Coal Seam Permeability Research Articles

Study on parameter optimization of deep hole cumulative blasting in low permeability coal seams

Coal seam gas extraction is an important means of exploiting and utilizing gas resources, as well as a means of preventing coal mine disasters. To improve gas extraction efficiency in high gas and low permeability coal seams while ensuring blasting security, deep hole cumulative blasting parameters were optimized. ANSYS/LS-DYNA software is used to establish a 3-dimensional cumulative blasting model. By comparing and analyzing the blasting stress nephograms, stress time-history curves, and crack expansion curves, the optimal blasthole diameter, charge position, and charge length are obtained. Based on the numerical simulation results, a field test was carried out in the No. 10 coal seam of the Pingdingshan coal mine. The test results show that after cumulative blasting, the gas concentration was increased by an average of 2.25 times, the gas purity was increased by an average of 3.78 times, the permeability coefficient of the coal seam was increased by 21 times, and the effective radius of blasting was up to 7 m. The positive effects of deep hole cumulative blasting parameter optimization on the pressure relief and permeability enhancement of a high gas and low permeability coal seam were determined, which can provide a reference for other similar working faces to implement this technology.

Mechanism of Coal Seam Permeability Enhancement and Gas Outburst Prevention under Hydraulic Fracturing Technology

In view of the high gas pressure and low permeability of deep coal seam, it is difficult to control gas, which affects the safety production of coal mine. The technical scheme of hydraulic fracturing to improve the permeability of coal seam is put forward, and the gas drainage technology is used to control the gas emission of coal seam. The fracturing effect under different water pressure, different gradient fracturing times, and in situ stress is analyzed by using 3DEC (3-Dimensional Distinct Element Code) discrete element software. The simulation analysis and field verification results show that the coal seam gas pressure increases linearly with the buried depth. In situ stress characteristics and hydraulic strength are the key factors affecting the effect of hydraulic fracturing. The fracturing radius increases with the increase of flow. When the construction pressure of hydraulic fracturing test is 18 MPa, the distance between fracturing hole and drainage hole is 8.5 m. The actual measurement shows that after hydraulic fracturing and gas drainage, the maximum gas emission is reduced by 51%, and the average gas emission is reduced by 58%.

Effects Of Matrix Swelling On Coal Permeability For Enhance Coalbed Methane (Ecbm) And Co2 Sequestration Assessment Part I: Laboratory Experiment

It has been reported that coal matrix swelling/shrinkage associated with CO2, adsorption/desorption are typically two to five times larger than that found for methane, yet there has been no direct measurements of this effect on permeability of coals to CO2. The feasibility of ECBM/CO2 sequestration technology depends very much on the magnitude of matrix swelling effect on permeability, especially in deep, low permeability coal seam reservoirs. The main objective of this research is to investigate the effects of coal matrix swelling induced by CO2 adsorption on the permeability of different coals which have been undergoing methane desorption under simulated reservoir conditions in the laboratory. Coal and reservoir properties which may impact upon this behaviour will be identified through extensive laboratory testing. This paper – first of two – presents the procedure for the laboratory tests as well as the findings. In the second part, a field permeability model for enhanced methane recovery and CO2 sequestration, incorporating the findings of the current laboratory tests, would be developed.

EFFECTS OF MATRIX SWELLING ON COAL PERMEABILITY FOR ENHANCE COALBED METHANE (ECBM) AND CO2 SEQUESTRATION ASSESSMENT PART I: LABORATORY EXPERIMENT

It has been reported that coal matrix swelling/shrinkage associated with CO2 , adsorption/desorption are typically two to five times larger than that found for methane, yet there has been no direct measurements of this effect on permeability of coals to CO2 . The feasibility of ECBM/CO2 sequestration technology depends very much on the magnitude of matrix swelling effect on permeability, especially in deep, low permeability coal seam reservoirs. The main objective of this research is to investigate the effects of coal matrix swelling induced by CO2 adsorption on the permeability of different coals which have been undergoing methane desorption under simulated reservoir conditions in the laboratory. Coal and reservoir properties which may impact upon this behaviour will be identified through extensive laboratory testing. This paper – first of two – presents the procedure for the laboratory tests as well as the findings. In the second part, a field permeability model for enhanced methane recovery and CO2 sequestration, incorporating the findings of the current laboratory tests, would be developed.

