Coal Reservoir Research Articles

Fractal and Multifractal Characteristics on Pore Structure of Coal-Based Sedimentary Rocks Using Nuclear Magnetic Resonance

Summary Unconventional reservoirs have nanoscale pores, complex pore structures, and heterogeneity that directly affect reservoir storage performance and fluid transport capacity. In this study, shale, mudstone, and sandstone, three typical coal sedimentary rocks from the Daqiang coal mine in the Tifa Basin, were selected for nuclear magnetic resonance (NMR) and scanning electron microscopy (SEM) investigation, with the aim to investigate the pore structure and multifractal characteristics of the coal sedimentary reservoirs and to qualitatively analyze the effects of the physical property parameters and the mineralogical compositions on the multifractal parameters. The distribution data of the NMR T2 spectra were analyzed. The results showed that (1) SEM analysis concluded that the pore system of the three different lithological samples (mudstone, shale, and sandstone) was dominated by mineral matrix pores (i.e., intergranular and intragranular pores) and in the sandstone samples, there were only a few biological pores found. (2) The distribution of the NMR T2 spectrum peaks indicates that the sandstone and shale T2 spectra are bimodal, dominated by micropores, and contain a small number of transitional pores; most of the T2 spectra of mudstone are single peaks in distribution, mainly dominated by micropores. (3) Multifractal parameters are positively correlated with porosity and significantly negatively correlated with permeability; multifractal parameters are significantly positively correlated with the content of clay minerals and kaolinite, which suggests that the increase in clay minerals and kaolinite content enhances the heterogeneity of the pore space. The negative correlation with the content of quartz suggests that the enrichment of quartz reduces the irregularity of the pore space.

Seismic Safety Analysis of Interlaminar Rock Mass in the Distributed Underground Reservoir of a Coal Mine

This study focuses on the seismic safety of distributed underground reservoirs in coal mines, especially in scenarios involving the establishment of multiple reservoirs within the same mining area, spanning different coal seams. Dynamic similarity model tests and numerical simulations are employed to investigate the construction and operation of these reservoirs under extreme conditions, such as mine tremors or earthquakes. Utilizing the Daliuta coal mine underground reservoir as a case study, a similarity material model test platform is established to represent both upper and lower coal mine underground reservoirs. Stability tests are conducted on the interlayer rock mass under various levels of seismic intensity, and the safety of the interlayer rock mass at different safety distances is comparatively analyzed. Meanwhile, using the finite element method, the responses of the upper and lower coal mine underground reservoirs under different seismic intensity levels are simulated with the same conditions of model tests. Through the two types of simulations, the mechanical response and safety of the surrounding rock of the Daliuta coal mine underground reservoir under the influence of different seismic intensities are systematically analyzed, and the reasonable safety distances between the upper and lower reservoirs are obtained. This study provides a valuable scientific insight into the safe design of underground reservoir embankments in coal mines.

3D pore structure characterization and permeability anisotropy visualization simulation of fusain

The Jurassic‒Cretaceous coal seams in northern China are rich in fusains, and the pore size of the residual plant tissue in fusains is much larger than the limit value of seepage pores and even larger than that of some microfractures in coal. Therefore, understanding the 3D pore structure and permeability of fusains is helpful for determining the physical characteristics of inertinite-rich coal reservoirs. The porosity of the fusain varies widely, the pore structure has obvious directivity, and this study summarizes three pore types. Based on the visual distribution module of fluid migration, three types of seepage models of fusains were established, including the tubular cell structure seepage model, the broken cell wall pore seepage model, and the complete cell structure surface barrier seepage model. The results of the physical simulation verify the rationality of the three seepage models and show that the degree of fusification and cell pore fragmentation strongly affect the permeability of the fusain. For a low-rank coal reservoir with a rich fusain, the overall pore permeability is high, which is conducive to the seepage production of coalbed methane and is an important reference value for high-permeability layer selection.

