Pore-fracture Structure Research Articles

High-pressure mercury intrusion analysis of pore structure in typical lithofacies shale

In this paper, statistical research on the characterization of pore and fracture structures is systematically conducted on shale reservoirs in different blocks and under different lithofacies conditions. The impact of lithological characteristics on shale pore fracture structures is revealed, and the coupling mechanism of "high-pressure mercury injection, pore fracture structure, and lithofacies" is clarified. Results show that: (a). When the capillary force is equal to 0.0035 MPa, the pore throat radius is equal to 209.661 μm. (b). There exist turning pressures both in injection curve and removal curve. When the capillary pressure is lower than 100 MPa, the slope of injection curve is high, while when the capillary pressure is higher than 100 MPa, the slope of injection curve becomes lower. (c). The majority of pores is distributed in the pore size interval range from 0.063 μm to 0.004 μm. (d). There are also a certain amount of regular pores in inorganic pores. Part of the pores are secondary intergranular dissolved pores formed by later dissolution.

Three-Dimensional Heterogeneity of the Pore and Fracture Development and Acoustic Emission Response Characteristics of Coal Rocks in the Yunnan Laochang Block

Studying the heterogeneity of coal reservoirs is significant to coal bed methane (CBM) exploitation. To investigate the development of the pore–fracture and acoustic emission response characteristics of the coal rock in the Yunnan Laochang block, four cores were extracted from the same coal rock in different directions. Through a comprehensive analysis using CT scanning and three-axis compression tests combined with synchronous acoustic emission experiments, a three-dimensional visualization of the pore–fracture structure and an analysis of the acoustic emission process during the elastic phase were conducted. Additionally, the impact of the heterogeneous development of pore–fractures on the acoustic emission characteristics was discussed. The results show that: there is strong heterogeneity in pore and fracture development within the coal rock, with the most significant development occurring along the direction of vertical stratification; the acoustic emission process in the elastic phase can be divided into three stages: strong–weak–strong; the development of pores and fractures affects the acoustic emission characteristics, with both counts and signal strength increasing as the percentage of voids rises; and the inferred in situ stress aligns with strike-slip faulting stress using acoustic emission. These results can provide a reference for the actual project.

Pore–Fracture Structure Characteristics of Low-Medium Rank Coals from Eastern Surat Basin by FE-SEM and NMR Experiments

Pore–Fracture Structure Characteristics of Low-Medium Rank Coals from Eastern Surat Basin by FE-SEM and NMR Experiments

Propagation of Combustion Front within Fractured Shale and Its Influence on Shale Structure and Crude Oil Properties: An Experimental Study

Summary The in-situ combustion (ISC) technique has emerged as a significant approach for shale oil production. However, currently, there is a lack of experimental evidence supporting the stable propagation of combustion front within fractured shale. This study aimed to investigate the combustion characteristics within fractured shale by using a self-designed combustion tube (CT) and an experimental scheme. Subsequently, an analysis of shale structure and oil properties was conducted. The results demonstrated that while the combustion front could propagate through shale with a single fracture width of approximately 43 μm, the combustion intensity gradually diminished, leading to an inability to sustain stable propagation in the later part of the oil-detritus mixtures. The combustion intensity within the shale was enhanced by preheating the shale at 250°C, resulting in an improved oil recovery from 67.8% to 77.9%. The findings indicated that the complex fractured shale allowed for the stable propagation of the combustion front without a significant decrease in combustion intensity. Moreover, the T2 spectrum analysis of shale revealed a gradual expansion of the pore-fracture structure and improved shale connectivity after combustion. The T1-T2 response illustrated the transformation of solid and heavy components into lighter components. Furthermore, the content of saturates and H in the oil increased after combustion, whereas there was a significant decrease in resins, O, and S. Overall, this study provided technical evidence supporting the feasibility of employing the ISC technique for the development of shale oil reservoirs with additional fractures.

