Coalbed Methane Reservoirs Research Articles

Experimental Study on the Effect of Coal Particle Size on the Mechanics, Pore Structure, and Permeability of Coal-like Materials for Low-Rank Coalbed Methane Reservoir Simulation

Coal-like materials have been widely used to simulate low-rank coalbed methane (CBM) reservoirs to develop fracturing stimulation technology since the transportation and preparation of low-rank coal produce easily secondary damages, causing large dispersion of test results. However, the coal particle sizes have a significant effect on the performance of coal-like materials among many factors. Hence, to accurately simulate low-rank CBM reservoirs for promoting the development of reservoir stimulation technologies, cement, gypsum, pulverized coal, and sand were selected as similar materials to make coal-like samples with different coal particle sizes to study the influence law of coal particle size on the mechanics, pore structure, and permeability of samples. The ultrasonic wave velocity, dynamic parameters, uniaxial compressive strength, elastic modulus, and uniaxial tensile strength of samples increased with increasing coal particle size in the range of 0–2 mm. However, these parameters suddenly decreased when the coal particle size increased to 2–3 mm. The porosity and permeability of the samples exhibited an opposite trend compared to the above parameters. The fracture occurred in cementing materials, and the crack extension direction was affected by the coal particle sizes. The fracture complexity of the coal-like samples decreased with increasing particle size under uniaxial compressive testing. The failure mode was mainly a tensile failure, supplemented by shear failure. The coal-like materials can well meet the performance requirements of low-rank CBM reservoirs.

A Fractal Permeability Model for Gas Transport in the Dual-Porosity Media of the Coalbed Methane Reservoir

A Fractal Permeability Model for Gas Transport in the Dual-Porosity Media of the Coalbed Methane Reservoir

Mechanism Exploration and application on Improving Coal Permeability by Heat Treating

The relationship between coal permeability and temperature under 200 °C was studied using a coal-heating experiment, and the mechanism of coal permeability increase was analyzed using X-ray diffraction analysis and electron microscope scanning. The results show that coal rock’s permeability could be increased from 0.5 to 15 md after heat treating. These mechanisms may be explained as follows. First, the desorption of methane molecules and the dehydration of gelatinous microcomponents in the matrix, which shrinks the coal matrix. Second, dehydration of coal rock and volatile coal, which will flow out and form new pores. Third, coal rock encounters inhomogeneous deformation under thermal stress, producing a small crack, unevenly. Fourth, coal rock generates hydrocarbon gases after oxidation, forming tiny cracks and reducing the degree of coal rock compaction. Finally, the study proposes a new mode of coalbed methane (CBM) exploitation to enhance CBM recovery by injecting thermal CO2. The above studies may be used to exploit CBM reservoirs with low permeability.

Fracture analysis using Stoneley waves in a coalbed methane reservoir

ABSTRACTThe production of coalbed methane gas depends upon various factors such as permeability, gas content, gas saturation and seam thickness. The low permeability in coal is a constraint in the gas extraction process. However, the presence of natural fractures and cleats in coal increases the permeability; therefore, its identification is necessary to establish the prospectivity. Hence, a methodology is used to process the Stoneley waveform in the Bokaro coalfield located in India to obtain slowness magnitude from the semblance analysis. The separation of directly transmitted and reflected Stoneley waves determines the reflection coefficient and attenuation, and its analysis emphasizes the presence of fractures. In coal seams, the average reflection coefficient ranges from 0.42 to 2.44 and attenuation ranges from 0.2 dB to 12.68 dB. The Stoneley slowness versus frequency plot shows a constant slowness with increasing frequency in the non‐fractured seams, whereas a dispersive trend is observed in the fractured seams. Fractures are also distinguished from the cross‐plot of compressional/shear versus Stoneley velocity. The observation of higher reflection coefficient, attenuation and slowness‐frequency dispersion in coal seams indicates extensive fractures/cleats that corroborate with the resistivity image log correlation. Thus, the study helps to distinguish between fractured and non‐fractured coal seams from the integrated approach of reflection coefficient, attenuation and dispersion analysis of Stoneley waves supported by the identification of fractures from resistivity image log for optimization in coalbed methane production in the development phase.

