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A special issue of Applied Sciences (ISSN 2076-3417). This special issue belongs to the section "Energy Science and Technology".

Deadline for manuscript submissions: 20 March 2025 | Viewed by 6180

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Dr. Run Chen

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Guest Editor

1. Jiangsu Key Laboratory of Coal-Based Greenhouse Gas Control and Utilization, Xuzhou 221008, China
2. Carbon Neutrality Institute, China University of Mining and Technology, Xuzhou 221008, China
Interests: unconventional natural gas; CCUS; carbon neutrality; underground coal gasification, CO₂ mineralization
Special Issues, Collections and Topics in MDPI journals

Dr. Zheng Zhang

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Guest Editor

1. Key Laboratory of Coalbed Methane Resources and Reservoir Formation Process, Ministry of Education, China University of Mining and Technology, Xuzhou 221008, China
2. School of Resources and Geosciences, China University of Mining and Technology, Xuzhou 221116, China
Interests: hydrogeochemistry; mine water environment; coal geology; hydrology; coal geochemistry
Special Issues, Collections and Topics in MDPI journals

Special Issue Information

Dear Colleagues,

Unconventional natural gas mainly involves shale gas, coalbed methane, tight gas, natural gas hydrate, etc. Globally, unconventional natural gas resources are abundant. Unconventional natural gas is the most realistic replacement resource for conventional natural gas and plays an important role in the world energy pattern. Currently, the development and utilization technologies are becoming increasingly advanced, and countries around the world attach great importance to the development and utilization of unconventional natural gas resources.

This Special Issue on “Advances in Unconventional Natural Gas: Exploration and Development” aims to cover the recent advances in the exploration and development of unconventional natural gas. Topics include, but are not limited to, the methods and/or applications in the following areas:

Key technologies for the exploration and development of unconventional natural gas;
Deep unconventional natural gas resources;
Numerical simulation techniques for unconventional natural gas reservoirs;
Recovery-enhancing techniques for unconventional natural gas;
Favorable area selection for unconventional natural gas.

Dr. Run Chen
Dr. Zheng Zhang
Guest Editors

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Keywords

unconventional natural gas
recovery-enhancing techniques
deep unconventional natural gas
numerical simulation
favorable area selection
reservoir evaluation

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Published Papers (8 papers)

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Research

24 pages, 1073 KiB

Open AccessArticle

Dynamic Prediction of Shale Gas Drilling Costs Based on Machine Learning

by Tianxiang Yang, Yuan Liang, Zhong Wang and Qingyun Ji

Appl. Sci. 2024, 14(23), 10984; https://doi.org/10.3390/app142310984 - 26 Nov 2024

Abstract

Shale gas, a significant recoverable natural gas resource trapped in shale formations, represents a significant energy reservoir. Although China has significant recoverable shale gas reserves, the challenge of controlling drilling costs remains a critical barrier to efficient development. This study presents a novel stacked ensemble learning model that integrates support vector machine (SVM) and long short-term memory (LSTM) networks to improve the accuracy of shale gas drilling cost prediction. The methodology consists of three main phases. First, we constructed a comprehensive, multidimensional spatiotemporal dataset of shale gas drilling costs. Second, we used Gradient Boosting Decision Tree (GBDT) modelling to rank the importance of various factors influencing drilling costs. Finally, we developed a stacked ensemble learning model combining SVM and LSTM architectures to achieve superior cost prediction accuracy. Experimental results demonstrate the effectiveness of the model, with the coefficient of determination (R²) improving from 0.25189/0.33834 (traditional SVM/LSTM models) to 0.55934. Model validation using selected well investment data from the Changning Block shows promising performance, achieving a Mean Absolute Percentage Error (MAPE) of 6.41%, with optimal prediction accuracy in the medium investment range (60–70 million yuan). This innovative approach provides a reliable tool for predicting shale gas drilling costs and offers new methodological perspectives for cost reduction strategies. The results contribute significantly to the sustainable development of shale gas resources and provide valuable insights for industry practitioners and researchers in the fields of energy economics and resource management. Full article

(This article belongs to the Special Issue Advances in Unconventional Natural Gas: Exploration and Development)

