Qinshui Basin Research Articles

Seismic attributes and fault-fold systems: A case study of quantitative analysis of coal seams in the Southern Qinshui Basin, China

The formation of folds often leads to the emergence of subsidiary faults, creating intricate fault-fold systems with fractures. Although existing literature on fault-fold systems predominantly relies on field observations, there has been scant research focusing on their subsurface structures. Using 3D seismic data, this study endeavors to identify and quantify the principal fault-fold systems within the no. 3 coal seam of the Shanxi Formation and the no. 15 coal seam of the Taiyuan Formation, in the southern Qinshui Basin, China. The findings reveal: (1) the presence of five principal fault-fold systems, with four exhibiting a north–south orientation and one aligning east–west; (2) the establishment of an asymmetric model for the fault-fold system through a quantitative analysis of seismic attributes, particularly focusing on the high-angle normal faults along the folds’ limbs; and (3) the asymmetrical fault damage zone width (no. 3 fault-fold system) is quantitatively measured at approximately 200 m on the west side, in contrast to the east side where widths range from 50 to 500 m. These initial assessments indicate the great potential of an attribute-enhanced geologic model, which may provide insights for analyses of the continuity of coal seams, the propagation of fractures, and the migration of gas within fractured reservoirs.

Influence of Pore Structure and Basic Coal Properties on Gas Efficient Desorption Capacity in Coal.

The gas desorption capacity is beneficial for predicting and optimizing gas recovery from coalbed methane reserves. In this study, coal samples were collected from the southern Qinshui Basin. The isothermal adsorption experiment and mercury injection capillary pressure test were adopted to study the gas desorption capacity and pore structure. The efficient desorption capacity definition was proposed to evaluate the efficient desorption stage. The results show that the micropores and transition pores are well developed in all of the coal samples. There are two fractal dimensions in coal: the percolation fractal dimension (D1) and the diffusion fractal dimension (D2). A larger pore volume and well-connected pore network facilitate efficient desorption and transport of methane, leading to an enhanced methane yield. D1 is positively correlated with efficient desorption capacity, while D2 has a weaker effect. In addition, the basic properties of coal samples, such as coal rank, vitrinite content, and inertinite content, also have a certain effect on the efficient desorption capacity, but the relationship is not clear. These findings enhance our understanding of coalbed methane desorption capacity.

The role of geological fluids on the distribution of lithium in anthracite, an example from the Yangquan Mining District, Qinshui Basin, northern China

The role of geological fluids on the distribution of lithium in anthracite, an example from the Yangquan Mining District, Qinshui Basin, northern China

Micromechanical response of multiphase CO2 injected into high-rank coal fractures in the Qinshui Basin via nanoindentation

The carbon dioxide-enhanced coalbed methane process enables CO2 sequestration and enhances coalbed methane (CBM) extraction efficiency, offering broad application potential in deep CBM development. The coal fracture surface is a significant medium for fluid transport, and its mechanical properties are significant factors for affecting the migration and embedment of proppants. The results show that as CO2 gradually transforms into the supercritical state, the surface indentation depth, creep distance, and residual depth increase gradually. Young's modulus, hardness, and fracture toughness decrease by 39.45%, 36%, and 31.5%, respectively, and are controlled by the indentation depth. The irreversible work ratio effectively reflects the mechanical weakening process and indirectly confirms that the plastic deformation of coal is an irreversible change. The macromolecular structure, surface free energy, and pore and fractures of coal are important factors contributing to the weakening of mechanical properties, mainly manifested in the destruction of the macromolecular structure, the reduction of surface free energy, and the expansion of fractures. In addition, the Burgers model better validates the mechanical properties of coal fracture surfaces during the creep stage. This greatly reduces the resistance to crack propagation in coal, thus decreasing the mechanical properties of coal. These findings provide important insights into the mechanical behavior of coal in the context of CO2 geological sequestration and related applications.