Experimental Study on Shear Characteristics of Coal Samples in Different Water-Bearing States

During the establishment of the coal mine goaf reservoir, the strength of the coal pillar dam was weakened by various water–rock interactions triggered by water movement. To explore the influence of the water-bearing state on the shear characteristics of coal pillar dams, the 2–2 coal seam of Shangwan Coal Mine in the Shendong Mining area was taken as the research object and was prepared to coal samples with different moisture contents and different drying-saturation cycles, and the variable angle shear test of coal samples was carried out. The law of water absorption of coal samples under natural and pressurized conditions is obtained. The research results show that the shear characteristics of coal samples change significantly under the action of water, with the increase of moisture content and drying-saturation cycles, the lower the shear strength of coal samples is, the larger the peak shear strain is, and the cohesion and internal friction angle of coal samples decrease linearly. According to the relationship between cohesion, internal friction angle, moisture content, and drying-saturation cycle times, the Mohr–Coulomb model of coal samples considering moisture content and immersion times is established, which contributes to the load-bearing capacity estimation of water-bearing coal mass. Additionally, scanning electron microscope analysis revealed significant changes in the microstructure of coal samples under different moisture states: as the moisture content and dry-saturation cycles increase, the microstructure of the coal samples gradually transforms from orderly and compact to disordered and murky, elucidating the mechanism by which moisture weakens the mechanical properties of the coal samples. The research results provide a useful reference for engineering problems related to the stability evaluation of coal bodies involving the repeated erosion effect of water.

Elaboration and application of the nested multi-coupling model test system for an artificial dam seepage simulation under combined action of dynamic and static loads

Outdated coal mining technologies involve considerable mine water resource waste. For coal mines located in water-starved western regions of China, this jeopardizes the sustainable coal mining development, local eco-environment, and agricultural production. Coal mine underground reservoirs furnish conservation and cyclic utilization of water resources in the coal mining process. The weakest links of underground reservoirs are artificial dams that are prone to frequent occurrence of water seepage and instability failures. Aiming to explore the deformation rules of artificial dams under fluid–solid coupling action, this study developed a nested multi-coupling test system comprising a model shelf, a mine pressure simulation device, a water pressure simulation device, and a monitoring system, The maximum size of the model shelf is 1400 mm * 1300 mm * 200 mm. The mine pressure simulation device was composed of static-load and vibration-load oil cylinders of various sizes. A servo motor mainly controlled the water pressure device; with a water pressure sensor and high-precision flowmeter, water pressure could be accurately adjusted. By taking the underground reservoir in the Wulanmulun Coal Mine as an example, the deformation failure characteristics of the MB-1 dam under fluid–solid coupling conditions were simulated to validate the reasonability and feasibility of the developed system. The obtained experimental results indicated that the artificial dam damage was induced by increased mine pressure, leading to shear compression failure. The artificial dam underwent plastic deformation at a water pressure of 11.43 kPa, accompanied with the generation of microfractures, which rapid propagation under vibration loading violated the dam’s stability.

Effect of ScCO2-H2O treatment duration on the microscopic structure of coal reservoirs: Implications for CO2 geological sequestration in coal

The microstructure of coal reservoirs, especially the pore structure, alters as a result of physicochemical reactions between the CO2 acidic fluid and coals. However, it is still unclear what controls this process and how the pore structure properties of coal alter by the action of CO2 acid fluids change over time. The objective of this study is to identify the underlying mechanism by analyzing the changes in the mineral composition, surface roughness, as well as micropore and mesopore structure of anthracite and bituminous coal when exposed to supercritical CO2 (ScCO2)-H2O. The results indicate that coal reservoirs primarily undergo processes of adsorption expansion and mineral dissolution/precipitation. Among these, adsorption expansion concluded or weakened after 7 days, with mineral precipitation gradually becoming the dominant factor, which results in more secure geological CO2 storage (GCS). Furthermore, comparative analysis of anthracite and bituminous coal with untreated and long-term treatment demonstrates that anthracite exhibits higher CO2 storage capacity and greater wettability. Consequently, anthracite is more suitable as a target reservoir for GCS compared to bituminous coal. These results, therefore, offer a theoretical foundation and a guide for choosing GCS sites in deep un-minable coal seams.

