Methane Reservoirs Research Articles

An Analytical Method for Timely Predicting of Coal Seam Pressure during Gas Production for Undersaturated Coalbed Methane Reservoirs

Coal seam pressure is an important parameter for production performance evaluation and prediction of coalbed methane (CBM). CBM production from undersaturated CBM reservoirs can be divided into two stages according to critical desorption pressure. At present, few prediction models of coal seam pressure performance consider the comprehensive influence of critical desorption pressure, dissolved gas, matrix shrinkage, and stress sensitivity. For the purpose of accurately predicting coal seam pressure during gas production for an undersaturated coalbed methane reservoir, the material balance principle is used to establish the analytical method for predicting coal seam pressure, considering the comprehensive influence of the critical desorption pressure, dissolved gas, matrix shrinkage, and stress sensitivity. Then, the proposed method is verified against a numerical simulation case using a computer modelling group (CMG) and two actual coalbed methane wells. Finally, the sensitivities of influencing factors on the coal seam pressure are analyzed. The results show that good agreements were obtained between the calculated coal seam pressures using the proposed method and those from the CMG-GEM simulation case and actual CBM wells, with the relative errors all being less than 1%. When ignoring the influence of critical desorption pressure and mistaking pd for pi as well as ignoring Cs, the relative error can reach as high as 31.3%. The main factors affecting the coal seam pressure are the critical desorption pressure and free gas saturation. The proposed method is simple to use, and without shutting-in the well, it can provide an important basis for production performance evaluation and development strategies.

Research on Fracturing Optimization of Coalbed Methane Wells Aiming at Economic Benefit—A Case Study of Liulin Block

Hydraulic fracturing is an essential technology in the development of coalbed methane reservoirs. Hydraulic fracturing can create a highly conductive fracture in the reservoir and increase its permeability. At present, the focus of coalbed methane reservoir fracturing optimization is gradually shifting to the fracturing scale. In the current development process, more and more coalbed methane blocks try to increase the fracturing scale to increase the gas production of coalbed methane wells. Field tests show that gas production of coalbed methane wells will increase to a certain extent with the increase of fracturing scale. However, the increase in the scale of fracturing also increases its cost. Therefore, the most economical fracturing scale is not necessarily the optimal fracturing scale for gas production. The field test usually pays more attention to the gas production effect, but the development of a coalbed methane field should pay more attention to the economic benefit, and the optimization of fracturing should take the economic benefit as the goal. Taking economic benefits as the starting point, this paper uses fracturing simulation to calculate the fracture extension under different geological conditions and different fracturing scales. It also uses numerical simulation to calculate the gas well productivity under different fracture extension conditions. The economic evaluation model was established to calculate the economic benefits under different fracturing scales, and the optimal fracturing scale was obtained. Finally, the typical maps of fracturing optimization under different geological conditions are formed. The optimization method of fracturing scale integrating economy, fracturing, and gas reservoir is realized. The research results have been successfully applied to the optimization scheme of Liulin block development, and very good results have been achieved. Because this method is targeted at different geological conditions, it can be used to guide the fracturing optimization of other coalbed methane blocks and has very important significance for the development and optimization of coalbed methane reservoirs.

Study on Seepage of Coalbed Methane in Pore-fracture Double - porosity Medium

The study of coalbed methane (CBM) seepage within dual-porosity medium constitutes a significant research endeavor in the field of geology and coalbed methane reservoir engineering. This research delves into the complex dynamics governing CBM flow through porous coal matrices within dual-porosity systems. Employing advanced methodologies including numerical simulation and experimental analysis, this investigation aims to elucidate the mechanisms underlying CBM migration, encompassing diffusion and adsorption phenomena within the coal matrix, as well as interactions with adjacent strata. The outcomes of this study carry substantial implications for optimizing CBM extraction techniques and promoting the sustainable utilization of this invaluable natural resource.

