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The State of the Art of Geo-Energy Technology in China

A topical collection in Energies (ISSN 1996-1073). This collection belongs to the section "H: Geo-Energy".

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Editors


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Collection Editor
School of Mechanics and Civil Engineering, China University of Mining and Technology, Xuzhou 221116, Jiangsu, China
Interests: rock mechanics; rock creep; underground engineering
Special Issues, Collections and Topics in MDPI journals

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Collection Editor
Key Laboratory of Deep Oil and Gas, School of Geoscience, China University of Petroleum (EastChina), Qingdao 266580, Shandong, China
Interests: shale oil; gas geology; petroleum resource evaluation; petroleum system modelling

E-Mail Website
Collection Editor
1. Geotechnical and Structural Engineering Research Center, Shandong University, Jinan 250061, China
2. Stake Key Laboratory for Geomechanics and Deep Underground Engineering, China University of Mining and Technology (Beijing), Beijing 100083, China
Interests: geotechnical engineering; mining engineering; surrounding rock control; rock testing
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Collection Editor
College of Construction Engineering, Jilin University, Changchun 130026, China
Interests: remote sensing for geological hazards; geological hazards evolution
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Collection Editor
School of Resources and Safety Engineering, Central South University, Changsha 410083, China
Interests: deep mining; rock mechanics; fracture mechanics
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Topical Collection Information

Dear Colleagues,

Geological energy has a long history in China. As early as 500 BC, China began to use coal as fuel. In the middle of the 19th century, China began to develop oil resources. After the foundation of new China, with the discovery of the Daqing oilfield, China's oil industry entered an era of great development. During the 21st century, with the development of drilling technology, the development of unconventional energy such as shale gas and shale oil has entered a new era. In recent years, the development of flammable ice has set off a wave of clean energy. With the carbon reduction plan proposed by the Chinese government, clean geo-energy has been granted unparalleled development space in the future.

China's geo-energy development technology used to lag behind that of major developed countries for a long time, but after years of development, it has become the world's leading edge in some fields. Therefore, we specially set up this collection to collect China's advanced geo-energy exploitation technology and development trends, whilst providing some new directions for thinking about geo-energy development in China and even the world. This collection seeks to contribute to such topics through enhanced scientific and multidisciplinary knowledge. We invite papers on innovative technical developments, reviews, case studies, as well as analytical and assessment papers from different disciplines that are relevant to the topic of geo-energy. The main topics of the section include, but are not limited to:

  • Reservoir Characterization and Modeling
    • Mathematical modeling of flow in porous media;
    • Naturally fractured reservoirs;
    • Reservoir simulation;
    • Reservoir management;
    • Reservoir properties and processes;
    • Traces studies;
    • Well spacing optimization.
  • Geomechanics for Energy and the Environment
    • Wellbore stability studies;
    • Automated drilling;
    • Hydraulic fracturing studies;
    • Fracture propagation models;
    • Proppant placement;
    • Fracture diagnostics;
    • Fracture characterization;
    • Naturally fractured in reservoirs.
  • Sequestration of Carbon Dioxide
    • CO2 storage in coal/shales/oil and gas reservoirs;
    • Regulation and legislation of carbon emissions;
    • Carbon sinks.
  • Geothermal Energy Extraction
    • Enhanced geothermal energy;
    • Integration of geothermal energy in energy systems;
    • Subsurface storage of heat and cold;
    • Geo fluids;
    • Geothermal power plants and heating systems;
    • Borehole heat exchanger;
    • Ground source heat pumps;
    • Energy from groundwater.
  • Petroleum Exploration and Production
    • Natural gas;
    • Shale gas and liquids;
    • Heavy oil EOR;
    • Petroleum economics;
    • Enhanced oil recovery;
    • Production engineering;
    • Optimization of lift systems.
  • Energy from Coal Formations
    • Environmentally friendly coal mining;
    • In situ coal combustion;
    • Coal gasification.