Effects Of Matrix Swelling On Coal Permeability For Enhance Coalbed Methane (Ecbm) And Co2 Sequestration Assessment Part Ii: Model Formulation And Field Application

In part II of this two-part paper series, a field permeability model for enhanced methane recovery and CO2 sequestration, incorporating the findings of the current laboratory tests presented in part I is presented. It has been reported that coal matrix swelling/shrinkage associated with CO2, adsorption/desorption are typically two to five times larger than that found for methane, yet there has been no direct measurements of this effect on permeability of coals to CO2. The feasibility of ECBM/CO2 sequestration technology depends very much on the magnitude of matrix swelling effect on permeability, especially in deep, low permeability coal seam reservoirs. The main objective of this research is to investigate and develop numerical models based on the the effects of coal matrix swelling induced by CO2 adsorption on the permeability of different coals which have been undergoing methane desorption under simulated reservoir conditions in the laboratory.

A numerical investigation of hydraulic fracturing on coal seam permeability based on PFC-COMSOL coupling method

Hydraulic fracturing and permeability enhancement are effective methods to improve low-permeability coal seams. However, few studies focused on methods to increase permeability, and there are no suitable prediction methods for engineering applications. In this work, PFC2D software was used to simulate coal seam hydraulic fracturing. The results were used in a coupled mathematical model of the interaction between coal seam deformation and gas flow. The results show that the displacement and velocity of particles increase in the direction of minimum principal stress, and the cracks propagate in the direction of maximum principal stress. The gas pressure drop rate and permeability increase rate of the fracture model are higher than that of the non-fracture model. Both parameters decrease rapidly with an increase in the drainage time and approach 0. The longer the hydraulic fracturing time, the more complex the fracture network is, and the faster the gas pressure drops. However, the impact of fracturing on the gas drainage effect declines over time. As the fracturing time increases, the difference between the horizontal and vertical permeability increases. However, this difference decreases as the gas drainage time increases. The higher the initial void pressure, the faster the gas pressure drops, and the greater the permeability increase is. However, the influence of the initial void pressure on the permeability declines over time. The research results provide guidance for predicting the anti-reflection effect of hydraulic fracturing in underground coal mines.

Synergistic ECBM extraction technology and engineering application based on hydraulic flushing combing gas injection displacement in low permeability coal seams

With a continuous increase in mining depth, high gassy coal seams with low permeability increasingly appear. Although hydraulic flushing (HF) is an efficient technology applied widely for enhanced coalbed methane (ECBM) in low permeability coal seams (LPCS), the methods utilized to eliminate the adverse effects of stress concentration (SC) zones generated by HF on gas drainage have not been proposed. Therefore, a synergistic ECBM extraction technology based on HF integrating with gas injection displacement (GD) has been put forward in this paper. The primary role of HF is to provide ample migration space for gases so that the gas injection resistance can be reduced in the following GD process. Accordingly, the GD is implemented to eliminate the negative effect of the SC zones on gas drainage by increasing the pressure gradient, and the permeability enhancement and pressure holding function of nitrogen-rich gas also take effect. Additionally, a coupled gas flow model was developed and validated to describe the synergistic process. The optimal engineering parameters that can provide theoretical reference for engineering application were determined systematically according to the modeling results. The on-site boreholes should be all flushed, and the actual drilling diameter should still be determined according to the geological and technical conditions in the field. Furthermore, low pressure (LP) is the best choice before the effects of HP being further investigated on fracturing coal and inducing outbursts. Meanwhile, the analysis results of the field experiment in the Chensilou coal mine indicate that the proposed technology can effectively promote the adsorbed methane remaining in the SC zones to desorb adequately and then migrate to the gas drainage boreholes. Especially for LPCS, this synergistic ECBM extraction technology integrating HF with GD possesses a great application prospect.

Interaction mechanism of water movement and gas desorption during spontaneous imbibition in gas-bearing coal

After a series of permeability enhancement and gas drainage measures are implemented in low permeability coal seams, the gas content still does not up to national standard. How to improve the transformation rate from adsorbed gas to free gas is very important. Coal seam water injection is a comprehensive disaster prevention measure. The flowing of water includes the processes of seepage and spontaneous imbibition during coal seam water injection. The process of seepage mainly displaces free gas, which shows the characteristics of rapid attenuation. The gas and water interact with each other during spontaneous imbibition which is an important way of water movement to the depth of coal seam. Spontaneous imbibition is the key to fully wetting, which provides guarantee for the water distribution in coal seam, and which establishes the foundation for the replacement of adsorbed gas. In this paper, a device integrated the isobaric water addition, isobaric desorption and spontaneous imbibition height measurement was developed. Isobaric desorption can ensure that the replaced gas is quantitatively tested. The change of electrical resistivity was grouped into the processes of sudden change, fast change and gradual change. The spontaneous imbibition height increases with increasing gas pressures during isobaric imbibition. Based on dimensionless analysis method, the mechanism that the greater the gas pressure around the capillary with one end opening, the more obvious the plugging effect is revealed. The lower the gas pressure, the better the wetting effect on the micropores and some mesopores. The equilibrium state of micropores and some mesopores determines whether the process of spontaneous imbibition is over. This study realizes the recognition of coal seam water injection technology.

