the equivalent permeability of coal samples can be approximately calculated considering coal fines migration. Furthermore, the influences of coal fines migration on the cleat opening and permeability variation are analyzed in the porous coal medium. The study will benefit CBM development and save pump maintenance costs. In this work, we found that maintaining the differential pressure for a longer period may result in new cleat openings and severe coal rock damage during the single-phase water flooding process. While coal fines may plug some natural cleats and pores, especially in the core samples with micro-cleats during the two-phase flooding stage, coal fines migration significantly reduces the equivalent permeability and dewatering ability of the coal rock in the earlier flooding. While enlarging the differential pressure in two-phase water and fines flooding, breakthrough of coal fines from the samples contributes to widened cleats. While coal fines are difficult to flood into the core pores for low-permeability core samples, coal fines gather in the inlet, and it is also difficult to reach the pseudo-steady status even under higher differential pressure. The damage to permeability mainly occurs in the early stage of coal fines migration, and an abrupt increase in the flow velocity can damage reservoirs and induce substantial coal fines generation. Thus, maintaining a stable effective strength and a controlled depressurization rate during drainage can effectively constrain coal fines output and decrease permeability damage within coal reservoirs.

Experimental Study of Supercritical CO2 Injected into Water Saturated Medium Rank Coal by X-ray MicroCT

Carbon dioxide geosequestration into deep unmineable coal seams is a technique which can mitigate anthropogenic greenhouse gas emissions. However, coal composition is always complex, and some minerals such as calcite chemically react when exposed to the acidic environment (which is created by scCO2 mixing with formation water). These reactive transport processes are still poorly understood. We thus imaged a water-bearing heterogeneous coal (calcite rich) core before and after scCO2 injection in-situ at high resolutions (3.43 µm) in 3D via X-ray in-situ microCT flooding system. Indeed, the calcite-coal mixed layer was partially dissolved, and absolute porosity and connectivity significantly increased. We thus suggested that such process could be used as an acidizing method in CO2 ECBM. However, such dissolved damage also can significantly affect the rock mechanical properties and potentially induce geohazards.

Microscopic damage field in coal induced by water jets

This paper uses experiments and simulations to investigate the fragmentation process of coal subjected to water jets. The experiments are conducted using computed tomography (CT) and digital image processing technology to reconstruct the damage field of an impacted coal core, and a numerical model is established based on the arbitrary Lagrange–Euler (ALE) algorithm under the same impact condition. By analyzing the variations of the mechanical parameters in the simulation, this study reveals the element fragmentation mechanism. The results show that the fragmentation of an element can be divided into three stages. In addition, this research explains the formation and development of the punching based on the strain states in the simulation and verifies the deductions using experiment results. Furthermore, the paper also arrives at certain conclusions regarding variations of the damage field with coordinates. In the vertical direction, due to the resistance of reflux and the friction between jets and the punching wall, the kinetic energy of water jets is consumed gradually. Accordingly, both the distribution range and the degree of damage decrease with increasing standard depth. In the horizontal direction, the distribution of microscopic damage is characterized by locality, which occurs because of the attenuation of the acting effective stress.

Development of Test Bench for Investigation of Methane Filtration Intensification in Coal Samples

The test bench and procedure are proposed for investigating intensification of methane release from coal samples under wave field action, which favors increase in degassing rate. The additional equipment is designed for delivering coal cores from mines and taking into account free-released methane volume.

Brittleness evaluation of coal based on statistical damage and energy evolution theory

Coal brittleness is an important index for evaluating whether coal can be fractured effectively. Coal with high brittleness can form a large complex network after fracturing, and also has better productivity and ultimate recovery. Therefore, accurate evaluation of coal brittleness can provide theoretical guidance for well and layer selection of coal seam fracturing. In this paper, first, uniaxial compression experiments have been carried out. According to the measured stress-strain curves and the core failure form, the brittleness of different coal cores have been analyzed. Considering the damage evolution of coal in loading process, a theoretical evaluation model of coal brittleness has been established based on the statistical damage theory and the energy evolution law of rock failure process. Brittleness indexes of experimental coal cores have been calculated by this model, the calculated results were in good agreement with the experimental results, which proved that the theoretical analysis method of coal brittleness proposed in this paper can be used to analyze the applicability of coal seam fracturing. This paper presents an evaluation method of coal brittleness and establishes a relevant theoretical model. The research results of this study have important academic merits by filling in the blank of theoretical study on the brittleness of coal, and could also further facilitate the field operations of coal fracturing.