Prof. Dr. Sheng-Qi Yang
Prof. Dr. Min Wang
Prof. Dr. Qi Wang
Prof. Dr. Wen Zhang
Prof. Dr. Kun Du
Dr. Chun Zhu
Collection Editors

Manuscript Submission Information

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Keywords

  • Petroleum 
  • Coal 
  • Natural gas 
  • Combustible ice
  • Geothermal

Published Papers (15 papers)

2023

Jump to: 2022, 2021

6 pages, 187 KiB  
Editorial
Advanced Progress of the Geo-Energy Technology in China
by Chun Zhu, Shengqi Yang, Yuanyuan Pu, Lijun Sun, Min Wang and Kun Du
Energies 2023, 16(19), 6812; https://doi.org/10.3390/en16196812 - 26 Sep 2023
Viewed by 738
Abstract
Geological energy has a long history in China [...] Full article

2022

Jump to: 2023, 2021

20 pages, 5043 KiB  
Article
Evolution of Biomarker Maturity Parameters and Feedback to the Pyrolysis Process for In Situ Conversion of Nongan Oil Shale in Songliao Basin
by Hao Zeng, Wentong He, Lihong Yang, Jianzheng Su, Xianglong Meng, Xueqi Cen and Wei Guo
Energies 2022, 15(10), 3715; https://doi.org/10.3390/en15103715 - 19 May 2022
Cited by 7 | Viewed by 1897
Abstract
In the oil shale in situ conversion project, it is urgent to solve the problem that the reaction degree of organic matter cannot be determined. The yield and composition of organic products in each stage of the oil shale pyrolysis reaction change regularly, [...] Read more.
In the oil shale in situ conversion project, it is urgent to solve the problem that the reaction degree of organic matter cannot be determined. The yield and composition of organic products in each stage of the oil shale pyrolysis reaction change regularly, so it is very important to master the process of the pyrolysis reaction and reservoir change for oil shale in situ conversion project. In the in situ conversion project, it is difficult to directly obtain cores through drilling for kerogen maturity testing, and the research on judging the reaction process of subsurface pyrolysis based on the maturity of oil products has not been carried out in-depth. The simulation experiments and geochemical analysis carried out in this study are based on the oil shale of the Nenjiang Formation in the Songliao Basin and the pyrolysis oil samples produced by the in situ conversion project. Additionally, this study aims to clarify the evolution characteristics of maturity parameters such as effective biomarker compounds during the evolution of oil shale pyrolysis hydrocarbon products and fit it with the kerogen maturity in the Nenjiang formation. The response relationship with the pyrolysis process of oil shale is established, and it lays a theoretical foundation for the efficient, economical and stable operation of oil shale in situ conversion projects. Full article
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14 pages, 3223 KiB  
Article
Analysis of Outburst Coal Structure Characteristics in Sanjia Coal Mine Based on FTIR and XRD
by Anjun Jiao, Shixiang Tian and Huaying Lin
Energies 2022, 15(6), 1956; https://doi.org/10.3390/en15061956 - 8 Mar 2022
Cited by 16 | Viewed by 2572
Abstract
In order to reveal the distribution characteristics of functional groups and the difference of microcrystalline structure parameters between outburst coal and primary coal, the coal samples inside and outside the outburst holes of the Sanjia coal mine were examined. The functional groups and [...] Read more.
In order to reveal the distribution characteristics of functional groups and the difference of microcrystalline structure parameters between outburst coal and primary coal, the coal samples inside and outside the outburst holes of the Sanjia coal mine were examined. The functional groups and microcrystalline structure parameters of outburst coal and primary coal in the Sanjia coal mine were studied by infrared spectroscopy, X-ray diffraction (XRD) experiment and peak-splitting fitting method. The results showed that the substitution mode of the benzene ring in an aromatic structure was mainly benzene ring tri-substituted, with primary coal of 32.71% and outburst coal of 31.6%. The primary coal contained more functional groups, from which hydrogen bonds can easily be formed, meaning that gas is not easily adsorbed by coal. The aromatic hydrogen rate (fHa) of the outburst coal was 0.271, the aromatic carbon rate (ƒC) was 0.986, the aromaticity I1 was 0.477, I2 was 0.373 and the length of the aliphatic branched chain (ACH2/ACH3) was 0.850. Compared with the primary coal, the aromatic hydrogen rate, aromatic carbon rate and the aromaticity of the outburst coal were higher, indicating that the hydrogen and carbon elements in the aromatic functional groups of outburst coal were higher and that the aliphatic functional group was higher than the aromatic structural functional group. ACH2/ACH3 and maturity (Csd) were slightly lower than those of primary coal, indicating that the coal has more straight chains than side chains, while aliphatic hydrocarbons are mostly short chains and have high branched degree. There were obvious 002 and 100 peaks in the XRD pattern. The d002 and d100 of outburst coal were 3.570 and 2.114, respectively, while the number of effective stacking aromatics was 3.089, which was lower than that of primary coal, indicating that the structure of the dense ring in the coal saw certain changes. Full article
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13 pages, 3056 KiB  
Article
Stable Crack Propagation Model of Rock Based on Crack Strain
by Xiao Huang, Chong Shi, Huaining Ruan, Yiping Zhang and Wei Zhao
Energies 2022, 15(5), 1885; https://doi.org/10.3390/en15051885 - 3 Mar 2022
Cited by 15 | Viewed by 2858
Abstract
