Acid fracturing technology is one of the most effective methods for resolving mineral plugging and for improving the pore structure of coal reservoirs. To investigate the characteristics of shallow and deep coal nanopore structures under the influence of acidic fracturing fluids, experiments using synchrotron radiation small-angle X-ray scattering were conducted on shallow and deep coal samples soaked in acidic fracturing fluids of different concentrations for varying durations. This quantitatively characterized the different nanoscale pore scattering intensity ratios (AI), fractal dimensions, and nanopore parameters. The research indicates that, under the influence of acidic fracturing fluids, the shallow coal nanopore structure tends to become more complex while that of deep coal becomes simpler. The impact of 20

Considering the difficulty of balancing real-time and simultaneous in previous studies on pore structure and microcrystalline structure during coal rising-temperature oxidation, this paper simulated the rising-temperature oxidation of three coals by setting the rising-temperature rate and range based on the sample heating furnace. The synchrotron radiation in-situ small-angle X-ray scattering and wide-angle X-ray scattering (SAXS-WAXS) simultaneous tests were carried out. The results show that as the temperature increases, the pore structure of long-flame coal tends to be simpler and more regular with more abundant macropores, and lean coal is relatively more stable, while anthracite is more active in the early period of rising-temperature oxidation with significant development of mesopores. The changes in mesopores of three coals during the entire rising-temperature oxidation are dominated by pores of 10 similar to 20 nm, while pores of 50 similar to 60 nm, 60 similar to 70 nm, and 70 similar to 80 nm dominate the changes in macropores of long-flame coal, lean coal, and anthracite, respectively. Furthermore, the changes in the microcrystalline structure of three coals show two stages: Physical desorption (20 similar to 200 degrees C): the removal of water molecules and weakly adsorbed gases causes the carbon skeleton structure to become relatively loose; thermal decomposition (200 similar to 300 degrees C): the intensification of pyrolysis reactions leads to a much higher polycondensation of aromatic layers.

Coal seam CO2 sequestration is an important option to address global warming. A better knowledge on coal pore structure evolution during gas adsorption can provide guidance for coal seams CO2 sequestration. However, few investigations on the pore structure evolution differences between the deep and shallow coal were conducted during gas adsorption. In this study, based on the real-time synchrotron radiation small-angle X-ray scattering (SAXS) observation, the average pore diameter and pore surface fractal dimension evolution differences between deep and shallow coal were investigated from the aspects of coal compositions and stress history. Two types of coal deformation (inner-swelling and outer-swelling) coexist during gas adsorption. Coal compositions have significant impact on the dominance of deformation type. The dominance of inner-swelling in deep coal is induced by the higher ash contents, and there is the decrease of average pore diameter during gas adsorption. The impact of stress-history (burial depth) on adsorption-induced deformation is more prominent than that of gas adsorption capacity. In deep coal, the surface fractal dimension evolution presents a negative correlation with the evolution of pore diameters. In shallow coal, the surface fractal dimension evolution presents a Langmuir-type correlation with the adsorption time.

Dust explosion is short-lived, but dangerous. Time-resolved small angle X-ray scattering (SAXS) with synchrotron radiation is an effective method to study the microscopic dynamics of dust explosion, but it needs high beam flux, fast detector and appropriate explosion equipment. The pink beam SAXS beamline at the upcoming high energy photon source (HEPS in Beijing, China) will possess high beam flux as 1015 phs/s and equip new advanced fast integral detector, which will provide ideal conditions for time-resolved experiment. This study aims to preliminarily develop an explosion equipment to integrate into the SAXS instrument at HEPS in the early future. The main body of the equipment is a 20 L Siwek sphere (spherical shell) of stainless steel to serve as the chamber for dust explosion. The sphere has a pair of single crystal diamond windows that are used to horizontally pass synchrotron radiation X-rays with the beam path slightly below the centre of the sphere. The powder is sprayed from bottom into chamber of the sphere and ignited by the ignition rods extending from the top to the centre of the sphere. A corresponding control system is used to conveniently integrate and logically control the dust explosion and the corresponding SAXS signals collection. The feasibility and safety of the system have been verified at the 1W2B experimental station at the Beijing Synchrotron Radiation Facility (BSRF).

