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14 pages, 5825 KiB

Open AccessArticle

Lacustrine Shale Oil Occurrence State and Its Controlling Factors: A Case Study from the Jurassic Lianggaoshan Formation in the Sichuan Basin

by Shaomin Zhang, Ruiying Guo, Qingsong Tang, Haitao Hong, Chunyu Qin, Shuangfang Lu, Pengfei Zhang, Tengqiang Wei, Keyu Pan and Zizhi Lin

Processes 2024, 12(12), 2617; https://doi.org/10.3390/pr12122617 - 21 Nov 2024

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Abstract

To reveal the shale oil occurrence state and its controlling factors of the Jurassic Lianggaoshan Formation in the Sichuan Basin, experimental analyses, including total organic content, X-ray diffraction, low-temperature nitrogen adsorption-desorption, nuclear magnetic resonance, conventional, and multistage rock-eval, were conducted on the shale [...] Read more.

To reveal the shale oil occurrence state and its controlling factors of the Jurassic Lianggaoshan Formation in the Sichuan Basin, experimental analyses, including total organic content, X-ray diffraction, low-temperature nitrogen adsorption-desorption, nuclear magnetic resonance, conventional, and multistage rock-eval, were conducted on the shale samples. The shale oil occurrence state, the amount/proportion of adsorbed/free oil, and their control factors were clarified. Moreover, the classification evaluation standard of shale oil resources was then determined. The results show that the selected shales are characterized by large oil contents. Total oil ranges from 0.08 mg/g to 10.06 mg/g (mean 2.82 mg/g). Adsorbed oil is between 0.03 mg/g and 5.66 mg/g (1.64 mg/g), while free oil spans from 0.05 mg/g to 4.94 mg/g (1.21 mg/g). The higher the total oil content, the higher the free oil content, indicating that the free oil sweet spot corresponds to the shale oil resource sweet spot. Shale oil is mainly adsorbed in organic matter; the larger TOC content results in the higher adsorbed oil content. Residual shale oil primarily occurs in pores less than 100 nm in diameter, and a higher pore volume corresponds to a higher total oil content. The shale oil enrichment resources refer to the shale with the TOC > 1.5%, S₁ > 1.5 mg/g, and S₁/TOC > 45 mg/g. This study is helpful for the prediction of shale oil resources and optimizing sweet spots in the Jurassic Lianggaoshan Formation of the Sichuan Basin. Full article

(This article belongs to the Special Issue Exploration, Exploitation and Utilization of Coal and Gas Resources)

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14 pages, 4668 KiB

Open AccessArticle

Pore-Fracture System Distribution Heterogeneity by Using the T₂ Spectral Curve under a Centrifugal State

by Tian Tian, Di Zhang, Yong Shi, Fangkai Quan and Zhenyuan Qin

Processes 2024, 12(9), 1812; https://doi.org/10.3390/pr12091812 - 26 Aug 2024

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Abstract

In this paper, 12 sandstone samples are collected from the Taiyuan Formation in Qinshui Basin, and sample types using the T₂ spectral under LF-NMR saturation and centrifugation conditions are classified. Moreover, single and multifractal models were used to calculate fractal parameters of [...] Read more.

In this paper, 12 sandstone samples are collected from the Taiyuan Formation in Qinshui Basin, and sample types using the T₂ spectral under LF-NMR saturation and centrifugation conditions are classified. Moreover, single and multifractal models were used to calculate fractal parameters of saturated and centrifugal T₂ spectra, and the correlation between different fractal parameters, pore structure, T_2cutoff value, and pore permeability parameters was studied. The results are as follows. (1) According to the T₂ spectrum curves under centrifugation and saturation conditions, all the samples can be divided into three types. There are significant differences in the uniform pore size distribution. However, the non-uniformity of small pore distribution in type B samples is stronger than that of other types, while heterogeneity of large pore distribution is weaker than that of different types. The centrifugal T₂ spectrum curve exhibits both single-fold and multifractal characteristics. The results of a single fractal by using a centrifugal T₂ spectrum are consistent with those of a saturated T₂ spectrum, indicating that single fractal features by using centrifugal and saturated T₂ spectra are consistent. Unlike the single fractal parameters, the correlation between the saturation and centrifugal T₂ spectrum’s multifractal parameters is weak. This suggests that the physical significance conveyed by the centrifugal T₂ spectrum’s multifractal parameters differs from that of the saturated T₂ spectrum. Full article

(This article belongs to the Special Issue Exploration, Exploitation and Utilization of Coal and Gas Resources)

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17 pages, 15960 KiB

Open AccessArticle

Analysis of Factors Influencing Tight Sandstone Gas Production and Identification of Favorable Gas Layers in the Shan 2³ Sub-Member of the Daning-Jixian Block, Eastern Ordos Basin

by Junyi Sun, Ming Chen, Bo Wang, Gang Wang, Haonian Tian, Jie Hou and Boning Zhu

Processes 2024, 12(9), 1810; https://doi.org/10.3390/pr12091810 - 26 Aug 2024

Cited by 1 | Viewed by 526

Abstract

The Daning-Jixian Block harbors abundant tight sandstone gas resources. However, significant variations in gas production exist among the different wells within the block. A comprehensive study was conducted on key factors such as sedimentary strata and petrophysical characteristics to elucidate their impact on [...] Read more.

The Daning-Jixian Block harbors abundant tight sandstone gas resources. However, significant variations in gas production exist among the different wells within the block. A comprehensive study was conducted on key factors such as sedimentary strata and petrophysical characteristics to elucidate their impact on gas reservoir productivity. Linear regression equations were employed to classify the favorable reservoirs within the study area. The analysis revealed that within the first 6 months of production from the Shan 2³ gas layer, daily gas production ranged from 2576.19 to 156,078.17 m³/d, averaging 24,037.9 m³/d. Over the first year, average daily production varied from 2185.05 to 136,806.99 m³/d, averaging 23,469.23 m³/d, indicating relatively stable production from the Shan 2³ layer alone. In the dominant central area of the underwater distributary channel delta front in Shan2³, the sand body exhibits a superimposed cutting type, resulting in high production rates. Conversely, the sand bodies on the periphery gradually transition to superimposed and isolated types, leading to decreased production. Through a correlation analysis of gas layer thickness, porosity, permeability, and initial gas well production, it was determined that gas production from the wells within the same layer is primarily influenced by gas layer thickness, porosity, and permeability. Gas saturation demonstrates a minimal impact on production according to single-factor analysis. The evaluated factors such as the gas productivity coefficient, energy storage coefficient, and enrichment coefficient exhibited similar distribution patterns across the study area. The high-value areas for the gas productivity coefficient, energy storage coefficient, and enrichment coefficient are concentrated in distributary channel zones and delta lobes. In contrast, regions with underdeveloped skeletal sand bodies generally display lower values for these parameters. The linear relationships between these parameters and the average gas production were calculated to further classify the favorable reservoirs in the study area. This study aimed to establish a scientific basis for the efficient development of the tight sandstone gas reservoirs within the Daning-Jixian Block. Full article