Experimental study of the moisture content influence on CH4 adsorption and deformation characteristics of cylindrical bituminous coal core

Moisture content in coal is an important factor affecting the coal seam gas extraction. It directly affects the storage and flow of gas in bituminous coal. In this paper, the cylindrical bituminous coal cores of Xutuan coal mine in Huaibei coal mine group were studied as experimental objects, using the laboratory self-designed experimental device Gas Adsorption and Strain Testing Apparatus system. The influence of the bituminous coal moisture content on gas adsorption characteristics was studied. Drying experiments of coal samples showed that they lose the original moisture content following the exponential decay function of time. At wetting, the saturated moisture content in coal samples increased following the Exponential Association function of time. The experimental results show that the average original moisture content and average saturated moisture content of raw coal samples are 1.3 and 2.4%, respectively. On this basis, the gas adsorption experiments on samples with different moisture contents under different gas pressures were carried out. With the moisture content increase, the gas adsorption capacity and saturation value decreased and the decrease rate gradually reduced. The single exponential decay function describes the gas adsorption capacity dependence on moisture content. Moisture content also affects the adsorption deformation of bituminous coal. At high moisture content, the adsorption deformation of bituminous coal is less.

Permeability enhancement of coal by chemical-free fracturing using high-voltage electrohydraulic discharge

Fractures, cleats or cracks provide the main fluid pathways for methane extraction from Coalbed Methane (CBM) gas reservoirs. For deep coal seams, where the cleats are often closed by the high overburden pressure, or some shallower but tight coals, fracturing operations to form new pathways or clean out existing but blocked natural cracks are necessary for CBM wells to produce commercially attractive gas rates. This is usually done by injecting fluids under high pressure into the subsurface, an operation that is controversial because of its possible environmental impacts.This study focused on developing and validating a novel stimulation technique, namely high-voltage electrohydraulic discharge (EHD), as an alternative method to effectively crack bulk materials without injecting any new fluids into the well. During the stimulation process, strong pressure shockwaves were generated by underwater electrical discharges, which then served to damage the associated solids, creating fractures and cracks. We used mortar (as a convenient, homogeneous, isotropic materials with consistent properties, and coal (which is naturally fractured and quite heterogeneous), as test materials. EHD significantly enhanced the permeability of mortar core (2.75–6.25 times) and coal core (3.6–10.8 times). X-ray CT scanning showed an obvious increase in fracture density, length and voids in the tested specimens, and mercury porosimetry confirmed that porosity increased at the micro (nm) and macro (μm) scales, in addition to the cracks which were both visible on the samples and at the in the X-ray images.The new cracks induced in the coal samples were directional in nature. The fractures tended to more easily occur parallel to the direction of bedding plane (i.e., opening in the weakest direction), or on the bright bands of the coal, which are brittle and fragile compared to the softer and more elastic dull bands. Based on these observations, we are encouraged that EHD has potential as a method for reservoir stimulation and gas development for tight source rocks such as coal or shales.

Recovery of tectonic traces and its influence on coalbed methane reservoirs: A case study in the Linxing area, eastern Ordos Basin, China