A new model for coal gas seepage based on fracture-pore fractal structure characteristics

Coal reservoir is a typical double pore medium, and the magnitude of its permeability is controlled by the parameters of the fracture pore structure. However, the relationship between fracture-pore parameters and permeability is not yet clear. Based on the fractal geometry theory, the fracture-pore morphology of coal body is characterized by tree bifurcation network and tortuous capillary bundle. This study also established a coal gas seepage flow model based on the fracture-pore fractal structure characteristics. The model includes the influence of coal structure parameters, different levels of original coal seam stress, and gas pressure on the gas seepage characteristics. Each parameter in the model has definite physical meaning and does not contain any empirical constants. Finally, the applicability of the model is evaluated by gas radial seepage experiment. It is shown that the gas permeability in the model is a function of fractal dimensions and structural parameters, and based on the sensitivity analysis of each parameter in the model, it is found that the tree diameter ratio β and length ratio γ of the bifurcation network have a significant impact on the permeability. When β increases from 0.1 to 0.9, the model permeability increases by 12 orders of magnitude. However, changes in the characteristic parameters of the pore structure have little effect on gas permeability. For dual porous media such as coal, the permeability of the fracture network dominates the overall permeability of the model, and small changes in the geometric structure of the fracture bifurcation network can lead to significant changes in permeability. The construction of this theoretical model provides a reliable basis for further improving coal-rock porous medium structure seepage models, as well as to better characterize the effect of gas drainage.

Paleotectonic Stress and Present Geostress Fields and Their Implications for Coalbed Methane Exploitation: A Case Study from Dahebian Block, Liupanshui Coalfield, Guizhou, China

The macroscopic structural fractures (joints) and geostress distribution characteristics of coal reservoirs are important factors affecting the exploitation of coalbed methane (CBM). In this study, the joints in the sedimentary strata of the Dahebian block in Liupanshui area, Guizhou Province were investigated. Directional coal samples were collected for observation and statistical analysis of coal microfractures, the paleotectonic stress fields of the study area were reconstructed, and the tectonic evolution was elucidated. The geostress distribution characteristics of the target coal seam (coal seam No. 11, P3l) in the study area were analyzed using the finite element numerical simulation method. The results indicate that the structural evolution of the Dahebian syncline in the study area can be divided into two stages. The Late Jurassic–Early Cretaceous stage (Early Yanshanian) is the first stage. Affected by the sinistral strike slip of the Weining–Ziyun–Luodian (WZL) fault zone, the derived stress field in the study area exhibits maximum principal stress (σ1) in the NEE–SWW direction. The Late Cretaceous stage (Late Yanshanian) is the second stage. Affected by the dextral strike slip of the WZL fault zone, the derived stress field exhibits σ1 in the NNW–SSE direction. The folds and faults formed in the first stage were modified by the structural deformation in the second stage. The dominant strikes of joints in the sedimentary strata are found to be in the NW–NNW (300°–360°) and NE (30°–60°) directions, with dip angles mostly ranging from 60° to 90°. The dominant strikes of coal microfractures are in the NW (285°–304°) and NE (43°–53°) directions. The distribution of geostress in the study area is characterized by high levels of geostress in the syncline center, decreasing towards the surrounding periphery. The overall trend of the geostress contour line is similar to the shape of the syncline and is influenced by folds and faults. The σ1 of coal seam No. 11 is vertical stress. The prediction results show that the joint density of coal seam No. 11 in the block is 36–50 joints/m, and the shape of the joint density contour line is also affected by the axial direction of the Dahebian syncline and the surrounding faults. The variation in coal seam joint density and the control effect of geostress on joints opening or closing affects the permeability of coal reservoirs. The study results provide significant guidance for the exploitation of CBM.

CFD estimation of gas production in tight carbonates using single and dual-porosity models

Tight Carbonate reservoirs are regarded as one of the most complex reservoir formations due to the heterogeneity and complexity of their mineral composition, pore structure, and storage model. It is uncommon to study the implementation of a transport model appropriate for such formation. Recent studies focused on tight reservoirs and developed models for shale or coal bed methane reservoirs. This study proposes a single and dual-porosity transport model that solely considers the tight matrix and acidized region to shed light on the transport models for tight carbonates. The numerical model included the effect of transport mechanisms such as Knudsen diffusion, desorption, and viscous flow. The proposed transport model includes the apparent permeability model defining these transport mechanisms. Finite element method analysis was conducted on the numerical model using COMSOL Multiphysics. Due to the presence of nanopores in both shale and tight Carbonate, transport models proposed for the former can be utilized to determine the fluid flow behavior in the latter. The adsorption isotherm, rock density, pore structure, porosity, and permeability of the tight carbonate reservoir, which contrasted with the shale results, were the defining features of the reservoir used in the transport model. The dual-porosity model yielded a peak production of 104,000 m3/day, whereas the proposed model represents a shallow production rate from the single-porosity reservoir. The results were validated with an analytical solution proposed in the literature. Based on the literature findings and the production profile, the desorption did not play a significant role in the total production due to calcite’s low affinity towards CH4.

