Coal Core Research Articles

Experimental study on proppant embedment depth and fracture conductivity in coal seam

The fracturing of coal seam in one of the blocks in Xinjiang is significantly different from that of conventional formation. After fracture closure, there is the proppant embedment, especially in the soft formation with high closure pressure. The mechanical parameters of the coal core leave out of consideration in conventional proppant embedment tests, so the effect of proppant embedment on the fracture conductivity is uncertain, which brings a challenge to the optimization of proppant concentration and proppant selection. In order to quantitatively evaluate the proppant embedment in coal seam, the core drilled on the coal is tested to measure the mechanical parameters of the coal core, and the proppant embedment morphology of the natural coal core is tested. The mechanical parameter test of natural coal core provides relatively accurate data under simulated coal seam conditions, and provides reference basis for the design of coal seam fracturing.

A 0-dimensional cavity growth submodel for use in reactor models of underground coal gasification

Modelling of the underground coal gasification process is dependent upon a range of sub-models. One of the most important is the calculation of the cavity growth rate as a function of various operating conditions and coal properties. While detailed 1-dimensional models of coal block gasification are available, it is not easy to couple them directly with reactor models, which aim to simulate the complete process. In this paper, a 0-dimensional cavity growth sub-model is presented. The model is based on the concept of a surface reaction and incorporates physics to account for moisture evaporation, water influx, coal pyrolysis, coal thermo-mechanical fragmentation and the build up of an ash layer on the char. The model is validated using measurements from laboratory experiments on coal cores and coal blocks. A comparison of calculated results from several UCG field trials shows that the model can provide good estimates of cavity growth rate for reasonable input parameters. Finally, simulation results of cavity growth in the combustion and gasification zones as a function of the bulk gas temperature, gas pressure, water influx rate, ash layer thickness and coal fragmentation behaviour are presented.

Heterogeneities of seepage pore and fracture of high volatile bituminous coal core: Implications on water invasion degree

High volatile bituminous coal (HVBC) reservoirs are now regarded as one of the most important targets for coalbed methane (CBM) exploration and development in northwestern China. Water invasion during coalbed methane reservoir fracturing can induce water blockage damage, leading to decreased relative permeability of gas and reduced production of CBM well. To study the influences of heterogeneities of seepage pore and fracture on water invasion degree (WID), water flooding experiment using HVBC core was conducted to simulate water invasion; magnetic resonance imaging and gray calculation were applied to quantitatively determine WID; heterogeneous parameters of seepage pore and fracture were extracted based on micro-CT and fractal characterization; finally implications of seepage pore and fracture heterogeneities on WID were discussed, and water invasion mechanism was analyzed.The results show that fracture porosity, connected fracture porosity, minerals’ filling porosity, displacement pressure, efficiency of mercury withdrawal, and fractal dimension as well as tortuosity of seepage pore, are closely correlated with WID. As fracture porosity increases, WID initially increases then tends to be unchanged. Minerals’ filling can be dominated factor resisting water invasion when severe filling occurs. Seepage pores with good connection and permeability are beneficial for water invasion, while those with complex structure and tortuosity are detrimental to water invasion. Reservoirs developing connected fractures and homogeneous seepage pores are not only beneficial for water invasion, but also beneficial for water elimination. Reservoirs developing unconnected fractures and homogeneous seepage pores are beneficial for water invasion while detrimental to water elimination. Reservoirs lacking fractures’ development but developing complex structural seepage pores are detrimental to both water invasion and elimination.

Influences of proppant on fracture making ability and fracture conductivity in coalbed methane

The fracture conductivity is one of the most important factors affecting coalbed methane production. The size of proppant, the number of proppant placement layers and the embedment all affect the permeability, thus affecting the fracture conductivity. However, exist research ignored the influence of fracture width. Therefore, according to the formula for calculating the fracture width of acid fracturing, the formula for calculating the fracture width is deduced. According to Carman-Kozeny formula, the fracture conductivity calculation models with and without embedment are respectively established. Through conducting the flow experiment on the coal core, the influence of the number of placement layers and embedment degree on the fracture conductivity is analyzed. The results show that the larger the particle size of proppant is, the more layers are laid, and the wider the fracture width is, thus the greater the fracture conductivity is. The smaller the embedment degree of proppant, the greater the fracture conductivity.

