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Spontaneous combustion of coal is a major problem in coal industry. In order to improve the environmental performance and effectiveness of existing inhibitors used to inhibit coal spontaneous combustion, this study utilized physical inhibitors such as chitosan (CS), acrylic acid (AA), and acrylamide (AM) and copolymerized them with the chemical inhibitor procyanidin (OPC). The resulting composite inhibitor was then in situ polymerized with modified nano-TiO2 to create an environmentally friendly gel-based composite inhibitor, referred to as TiO2/OPC/CPAAM. Scanning electron microscopy analysis revealed a significant increase in the number of laminar folds and pores within the inhibited coal samples. Additionally, thermogravimetric analysis showed that all characteristic temperature points of the inhibited coal samples were raised, with an average increase of 21.34%. Differential scanning calorimetry also indicated a reduction in the total heat release and heat release intervals of the inhibited coal samples. Gas infrared spectroscopy provided evidence that the inhibitory treatment significantly slowed the growth of CO and CO2 during the heating process of the coal samples, leading to an improvement in environmental performance. Furthermore, diffuse reflection infrared spectroscopy testing demonstrated that the inhibitory treatment resulted in the passivation of active groups' free hydroxyls and an increase in the content of stable group ether bonds. Overall, the results of this study indicate that the composite inhibitor TiO2/OPC/CPAAM offers superior inhibition effects on coal spontaneous combustion. [ABSTRACT FROM AUTHOR]

In-situ stress plays a pivotal role in influencing the desorption, adsorption, and transportation of coalbed methane. The reservoir gas content represents a pivotal physical parameter, encapsulating both the coalbed methane enrichment capacity and the underlying enrichment law of the reservoir. This investigation collates, computes, and consolidates data concerning pore pressure, breakdown pressure, closure pressure, triaxial principal stress, gas content, lateral pressure coefficient, and other pertinent variables from coal reservoirs within several coal-bearing synclines in the Liupanshui coalfield, China. This study elucidates the characteristics of longitudinal stress development in the study area, the gas content of the longitudinal reservoirs and their interrelationships. The study area is situated within the middle-high stress zone, exhibiting discernible evolution patterns from reverse fault mechanism to strike-slip fault mechanism to normal fault mechanism, progressing from shallow to deep. In the deeper stratigraphy, a strike-slip-normal fault mechanism emerges. The relationship between burial depth and triaxial principal stress is subjected to linear regression, resulting in the proposal of a simplified model for vertical in-situ stress. The hyperbolic regression algorithm was employed in order to derive both the envelope and median formulas for lateral pressure coefficient (k values). The k value displays discrete behavior along the vertical axis in shallow depths, gradually converging in deeper strata and ultimately stabilising at approximately 0.65 with increasing depth. A comprehensive examination of the k value substantiates the efficacy of the simplified in-situ stress model along the vertical axis, offering profound insights into the vertical interrelationships and evolving patterns of the triaxial principal stresses. The mean gas content in the study area was found to be 11.89 m³/t, exhibiting a general increase in depth, followed by a subsequent decrease. The pore pressure (Pp) displays a discernible positive correlation with gas content. This study comprehensively elucidates the developmental patterns of the stress field, the simplified model of vertical in-situ stress, the attributes of the stress ratio (KH, kh, lateral pressure coefficient k), the characteristics of reservoir gas content, and the corresponding and transformative relationships between coupled geostress field parameters and gas content. The lateral pressure coefficient conversion depth, in-situ stress conversion depth, and gas inversion depth are delineated, accompanied by a detailed exposition of their definition process, physical significance, and interrelations. Within the study area, the lateral pressure coefficient conversion depth is estimated to range between 450 and 500 m, while the critical depth for in-situ stress conversion is approximately 670 m. Moreover, the critical depth for gas content conversion falls within the range of 700–800 m. It is noteworthy that the critical depth for deep coalbed methane within the Liupanshui coalfield has been identified as approximately 800 m. Subsequently, a vertical "in-situ stress-gas content mode" relationship model for coalbed methane development was formulated, thereby providing a structured framework for understanding the dynamic interactions between vertical in-situ stress and gas content. [ABSTRACT FROM AUTHOR]