Pore-pressure and stress-coupled creep behavior in deep coal: Insights from real-time NMR analysis

Understanding the variations in microscopic pore-fracture structures (MPFS) during coal creep under pore pressure and stress coupling is crucial for coal mining and effective gas treatment. In this manuscript, a triaxial creep test on deep coal at various pore pressures using a test system that combines in-situ mechanical loading with real-time nuclear magnetic resonance (NMR) detection was conducted. Full-scale quantitative characterization, online real-time detection, and visualization of MPFS during coal creep influenced by pore pressure and stress coupling were performed using NMR and NMR imaging (NMRI) techniques. The results revealed that seepage pores and microfractures (SPM) undergo the most significant changes during coal creep, with creep failure gradually expanding from dense primary pore fractures. Pore pressure presence promotes MPFS development primarily by inhibiting SPM compression and encouraging adsorption pores (AP) to evolve into SPM. Coal enters the accelerated creep stage earlier at lower stress levels, resulting in more pronounced creep deformation. The connection between the micro and macro values was established, demonstrating that increased porosity at different pore pressures leads to a negative exponential decay of the viscosity coefficient. The Newton dashpot in the ideal viscoplastic body and the Burgers model was improved using NMR experimental results, and a creep model that considers pore pressure and stress coupling using variable-order fractional operators was developed. The model’s reasonableness was confirmed using creep experimental data. The damage-state adjustment factors ω and β were identified through a parameter sensitivity analysis to characterize the effect of pore pressure and stress coupling on the creep damage characteristics (size and degree of difficulty) of coal.

Dynamic Evolution of Coal Pore-Fracture Structure and Its Fractal Characteristics under the Action of Salty Solution

The instability and failure of coal pillars is one of the important factors leading to the catastrophic consequences of coal mine goaf collapse. Coal mine water has the characteristics of high salinity. Long-term mine water erosion can easily deform the coal pillar structure, eventually leading to instability and damage. This study carried out tests on coal samples soaked in salt solutions with different concentrations, and the nuclear magnetic resonance (NMR) method was used to obtain the dynamic evolution of the pore-fracture structure of coal. On the basis of fractal theory, the changes in fractal dimension of pore structure during the soaking process were discussed. The damage variable based on the pore fractal dimension was defined and the evolution relationship between the damage variable and immersion time was characterized. The findings demonstrated that the porosity change rate has an exponentially increasing relationship with the immersion time, and with the increasement of concentration of salt solution, the porosity change rate also shows increasing trends. The number of seepage pores and total pores increased with the immersion time. While, with the extension of soaking time, the number of adsorption pores first increased and then decreased. The connectivity between pores was enhanced. The relationship between the fractal dimension and the immersion time is linearly decreasing. The damage variable showed an increasing trend with the immersion time. As the concentration of salt solution increased, the damage of coal increased. The research results are of great significance for rationally evaluating the stability of coal pillars and ensuring the safe operation of underground engineering.

Dynamic pore-fracture characteristic and evolution influenced by the underground mining considering the in-situ stress

The in-situ pore-fracture development is crucial for the methane drainage during underground mining. In this study, the dynamic characteristic and evolution mechanism of the internal pore-fracture structure located at the mining face were investigated by a self-developed true triaxial coupling nuclear magnetic resonance (NMR) experiment system, considering the mining-induced in-situ stress. The mechanical response characteristics, pore-fracture distribution, morphology and connectivity of coal were analyzed through the T2 spectrum and magnetic resonance imaging (MRI). The result shows that the pore-fracture of the coal is composed of adsorption pore (58%–67 %, AP＜1.5 ms), seepage pore (28%–43 %, 1.5 ms＜SP＜98 ms) and migration pore (2.3%–7.5 %, 98 ms＜MP). The compression, pore-dilation, and crack and fracture creation are successively occurred responding to the stress loading. The dynamic drop, slight increase and stabilization were sequentially presented in pore volume, while pore distribution gradually approached the homogeneity, which was mainly contributed by the evolution of SP and MP. The development-, transformation- and mixture-type pore-fracture, occurred at lower in-situ stress (LISS), middle in-situ stress (MISS), and high in-situ stress (HISS), was dominated by the SP-MP, AP-SP-MP, and SP-MP respectively. The fractal dimension experienced the sharp drop, dynamic increase and dramatic decrease with the dynamic- and slight-variation amplitude were observed at LISS, HISS and MISS, respectively. A slight increase of the number in connected channels was formed by the transformation-dependent AP and mixture-dependent AP and MP for LISS and HISS, while the dynamic increase of connective channel was created by the developed crack and fracture. The optimal methane channels were presented the elastic-plastic period with mature SP-MP pore-fracture in HISS, AP in MISS, and SP-MP in LISS. The finding provides insight into the co-mining of coal and methane.