(This article belongs to the Special Issue Exploration, Exploitation and Utilization of Coal and Gas Resources)

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16 pages, 3102 KiB

Open AccessArticle

Mineralogical and Geochemical Composition of Late Permian Coals from Dengfeng Coalfield, North China: Conversion of Clay Minerals in Coal during Coalification

by Shuyuan Ning, Zhenzhi Wang, Hui Wang, Chunxiang Chen, Hui Zhao, Bo Huang and Qiming Zheng

Processes 2024, 12(8), 1688; https://doi.org/10.3390/pr12081688 - 13 Aug 2024

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Abstract

Dengfeng Coalfield represents a significant coalfield in Henan Province, North China. It is therefore essential to gain an understanding of the mineralogy and geochemistry of the Dengfeng coal, both from a geochemical perspective and in terms of the wider environmental context. In this [...] Read more.

Dengfeng Coalfield represents a significant coalfield in Henan Province, North China. It is therefore essential to gain an understanding of the mineralogy and geochemistry of the Dengfeng coal, both from a geochemical perspective and in terms of the wider environmental context. In this study, a total of 27 coal bench samples were collected from the No. II1 coal of the Dengfeng Coalfield. The mineral species and major elements were quantitatively analysed using the X-ray diffraction and X-ray fluorescence methods, respectively. The minerals in the Dengfeng coal are dominated by ammonian illite and kaolinite with average contents of 3.73% and 7.47%, respectively. These are followed by calcite (2.74% on average) and ankerite (0.49%). The mean value of the kaolinite Hinkley index, which is a quantitative measure of kaolinite crystallinity, is 1.26. This suggests that kaolinite formation is primarily driven by diagenetic recrystallisation. The ammonian illite exhibits an average d₀₀₁ of 10.2995 Å, indicative of a prevalence of NH₄⁺ interlayer cations, with K⁺ also present in notable quantities. The ratio of NH₄⁺ to (NH₄⁺ + K⁺) has an average value of 0.90, which is indicative of the predominance of NH₄⁺. The mean value of the illite Kübler index, which is a quantitative measure of illite crystallinity, is 0.264. This suggests that the diagenetic conditions correspond to the rank of the Dengfeng coal. The kaolinite present in the Dengfeng coal is suggested to have been derived from terrigenous detritus and subsequently subjected to diagenetic recrystallisation, resulting in a relatively high Hinkley index. The ammonian illite in the Dengfeng coal was predominantly formed through the conversion of the precursor kaolinite, with the influence of seawater during peat accumulation favouring the conversion of kaolinite to ammonian illite. Full article

(This article belongs to the Special Issue Exploration, Exploitation and Utilization of Coal and Gas Resources)

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17 pages, 24979 KiB

Open AccessArticle

Segmentation Differences of the Salt-Related Qiulitage Fold and Thrust Belt in the Kuqa Foreland Basin

by Yingzhong Zhu, Chuanxin Li, Yuhang Zhang, Yibo Zhao and Tulujun Gulifeire

Processes 2024, 12(8), 1672; https://doi.org/10.3390/pr12081672 - 9 Aug 2024

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Abstract

The Qiulitage fold and thrust belt (QFTB) is situated in the Kuqa Depression, exhibiting spectacular salt structures with well-defined geometric and kinematic characteristics and thereby playing a significant role in advancing the study of salt structures worldwide. This research, based on regional geology, [...] Read more.

The Qiulitage fold and thrust belt (QFTB) is situated in the Kuqa Depression, exhibiting spectacular salt structures with well-defined geometric and kinematic characteristics and thereby playing a significant role in advancing the study of salt structures worldwide. This research, based on regional geology, well logging, and newly acquired three-dimensional seismic data, applies principles of salt-related fault structures to interpret seismic data and restore structural equilibrium in the Qiulitage fold and thrust belt within the Kuqa Depression by conducting quantitative studies on structural geometry and kinematics. Results indicate clear differences in salt structures between the eastern and western segments of it, vertically divided into upper salt, salt layer, and lower salt and horizontally into four parts. The Dina segment features a single-row basement-involved thrust fault, the East QFTB segment displays detachment thrust faults involving cover layers, the Central QFTB segment exhibits detachment thrust faults involving multiple rows of cover layers, the leading edge forms structural wedges, and the West QFTB segment develops blind-thrust faults. During the deposition of the Kangcun formation, the eastern profile experiences an 18% shortening rate, 14% in the central part, and 9% in the western part. For the Kuqa formation, the eastern profile experiences a 10% shortening rate, 9% in the central part, and 3% in the western part, indicating more significant deformation in the east than in the west. Quantitative statistical analysis reveals that different types of detachments, paleogeomorphology, and northeast-directed compressive stress exert control over the Qiulitage fold-thrust belt. Full article

(This article belongs to the Special Issue Exploration, Exploitation and Utilization of Coal and Gas Resources)

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16 pages, 9287 KiB

Open AccessArticle

The Influence of Coal Body Structure on Coal Fines’ Output Characteristics in the Southern Qinshui Basin

by Junshan Ren, Zhou Zhang, Liru Xing, Pengxiang Wang, Wanying Yu and Piao Long

Processes 2024, 12(4), 656; https://doi.org/10.3390/pr12040656 - 26 Mar 2024

Cited by 1 | Viewed by 858

Abstract

Large amounts of coal fines are discharged from coalbed methane wellheads in the Qinshui Basin, obstructing the continuity of drainage; their extraction poses significant hazards. This paper recognized the coal body structure of 30 coalbed methane wells in the study region, using the [...] Read more.

Large amounts of coal fines are discharged from coalbed methane wellheads in the Qinshui Basin, obstructing the continuity of drainage; their extraction poses significant hazards. This paper recognized the coal body structure of 30 coalbed methane wells in the study region, using the integrated identification method of logging curve and tectonic curvature. The research found that the primary structural coal output of coal fines concentration averaged 0.237 g/L, the average content of particle size 10–100 μm was 58.88%, the average range of particle size 1–10 μm was 22.91%, and the main form was irregular columns and lumps. The average concentration of fractured structural coal fines was 1.169 g/L, the average content of particle size 10–100 μm was 41.73%, the average range of particle size 1–10 μm was 31.77%, and the main form was balls and lumps. The average concentration of granulated-mylonitic structured coal fines was 3.156 g/L, the average content of particle size 10–100 μm was 25.26%, the average range of particle size 1–10 μm was 57.59%, and the coal fines were mainly in the form of clusters and flaky aggregates. Full article

(This article belongs to the Special Issue Exploration, Exploitation and Utilization of Coal and Gas Resources)

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20 pages, 13576 KiB

Open AccessFeature PaperArticle

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

by Shunxi Liu, Jie Yang, Yi Jin, Huibo Song, Baoyu Wang, Jiabin Dong and Junling Zheng

Processes 2024, 12(3), 602; https://doi.org/10.3390/pr12030602 - 18 Mar 2024

Cited by 1 | Viewed by 997

Abstract

The parameters of coalbed methane reservoirs have large differences, and the precise values cannot represent the resource and production characteristics of the whole block. In order to address these problems, an index system for evaluating the production potential of coalbed methane blocks was [...] Read more.