Application of integrated drilling and stamping technology in gas extraction through layer drilling

In coal mine gas extraction in China, the current hydraulic permeability enhancement measures generally have complex processes, low adaptability, long operation time, and high labor intensity. This makes it challenging to meet the requirements of coal mining enterprises for safe and efficient mining, and there is an urgent need to develop new technologies to enhance gas extraction and production. This paper proposed an integrated drilling and stamping technology to enhance gas extraction and production and integrates drilling, hydraulic jet fracturing, and hydraulic punching to enhance the permeability of coal seams. A field test was conducted using the specially-developed integrated drilling and stamping equipment to extract gas from the 16101 bottom pumping lane penetration hole in the Jiulishan Mine, Jiaozuo, Henan. The test results show that compared with the hydraulic punching technology previously used in the mine, the punching time of the soft coal seam was shortened by 66–75%, the coal output was increased by 1.7 times, and the punching hole aperture was increased by 1.3 times. Hydraulic injection fracturing was successfully performed to increase the permeability of the hard coal seams, and the fracturing formed a shot hole depth of 345–539 mm. After the integrated drilling and stamping of the drill holes, the coal output, gas extraction concentration, pure gas extraction volume, and coal seam permeability coefficient increased by 2.4, 2.2, 4, and 37.3 times, respectively. The gas flow decay coefficient of the drills holes was also further reduced, which significantly improved the extractionefficiency. Thus the integrated drilling and stamping technology can transform the process of gas extraction from regular extraction to quick, economic, pure, and clean extraction. Thus, this technology has large applicable value for underground gas management in coal mines.

A New Fractal Permeability Model Considering Tortuosity of Rock Fractures

During methane extraction, the permeability of a coal seam is the vital factor affecting recovery. Although the permeability of a coal seam and its relationship with porosity have been studied in a few works, the calculation process of coal seam permeability is usually too simplistic or neglects the influence of microscopic fracture structures. Statistical research shows that the permeability of coal seams with the same porosity and different fracture structures is quite different. For the purpose of quantitatively investigating the contribution of fractures and pore structure in coal seams, a fractal permeability model considering the microstructure of coal seam fracture is established in this paper. The correctness of the model is verified by comparing with the previous research results. Then, the influence of the microscopic fracture structure on the equivalent permeability is analyzed. The simulation results show that the permeability of fractured coal is directly proportional to the fractal dimension of the fracture, the maximum fracture length and the azimuth. It is inversely proportional to the tortuous fractal dimension and the dip angle of the fracture surface. This conclusion provides the foundation for revealing the microstructure mechanisms of macroscopic seepage characteristics of coal seams, and implementing effective strategies to enhance gas recovery rates under different geological structures.

Effect of Liquid Nitrogen Freeze-Thaw Cycles on Pore Structure Development and Mechanical Properties of Coal.

To improve the mining efficiency of coalbed methane, liquid nitrogen freeze–thawing experiments were performed to improve coal seam permeability and to study its influence on coal pore structure development and mechanical properties. Mechanical properties and nuclear magnetic resonance tests of coal samples were performed with 0, 5, 10, and 15 freeze–thaw cycles of liquid nitrogen. The results show that the number of freeze–thaw cycles caused the change of uniaxial compressive strength and elastic modulus of coal, and the change effect decreased significantly after 11–15 freeze–thaw cycles. Between 0 and 5 freeze–thaw cycles, the base growth rate of the transverse relaxation time T2 spectral area of the full pore of coal is 44.1%, and that of the transverse relaxation time T2 spectral area of adsorption pore is 71.5%. After 6–10 freeze–thaw cycles, the fixed base growth rate of the transverse relaxation time T2 spectral area of the full hole of coal is 269.0%, and the chain growth rate is 156.2%. In this stage, the chain growth rate of the transverse relaxation time T2 spectral area of the seepage hole is 198.4%, which is mainly the growth of seepage hole volume. After 11–15 freeze–thaw cycles, the chain growth rate of the full pore of coal transverse relaxation time T2 spectrum area is 20.1%, the chain growth rate of adsorption pore is 4.8%, the chain growth rate of seepage pore is 22.2%, and the growth rate of the pore volume is greatly reduced. Comparing the changes of pore and coal mechanical properties in different pore sizes, it can be seen that the change of adsorption pore volume has a greater impact on coal mechanical properties.