The physical properties of coal reservoirs, i.e., porosity, permeability, fractures, and coal structures, are generally evaluated based on its structural morphology, however, its tectonic evolution history plays a much more important role. The key problems associated include the time when the dominant variations on properties of coal occurs and how to quantitatively characterize the influences of different tectonic movements. Based on field joint observations and well logging (micro-resistivity imaging logging (MIL), interval transit time (DT24), density (DEN), and natural gamma-ray (GR)) interpretations, the paleotectonic stress fields and tectonic traces were recovered and the present coal structures were identified. Subsequently, the quantitative correlation between the tectonic deformation and coal reservoir property were established with fluid intrusion tested porosity and permeability and scanning electron microscope (SEM) results.The results showed that the maximum horizontal compressive stress was oriented at the NW-SE direction in Yanshanian and at the NEE-SWW direction in the Himalayan period as obtained from hundreds of joint measurement and fracture interpretation results. Six combinations (by superposition of anticline, syncline, and the flanks) from the Yanshanian and Himalayan movements were found, which indicate different deformation degrees. The maximum tectonic curvature (r'), which occurred in either the Yanshanian or Himalayan period, shows differences at different structural areas and reflects the real deformation degree of coal seam in geological history. When r'< 12 × 10−6 m−1, primary coals are developed; when 12 × 10−6 m−1 <r' < 35 × 10−6 m−1, cataclastic coals are generally developed; when r'> 35 × 10−6 m−1, granulated coals are dominant. Mylonitized coals are barely developed in the study area. With the increase of deformation degrees of coal seams, the number of microfractures increases and fracture stretches tend to be more complicated. When r' < 25 × 10−6 m−1 (if r' > 25 × 10−6 m−1, the coal cores are much too broken for drilling core samples), with the increase of r', the displacement pressure (Pd) achieved by mercury intrusion method decreases in an linear form, and the permeability tested by helium presents a linear growth. However, the porosity shows a weak correlation with r', mainly caused by the great burial depth and long-term compaction resulting in low porosity ranges (2.8%–8.1%) with little difference shown. This presents an applicable method for evaluating the influence of tectonic movement on the physical properties of coal during different times and would benefit the evaluation of coalbed methane reservoirs in the study area as well as other coal basins in the world.

The identification of coal texture in different rank coal reservoirs by using geophysical logging data in northwest Guizhou, China: Investigation by principal component analysis

Coal texture properties are one of important factors for determining the gas sorption capacity and transport properties. The recognition of coal texture with drilled cores or mining seam observation is direct and effective methods, but both methods are expensive and impossible for unexplored coal seams. The cut-off values logging data were applied in a few coal basins while the method ignores the effect of different coal ranks and is complicated with blurry boundary. In this study, 174 coal cores data obtained from 18 CBM wells were correlated with their geophysical logging responses in northwestern Guizhou Province. Four well-logging curves of the caliper logging (CAL), densities (DEN), natural gamma (GR) and deep lateral resistivity (LLD) were chosen to analyze coal textures. With progressive damage of coal, both values of CAL and LLD gradually increase while the DEN and GR tend to decrease. The identification index was reconstructed by using the principal component analysis (PCA) to identify coal texture of different coal ranks to improve the qualities of coal texture identification and reduce multiple solutions. The logging evaluation method for coal texture identification were applied in multiple coal seams in western Guizhou Province to validate the prediction method of logging data. The results show that PCA is feasible tool to analyze coal texture with improving accuracy and the well logging identification coal texture is good consistency with core identification in different rank coal.

The wellbore instability control mechanism of fuzzy ball drilling fluids for coal bed methane wells via bonding formation

As a novel drilling fluid, Fuzzy-ball Drilling Fluid has been successfully utilized to control the collapse and lost circulation in approximately 1000 coal bed methane wells by now. However, in terms of current research, its mechanism of controlling wellbore instability has not yet been revealed clearly enough to satisfy the requirement from field applications. In order to achieve further understanding, (1) fuzzy-ball drilling fluid was firstly prepared in the laboratory with 2.0% fuzzy-ball coating, 0.5% fuzzy-ball floss, 0.4% fuzzy-ball core, and 0.4% fuzzy-ball membrane blended by Warring Blender at 7000 rpm, whose density and apparent viscosity are respectively 0.85 g/cm3 and 40mPa·s , close to the in-situ reality.(2) Secondly, both uniaxial and triaxial compressive tests under 5MPa confining pressure were conducted to measure the strengths and obtain the stress-strain curves of different coal samples after the separate injections of 2% potassium chloride solution, low solid-phase polymer drilling fluid and fuzzy-ball drilling fluid, as well as the coal plunger (Φ50) without any treatment serving as the control group respectively for three times. (3) Next, the mechanical parameters which influenced the collapse pressure were figured out with regards to sampling cores acquired from the coal reservoir. Through aforementioned experimental procedures, incorporating with a classical collapse pressure prediction mathematical model that contains these mechanical parameters, the collapse pressures corresponding to coal seams subject to different types of drilling fluids were calculated. (4) Finally, two cases, Hancheng and Qinping were employed to verify the results in the laboratory. That is, collapse and loss simultaneously took place in sites, where both issues were tackled in the same well. The findings are as follows. (1) The uniaxial compressive tests show that the strength of natural coal cores after the injection of fuzzy ball drilling fluid rises from 2.6 to 3.6MPa in average while that after the injection of KCl displays a decline and that after the injection of polymer witnesses a modest alteration. (2) The calculation results based on the stress-strain curves illustrate that the Young's modulus (E), Poisson ratio (υ) and cohesion (Cm) of the cores being injected with fuzzy ball drilling fluid change from 3.0665 to 3.9385GPa, 0.229 to 0.318 and 0.5 to 0.9 MPa respectively, whereas those mechanical parameters being injected with other fluids manifest only slight changes. Similar results can be observed in the experiments with reconstructed artificial coal cores. (3) The in-situ application demonstrates that fuzzy ball drilling fluid is capable of overcoming the dilemma brought by collapse and loss taking place simultaneously in the same well. In conclusion, the wellbore instability control mechanism of CBM fuzzy ball drilling fluid lies in that through bonding formation, CBM fuzzy ball drilling fluid changes the rock mechanical parameters, enhancing the strength of rocks microscopically and thereby magnifying the safety density windows macroscopically so as to finally realize the control of wellbore instability.