Stress-seepage mechanism of surrounding rocks of the coalbed methane well and the bottomhole pressure control during dewatering and depressurization

In coalbed methane (CBM) well production, “bottomhole flowing pressure control” is key to improving production. A reasonable bottomhole pressure range is essential for optimizing CBM well drainage. In this paper, the stress–permeability mechanism of the surrounding rocks of CBM wells and the method of controlling the bottomhole pressure were investigated through experimental study, theoretical analysis, and field applications. Their characteristics and correlations were analyzed. The research results indicate that the deformation and failure process of coal samples can be divided into four stages: initial compacting stage, elastic deformation phase, plastic deformation stage, and failure stage, revealing the stress–strain–permeability characteristics of coal rock during the deformation and failure process. The surrounding rock of a CBM well can be divided into elastic, plastic, and fracture zones based on elastic–plastic theory, and theoretical calculation models for stress seepage in three zones of surrounding rock of coalbed methane wells have been derived. The stress and range condition expressions for each zone are obtained, and analysis shows that the surrounding rock has the highest stress at θ=0∘ and the lowest stress at θ=90∘. A calculation model for bottomhole pressure was derived, and a method for determining the range of bottomhole pressure in CBM well drainage was established. The variation law of bottomhole pressure during coalbed methane well drainage was revealed. The theoretical model was validated using production data from six vertical wells in the Zhengzhuang block of the southern Qinshui Basin, and good application results were achieved.

Proppant embedding-based optimal design method for fracturing coalbed methane wells in Qingshui basin

Fracturing design needs to consider the effect of proppant embedding as the production decreases rapidly due to proppant embedding after fracturing of coalbed methane wells. Coal fracture conductivity experiments were carried out to analyze the changes in coal fracture conductivity due to proppant embedding. A model of coal fracture conductivity was established considering proppant embedding. A proppant index method, which considers the dynamic change of conductivity over time, was adopted to establish a proppant embedding-based optimal design method for fracturing coal bed methane wells. The optimal design of coalbed methane wells in the southern block of Qinshui Basin was carried out. The results show that the fracture conductivity decreased with the increase of formation closure pressure. It increased with the rise of proppant grain size, number of placement layers, and Young’s modulus of the formation. The simulation results were in good agreement with the indoor fracture conductivity experiments. The proppant embedding reduced the fracture width and fracture conductivity. The optimized fracture length considering proppant embedding decreased with an average decrease of 10.6%, and the fracture width increased with an average increase of 11.5%. Consideration of proppant embedment during fracture optimization of coalbed methane wells compared to no consideration resulted in a decrease in design fracture length, an increase in design fracture width, and a relative increase in average production. The validity and practicability of the method were verified, and it was of great guiding significance for the optimal design of fracturing in coalbed methane wells.

Paleoenvironmental Controls and Economic Potential of Li-REY Enrichment in the Upper Carboniferous Coal-Bearing “Si–Al–Fe” Strata, Northeastern Qinshui Basin

Critical metals in coal-bearing strata have recently emerged as a frontier hotspot in both coal geology and ore deposit research. In the Upper Carboniferous coal-bearing “Si–Al–Fe” strata (Benxi Formation) of the North China Craton (NCC), several critical metals, including Li, Ga, Sc, V, and rare earth elements and Y (REY or REE + Y), have been discovered, with notable mineralization anomalies observed across northern, central, and southern Shanxi Province. However, despite the widespread occurrence of outcrops of the “Si–Al–Fe” strata in the northeastern Qinshui Basin of eastern Shanxi, there has been no prior report on the critical metal content in this region. Traditionally, the “Si–Al–Fe” strata have been regarded as a primary source of clastic material for the surrounding coal seams of the Carboniferous–Permian Taiyuan and Shanxi Formations, which are known to display critical metal anomalies (e.g., Li and Ga). Given these observations, it is hypothesized that the “Si–Al–Fe” strata in the northeastern Qinshui Basin may also contain critical metal mineralization. To evaluate this hypothesis, new outcrop samples from the “Si–Al–Fe” strata of the Benxi Formation in the Yangquan area of the northeastern Qinshui Basin were collected. Detailed studies on critical metal enrichment were assessed using petrographic observations, mineralogy (XRD, X-ray diffractometer), and geochemistry (XRF, X-ray fluorescence spectrometer, and ICP-MS, inductively coupled plasma mass spectrometer). The results indicate that the siliceous, ferruginous, and aluminous rocks within the study strata exhibit varying degrees of critical metal mineralization, mainly consisting of Li and REY, with minor associated Nb, Zr, and Ga. The Al2O3/TiO2, Nb/Y vs. Zr/TiO2, and Nb/Yb vs. Al2O3/TiO2 diagrams suggest that these critical metal-enriched layers likely have a mixed origin, comprising both intermediate–felsic magmatic rocks and metamorphic rocks derived from the NCC, as well as alkaline volcaniclastics associated with the Tarim Large Igneous Province (TLIP). Furthermore, combined geochemical parameters, such as the CIA (chemical index of alteration), Sr/Cu vs. Ga/Rb, Th/U, and Ni/Co vs. V/(V + Ni), indicate that the “Si–Al–Fe” strata in the northeastern Qinshui Basin were deposited under warm-to-hot, humid climate conditions, likely in suboxic-to-anoxic environments. Additionally, an economic evaluation suggests that the “Si–Al–Fe” strata in the northeastern Qinshui Basin hold considerable potential as a resource for the industrial extraction of Li, REY, Nb, Zr, and Ga.