The establishment of a rock constitutive model considering microcrack propagation characteristics is an important channel to reflect the progressive damage and failure of rocks. The prepeak crack strain evolution curve of rock is divided into three stages based on the triaxial compression test [...] Read more.
The establishment of a rock constitutive model considering microcrack propagation characteristics is an important channel to reflect the progressive damage and failure of rocks. The prepeak crack strain evolution curve of rock is divided into three stages based on the triaxial compression test results of granite and the definition of crack strain. According to the nonlinear variation characteristics of crack strain in the stage of rock crack stable propagation, rock deformation is expressed as the sum of matrix strain and crack strain. Then, the exponential constitutive relationship of rock crack stable propagation is deduced. The axial crack strains of the rock sample and its longitudinal section are equal. Thus, the longitudinal symmetry plane of the rock sample is abstracted as a model containing sliding crack structure in an elastic body, and the evolution equation of crack geometric parameters in the process of stable crack propagation is obtained. Compared with the experimental data, results show that the rock crack stable propagation model based on crack strain can adequately describe the evolution law of crack strain and wing crack length. In addition, the wing crack propagates easily when the elastic body with small width contains an initial crack with a large length and an axial dip angle of 45° under compressive load. This study provides a new idea for the analysis of the stable propagation characteristics and laws of rock cracks under compressive load. Full article
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22 pages, 14685 KiB  
Article
Seismic-Geological Integrated Study on Sedimentary Evolution and Peat Accumulation Regularity of the Shanxi Formation in Xinjing Mining Area, Qinshui Basin
by Bo Liu, Suoliang Chang, Sheng Zhang, Yanrong Li, Zhihua Yang, Zuiliang Liu and Qiang Chen
Energies 2022, 15(5), 1851; https://doi.org/10.3390/en15051851 - 2 Mar 2022
Cited by 5 | Viewed by 2221
Abstract
Accurate identification of the lithofacies and sedimentary facies of coal-bearing series is significant in the study of peat accumulation, coal thickness variation and coal-measured unconventional gas. This research integrated core, logging and 3D seismic data to conduct a comprehensive seismic–geological study on the [...] Read more.
Accurate identification of the lithofacies and sedimentary facies of coal-bearing series is significant in the study of peat accumulation, coal thickness variation and coal-measured unconventional gas. This research integrated core, logging and 3D seismic data to conduct a comprehensive seismic–geological study on the sedimentary evolution characteristics and peat accumulation regularity of the Shanxi Formation in the Xinjing mining area of the Qinshui Basin. Firstly, the high-resolution sequence interface was identified, and the isochronous stratigraphic framework of the coal-bearing series was constructed. Then, the temporal and spatial evolution of sedimentary filling and sedimentary facies was dynamically analyzed using waveform clustering, phase rotation, stratal slice and frequency–division amplitude fusion methods. The results show that the Shanxi Formation in the study area can be divided into one third-order sequence and two fourth-order sequences. It developed a river-dominated deltaic system, mainly with delta plain deposits, and underwent a constructive–abandoned–constructive development stage. The locally distributed No. 6 coal seam was formed in a backswamp environment with distribution constrained by the distributary channels. The delta was abandoned at the later stage of the SS1 sequence, and the peat accumulation rate was balanced with the growth rate of the accommodation, forming a large-area distributed No. 3 thick coal seam. During the formation of the SS2 sequence, the No. 3 coal seam was locally thinned by epigenetic erosion of the river, and the thin coal belt caused by erosion is controlled by the location of the distributary channels and their extension direction. This study can provide a reference for the research on the distribution of thin sand bodies, sedimentary evolution and peat accumulation regularity in the coal-bearing series under the marine–continental transitional environment. Full article
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20 pages, 5491 KiB  
Article
Numerical Study and Force Chain Network Analysis of Sand Production Process Using Coupled LBM-DEM
by Tian Xia, Qihong Feng, Sen Wang, Jiyuan Zhang, Wei Zhang and Xianmin Zhang
Energies 2022, 15(5), 1788; https://doi.org/10.3390/en15051788 - 28 Feb 2022
Cited by 4 | Viewed by 1869
Abstract
Sand production has caused many serious problems in weakly consolidated reservoirs. Therefore, it is very urgent to find out the mechanism for this process. This paper employs a coupled lattice Boltzmann method and discrete element method (LBM-DEM) to study the sand production process [...] Read more.
Sand production has caused many serious problems in weakly consolidated reservoirs. Therefore, it is very urgent to find out the mechanism for this process. This paper employs a coupled lattice Boltzmann method and discrete element method (LBM-DEM) to study the sand production process of the porous media. Simulation of the sand production process is conducted and the force chain network evolvement is analyzed. Absolute and relative permeability changes before and after the sand production process are studied. The effect of injection flow rate, cementation strength, and confining pressure are investigated. During the simulation, strong force chain rupture and force chain reorganization can be identified. The mean shortest-path distance of the porous media reduces gradually after an initial sharp decrease while the mean degree and clustering coefficient increase in the same way. Furthermore, the degree of preferential wettability for water increases after the sand production process. Moreover, a critical flow rate below which porous media can reach a steady state exists. Results also show that porous media under higher confining pressure will be more stable due to the higher friction resistance between particles to prevent sand production. Full article