Profiling of inorganic elements in coal seams using laboratory-based core scanning X-ray fluorescence techniques

The major inorganic elements in cores of coal and interbedded non-coal materials, ranging from 0.9 to 1.7 m in length, have been evaluated using energy-dispersive X-ray fluorescence (ED-XRF) measurement at close-spaced (2 mm) intervals in a laboratory-based Itrax core scanner, to evaluate the potential of this type of technology for use in quality assessment of potentially mineable coal seams in exploration and mine geology programs. The ED-XRF measurements from the scanner were calibrated against a suite of independently-analysed coals to provide quantitative data on the percentages of Si, Al, K, Ca, Fe, Ti and S at each individual point. With the exception of (total) S, also used in quality assessment, these values were recalculated as percentages of the relevant oxides, and combined to provide an estimate of the expected ash yield. Profiles were prepared illustrating the variation in the different components through the seam section, plotted alongside optical and X-radiograph images of the scanned core. Samples of selected intervals were analysed by conventional techniques and the ash yield, total sulphur and individual oxide percentages compared to those indicated for the same intervals by the core scan data. Despite difficulties due to precipitated salts and anomalous element concentrations on or near some core surfaces, as well as differences in moisture content between the scanned core and the conventionally-analysed samples, the results of this cross-checking have shown a general level of agreement for the different coal and non-coal materials involved. While the precision is not as high as that from conventional methods, the scan data allow the materials in the cores, including composites representing potentially mineable intervals, to be categorised to an extent that is of significant value for many exploration and mine geology applications.

In-situ inorganic analysis of coal seams using a hand-held field-portable XRF Analyser

The major inorganic elements in cores of coal and associated non-coal materials, ranging up to 7.6 m in length, as well as in large, naturally-broken coal blocks, have been measured using a hand-held, field-portable energy-dispersive X-ray fluorescence (fp-XRF) analyser, to evaluate the potential of such technology in quality assessment of coal seams for exploration and mine geology programs. Calibration against a suite of independently-analysed coals was used to provide element-specific user factors, which were incorporated into the individual fp-XRF analyses to obtain data on the percentages of Si, Al, K, Ca, Fe, Ti and S, and in some cases Mg and P, at each measurement point. With the exception of (total) S, these values were then recalculated as percentages of the relevant oxides, and combined to provide an estimate of the expected ash yield from the materials concerned. Profiles were prepared illustrating the variation in the different components through seam sections in three different drill cores, based on measurement of points at a 10 mm spacing. Samples of selected core intervals were analysed by conventional techniques, and the ash yield, total sulphur and individual oxide percentages compared to those indicated for the same intervals by the fp-XRF data. Although difficulties were encountered due to precipitated salts and anomalous element concentrations on or near some core surfaces, the results have shown a general level of agreement for the different coal and non-coal materials involved. While the accuracy is not as high as that from conventional methods, the fp-XRF data allow the materials in cores and exposed coal faces to be categorised to an extent that is of value for many research and industry applications.

