Coalbed Methane Research Articles

A Comprehensive Methodology for Investigating Cocurrent Spontaneous Imbibition in Coal.

Visualizing and quantifying fluid distribution during spontaneous imbibition at the nanomicro scale is vital for understanding microfluid flow and dynamic wettability in coalbed methane (CBM) reservoirs, which could serve as a fundamental basis for optimizing the parameters of the hydraulic fracturing process. In this study, fluid distribution and flow behavior can be acquired by combining nuclear magnetic resonance (NMR) and in situ X-ray microcomputed tomography (μ-CT) technologies. Meanwhile, spontaneous imbibition stages were studied to analyze gas-water exchange efficiency. Additionally, dynamic wettability of gas-water was calculated during the process of spontaneous imbibition based on NMR. The results show that imbibition characteristics can be divided into three categories based on NMR. In type I, changes in imbibition fluid within nanopores are not obvious, while significant changes occur in micro pore-fractures, especially during the early stage of imbibition (0-2 h) for type I, which is almost the opposite of type II. Coal samples of type III exhibit low porosity and permeability, making it difficult for water to flow through the pores in coal due to capillary forces. The dynamic process of spontaneous imbibition can be divided into four stages using μ-CT equipment, with the third stage displaying the highest imbibition efficiency. This stage is characterized by frequent imbibition fluid exchange among various-scale fractures, which is related to the wettability and pore-fracture of the coal sample. A negative correlation between contact angle and T 2g was observed, where hydrophobic samples corresponded to a smaller geometric average of T 2g in the sample. In addition, the wettability of coal samples changed dynamically during the process of imbibition, with the contact angle gradually decreasing as imbibition time increased, which may be related to the formation of water film and hydration reactions.

Research on an Equivalent Algorithm for Predicting Gas Content in Deep Coal Seams

This document introduces a novel equivalent algorithm for forecasting gas content within deep coal seams, which is subject to constraints stemming from the advancements and precision achieved in well and roadway engineering endeavors. This algorithm meticulously acknowledges that coal seam gas content comprises three fundamental components: the inherent gas emission rate of the equivalent stratum, the residual gas content retained within the coal seam itself, and the influence imparted by the gas content within the coal seam. Furthermore, the approach thoroughly considers variations in the level of porosity development within the coal seam and its surrounding rock formations, as well as the occurrence of gas within these structures. The equivalent layer is classified into two distinct groups: the sandstone zone and the clay zone. The sandstone zone utilizes pertinent parameters pertaining to fine sandstone, whereas the clay zone distinguishes between clay rock and thick mudstone. The influencing factor considerations solely encompass natural elements, such as the coal seam’s occurrence and geological structure. The residual gas content employs either existing measured parameters or acknowledged experimental parameters specific to the coal seam. Based on this predictive approach, an intelligent auxiliary software (V1.0) for mine gas forecasting was devised. The software calculates the gas content of deep coal seams within the mine at intervals of 100 m × 100 m, subsequently fitting the contour lines of gas content across the entire area. The gas content predictions derived from this equivalent algorithm demonstrate robust adaptability to variations in gas content caused by construction activities, and the prediction results exhibit an acceptable level of error on-site. Notably, the prediction process is not constrained by the progress of tunnel engineering, ensuring that the prediction outcomes can accurately represent the distribution characteristics of deep coal seam gas content. After a year of application, the prediction results have consistently met on-site requirements, providing a scientific foundation for the implementation of effective gas prevention and control measures in the mining area. Furthermore, this approach can effectively guide the formulation of medium- and long-term gas prevention and control plans for mines.

Fabricating a Robust Ultramicroporous Metal-Organic Framework for Purifying Natural Gas and Coal Mine Methane.

Purifying methane (CH4) from natural gas and coal mine methane (CMM) is of great significance but challenging in the chemical industry. Herein, a robust ultramicroporous metal-organic framework (MOF) is reported, which can be synthesized on a gram scale by stirring under room temperature. Single-component adsorption isotherms of gases (CH4, ethane (C2H6), propane (C3H8), nitrogen (N2)) and breakthrough experiments indicate that the MOF can separate CH4 efficiently from CH4/C2H6/C3H8 ternary mixture, with super high purity-CH4 production of 154.7 cm3g-1. Additionally, the MOF shows higher CH4 capacity than N2, resulting in excellent separation performance for the CH4/N2 mixture. Notably, the binding sites of gases can be precisely determined by single-crystal X-ray data, further confirmed by molecular simulation. It is found that there are multiple hydrogen bonds and C─H···π interactions between the gases and the framework. This work offers an excellent candidate material for CH4 purification with both high capacity and separation efficiency.