14 pages, 2468 KiB

Open AccessArticle

Phase Behavior of Fluid Composition in Coalbed Methane Wells Pre- and Post-Workover: An Examination of the Panzhuang Block, Qinshui Basin, Shanxi, China

by Qingwei Wang, Qiang Yan, Yan Zhang, Xiafan Xing and Cailian Hao

Appl. Sci. 2024, 14(16), 7207; https://doi.org/10.3390/app14167207 - 16 Aug 2024

Viewed by 574

Abstract

Workover operations significantly impact the service life and gas production capacity of coalbed methane (CBM) wells and are crucial for optimizing resource exploitation. To investigate workover operations’ impact on coal seam reservoirs, the authors designed a series of experiments and obtained the following results: (1) The workover operation induced a phase transition in the solid-liquid composition produced by the CBM well, indicating changes in the coal reservoir’s internal structure. (2) During the stable production stage before and after the workover, the proportion of Na⁺, Cl⁻, Ca²⁺, and Total Dissolved Solids (TDS) in the water samples showed a downward trend as a whole, while the HCO₃⁻; after the workover, the Na⁺, Cl⁻, Ca²⁺, and TDS all increased suddenly, while the HCO₃⁻ decreased. (3) While inorganic minerals predominated in the precipitation material during the stable production stage pre-workover, their proportion decreased post-workover, with a noticeable shift in their qualitative composition. (4) It is an indisputable fact that workover operations cause physical and chemical damage to coal seam reservoirs. During workover operation, how to avoid damage and conduct benign reconstruction to the reservoir will be the direction of our future efforts. The experimental results provide valuable insights that can guide the optimization of CBM workover operations and inform the strategic planning of subsequent drainage activities. Full article

(This article belongs to the Special Issue Advances in Unconventional Natural Gas: Exploration and Development)

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21 pages, 14878 KiB

Open AccessArticle

Reservoir Characteristics of Marine–Continental Transitional Taiyuan Formation Shale and Its Influence on Methane Adsorption Capacity: A Case Study in Southern North China Basin

by Wei Jiang and Yang Hu

Appl. Sci. 2024, 14(15), 6577; https://doi.org/10.3390/app14156577 - 27 Jul 2024

Viewed by 635

Abstract

To further study the reservoir characteristics and adsorption capacity of the Taiyuan Formation shale in the South North China Basin (SNCB), the pore structure and adsorption capacity of shale are discussed using various analysis tests, including elemental geochemistry, organic geochemistry, mineral composition, low-temperature nitrogen adsorption (LTNA), and methane adsorption experiments. The results indicate that the Taiyuan Formation shale formed in a poor oxygen and anaerobic sedimentary environment in still water. The average value of total organic carbon (TOC) content is 2.37%. The organic matter type mainly consists of type III kerogen. The vitinite reflectance (Ro) ranges from 3.11% to 3.50%. The clay mineral content varies greatly, averaging at 40.7%, while the quartz content averages at 37.7%. The Taiyuan Formation shale mainly develops interparticle (InterP) pores, followed by organic pores, intraparticle (IntraP) pores, solution pores, and microfractures. BET specific surface area (SSA) is between 9.47 m²/g and 22.14 m²/g, while pore volume (PV) ranges from 0.0098 cm³/g to 0.022 cm³/g, indicating favorable conditions for shale gas storage. According to the results of the CH₄ adsorption experiment, Langmuir volume from Taiyuan Formation shales exhibits 1.35~4.30 cm³/g, indicating excellent adsorption capacity. TOC content shows a positive correlation with both Langmuir volume and BET SSA from Taiyuan Formation shales, suggesting that TOC plays a crucial role in controlling microscopic pores and gas adsorption capacity. Organic matter enhances the shale adsorption capacity by providing abundant pore SSA. Due to formation compaction, the pore size of clay minerals decreases, leading to an increase in pore SSA, while kaolinite exhibits weak hydrophilic ability. Consequently, with the increase in clay minerals and kaolinite content, the shale adsorption capacity is enhanced to a certain extent. However, an increase in the carbonate mineral content may result in a decrease in the proportion of clay minerals, therefore reducing the CH₄ adsorption capacity of shale. Full article

(This article belongs to the Special Issue Advances in Unconventional Natural Gas: Exploration and Development)

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11 pages, 5279 KiB

Open AccessArticle

Physical Simulation Experiment for Visualizing Pulverized Coal Transport in Propped Fractures