Room temperature oxidation of active sites in coal under multi-factor conditions and corresponding reaction mechanism

The thermal decomposition of oxygen-containing functional groups can produce active sites that accelerate coal spontaneous combustion. To investigate the room temperature oxidation of the active sites in coal by varying multiple experimental conditions (e.g., pyrolysis temperature, pyrolysis duration, oxygen concentration, and pyrolysis-oxidation times), pyrolysis-room temperature oxidation experiments and corresponding infrared spectroscopy experiments were carried out. First, by analysing the evolution of gas generation and the coal core temperature as a function of time during the room temperature oxidation of coal samples, it is found that the active sites oxidize more intensely under the conditions of higher the pyrolysis temperature, longer the pyrolysis duration and higher oxygen concentration. Then, the continuous pyrolysis-room temperature oxidation experiment and the changes in the chemical structure prove that the oxygen-containing functional groups can be converted into the active sites and vice versa. In addition, the concentration of the gaseous products evolved in each room temperature experiment shows that the amount of CO2 produced is higher than that of CO, and the molar ratio of CO2 to CO is approximately 3.3:1, which was found by using a statistical method and indicated that the two gas products are derived from different reaction pathways of the identical substance. Finally, a possible microscopic reaction mechanism of the room temperature oxidation of the active sites is proposed. The research results are of great value not only for revealing the coal spontaneous combustion mechanism, but also for preventing the spontaneous combustion of heat-invaded coal in the subsequent mining process.

Experimental investigation of effects of CO2 injection on enhanced methane recovery in coal seam reservoirs

Enhanced coalbed methane (ECBM) recovery performed by injecting CO2 into coal reservoirs has been found to be a viable option for methane recovery, while CO2 injection into coal seams, especially unmineable ones, has great potential to sequestrate large volumes of CO2 to mitigate greenhouse gas accumulation in the atmosphere. This study was therefore initiated to investigate the effect of CO2 flooding on ECBM recovery and CO2 storage in coal. A series of CO2 core flooding tests was carried out on a high rank bituminous coal core with CO2 injection pressures from 6 to 10 MPa. CO2 injection rate, methane and CO2 production rates, methane displacement efficiency and the volumetric strains of the core were calculated and recorded to enable interpretation of the results. It was observed that, compared with natural methane production in which only around 51.73% of methane is recovered, CO2 injection provides excellent methane displacement efficiency (over 90%), especially for high injection pressures (greater than 8 MPa), which drive methane completely out of the coal sample. Although higher CO2 flooding pressure favours greater CO2 injection rates and methane production rates for any coal types, high rank coal shows faster and greater methane production and CO2 storage capacity. The observed rapid breakthrough of CO2 in the gas produced by elevated CO2 injection, especially for high injection pressures, may complicate the gas separation process. Adsorption-induced volumetric strain is approximately proportional to the CO2 storage capacity in coal which increases with CO2 pressure, and the strain reduces coal permeability and therefore CO2 injectivity and methane production.

Insight into the chemical reaction process of coal self-heating after N2 drying

It is generally considered that changes in physical rather than chemical structures are responsible for the increase in the spontaneous combustion tendency of coal after inert drying. In this paper, an isothermal flow reactor was combined with FTIR and ESR techniques to study the room temperature oxidation of coal samples under the effects of drying temperatures, numbers of drying-oxidation cycles and anaerobic heating temperatures. Macro-physical parameters such as gaseous generation, oxygen consumption, coal core temperature and micro-chemical structural parameters such as functional groups and free radicals were examined. After coal drying, the thermal decomposition of oxygen-containing functional groups generates free radical active sites, which can be oxidized and are exothermic at room temperature, accompanied by the generation of large amounts of CO and CO2 gaseous products. In addition, the active sites and oxygen-containing functional groups can be transformed into one another under certain conditions. The results indicate that the inherent reason responsible for self-heating and uncontrolled spontaneous combustion of N2-dried coal is the room temperature oxidation of the active sites, especially the room temperature oxidation of the secondary active sites generated by thermal decomposition of the oxygen-containing functional groups. The results presented in this research have applications to reveal the chemical reaction mechanism determining why lignite is more prone to spontaneous combustion after inert drying.