Comparison of elastic properties of tectonic coals along the face and butt cleat directions

AbstractCoalbed methane is a hot spot for gas exploration at present, and fractures are widely developed in the coals. However, despite being essential for a number of geophysical applications such as reservoir prediction and hydraulic fracturing, the influence of fractures on the elastic properties and anisotropy of coals is still poorly understood. Therefore, three groups of cylindrical coals were drilled along the face and butt cleat directions to study the effects of pressure and fracture on the elastic properties and anisotropy of tectonic coals. The velocity along the face cleat direction is less sensitive to the pressure than that along the butt cleat direction due to the difference in compliant pore and fracture content. Coals display a strong directional dependence in petrophysical and elastic properties. The permeabilities and velocities of samples along the butt cleat direction are much lower than those along the face cleat direction. The fracture densities are quantitatively characterized by the theoretical model. The values along the face cleat direction are much smaller than those along the butt cleat direction. The influence of fractures on the elastic properties and anisotropy is comprehensively studied by theoretical and laboratory methods. The mineralogy and fracture effects are important factors to the elastic anisotropy of coals, the latter can be a dominant factor to the coal elastic anisotropy. The results can contribute to the understanding of the cleat system of coalbed methane reservoir and can provide a critical rock physics basis for micro‐seismic fracture monitoring and reservoir prediction.

A Method for Evaluating Coalbed Methane Reservoir Productivity Considering Drilling Fluid Damage

In the process of coalbed methane development, drilling fluid and fracturing fluid cannot achieve absolute compatibility with formation. The incompatibility between the working fluid and reservoir will lead to the intrusion of working fluid into the reservoir and cause reservoir pollution. This is a very common phenomenon. There is a large amount of pulverized coal in the coal seam, and the intrusion of working liquid will be combined with the pulverized coal to form cement to block the seepage space in the reservoir. Since pressure relief and fracturing fluid backflow will be performed at the first time after fracturing, the intrusion range of the working fluid is small, generally reaching 10 m to 50 m. Compared with a conventional gas reservoir or shale gas reservoir, the working fluid loss during CBM development will seriously affect the subsequent production project and even make the gas well lose production capacity. On the other hand, in order to avoid this phenomenon, measures such as acidification or volumetric fracturing are sometimes used to improve the seepage environment near the well and near the fracture. The purpose of this study is to quantitatively evaluate the impacts of working fluid filtration and reservoir reconstruction on production. In this study, a single well productivity evaluation model and sensitivity analysis method considering drilling fluid filtration loss, fracturing fluid filtration loss, reservoir reconstruction and other processes is proposed. The formation mechanism of fluid loss during drilling and fracturing is described, and the productivity evaluation model considering fluid loss is combined with the Langmuir isothermal adsorption equation, steady-state diffusion law, Darcy’s seepage law and Duhamel convolution formation. Combined with the distribution of actual gas reservoir flow characteristics, the sensitivity of single well productivity to gas reservoir porosity, gas reservoir permeability, coal seam adsorption coefficient, working fluid filtration loss and reservoir reconstruction measures are analyzed. Through the analysis and fitting of the actual production data on site, the relationship curve can better fit the field production data, and the evaluation results are in line with the drilling and fracturing conditions at that time and the subsequent production conditions, with small errors. The obtained method is suitable for predicting the productivity of fractured vertical wells in different working conditions and provides a basis for the development and productivity prediction of CBM reservoirs in China and in international cooperation.

Reservoir Characteristics of the Main Coal Seams in the Longtan Formation, Guxu Coal Mining Area, Sichuan

To facilitate the efficient exploration and development of coalbed methane of the Longtan Formation in the Guxu coal mining area, Sichuan, it is essential to evaluate the reservoir characteristics of the main production layers. In this study, scanning electron microscopy, low-temperature liquid nitrogen adsorption experiments, high-pressure mercury pressure experiments, and isothermal adsorption experiments were conducted to investigate the reservoir characteristics of favorable coalbed methane reservoirs. The results indicate that the main coal seams in the Longtan Formation are medium-to-high ash content anthracite with a well-developed pore–fracture system. The pore size distribution exhibits both unimodal and bimodal types, while the pore morphology includes impermeable pores closed at one end, open permeable pores, and ink bottle-shaped pores. It shows that the middle part of Longtan Formation acts as an enrichment zone for coalbed methane, which is characterized by a stronger adsorption capacity, high Langmuir volume, high gas content, and high gas saturation distribution. The pH value, mineralization degree, and hydrogen–oxygen isotope of the produced water in the main coal seams indicate that the enrichment zone is typically located in a stagnant flow zone with a reducing environment and favorable storage conditions.