Fractures influence the mechanical strength of coal roof and floor, constraining the design of hydraulic fracturing for coalbed methane production. Currently, the predominant approach involves the integration of petrophysical logging with machine learning for fracture prediction. Nevertheless, challenges exist regarding the model's accuracy. In this study, we present a novel approach to predict fracture density. Our method optimises a back‐propagation (BP) neural network and utilises principal component analysis for feature extraction. We employ logging parameters (density, compensated neutron and acoustic time difference) obtained from Shouyang Block well SY‐1 and fracture density data from electrical imaging logging to construct the FVDC model's dataset. The BP neural network model is optimised using the Sparrow Search algorithm and Tent Chaotic Mapping. The results demonstrate a substantial enhancement over the BP neural network model, with reductions of 80.102% in mean absolute error, 94.182% in mean square error, 75.879% in root mean square error and 79.764% in mean absolute percentage error. When considering accuracy, the optimised model (97.098%) surpasses the support vector regression model (96.478%), the random forest model (94.404%) and the BP neural network model (85.657%). Scalability testing for the optimised model was conducted using data from well SY‐2, yielding a remarkable prediction accuracy of 96.775%. This performance exceeds that of the BP neural network (with an accuracy of 85.102%), as well as the random forest and support vector regression models (with accuracies of 91.234% and 90.384%, respectively). These results underscore the potential of well logging and machine learning in FVDC prediction. [ABSTRACT FROM AUTHOR]

[Display omitted] • CO 2 -rich industrial waste gas injection can displace unconventional gas. • Adsorption energy of waste gas components/CH 4 on diverse reservoirs is compared. • Adsorption selectivity is calculated under reservoir temperature–pressure conditions. • The gas adsorption modes in different unconventional gas reservoirs are analyzed. Direct injection of CO 2 -rich industrial waste gas to displace unconventional natural gas is proposed as an energy-saving and green development method. In this work, the Giant canonical Monte Carlo and molecular dynamics methods were employed to compare the adsorption mechanisms of this method in three typical unconventional gas reservoirs, which are coal seam, shale and tight sandstone. It is found in the adsorption configurations that the proportion of gas molecules in the free state is higher in tight sandstone than in shale and coal seam. The adsorption energy of CH 4 /waste gas components on all reservoirs is in the order of CH 4 /SO 2 > CH 4 /CO 2 > CH 4 /NO > CH 4 /N 2 , among which the absolute value of the CH 4 /SO 2 adsorption energy on the coal seam reaches the highest 64.220 kJ/mol. The adsorption selectivity of SO 2 , CO 2 and NO over CH 4 is greater than 1.0 in all reservoirs at depths of 0.5–4.0 km, and the competitive adsorption phenomenon in the deep reservoirs has a severe negative effect on it. Gas adsorption mode in the tight sandstone is dominated by filling pore structures, while gas adsorption in the coal seam is highly correlated with adsorption sites. This study is instructive for enhanced unconventional natural gas recovery by CO 2 -rich industrial waste gas injection from the perspective of gas adsorption. [ABSTRACT FROM AUTHOR]

Accurate parameter prediction in the coalbed methane (CBM) pre-extraction process is crucial for formulating effective control measures and preventing CBM-related accidents. Traditional prediction methods rely on feature extraction or complex physical model parameter calculations, which require extensive manual intervention and have limited practical applicability. Additionally, simple neural network methods are prone to overfitting and gradient vanishing when handling parameters, and they lack the capability to dynamically monitor gas pressure during extraction, leading to inefficient and blind extraction operations. This study proposes a CBM pre-extraction parameter and completion time prediction method based on the Transformer model. By integrating autoregressive models and wavelet denoising techniques, the approach effectively captures temporal features and long-term dependencies in CBM data. Experimental results demonstrate that the proposed model outperforms traditional methods in short-, medium-, and long-term predictions, with a median R2 value of 0.99072, and 76% of the training results exceeding 0.9. Furthermore, a CBM pressure inversion model was developed, combining dimensional analysis and physical similarity principles with the Transformer model, enabling the dynamic detection of high- and low-pressure regions in coal seams. In single borehole compliance time predictions, the median compliance time for the first stage is 4 days, with an average of 49 days and a maximum of 277 days, providing adjustment guidance for boreholes with extended compliance times. The proposed model significantly improves prediction accuracy and stability, offering critical support for developing scientifically sustainable pre-extraction plans and advancing intelligent CBM management. [ABSTRACT FROM AUTHOR]