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

(This article belongs to the Special Issue Exploration, Exploitation and Utilization of Coal and Gas Resources)

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12 pages, 3413 KiB

Open AccessArticle

The Wetting Characteristics and Microscopic Wetting Mechanism of Coal under High-Pressure Nitrogen Environment

by Piao Long, Bin Shi, Yunxing Cao, Yufei Qi, Xinyi Chen and Liuyang Li

Processes 2024, 12(3), 568; https://doi.org/10.3390/pr12030568 - 13 Mar 2024

Viewed by 1009

Abstract

The wettability of coal is an important factor influencing hydraulic stimulation. Field-trial data has proven that high-pressure N₂ injection plays a positive role in increasing the coalbed methane (CBM) production rate. For the purpose of investigating the mechanism by which N₂ [...] Read more.

The wettability of coal is an important factor influencing hydraulic stimulation. Field-trial data has proven that high-pressure N₂ injection plays a positive role in increasing the coalbed methane (CBM) production rate. For the purpose of investigating the mechanism by which N₂ promotes the gas rate, multiple experiments were conducted sequentially on the wettability of anthracite under different N₂ pressures. Testing of the coal surface contact angle was conducted under 0.1–8 MPa nitrogen pressure using a newly built contact angle measuring device. The coal samples were collected from the Xinjing Coal Mine in the Qinshui Basin, China. The test results revealed that the contact angle increased with increasing N₂ pressure. That is, the contact angle was 77.9° at an N₂ pressure of 0.1 MPa and gradually increased to 101.4° at an infinite N₂ pressure. In contrast, the capillary pressure decreased with an increasing N₂ pressure, from 0.298 MPa to −0.281 MPa. The relationship between contact angle and N₂ pressure indicated that the wettability was reversed at a N₂ pressure of 5.26 MPa, with a contact angle of 90° and a capillary pressure of 0 MPa. The capillary pressure reversed to a negative value as the N₂ pressure increased. At the microlevel, a high N₂ pressure increases the surface roughness of coal, which improves the ability of the coal matrix to adsorb N₂, forming the gas barrier that hinders the intrusion of water into the pores of the coal matrix. The results of this study provide laboratory evidence that high-pressure N₂ injection can prevent water contamination and reduce the capillary pressure, thus benefiting coalbed methane production. Full article

(This article belongs to the Special Issue Exploration, Exploitation and Utilization of Coal and Gas Resources)

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20 pages, 13273 KiB

Open AccessArticle

Numerical Simulation Study on the Mechanics and Pore Characteristics of Tectonically Deformed Coal under Multi-Level and Multi-Cycle Loading and Unloading Conditions

by He Wang, Shuxun Sang, Shiqi Liu, Ziliang Wang and Wenkai Wang

Processes 2024, 12(2), 362; https://doi.org/10.3390/pr12020362 - 9 Feb 2024

Cited by 1 | Viewed by 951

Abstract

Horizontal well cavern completion and stress release is considered a potential technique for efficient development of coalbed methane in tectonically deformed coal (TDC). Pulsating loading and unloading is a key technique for the controlled expansion of caverns and broader stress release within the [...] Read more.

Horizontal well cavern completion and stress release is considered a potential technique for efficient development of coalbed methane in tectonically deformed coal (TDC). Pulsating loading and unloading is a key technique for the controlled expansion of caverns and broader stress release within the reservoir. However, current understanding of the mechanical characteristics and pore network structure evolution of TDC under cyclic loading and unloading conditions is still limited. This paper employs numerical simulation methods to study the mechanical behavior and damage characteristics of TDC under cyclic loading and unloading. After obtaining a set of micromechanical parameters reflecting the behavior of TDC samples under triaxial compression in high-stress states, the effects of different stress gradients and cyclic amplitudes on the stress–strain curve, porosity changes, and crack propagation in TDC samples were analyzed. The study results indicate that under various cyclic loading and unloading conditions, the mechanical response characteristics of TDC samples are broadly similar, primarily divided into compression, slow expansion, and accelerated expansion phases. Under low unloading level conditions, the volume expansion of TDC samples is minimal. Also, at the same unloading level, the strain increment decreases with an increasing number of cycles. Correspondingly, under these conditions, the porosity and microcrack expansion in TDC are less than in high-stress gradient scenarios. Under the same unloading level but different amplitudes, the volume expansion rate at 50% unloading amplitude is higher than at 1 MPa unloading amplitude for TDC, with an increased number of crack expansions. Therefore, under cyclic loading conditions, the sensitivity of crack propagation within TDC samples to amplitude is greater than that to unloading level. Under actual pulsating excitation conditions, a low-amplitude, low-stress gradient pulsation method should be used to maintain the stability of horizontal well caverns, and gradually increase the cyclic amplitude to achieve the efficient extraction of coalbed methane in TDC reservoirs. The findings of this study can serve as an important reference for optimizing process parameters in cyclic pulsating stress release engineering for TDC. Full article

(This article belongs to the Special Issue Exploration, Exploitation and Utilization of Coal and Gas Resources)

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24 pages, 31262 KiB

Open AccessArticle

Hydrocarbon Source Rock Evaluation of the Lucaogou Shale in the Periphery of Bogeda Mountain (SE Junggar Basin, China) and Its Implications for Shale Oil Exploration: Insights from Organic Geochemistry, Petrology, and Kinetics Pyrolysis

by Guanlong Zhang, Yuqiang Yang, Tianjiao Liu, Youde Xu, Xiangchun Chang, Yansheng Qu, Bingbing Shi, Bo Yang and Tao Song

Processes 2024, 12(2), 356; https://doi.org/10.3390/pr12020356 - 8 Feb 2024

Cited by 1 | Viewed by 1029

Abstract

Since the discovery of the vast Jimusaer shale oilfield in the southeastern Junggar Basin in 2012, there has been considerable interest in neighboring areas around Bogeda Mountain that have shale oil potential. The primary productive interval in the basin, the Middle Permian Lucaogou [...] Read more.