Characterization for 3D pore-fracture systems in Hot-Dry-Rock samples from Gonghe Basin, China

As a new type of renewable energy, Hot-Dry-Rock (HDR) is gaining increasing attention. The physical properties of rock influenced by the pore-fracture structure are vital for the hydraulic and chemical characteristics of HDR reservoirs. However, the natural pore-fracture structure in HDR has not been sufficiently studied. To fulfill this gap, we used focused ion beam-scanning electron microscopy (FIB-SEM) to construct three-dimensional (3D) digital models for four granite samples from the deep HDR reservoir in Gonghe Basin, Qinghai Province, China, and obtained the nanoscale structure of the pores and fractures in HDR reservoir for the first time. Our analysis based on the 3D models indicates that the heat transfer area in the HDR reservoir is mainly from fractures and crack-like pores, but a large amount of small but regular pores could be neglected when heat transfer in HDR is considered. The permeability of HDR reservoirs at nanoscale varies from 9.414 nD to 0.082 mD, and the high permeability in this study is accompanied by throughgoing fractures. The variation of porosity and permeability of deep HDR at nanoscale in this study is much more severe than that of shallow granites previously studied at microscale. The performance of geothermal field is significantly influenced by the size of throats and the average size and tortuosity of the interconnected pore-fracture network in HDR. The hydraulic aperture could be 32-58% lower than the mechanical aperture of the natural fractures in HDR reservoirs, which is a vital clue for the numerical simulation associated with enhanced geothermal systems.

Investigation on physico-mechanical properties and microstructural evolution patterns of heated granite after liquid nitrogen cooling

Liquid nitrogen (LN2) fracturing is recognized as an innovative reservoir enhancement technique, demonstrating significant potential in establishing high permeability pathways in geothermal development. Understanding the physico-mechanical properties and microscopic mechanisms of hot dry rock post LN2 cooling treatment is pivotal for promoting the effective deployment of LN2 fracturing. This study undertook physical and mechanical testing on a quintessential hot dry rock type-granite-post various temperature and LN2 cooling treatments. Subsequently, a multi-scale investigation was conducted on the thermal fracture-pore structure's morphological features and the evolution of mineral components. The experimental findings indicated that LN2 cooling exerts a pronounced impact on the physico-mechanical properties of high-temperature granite. Temperature-induced damage to granite can be delineated into three phases: initial phase up to 200 °C where its effects are relatively imperceptible; an intermediate phase from 200 to 500 °C characterized by gradual deterioration; and a pronounced degradation phase evident at 600 °C. Correlations between the changes in the micro pore-fracture structure of granite and its physico-mechanical property degradation were subsequently established. As temperature escalated, the area of micro-defects in the CT-reconstructed spatial distribution increased progressively. The volumetric porosity damage ratio and fractal dimensions were quantitatively introduced to characterize the fractal geometric features and distribution patterns of internal thermal fractures in granite post thermal shock. Concurrently, NMR experiments indicated that with the escalation of temperature, micropore develop and interlink to form mesopore and macropore, enhancing rock porosity connectivity. These macropores significantly influence granite's physico-mechanical and permeability characteristics. A mineral stability analysis revealed that various minerals undergo physico-chemical reactions, such as dehydration, phase transitions, decomposition, and chemical bond fractures at elevated temperatures, altering mineralogical composition and mineral strength, thus impacting the rock's macroscopic properties. Ultimately, the study delved into the damage mechanisms of granite's physico-mechanical properties from both heating and cooling perspectives during various temperature phases. The insights garnered from this investigation offer novel value in comprehending the thermal damage characteristics induced by LN2 cooling.