by Yufang Liu, Longbin Yang, Jinxing Song, Junke Shi and Qian Wang

Appl. Sci. 2024, 14(14), 6114; https://doi.org/10.3390/app14146114 - 13 Jul 2024

Cited by 1 | Viewed by 1040

Abstract

The issue of pulverized coal in coalbed methane wells during the discharge and mining process spans all stages, and it is a key factor constraining the continuous and stable discharge and production capacity of coalbed methane. Among these stages, the single-phase water flow stage features a high incidence of pulverized coal. Consequently, this paper presents a physical simulation experiment within the propped fractures during the single-phase water flow stage. The results of this study reveal the following: (1) Within the propped fracture channel, when pulverized coal is deposited along the flow line without causing blockage, the front end of the deposition exhibits a strip-like dispersion, evolving into “block deposition”, “flame-like accumulation”, “linear accumulation”, and “dispersed point-like accumulation”. (2) Agglomerated fracturing fluid can effectively mitigate the permeability damage caused by pulverized coal to the propped fractures. Both the driving speed and particle size of pulverized coal significantly influence pulverized coal transportation. The injury rate of propped fracture conductivity increases with increasing driving speeds, while the output of pulverized coal first increases and then decreases with increasing driving speed. Moreover, larger pulverized coal particle sizes result in notably greater damage to propped fracture conductivity than smaller particle sizes. Correspondingly, larger particles exhibit significantly lower output of pulverized coal compared to smaller particles, and transportation and output time are prolonged for larger particles. These findings underscore the importance of particle size and driving speed in the transportation dynamics of pulverized coal. The research results provide a theoretical basis for developing strategies for the prevention and control of pulverized coal during the single-phase water flow stage, thereby offering substantial scientific and practical value for the economic and efficient development of coalbed methane. Full article

(This article belongs to the Special Issue Advances in Unconventional Natural Gas: Exploration and Development)

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19 pages, 4462 KiB

Open AccessArticle

Effect of Lateral Confining Pressure on Shale’s Mechanical Properties and Its Implications for Fracture Conductivity

by Jinliang Song, Yuan Liu, Yujie Luo, Fujian Yang and Dawei Hu

Appl. Sci. 2024, 14(13), 5825; https://doi.org/10.3390/app14135825 - 3 Jul 2024

Viewed by 807

Abstract

The field stress of the shale affects the proppant embedment, fracture conductivity, well production rate, and ultimately the recovery of hydrocarbons from reservoir formations. This paper presents, for the first time, an experimental study investigating the mechanical characteristics of a shale under confining pressures that simulate the in situ stress state in deep reservoirs. Bidirectional but equal confining pressures were applied to the shale sample to replicate its field stress state. Microindentation tests were conducted to assess the alterations of mechanical properties resulting from the application of confining pressures. The results demonstrate a significant increase in Young’s modulus, hardness, and fracture toughness for the samples subjected to confining pressure. Considering the effect of confining pressure, the decrease in proppant embedment is proportional to Young’s modulus of the shale. For larger-sized proppants (e.g., D = 2.50 mm), the influence of confining pressure on fracture conductivity is relatively minor. However, when smaller-sized proppants (e.g., D = 1.00 mm) are used, particularly in scenarios involving shale debris swelling due to prolonged interaction with fracturing fluid, there is a noticeable improvement in fracture conductivity. Importantly, previous computational models have tended to overestimate proppant embedment depth while underestimating fracture conductivity. The findings from this study contribute to advancing the understanding of shale’s mechanical characteristics under in situ reservoir conditions and support the optimization of proppant embedment and fracture conductivity calculation models for the efficient extraction of shale gas. Full article

(This article belongs to the Special Issue Advances in Unconventional Natural Gas: Exploration and Development)

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18 pages, 4562 KiB

Open AccessArticle

Analysis and Characterization of Micro–Nano Pores in Coal Reservoirs of Different Coal Ranks

by Jinxing Song, Yulu Yue and Yufang Liu

Appl. Sci. 2024, 14(12), 5198; https://doi.org/10.3390/app14125198 - 14 Jun 2024

Cited by 1 | Viewed by 653

Abstract

Coalbed methane represents a promising source of clean and efficient unconventional energy. The intricate network of micro–nano pores within coal serves as the primary adsorption space for gas, contributing to the complexity of gas migration channels. In this study, based on the box-counting method, three coal samples representing low, medium, and high ranks were subjected to high-precision micro-CT scanning and nano-CT scanning to generate three-dimensional (3D) pore network models using Avizo visualization software. This facilitated the accurate and quantitative characterization of the micro–nano pore structures within coal reservoirs. The results indicated that the face rate distribution range of each sample was large, indicating relatively strong heterogeneity in each sample. The volume fractal dimension of each sample, determined through micro–nano-CT scanning, was around 2.5, while the surface fractal dimension exhibited oscillatory characteristics with moderate uniformity. The pore equivalent radius and throat equivalent radius distributions were unimodal across all the samples, with the micro-CT scanning revealing a concentration primarily within the range of 100–400 μm for the pore equivalent radius and within 200 μm for the throat equivalent radius. Conversely, the nano-CT scanning exhibited concentrations primarily within the range of 500–2500 nm for the pore equivalent radius and within 2000 nm for the throat equivalent radius. The analysis of the 3D reconstruction structures indicated that the middle-rank coal exhibited more developed large–medium pores compared with the low-rank and high-rank coal, while the low-rank and high-rank coal exhibited relatively more micro–small pores. Furthermore, the low-rank coal exhibited the fewest number of pores but the largest average pore equivalent radius and throat radius. Additionally, the middle–high-rank coal exhibited a relatively larger number of pores. Despite the complex topological structures observed in each sample, a significant proportion indicated a coordination number of 0–20, indicating excellent connectivity within the coal samples. This study is conducive to the optimization of coalbed methane surface development blocks and the formulation of reasonable development plans. Full article