Research on Coal Structure Prediction Method Based on Genetic Algorithm–BP Neural Network

This paper proposes a coal structure prediction technology based on deep learning, which uses logging data to achieve single-well prediction of the coal structure. This paper introduces the genetic algorithm (GA) to optimize the BP neural network, which can speed up its convergence to the global optimal solution, improve its training speed, and avoid the problems of easily producing the local optimal value and requiring a long training time. Taking the main coal seam of the Shizhuang block in the south of the Qinshui Basin as the research object and using the coal core data and logging data of nine parameter wells, the mapping relationship between the logging curve and coal structure is constructed based on the GA-BP neural network structure, and the coal structure is predicted. The prediction results are highly consistent with the coal structure measured from coal core sampling, with only a small error, and the prediction accuracy is 90%. It is shown that the GA-BP neural network structure can be used to effectively identify the coal structure, as well as predict the coal structure of uncored wells. Moreover, the findings of this study will be helpful for efforts to study the distribution law of the coal structure.

Characteristics of Coal-Bearing Shale Reservoirs and Gas Content Features in the Carboniferous–Permian System of the Qinshui Basin, Shanxi Province, China

The evaluation of reservoir properties and gas-bearing characteristics is critical for assessing shale gas accumulation. This study aimed to improve the precision of characterizing the properties and gas-bearing features of the Carboniferous and Permian shale reservoirs within the Qinshui Basin, Shanxi Province, China. It specifically focuses on the shale from the Late Carboniferous to Early Permian Shanxi and Taiyuan formations at Well Z1, located in the mid-eastern region of the basin. A comprehensive suite of analytical techniques, including organic geochemical analysis, scanning electron microscopy (SEM), X-ray diffraction (XRD), high-pressure mercury intrusion, low-temperature nitrogen adsorption, isothermal adsorption experiments, and gas content measurements, was used to systematically evaluate the reservoir properties and gas-bearing characteristics of the Carboniferous–Permian shale in Well Z1. The findings reveal the following. (1) The organic matter in the Shanxi and Taiyuan formations of Well Z1 is predominantly Type III humic kerogen, exhibiting high maturity and abundance. Specifically, 67.40% of the samples have TOC > 1.00%, classifying them as medium- to high-quality source rocks. The vitrinite reflectance (Ro) ranges from 1.99% to 2.55%, and Tmax varies from 322.01 °C to 542.01 °C, indicating a high to over-mature stage. (2) The mineral composition of the shale is dominated by kaolinite, illite, and quartz, with a moderate brittleness index. The average clay mineral content is 52.12%, while quartz averages 45.53%, and the brittleness index averages 42.34. (3) The pore types in the shale are predominantly macropores, with varying peak intervals among different samples. (4) The surface area and specific pore volume of macropores show positive relationships with TOC, Tmax, kaolinite, and the amount of desorbed gas, while they are negatively correlated with quartz. In contrast, mesopores exhibit positive correlations with TOC and illite. (5) Desorbed gas content exhibits a positive correlation with porosity, Ro, and illite. These insights enhance the comprehension of the reservoir’s properties, the characteristics of gas presence, and the determinant factors for the Carboniferous–Permian shale located in the Qinshui Basin, providing a robust practical procedure for the exploration and extraction of coal-measure shale gas resources within this area.