An evaluation method for types of low-production coalbed methane reservoirs and its application

At present, the macro-parameters such as critical desorption pressure, comprehensive diffusion coefficient, and permeability are commonly used to characterize the properties of a reservoir, and the productivity of coalbed methane (CBM) wells can be predicted through numerical simulation or established mathematical models, and the types of low-producing CBM reservoirs can be classified. However, low production from CBM wells is often caused by the weakness of one (or more) of desorption, diffusion, and seepage in CBM reservoirs, and the evaluation of a single link and macro-parametric characterization of reservoir properties cannot really ascertain the cause of the low production from a given CBM well.In the present work, the influences of pores and fracture structure, fluid flow dynamics and resistance on the desorption, diffusion, and seepage of CBM were considered from the perspective of system theory, and the evaluative parameter index system of low-productivity reservoirs was established. This avoids the one-sidedness of the evaluative results arising when only considering macro-parameters. New methods of obtaining the evaluative parameters pertaining to desorption, diffusion, and seepage are proposed: the methods include isothermal adsorption experiments, the use of logging curves, drainage data, mercury injection, and a classical mathematical model. These methods combined the data from experiments with in-situ data, and ensure the accuracy and practicability of the resulting evaluative parameters. On this basis, types of low-production reservoirs were classified using an analytic hierarchy process with multi-parameter superposition.The evaluative results pertaining to Shizhuang North block in the Qinshui Basin, Shanxi Province, China show that there are six types of low-yield reservoirs among the evaluative wells tested. Among them, diffusion is the main factor governing the low yield of CBM wells, followed by desorption and seepage. Herein, the reservoir constraint on CBM production is comprehensively evaluated, identifying the main controlling link affecting the low productivity of CBM wells, and laying a theoretical foundation for targeted transformation measures.

Petroleum rock mechanics: An area worthy of focus in geo-energy research

Petroleum rock mechanics: An area worthy of focus in geo-energy research

Combining K-Means Clustering and Random Forest to Evaluate the Gas Content of Coalbed Bed Methane Reservoirs

The accurate calculation of the gas content of coalbed bed methane (CBM) reservoirs is of great significance. However, due to the weak correlation between the logging response of coalbed methane reservoirs and the gas content parameters and strong nonlinear characteristics, it is difficult for conventional gas content calculation algorithms to obtain more reliable results. This paper proposes a CBM reservoir gas content assessment method combining K-means clustering and random forest. The K-means clustering is used to divide the reservoirs and distinguish the types to establish a random forest model. Judging from the evaluation effect of the research block, the prediction accuracy of the new method is significantly higher than that of the original method, and more accurate gas content prediction values can be obtained for different types of reservoirs. Studies have shown that this method can help the gas content evaluation of CBM reservoirs, improve the accuracy of gas content evaluation, and better support the exploration and development of CBM reservoirs. The results of this study show that the random forest method based on clustering can effectively distinguish the relationship between different logging responses and gas content. On this basis, the random forest algorithm modeling can effectively characterize the complex relationship between gas content and logging curve response. In the case of poor correlation between gas content and logging curve, the gas content of the reservoir can also be accurately calculated.

Experimental and Numerical Simulation Study of Hydraulic Fracture Propagation during Coalbed Methane Development

The extraction of low-permeability coalbed methane (CBM) has the dual significance of energy utilization and safe mining. Understanding hydraulic fracturing mechanism is vital to successful development of CBM. Therefore, it is important to improve the law of hydraulic fracture propagation in coal and rigorously study the influencing factors. In this paper, laboratory experiments and numerical simulation methods were used to investigate the hydraulic fracture propagation law of coal in coalbed methane reservoir with natural fractures. The results show that the maximum and minimum horizontal in situ stress and the difference in stress significantly affect the direction of crack propagation. The elastic modulus of coal, the mechanical properties of natural fractures, and the injection rate can affect the fracture length, fracture width, and the amount of fracturing fluid injected. To ensure the effectiveness of hydraulic fracturing, a reservoir environment with a certain horizontal stress difference under specific reservoir conditions can ensure the increase of fractured reservoir and the controllability of fracture expansion direction. In order to increase the volume of fractured reservoir and fracture length, the pumping speed of fracturing fluid should not be too high. The existence of stress shadow effect causes the hydraulic fracture to propagate along the main fracture track, where the branch fracture cannot extend too far. Complex fractures are the main hydraulic fracture typology in coalbed methane reservoir with natural fractures. The results can provide a benchmark for optimal design of hydraulic fracturing in coalbed methane reservoirs.