(This article belongs to the Special Issue Advances in Unconventional Natural Gas: Exploration and Development)

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18 pages, 10152 KiB

Open AccessArticle

Characteristics and Sources of CBM Well-Produced Water in the Shouyang Block, China

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

Appl. Sci. 2024, 14(10), 4218; https://doi.org/10.3390/app14104218 - 16 May 2024

Viewed by 709

Abstract

The Shouyang Block was selected as the research subject. Comprehensive analysis was conducted using coalbed methane (CBM) well production data, geochemical test data on water produced from the coalbed methane well, and fundamental geological information. The findings reveal the water dynamics in the Shouyang Block are characterized by weak groundwater runoff or retention in most areas. The groundwater head height exhibits a gradual decrease from the north to south, which is closely associated with the monoclinic structure of the Shouyang Block. Overall, water production is relatively high. As the average water production increases, the average gas production gradually decreases. A concentration of high water production wells is observed in the northern part of the Shouyang Block, which gradually increases towards the southeast direction. A comprehensive analysis was conducted on the factors influencing water production, including total water content of coal seams, coal seam porosity, groundwater stability index, groundwater sealing coefficient, D value of the fracture fractal dimension, fault fractal dimension, and sand–mud ratio. The correlation degree was calculated and ranked in order of magnitude through grey correlation analysis. The order of factors that influence water production, from strongest to weakest, is as follows: sand–mud ratio > porosity > fractal dimension of fault > fracture fractal dimension D value > groundwater sealing coefficient > groundwater stability index > total water content of coal seams. The dissolution amounts of carbonate and sulfate are both small, and the water source may mainly come from the sandstone aquifer. Attention should be paid to the distribution and lithological combination of sandstone aquifers in coal-bearing strata in the future exploration and development process of the Shouyang Block. This will help to avoid the potential influence of fault structures and enable the identification of favorable areas for low water and high gas production. Full article

(This article belongs to the Special Issue Advances in Unconventional Natural Gas: Exploration and Development)

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11 pages, 2864 KiB

Open AccessArticle

Study of the Methane Adsorption Characteristics in a Deep Coal Reservoir Using Adsorption Potential Theory

by Zhengjiang Long, Xushuang Zhu, Junqiao Liao, Dingnan Ye and Run Chen

Appl. Sci. 2024, 14(8), 3478; https://doi.org/10.3390/app14083478 - 20 Apr 2024

Cited by 1 | Viewed by 753

Abstract

The gas adsorption characteristics in deep coal reservoirs are the focus of deep coalbed methane geology research. In order to reveal the adsorption characteristics in deep coal reservoirs and quantitatively characterize the amount of adsorbed methane in the deep coal seams, four coals were collected from the Permian Longtan Formation in southern Sichuan Province. Methane isothermal adsorption tests were carried out on the collected coal samples at 30 °C. The adsorption characteristic curve was established based on the data of the isothermal adsorption. The adsorption potential theory was used to predict the isothermal adsorption curves under different temperatures and the evolutionary relationship between the methane adsorption capacity and the coal seam burial depth in the C₁₇ and C₂₅ coal seams of the Permian in southern Sichuan Province, China. The results showed that the methane isothermal adsorption curve at 30 °C belonged to the Type I isotherm adsorption curve. The methane isothermal adsorption curves for various samples at 45 °C, 60 °C, and 75 °C were predicted based on the uniqueness of the methane adsorption characteristic curve. The amount of adsorbed gas in deep coal reservoirs was comprehensively controlled by pressure and temperature. The pressure showed a positive effect on the amount of methane adsorbed, while the temperature showed a negative effect on the adsorption of methane. The negative effect of temperature became more significant with the increase in pressure. The results of the study are beneficial for further promoting the exploration and development of deep coalbed methane in the southern Sichuan Province of China. Full article

(This article belongs to the Special Issue Advances in Unconventional Natural Gas: Exploration and Development)

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