Digital rock characterization and CO2 flow simulation of high-volatile bituminous coal: An application to carbon geosequestration

The permeability of CO2 sequestration is influenced by the characteristics of pore–fracture structures in coal, yet the flow simulation via the pore network model (PNM) and the controls of topology and morphology on permeability remain to be understood. First, the pore–fracture structure of high-volatile bituminous coal (HVBC) is characterized using micro-computed tomography, and the flow characteristics of single-phase CO2 in the connected pore–fracture network are simulated by the PNM. Second, the surface roughness of the connected pore–fracture is extracted by fractal characterization, and morphological algorithms are applied to accurately present the pore–fracture network structure. Finally, the implications of structural parameters (pore or throat diameter, coordination number, tortuosity, and sphericity) on CO2 transport are discussed, and the mechanism of pore–fracture structure response due to mineral dissolution in CO2 injection is analyzed. The results shows that the HVBC sample provides significant pore–fracture space (porosity of 10.87%) and flow path (connectivity of 66.50%) for hydrogen and carbon storage. CO2 flow simulation results demonstrate anisotropic flow, with higher CO2 permeability observed in the Z-axis direction (0.061 × 10−3 μm2) compared to the X- and Y-axis directions (0.044 × 10−3 μm2 and 0.059 × 10−3 μm2, respectively). Moreover, the fractures perpendicular to the coal bedding plane in the coal seam with a large tectonic dip strongly influence the flow of CO2 injection. Fractal analysis reveals a positive correlation between fractal dimension and porosity, indicating that structures with higher surface roughness are not convenient for CO2 transport. Significant pore angle characteristics (sphericity average of 0.58), high pore–throat connectivity (coordination number average of 4.31), and low capillary resistance (tortuosity average of 1.19) collectively affect the flow of CO2. Overall, the strongly anisotropic pore–fracture structure contributes to the inhomogeneous flow pattern in CO2 geosequestration. Changes in pore–fracture structure resulting from mineral dissolution during the early stage of CO2–coal matrix interaction in the HVBC reservoir can significantly enhance storage potential. This study contributes to the existing understanding of flow characteristics and provides insights for optimizing CO2 injection efficiency in carbon geosequestration.

Drill-In Fluid Optimization for Formation Damage Control Considering Salt Dissolution in Saline-Lacustrine Reservoirs

Summary Salt dissolution induced by drill-in fluid loss is a frequent occurrence in saline-lacustrine reservoirs, which can potentially result in serious formation damage. In light of this, an experimental study was conducted to investigate the salt mineral dissolution and dynamic damage in the rock samples collected from a saline-lacustrine carbonate reservoir and the response of pore-fracture structures using the in-situ drill-in fluids. The study further involved analyzing the formation-damage-control (FDC) ability of the in-situ drill-in fluids. The results indicated that although salt dissolution significantly increased the pore size of the tight matrix and the width of natural fractures, improving the conductivity of seepage channels, the increase in pore-fracture size may have greatly aggravated the drill-in fluid loss during the process. The continuous serious filtrate loss, lower pressure-bearing capacity of the plugging zone, and lower permeability recovery rate (PRR) of rock indicated poor FDC performance of in-situ brine drilling fluids for the salt-dissolved core samples. The FDC performance of drill-in fluids for saline-lacustrine carbonate reservoirs was optimized based on the response of reservoir pore-fracture structure to salt dissolution and the theory of slightly underbalanced activity. The experimental results showed that the optimized drill-in fluids had better FDC ability, with an average PRR increase of 14.04%. Field application indicated that the optimized drill-in fluids reduced the drill-in fluid loss by 76.48%, shortened the drilling cycle by 45.20%, and increased the initial production capacity per well by 7.70%. This study can provide insightful guidance to optimize the FDC performance of drill-in fluids for saline-lacustrine hydrocarbon reservoirs during drilling.