Evolution of the Pore-Fracture System of Coal: A Case Study of Zhaozhuang Colliery, Qinshui Basin.

The structure and evolution of coal are intricately linked to its properties at both nanometer and micrometer scales. The refinement of pores and fractures is crucial for assessing outburst risks, evaluating coalbed methane (CBM) reservoirs, and improving the CBM recovery efficiency. This study involved collecting four different coal-body structure samples from the Zhaozhuang colliery in the southern Qinshui Basin. We conducted laboratory analyses to determine the physical properties of the coal and employed X-ray computed tomography (CT) to quantitatively assess the pore size distribution (PSD), volume contribution, morphology, and connectivity across various coal structures. The evolution and comparative characteristics of these coal structures under different tectonic stresses is discussed. Results reveal that the hysteresis loop type of 3# anthracite primarily aligns with the H3 type as per IUPAC classification, featuring predominantly plate-like and wedge-shaped pores at the nanometer scale. A shape factor was introduced to quantitatively categorize the pore types, highlighting its sensitivity to the coal structure. Spherical and tubular pores are mainly present in the aperture range of 0-25 μm, while larger apertures appeared as prolate spheroids and flat fractures. Primary coal contains more spherical or tubular pores, whereas tectonic coal shows a prevalence of slit pores and flat fractures, suggesting that pores and fractures undergo progressive deformation, either breaking or elongating, under tectonic stress. The elastic property is affected by the multifactor of the increased pore volume and changes in pore morphology. Permeability is influenced by PSD and connectivity, demonstrating a quadratic positive correlation with porosity, aperture, and specific surface area. Granular coal exhibited the most favorable permeability characteristics for CBM extraction. The distribution trend of morphology and comprehensive data highlight an evolutionary pattern of different coal structures. The evolution of the coal structure is mainly shaped by brittle and ductile deformation mechanisms. Cataclastic coal is characterized by an increase in new fractures, and granular coal undergoes rapid new fracture formation and enhanced connectivity under strong stress conditions. Mylonitic coal develops under ductile deformation mechanisms. These insights into the properties of various coal structures can significantly enhance our understanding of the CBM recovery efficiency from both microscopic and mesoscopic perspectives.

Dynamic evolution and differential enrichment of deep coalbed methane: A case study in Qinshui Basin

Dynamic evolution and differential enrichment of deep coalbed methane: A case study in Qinshui Basin

A Study on the Controlling Effect of Geological Structures on Coalbed Methane Occurrence in the Northeast Margin of Qinshui Basin, North China

This study investigated the patterns of gas occurrence in the coal seam structural zones of the northeastern margin of the Qinshui Basin, with a focus on the Sijiazhuang Coal Mine. Using laboratory experiments, theoretical analysis, and field exploration, we examined how geological structures influence gas distribution. The results show that gas content and pressure near normal faults are generally higher than those in reverse fault areas. However, fault-induced gas occurrence is complex, with stress superposition potentially reversing this trend. When a normal fault intersects modern tectonic stress at a perpendicular or large angle, the fault zone may transition to a compressional state, enhancing gas preservation. Fold structures were found to play a significant role in gas distribution, with anticline zones exhibiting the highest gas content, followed by syncline and normal zones. Collapse columns were shown to affect gas occurrence within a range of 15 to 180 m, with the impact depending on factors such as surrounding rock properties, hydrogeological conditions, and fault activity during collapse formation. Additionally, mirror-like sliding surfaces, formed by multiple factors, are prevalent in the coal seam structures of this region. These sliding surfaces are closely linked to structural zones and serve as valuable indicators for geological predictions in coal seam development.