Error analysis and improvement of water displacement method in measuring gas desorption volume from coal particles

The water displacement method is widely employed in experiments to investigate the adsorption and desorption characteristics of coal gas. However, conventional desorption apparatus faces challenges in accurately quantifying gases with low desorption rates, leading to significant inaccuracies in experimental assessments of residual gas content and adsorption–desorption hysteresis. In order to solve the issue, a new desorption device was invented to carry out isothermal ultimate desorption experiments together with the conventional device under two different equilibrium pressures with coal samples of four particle sizes. The results reveal that after 100 min, the gas desorption rate gradually decreases, with the desorption percentage of the conventional device remaining relatively constant, while that of the new device continues to rise. By the end of the experiment, the conventional device measured gas residual percentages ranging from 30 % to 50 %, whereas the new device recorded percentages between 10 % and 25 %. The lower residual percentages obtained by the new device prove the effectiveness to solve the issue that the conventional devices struggle to quantify low-rate desorption gas.When applying the conventional device, as desorption progresses, the desorption rate decreases, and the pressure within the coal sample tank increases more slowly, which results in the difficulty for the gas to reach a pressure of 71.18 Pa to overcome fluidic constraints and enter into the graduated cylinder and ultimately accumulates in the soft, large-volume silicone pipeline. In contrast, the new device, designed with its gas outlet strategically positioned above the liquid level, bypasses the constraints of liquid forces. Additionally, the modest inner diameter of the rigid tube undergoes minimal deformation under experimental conditions. Differences in liquid forces and piping allow the new device to accumulate 71 mL less gas than the conventional device. The utilization of the new device in coal gas desorption experiments effectively mitigates experimental errors stemming from low desorption rates, thereby driving advancements in the investigation of coal seam gas parameters and residual gas content.

Study on I/III mixed fracture characteristics of coal in different regions of China

The fracture mechanical property of coal rock constitutes an important mechanical constraint for hydraulic fracturing of coal reservoir, and its three-dimensional fracture characteristics hold great significance for coal-bed methane mining. In this study, non-invasive methods were employed to investigate the pore structure and permeability of coal, aiming to understand the gas storage, distribution, and mobility of coal from various regions. To assess the influence of geological stress on coal during coal-bed methane mining, a three-point bending test was conducted to determine the characteristics of coal I/III mixed fractures. The experimental results indicate that with the decrease of the modal mixing parameter Me, the fracture area expends, the fracture load Pf and fracture energy Ef increase, while the fracture toughness declines. The 3D fracture criterion is utilized to predict the toughness ratio of coal rock I/III mixed fracture. The 3D-MTS and 3D-MMTSN criteria offer the upper and lower limits of prediction respectively. The 3D-MTSED criterion proves to be a more appropriate fracture criterion for analyzing the 3D fracture.

Study on failure mechanism of cracked coal rock and law of gas migration

China possesses abundant coal resources and has extensive potential for exploitation. Nevertheless, the coal rock exhibits low strength, and the coal seam fractures due to mining activities, leading to an increased rate of gas emission from the coal seam. This poses significant obstacles to the exploration and development of the coal seam. This paper focuses on studying the failure mechanism of fractured coal rock by conducting uniaxial and triaxial compression experiments on the coal rock found at the Wangpo coal mine site. Simultaneously, in conjunction with the findings from the field experiment, a gas migration model of the mining fracture field is constructed to elucidate the pattern of coal seam gas distribution during mining-induced disturbances. The study structure reveals that coal rock exhibits three distinct failure modes: tensile failure, shear failure, and tension-shear failure. The intricate fissure in the rock layer will intensify the unpredictability of rock collapse patterns. The compressive strength of coal rock diminishes as the confining pressure drops. The coal rock in the working face area will collapse as a result of the lack of confining pressure. In the rock strata above the mining fracture zone, the gas pressure is first higher and then significantly falls with time. After 100 days of ventilation, the low gas pressure area changes little, so to ensure the safety of the project, the ventilation time of the fully mechanized mining surface is at least 100 days. The research results will help to establish the core technology system of coal seam development and improve the competitiveness of coal seam resources in China.