Dust explosions are common and highly hazardous industrial accidents that occur rapidly and involve complex changes in particle structure. Traditional experimental methods struggle to capture the time-resolved information of the explosion process, limiting the in-depth study of its dynamic mechanisms. Synchrotron radiation small-angle x-ray scattering (SAXS) offers a novel method to study the microscopic dynamics of dust particles during explosions. Utilizing high-flux x-rays from synchrotron facilities, coupled with fast detectors and specialized explosive chambers, SAXS provides high temporal and spatial resolution alongside nondestructive detection capabilities. It is expected to obtain information on particle morphology changes during dust explosions. This contribution first introduces the basic characteristics and hazards of dust explosions, reviews the current research status and challenges, and then expounds on the basic principles and advantages of synchrotron radiation SAXS. Finally, the potential of this method in real-time monitoring of dust explosion characteristics and enhancing understanding of explosion mechanisms was explored.

ABSTRACT: This study explores the potential of using supercritical CO2 (ScCO2) injection for carbon sequestration in shale gas reservoirs, focusing on the impact of structural heterogeneities on CO2 storage. This investigation presents a fully coupled thermo-hydro-mechanical (THM) analysis of ScCO2 injection in such reservoirs, aiming to unravel the complexities imposed by multiscale structural heterogeneities on the process of ScCO2 sequestration, encompassing the intricate shale matrix and the discrete fracture network. The model incorporates a fractal-based paradigm for accurately delineating shale matrix porosity and the evolution of pore diameters, which are instrumental in refining the permeability model within these geologically complex structures. The model considers the unique properties of ScCO2 and uses the Navier-Cauchy equation to simulate the mechanical behavior of shale formations during ScCO2injection. A thorough sensitivity analysis assesses the influence of various factors, such as injection pressure, temperature and principal stresses, on the process's efficiency. Our findings aim to improve carbon capture and storage (CCS) techniques, contributing to NetZero goals by optimizing ScCO2 sequestration in unconventional reservoirs. 1. INTRODUCTION The global commitment to achieving net-zero emissions is a cornerstone in the fight against climate change, necessitating innovative strategies to capture and store carbon dioxide (CO2) effectively (Alcalde et al., 2018). CO2 storage, particularly in geological formations, emerges as a pivotal solution to mitigate atmospheric CO2 levels. Among various geological formations, shale reservoirs offer unique advantages for CO2 storage. Their abundant organic content and low permeability make them potential candidates for secure and long-term CO2 sequestration. Moreover, the injection of supercritical CO2 (ScCO2) into shale formations not only facilitates storage but also enhances gas recovery, thereby presenting a dual benefit of mitigating climate change while improving energy resource utilization (Iddphonce et al., 2020). The complexity of subsurface environments, especially in shale reservoirs, demands sophisticated numerical models to accurately simulate CO2 storage processes. These models encompass various physical and chemical interactions through multiphysics coupling, addressing the thermal (T), hydraulic (H), and mechanical (M) responses of the reservoir to CO2 injection (Gaus et al., 2008). Recent studies have highlighted the significance of such THM coupled models in predicting the behavior of CO2 within shale formations, considering aspects like CO2 plume migration, pressure build-up, and the potential for inducing seismic events (Cheng et al., 2023; Gholami et al., 2021). For instance, Zhang et al. (2016) and Lei et al. (2015) have made substantial contributions by developing models that integrate these multiphysics aspects, offering insights into the optimal strategies for CO2 injection and storage in subsurface rock formations.

Coal spontaneous combustion is a common problem faced by many coal mines. Spontaneous combustion in goaf releases a large amount of harmful gases, polluting the environment while causing a large amount of wasted resources, and even endangering the lives of workers. Due to the collapse of the interior of the mining area, it is impossible to measure the internal gas composition directly. In order to more accurately predict the spontaneous combustion state inside the mining airspace, this paper obtains the CO generation law and the main source of the working face through the combination of laboratory experiments and on-site monitoring. The CO concentration prediction model of the return corner is established with CO as the index gas. Finally, the safe concentration and warning concentration of the working face are calculated according to the example, which provides theoretical basis for the prediction of spontaneous combustion of coal.