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21 pages, 4682 KiB  
Article
Establishment and Solution of Four Variable Water Hammer Mathematical Model for Conveying Pipe
by Jiehao Duan, Changjun Li and Jin Jin
Energies 2022, 15(4), 1387; https://doi.org/10.3390/en15041387 - 14 Feb 2022
Cited by 3 | Viewed by 2204
Abstract
Transient flow in pipe is a much debated topic in the field of hydrodynamics. The water hammer effect caused by instantaneous valve closing is an important branch of transient flow. At present, the fluid density is regarded as a constant in the study [...] Read more.
Transient flow in pipe is a much debated topic in the field of hydrodynamics. The water hammer effect caused by instantaneous valve closing is an important branch of transient flow. At present, the fluid density is regarded as a constant in the study of the water hammer effect in pipe. When there is gas in the pipe, the variation range of density is large, and the pressure-wave velocity should also change continuously along the pipe. This study considers the interaction between pipeline fluid motion and water hammer wave propagation based on the essence of water hammer, with the pressure, velocity, density and overflow area set as variables. A new set of water hammer calculation equations was deduced and solved numerically. The effects of different valve closing time, flow rate and gas content on pressure distribution and the water hammer effect were studied. It was found that with the increase in valve closing time, the maximum fluctuating pressure at the pipe end decreased, and the time of peak value also lagged behind. When the valve closing time increased from 5 s to 25 s, the difference in water hammer pressure was 0.72 MPa, and the difference in velocity fluctuation amplitude was 0.076 m/s. The findings confirm: the greater the flow, the greater the pressure change at the pipe end; the faster the speed change, the more obvious the water hammer effect. High-volume flows were greatly disturbed by instantaneous obstacles such as valve closing. With the increase of time, the pressure fluctuation gradually attenuated along the pipe length. The place with the greatest water hammer effect was near the valve. Under the coupling effect of time and tube length, the shorter the time and the shorter the tube length, the more obvious the pressure fluctuation. Findings also confirm: the larger the gas content, the smaller the fluctuation peak of pipe end pressure; the longer the water hammer cycle, the smaller the pressure-wave velocity. The actual pressure fluctuation value was obviously lower than that without gas, and the size of the pressure wave mainly depended on the gas content. When the gas content increased from 1% to 9%, the difference of water hammer pressure was 0.41 MPa. Full article
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16 pages, 6549 KiB  
Article
Modal Analysis of Tubing Considering the Effect of Fluid–Structure Interaction
by Jiehao Duan, Changjun Li and Jin Jin
Energies 2022, 15(2), 670; https://doi.org/10.3390/en15020670 - 17 Jan 2022
Cited by 12 | Viewed by 2848
Abstract
When tubing is in a high-temperature and high-pressure environment, it will be affected by the impact of non-constant fluid and other dynamic loads, which will easily cause the tubing to vibrate or even resonate, affecting the integrity of the wellbore and safe production. [...] Read more.
When tubing is in a high-temperature and high-pressure environment, it will be affected by the impact of non-constant fluid and other dynamic loads, which will easily cause the tubing to vibrate or even resonate, affecting the integrity of the wellbore and safe production. In the structural modal analysis of the tubing, the coupling effect of the fluid and the tubing needs to be considered at the same time. In this paper, a single tubing is taken as an example to simulate and analyze the modal changes of the tubing under dry mode and wet mode respectively, and the effects of fluid solid coupling effect, inlet pressure, and ambient temperature on the modal of the tubing are discussed. After considering the fluid–structure interaction effect, the natural frequency of tubing decreases, but the displacement is slightly larger. The greater the pressure in the tubing, the greater the equivalent stress on the tubing body, so the natural frequency is lower. Furthermore, after considering the fluid–solid coupling effect, the pressure in the tubing is the true pulsating pressure of the fluid. The prestress applied to the tubing wall changes with time, and the pressures at different parts are different. At this time, the tubing is changed at different frequencies. Vibration is prone to occur, that is, the natural frequency is smaller than the dry mode. The higher the temperature, the lower the rigidity of the tubing and the faster the strength attenuation, so the natural frequency is lower, and tubing is more prone to vibration. Both the stress intensity and the elastic strain increase with the increase of temperature, so the displacement of the tubing also increases. Full article
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2021