Characterisation and evaluation of shockwave generation in water conditions for coal fracturing

We investigated the effect of circuit parameters on shockwaves generated by electrohydraulic discharge (EHD), and the application of EHD to stimulation of coal seam gas reservoirs. The discharge circuit analyses indicate that the deposited electrical energy used for shockwave generation is not simply determined by charging voltage or capacitance. The discharge efficiency is also affected by mutual interactions among inductance, resistance and capacitance as well as real-time current change. Furthermore, the relation between charging voltage and peak pressure strength revealed that voltage at breakdown point, ranging from 9 kV to 28 kV, was more closely associated to shock strength rather than the initial charge voltage. Generally, a higher breakdown voltage tends to lead to a stronger shockwave. Finally the EHD fracturing stimulation conducted on coal cubes showed the coal was effectively fractured with enormous cracks and voids. By using three-dimension computed-tomography (CT), the fracture extension and propagation in the coals are found to be obvious and extensive. Though the existing fracture greatly affects the new fracture development in vertical direction, the horizontal fractures parallel with the bedding plane direction are more likely to occur and even expand across the entire tested coal core. The dramatic coal permeability improvement (over a hundred times) after EHD also validates the cutting-through fractures, as does the enhancement of pore and porosity observed by morphology and structural characterisation via scanning electron microscopy (SEM) and mercury intrusion porosimetry (MIP).

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.

Damage mechanism of cement slurry to CBM reservoirs with developed fractures and cleats: A case study from eastern Yunnan and western Guizhou in China

The coalbed methane (CBM) reservoirs in the areas of eastern Yunnan and western Guizhou are characterized by developed cleats and fractures and low fracturing pressures, so cementing slurry (“slurry” for short) can invade into CBM reservoir easily, resulting in reservoir damage and abnormal increase of reservoir transformation fracturing pressure. In order to reveal the damage mechanisms of slurry to this type of coal reservoirs, we analyzed the physical and chemical properties and potential damage modes of coal rocks. Then, the development situations of fractures and pores before and after the coal core samples were internally contaminated and the invasion and plugging situations of slurry in fractures and pores were analyzed intuitively by means of CT scanning and scanning electron microscope (SEM), and the percentage of slurry and fractures in coal core volume was calculated. In this way, a method to quantitatively evaluate the damage of slurry to coal reservoirs was established. And the following research results were obtained. First, under the effect of differential pressure, slurry and its filtrate invade into coal reservoirs along the fractures. The invasion degree varies with the development degree of fractures and pores. The more developed the fractures and pores, the higher the invasion degree. Second, the cement products formed after the slurry in the reservoirs gets cemented and solidified fill the fractures and pores tightly and cover the surface of coal core samples densely, so CBM flowing channels are blocked severely. Consequently, the permeability of coal core samples decreases and the compressive strength of coal rocks increase, leading to the abnormal increase of subsequent fracturing pressure and impacting the fracturing stimulation effects. Third, the effect of slurry filtrate on the alkali sensitivity and velocity sensitivity of coal rocks is much less than the damage degree of slurry invasion to coal rocks. In conclusion, this newly developed quantitative evaluation method for the damage of slurry to coal reservoirs is of guiding significance to improving the cement job quality of coal reservoirs and ensuring the efficient CBM development.

Swelling of clay minerals and its effect on coal permeability and gas production: A case study of southern Qinshui Basin, China

AbstractThe permeability damage of high‐rank coal reservoirs is the main factor affecting the development of coalbed methane (CBM) in the late production stage. Coal core samples from southern Qinshui Basin were saturated with actual formation water, 1/2 salinity formation water, and distilled water to study the swelling of clay minerals. Then, the mechanism of permeability change caused by water sensitivity and its effect on gas production was discussed. The results show that the permeability decrease caused by flow velocity sensitivity did not occur during the water sensitivity experiments. The permeability damage caused by water sensitivity is mainly the static damage from swelling of clay minerals, especially illite/smectite mixed clays and chlorite. During the process of CBM development, the water sensitivity of coal reservoirs will lead to the sharp increase of Na+ concentration and a decrease of Mg2+ concentration in the produced water. Meanwhile, the clay minerals swell due to the adsorption of more Mg2+ and Ca2+, decreasing the reservoir permeability and gas production rate.