Hydraulic fracture initiation and propagation in deep coalbed methane reservoirs considering weak plane: CT scan testing

Hydraulic fracturing technology in coal reservoirs helps increase coalbed methane (CBM) production, and the development of hydraulic fractures (HFs) is influenced by in situ stress and original fractures. Therefore, in this study, triaxial HF tests and computed tomography (CT) were used to determine HF initiation and propagation in coal, considering weak structural planes and in situ stress. The experimental results show that weak structural planes control HF initiation and propagation in coal under a deep-stress environment. Most HFs occurred along bedding planes, followed by exogenetic fractures. The influence of the horizontal stress difference coefficient k, which is equal to the maximum horizontal stress minus the minimum horizontal stress and divided by the minimum horizontal stress, on the propagation of the HF was studied. When k increases to 1.04, the HF becomes perpendicular to the direction of the minimum principal stress. When k < 1.04, the main controlling factor of HF propagation is a weak structural plane, and when k ≥ 1.04, the main controlling factor of HF propagation is in situ stress. Based on the CT reconstruction model, the average aperture distributions of the discrete bedding fractures and exogenetic fractures of coal in this experiment were calculated to be 0.0947 mm and 0.1940 mm, respectively. This determines the different strength properties of exogenetic fractures and bedding planes, and determines the priority initiation position of HF in coal. During the HF initiation and propagation, the average energy along the exogenous fracture was lower than that along the bedding planes. On this basis, this study predicts that the main factor controlling HF propagation in a deep coal seam in the Qin-Shui Basin, China, is the weak structural plane in the coal seam.

WITHDRAWN: Corrigendum to ‘Permeability enhancement of coal-bearing propped fractures using blockage removal agent treatments in coalbed methane reservoirs’ [Gas Science and Engineering 124 (2024) 205259]

WITHDRAWN: Corrigendum to ‘Permeability enhancement of coal-bearing propped fractures using blockage removal agent treatments in coalbed methane reservoirs’ [Gas Science and Engineering 124 (2024) 205259]

Synthetic aperture radar (SAR) detects large gas seeps in Alaska lakes

Reservoirs of 14C-depleted methane (CH4), a potent greenhouse gas, residing beneath permafrost are vulnerable to escape where permafrost thaw creates open-talik conduits. However, little is known about the magnitude and variability of this methane source or its response to climate change. Remote-sensing detection of large gas seeps would be useful for establishing a baseline understanding of sub-permafrost methane seepage, as well as for monitoring these seeps over time. Here we explored synthetic aperture radar’s (SAR) response to large sub-permafrost gas seeps in an interior Alaskan lake. In SAR scenes from 1992 to 2011, we observed high perennial SAR L-band backscatter (σ 0) from a ∼90 m-wide feature in the winter ice of interior Alaska’s North Blair Lake (NBL). Spring and fall optical imagery showed holes in the ice at the same location as the SAR anomaly. Through field work we (1) confirmed gas bubbling at this location from a large pockmark in the lakebed, (2) measured flux at the location of densest bubbles (1713 ± 290 mg CH4 m−2 d−1), and (3) determined the bubbles’ methane mixing ratio (6.6%), radiocarbon age (18 470 ± 50 years BP), and δ13CCH4 values (−44.5 ± 0.1‰), which together may represent a mixture of sources and processes. We performed a first order comparison of SAR σ 0 from the NBL seep and other known sub-permafrost methane seeps with diverse ice/water interface shapes in order to evaluate the variability of SAR signals from a variety of seep types. Results from single-polarized intensity and polarimetric L-band SAR decompositions as well as dual-polarized C-band SAR are presented with the aim to find the optimal SAR imaging parameters to detect large methane seeps in frozen lakes. Our study indicates the potential for SAR remote sensing to be used to detect and monitor large, sub-permafrost gas seeps in Arctic and sub-Arctic lakes.