The Influence of CO2 Saturated Brine on Microstructure of Coal: Implications for Carbon Geo-Sequestration

Carbon dioxide geological storage in deep underground oil and gas reservoirs or coal seam is considered a key technology to effectively mitigate carbon emissions. Deep coal seams are the potential host rock for CO2 storage, with chemical reactions occurring when coal is exposed to CO2-saturated brine. However, limited studies have been conducted to reveal the effect of chemical reaction, particularly to unveil how this importantly affects the coal seam porosity at the micrometer scale. In this study, the CO2 geological storage is simulated using a high-temperature and high-pressure reactor containing CO2-saturated brine and coal samples. The samples in the previous study on the effect of CO2 on coal mostly contained high content of carbonate minerals and sulfide minerals. However, the tested Collie coal samples, which are characterized by high content of kaolinite and siderite, are rarely used. This coal was quantitatively analyzed using a scanning electron microscope before and after reaction. The results show that the microstructure of coal matrix was largely changed due to acid exposure. Kaolinite and siderite in the coal matrix were dissolved, the size and morphology of the cleats increased, and the absolute effective porosity also increased significantly. Most importantly, the connectivity of cleat network was improved, leading to an increase in CO2 storage space and coal seam permeability. Therefore, it is concluded that the microstructure changes of coal can be measured on a microscopic scale, and it is better to be quantitatively evaluated to improve the accuracy and reliability of CO2 storage in deep coal seams.

Research on Influencing Factors of Effective Gas Extraction Radius in Coal Mine Based on Multiple Linear Regression

In order to analyze the influence degree of different factors on gas extraction radius, on the basis of coal seam gas flow theory and elastic mechanics, considering the influence of coal seepage rate change on gas flow, the COMSOL Multiphysics numerical simulation software is secondarily developed by using the stress-seepage coupling mathematical model of coal and rock containing gas. Taking coal seam No.2 of Juji Coal Mine as an example, the effective extraction radius under different extraction negative pressure, extraction borehole diameter, extraction time, and initial permeability of coal seam are simulated, respectively. Then, the orthogonal test method is used to design different calculation schemes, and the effective extraction radius is used as the judgment index. The simulation software is used to calculate the effective extraction radius corresponding to different influencing factors. SPSS statistical software is used to analyze the parameters. The multivariate linear regression equation of the influencing factors of the effective extraction radius is obtained, and the multivariate linear regression model is established. The results show that the negative pressure of extraction has little effect on the effective extraction radius; the effective extraction radius gradually expands with the increase of extraction time and extraction aperture, but it has certain timeliness. The initial permeability of the coal seam has a great influence on the effective extraction radius. The influence degree of different influencing factors from large to small is the initial permeability of coal seam, extraction time, borehole diameter, and extraction negative pressure. Therefore, the key to improving the effectiveness of gas extraction is to increase the initial permeability of the coal seam before preextraction, ensure sufficient extraction time, appropriately increase the borehole diameter, and improve the negative pressure of extraction. Finally, the gas flow rate is used for a field test, and the measured results are basically consistent with the simulation results, which proves the correctness of the mathematical model of gas-containing coal and the feasibility of using numerical simulation methods to study the effective extraction radius of coal seams.

Regional Characteristics of Porosity and Permeability of Dahebian Syncline Coal and Its Application