Three-Dimensional Geological Modeling of Coal Reservoirs and Analysis of Sensitivity Factors for Combined Mining Capacity

Due to the large non-homogeneity of coal reservoirs, there is a large uncertainty about the extent of the impact on coal bed methane production capacity. The Hanchengbei Block has the problems of early exploration, less available production data, and large variations in developed production capacity within a single well group during test production. Therefore, how to use the existing data to analyze the geological factors affecting the development of coalbed methane in the Hanchengbei Block is particularly important. In this paper, based on the coal seam properties and production characteristics of the Hanchengbei Block, a three-dimensional geological model of the area was meticulously constructed using Petrel 2015 modeling software. Through the utilization of stochastic modeling techniques, reservoir attributes were visualized in three dimensions, and probability distribution functions as well as confidence intervals for different geological parameters were derived through geological statistics. Building upon this foundation, a dual-layer geological model incorporating multiple factors was established using Comet3.0 numerical simulation software. Monte Carlo simulation methods were then employed to simulate the effects of various geological parameters on gas production, yielding corresponding simulation results. Through normalization processes, parameter sensitivity was analyzed to determine the primary controlling factors influencing production capacity. The results show that the thickness of the No. 5 coal seam in the Hanchengbei Block is mainly distributed in the range of 1.35–6.89 m; the gas content is 10.28–15.52 m3/t; and the permeability is 0.014–0.048 mD. Under their joint influence, the average gas production of Hanchengbei Block is between 310–720 m3/d. The main factors affecting the capacity of Hanchengbei Block are the thickness and gas content of the coal seam. This study can provide a basis for the subsequent optimization of favorable areas, the formulation of drainage systems, and the design and optimization of development well networks.

Reformation of coal reservoirs by microorganisms and its significance in CBM exploitation

The study investigates the variations in surface morphology and micro-nano-scale pore structure of coal matrix during the process of biodegradation. Comprehensive reservoir characterization techniques, including field emission scanning electron microscope, mercury intrusion porosimetry, and low-temperature N2 adsorption, are employed on four anthracite samples. The significance of the relationship between coal reservoir bio-reformation and coalbed methane (CBM) exploitation is proposed. The main findings indicate that organic matter on the surface of the coal matrix can undergo biodegradation, resulting in the generation of new pores. Microbial metabolism does not alter pore type but only modifies pore size and volume of coal. After microbial degradation, there is an increase in average pore width, while both Brunauer-Emmett-Teller specific surface area and Barrett-Joyner-Halenda pore volume decrease. Furthermore, bioconversion leads to a reduction in both surface and volume dimensions of coal, resulting in a smoother coal matrix surface and simplified complexity of its pore structure. This benefits desorption, migration, and exploitation of CBM. These findings further reinforce the pivotal role of microbial-enhanced coalbed methane production technology in improving CBM production and promoting the clean utilization of coal resources.

Numerical analysis of temperature effect on CO2 storage capacity and CH4 production capacity during the CO2-ECBM process

Temperature is a key factor affecting the CO2 injection efficiency and CH4 production efficiency, so it is of great significance to clarify the evolution diagram of temperature change during the CO2-ECBM process. In this study, the crushed soft coal seam with low permeability in Huainan coalfield was taken as the research object. Firstly, the fully coupled mathematical models of CO2-ECBM process were constructed, and the influence of temperature field on CO2-ECBM process was emphasized. Secondly, the evolution law of CO2-ECBM process at different time was revealed. Then, the effects of injection temperature and initial temperature on the CO2-ECBM process were analyzed. Finally, the evolution mechanism of reservoir temperature during the CO2-ECBM process was illustrated, and the effect of temperature on temperature field of CO2-ECBM process was further illustrated. The results show that the longer the displacement time, the CH4 concentration gradually decreased, and the CO2 concentration gradually increased. The area where the temperature changes are more concentrated focuses on the vicinity of CO2 injection well, which are mainly determined by the dominant reaction of adsorption and desorption reactions of binary gas. Reservoir temperature and permeability are positively and negatively correlated with injection temperature, respectively. Both CO2 storage capacity and CH4 production capacity increase with the increase of injection temperature, and the sensitivity of CO2 storage capacity to temperature was greater than that of CH4 production capacity. With the increase of displacement time and injection volume, the effective displacement radius increases, but the increment of influencing radius gradually decreases. The lower the initial temperature of coal reservoir, the larger the influencing radius, and which increases with the increase of displacement time. CO2 storage capacity and CH4 production capacity decrease with the increase of initial temperature. Young's modulus and Poisson's ratio can effectively change the reservoir strength. CO2 storage capacity and CH4 production capacity decrease with the increase of Young's modulus, and increase with the increase of Poisson's ratio. This study can support the engineering landing of CO2-ECBM technology in crushed soft coal seam with low permeability, and promote the realization of China's “Dual Carbon” strategic goal.