Small angle X-ray scattering (SAXS) and small angle neutron scattering (SANS) are two of the main methods to investigate the pore structure of materials. The development of SAXS and SANS theory and the improvement of experimental methods have been mutually complementary and confirmed, and now a mature theoretical system has formed. The characterization of geomaterials microstructure is extremely important for unconventional resources development and carbon sequestration. As the nondestructive pore structure testing methods, SAXS and SANS are relatively new techniques that can be applied to obtain the nanoscale structure in geomaterials. This chapter describes the fundamental theory of SAXS and SANS.

The characteristics of the geomaterials pore structure play an important role in various aspects, including unconventional resources development, underground mining, carbon sequestration, and natural hazard prevention. A method to effectively determine nanopores in geomaterials through experiments has been a hot topic and a difficult problem in the study of geomaterials microstructures. Various techniques have been employed to characterize the pore structure in geomaterials. These methods have their own advantages and disadvantages. Considering the complementarity of various techniques in the pore structure detection range and detection dimension, in this chapter, connections between small-angle scattering and other techniques, including NMR cryoporometry, transmission electron microscopy, low-pressure N2 and CO2 adsorption, and synchrotron radiation nano-CT, are described particularly.

Coalbed methane (CBM) development requires dewatering until the reservoir pressure is less than the critical desorption pressure. Significant quantities of CBM in China are buried >1000 m deep. Therefore, the desorption characteristics of deep CBM reservoirs must be investigated for the further development of deep CBM. In this study, the variation laws of adsorbed and free CH4 during adsorption in dry samples and during desorption via dewatering are investigated using nuclear magnetic resonance. During CH4 adsorption in dry samples by increasing CH4 pressure and during CH4 desorption in water-injected samples by dewatering, a Langmuir relationship exists between the volume of adsorbed CH4 and the pressure in deep and shallow coals, and the volume of free CH4 and the pressure are linearly related. When the pressure is the same, the volume of adsorbed CH4 in the dry coal samples during adsorption is larger than that in the water-injected samples during desorption by dewatering. When the pressure is the same, for the difference in the adsorbed CH4 volume between adsorption and desorption isotherms, shallow coal is less significant than deep coal. The slopes of free CH4 in deep coal are lower than those in shallow coal during adsorption and desorption.

SAXS studies usually use synchrotron radiation light source (SRLS), and it has the advantages of high intensity, high brightness, good collimation, and high purity. There are more than 50 synchrotron radiation light sources throughout all of world (operational or under construction). SANS studies must use neutron sources. The neutron sources can be divided into two types: steady-state reactor and spallation neutron source. This chapter introduces representative SAXS and SANS facilities across the Globe, mainly relating Beijing Synchrotron Radiation Facility (BSRF), Shanghai Synchrotron Radiation Facility (BSRF), Advanced Photon Source (APS), SPring-8 for SAXS facilities, and China Mianyang Research Reactor (CMRR), China Spallation Neutron Source (CSNS), General Purpose SANS (GP-SANS), and Extended Q-Range SANS (EQ-SANS) for SANS facilities.

Liquid nitrogen freeze–thaw (LNFT) is an environmentally-friendly permeability enhancement technology. There are significant differences between shallow and deep coal in their composition and stress history. Therefore, it is necessary to reveal the difference in the impact of LNFT on the multistage gas flow between shallow and deep coal. In this paper, the differences in multistage gas flow were revealed from the perspective of a multiscale structure evolution, and the differences in the multiscale structure evolution were analyzed from the coal compositions and stress history. Nuclear magnetic resonance method was used in the multiscale structure evolution characterization. As for the multistage gas flow evolution, gas ad/de-sorption, diffusion, and seepage evolution characterization were investigated. There is a quadratic relationship between the total porosity and LNFT cycles in both shallow and deep coal. The fracture expansion is dominant in deep coal, while pores connectivity enhancement is dominant in shallow coal. The permeability of both shallow and deep coal increases during the LNFT cycling, presenting a logarithmic correlation between permeability and the number of cycles. With the same porosity, shallow coal has a higher permeability. LNFT cycling can improve the Langmuir volume. Langmuir volume and gas diffusion coefficients of deep coal is smaller both pre- and post-LNFT cycling. Except for CH4 in deep coal, the Langmuir pressure and gas diffusion coefficients in both coals are improved by LNFT cycling.