Using digital cores and nuclear magnetic resonance to study pore-fracture structure and fluid mobility in tight volcanic rock reservoirs

Pore-fracture structure characterization and fluid mobility analysis are key to the effective development of tight reservoirs. However, it is difficult to accurately characterize the pore-fracture structure of tight volcanic rocks using a single method, and the mechanisms of fluid mobility are not fully understood. A combination of X-ray diffraction, casting thin section observations, scanning electron microscopy, high-pressure mercury injection, constant-speed mercury injection, nuclear magnetic resonance (NMR), X-ray computed tomography, and the advanced mathematical algorithms in the AVIZO visualization software was used to analyze the Permian tight volcanic rock in the ZhongGuai area of the Junggar Basin to investigate its overall pore-fracture structure characteristics. On this basis, NMR centrifugation experiments were conducted to monitor the fluid migration dynamics in the tight volcanic rocks, and the mobile fluid migration characteristics were studied based on the NMR T2 spectra. The results show that the porosity and permeability of the samples are 0.2–18.7 % (avg. of 8.3 %) and 0.001–6.680 × 10−3μm2 (avg. of 1.602 × 10−3μm2), respectively, and the various pore types are due to the differences in cementation and compaction. The distribution of the pore throats are mainly contiguous and isolated, and the connected pores are mainly distributed in enriched bands, which is due to the interconnection of gas pores, intergranular pores, and dissolution fractures; while the disconnected pores are mainly isolated, which is related to the development of inter-gravel dissolved pores and matrix dissolution pores. In addition, the contribution of the pore connectivity to seepage is greater than that at the pore scale. The pores in the tight volcanic rock have a wide range of sizes, from 3 nm to 120 μm, and are dominated by Gaussian and bimodal distribution patterns. The micron-scale pore radius in this area is mainly 4.47–31.56 μm. Also, the fractures can be divided into three types according to their occurrence and openings. They are mainly high-angle structural fractures and vertical fractures; the pore-fracture structures have strong heterogeneity, and the fractures play a larger role on the seepage of oil and gas. The connectivity of the pore throats in the tight matrix is poor, and the pore throat structure has a great influence on fluid mobility. Subsequently, the movable fluid saturation increases with increasing permeability, and the fractures and micropores have less flow resistance and are more conducive to water flow than small pores. This study provides new insights for the exploitation of similar tight reservoirs.

Multiscale Fractal Characterization of Pore–Fracture Structure of Tectonically Deformed Coal Compared to Primary Undeformed Coal: Implications for CO2 Geological Sequestration in Coal Seams

The tectonically deformed coal (TDC) reservoirs with abundant gas resources and low permeability are expected to become one of the target coal seams for carbon dioxide geological storage-enhanced coalbed methane recovery (CO2-ECBM). The pore–fracture structure plays a crucial role in determining the effectiveness of CO2 storage. Fractal analysis provides a valuable approach to quantitatively describe the complex and heterogeneous pore–fracture structures across various scales in coal matrixes. Accordingly, the TDC samples in the Huainan–Huaibei coalfield and primary-undeformed coal (PUC) samples in the Qinshui Basin were selected for pore–fracture structure parameter tests using the mercury intrusion porosimetry (MIP) and low–temperature nitrogen adsorption (LNA) methods. Their multiscale pore–fracture parameters were analyzed using different fractal methods based on pore diameter. According to the fractal results, a multiscale classification standard for pore–fracture structures was devised in this study that is suitable for the controlling gas migration process. A parameter of 8 nm is set as the separating pore diameter for gas migration and storage. It was observed that the connectivity of migration pores (>8 nm) in TDC samples was stronger compared to PUC samples, reflected in larger pore volumes and smaller fractal dimensions. However, its complex development of seepage pores (150–300 nm) may hinder the flow of CO2 injection. As for the storage pores (<8 nm), the fractal dimension of the 2–8 nm pores in TDC was found to be similar to that of PUC but with larger pore volumes. The fractal dimension of the filling pores (<2 nm) in TDC samples was relatively lower, which facilitates efficient gas volume filling. Therefore, the pore–fracture structure of the TDC samples is found to be more advantages for CO2 injection and storage compared to the PUC. This suggests that TDC reservoirs holds promising geological potential for CO2-ECBM implementation.