A Study on the Production Simulation of Coal–Shale Interbedded Coal Measure Superimposed Gas Reservoirs under Different Drainage Methods

To study the influence of different drainage methods on the production performance of coal measure gas wells, the interbedded reservoir composed of coal and shale in the Longtan Formation of the Dahebian block was used as the research object. Considering the influence of coal and shale matrix shrinkage, effective stress, and interlayer fluid flow on reservoir properties such as fluid migration behavior and permeability, a fluid–solid coupling mathematical model of coal measure superimposed gas reservoirs was established. Numerical simulations of coal measure gas production under different drainage and production modes were conducted to analyze the evolution of reservoir pressure, gas content in the matrix, permeability, and other characteristic parameters of the superimposed reservoir, as well as differences in interlayer flow. The results showed that, compared to single-layer drainage, cumulative gas production increased by 33% under multi-layer drainage. Both drainage methods involve interlayer energy and substance transfer. Due to the influence of permeability, porosity, and mechanical properties, significant differences exist in reservoir pressure distribution, preferential flow direction, gas content in the matrix, and permeability ratio between coal and shale reservoirs under different drainage and production modes. Multi-layer drainage effectively alleviates the influence of vertical reservoir pressure differences between reservoir layers, facilitates reservoir pressure transmission in shale reservoirs, enhances methane desorption in shale matrices, promotes matrix shrinkage, and induces the rebound of shale reservoir permeability, thus improving overall gas production.

Molecular Simulation Study on the Effect of Co-Associated Minerals on Methane Adsorption and Mechanical Properties of Coal

When rockbursts and coal and gas outbursts simultaneously occur in a coal mine, changes in gas adsorption (concentration of ambient methane) and displacement of coal and rock must occur. The co-associated minerals in coal reservoirs can affect the mechanical properties and methane adsorption capacity, which are commonly disregarded. It is important to construct compound molecular structure models of coal and rock and conduct molecular dynamic simulations to gain a microscopic understanding of underground disasters. In this work, the molecular structure models of anthracite and coking coal–rock compound models containing different contents of calcite and kaolinite were constructed, and the methane adsorption amount and mechanical properties considering temperature, pressure, and mineral contents were simulated and analysed. The results showed that the methane adsorption amount of the compound models increased rapidly, then increased moderately, and stabilized eventually with increasing adsorption pressure, and the Langmuir fitting findings were good. The saturation adsorption amount of methane in the coal models linearly decreased with increasing temperature, while the methane adsorption heat increased. The presence of minerals adsorbed a certain amount of methane, and the methane adsorption amount increased with increasing mineral contents. The mechanical properties of coal molecules changed when mineral molecules such as calcite and kaolinite were present, which had opposite contribution effects. The addition of kaolinite minerals to the coal molecular model always increased the bulk modulus and shear modulus, while the addition of calcite decreased the bulk modulus of the anthracite, causing an increase in the brittleness of the models. The results of the study further explain the adsorption behaviour and mechanical properties of methane in coal and minerals.

Research on controllable shock wave technology for in-situ development of tar-rich coal

The world is rich in tar-rich coal resources, which have broad exploration and development prospects. At present, the academic community has relatively weak research on the impact of shock waves on the permeability of target coal bodies. This article focuses on the development technology of controllable shock waves in tar-rich coal reservoirs. Firstly, a systematic analysis was conducted on the drilling situation and physical parameters of the target coal seam. Secondly, the impact of shock waves on permeability of target coal was systematically analyzed. Thirdly, a systematic summary was conducted on the process parameters and measures of coal seam operations. Research has shown that: (a) The permeability of coal samples after shock wave action is much greater than the permeability of coal samples in their original state. (b) Under the same axial and confining pressure conditions, as the gas pressure increases, the increase in the permeability of coal under the action of shock waves shows a decreasing trend. (c) The shock wave generates a large number of crack structures inside the coal body. These cracks will undergo very obvious closure under external pressure. (d) Under different axial and confining pressures, when the gas pressure is 1 MPa, the maximum increase in permeability of coal samples under shock wave action can reach 90–200 times. When the gas pressure is 2 MPa, the increase in permeability before and after shock wave action is the smallest, reaching 30–90.