Numerical Simulation Research on Improvement Effect of Ultrasonic Waves on Seepage Characteristics of Coalbed Methane Reservoir

The matrix pores of a coalbed methane (CBM) reservoir are mostly nanoscale pores, with tiny pore throats and poor connectivity, which belong to the category of low–permeability gas reservoirs. The matrix particles and organic pore surfaces adsorb a large amount of CBM. These problems are the main reasons that limit the increase in CBM production. At present, the primary measure to increase CBM production is hydraulic fracturing. However, due to the technical characteristics and geological conditions of CBM reservoirs, applying this technology to CBM exploitation still has some key issues that need to be resolved. Therefore, it is essential to develop a new technology that can effectively increase the production of CBM. This paper proposed a method that uses ultrasonic waves to improve the seepage characteristics of CBM reservoir and theoretically verifies the feasibility of this idea using numerical simulation. In this paper, we firstly coupled the temperature, pressure, and seepage parameters of the CBM reservoir and built a CBM seepage model under the action of ultrasonic waves. Secondly, by comparing the numerical simulation results with the experiment, we verified the accuracy of the model. Finally, on the basis of the mathematical model, we simulated the change characteristics of pore pressure, reservoir temperature, permeability, and porosity under the action of ultrasonic waves. Research results show that under the action of ultrasonic waves, the pressure-drop funnel of CBM reservoir becomes more apparent. The boundary affected by the pressure drop also increases. With the increase of the action time of ultrasonic waves, the temperature of CBM reservoir also increases, and the action distance is about 4 m. With decreased pore pressure, the permeability and porosity of CBM reservoir significantly increase under the action of ultrasonic waves. With increased ultrasonic power, its effect on reservoir permeability and porosity becomes more significant.

Analysis of Enhancing Coalbed Methane Recovery and Improving Coal Mining Safety by CO2 Injection

Coalbed methane (CBM) is produced before coal mining at the Qinshui Basin in China to utilize CBM and reduce CH4 volume fraction for coal mining. However, the volume fraction of CH4 often reaches the range between lower and upper explosion limits after CBM production, which is a great threat to coal mining safety. In previous work, we analyzed the feasibility of injecting CO2 into coalbeds to control CH4 volume fraction for mining safety and simultaneously enhancing CBM recovery. In this paper, we extended our work to propose a model to calculate the critical CO2 volume fraction for CO2 injection. We simplified the gas mixture during coal mining as the CO2/CH4/air mixture. The model of the critical CO2 volume fraction was then built based on the explosion limit formula for the CO2/CH4/N2 mixture. The formula for the critical CO2 volume was derived using the critical CO2 volume fraction. The model of the critical CO2 volume fraction was applied in a CBM reservoir at the South Shizhuang Block in the Qinshui Basin. The CO2 injection rate for this block was optimized to obtain the highest CBM recovery using the reservoir simulation method. Results show that the critical CO2 volume fraction is 7.97%, which makes the CH4 volume fraction out of the explosion limits. The optimum CO2 injection rate for this block is 8000m3/d which improves the CBM recovery up to 86.24%.

Effects of Proppant Wettability and Size on Transport and Retention of Coal Fines in Saturated Proppant Packs: Experimental and Theoretical Studies

Hydraulic fracturing is the main stimulation technique that enables the economic production of coalbed methane reservoirs. However, massive invasion and retention of coal fines in proppant packs can clog the pore spaces and lead to an impairment of fracture conductivity. In this study, the effects of proppant wettability and size on retention of coal fines in proppant packs during the dewatering stage were investigated, for their optimization to achieve a long-term and effective control of fines retention. First, a series of transport and retention experiments of coal fines were performed on saturated proppant packs with different proppant sizes and wettabilities. Herein, standard quartz sand was used as the proppant carrier, and its surface wettability was modified to either hydrophobicity or stronger hydrophilicity by treatment with fluorocarbon resin and piranha solution, respectively. The experimental results indicated that both the retained amount and the deposition rate coefficient of coal fines decreased with increasing proppant hydrophilicity (or decreasing proppant hydrophobicity) and size. For the proppants treated with piranha solution, almost no coal fines were retained in proppant packs. Second, the forces/torques acting on coal fines in the vicinity of proppants were analyzed to elucidate the underlying mechanisms responsible for the influence of proppant wettability and size on fines retention. The effective filter surface (EFS) parameter was estimated by analyzing forces/torques to evaluate the filtration performance of proppant packs. Theoretical results illustrated that the EFS value decreased with increasing proppant hydrophilicity and size. The EFS value for proppants treated with piranha solution was near zero, which was due to the chemically “unfavorable” retention condition induced by piranha solution treatment. The EFS exhibited a similar tendency to the retention of coal fines observed in experiments, which validated the model. Finally, the recommendations on fines retention control in proppant packs were provided.