Radial Jet Drilling (RJD) provides favorable conditions for exploitation of coalbed methane (CBM). However, the jetting medium always remains in a high-shear state (Re >> 1 × 105) during the construction of RJD, with significant pressure loss that is difficult to control. In this paper, two surfactant-polymer composite systems for high-shear conditions are firstly preferred: CTAC-PEO and OTAC-PEO, with drag reduction rates (DR) of 31.92 % and 21.97 %, respectively. Secondly, three fluids were further optimized by adjusting the ratio and concentration of drag reducers: CP11–1000, OP41–1000, and OP41–3000. Their drag reduction characteristics were investigated at different temperatures and shear durations. Among them, the average DR of OP41–3000 at 55 °C is 2.28 and 2.95 times that of CP11–1000 and OP41–1000, respectively. Moreover, terminated DR after 30 mins of cyclic shear is 4.74 and 7.5 times greater than that of them, with a drag reduction retention rate exceeding 80 %. Finally, the "laminae encapsulate chains-network structure" was proposed to explain the mechanism of drag reduction under high shear. This study is the first to investigate drag reduction from the perspective of the jetting medium. The research findings will improve the future advancement of CBM mining utilizing RJD. Additionally, this study can assist in the selection of appropriate drag reducers for high-shear environment and the exploration of drag reduction mechanism. [ABSTRACT FROM AUTHOR]

Permeability is a critical parameter in coalbed methane (CBM) recovery and received increasing attention in recent years. The slip effect and effective stress exert competing influences on permeability, with coal exhibiting varying sensitivities to effective stress depending on their pore structures. The presence of water further complicates these interactions, affecting both the slip effect and permeability. This study investigates the pore structure and permeability characteristics of four coal cores at varying water contents using low-field nuclear magnetic resonance (NMR) and pulse pressure decay (PPD) methods. An enhanced apparent permeability model was developed by incorporating water content, effective stress, and the slip effect. The dynamic variations of compressibility coefficient, slip coefficient, and intrinsic permeability for Cores C-F were theoretically examined based on the refined model, and the critical pore pressures at which the slip effect becomes significant were identified. The results indicate that cores with larger average pore sizes exhibit more pronounced changes in fracture compressibility coefficients as water content increases. Additionally, the slip coefficient decreases with increasing pore pressure and is notably lower at reduced water contents. Intrinsic permeability increases more significantly with pore pressure at higher water content, with cores having larger average pore diameters showing greater sensitivity to these changes. The critical pore pressure, where the slip effect becomes significant, increases with water content and is higher in cores with smaller average pore sizes. Finally, various coefficients are proposed to quantitatively assess changes in fracture compressibility, slip coefficients, intrinsic permeability, and critical pore pressures under varying water content conditions, enabling more accurate analysis of permeability behavior. [ABSTRACT FROM AUTHOR]

• The thermo-hydro-mechanical coupling model for gas migration was constructed. • The gas pressure and temperature evolution were investigated. • The transition of dominated factors during gas migration process was discussed. Gas extraction by boreholes is one of the most effective means to eliminate gas related disasters and utilize gas resources. Due to the complexity of coal matrix and fracture structure, the mechanism of gas migration in coal and its dominated factors are not clear. In this paper, a thermo-hydro-mechanical coupling model was constructed, in particular, the heat conduction of the roof and floor to the coal seam was considered. The results indicated that the temperature of the coal seam decreases gradually and then increases. Due to the heat conductivity of the roof and floor of coal seam, the temperature isoline of the coal seam surrounding the borehole varies from circle-shape to elliptical-shape. The initial permeability of coal, initial gas diffusion coefficient, residual gas diffusion coefficient, and suction negative pressure are positively correlated with gas extraction quantity, while the gas diffusion attenuation coefficient and fracture spacing are negatively correlated with gas extraction quantity. During gas extraction process, the dominate factors transition from the fracture system to the matrix system, as a consequence, the increase of permeability of coal can significantly improve the gas extraction quantity in the early stage, while in the later stage, measures should be taken to enhance the diffusivity, such as crushing the coal body, the high-voltage electric pulse technology, and ultrasonic stimulation technology. The research of this paper provides a deeper understanding of the gas migration mechanism during gas extraction process. [ABSTRACT FROM AUTHOR]

• The conventional water displacement method used in gas desorption experiments is inadequate for measuring low-rate gas because of fluid binding forces and expanded tubes, leading to significant errors. • A novel method has been developed to enhance the accuracy of gas measurement, particularly in detecting low-rate gas, thereby improving upon the limitations of the conventional approach. • The reliability of the novel method is demonstrated through a comprehensive evaluation utilizing both experimental validation and theoretical analysis. 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. [ABSTRACT FROM AUTHOR]