A Fractional-Order Creep Model of Water-Immersed Coal

The long-term stability of a coal pillar dam is a serious concern for coal mine underground reservoirs because of the creep behavior of coal in complex water immersion and mechanical environments. In order to investigate the characteristics of creep deformation of water-immersed coal and develop a proper creep model, this paper implemented a series of creep experiments of coal via multistage loading at various water-immersion times. The experiment data were analyzed, in terms of immersion-induced damage, elasto-plastic performance, creep behavior, etc., suggesting obvious mechanical properties’ degradation of coal by water. The elastic modulus and peak strength of water-immersed coal decrease exponentially with the immersion time, while the creep rate of coal shows an upward tendency with the promoted immersion time. According to the remarked relationships of elastic, viscoelastic, and viscoplastic properties versus the stress levels and water-immersion time, a creep model based on conformable fractional derivatives is proposed, considering the influence of the water-immersion time and variable stress level. The proposed model was verified using the experiment data, showing a good capacity of the creep model for reproducing the creep process of water-immersed coal. This paper provides a fundamental model for further studying the stability of coal pillars and their influence on the safety of underground water reservoirs.

Microstructure evolution law and dissolution time effect of bituminous coal under acetic acid acidification: Based on Micro-CT image analysis

Undeveloped pores and fractures in coal seams constrain coalbed methane mining and dust disaster prevention significantly. The existing hydraulic fracturing technology has achieved some effects in coal seam modification. Still, there are hidden dangers in inducing microseismicity and other aspects. Applying acidification technology for pore and fracture modification improves the efficiency of coalbed methane mining and dust suppression. In this paper, 75% concentration of acetic acid was utilized to treat columnar coal samples, and the experimental method of X-ray computed tomography was applied to study the role of dissolution time on the microcomponents and structure of samples. The experimental results showed that the porosity increased with the dissolution time; the effect of acetic acid on the pore expansion and mineral dissolution showed three periods, i.e., the beginning of the reaction period (0 ∼ 12 h), the full reaction period (12 ∼ 48 h), and the reaction slowing down period (>48 h). The mineral volume fraction showed an overall decreasing trend; before 36 h, the dissolution of acetic acid on the mineral structure within the coal was faster, making it loose, which in turn accelerated the dissolution of coal, and after 36 h, the dissolution effect of acetic acid weakened, and the mineral was gradually stabilized. Acetic acid first reacts with minerals in large quantities. With the increase of products, the solution's ionic complexity increases, so acetic acid accelerates the ionization of more H+ ions, and the solution's acidity increases. After a while of dissolution, the content of acetic acid decreases, the ionized H+ ions are not enough to continue to react with the minerals, and the rate of the mineral's crystal particle size reduction slows down gradually. In conclusion, acetic acid has the potential for pore and fracture modification of coal reservoirs. It can affect the mineral through dissolution, at the same time, improve the porosity. The research results provide an essential reference for using acidification technology to modify the pores and fractures in coal seams.

The geological factors affecting gas content and permeability of coal seam and reservoir characteristics in Wenjiaba block, Guizhou province

The gas content and permeability of coal reservoirs are the main factors affecting the productivity of coalbed methane. To explore the law of gas content and permeability of coal reservoirs in the Zhijin area of Guizhou, taking No.16, No.27 and No.30 coal seams in Wenjiaba mining area of Guizhou as the engineering background, based on the relevant data of coalbed methane exploration in Wenjiaba block, the geological structure, coal seam thickness, coal quality characteristics,coal seam gas content and permeability of the area were studied utilizing geological exploration, analysis of coal components and methane adsorption test. The results show that the average thickness of coal seams in this area is between 1.32 and 1.85 m; the average buried depth of the coal seam is in the range of 301.3–384.2 m; the gas content of No.16 and No.27 coal seams is higher in the syncline core. The gas content of the No.30 coal seam forms a gas-rich center in the south of the mining area. The buried depth and gas content of coal seams in the study area show a strong positive correlation. Under the same pressure conditions, the adsorption capacity of dry ash-free basis is significantly higher than that of air-dried coal. The permeability decreases exponentially with the horizontal maximum principal stress and the horizontal minimum principal stress. The horizontal maximum primary stress and the flat minimum prominent stress increase with the increase of the buried depth of the coal seam. The permeability and coal seam burial depth decrease exponentially. This work can provide engineering reference and theoretical support for selecting high-yield target areas for CBM enrichment in the block.