Research on methane Hazard interval prediction method based on hybrid “model-data”driven strategy

Safe mining of coal has an important impact on energy security, while effective control of methane hazard is the key to ensuring safe coal mining. Methane concentration is the main factor determining the hazard of methane in coal mines, and in order to limit the impact of methane on coal mine safety, this study proposes a methane concentration interval prediction method based on a hybrid “model-data” driven idea. Firstly, by analyzing the data and constructing a methane concentration prediction method based on model-driven, which reduces the influence of multicollinearity in the methane concentration series on the prediction effect, and then, in combination with the deep learning technique, a method based on the Wasserstein distance to improve the Informer model is proposed, and finally a hybrid-driven methane concentration interval prediction model is established by introducing the IOWGA operator and the statistical method. After an example analysis of a coal mine in Guizhou Province, China, the hybrid-driven model proposed in this study has better applicability and prediction accuracy in the methane concentration prediction task, which can effectively prevent the occurrence of coal mine accidents and is more in line with the needs of coal mine safety production.

Clumped isotopes constrain thermogenic and secondary microbial methane origins in coal bed methane

Methane is an economic energy resource and potent greenhouse gas. Distinguishing secondary microbial methane from thermogenic gas is important for natural gas exploration and consideration of subsurface microbial activity in the global carbon cycle, but remains challenging. To understand controls on methane origins in natural gas systems, we investigated the methane clumped isotopologue distributions in the Qinshui Basin high-thermal maturity coal bed methane (CBM) reservoir. Here, near-equilibrium clumped isotopologues distribution (Δ13CH3D and Δ12CH2D2) inferred a temperature interval of 21.6–252.3 °C. The high-temperature thermodynamic equilibrium most likely represents original thermogenic CBM characteristics during coalification. The low-temperature equilibrium clumped isotopologue distributions suggest microbial alteration to CH4 isotopic bond ordering by increased enzymatically catalyzed isotopic exchange. The independent constraints from clumped isotopes, integrated with other geochemical and genomic evidence, confirm notable secondary microbial methane from biodegradation in the highly mature reservoir. Thus, methane clumped isotopes can be used as unequivocal tracers to distinguish secondary microbial methane from thermogenic gases and hence provide the ability to incorporate them separately into global methane budgets.

Microscopic adsorption study from coal rank comparison on the feasibility of CO2-rich industrial waste gas injection enhanced coal bed methane recovery

A new method of injecting CO2-rich industrial waste gas (CO2-rich IWG) to displace coal bed methane (CBM) is proposed for energy conservation and green growth. In this study, theadsorption mechanisms of this method were studied by the Giant canonical Monte Carlo and molecular dynamics methods in macromolecular coal models with three ranks (brown, bituminous, and anthracite coal). It is found that the proportion of gas molecules in the adsorbed state is higher in the anthracite coal slit than in the brown and bituminous slits, indicating that anthracite coal seams are more appropriate for waste gas sequestration. The adsorption selectivity of NO and CO2 is consistent among the three coal ranks, with the order of brown > bituminous > anthracite > 1.0, and that of N2 orders as 1.0 > bituminous > anthracite > brown.This is due to the different dominant factors affecting the competitive adsorption between different waste gas components and CBM, namely competition for adsorption sites in the NO and CO2 systems and thefilling effect in the N2 system. There is a positive correlation between the number of major competitive adsorption sites and the adsorption selectivity of theCO2(NO) system. CO2 and CH4 compete for COC structures in brown and bituminous coal and OH structures in anthracite coal, and OH structures in the three coals are the major adsorption sites for NO systems. Our work advances the understanding of the microscopic adsorption mechanisms of CO2-rich IWG in the CBM reservoirs, which provides a theoretical basis for CO2-rich injection-enhanced CBM recovery technology.