On the fundamental difference of adsorption-pores systems between vitrinite- and inertinite-rich anthracite derived from the southern Sichuan basin, China

To further understand the fundamental difference of adsorption-pores systems between vitrinite- and inertinite-rich anthracite, six coal core samples with >85% vitrinite and three samples with >80% inertinite were collected from coalbed methane wells in the southern Sichuan basin, China, by macerals analysis method. The differences of pore genetic types, pores shapes, pore surfaces roughness, pore-size distribution, specific surface area, total pore volume, physical properties, and CH4 adsorption capacity between vitrinite- and inertinite-rich samples, were studied via ESEM observation, low-temperature N2 adsorption experiments, NMR tests, and CH4 isothermal adsorption experiments. Results show that plant tissue holes are easier to observe in inertinite than vitrinite, and blowholes, breccia pores and broken pores are all common in vitrinite and rare in inertinite of the study coals. Both vitrinite- and inertinite-rich coal samples exhibit complex nano-pore structures, and pore shapes in inertinite-rich coals are more special than those in vitrinite-rich samples. Fractal dimensions analysis from the N2 adsorption isotherms indicates that inertinite-rich coals have the higher surfaces roughness of irregular pores than vitrinite-rich coals in the P/Po intervals of 0.5–1. In addition, it can be predicted that pores of D2 and D3 type hysteresis loops with diameters of <2.76 nm each are almost semi-closed pores, and the narrow neck in “ink bottle” pores generally measure 2.76 nm. Nano-pores measuring <4 nm comprise the largest proportion among all adsorption-pores. The proportion of pores with diameters of <0.64 nm each in inertinite-rich coals is greater than that in vitrinite-rich coals. NMR porosities, permeabilities, and irreducible water saturations between vitrinite- and inertinite-rich coals are all similar, and the porosities display a good exponential positive correlation with the permeabilities. Vitrinite- and inertinite-rich samples are both characterized by a large VL, and there are positive relationships of VL with vitrinite and inertinite, indicating the CH4 adsorption capacity of organic macerals is much stronger than that of inorganic minerals in anthracite.

Oxidant stimulation for enhancing coal seam permeability: Swelling and solubilisation behaviour of unconfined coal particles in oxidants

Low permeability renders a significant fraction of coal seam gas (CSG) resources sub-economic. An effective permeability enhancement strategy is thereby crucial in monetising a large proportion of low permeability CSG resources. This paper introduces the concept of using oxidants for permeability enhancement, describes a practical screening method to evaluate potential oxidants and provides knowledge about the coal behaviour in oxidants. A test based on time-lapse photography and image analysis of coal particles immersed in liquid oxidants was used to assess the extent and rate of change of coal particle size. Complementary leaching tests determined the extent of coal solubilisation by quantifying the change in coal mass and leachate organic content. The swelling profiles of coal particles hand-picked from a low permeability CSG coal core (Bowen Basin, Australia) were first examined in solutions of potassium chloride, and then pyridine for the purpose of method development and validation. Finally, the swelling ratio, SR, and rate of swelling Sr (% area change per 6 h), of coal particles immersed in oxidising solutions of sodium hypochlorite (0.1%, 1% and 10% NaClO), potassium permanganate (0.015%, 0.03%, 0.1%, 1%, 3% and 5% KMnO4), hydrogen peroxide (1%, 3%, 10% and 30% H2O2) and potassium persulfate (1% and 3% K2S2O8) were examined.Results provide evidence for coal solubilisation (maximum mass loss = 15%) and the propensity to swell (maximum particle size increase = 15%) in all the candidate oxidant stimulants as well as coal breakage in specific oxidants and at specific concentrations (1% NaClO and 3%, 5% KMnO4). The swelling and solubilisation of the coals used in this study tends to increase with higher oxidant concentrations. Anisotropic swelling was also clearly observed in 1% NaClO. Coal reacted vigorously with NaClO and KMnO4, but only slightly with K2S2O8 and H2O2. Massive coal solubilisation occurs in NaClO and KMnO4, but negligible in K2S2O8 and H2O2. In terms of coal oxidation to enhance permeability, NaClO and KMnO4 seem to be more promising than K2S2O8 and H2O2.For the situation of in situ application, it remains unclear if the net effect of coal swelling and coal solubilisation will increase or decrease permeability. In addition, coal breakage may lead to void space or new cracks in the coal matrix, which could have the potential to increase the coal permeability. Confined core-flooding tests that simulate in situ conditions are required to elucidate this behaviour.