Prediction of velocity, gas content from neural network modeling and estimation of coal bed permeability from image log in coal bed methane reservoirs: Case study of South Karanpura Coalfield, India

P-wave velocity and gas content of major coal seams are predicted from three wells from the South Karanpura coalfield, India. Multilayered feed-forward neural network (MLFN) model is developed for prediction P-wave velocity from two wells using three input log parameters: gamma ray, resistivity and bulk density. The model is tested on coal seams: Sayal, Balkudra, Banasgarah and Argada of three wells namely; A, B and C with satisfactory goodness of fit (R2 = 0.77). The ratio of P-wave and S-wave velocities (Vp/Vs) ranges from 1.68 to 3.10 in the coal seams of this field. Cleat density using image log is obtained from four major coal seams namely; Balkudra, Kurse, Hathidari, Banasgarah varying from 1/m to 11/m. Ash content varies from 15 to 51% whereas gas content of major coal seams varies from 2.30 to 4.4 10−3 kg/m3. There is a good correlation between ash content and Vp/Vs of coal seams under the study area. MLFN model for prediction of gas content of the above-mentioned coal seams from two well is trained with input parameters such as: Vp/Vs, bulk density, cleat density and ash content. The model estimates gas content of other coal seams: Argada from well A, Hathidari from well B and Balkudra, Kurse, Banasgarah from well C with satisfactory R2 = 0.77. Maximum horizontal stress orientation is observed from azimuthal shear wave anisotropy from cross multipole array acoustic (XMAC) log for a well A. It varies from N110° to N115° in these coal seams. Permeability of coal seams are computed from X-tended Range Microresistivity Imager (XRMI) tool from a well C. Permeability of coal is varying from 0.5 md in Saunda seam to 17.29 md in Banasgarah seam. The estimated permeability matches well with the pre-fracture permeability of these seams. It is observed that increase in cleat density enhances coal seam fracture permeability in coal seam.

Coupled Effects of Stress, Moisture Content and Gas Pressure on the Permeability Evolution of Coal Samples: A Case Study of the Coking Coal Resourced from Tunlan Coalmine

Deep coalbed methane (CBM) is widely distributed in China and is mainly commercially exploited in the Qinshui basin. The in situ stress and moisture content are key factors affecting the permeability of CH4-containing coal samples. Therefore, considering the coupled effects of compressing and infiltrating on the gas permeability of coal could be more accurate to reveal the CH4 gas seepage characteristics in CBM reservoirs. In this study, coal samples sourced from Tunlan coalmine were employed to conduct the triaxial loading and gas seepage tests. Several findings were concluded: (1) In this triaxial test, the effect of confining stress on the permeability of gas-containing coal samples is greater than that of axial stress. (2) The permeability versus gas pressure curve of coal presents a ‘V’ shape evolution trend, in which the minimum gas permeability was obtained at a gas pressure of 1.1MPa. (3) The gas permeability of coal samples decreased exponentially with increasing moisture content. Specifically, as the moisture content increasing from 0.18% to 3.15%, the gas permeability decreased by about 70%. These results are expected to provide a foundation for the efficient exploitation of CBM in Qinshui basin.

Influence of Thermal Cracking Permeability Enhancement in Spontaneously Combustible Coalbed Methane Reservoirs on the Characteristics of Coal Spontaneous Combustion

ABSTRACT Thermal cracking permeability enhancement (TCPE) is a method to improve the coalbed methane (CBM) extraction efficiency and yield in low-permeability CBM reservoirs, but this method can change the physical and chemical properties of coal and thus affects the characteristics of coal spontaneous combustion. In order to explore the influence mechanism of TCPE on the characteristics of coal spontaneous combustion, the evolutions of oxygen adsorption capacity and chemical groups of coal under different thermal cracking parameters were measured by experiments. The following research results were obtained: On the whole, the pore and free radical structures of coal are changed after TCPE at low temperature and for a short time. The oxygen adsorption capacity and chemical activity of coal are increased, which leads to the increase of the risk of coal spontaneous combustion. However, when the coal is subjected to long-term high-temperature TCPE, the oxygen absorption capacity, and chemical reactivity are weakened. Ultimately, the risk of coal spontaneous combustion and the intensity of oxygen composite reaction become lower compared with the original state of coal. Based on these findings, it is suggested to consider long-term high-temperature thermal cracking during TCPE in CBM reservoirs.