Since the discovery of the vast Jimusaer shale oilfield in the southeastern Junggar Basin in 2012, there has been considerable interest in neighboring areas around Bogeda Mountain that have shale oil potential. The primary productive interval in the basin, the Middle Permian Lucaogou Formation (P₂l), is well-developed in the areas of Qitai, Mulei, Shiqiantan, Chaiwopu, and Miquan. In this study, we conducted an assessment of the hydrocarbon generation potential of the P₂l in these five areas and compared it with that of the P₂l in the Jimusaer oilfield, which were determined by GC-MS, total organic carbon (TOC) and vitrinite reflectance (Ro) measurements, Rock-Eval pyrolysis, and organic petrology to investigate the type, origin, thermal maturity, hydrocarbon potential, and oil/gas proneness of organic matter in the P₂l. Additionally, we applied open-system pyrolysis of hydrocarbon generation kinetics to explore differences in hydrocarbon generation and expulsion across various P₂l mudstone/shale in the southeastern Junggar Basin. The findings of this study revealed that the P₂l shale in Qitai and Miquan areas contains more abundant and lower thermally mature organic matter (early mature–mature stage), characterized by primarily Type II₁–I kerogen, similar to that found in the P₂l shale of the Jimusaer oilfield. Conversely, the P₂l shale in Mulei, Shiqiantan, and Chaiwopu contains less abundant and more thermally mature organic matter (mainly mature–highly mature stage), dominated by Type II₂–III kerogen. Consequently, shale in these areas is considerably less desirable for oil exploration compared to the Jimusaer shale. The semi-deep to deep lake facies in Miquan and Qitai exhibit the most promising exploration potential. This study can serve as a guide for shale oil exploration in the southeastern Junggar Basin. Full article

(This article belongs to the Special Issue Exploration, Exploitation and Utilization of Coal and Gas Resources)

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23 pages, 10584 KiB

Open AccessArticle

Prevention of Blowout Tests in Large-Diameter Boreholes with Soundless Chemical Demolition Agents and Fracturing Characteristics of Hard Sandstones

by Junjie Wu, Zhuo Dong, Ruifu Yuan, Shuaishuai Xie and Junhao Deng

Processes 2024, 12(2), 336; https://doi.org/10.3390/pr12020336 - 4 Feb 2024

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Abstract

Increasing the diameter of the drillhole can facilitate drillhole breakage using soundless chemical demolition agents, but it is prone to cause drillhole blowout, resulting in crushing failure. This paper conducted a blowhole prevention test on a large borehole using the internal insertion cooling [...] Read more.

Increasing the diameter of the drillhole can facilitate drillhole breakage using soundless chemical demolition agents, but it is prone to cause drillhole blowout, resulting in crushing failure. This paper conducted a blowhole prevention test on a large borehole using the internal insertion cooling pipe method (ICBPM) to test the expansion pressure of cooling pipes with different diameters. During this test, a fracture occurred in a hole with a 75 mm inner diameter in the rectangular sandstone specimens with high strength. It was found that utilizing the ICBPM can effectively hinder the development of blowholes. Expansion and blowhole prevention are optimized with a 0.14 mass ratio of the cooling water to demolition agent and a maximum expansion stress of 49.0 MPa. The guiding effect of the minimum resistance line is significant. In repeated tests, all fissures are distributed in a Y-shape on the free surface where the minimum resistance line is located. The acoustic emission signals from statically fractured hard rock increase abruptly before damage, and the development of rock expansion and fracturing can be obtained through strain monitoring. These results suggest that the ICBPM can reduce the expansion time with a strong crushing effect, satisfying the need to process more crushing projects. Full article

(This article belongs to the Special Issue Exploration, Exploitation and Utilization of Coal and Gas Resources)

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17 pages, 3697 KiB

Open AccessArticle

Exploitation and Utilization of Generated Oil and Gas by Pyrolysis Simulation Modeling of Shale Source Rocks under the Condition of In Situ Conversion

by Xia Luo, Lianhua Hou, Zhongying Zhao, Senhu Lin, Zhenglian Pang, Lijun Zhang, Wenxue Han and Yong Li

Processes 2024, 12(1), 121; https://doi.org/10.3390/pr12010121 - 3 Jan 2024

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Abstract

Previous studies have mainly focused on the source rocks of the 7th Member of Yanchang Formation (Chang 7 Member) in the Ordos Basin, with very few studies focusing on the extracts from the source rocks. These extracts have important guiding significance for studying [...] Read more.

Previous studies have mainly focused on the source rocks of the 7th Member of Yanchang Formation (Chang 7 Member) in the Ordos Basin, with very few studies focusing on the extracts from the source rocks. These extracts have important guiding significance for studying the in situ conversion process of shale oil. Taking the shale source rock of the Chang 7 Member as an example, this paper selected the extract of shale source rock (i.e., retained oil), which has been less studied previously, as the sample to carry out the hydrocarbon-generating pyrolysis simulation experiment of a semi-open–semi-closed system. Seven groups of parallel simulation experiments were designed with a pressure of 20 MPa. The generated oil and gas were collected and quantified, and their geochemical characteristics were researched. In addition, the generated oil and gas were investigated from aspects of cumulative yield and net increased yield, and the chromatographic and mass spectral characteristics of the generated oil were also researched. Based on this, an inductive hydrocarbon generation model of retained oil in shale source rocks was established: slow hydrocarbon generation stage (300–320 °C), rapid hydrocarbon generation stage (320–360 °C), and residual oil pyrolysis stage (0.79%Ro–1.47%Ro). This study is of important significance to guide the research on the in situ conversion process of shale source rock. Full article

(This article belongs to the Special Issue Exploration, Exploitation and Utilization of Coal and Gas Resources)

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16 pages, 25401 KiB

Open AccessArticle

Study on the Extraction Mechanism of Metal Ions on Small Molecular Phase of Tar-Rich Coal under Ultrasonic Loading

by Zetang Wang, Yuan Bao, Chaoyong Wang and Yiliang Hu

Processes 2024, 12(1), 104; https://doi.org/10.3390/pr12010104 - 1 Jan 2024

Cited by 1 | Viewed by 1323

Abstract

This study aims to elucidate the mechanism by which the ultrasonic loading of metal affects the extraction of small molecular phase substances (low molecular compounds) in tar-rich coal. Tar-rich coal samples were collected from the Huangling mining area in the southeastern Ordos Basin, [...] Read more.