An Evaluation of the Coalbed Methane Mining Potential of Shoushan I Mine Based on the Subject–Object Combination Weighting Method

The parameters of coalbed methane reservoirs have large differences, and the precise values cannot represent the resource and production characteristics of the whole block. In order to address these problems, an index system for evaluating the production potential of coalbed methane blocks was constructed, the weights of evaluation parameters were determined, and a model for the preferential selection of coalbed methane blocks based on the subjective–objective combination of weights method was established. The main coal seams (No. 2-1 and No. 4-2) of the Pingdingshan-Shoushan I Mine Block were taken as the research objects to rank the development potential of CBM blocks in a preferential way. The results show that the six resource and production parameters of No. 2-1 coal are gas content, top and bottom rock properties, coal seam thickness, coal seam depth, coal body structure, and tectonic conditions, in descending order of importance, and the parameters of No. 4-2 coal are gas content, coal body structure, coal seam thickness, top and bottom rock properties, coal seam depth, and tectonic conditions, in descending order of importance. It is predicted that the favorable CBM gas development sweet spot areas of the No. 2-1 coal seam and No. 4-2 coal seam will be located along the exploration wells W15–W29 and W31, respectively. This paper aims to make a multi-dimensional and more comprehensive evaluation of coalbed methane mining potential in the Shoushan I mine, and provide a technical basis for the next step of coalbed methane mining in the study area.

Permeability enhancement of coal-bearing propped fractures using blockage removal agent treatments in coalbed methane reservoirs

The generation and transport of coal powder has caused great harm to coal reservoir channeling conductivity, affecting the capacity of coalbed methane wells, causing the failure of engineering equipment and great distress to production, while current unblocking methods have many shortcomings. Dispersants, foam stabilizers, and inorganic salt electrolytes were selected in this study as the main components of blockage removal agents, and the wettability and synergistic effects of each component were tested. The results showed that the wettability was best when the ratio of the blockage removal agent was 0.8% SDS + 0.6% BS-12 + 0.8% NaCl, which was 1.57 times higher than that of a single dispersant. The surface tension of blockage removal agent was 64.75% lower than that of clear water. The discharge rate of the pulverized coal particles under the action of the optimum blockage removal agent reached more than 40%, which was more than twice that of clear water. The discharge of the pulverized coal was more than 1.2 times that of the common dispersant formula. The gas-liquid permeability was 4.21 times higher than that obtained with water injections, and 1.45 to 2.91 times higher than that of the commonly used dispersant formulations. This work is helpful to provide a practical guide for the efficient cleaning of sedimentary retained pulverized coal in low-yield coalbed methane wells. This research result is very important to ensure the efficient and stable development of coalbed methane.

Research on the Multiscale Microscopic Pore Structure of a Coalbed Methane Reservoir

Coal rock pores are the space in which coalbed gas is stored and flows. Accurately characterizing the pore structure of coalbed gas is the foundation of coalbed gas reserve assessment and production forecasting. Traditional experimental methods are unable to characterize the multi-scale pore structure characteristics of coal rock. In this paper, a multi-scale pore structure characterization method is proposed by coupling various experimental methods, including low-pressure nitrogen gas adsorption experiments, X-ray computed tomography (XCT) imaging technology, and scanning electron microscopy (SEM). Using Zhengzhuang coalbed gas as an example, the micro-pore structure of coalbed gas reservoirs is characterized and depicted from a multi-scale perspective. The results indicate that a single experimental approach can only partially reveal the microstructure of coal rock pores. The combined use of multiple methods can accurately reveal the full-scale microstructure of coal rock pores. The pore structure of the experimental coal rock samples exhibits multi-scale characteristics, with a complex variety of pore types, including inorganic pores, organic pores, and fractures. Organic pores are predominant, with a small number of inorganic pores, and their sizes range from 2 nm to 50 μm. Mineral particles and fractures are observed at both the nanoscale and microscale, exhibiting typical multi-scale characteristics, with quartz being the predominant mineral.

Study on the Complex Electrical Response Characteristics of Loaded Coal and the Control Mechanism of the Main Fracture System Structure.