• The gas migration control equations under the coupling effect were established. • The calculation models of energy considering the coupling effects were proposed. • Outburst mechanism of composite coal seam under the coupling effect was explained. As a fossil fuel, coal dominates global energy consumption. In the process of coal formation, due to the influence of geological structure, there are many phenomena of stratification of primary coal and tectonic coal. This also makes the occurrence and migration of gas in different coal seams different, which sharply increases the outburst risk of coal seams. In this paper, the control equation of gas migration was established according to the diffusion state of gas in damaged composite coal. The mechanical instability law of composite coal under the coupling damage and unsteady diffusion was simulated and analyzed. It is found that the existence of tectonic coal can effectively promote the gas desorption in the primary coal seam. Then, the calculation model of outburst energy under coupling effect was established, and the role of adsorption gas expansion energy in composite coal seam and single coal seam was analyzed from the perspective of energy. It is proposed that in the single coal seam of tectonic coal, the adsorption gas expansion energy is mainly used to transport coal and maintain the development of outburst. In the composite coal seam, it is mainly used to crush the coal, resulting in the occurrence of outburst. Finally, from the influence of tectonism, pore damage path and gas migration path, the outburst mechanism of composite coal seam under coupling effect was comprehensively explained. The research results provide a theoretical basis for the prevention and control of the outburst of composite coal seam. [ABSTRACT FROM AUTHOR]

• The influence tendency of limonene on the coals molecular structure is the same. • The extent of the effect of lemonene on coals molecular structure is different. • The overall orderliness quantifies the orientation of all aromatic lattice fringes. • Limonene liquid leaching is beneficial to clean production of coalbed methane. Variations in the surface chemical composition and chemical structure of coal induced by limonene leaching are crucial for realizing its application in clean and efficient production of coalbed methane (CBM). By quantitatively analyzing the surface groups, microcrystalline structural features, and morphological information of the aromatic lattice fringes in selected coals, this study investigated the mechanisms underlying the effects of limonene leaching on methane (CH 4) adsorption–desorption behaviors. The results indicated that limonene leaching disrupted aliphatic hydrocarbons, interstructural C=C bonds of aromatic hydrocarbons, and π-π stacking between aromatic rings in selected rank coals. This disruption caused the largest decrease in the average length of the aromatic lattice fringes by 21.9 % in SH coal, the greatest reduction in the length and degree of branching of aliphatic side chains (CH 2 / CH 3) by 19.5 % in SQ coal, and the most significant decline in the overall orderliness of the aromatic lattice fringes by 193.6 % in LD coal after leaching with a 20 wt% limonene solution (LS). These variations made limonene leaching more effective in improving the CH 4 adsorption–desorption behaviors of medium- and high-rank coals for clean and efficient CBM production. [ABSTRACT FROM AUTHOR]

The classic embedded discrete fracture model (EDFM) may cause certain errors when simulating the transient flow process within the multi-scale fractured system. In this study, the transient transmissibility was derived by using the point (matrix-fracture) and line (fracture-fracture) source functions. The transient source-function-based EDFM (tSEDFM) is established via this method. Then, based on the dual criteria that d NNC is less than a certain distance and its projection point is within the corresponding fracture domain, a 3D fracture meshing method for arbitrary fracture geometry and orientation is established. The finite volume method is used to discrete the fluid flow equations for matrix and fracture systems, considering the multi-deformation-induced permeability evolution. Consequently, the numerical simulation framework of FVM-tSEDFM is proposed to investigate the transient mass transfer characteristics. Compared with the current pEDFM, the pressure drop near fractures of the proposed model is higher and the gap narrows as pressure wave further spreads. Compared with AEDFM, the T mf of tSEDFM is almost the same, but AEDFM's T ff is substantially higher in the initial stage and the attenuation rate is slower, resulting in an error of nearly 15% at the hydraulic fracture location. As fracture density increases, more transient flow makes AEDFM- and EDFM-calculated BHP values deviate from tSEDFM. History matching using the tSEDFM with only 10000 meshes shows better results where the mismatching rate is larger than 90%. Simulation of CBM extraction indicate that sharp pressure decline occurs near the fractures, resulting in gas desorption enhancement and rapid productivity increment. The tSEDFM demonstrates good practicality in calculating the production and long-term permeability evolution. Results also show that the tSEDFM can correct the underestimation of mass exchange in the early stage and overestimation since the pseudo-steady stage, and can be better applied to the numerical simulation of multi-scale fractured reservoirs. As the dominant factor for gas extraction, sensitivity analyses of fracture properties are also documented. • A transient source-function-based embedded discrete fracture model is established. • A 3D fracture meshing method for arbitrary geometry and orientation is established. • A numerical simulation framework of FVM-tSEDFM is proposed to evaluate CBM extraction. • The proposed tSEDFM only requires 10000 grids to achieve a fitting accuracy of less than 10%. • The synergistic pressure-permeability evolution mechanism is revealed. [ABSTRACT FROM AUTHOR]