This study aims to elucidate the mechanism by which the ultrasonic loading of metal affects the extraction of small molecular phase substances (low molecular compounds) in tar-rich coal. Tar-rich coal samples were collected from the Huangling mining area in the southeastern Ordos Basin, China. The coal, the leaching solution of the coal, the extraction products, and the extraction residual coal samples with different metal ions loaded by ultrasound were analyzed using field emission scanning electron microscopy, pH detection, gas chromatography–mass spectrometry, a Fourier transform infrared spectrometer, and an X-ray diffractometer. The obtained results indicated that the ultrasonic loading of coal samples with different metal ions (Mn²⁺, Co²⁺, Cu²⁺, Fe²⁺, and Ni²⁺) promoted the extraction of small molecular phase substances in coal and increased the proportion of extracted aliphatic hydrocarbons, alkylbenzene, naphthalene, phenanthrene, and other compounds. The extraction rate of Mn²⁺ was the highest. Compared with the control group, the extraction rate was increased by 212%. After the ultrasonic loading of metal ions, the physical structure of the coal was loose and the contact area of the solvent increased; the degree of branching and the hydrogen enrichment of the residual coal decreased, the aromaticity increased, the interlayer spacing and stacking layers decreased, and the stacking degree and ductility increased. Metal ions exchanged with hydrogen ions in the coal molecules. At the same time, the metal ions were adsorbed in the coal molecules and effectively combined with free electrons in the coal molecules to catalyze; thus, the extraction effect of the small molecular phase of tar-rich coal was improved. This provides a new method for the clean and efficient utilization of tar-rich coal. Full article

(This article belongs to the Special Issue Exploration, Exploitation and Utilization of Coal and Gas Resources)

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20 pages, 5841 KiB

Open AccessArticle

Theoretical Simulation of the Resistivity and Fractured–Cavernous Structures of Carbonate Reservoirs

by Zhaohui Zhang, Chuqiao Gao, Yongde Gao, Chunzhen Niu and Shenglun Ma

Processes 2024, 12(1), 43; https://doi.org/10.3390/pr12010043 - 23 Dec 2023

Viewed by 968

Abstract

Recently, theoretical modeling based on rock physics has emerged as a pivotal approach to studying the resistivity of complex fractured–cavernous microstructures. In this work, to study the effects of fractured–cavernous structures on carbonate reservoir resistivity, electrical conductivity models were developed based on the [...] Read more.

Recently, theoretical modeling based on rock physics has emerged as a pivotal approach to studying the resistivity of complex fractured–cavernous microstructures. In this work, to study the effects of fractured–cavernous structures on carbonate reservoir resistivity, electrical conductivity models were developed based on the effective medium theory and Ohm’s Law, and theoretical simulations were performed to examine how the porosity and resistivity of the rock matrix, the formation water resistivity, and the parameters of the fractured–cavernous microstructure affect the resistivity of rocks saturated with petroleum or water. Furthermore, the modeling results revealed the specific relationships between these factors in petroleum-saturated and water-saturated rocks. For vuggy reservoirs, a significant negative correlation between throat diameter and resistivity was revealed when variations in the rock matrix and formation water resistivity were negligible. Furthermore, the pore shape—especially the extension of pores in the direction of the current—severely reduced the resistivity of petroleum-saturated rocks. For fractured reservoirs, the porosity and resistivity of the rock matrix were the primary factors affecting resistivity, with the fracture inclination angle and width also exhibiting pronounced effects on the resistivity of water-saturated rocks. The rock cementation exponent was much smaller when the matrix pores were interconnected through fractures than when they were interconnected through throats. The findings reveal that the effects of the structural parameters of fractured–cavernous carbonate reservoirs on reservoir resistivity differ between petroleum-saturated and water-saturated rocks. The conventional Archie’s equation is insufficient for evaluating fluid saturation in carbonate reservoirs. A saturation evaluation model with a variable rock cementation exponent tailored to the specific reservoir type should thus be developed. Full article

(This article belongs to the Special Issue Exploration, Exploitation and Utilization of Coal and Gas Resources)

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16 pages, 39266 KiB

Open AccessArticle

Analysis of Coalbed Methane Production Characteristics and Influencing Factors of No. 15 Coal Seam in the Shouyang Block

by Bing Zhang, Wei Li, Gang Wang and Xinglong Jiao

Processes 2023, 11(12), 3269; https://doi.org/10.3390/pr11123269 - 22 Nov 2023

Cited by 2 | Viewed by 1018

Abstract

Based on the basic geological data and production data of coalbed methane wells in the Shouyang Block, the characteristics and influencing factors of coalbed methane well production were analyzed, and the primary controlling factors were identified by the grey correlation method. The results [...] Read more.

Based on the basic geological data and production data of coalbed methane wells in the Shouyang Block, the characteristics and influencing factors of coalbed methane well production were analyzed, and the primary controlling factors were identified by the grey correlation method. The results show that the average daily gas production of the coalbed methane wells in the study area for the single mining of No. 15 coal ranges from 0 to 604.34 m³/d, with an average of 116.82 m³/d. The overall average gas production is relatively low, with only 7 of the 42 wells having an average gas production greater than 200 m³/d. Gas production tends to increase as the gas content increases. There is a significant positive correlation between gas saturation and average gas production. Burial depth and coal seam thickness also show a positive correlation with average gas production. On the other hand, there is a negative exponential relationship between average gas production and critical desorption pressure. Permeability, as determined by well tests in the area, exhibits a negative correlation with the gas production of coalbed methane wells. The correlation between gas production and the mean three-dimensional stress is weak. As the fractal dimension D value of fractures increases, gas production decreases. A smaller difference in horizontal principal stress is more favorable for the formation of network fractures, facilitating reservoir fracturing and resulting in better reconstructive properties. Moreover, an increase in the sand–mud ratio leads to a decrease in average gas production. The correlation between fault fractal dimension and average gas production is weak. The grey correlation method was employed to determine the controlling factors of coalbed methane production in the study area, ranked from strong to weak, as follows: coal thickness > fracture fractal dimension D value > gas saturation > coal seam gas content > horizontal stress difference coefficient > permeability > critical desorption pressure > mean value of three-dimensional principal stress > coal seam burial depth > sand–mud ratio > fault fractal dimension. Full article

(This article belongs to the Special Issue Exploration, Exploitation and Utilization of Coal and Gas Resources)

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17 pages, 3462 KiB

Open AccessArticle

Physical Characteristics and Controlling Factors of Coal Gas Reservoir in Pingdingshan No. 10 Coal Mine

by Dejie Zhou, Juan Wang, Baoyu Wang, Di Gao and Junjie Zhao

Processes 2023, 11(11), 3130; https://doi.org/10.3390/pr11113130 - 1 Nov 2023

Cited by 1 | Viewed by 1172

Abstract

The physical properties of coal reservoirs are the main restrictions to exploration and development of Coalbed methane (CBM). The study of the physical characteristics of coal reservoirs and their controlling factors is of great significance to the safe and efficient development and utilization [...] Read more.