Coalbed methane (CBM) and shale gas become two most important unconventional natural gas resources. The fractal dimension is an important parameter to quantitatively characterize gas storage capacity and gas transport properties in pores of geomaterials. Water condensation in geomaterials impacts its hydro-mechanical response. A mechanistic understanding of the pore-water system is made more challenging by significant anisotropy of pore architecture and nano-scale heterogeneity of pore surfaces. In this chapter, fractal evolution under in situ pressure and sorption conditions for coal and shale is introduced, and then the interactions between pore structures and fluid behaviors in geomaterials are described.

Gas drainage is an important technology to prevent coal and gas outburst, and the drained gas is a kind of clean energy. The gas pressure can characterize gas drainage effectiveness. In this paper, we investigated the effectiveness of gas drainage by gas pressure. Determined by the space shape of the gas flow field, the gas flow state surrounding the drainage boreholes is radial flow. According to the basic equations of radial flow, discrete equations were achieved by the implicit difference scheme, and then we obtained the gas pressure surrounding the drainage boreholes. Results showed that the midpoint between two holes presents the highest gas pressure, and gas pressure declined from the midpoint of two boreholes to both sides. The midpoint gas pressure of the two holes reflects gas drainage effectiveness in a certain degree. Gas pressure declined with segmented characteristics in the first period decline curve in the form of a cubic curve, and the second period decline curve in the form of a straight line. When the drainage pressure reaches a certain value, the decline rate of gas pressure had little relationship with the drainage negative pressure, mainly influenced by the permeability coefficient. To improve the drainage effectiveness, anti-reflection measures are feasible, instead of increasing the drainage negative pressure. Moreover, the conclusion was verified by field data.

This chapter focuses on the outlook of small angle X-ray and neutron scattering (SAXS/SANS) techniques for future applications in characterizing geomaterials, specifically coal and shale. The chapter discusses the importance of understanding the pore structure and fluid-rock interactions in these materials for applications such as natural gas production, energy storage, and CO2 sequestration. The complementary nature of X-ray and neutron scattering is highlighted, with X-ray being sensitive to mineral matters and neutron being highly sensitive to hydrocarbons. The potential future applications of SAXS/SANS in characterizing the anisotropic properties of coal and shale, as well as rock-fluid interactions, are recommended for future characterizations. The chapter emphasizes the needs for quantifying anisotropic pore structure, fluid flow behavior, and the influence of factors such as rock properties, moisture, heating, and pressurization on pore structure and rock alterations. The use of SAXS/SANS as nondestructive techniques for studying these phenomena is discussed, along with the importance of correlating scattering results with other microscale characterization techniques. The chapter concludes by highlighting the potential of SAXS/SANS for advancing subsurface engineering in coal and shale reservoirs and providing a foundation for fluid flow modeling and prediction.

Gas diffusion in coal is controlled by nanostructure of the pores. The accessibility of pores not only determines the dynamics of gas transport in the coal matrix but also influences the mechanical strength. Small angle X-ray scattering (SAXS) and small angle neutron scattering (SANS) have significant advantages in resolution and in situ characterization. In this chapter, the pore accessibility characterization for natural rocks will be described from four aspects, quantitative evaluation of pore structure heterogeneity and anisotropy, quantification of pore modification in coals due to pulverization, estimation and modeling of coal pore accessibility, and nanoscale coal deformation and alteration of porosity and pore orientation under uniaxial compression.