The structure of coal seam fractures is the main physical property of coalbed methane reservoir evaluation, and the complex resistivity method is a potential geophysical evaluation method for coal seam fractures. In this study, cylindrical coal samples with axial directions perpendicular to the bedding, face cleat, and butt cleat were prepared. The complex electrical parameters of the loaded specimens were tested with test frequencies ranging from 1 Hz to 10 kHz. The complex electrical response characteristics of the loaded coal are summarized, and the control mechanism of the main fracture system structure is analyzed. The results indicated that (1) as the loading pressure increased, the resistance R and the absolute values of reactance X(|X|) gradually decreased, especially in the frequency band where R slowly decreased and the characteristic frequency of X, the decreased amplitude was more significant, and the cutoff frequency of R and the characteristic frequency of X all gradually increased. (2) The complex electrical properties of coal show obvious anisotropic characteristics. Both R and |X| decreased sequentially according to the direction perpendicular to the bedding, face cleat, and butt cleat; the cutoff frequency of R and the characteristic frequency of X all increased sequentially. (3) The dispersion phenomenon of the complex electrical properties of coal is attributed to the induced polarization; the elevated loading stress enhances the polarization effects of the molecular-induced moments of the coal skeleton, and the anisotropic difference of the complex electrical properties is due to the difficulty in the degree of transport of charged particles induced by structural differences of the main fracture system. (4) The resistance R3 and capacitance Xc were selected as the complex electrically sensitive parameters of the loaded coal orthogonal fracture structures. A logarithmic inversion model reflecting the main fracture system structure of coal was constructed. This provides a certain theoretical basis for efficient electrical exploration of coal reservoir fracture structures.

Biogeochemical investigations of methane seepage at the ultraslow-spreading Arctic mid-ocean ridge: Svyatogor ridge, Fram Strait

The Arctic continental shelves are important reservoirs of methane stored in gas hydrates and deeper geological formations. However, little is known about methane dynamics in deeper oceanic settings such as mid-ocean ridges, mainly due to operational challenges related to their remoteness. This study investigates a recently discovered methane seepage environment at Svyatogor Ridge, a sediment-covered transform fault on the western flank of the Arctic mid-ocean ridge west of Svalbard. Svyatogor Ridge was previously hypothesized to host deep gas hydrates and active fluid flow systems, which is a unique combination for this setting and requires geochemical evidence. Based on sediment and foraminiferal geochemistry, we demonstrate that Svyatogor Ridge hosts a shallow methane cycle with anaerobic oxidation of microbial methane, sustaining chemosynthetic communities at the seafloor. Our geochemical datasets also suggest that methane fluxes were higher in the past than today and long-lasting, with episodes of intense methane oxidation recorded in pre-Holocene sediments. Methane seepage is still ongoing at this location, although no evidence exists that methane is reaching the sea surface. The results provide the first evidence of a methane seepage environment ever reported from the ultra-slow spreading Arctic mid-ocean ridge, thus calling for a reevaluation of the role of this type of ridge in the ocean methane cycle.

Influence of Imbibition Fracturing Fluid on the Original Water and Methane Occurrence in Actual Coalbed Methane Reservoirs Using the Integrated Device of Displacement and Low-Field Nuclear Magnetic Resonance.

The original water in the coal rock pores plays a controlling role in the occurrence of gas. Furthermore, during the hydraulic fracturing process, pressurized fracturing fluid with a higher pressure than the original pore pressure in the fractures drives the fracturing fluid to infiltrate into the coal rock pores, thereby altering the occurrence pattern of gas and water in the original pores. However, due to the limitations of the indoor simulation device, a systematic conclusion on the impact of the original pore water and imbibition fracturing fluid on coalbed methane reservoirs has not yet been formed. In this paper, an integrated device combining displacement and low-field nuclear magnetic resonance was employed using underground cylindrical coal rock samples as experimental subjects. Experimental conditions were maintained at a temperature of 30 °C, a confining pressure of 23 MPa, and an approximate reservoir pressure of 15 MPa. The initial water saturation levels were altered to 0, 27.88, and 42.18% to replicate the conditions of a coalbed methane reservoir at a depth of approximately 1200 m. Fracturing fluid with a pressure of 18 MPa was injected into the experimental samples to simulate the impact of the fracturing fluid on the original reservoir during hydraulic fracturing. This allowed for a realistic assessment of the influence of initial water saturation and fracturing fluid absorption on the coalbed methane recovery rate in the reservoir. The experimental results indicate that the imbibition process promotes the desorption of adsorbed gas, and the desorption amount of adsorbed gas increases with the increase in the original water saturation. This will result in an increase in the gas pressure within the pore system. The conditions of this experiment, in comparison to the previous ones, more closely resemble real reservoir conditions. This enables a realistic assessment of how the presence of the original water content and the absorption of the fracturing fluid affect gas occurrence within the reservoir.