The physical properties of coal reservoirs are the main restrictions to exploration and development of Coalbed methane (CBM). The study of the physical characteristics of coal reservoirs and their controlling factors is of great significance to the safe and efficient development and utilization of CBM resources in the mining area. The Pingdingshan No. 10 coal mine was chosen for this study because of its better gas production effect at the test wells. The reservoir properties of the main coal seams of the No. 10 coal mine were tested and analyzed to comprehensively evaluate the reservoir properties, and the physical characteristics and controlling factors of the coal reservoir in Pingdingshan No. 10 coal mine were studied. The results indicate the following: (1) The gas content and methane purity of the No. 4 coal seam were significantly higher than the No. 2 seam, and therefore they have better development potential than the No. 2 seam. (2) The average adsorption time of the No. 2 coal seam was less than that of the No. 4 seam, making it easier to reach the peak production capacity of CBM wells in the short term, but was not conducive to long-term stable production of CBM. The Langmuir volume of the No. 2 coal seam samples was significantly greater than for the No. 4 coal seam; however, the No. 4 coal seam contains about three times the amount of gas in the No. 2 seam. (3) All three coal seams have high porosity, which was favorable for large amounts of CBM adsorption and storage. Micropores predominated; transitional pores were less frequent, and a few mesopores occurred; macropores were the least common. Samples from the No. 4 seam contained the highest proportion of micropores. (4) Organic pores were common in all the coal samples, with pore diameters not more than 30 µm, mainly concentrated between 50.5 and 1000 nm. Microfractures with apertures less than 70 nm were relatively frequent, mainly in the 50–65 nm range. A large number of the nanoscale microfractures were curved or jagged. (5) Fractures in the No. 2 and No. 4 coal seam samples with widths of 50 nm to 20 µm were more developed, and many were filled with kaolinite, quartz, and other minerals. (6) The samples contained mostly layered silicate minerals (kaolinite), with hard granular minerals (quartz) next, and a very small quantity of iron-type minerals, such as siderite and pyrite in dendritic form. The results of this study can provide a reference basis for the large-scale development and utilization of subsequent CBM wells. Full article

(This article belongs to the Special Issue Exploration, Exploitation and Utilization of Coal and Gas Resources)

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18 pages, 5131 KiB

Open AccessArticle

Heterogeneity of Pore and Fracture Structure in Coal Reservoirs by Using High-Pressure Mercury Intrusion and Removal Curve

by Zhixiang Niu, Jie Li, Peng Yao, Junjian Zhang, Shuang Sun, Wenlong Liu, Shuangshuang Cui and Jingli Sun

Processes 2023, 11(11), 3120; https://doi.org/10.3390/pr11113120 - 31 Oct 2023

Cited by 1 | Viewed by 1028

Abstract

The pore structure determines the desorption, diffusion and migration of coalbed methane, and the heterogeneity of the pore structure seriously restricts the diffusion and seepage process and productivity of coalbed methane. Therefore, this paper takes eight coal samples in the Linxing area as [...] Read more.

The pore structure determines the desorption, diffusion and migration of coalbed methane, and the heterogeneity of the pore structure seriously restricts the diffusion and seepage process and productivity of coalbed methane. Therefore, this paper takes eight coal samples in the Linxing area as the research target and uses the high-pressure mercury injection test to describe the pore structure distribution. On this basis, three kinds of single and multifractal models are used to calculate the progressive mercury removal curve, and the correlation analysis is carried out to determine the physical significance of the mercury removal fractal dimension. Finally, the relationship between the fractal dimension of the mercury curve and the pore structure parameters is defined, and the applicability of fractal models in characterizing pore structure heterogeneity is discussed. The conclusions of this paper are as follows. (1) Samples can be divided into two categories according to porosity and mercury removal efficiency. Among them, the mercury removal efficiency of sample 1–3 is higher than 35%, and porosity is less than 9.5%, while those of sample 4–8 are the opposite. The seepage pore volume percentage of sample 1–3 is 35–60%, which is higher than that in sample 4–8. (2) The difference of the samples’ fractal dimension calculated with the Menger and Sierpinski models is small, indicating that the pore structure distribution heterogeneity of the two types is similar. The multifractal model shows that the adsorption pore and macro-pore heterogeneity of sample 4–8 are stronger than those of sample 1–3, and the pore distribution heterogeneity is controlled by the low value of pore volume. (3) The results of the two single fractal calculations show that the pore structure distribution heterogeneity of sample 4–8 is stronger than that of sample 1–3. The multifractal model calculation shows that the adsorption pore distribution heterogeneity of sample 4–8 is stronger, and the low value of pore volume controls the pore distribution heterogeneity. (4) The mercury fractals based on the Menger model can reflect the adsorption pore distribution and macro-pore distribution heterogeneity, while the Sierpinski model can only reflect the adsorption pore distribution heterogeneity at the mercury inlet stage. Full article

(This article belongs to the Special Issue Exploration, Exploitation and Utilization of Coal and Gas Resources)

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19 pages, 5330 KiB

Open AccessArticle

Quantitative Prediction of Deep Coalbed Methane Content in Daning-Jixian Block, Ordos Basin, China

by Zheyuan Ouyang, Haichao Wang, Bin Sun, Yunxuan Liu, Xuehai Fu, Wei Dou, Liang Du, Beixi Zhang, Bing Luo, Mengmeng Yang and Zhiwei Zeng

Processes 2023, 11(11), 3093; https://doi.org/10.3390/pr11113093 - 27 Oct 2023

Cited by 10 | Viewed by 1402

Abstract

The potential of deep coalbed methane resources is substantial. Gas content is an important parameter for resource assessment. At present, the gas content test method commonly used for shallow coal reservoirs is not suitable for determining deep coalbed methane gas content. Therefore, it [...] Read more.

The potential of deep coalbed methane resources is substantial. Gas content is an important parameter for resource assessment. At present, the gas content test method commonly used for shallow coal reservoirs is not suitable for determining deep coalbed methane gas content. Therefore, it is urgent to establish a prediction method for deep coalbed methane gas content. This study aims to quantitatively predict the gas content of coalbed methane in deep coal reservoirs and uncover its influencing factors. For this purpose, we selected the Daning-Jixian area, a region in China with relatively advanced development of deep coalbed methane, as a case study. It established a prediction model for adsorbed gas and free gas content in deep coal reservoirs through a series of experimental tests, encompassing gas content, coal quality, isothermal adsorption, and nuclear magnetic resonance. The model sheds light on the impact of coal-rock characteristics, coal quality attributes, and pore characteristics on adsorbed gas and free gas content. The results show that adsorbed gas dominates in deep coal reservoirs with a high metamorphic degree and that the average proportion of adsorbed gas under reservoir burial depth is 80.15%. At a depth of approximately 1800~2000 m, a turning point is observed where adsorbed gas content begins to decline. Beyond this depth range, the detrimental effect of temperature on coalbed methane content surpasses the beneficial impact of pressure. Coal quality characteristics play a major role in controlling adsorbed gas content, and an increase in water content and ash yield significantly reduces the adsorption performance of coal reservoirs. The content of free gas increases with the increase in burial depth, with its controlling factors primarily being confining pressure and porosity. The increase in the proportion of micropores in the pores of deep coal reservoirs has an adverse effect on the content of free gas. The proportion of adsorbed gas in deep coal reservoirs gradually decreases with the increase in burial depth, while the proportion of free gas gradually rises with the increase in burial depth. The development potential of free gas cannot be overlooked in the exploration and development of deep coalbed methane. Full article

(This article belongs to the Special Issue Exploration, Exploitation and Utilization of Coal and Gas Resources)

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16 pages, 6974 KiB

Open AccessArticle

The Coal-Forming Environment at the End of the Late Permian and Its Control on Trace Elements: The Upper Xuanwei Formation in Eastern Yunnan, China

by Juan Wang, Longyi Shao and Xuetian Wang

Processes 2023, 11(10), 2936; https://doi.org/10.3390/pr11102936 - 9 Oct 2023

Cited by 1 | Viewed by 1384

Abstract

Forming environments have important effects on the dispersion and enrichment of trace elements in coal. The C₃ coal seam of the Xuanwei Formation in eastern Yunnan was used as a case study to reconstruct the peat-forming environment based on coal facies parameters [...] Read more.