DigiCoal: A computational package for characterisation of coal cores

Coal cores contain a complex fracture system, known as cleats, that highly impacts its petrophysical properties. We present, DigiCoal computational toolbox, an integrated set of numerical functions, for analysing 3D micro computed tomography (micro-CT) images of coal and reconstructing representative digital models. The method presented in this paper can be applied to other fractured rocks as well. Coal characteristics analysed in this paper include cleat geometry, topology of cleat network, degree of mineralisation, and roughness of cleat surfaces. Micro-CT scanning procedures to obtain high-quality 3D images of coal are explained. Then, the steps to computationally analyse the images are described: CT image pre-processing, statistics extraction, digital coal modelling and structural analyses. This paper offers an overview of the structure and techniques used in the creation of the computational methods, together with code snippets and applications.

Variable Pore Structure and Gas Permeability of Coal Cores after Microwave Irradiation

The experimental work in this paper investigated the effect of microwave energy on pore structure and gas permeability of coal cores. Fifteen coal samples were irradiated under the condition of 2.45 GHz with 6 kW. The effect of microwave irradiation on the pore structure of coal samples was evaluated by nuclear magnetic resonance (NMR). The water saturation degree has little influence on transverse relaxation time (T2) distribution before the microwave treatment. By contrast, water saturation degree obviously affects the T2 distribution after the microwave energy treatment. Coal permeability increased after microwave energy treatment. But fractal dimension decreased after microwave energy treatment. The results show that microwave energy has a potential for degassing coal seams.

Changes in pore structure and permeability of anthracite coal before and after high-voltage electrical pulses treatment

High-voltage electrical pulses (HVEP) can potentially be used to increase the yield of low-permeability coalbed methane (CBM) reservoirs. This study highlights the pore change and permeability evolution characteristics of anthracite coal samples using HVEP treatment. An experimental system was developed to study the variation of coal core permeability by subjecting the coal samples to HVEP treatment under confining conditions. Scanning electron microscopy (SEM) and mercury intrusion porosimetry (MIP) analyses were adopted to gain a clearer insight into the changes in pore structure. SEM images revealed that the surface morphologys of the raw coal samples were complete and smooth, whereas those of the coal samples subjected to HVEP developed many pores and cracks. The pores and cracks generated enhanced the permeability of HVEP-treated coal samples. MIP test results showed that the cumulative pore volume of the coal samples subjected to HVEP was significantly greater than that of raw coal samples. Importantly, the HVEP technology had a significant effect on mesopores and macropores, compared with micropores. Because the mesopores and macropores are critical for gas transport, the technology will be helpful for enhancing CBM recovery. Seepage experiments results showed that with the variation of CH4 injection pressure from 0.4 MPa to 1.2 MPa, the permeability of both raw coal samples and coal samples subjected to HVEP showed a decline at a particular confining stress. The permeability of Yangzhuang (YZ) coal sample subjected to HVEP was 1.6–2.2 times that of YZ raw coal under a constant confining pressure, and the permeability of Guhanshan (GHS) coal sample subjected to HVEP is 1.8–2.4 times that of the GHS raw coal sample. At the same confining pressure and CH4 injection pressure, the permeability of the coal samples subjected to HVEP was obviously higher than that of the raw coal samples.