Geochemical characteristics of Jurassic coal and its paleoenvironmental implication in the eastern Junggar Basin, China

This study is the first to provide geochemical and paleoenvironmental information of Jurassic Xishanyao Formation coal in the eastern Junggar Basin, China. Twenty-seven coal core samples were collected from the zk0413 boreholes in Dajing area to determine the geochemical variability. Proximate analyses show that the Xishanyao Formation coal has the medium moisture content (average 13%), special low ash yield (average 6%) and high volatile matter (average 34%), which slightly fluctuate from bottom to top of coal seam. Organic matter in the coal are type III kerogen, with coalification degrees determined as subbituminous A or high volatile bituminous C. All coal samples have poor hydrocarbon generating potential with the yield between 31.93 and 72.36 (average of 43.57) mg HC/g.rock. The primary input of organic matter for coal-formation was higher terrestrial plants, with greater input from algae and micro-organisms and greater role of bacteria in plant degradation higher in the seam. The process of peatification in the lower section of the seam occurred under more acidic and oxic conditions. Coal accumulated in a lacustrine freshwater environment with the rise of the (ground) water table from bottom to top. There is evidence of higher salinity of water within the lower seam and decreased salinity (brackish to fresh water) within the middle of the seam, then an increase to moderate salinity (fresh water to brackish). The increasing stratified water column upward with reduced oxidation in the coal seams may result from the increased temperature in periods of a relatively hot and dry climate. The environment changes demonstrated in this study illustrate the importance of studying combined elemental and organic geochemistry to reconstruct paleoenvironment.

Experimental study of permeability behaviour for proppant supported coal fracture

Proppants are typically added in the hydraulic fracture to maintain the fracture aperture and increase conductivity for gas production from coal seams. However, the presence of proppants complicate the permeability behaviour of the coal. Understanding permeability evolution of proppant supported fractures under dynamic stress conditions are necessary to predict the production of coalbed methane. In this work, a series of laboratory experiments were conducted on a cylindrical coal core from Chongqing, China, under hydrostatic condition. Permeabilities at various gas pressures and confining stresses were measured on the original sample, as well as the sample with a proppant supported fracture. Gas adsorption, matrix swelling behaviour were also investigated by injecting non-adsorbing (He) and adsorbing (CH4) gases. The results show that proppant supported fracture has little effect on adsorption capacity, as well as the swelling behaviour due to gas adsorption. However, the proppant supported fracture can significantly enhance permeability about 2-3 orders of magnitude higher than original sample depending on proppant type and its distribution in the fracture. Fracture compressibility may be decreased by 1 order of magnitude, suggesting that the permeability for the proppant supported fracture is less sensitive to stress. It was also found that sparsely placed monolayer proppant can have comparable permeability with densely packed multilayer proppant.

Coal petrography, mineralogy, elemental compositions and palaeoenvironmental interpretation of Late Carboniferous coal seams in three wells from the Kozlu coalfield (Zonguldak Basin, NW Turkey)