Revealing the microstructures and seepage characteristics in the uncured rubber-cord composites using micro-computed tomography and lattice Boltzmann approach

The internal microstructure distribution of cord-rubber-air during the processing of uncured rubber-cord composites (URCs) determines the finished product's performance. For the first time, we used computed tomography (CT) and the lattice Boltzmann method (LBM) to establish a geometrical representation model of the real microscopic pore-fracture structures of URCs and investigate the seepage law of fluid in porous URCs, where the reinforced rubber formula was originally designed to reduce CT artifacts of cord. The results showed that the average porosity and pore radius of the original cord (0.2711 and 15.53 μm, respectively) were considerably larger than those of the URCs (0.0509 and 4.46 μm, respectively); the pore number of the cord was the largest when the pore radius was 5–10 μm, accounting for 29.36% of the total number; however, the pore number accounted for 31.36% of the total number of the URCs when the pore radius was 2–3 μm. Moreover, the characteristics of the pore/throat surface area and pore volume/throat length exhibited perfect consistent patterns with those of the pore radius. Furthermore, the fluid flow velocity increased in both cord and URCs as the displacement differential pressure increased, but decreased dramatically as the fluid kinematic viscosity increased. The critical values of displacement differential pressure and kinematic viscosity were different in the cord and URCs samples, presenting 11.1209 Pa/1.3696 × 10−3 m2/s and 14.2984 Pa/2.8869 × 10−4 m2/s, respectively. This phenomenon should be attributed that when the uncured rubber was injected into the original cord sample, its porosity decreased, its pore radius decreased, the number of micro-scale pores increased, and flow resistance became larger, resulting in a higher critical value of displacement differential pressure and a lower critical value of kinematic viscosity.

Experimental study on dynamic pore-fracture evolution and multiphase flow in oil-saturated coal considering the influence of in-situ stress

The methane drainage in liquid-saturated coal is directly related to the multiphase flow in complex geological structures. In this study, the pore-fracture dynamic evolution induced by in-situ stress and the corresponding gas-liquid migration and morphology characteristics were investigated by a self-developed online LF-NMR triaxial stress system. The dynamic variation, obvious change, and slight response were occurred at the migration pore (MP＞3ms), percolation pore (0.3ms < PP < 3ms), and adsorption pore (AP<0.3ms) with the increased stress, and development of gas channel was contributed by MP (15%–20%), AP (5%–10%), and PP(0–5%) responding to the gas flooding. The pore-fracture successively experienced compression, dilation, expansion, and coalescence, and PP and MP dominated the fracture initialization and connection. The permeability is negatively correlated with the fractal dimension of the pore-fracture structure and is exponentially related to the porosity and MP porosity. The longitudinal shear fracture is beneficial for the gas-liquid migration, and the horizontal tension crack connects the scattered pore and branch fracture. The gas channel was influenced by the liquid capillary pressure and gas jiamin effect in poor pore-fracture and mainly influenced by the gas slippage effect, immiscible behavior, and fracture surface tension force in mature pore-fracture. The optimal gas channel occurred at the crack-propagation period, which was located at around 70% peak strength with 1.5md permeability, high liquid relative permeability and lower gas relative permeability. The findings provide significant insight into methane extraction.