Moisture dependence of responses in physical–chemical property and CH4 ad-/desorption capability of coals to microwave radiation

For purpose of understanding feasibility of coalbed methane (CBM) production in practical water (H2O)-containing reservoirs by using microwave radiation, the dielectric property highly associated with microwave-absorbing and resultant temperature-rising capabilities of coals were addressed primarily; afterward, the temperature-rising rule of coals and its dependence on moisture were examined; finally, the impacts of moisture on changes about main physical–chemical property and resultant CH4 ad-/desorption performance of coals were investigated under microwave field. Results indicated that the coals display microwave-absorbing potential to raise temperature. The moisture initially increases the temperature-rising rate owing to its strong microwave-absorbing capability while decreases the final bulk temperature because of its endothermal evaporation. The resultant temperature decomposes and transforms minerals. Moreover, the temperature rise upgrades crystallite structure of the dried coals. As for the wetted coals, the rapid temperature rise disorders crystallite structure of the Sihe and Yima coals due to potential steam explosion. Furthermore, the radiation decreases the aromaticity and the total surface oxygenic groups of the dried coals; the moisture whittles down these decreasing trends. In general, the micro- and mesopore surface area and volume of the coals drops after radiation. The microwave radiation smooths pore surface of the wetted coals while roughens that of the dried coals. The complexity of the coal pore structure almost reduces after radiation. Besides, the microwave radiation significantly reduces the CH4 adsorption capability of the dried coals by 6.74–36.69%; the moisture further reduces the CH4 adsorption capability by 20.48–43.03%. Ultimately, the microwave radiation almost enhances the ad-/desorption hysteresis of CH4 on the dried coals while weakens that of the wetted coals. Such alterations depend on the responses of pore abundance and pore surface roughness to microwave. These findings confirm that microwave radiation is a promising option to produce CBM, particularly for H2O-bearing CBM reservoirs.

Analysis of dominant flow in tectonic coal during coalbed methane transport

Diffusion and seepage are the main flow forms of coal seam gas transport, and are one of the key factors in the selection of gas extraction improvement methods. Changes in the physical structure of tectonic coal make gas transport more complex during coalbed methane extraction. In this paper, we develop a multi-field coupled model of methane transport in coal seams, taking into account the effects of tectonics, and theoretically analyze the dominant flow patterns for methane extraction. Then, the evolution of gas dominated flow is analyzed for different initial pressures, initial permeabilities, and initial diffusion coefficients of tectonic and intact coal seams. The results show that the amount of daily methane seepage in tectonic coal increases with the initial pressure of the coal reservoir, but decreases with the initial diffusion coefficient of the coal reservoir. Methane seepage in tectonic coal has a longer control time than in intact coal at different initial pressures, initial permeabilities, and initial diffusion coefficients of the coal reservoir. For different coal reservoir initial pressures, coal reservoir initial permeabilities, and coal reservoir initial diffusion coefficients, the maximum seepage control time for tectonic coal is 20, 17, and 15 times longer than for intact coal, respectively. Finally, the discrepancies of methane dominant flow in tectonic coal and intact coal during methane extraction were analyzed by using the double bottleneck flow model, and methods for methane enhanced extraction in tectonic coal and intact coal were discussed. The results presented in this paper may provide a theoretical reference for the extraction of differentiated gas in coal seams.

Microstructural response of coal fracture surface induced by ScCO2 injection measured with AFM

Fractures in coal serve as the principal channels for the migration of coalbed methane (CBM). The microscopic traits of coal fracture surface are of significance for evaluating reservoir permeability and directly influence the outcome of CO2 storage and displacement. To comprehend the impact of CO2 on the coal fracture surface, the alterations in the coal fracture surface before and after CO2 injection are investigated vis atomic force microscopy. As CO2 gradually transforms into a supercritical state, the fracture density and aperture on the coal fracture surface gradually escalate. The mean pore size and porosity of the coal fracture surfaces rose from 5.45 nm to 6.55 nm and from 4.66% to 6.85%, respectively. The proportion of micropores (<2 nm) gradually diminished. The proportions of mesopores (2∼50 nm) and macropores (>50 nm) proportions gradually. Swelling, extraction and mineral dissolution are the predominant reasons for the alteration in the pore structure of the coal fracture surface. CO2 injection modifies the roughness and fractal characteristics of the coal fracture surface. The mean roughness Ra dropped from 6.54 nm to 4.03 nm, and the fractal dimension Ds declined from 2.45 to 2.28. With the fixed scanning range, the injection of CO2 has been shown to reduce the roughness of coal fracture surface and lower their fractal dimension, thereby has a positive influencing permeability enhancement. Therefore, in CO2-ECBM engineering, it is essential not only to analyse the physical properties of the target reservoir, but also to conduct a comprehensive evaluation of permeability with a suitably defined area of the reservoir.

Optimizing Methane Control in Composite Goaf Areas: Insights from Multisource Emission Analysis and Numerical Modeling in the Tongxin Coal Mine.