The behavior and efficiency of methane displaced by CO2 in different coals and experimental conditions

CO2 sequestration into coalbed methane reservoirs not only reduces the greenhouse effect by removing CO2 from the atmosphere, but also recovers additional methane from coal which makes coalbed methane (CBM) production more economically attractive. An experimental study on the displacement of methane by CO2 has been carried out using eight bituminous and anthracite coals, to evaluate the displacement efficiency and utilization ratio of CO2. In our experiments, the coal samples were partially saturated with methane, and different amounts of CO2 were then injected. After every completed displacement, the T2 distributions were measured using low-field nuclear magnetic resonance (NMR). The results show that the P1 of the T2 spectra clearly decreased, which indicates that methane was desorbed as a result of CO2 injection. The amount of the adsorbed CO2 and desorbed methane were obtained from a combination of the T2 spectra and the CO2 gas pressure. By comparing the amounts of adsorbed CO2 and desorbed methane, an effective displacement capacity of CO2 has been defined, which varies with coal properties, temperature, and moisture content. The higher rank coals have a lower VL-CO2/VL-CH4 and show therefore a stronger effective displacement capacity. In addition, high temperature, and high moisture content weaken the effective displacement capacity of CO2. The study suggests that the utilization ratio of CO2 is higher in high-rank coals, which makes them more suitable for CO2-enhanced CBM production.

In Situ Stress Distribution and Its Control on the Coalbed Methane Reservoir Permeability in Liulin Area, Eastern Ordos Basin, China

Permeability is one of the important factors that affect the production efficiency of coalbed methane, and it is mainly controlled by in situ stress. Therefore, it is very essential to study the in situ stress and permeability for the extraction of coalbed methane. Based on the injection/falloff well test and in situ stress measurement of 35 coalbed methane wells in the Liulin area in the east of the Ordos basin, the correlations between initial reservoir pressure, in situ stress, lateral stress coefficient, permeability, and burial depth were determined. Finally, the distribution characteristics of in situ stress and its influence on permeability were analyzed systematically. The results show that with the increase of burial depth, the initial reservoir pressure and in situ stress both increase, while the lateral stress coefficient decreases. The permeability variation is related to the type of stress field in different burial depths, and its essence is the deformation and destruction of coal pore structures caused by stress. The distribution characteristics of in situ stress at different depths and its effect on permeability are as follows: at depths < 800 m , the horizontal principal stress is dominant ( σ H ≥ σ v > σ h ) and the permeability is a simple decreasing process with the increase of the depth; at depths > 800 m , the vertical stress is dominant ( σ v ≥ σ H > σ h ). The permeability of most coal is very small due to the large in situ stresses in this depth zone. However, because of the stress release at the syncline axis, coal with high permeability is still possible at this depth zone. Due to the existence of high permeability data points at burial depth (>800 m) and the fitting relationship between permeability and vertical stress, the maximum and minimum horizontal principal stress is poor. However, the coal permeability and lateral stress coefficient show a good negative exponential relationship. This indicates that the lateral stress coefficient can be used to predict permeability better.

Current Status and Effective Suggestions for Efficient Exploitation of Coalbed Methane in China: A Review

The commercial production of coalbed methane (CBM) in China has made significant achievements. To further improve the production of CBM and realize its development, it is necessary to establish a unique, high-efficiency CBM production method that considers the characteristics of coal reservoirs in China. This paper presents the current status of China’s CBM exploitation over the past 15 years from three aspects: reservoir characteristics, production mode, and exploitation technology. The achievements, in terms of exploration and development of CBM resources in China, were summarized. The CBM reservoirs in China are characterized by high coal rank, low permeability, high gas content, and low reservoir pressure gradient. Strategic measures for coal and CBM co-mining were proposed creatively as “Huainan”, “Jincheng”, and “Songzao” modes. CBM production enhancement measures have achieved significant innovations and breakthroughs in drilling and completion technologies and reservoir reconstruction, effectively reducing development costs and increasing single well production. Finally, based on existing basic research, this study proposes efficient CBM exploitation technologies suitable for the characteristics of CBM reservoirs in China. In particular, the uniform permeability enhancement measures of hydraulic grid slotting and fracturing have the advantages of safety, high efficiency, economy, feasibility, and broad application prospects under complex mining conditions. It is of great strategic significance to improve the CBM recovery rate and optimize China’s energy structure.