Characterization of dynamic mechanical alterations of supercritical CO2-interacted coal through gamma-ray attenuation, ultrasonic and X-ray computed tomography techniques

Coal-Supercritical CO2 (S-CO2) interaction causes severe mechanical alterations in coal that can possibly affect the mechanical competency and the coal seam integrity. Conventional static mechanical testings often fail to conduct repetitive and temporal measurements on coal specimens in the process of coal-S-CO2 interaction, thus unable to attain at decisive and reliable conclusions. Moreover, the said techniques fail to capture the mechanical anisotropies of heterogeneous coal mass that arise due to complex spatial distribution of mineral/maceral phases and fracture network. We combine three non-destructive techniques, viz. gamma-ray attenuation, ultrasonic technique and micro computed tomography to evaluate the S-CO2 interaction induced localized and anisotropic micro-structural alterations and the consequent mechanical response of a heterogeneous coal core specimen. Dynamic moduli were computed at radial and axial directions at natural condition, after 14 days and 45 days of S-CO2 interaction, and the resultant mechanical alterations were elucidated with the 3D-reconstructed micro-CT data collected at the same stages. Visualization and quantification of coal micro-structure emphasize that S-CO2-interacted coal mass is subjected to severe micro-cracking due to differential and free swelling, drying induced shrinkage and surface energy reduction, which can possibly lead to mechanical degradation. Spatial distribution of minerals and fractures at specific orientations significantly affects the gamma and ultrasonic wave propagation, thus can be successfully utilized to reflect the mechanical anisotropies. For same interaction condition and period, coal mass exhibits mechanical anisotropies and strength alterations at different levels, in which more heterogeneous mineral/maceral distribution causes significant localized mechanical alterations in mineral-rich areas, compared to that of comparatively homogeneous coal-rich regions. Temporal evaluation of strength parameters suggests that regardless of the considered orientations, significant strength alterations occur at initial stage of the coal-S-CO2 interaction process and the longer interaction causes only a slight strength reduction. Overall results conclude that S-CO2 induced mechanical property alterations in heterogeneous coal are highly localized and anisotropic, thus should be carefully evaluated in CO2 sequestration and ECBM extraction processes.

The variation in produced gas composition from mixed gas coal seam reservoirs

This paper investigates predictive modelling of gas production from coal seam reservoirs that contain mixtures of CO2 and CH4. A series of laboratory experiments are presented that are analogues of the gas production process where gas is drained at a controlled rate from a coal core sample with a known initial gas content and composition. The experiments were performed at two initial gas compositions and for two outflow rates. There was excellent agreement in the mass balance of the experiments with the difference between the initial gas volume and cumulative outflow ranging between 0.8 and 1.8% of the initial volume. During the early stages of gas desorption the CO2 concentration of the outflow gas was significantly lower than that in the adsorbed gas but increased significantly at low pore pressures. Thus, the CH4 was preferentially desorbed relative to CO2. For example, in an experiment in which the initial gas content of the coal was ~30% CO2, the CO2 concentration of the produced gas was only ~13% and did not increase significantly until the pore pressure fell below 0.8 MPa, ultimately increasing to ~95%. A reservoir simulator was used to match the desorption observations, and the results indicate, in a finding consistent with previous work, that the extended Langmuir adsorption model based on pure gas isotherms does not provide accurate predictions. A modified extended Langmuir approach was developed, based on using measurements of the initial gas content and its composition, provided a more accurate fit to the experimental observations of gas desorption. A fully predictive procedure using the 2D Equation of State adsorption model with the pure gas adsorption isotherms provided good agreement with gas desorption observations. A series of hypothetical reservoir simulation case studies are presented that illustrate changes of CO2 concentration with time for a variety of initial compositions.

Anisotropic coal permeability and its stress sensitivity

This paper focuses on anisotropy of coal permeability and its stress sensitivity. Coal blocks were taken from Xinjing Coal Mine in Yangquan Coal District. Coal cores were then drilled along the strike, dip, and vertical directions. Coal permeabilities were measured with respect to stress by using a self-developed coal permeability measurement system. The used samples exhibited significant permeability anisotropy. The permeability along the strike direction was greatest among the three directions, the permeability along the vertical direction was the smallest, and the permeability along the dip direction was between the other two directions. The sensitivity of coal permeability to stress was transversely isotropic. The stress sensitivity coefficient was greater along the horizontal directions than along the vertical directions. Coal permeability exhibited anisotropic stress sensitivity due to anisotropy in Young’s modulus and porosity. The results obtained in this study are useful for optimizing the arrangement of pre-drainage boreholes.

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.