Geological structure identification of coalbed methane reservoir based on trend surface and curvature analysis algorithms

Geological structure identification of coalbed methane reservoir based on trend surface and curvature analysis algorithms

Experimental Investigation of Fracture Propagation in Clayey Silt Hydrate-Bearing Sediments

More than 90% of the natural gas hydrate resources are reserved as marine clayey silt sediments. It is of great significance to efficiently develop a clayey silt hydrate. At present, there are problems of low single well production and small depressurization range in its production test, which is still a long way from commercial exploitation. The combination of hydraulic fracturing technology and other methods such as depressurization method is regarded as one of the potential technical means to achieve the commercial exploitation of the hydrate. However, compared with shale gas reservoirs and coalbed methane reservoirs, clayey silt hydrate reservoirs have special mechanical properties, resulting in unique hydraulic fracturing processes. Therefore, it is necessary to study the fracture initiation and propagation laws of clayey silt hydrate reservoirs. To this end, we carried out large-scale (30 × 30 × 30 cm) true triaxial hydraulic fracturing experiments using a simulated material with similar mechanics, porosity, and permeability to clayey silt hydrate-bearing sediments. The effects of completion method, fracturing method, and fracturing fluid displacement on hydraulic fracture propagation of clayey silt hydrate-bearing sediments were studied. The results showed that a perforated completion can significantly increase the fracture reconstruction area and decrease the fracture initiation pressure compared to an open hole completion. Due to the small horizontal stress difference, it is feasible to carry out temporary plugging fracturing in clayey silt hydrate reservoirs. Temporary plugging fracturing can form steering fractures and significantly improve fracture complexity and fracture area. Increasing the fracturing fluid displacement can significantly increase the fracture area as well. When conducting fracturing in clayey silt hydrate-bearing sediments, the fracturing fluid filtration area is obviously larger than the fracture propagation area. Therefore, it is recommended to use a high-viscosity fracturing fluid to reduce the filtration of the fracturing fluid and improve the fracturing fluid efficiency. This study preliminarily explores the feasibility of temporary plugging fracturing in clayey silt hydrate reservoirs and analyzes the effect of completion methods on the propagation of fracturing fractures, which can provide a reference for the research conducted on the fracturing stimulation of clayey silt hydrate reservoirs.

Theoretical analysis and numerical simulation of methane adsorption behavior on rough surfaces featuring fractal property

This work focuses on the control mechanism of adsorption behavior from rough surfaces featuring fractal property numerically. For that, we develop an equivalent characterization method for rough fractal surfaces, reproduce the adsorption process of methane on the surface by combining molecular dynamics and Grand canonical Monte Carlo (GCMC) simulations, analyze the control mechanisms of complex types of surface geometry on the methane adsorption process, and clarity the ratio of vertical roughness to horizontal complexity as key parameter to describe the adsorption capability. Our numerical results indicate that the adsorption density of methane in fractal and non-fractal slit pores varies with the fractal behavior of surface geometry, while the density of gas phase methane remains unaffected (all distributed around 0.06 g/cm3). And, the complexities of surface geometry process various effects on occurrence mode, adsorption potential energy (ɛ), and adsorption amounts. The surface original complexity will influence the force field, and alter the occurrence mode from monolayer adsorption to pore filling. Meanwhile, the width-to-depth ratio of surface original complexity has a negatively effects the ɛ of methane. The behavioral complexity of the surface reflects the number of small pores on it. For fractal slit pores, the maximum adsorption amounts are 2405.77, 2634.90, and 2749.46 mol/m3, respectively. They are smaller than that of the non-fractal slit pore, which is due to the increase of non-adsorption pores for the small diameter of pores. Above results give a clear explanation to the effects of fractal property on methane adsorption, which will provide a reference to study the adsorption process of methane in Coalbed Methane reservoir.