Analysis of geological characteristics and potential factors of formation damage in coalbed methane reservoir in Northern Qinshui basin

Given the suboptimal physical properties and distinctive geological conditions of deep coalbed methane reservoirs, any reservoir damage that occurs becomes irreversible. Consequently, the protection of these deep coalbed methane reservoirs is of paramount importance. This study employs experimental techniques such as scanning electron microscopy, X-ray diffraction, and micro-CT imaging to conduct a comprehensive analysis of the pore structure, mineral composition, fluid characteristics, and wettability of coal seams 3# and 15# in the northern Qinshui Basin of China. The objective is to elucidate the types of reservoir damage induced by fracturing fluid intrusion along with potential contributing factors. This research is critical for ensuring safe drilling practices, effective gas injection, and efficient development strategies for coalbed methane reservoirs. The findings indicate that the mineral composition of the coal rock consists of 18.52% clay minerals, 34% quartz, and 8.98% calcite. Furthermore, hydrophilicity and natural fractures within the coal rock may lead to water-sensitivity, velocity- sensitivity, alkali- sensitivity, and acid- sensitivity damages to the coalbed methane reservoir. There exists good compatibility between fracturing fluids and both coal rock as well as formation water. The fine particles generated from hydraulic fracturing are prone to transport through the coal seam while obstructing pore throats. Thus exhibiting pronounced velocity sensitivity characteristics in this reservoir type. Coal rock demonstrates pronounced stress sensitivity. As the effective stress escalates from 2 MPa to 10 MPa, there is a marked decrease in the permeability of coal rock. With increasing effective stress, the pore structure and natural fractures within the coal rock are compressed more tightly, resulting in a diminished permeability of the coal rock. When exposed to fracturing fluid saturation, not only does the volume of these particles expand but they can also cause blockages that result in up to a 60% reduction in fracture flow capacity. These insights are vital for optimizing fracturing designs aimed at protecting reservoir integrity.

A Relative Permeability Model of Coal Based on Fractal Capillary Bundle Assumption

Summary As the pore and fracture structure of coal significantly influence gas-water relative permeability (GWRP), it is crucial to study the GWRP in coal reservoirs for optimizing gas production. This paper provided parameters such as pore size range and capillary bundle porosity by referring to existing mercury intrusion porosimetry (MIP) experiments. The effective porosity coefficient and gas-water phase critical pore size were introduced to improve the GWRP model for coal based on the assumption of fractal capillary bundle. The GWRP model depends on changes in phase saturation, maximum and minimum capillary tube pore diameters, porosity, capillary size distribution dimension Df, and fractal dimension of tortuosity Dt. It demonstrated that models for various coal samples from the southern Qinshui Basin exhibit good agreement with the GWRP experimental data. In addition, the improved GWRP model was used to simulate coalbed methane (CBM) production and water production. The findings suggested that as water and gas are continuously extracted, effective stress rises as reservoir pressure and water saturation decline, leading to a more even distribution of capillary diameter and an increase in capillary degree. Furthermore, the effect of structural parameters on CBM production was also discussed.

Research on the Construction of Coal Powder Settling Final Velocity Model for Coalbed Methane Wells in Panhe Block

ABSTRACTIn response to the severe problem of coal powder production in the Panhe block coalbed methane wells in the southern Qinshui Basin, the characteristics of coal powder production in the study area were identified through sample testing, indoor experiments, and theoretical calculations. The settling velocity of coal powder with different mesh sizes was clarified, and a correction factor α was proposed for the experimental and theoretical results of settling velocity. The research results indicate that the coal powder concentration in the Panhe block ranges from 0.03 to 7.14 g/L, with an average of 1.26 g/L. The particle size of the coal powder produced was 4.10–237.64 μm, with an average of 35.82 μm. The settling velocity of coal powder particles with a mesh size of 40–400 is between 0.0041 and 0.029 m/s. The larger the particle size of coal powder particles, the higher the settling velocity of coal powder; the correction coefficient ranges from 2.3 to 13.67. A corrected settling velocity calculation model was obtained by fitting the coal powder particle size data to the correction coefficient. The research results provide a theoretical basis for developing production conditions, coal powder prevention, and control measures for coalbed methane wells in the Panhe area.