Coal mine methane poses a threat to both coal production and the well-being of workers. This study investigates the complex challenge of managing methane emissions from multiple sources in coal mines prone to gas. Coal mining creates multiple air leakage channels in the mine goaf, increasing the risk of gas explosions. Tracer SF6 measurements were employed to identify multiple air leakage channels. A comprehensive numerical model, incorporating multiple air leakage channels, was developed to simulate methane distribution in the goaf and upper corner. By adjusting the combination of gas drainage rate (U) and working face ventilation flux (Q), the concentration of methane at the upper corner is expected to be decrease, ensuring safe production at the Tongxin mine. A total of 48 simulation cases were conducted, determining that U = 1080 m3/min and Q = 2250 m3/min are recommended for the 8201 working face in the Tongxin coal mine. This combination provides not only an effective solution but also a cost-efficient one for methane control. The findings offer a new insight into effective methane control providing a foundation for improving mine safety.

Study on the Spontaneous Combustion Law of Coal Body around a Borehole Induced by Pre-extraction of Coalbed Methane.

To address the challenges associated with the high gas content, high pressure, and low permeability coefficient in deep coal seams, strategies such as infilling boreholes and increasing the negative pressure of extraction are commonly implemented to alleviate issues related to coalbed methane extraction. However, long-term mining pressure can lead to the development of cracks in the coal seam near the borehole, thereby creating air leakage channels, which could potentially impact the oxygen supply during the extraction process. This leads to secondary disasters such as the spontaneous combustion of coal and gas explosions, considerably impacting the life and health of underground workers. To solve this issue, a thermal-fluid-solid coupling model for the working surface was constructed based on numerical simulation software, taking into account the multimechanism coupling effect of coal seam gas. The laws of coal oxidation and spontaneous combustion induced by coalbed methane extraction around boreholes were studied. The variation laws of the oxygen concentration, coal temperature, and oxidation heating zone around the borehole under different extraction conditions were simulated and analyzed. The findings demonstrate that the negative extraction pressure enables the gas to penetrate the fracture zone of the borehole, leading to an increase in the oxygen consumption rate and coal temperature around the borehole with an increase in negative extraction pressure. The coal gas leakage surrounding the borehole reduces as the sealing depth increases, and both the heating rate of coal and oxygen volume fraction show a downward trend. The fitting relationship between the negative pressure of drainage, depth of sealing, and temperature change in the coal body surrounding the boreholes was identified. It was determined that the negative pressure of 13 kPa for borehole drainage and a sealing depth >18 m are the optimal extraction parameters. The range of the oxidation zone and the position of the boundary line under this parameter were predicted, and the position function of the dangerous area of oxidation heating was defined. The research results have remarkable implications for the coordinated prevention and control of gas and coal spontaneous combustion in coalbed methane predrainage boreholes, as well as for efficient prevention and control of CO in on-site gas extraction boreholes, thus ensuring efficient and safe gas extraction.

Effects of oxygen consumption characteristics of goaf on the low oxygen formation mechanism in the working face

Low oxygen at a coal mine's working face has a detrimental impact on working conditions and productivity. This study conducted an experiment on oxygen consumption at low temperatures, quantified the rate of air infiltration, and scrutinised and examined the zoning division of coal seam gas occurrences. The gas change rules in the return air corner and the goaf and the gas outflow rules in the goaf to the working face are analysed. Experiment results reveal that residual coal predominantly consumes oxygen through a combination of physical adsorption and chemical reaction, which is the key factor in the decrease in oxygen concentration. The examination showed, that the gas emission rate of air from the surface cracks to the working face is 0.09–0.13 m·s−1. Air leakage in the goaf results in the oxidation of residual coal and consequently leads to significant oxygen consumption. The release of low-oxygen gases from the oxidation of residual coal in the goaf to the working face is facilitated by atmospheric pressure and negative pressure ventilation methods, which act as sources of power. The research findings offer direction for examining and managing the problem of low oxygen in coal mine working faces.