In order to study the basic properties of coal seam thus to provide reference for coalbed methane (CBM) production, the porosity and permeability of coal from DH and WJZ regions of Dahebian syncline in Liupanshui Coalfield were systematically studied. The results shows that the porosity and permeability of coal samples in DH region are between 7.4–10.1% and 0.01 mD∼0.04 mD respectively. The permeability is obviously anisotropic, and it is distributed in a U-shape along bedding angle. The porosity and permeability in the WJZ region are between 12.4–24.85% and 0.3 mD–4.8 mD, but the correlation of permeability and bedding angle is not obvious. There are many primary cracks in coal matrix of WJZ region, while the coal of DH region is relatively complete. As a result, the porosity and permeability of WJZ coal samples are much higher than that of DH coal samples. With the change of confining pressure, obvious change has taken place in the permeability of coal in the two regions. When the confining pressure decreases from 2 to 8 MPa, the permeability loss rate of coal samples is close to 80%. The internal mechanism of the permeability loss is related to crack closure and plastic deformation caused by confining pressure. In contrast, the porosity and permeability of coal in WJZ region are much higher than those in DH region, which indicates that the permeability of the same coal seam in the same structural will have great differences and inhomogeneities. Therefore, the WJZ region is the preferred area, which can be included in the preferential exploitation plan of CBM. In addition, for the typical syncline gas reservoir in Liupanshui Coalfield, it is necessary to consider the block division according to the porosity and permeability indexes, and select the optimal block and give priority to development of CBM. Furthermore, we should pay attention to the difference and adaptability of CBM development in different regions.

Molecular insight into the diffusion/flow potential properties initiated by methane adsorption in coal matrix: taking the factor of moisture contents into account.

Coal seam permeability is one of the key parameters affecting coalbed methane (CBM), and plays an important role in resource evaluation and regional selection. To fully explore the diffusion/flow potential properties initiated by methane adsorption beneath diverse moisture contents (1-5%) in coal molecules. The pore size distribution and methane adsorption capacities were discussed based on Monte Carlo (MC) and molecular dynamics (MD) methods. The potential properties of diffusion/flow induced by methane adsorption were investigated using the maximum absolute adsorption capacities as benchmark. The variation patterns of the pore structure were analyzed using SEM scanning experiment to verify the results of simulation analysis. It is found that the free pores facilitate methane molecular adsorption and increase adsorption spaces; the skeleton pores restrict the flow and transport of water molecules. Reduction values in surface free energies increase at different temperatures, and released heat diffusion coefficients and permeabilities for methane molecules drop as moisture contents increase. Interestingly, however, enhancements in temperatures increase the methane molecular diffusion coefficients. The lower the activation energies, the easier they are to diffuse. Sufficiently, the optimum conditions for gas drainage of coal seam are at temperature of 293K and moisture content of 5%, indicating greater contributions to gas pressure relief for coal seam. By comparing the results of molecular simulation and SEM scanning, trend of change is basically the same. Moreover, it is explored that hydraulic measure was the most significant to the CBM stimulation technology through field engineering application. This research is expected to provide guidance for facilitating the effectiveness of gas extraction for coal seam.

Design and Optimization of High-Pressure Water Jet for Coal Breaking and Punching Nozzle Considering Structural Parameter Interaction

The technology of increasing coal seam permeability by high-pressure water jet has significant advantages in preventing and controlling gas disasters in low-permeability coal seam. The structural parameters of a nozzle are the key to its jet performance. The majority of the current studies take strike velocity as the evaluation index, and the influence of the interaction between the nozzle’s structural parameters on its jet performance is not fully considered. In practice, strike velocity and strike area will affect gas release in the process of coal breaking and punching. To further optimize the structural parameters of coal breaking and punching nozzle, and improve water jet performance, some crucial parameters such as the contraction angle, outlet divergence angle, and length-to-diameter ratio are selected. Meanwhile, the maximum X-axis velocity and effective Y-axis extension distance are used as evaluation indexes. The effect of each key factor on the water jet performance is analyzed by numerical simulation using the single factor method. The significance and importance effect of each factor and their interaction on the water jet performance are quantitatively analyzed using the orthogonal experiment method. Moreover, three optimal combinations are selected for experimental verification. Results show that with an increase in contraction angle, outlet divergence angle, and length-to-diameter ratio, the maximum X-axis velocity increases initially and decreases thereafter. The Y-direction expansion distance of the jet will be improved significantly with an increase in the outlet divergence angle. Through field experiments, the jet performance of the improved nozzle 3 is the best. After optimization, the coal breaking and punching diameter of the nozzle is increased by 118%, and the punching depth is increased by 17.46%.

Numerical Simulation of Integrated Mechanics of Drilling and Mechanical Cavitation in Coal Seam.