Identification of Fracture Extension Modes During Hydraulic Fracturing in Coalbed Methane Vertical Wells: A Case Study From the Southern Shizhuang Area of the Qinshui Basin, China

ABSTRACTThe accurate identification of fracture extension patterns in hydraulic fracturing can provide important guidance for the optimisation of fracturing parameters. In this paper, factors such as effective hole friction and wellbore flow friction during fracturing are fully considered, and a calculation model of net bottom‐hole pressure of fracturing is constructed. By introducing the change rate of net bottom‐hole pressure and the changing characteristics of the fracturing curve, seven fracture extension modes during hydraulic fracturing in coalbed methane vertical wells are established. The accuracy of the identification method is verified by the fracture monitoring and production results in Shizhuang South Block. The results show that fracture elongation is mainly controlled by in situ stress difference, angle between natural fracture and maximum principal stress, coal tensile strength, fracturing time, proppant and angle between other factors. When the fracture construction parameters are fixed, the smaller the difference between maximum and minimum horizontal principal stresses and the smaller the natural fractures and maximum horizontal principal stresses. When the reservoir potential is similar, the effective extension index is positively correlated with the gas production effect, and the effective extension index can effectively judge the fracturing effect. The higher the proportion of effective extension mode, the longer the extension time and the higher the stable daily gas production. The research results provide a method and reference for clearly identifying the fracture extension and the occurrence conditions of different extension modes in the hydraulic fracturing process.

Numerical Simulation of the Coal Measure Gas Accumulation Process in Well Z-7 in Qinshui Basin

The process of coal measure gas accumulation is relatively complex, involving multiple physicochemical processes such as migration, adsorption, desorption, and seepage of multiphase fluids (e.g., methane and water) in coal measure strata. This process is constrained by multiple factors, including geological structure, reservoir physical properties, fluid pressure, and temperature. This study used Well Z-7 in the Qinshui Basin as the research object as well as numerical simulations to reveal the processes of methane generation, migration, accumulation, and dissipation in the geological history. The results indicate that the gas content of the reservoir was basically zero in the early stage (before 25 Ma), and the gas content peaks all appeared after the peak of hydrocarbon generation (after 208 Ma). During the peak gas generation stage, the gas content increased sharply in the early stages. In the later stage, because of the pressurization of the hydrocarbon generation, the caprock broke through and was lost, and the gas content decreased in a zigzag manner. The reservoirs in the middle and upper parts of the coal measure were easily charged, which was consistent with the upward trend of diffusion and dissipation and had a certain relationship with the cumulative breakout and seepage dissipation. The gas contents of coal, shale, and tight sandstone reservoirs were positively correlated with the mature hydrocarbon generation of organic matter in coal seams, with the differences between different reservoirs gradually narrowing over time.

Clumped isotopes constrain thermogenic and secondary microbial methane origins in coal bed methane

Clumped isotopes constrain thermogenic and secondary microbial methane origins in coal bed methane

Matrix Compression and Pore Heterogeneity in the Coal-Measure Shale Reservoirs of the Qinshui Basin: A Multifractal Analysis

The application of high-pressure fluid induces the closure of isolated pores inside the matrix and promotes the generation of new fractures, resulting in a compressive effect on the matrix. To examine the compressibility of coal-measure shale samples, the compression of the coal–shale matrix in the high-pressure stage was analyzed by a low-pressure nitrogen gas adsorption and mercury intrusion porosimetry experiment. The quantitative parameters describing the heterogeneity of the pore-size distribution of coal-measure shale are obtained using multifractal theory. The results indicate that the samples exhibit compressibility values ranging from 0.154 × 10−5 MPa−1 to 4.74 × 10−5 MPa−1 across a pressure range of 12–413 MPa. The presence of pliable clay minerals enhances the matrix compressibility, whereas inflexible brittle minerals exhibit resistance to matrix compression. There is a reduction in local fluctuations of pore volume across different pore sizes, an improvement in the autocorrelation of PSD, and a mitigation of nonuniformity after correction. Singular and dimension spectra have advantages in multifractal characterization. The left and right spectral width parameters of the singular spectrum emphasize the local differences between the high- and low-value pore volume areas, respectively, whereas the dimensional spectrum width is more suitable for reflecting the overall heterogeneity of the PSD.

Prototypical basin reconstruction and evolutionary models for the Precambrian of the Qinshui Basin, North China craton

Prototypical basin reconstruction and evolutionary models for the Precambrian of the Qinshui Basin, North China craton