Study on oil seepage mechanisms in lamellar shale by using the lattice Boltzmann method

The shale oil and gas resources in the world are rich and are an important part of unconventional oil and gas resources. In this paper, firstly, by comprehensively using CT scanning technology, the three-dimensional pore-fracture structure characteristics of shale core samples of different sizes is characterized, and a three-dimensional digital core is established; Secondly, SEM and FIB-SEM techniques are systematically used to describe the microstructure and fracture network development characteristics of shale matrix pores and fractures, and a three-dimensional pore and fracture network model is established; Thirdly, Combined with LBZ simulation method, the mechanism of two-phase fluid transport in shale cross-scale fracture network is explored. Results show that: (a) The relatively dispersed mesopores and macropores in space constitute the main storage space of methane. Some pores in the study area of shale samples have good connectivity, and some samples only have connected pores in the study area of their magnification. (b) A large number of intergranular pores and fissures developed together with the intergranular pores of clay minerals, forming a good network connection system between microscopic pores in laminated shale. (c) when the hydrophilic mineral proportion is increased from 30% to 70%, the relative permeability of oil phase at water saturations of 20%, 40% and 60% has an increase of 6.8%, 7.8% and 4.2%, respectively, and the relative permeability of water phase at water saturations of 20%, 40% and 60% has a decrease of 1.8%, 2.6% and 6.5%, respectively. (d) When the oil phase contact angle is increased from 30° to 70°, the relative permeability of oil phase at water saturations of 20%, 40% and 60% has an increase of 1.9%, 2.7% and 2.2%, respectively, and the relative permeability of water phase at water saturations of 20%, 40% and 60% has a decrease of 1.2%, 2.1% and 3.5%, respectively.

Research on Pore-Fracture Characteristics and Adsorption Performance of Main Coal Seams in Lvjiatuo Coal Mine

Gas prevention and control have always been the focus of coal mine safety. The pore structure characteristics and gas adsorption characteristics of coal seams are the key factors affecting gas adsorption and diffusion in coal seams. Lvjiatuo Mine has the characteristics of a high gas content when it enters deep mining. In order to clarify the influence of the pore-fracture structure characteristics of main coal seams in the research area on coal seam gas adsorption and diffusion, and to study the differences in gas adsorption and diffusion ability in different coal seams, low-temperature nitrogen adsorption (LT-N2GA), high-pressure mercury intrusion (MIP) and computerized tomography (μ-CT) were used as characterization methods, and methane isothermal adsorption experiments were carried out to systematically study the pore structure characteristics of five groups of coal samples, and the pore-fracture structure characteristics and gas adsorption characteristics of each main coal seam were obtained. The results show that: (1) in the LT-N2GA experiment, the adsorption–desorption curves of all coal samples are of type III, and mainly develop cone-shaped pores or wedge-shaped semi-closed pores, with an average pore size of 1.84~4.84 nm, a total pore volume of 0.0010~0.0023 mL/g, a total specific surface area of 0.16~0.24 m2/g, and a fractal dimension D1 of 1.39~1.87 and D2 of 2.44~2.60. The micropores of L12 are more developed, and the mesopores and macropores of L9 are more developed. (2) In the MIP experiment, the porosity of coal samples is 3.79~6.94%. The porosity of L9 is the highest, the macropore ratio is the highest, and the gas diffusion ability is also the strongest. (3) In the μ-CT experiment, the porosity of L8-2 and L12 is 12.12% and 10.41%, the connectivity is 51.22% and 61.59%, and the Df is 2.39 and 2.30, respectively. The fracture of L12 is more developed, the connectivity is better, and the heterogeneity of the pore of L8-2 is higher. (4) In the isothermal adsorption experiment of methane, the gas adsorption capacity basically increases with the increase in the buried depth of the coal seam, and the gas adsorption capacity of the No.12 coal seam is the highest. Based on the pore-fracture structure characteristics and gas adsorption characteristics of the main coal seams in the research area, the gas outburst risk of each coal seam is ranked as follows: No.12 coal seam &gt; No.8 coal seam &gt; No.7 coal seam &gt; No.9 coal seam. The experimental results provide important help for researching the structural characteristics of coal seam pore fractures and preventing gas outbursts during deep coal seam mining.