The Gas Production Characteristics of No. 3 Coal Seam Coalbed Methane Well in the Zhengbei Block and the Optimization of Favorable Development Areas

The characteristics and influencing factors of gas production in CBM wells are analyzed based on the field geological data and the productivity data of coalbed methane (CBM) wells in the Zhengbei block, and then the favorable areas are divided. The results show that the average gas production of No. 3 coal seam CBM wells in the study area is in the range of 0~1793 m3/d, with an average of 250.97 m3/d; 80% of the wells are less than 500 m3/d, and there are fewer wells above 1000 m3/d. The average gas production is positively correlated with gas content, critical desorption pressure, permeability, Young’s modulus, and Schlumberger ratio, and negatively correlated with fracture index, fault fractal dimension, Poisson’s ratio, and horizontal stress difference coefficient. The relationship between coal seam thickness and the minimum horizontal principal stress is not strong. The low-yield wells have the characteristics of multiple pump-stopping disturbances, unstable casing pressure control, overly rapid pressure reduction in the single-phase flow stage, sand and pulverized coal production, and high-yield water in the later stage during the drainage process. It may be caused by the small difference in compressive strength between the roof and floor and the coal seam, and the small difference in the Young’s modulus of the floor. The difference between the two high-yield wells is large, and the fracturing cracks are easily controlled in the coal seam and extend along the level. The production control factors from strong to weak are as follows: critical desorption pressure, permeability, Schlumberger ratio, fault fractal dimension, Young’s modulus, horizontal stress difference coefficient, minimum horizontal principal stress, gas content, Poisson’s ratio, fracture index, coal seam thickness. The type I development unit (development of favorable areas) of the Zhengbei block is interspersed with the north and south of the block on the plane, and the III development unit is mainly located in the east of the block and near the Z-56 well. The comprehensive index has a significant positive correlation with the gas production, and the prediction results are accurate.

Response Patterns of Acoustic Wave Characteristics to Reservoir Petrophysical Parameters in Different Bedding Directions: A Case Study of Low- and Middle-Rank Coals in Xinjiang, China.

The physical properties of coal reservoirs, important parameters for evaluating the production potential of coalbed methane (CBM) resources, can be assessed nondestructively and in real-time using acoustic wave technology. In this study, we collected 48 low- and middle-rank coal samples oriented in different bedding directions from seven typical coal mines, encompassing the Zhunan, Tuha, and Kuqa-Bay coalfields in Xinjiang, China. We clarified the characteristics of the physical parameters (apparent density, fracture, porosity, and permeability) and acoustic wave of coal variations through acoustic wave, porosity, and permeability experiments, revealing the response law of acoustic wave characteristics to the physical parameters of coal. The results indicated that the acoustic wave velocity and dynamic elastic modulus (E d) of coal samples oriented in the perpendicular bedding direction are larger than those oriented in the parallel bedding direction; however, the dynamic Poisson's ratio (μd) of coal samples oriented in different bedding directions does not significantly differ. The existence of fractures significantly reduces the acoustic wave velocity and E d of the coal. The greater the apparent density of coal, the tighter its structure, resulting in a faster acoustic wave velocity. The larger the porosity of coal, the greater its internal voids, leading to a more pronounced attenuation of acoustic energy and a slower acoustic wave velocity. The more developed and interconnected the bedding fractures of coal bodies oriented in the parallel bedding direction, the higher their permeability, resulting in a smaller decrease in acoustic wave velocity. Conversely, the more developed the bedding fractures of coal bodies oriented in the perpendicular bedding direction, the more pronounced their attenuation of acoustic wave velocity. Finally, the regression equations for E d with the square of P-wave velocity (V P 2) and μd with the square ratio of V P to S-wave velocity (V P 2/V S 2) were established for coal. The study findings can help evaluate and predict the reservoir quality of coal seams, assess CBM, and improve the safety and efficiency of its extraction.

Dynamic gas emission during coal seam drilling under the thermo-hydro-mechanical coupling effect: A theoretical model and numerical simulations

The potential risk of coal and gas outbursts has increased with the depth of coal mining, posing the threat of heavy casualties, vast economic losses, and harm to the ecological environment. Considering the limitations of existing coal and gas outburst prediction methods, novel indicators or methods are needed. In this study, a fully coupled dynamic drilling gas emission THM model of gas diffusion, seepage, and flow is established, and the reliability of the model was verified by the measured data. The validated model was applied to single-factor and multi-factor impact analysis of dynamic drilling gas emission. The simulation results showed that (1) During the drilling process, the increase in permeability is mainly attributed to the coal seam damage and disturbance caused by the borehole and the coupling effects of the THM model, which increase the porosity around the borehole. Driven by the pressure gradient and temperature gradient, the gas pressure and temperature of the coal seam around the borehole show a conical distribution. (2) In the single-factor analysis, increasing the values of coal seam parameters (initial gas pressure, initial permeability, and initial coal temperature) and drilling parameters (borehole radius) will promote gas emissions in the drilling process to different degrees. The most significant promoting effect is the initial permeability, and the weakest is the initial coal temperature. (3) The response surface analysis results of the four factors show that the interaction between initial coal temperature C and borehole radius D is the least significant, and the interaction between initial gas pressure A and initial permeability B is the most significant. Meanwhile, the established multiple regression model characterizes the relationship between borehole gas emissions and initial coal seam gas pressure, and the idea of using drilling gas emission volume to predict coal and gas outbursts was proposed.