Climatic factors and fertilization rates co-regulate anaerobic methane oxidation driven by multiple electron acceptors in Chinese paddy fields

Paddy fields constitute a substantial anthropogenic reservoir of methane, with their inundation management fostering an ideal habitat for anaerobic methane oxidation. Within this context, a novel clade of anaerobic methanotrophic (ANME) archaea, known as ANME-2d, has been identified as capable of catalyzing anaerobic methane oxidation in conjunction with nitrate and metal oxide reduction processes within paddy fields. Nevertheless, our comprehension of the mechanisms governing anaerobic methane oxidation and its pivotal role in regulating methane emissions within rice paddies remains limited. This study quantified the rates of nitrate- and iron (III)-driven anaerobic methane oxidation through 13C-labeled stable isotope tracing experiments in Chinese paddy fields spanning diverse climate zones. Additionally, it investigated the ANME-2d archaeal community using quantitative polymerase chain reaction and high-throughput sequencing techniques. The nitrate-driven anaerobic methane oxidation contributed 10.9% to methane emission reduction. This contribution is equal to the previously identified contribution of nitrite-driven anaerobic methane oxidation (11.2%) mediated via NC10 bacteria, but played more important roles than iron-driven one (4.1%). The rates of nitrate- and nitrite-driven anaerobic methane oxidation differed significantly among climate zones and showed positive correlation with the mean annual temperature. Furthermore, their rates were more sensitive to temperature increases at higher and lower latitudes, respectively, under both representative concentration pathways 2.6 and 8.5. The rate of anaerobic methane oxidation driven by nitrate exhibited a positive correlation with nitrogen fertilization rate but displayed a negative correlation with phosphorus fertilization rate. Conversely, the rate of anaerobic methane oxidation driven by iron showed no significant correlation with either nitrogen or phosphorus fertilization rates. This study underscores the great potential of anaerobic methane oxidation in mitigating global warming, particularly under the conditions of future climate change and elevated nitrogen loading. These findings underline the necessity of incorporating anaerobic methane oxidation as a crucial parameter in methane emission prediction models.

The Influence of Coal Lithotype and Metamorphism on the Development Characteristics of Pore and Fracture in Coalbed Methane Reservoir in Ordos Basin and Qinshui Basin

To study the characteristics of pores and fractures development in coalbed methane reservoir controlled by coal lithotype and metamorphism, a total of 26 coals from 16 coal mines in the Ordos Basin and Qinshui Basin were tested for kerogenic components, scanning electron microscopy, mercury intrusion experiments, low‐temperature nitrogen adsorption experiments, and Fourier transform infrared spectroscopy, which summarized and analyzed the coal lithotype and metamorphism constraints on the development of pores and fractures and came to the following conclusions: At the early stage of coalification, with the aliphatic hydrocarbon chains in coals are gradually shortened, and the branched chains and oxygen‐containing functional groups are continuously shed and reduced, resulting in asphaltenes filling the pores, which leads to an overall decrease in the porosity of coal and an increase in the inhomogeneity of the pore structure; at the transition stage of coals to high maturity, asphaltenes in coal are gradually reduced, the filling effect is reduced, aromatic lamellae are continuously oriented, and aromatic rings are continuously condensed. In the transition stage of coal to high maturity, the filling effect decreases, the aromatic lamellae keep orienting and arranging, and the aromatic rings keep condensing, which makes the porosity, pore volume, and specific surface area of coal rebound, and the nonhomogeneity of pore structure weakens; the density of microfracture decreases with the increase of coal rank, which may be the effect of compaction and asphaltene effect; the mirror group in coal is favorable to the development of pore fracture, while the inert group is opposite; the density of microfracture is positively correlated with the content of vitrite and vitrinertite, and negatively correlated with the content of inertite. The higher the content, especially the content of microscopic coal, the more developed the microfractures.