Forming environments have important effects on the dispersion and enrichment of trace elements in coal. The C₃ coal seam of the Xuanwei Formation in eastern Yunnan was used as a case study to reconstruct the peat-forming environment based on coal facies parameters and geochemical characteristics, and its influence on trace element (including rare earth elements and yttrium, REY) enrichment was investigated. The C₃ coal was classified as medium rank bituminous coal with an ultra-low moisture content, medium-high ash yield, and medium-low volatile content. Compared to the average values for Chinese coal, Cu and V were enriched and Co was slightly enriched in the C₃ coal. Compared with the average values for world coal, Cu and V were enriched while several other trace elements were slightly enriched in the C₃ coal, including Co, Hf, Nb, Sc, Ta, Zn, and Zr. The C₃ coal was deposited in the limno-telmatic environment with fresh water, and reducing conditions. Trace elements, including Cu, V, Hf, Nb, Sc, Ta, Zr, Zn, Co, and REY, were typically enriched in the limno-telmatic environment with fresh water and reducing conditions. Additionally, REY and V were also significantly enriched in brackish water limno-telmatic conditions with the same depositional environment. Full article

(This article belongs to the Special Issue Exploration, Exploitation and Utilization of Coal and Gas Resources)

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21 pages, 5333 KiB

Open AccessArticle

Experimental Study on Methane Diffusion Characteristics of Different Metamorphic Deformed Coals Based on the Counter Diffusion Method

by Jiangang Ren, Liang Gao, Zhihui Wen, Hongbo Weng, Jianbao Liu, Runsheng Lv, Yanwei Qu, Zhimin Song, Yongwang Zhang and Bing Li

Processes 2023, 11(9), 2808; https://doi.org/10.3390/pr11092808 - 21 Sep 2023

Viewed by 1029

Abstract

The diffusion coefficient (D) is a key parameter that characterizes the gas transport occurring in coal seams. Typically, D is calculated using the desorption curve of particle coal. However, this method cannot accurately reflect the diffusion characteristics under the stress constraint [...] Read more.

The diffusion coefficient (D) is a key parameter that characterizes the gas transport occurring in coal seams. Typically, D is calculated using the desorption curve of particle coal. However, this method cannot accurately reflect the diffusion characteristics under the stress constraint conditions of in situ coal seams. In this study, different metamorphic deformed coals of medium and high coal rank were considered based on Fick’s law of counter diffusion. The change laws of D under different confining pressures, gas pressures, and temperature conditions were tested and analyzed, and the influencing mechanisms on D are discussed. The results showed that D of different metamorphic deformed coals exponentially decreased with an increase in confining pressures, and exponentially increased with increases in gas pressures and temperature. There is a limit diffusion coefficient. The influence of the confining pressure on D can essentially be determined by changes in the effective stress, and D negatively affects the effective stress, similar to permeability. The effect of gas pressure on D involves two mechanisms: mechanical and adsorption effects, which are jointly restricted by the effective stress and the shrinkage and expansion deformation of coal particles. Temperature mainly affects D by changing the root-mean-square speed and average free path of the gas molecules. Under the same temperature and pressure conditions, D first increased and then decreased with an increase in the degree of deformation. D of the fragmented coal was the largest. Under similar deformation conditions, D of the high-rank anthracite was larger than that of the medium-rank fat coal. Porosity is a key factor affecting the change in D in different metamorphic deformed coals. Full article

(This article belongs to the Special Issue Exploration, Exploitation and Utilization of Coal and Gas Resources)

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22 pages, 25050 KiB

Open AccessArticle

Response and Mechanism of Coal Fine Production to Differential Fluid Action in the Baode Block, Ordos Basin

by Boyang Wang, Yunfei Cui, Jingjing Li, Junjian Zhang, Longhui Bai and Liu Wang

Processes 2023, 11(8), 2476; https://doi.org/10.3390/pr11082476 - 17 Aug 2023

Cited by 1 | Viewed by 1070

Abstract

The Baode Block in the Ordos Basin is currently one of the most successfully developed and largest gas field of low–medium rank coal in China. However, the production of coal fine has affected the continuous and stable drainage and efficient development of this [...] Read more.

The Baode Block in the Ordos Basin is currently one of the most successfully developed and largest gas field of low–medium rank coal in China. However, the production of coal fine has affected the continuous and stable drainage and efficient development of this area. The special response and mechanism of differential fluid action during the drainage process is one of the scientific issues that must be faced to solve this production problem. In view of this, the evolution laws of a reservoir’s macro–micro physical characteristics under different fluid conditions (fluid pressure, salinity) have been revealed, and the response mechanism of coal fine migration-induced reservoir damage has been elucidated through a nuclear magnetic resonance online displacement system. The results indicated that pores at different scales exhibited varying patterns with increasing displacement pressure. The proportion of the mesopore and transition pore is not affected by salinity and is positively correlated with displacement pressure. When the salinity is between 3000 mg/L and 8000 mg/L, the proportion of macropore and micropore showed parabolic changes with increasing displacement pressure, and there was a lowest point. The evolution law of pore fractal dimension and permeability change rate under the action of different fluids jointly showed that there was an optimal salinity for the strongest reservoir sensitivity enhancement effect. The mechanical and chemical effects of fluid together determined the damage degree of coal reservoir induced by coal fine migration. Full article

(This article belongs to the Special Issue Exploration, Exploitation and Utilization of Coal and Gas Resources)

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16 pages, 14686 KiB

Open AccessArticle

Changing Law of Permeability of Coal Reservoirs under Variable Pressure Conditions and Its Influence on Extraction Efficiency of Coalbed Methane

by Jianbao Liu, Zhimin Song, Chengtao Yang, Bing Li, Jiangang Ren and Shengjie Chen

Processes 2023, 11(8), 2455; https://doi.org/10.3390/pr11082455 - 15 Aug 2023

Viewed by 1030

Abstract

Coal permeability data are critical in the prevention and control of coal and gas outbursts in mines and are an important reservoir parameter for the development of coalbed methane. The mechanism by which permeability is affected by gas pressure is complex. We used [...] Read more.