A number of coal seams of Late Carboniferous age in the coal-bearing Karadon, Kozlu, and Alacaağzı formations have been mined in several deep underground mines in the Zonguldak Basin (NW Turkey). For this study, a total of 154 coal core samples were collected from three research wells that were drilled in the tectonically complex settings of the Kozlu coalfield. All the coal samples were studied by means of coal petrography, mineralogy, and geochemistry. Special attention was given to the compositions and origins of fracture/cleat-filling minerals that were identified by SEM-EDX on polished blocks. The coal seams investigated are of bituminous rank. They are characterized by low total sulphur contents (≤1.0%, on dry basis) and high gross calorific values (22.5–40.2MJ/kg, on dry, ash-free basis), whereas ash yields and volatile matter contents are variable. X-ray powder diffraction data indicate that the mineralogical compositions of the studied coal seams are rather similar, and clay minerals, along with quartz and carbonate minerals, were identified in almost all seams. The statistical evaluations and SEM data show that some elements such as Mg, Ti, Zr, Nb, and Th are generally affiliated with silicate minerals, whereas elements like Cu, Zn, Mo, and Pb are mainly associated with sulphide minerals. The silicate minerals are generally detrital, and the majority of carbonate minerals are of epigenetic origin. The epigenetic carbonate and sulphide minerals suggest that circulated solutions within coal seams were mainly derived from Ca2+-rich solutions leached from overlying formations such as Cretaceous aged Zonguldak formations and from hydrothermal solutions that likely were derived from andesitic dykes within the basin. The regional tectonic activities during coalification seem to have deformed coal-bearing formations and, therefore, intra-basinal solutions could have penetrated into the studied coal seams. The epigenetic mineralisation also resulted in elemental enrichments and elevated ash yields, total sulphur, and volatile matter contents in some coal seams. Thus, deformation during coalification controlled some coal quality parameters in the studied wells. The coal facies diagrams suggest that palaeomires mostly accumulated under limno-telmatic conditions, whereas a telmatic environment was more common in palaeomires of the Kozlu Formation. Coal petrography suggests that peat-forming vegetation and water tables in palaeomires were changeable during the Late Carboniferous.

Geochemical and organo-petrographic characterization for hydrocarbon generation from Barakar Formation in Auranga Basin, India

Keeping in pace with global studies of shale gas/oil, the authors had selected fifty-one (41 shales and 10 coal) core samples of Barakar Formation (lower Sakmarian–Kungurian) from four boreholes of Auranga basin, to understand hydrocarbon generation potential. The Rock-Eval pyrolysis along with total organic carbon content (TOC), organo-petrographic, vitrinite reflectance, Fourier Transform infrared spectroscopy (FTIR), X-ray fluorescence (XRF) analyses were carried. These studies indicate that all the samples (coal and shale) have fair to excellent hydrocarbon generation in respect of TOC and S2 (under Rock-Eval pyrolysis). Further, the thermal maturity (Tmax) of shales and coals are placed in ranges from 409 to 468 and 420–426°C respectively. However, vitrinite grains having reflectance>0.60% (↑Vt60%) may indicate the onset of thermogenic oil generation could possible in the majority of the samples. The presence of alginite, sporinite, resinite, liptodetrinite and suberinite along with perhydrous vitrinite and inertinite macerals is observed. The linear relationship of total maceral content (TMC) with TOC may help in their determination empirically. Multiple regression analysis denote maceral reactivity in the generation of S2. In addition, A-factor (aliphatic/aromatic peak intensity) and C-factor (carbonyl/aromatic peak intensity) under FTIR spectra, suggest dominance of kerogen type III, IV and II/III. The geochemical indices like CIA (chemical index of alteration), CIW (chemical index of weathering) and ICV (index of compositional variation) indicate strong to intermediate weathering, under passive continental margin depositional environment supporting luxurious vegetation and organic matter preservation.

3D Modelling of Coal Deformation under Fluid Pressure using COMSOL Multiphysics

Gas transportation in coal bed is a complex physical process. The Adsorption/desorption process is associated with the migration of gas in coal. Adsorption/desorption of CO2/CH4 in coal bed includes diffusion of gas from coal matrix and its flow through natural fractures. Therefore, it is necessary to have a clear concept of the flow behavior of gas in coal beds for successful coal bed methane (CBM) production and carbon dioxide sequestration. The flow of gas through natural cracks results in the deformation of the coal bed. Injection of CO2 in enhanced coal bed methane (ECBM) depends on the permeability and fluid flow behavior. Deformation in solid coal induced by fluid pressure during CBM and ECBM process is still not clearly understood. Numerical models and Multiphysics are dominant in the modeling of a complex study of flow-induced deformation. Such modeling is required for a detailed understanding of deformation as well as flow parameters. In this study 3D model of fractured coal, the core was developed. The deformation in the coal core at multiple injected fluid pressures was analyzed. Geo-mechanical parameters (Stress, Strain, Deformation, Pore pressure, and Darcy’s Velocity, etc.) were determined at varying injection pressures. The relations between different Geo-mechanical parameters were established using a statistical approach.y]