Study on microscale pore structure and bedding fracture characteristics of shale oil reservoir

Accurately characterizing the cross-scale structure of pores and fractures in shale reservoirs is the theoretical basis for efficient exploration and development of shale oil and gas. In this paper, (a) Based on the research background of different shale maturity and sedimentary characteristics, a three-dimensional model of cross-scale pore-fracture structure is established; (b) Based on a large number of CT characterization data of core pore-fracture structure, the characteristics of pore-fracture development under different geometric dimensions (25 mm and 2 mm) are statistically analyzed; (c) Pore and fracture structures of mixed shale, felsic shale and laminated felsic shale is comparatively studied, and the pore structure and bedding fracture characteristics of shale oil reservoir is revealed. Results show that: (a) for the bedding fracture porosity, felsic shale in Group I is 0.36 %–0.89%; mixed shale in Group I is 0.34 %–0.47%; mixed shale in Group III is 0.12 %–0.86%; mixed shale gravelly dolomite in Group III is 0.13 %–0.15%. (b) Compared with block 1, the laminated felsic shale bedding in block 2 is more developed. (c) CT imaging is based on density difference. Different components of shale samples are distinguished by gray scale.

Interactions of CO2–H2O-coal and its impact on micro mechanical strength of coal

High temperature flue gas enhanced coalbed methane (CBM) recovery is a potential technique for improving CBM recovery efficiency and carbon sequestration, and it is of significance to carbon emission reduction in coal mine area. Previous research has demonstrated that the injection of CO2 could markedly alter the mechanical properties of coal seam, which further changes the gas flow and long-term safety of CO2 storage. Aiming at this issue, we studied the coupling effect of CO2–H2O on micro mechanical properties of coal with nanoindentation method. And the pore-fracture structure and minerals of the target sample was systematically characterized with LF-NMR and SEM to explore the influence mechanism of CO2–H2O on coal. The results show that (1) the micro strength of coal is obviously weakened after the treatment of CO2–H2O, and with an increase of the time, both the hardness and reduced modulus decreases, while the indentation depth shows an opposite trend. (2) Compared with the control group, the volumes of micropore, minipore, mesopore and macropore/microfracture in coal treated by CO2–H2O all increase in varying degrees. The increase of micropore volume is generally less than 20%. While the increases of the volumes of mesopore and macropore/microfractures are significant, with the maximum increase of 44.13% for the mesopore, and 299.58% for the macropore/microfracture. (3) Ulan coal contains a large number of calcareous minerals, which undergo chemical reaction after encountering the acid solution formed by CO2–H2O, resulting in dissolution of part of the minerals, which can be evidenced by the corrosion pits observed on the coal surface. (4) The injection of CO2 results in nonuniform swelling of the coal, which further leads to the formation of new cracks and the resultant coal strength deterioration. In addition, the acid solution formed by CO2–H2O reacts with the minerals in coal, resulting in dissolution of carbonate and other minerals in the coal, thus weakening the coal strength. The research results in this work provide basic support for the evaluation of the effectiveness of high temperature flue gas enhanced CBM and the safety of long-term CO2 storage.

μCT quantitative assessment of the pore–fracture structures and permeability behaviors of long-flame coal treated by infrared rapid heating

To explore the evolution features of the pore–fracture structures and permeability behaviors of coal under high temperatures and nitrogen atmospheres, an infrared rapid heating apparatus was employed to heat coal samples up to 100–500 °C at intervals of 100 °C. The coal samples were then scanned using high-precision micro-CT, and a three-dimensional (3D) pore–fracture structure and equivalent pore network model of the coal samples were established. The changing laws of Ep, Cn, Et, and Lc that varied with the heating temperature were first quantitatively analyzed. The distribution of the surface area and volume of the pore fracture was then statistically characterized. The results showed that the number of pore fractures and throats reached a maximum at 200 °C. Pore and fracture structures in coal tend to develop more with increasing temperature. In addition, the variations in the fractal dimension, porosity, connectivity, and permeability of coal with temperature are discussed. The fractal dimension and porosity increased significantly with increasing temperature. Dc and φc are more linearly dependent on temperature than Df and. φf The interconnection between the pores and fissures caused by the temperature increase is responsible for the increase in connectivity and permeability. This study provides theoretical guidance for coal fire prevention and coalbed methane extraction.