Coal permeability data are critical in the prevention and control of coal and gas outbursts in mines and are an important reservoir parameter for the development of coalbed methane. The mechanism by which permeability is affected by gas pressure is complex. We used a self-developed true triaxial seepage experimental device that collects lignite and anthracite coal samples, sets fixed axial pressure and confining pressure, and changes gas pressure by changing the orientation of the coal seam to study the influence of the gas pressure on the permeability of the coal seam under the conditions of different coal types and different bedding orientations. Coal permeability decreased rapidly and then decreased slowly and tended to be stable with the increase in gas pressure. This conformed to the power exponential fitting relationship, and the fitting degree reached more than 99%. The comparison of the two anthracite coal samples showed that the sample’s permeability with a bedding plane vertical to the seepage direction was significantly lower than that of the bedding plane parallel to the seepage direction, indicating that gas seeped more easily along the bedding. The sensitivity coefficient of permeability with the change in gas pressure was calculated. The analysis showed that coal permeability was sensitive to changes in gas pressure during the low-pressure stage. When the gas pressure was greater than 0.8 MPa, the sensitivity coefficient was significantly reduced, which may have been related to the slow increase in the amount of gas absorbed by the coal seam in the high-pressure stage. A theoretical calculation model of coal seam permeability considering adsorption/desorption and seepage effects was proposed and then verified with experimental results showing that the theoretical model better reflected the permeability characteristics of coal and predicted its permeability. Using the finite element simulation software COMSOL, the extraction efficiency of the coal seam gas under different gas pressure conditions was simulated. The results showed that coal permeability and extraction efficiency decreased with an increase in gas pressure. In the low-pressure stage, the reduction in the extraction efficiency was more evident than that in the high-pressure stage. Full article

(This article belongs to the Special Issue Exploration, Exploitation and Utilization of Coal and Gas Resources)

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16 pages, 6504 KiB

Open AccessArticle

Theoretical Basis and Technical Method of Permeability Enhancement of Tectonic Coal Seam by High Intensity Acoustic Wave In Situ

by Weidong Li, Yongmin Zhang, Dalong Wang, Cunqiang Chen, Yongyuan Li, Youzhi Zhao, Shuo Zhang, Jing Ren and Yong Qin

Processes 2023, 11(8), 2372; https://doi.org/10.3390/pr11082372 - 7 Aug 2023

Cited by 1 | Viewed by 1059

Abstract

Tectonic coal seams are characterized by soft, low permeability and high gas outburst. The traditional gas control method is the intensive drilling and extraction in this seam, which is not only large in engineering quantity, high in cost, difficult to form holes and [...] Read more.

Tectonic coal seams are characterized by soft, low permeability and high gas outburst. The traditional gas control method is the intensive drilling and extraction in this seam, which is not only large in engineering quantity, high in cost, difficult to form holes and low in extraction efficiency, but also easy to induce coal and gas outburst, which is a difficult problem for global coal mine gas control. To solve this difficult problem, the controllable shockwave equipment developed by the author’s team and successfully applied in the practice of permeability enhancement of coal seam, combined with the principles of shock vibration sound wave generation and shock wave attenuation and evolution in the rock stratum, a new idea of loading a controllable shock wave in the roof and floor of coal seam is proposed. The shock wave first attenuates and evolves into a high-strength sound wave in the roof and floor rock stratum, and then enters and loads into the coal seam to achieve the purpose of increasing permeability without damaging the physical properties of the tectonic coal seam and facilitating the opening of the original fractures. According to the new technical ideas, the implementation scheme and key parameters of the gas pre-extraction models in tectonic coal seam are designed, including the penetration drilling, roof and floor horizontal holes, shield tunneling and the high-strength acoustic wave of the working face, which provides a new technical approach to solve the problem of high efficiency and low cost gas extraction in the tectonic coal seam. Full article

(This article belongs to the Special Issue Exploration, Exploitation and Utilization of Coal and Gas Resources)

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Review

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23 pages, 3638 KiB

Open AccessReview

Mechanism and Model Analysis of Ultralow-Temperature Fluid Fracturing in Low-Permeability Reservoir: Insights from Liquid Nitrogen Fracturing

by Haifeng Wang, Yunbo Li, Dangyu Song, Meng Lin, Xingxin Guo and Xiaowei Shi

Processes 2024, 12(6), 1117; https://doi.org/10.3390/pr12061117 - 29 May 2024

Viewed by 838

Abstract

Ultralow-temperature fluids (such as liquid nitrogen, liquid CO₂) are novel waterless fracturing technologies designed for dry, water-sensitive reservoirs. Due to their ultralow temperatures, high compression ratios, strong frost heaving forces, and low viscosities, they offer a solution for enhancing the fracturing [...] Read more.

Ultralow-temperature fluids (such as liquid nitrogen, liquid CO₂) are novel waterless fracturing technologies designed for dry, water-sensitive reservoirs. Due to their ultralow temperatures, high compression ratios, strong frost heaving forces, and low viscosities, they offer a solution for enhancing the fracturing and permeability of low-permeability reservoirs. In this study, we focus on the combined effects of high-pressure fluid rock breaking, low-temperature freeze-thaw fracturing, and liquid-gas phase transformation expansion on coal-rock in low-permeability reservoirs during liquid nitrogen fracturing (LNF). We systematically analyze the factors that limit the LNF effectiveness, and we discuss the pore fracture process induced by low-temperature fracturing in coal-rock and its impact on the permeability. Based on this analysis, we propose a model and flow for fracturing low-permeability reservoirs with low-temperature fluids. The analysis suggests that the Leidenfrost effect and phase change after ultralow-temperature fluids enter the coal support the theoretical feasibility of high-pressure fluid rock breaking. The thermal impact and temperature exchange rate between the fluid and coal determine the temperature difference gradient, which directly affects the mismatch deformation and fracture development scale of different coal-rock structures. The low-temperature phase change coupling fracturing of ultralow-temperature fluids is the key to the formation of reservoir fracture networks. The coal-rock components, natural fissures, temperature difference gradients, and number of cycles are the key factors in low-temperature fracturing. In contrast to those in conventional hydraulic fracturing, the propagation and interaction of fractures under low-temperature conditions involve multifield coupling and synergistic temperature, fluid flow, fracture development, and stress distribution processes. The key factors determining the feasibility of the large-scale application of ultralow-temperature fluid fracturing in the future are the reconstruction of fracture networks and the enhancement of the permeability response in low-permeability reservoirs. Based on these considerations, we propose a model and process for LNF in low-permeability reservoirs. The research findings presented herein provide theoretical insights and practical guidance for understanding waterless fracturing mechanisms in deep reservoirs. Full article

(This article belongs to the Special Issue Exploration, Exploitation and Utilization of Coal and Gas Resources)

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