Controlled mechanism of dynamic behavior of water-based ultrasonic cavitation in coal pore microstructure

Coal pores are sealed as a result of water lock effect, which in turn inhibits coalbed methane (CBM) from desorbing and diffusing. It seriously restricts the efficient development of CBM in the complex geological environment. Relieving pore water lock (RPWL) in coal reservoirs is a scientific problem that puzzles CBM development. Currently, the widely-interested technology of enhancing coalbed methane recovery (ECBM) can not effectively break through the pore water lock. Ultrasonic cavitation can generate shaped charge micro-jets, high temperature, and high pressure, which may be a potential strategy to realize RPWL in coal reservoirs. It has been maturely applied in numerous fields such as medicine, chemical industry, and manufacturing industry. Therefore, water-based ultrasonic cavitation-enhanced coalbed methane recovery (WUC-ECBM) was proposed in this study. This technology has enormous application potential for the efficient and green development of CBM. However, as the theoretical key to the successful application of WUC-ECBM, its RPWL mechanism is still blank. In this paper, X-ray nondestructive testing and fluid invasion methods were used to reconstruct the digital model of coal pore microstructure. Through ultrasonic cavitation numerical simulation, the dynamic behavior mechanism of water-based ultrasonic cavitation was studied in coal pore systems, revealing the microscopic RPWL mechanism of WUC-ECBM. Results show that low metamorphic coal has a larger and fuller pore system, which reflects a stronger topological advantage. Small pores and throats are the main locations of ultrasonic cavitation. This spatial feature is consistent with pore water locking phenomenon. During the formation of a cavitation bubble, ultrasonic energy instantly weakens the surface tension of the water film and activates RPWL. When the cavitation bubble collapses, the micro-jets, high temperature, and high pressure wreak mechanical damage and thermal shock to pore water, accelerating the RPWL process. Ultrasonic frequency affects the cavitation threshold. Increasing ultrasonic power will significantly boost cavitation intensity in pores, particularly in high metamorphic coal. The research results contribute to the green, efficient and intelligent development of CBM in the harsh geological environment, and provide theoretical guidance for WUC-ECBM.

3D Reconstruction of Pore and Fracture Structures in Crushed Soft Coal with Low Permeability and Its Permeability Analysis: Implications for the Design of CO2 Injection and CH4 Production.

In order to improve the CO2 injection and CH4 production efficiencies during the CO2-ECBM process, it is necessary to clarify the relationship among the complexity of pore and fracture structures, the typicality of the fluid migration path, and the heterogeneity of reservoir permeability. In this study, crushed soft coal with low permeability from Huainan and Huaibei coalfields of China was taken as the research object. First, the three-dimensional (3D) visualization reconstruction of pore and fracture structures was realized. Second, the equivalent pore and fracture network model was constructed. Finally, the permeability evolution and its anisotropy of the coal reservoir were dynamically demonstrated. In this study, the implication of surface porosity on the heterogeneity of pore and fracture structures was first discussed, followed by the implication of coordination number on the anisotropy of fluid flow, and finally, the influence of the anisotropy of fluid flow on the CO2-ECBM process was discussed. The results show that the equivalent pore and fracture network models of the reservoir structure can be constructed based on the digital rock physics technology. The analysis results of porosity, interconnected porosity, typical path of fluid migration, absolute permeability, and surface porosity of each sample have good consistency in characterizing the complexity of pore and fracture structures and the heterogeneity of permeability. The average coordination numbers of RL and LZ samples are 5.99 and 5.78, respectively, and the number of pores and throats is well-balanced, which indicates that LZ and RL collieries are suitable for the development of CO2-ECBM industrial tests. When the interconnected pores and fractures are mainly developed vertically and horizontally, the construction of drilling technology of the CO2-ECBM process should be mainly designed for vertical wells and horizontal wells, respectively. This study has important theoretical and practical significance for the industrial testing and commercialization of CO2-ECBM technology in crushed soft coal with low permeability.