High-Order Models for Gas Transport in Multiscale Coal Rock.

The coal reservoirs exhibit great heterogeneity and strong anisotropy in multiscale pore/fracture structures. Developing highly accurate multiscale models for real-time prediction of microgaseous flow in complicated porous media with pronounced contrast in transport coefficients is crucial but not yet available, which is time-consuming, expensive, and even computationally impossible. In this study, a multiscale approximate solution of the gas flow and pressure field is derived in this paper for predicting coalbed methane (CBM) transport in macro-microscopic two-scale porous media of typical coal rocks in Guizhou Province, China, and the detailed finite element algorithm is established. The multiscale approximate solutions are constructed from the first- and second-order auxiliary cell functions and low- or high-order derivatives of the homogenized pressure field, which can accurately approximate the solution of the original problem to a certain extent. The numerical accuracy and validity of the multiscale approximate solutions under various working conditions are analyzed in detail by comparing and verifying the results with the direct numerical simulation results of fine-mesh high-precision finite elements. The results show that the homogenized approximate solution can only roughly capture the characteristics of the macroscopic pressure evolution, the first-order approximate solution can partially reproduce the macro-microscopic pressure oscillation phenomenon, and the second-order approximate solution can accurately predict the pressure profile and its evolution law with time in porous media with macro-micro configuration under various complex conditions. The second- and high-order two-scale approximate solutions constructed in this paper exhibit high numerical accuracy and excellent computational efficiency. We also develop multiscale approximate solutions for predicting microgaseous flow at a low reservoir pressure where the slippage effects are very pronounced. Potential applications of our high-order models to the coal reservoirs with a hierarchical matrix and fractures are discussed. The developed multiscale models exhibit excellent applicability to investigating the crossflow effects and coupling mechanisms between the matrix and cleats or fractures with multiple configurations in the CBM and shale gas reservoirs, which is crucial for the effective implementation of hydraulic fracturing technology. This study provides a fundamental understanding of gas transport in multiscale matrix-fracture networks, which helps to improve the optimization design of hydraulic fracturing in tight reservoirs.

Aquifer Leakage Recharge Controls on CBM Production: A Case Study in the Sanjiao Block, Eastern Ordos Basin, China.

Aquifer leakage recharge poses a prevalent challenge in coalbed methane (CBM) development, severely impeding its efficient production. This study focuses on the Sanjiao Block, located on the eastern margin of the Ordos Basin, and provides a comprehensive evaluation of the impact of aquifer leakage recharge on CBM well productivity, employing hydrochemical characteristics and numerical simulation. Fisher's discriminant analysis reveals a close association between external water sources for CBM wells in the Taiyuan Formation and the hydrodynamic environment: in the western stagnant zone, hydrochemical characteristics resemble those from the Shanxi Formation; in the eastern strong runoff zone, water from the Taiyuan Formation directly contributes to CBM well development; in the central weak runoff zone, hydrochemical characteristics suggest mixed water sources from the Shanxi Formation and Taiyuan Formation. The numerical simulation employs orthogonal experiments to quantify the sensitivity of aquifer geological parameters to CBM production, ranked from strong to weak for aquifer types, leakage channel properties, and aquifer physical properties. The fundamental reason for disparities in the efficacy of leakage recharge lies in categorizing aquifers into finite and infinite recharges based on their water supply capacity. The properties of leakage channels, including the location and scale, manifest as effects on the magnitude and shape of gas production characteristics in infinite and finite recharge aquifers, respectively. Furthermore, a discriminant flowchart of CBM production is presented, delineating the production characteristics of CBM wells under the influence of aquifer leakage recharge into six patterns and illustrating their distribution in the study area. This discriminant process provides scientific guidance for analyzing CBM production characteristics and evaluating potential under the influence of aquifer leakage recharge.