Combining experimental and logging data for calculation of the true skin factor of a horizontal well

AbstractHorizontal wells are an effective way to develop oil and gas fields with large drain areas, leading to high production rates and recoveries. They have been widely used in low permeability oil‐gas reservoirs, shale gas reservoirs, coalbed methane reservoirs, and so on. To achieve the best production rate of horizontal wells, it is essential to further reduce drilling completion fluid damage to protect the reservoir. The skin factor is the main indicator for evaluating the degree of damage of the drilling completion fluid on the reservoir. However, the skin factor obtained by field tests is the sum of the pseudo‐skin factor and the true skin factor, which cannot precisely evaluate the degree of damage. Therefore, it is important to calculate the true skin factor to correctly evaluate the degree of damage of the drilling completion fluid on a reservoir during the drilling completion process of horizontal wells. In this work, a combined method based on experimental data and logging data is proposed to calculate the true skin factor. First, based on a drilling completion fluid dynamic damage experiment, a relational expression between the initial permeability and the damaged permeability is equated, and then, a relational expression between the permeability of rock samples and the filtration velocity of the drilling completion fluid is formulated. Second, after combining experimental results with logging data, the horizontal reservoir intervals are subdivided and both the damaged permeability of the partial reservoir intervals and the major semiaxis of the damaged ellipse are calculated. Finally, the true skin factor of the horizontal well is computed. This metric provides a more precise evaluation of the effect of drilling completion fluid on a reservoir.

Promotion effects of microwave heating on coalbed methane desorption compared with conductive heating

As a clean energy resource, coalbed methane (CBM) has drawn worldwide attention. However, the CBM reservoir has strong adsorption capacity and low permeability and thus requires stimulation. As a means to stimulate coalbed methane recovery, thermal injection faces geological and economic challenges because it uses conventional conductive heating (CH) to transfer heat. Realized by the conversion of the electromagnetic energy into the thermal energy, microwave heating (MH) may be a sound stimulation method. Although previous research suggested that MH had potential as a stimulation method for coalbed methane recovery, it is not clear if MH is superior to CH for enhancing coalbed methane recovery. This paper compares the effect of MH and CH on methane desorption from coal using purpose-built experimental equipment. To compare the MH and CH experimental results, the desorption temperature for each CH desorption test was set to the maximum temperature reached in the correlative MH desorption test. The results show that although the cumulative desorbed volume (CDV) of methane under MH was less than that desorbed by CH in the initial desorption stage, the final total CDV under MH for the three different power settings was ~ 12% to ~ 21% more than that desorbed by CH at the same temperatures. CH and MH both change the sample’s microstructure but MH enlarges the pores, decreases methane adsorption, promotes methane diffusion, and improves permeability more effectively than CH. Rapid temperature rise and the changes in the coal’s microstructure caused by MH were the main reasons for its superior performance. These findings may provide reference for selecting the most appropriate type of heating for thermal injection assisted coalbed methane recovery.

Experimental investigation on coal-breakage performances by abrasive nitrogen-gas jet with a conical nozzle

With the increasing energy and environmental protection demands, coalbed methane (CBM) have attracted worldwide attentions. A cost-effective way of using nitrogen-gas as the fluid media of abrasive jet for radial jet drilling (RJD) in CBM exploitation was proposed in this paper. Laboratory experiments were carried out to investigate the influence of parameters including jetting pressure, nozzle diameter, dimensionless standoff distance, bedding direction, abrasive diameter, duration time and coal strength on coal-breakage efficiency. Besides, sensitivity analysis was conducted to evaluate the effect of jetting parameters on erosion performance. Moreover, the surface morphologies of jet-drilled holes were measured by a 3D profilometer. Finally, the coal-breakage mechanisms of abrasive nitrogen-gas jet were illustrated. The results indicated that the key factors for improving coal-breakage efficiency by abrasive nitrogen-gas jet were dimensionless standoff distance and nozzle diameter. Four major factors that contribute to the coal-breakage mechanisms can be summarized as: fractures generation, edge chipping, repeated erosion and gas entrainment effect. Moreover, jetting direction plays an important role in generating fractures. Fractures tend to be generated along the parallel bedding direction on the coal since tensile failures are easier to be occurred. Furthermore, a regular and smooth hole generated by the abrasive nitrogen-gas jet made it suitable for RJD drilling in CBM wells. The experimental results are expected to provide a viable alternative for efficient, economical and safe exploitations in CBM reservoirs.