For coal and gas outburst and difficult extraction in soft, low-permeability, and high-gas seam, the integration technology of drilling and mechanical cavitation is put forward to relieve pressure and improve permeability of coal seam. Through the test of mechanical parameters of the coal body, the physical model of drilling and cave-making in coal seam is established, and the mechanical characteristics of drilling and mechanical cavitation are simulated numerically. The results show that the peak strength of coal increases linearly with confining pressure. When the drill bit just touches the coal, the maximum stress occurs at the center of the coal sample. With the increase in the drilling depth of the drill bit, the maximum stress point shifts to the depth. When the coal at the center of the sample is broken, the position of the maximum stress shifts to the surrounding. With the increase of drilling depth, the maximum contact number between the drill bit and coal body increases sharply. When the bit body basically enters the coal body, the maximum contact number between the drill bit and coal body remains unchanged. When the drill pipe rotates, the reaming tool collides with the hole wall at the reaming position on the axis. In general, the contact area increases with the opening of the reaming tool, but the contact between the reaming tool and the hole wall is random. As time increases, the contact force begins to increase and then basically stabilizes; at this time the reaming tool has been fully opened.

Effect of self-diverting acid viscosity and the chemical structure of coal under different acid environment

In order to solve the poor water infusion problem for low permeability coal seams, this paper studied the self-diverting acid to uniformly distribute acid. The results show that massive Ca2+ generated by the reaction between HCl and calcite can enhance the crosslinking of micelles. With the increase of HCl concentration, the viscosity increased and then decreased, which could be divided into “low viscosity zone” (0.1%–2%), “viscosity rapid increase zone” (2%–5%) and “viscosity decrease zone” (5%–15%). After treated with self-diverting acid, the fracture width and connectivity of coal was increased. Massive clay minerals were dissolved, the distribution range of pore size and pore volume gradually increased, and a large number of pores caused by corrosion appeared. Due to the influence of viscosity, the wettability of self-diverting acid for the coal was gradually reduced during infusion process. However, compared with distilled water, the overall wettability of coal still increased by 6.91%.

Research method of pressure relief and permeability enhancement in low permeability coal seam: A review

Gas drainage can prevent accidents such as coal and gas outbursts and gas explosions, which is of great significance for coal mine safety production. The level of permeability directly determines the effect of gas drainage. Therefore, experts and scholars in related fields have conducted extensive research with regard to adopting different measures to increase the permeability of coal and thereby improve the efficiency of gas drainage. However, in analyzing, comparing, and summarizing the advantages, disadvantages, and adaptability of various methods, there are few review studies in the literature. First, this Review investigates the principles, processes, and effects of different methods and finds that each method has its own specific application conditions and limitations. Simultaneously, it classifies different stimulation methods according to the principle of stimulation, which are mainly divided into physical stimulation methods, chemical stimulation methods, microbial stimulation methods, and thermal stimulation methods. Then, for better field applications, this Review investigates emphatically these methods that are currently popular but still in the laboratory stage, such as acidification stimulation and freeze–thaw stimulation, and analyzes the experimental methods, principles, experimental effects, influencing factors, etc. Permeability represents the flow capacity of gas in porous media and thereby affects the efficiency of gas extraction. Therefore, this Review summarizes the influencing factor of permeability. Finally, this Review discusses the aspect that needs to be perfected and improved for different methods and points out the problems and possible development directions in the future.

Dissolution behaviour of different rank coals in L‐glutamic acid N, N‐diacetic acid chelating agent: Implications to enhance coalbed methane recovery by acid stimulation

AbstractAcid stimulation is a promising method in improving coal seam permeability by demineralization in coal cleats. However, using conventional acids such as HCl and mud acid (12 wt.% HCl + 3 wt.% HF) may cause clay breakage, migration, and precipitation, leading to formation damage. In this study, we investigated the effect of L‐glutamic acid N, N‐diacetic acid (GLDA) chelating agent treatment on the dissolution behaviour of lignite (Coal L) and anthracite (Coal A), collected from Er′Lian Basin and Qinshui Basin, China. Characteristics of the raw and acid treated coal samples, including mercury intrusion porosimetry (MIP) results as well as metal elements distribution analysis of reacted leachates, micromorphology, mineral composition, and mass change were taken into consideration. It was found that high‐rank Coal A is more suitable for chemical stimulation. For both Coal L and Coal A, 5 wt.% GLDA solution is preferred to avoid sticking reaction products. Total porosities of Coal L and Coal A were increased by 3.9 and 1.1 times, respectively. Based on the water chemistry analysis of the reacted solutions, we found that acidic condition is beneficial for clay minerals and carbonates dissolution, while alkaline condition is favourable for the dissolution of quartz. Besides, the dissolution of minerals can trigger the release of trace elements, which is a potential environmental risk that needs to be taken into consideration before its in‐situ application in coal fields.