Numerous historical buildings exist in Shanxi Province, a major coal producing area in China, so there exist many overlapping areas between ancient wooden buildings and coal mining. Coal mining in overlapping areas will lead to surface subsidence, which will have an impact on historical buildings. Based on the distribution of historical buildings and the distribution and mining of coal resources in Shanxi Province, this paper concludes that the overlapping areas of coal mining and ancient wooden buildings in Shanxi Province are mainly concentrated in Changzhi City, and the Lu'an mining area in Changzhi City is selected as the research object. In addition, using the gray correlation analysis method, the surface subsidence coefficient, which characterizes the intensity of mining subsidence, is used as the reference sequence. Seven factors selected from the geological conditions and mining conditions of the Lu'an mining area are used as the comparison sequence to calculate the gray correlation between each influencing factor and the surface subsidence coefficient, and to obtain that geological factors such as the nature of the overlying rock layer, bedrock thickness and dip angle of the coal seam, and mining factors such as mining height, average mining depth and working face size largely determine the surface subsidence coefficient. The surface subsidence in the overlap area could largely be influenced by geological factors such as the nature of the overlying rock layer, bedrock thickness and coal seam inclination, and mining factors such as mining height, average mining depth and working face size. Finally, we investigate the possible effects of surface subsidence on ancient wooden buildings in the overlapping area with the surface subsidence and formation mechanism and propose technical measures to reduce the effects of surface subsidence due to coal mining on historical buildings in the overlapping area.

Permeability of subsurface porous media is one of the primary factors that affect fluid transport in porous rock. However, accurate prediction of rock permeability is a challenging task due to its intricate pore network. Development of digital rocks provides an effective approach to reveal and characterize the pore network. In this paper, a combination of digital rock petrophysics and ensemble machine learning (ML) models is proposed to improve the permeability prediction of subsurface porous media. The permeability of the numerically generated porous samples as outputs was determined by the lattice Boltzmann method (LBM). The five most important parameters (porosity, tortuosity, fractal dimension, average pore diameter, and coordination number) were selected as inputs for the permeability prediction. To improve the accuracy, feature selection and ML methods comparisons were conducted. Three feature selection methods based on expert knowledge, correlation coefficient, and importance score were compared. Moreover, a comparison was performed on six ML methods (support vector machine, artificial neural network, decision tree, random forest, gradient-boosting machine, and Bayesian ridge regression) that were optimized by particle swarm optimization (PSO). The results indicated that (1) the feature selection based on the expert knowledge obtained a higher performance than the groups based on the correlation coefficient and importance score, implying the importance of expert knowledge on feature selection, and thus on ML performance; (2) artificial neural network with hyperparameter tuning achieved the best performance in predicting permeability; and (3) the optimized ML method outperformed the empirical equations in predicting permeability. In conclusion, this study provides a fast and reliable approach predicting permeability of subsurface porous media based on numerically generated porous images. Moreover, the proposed framework can be further extended to determine other petrophysical properties, for example, the relative permeability and thermal conductivity.

Aiming at the gas flow problem in fractured coal–rock assemblage during L-shaped horizontal well hydraulic fracturing and gas extraction in coalbed methane development, permeability–stress test of single fractured coal–rock assemblages was carried out. The influence of gas pressure, effective stress and individual permeability of fractured coal–rock mass on the combination permeability were analyzed. With increase in effective stress, the permeability and its stress sensitivity of the single fractured coal–rock assemblage decreased gradually. Due to the existence of slippage effect, the permeability of coal–rock assemblage decreased gradually with increase in gas pressure within the range of study gas pressures. The gas adsorption of the coal caused the fracture aperture to decrease and led to the uneven deformation of coal–rock assemblage because the adsorption deformation of rock mass can be neglected. Comparison of the permeability variation of single fractured rock and coal sample with the increase of gas pressure further verified that the coal adsorption reduced the permeability of coal–rock assemblage. Finally, the permeability calculation model of single fractured coal–rock combination is presented in this paper. This model can well match the laboratory measured data, and it can clarify the contribution of each permeability part to the overall permeability. The permeability model indicated that the permeability of coal–rock assemblage depended on the smaller part of permeability. The contribution of the high permeability part to the overall permeability can only be increased a little on the basis of the low permeability part.