Jump to: 2023, 2022

17 pages, 10474 KiB  
Article
Velocity Structure Revealing a Likely Mud Volcano off the Dongsha Island, the Northern South China Sea
by Yuning Yan, Jianping Liao, Junhui Yu, Changliang Chen, Guangjian Zhong, Yanlin Wang and Lixin Wang
Energies 2022, 15(1), 195; https://doi.org/10.3390/en15010195 - 28 Dec 2021
Cited by 11 | Viewed by 2478
Abstract
The Dongsha Island (DS) is located in the mid-northern South China Sea continental margin. The waters around it are underlain by the Chaoshan Depression, a relict Mesozoic sedimentary basin, blanketed by thin Cenozoic sediments but populated with numerous submarine hills with yet less-known [...] Read more.
The Dongsha Island (DS) is located in the mid-northern South China Sea continental margin. The waters around it are underlain by the Chaoshan Depression, a relict Mesozoic sedimentary basin, blanketed by thin Cenozoic sediments but populated with numerous submarine hills with yet less-known nature. A large hill, H110, 300 m high, 10 km wide, appearing in the southeast to the Dongsha Island, is crossed by an ocean bottom seismic and multiple channel seismic surveying lines. The first arrival tomography, using ocean bottom seismic data, showed two obvious phenomena below it: (1) a low-velocity (3.3 to 4 km/s) zone, with size of 20 × 3 km2, centering at ~4.5 km depth and (2) an underlying high-velocity (5.5 to 6.3 km/s) zone of comparable size at ~7 km depth. MCS profiles show much-fragmented Cenozoic sequences, covering a wide chaotic reflection zone within the Mesozoic strata below hill H110. The low-velocity zone corresponds to the chaotic reflection zone and can be interpreted as of highly-fractured and fluid-rich Mesozoic layers. Samples dredged from H110 comprised of illite-bearing authigenic carbonate nodules and rich, deep-water organisms are indicative of hydrocarbon seepage from deep source. Therefore, H110 can be inferred as a mud volcano. The high-velocity zone is interpreted as of magma intrusion, considering that young magmatism was found enhanced over the southern CSD. Furthermore, the origin of H110 can be speculated as thermodynamically driven, i.e., magma from the depths intrudes into the thick Mesozoic strata and promotes petroleum generation, thus, driving mud volcanism. Mud volcanism at H110 and the occurrence of a low-velocity zone below it likely indicates the existence of Mesozoic hydrocarbon reservoir, which is in favor of the petroleum exploration. Full article
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22 pages, 6181 KiB  
Article
Prediction of Conformance Control Performance for Cyclic-Steam-Stimulated Horizontal Well Using the XGBoost: A Case Study in the Chunfeng Heavy Oil Reservoir
by Zehao Xie, Qihong Feng, Jiyuan Zhang, Xiaoxuan Shao, Xianmin Zhang and Zenglin Wang
Energies 2021, 14(23), 8161; https://doi.org/10.3390/en14238161 - 5 Dec 2021
Cited by 5 | Viewed by 2490
Abstract
Conformance control is an effective method to enhance heavy oil recovery for cyclic-steam-stimulated horizontal wells. The numerical simulation technique is frequently used prior to field applications to evaluate the incremental oil production with conformance control in order to ensure cost-efficiency. However, conventional numerical [...] Read more.
Conformance control is an effective method to enhance heavy oil recovery for cyclic-steam-stimulated horizontal wells. The numerical simulation technique is frequently used prior to field applications to evaluate the incremental oil production with conformance control in order to ensure cost-efficiency. However, conventional numerical simulations require the use of specific thermal numerical simulators that are usually expensive and computationally inefficient. This paper proposed the use of the extreme gradient boosting (XGBoost) trees to estimate the incremental oil production of conformance control with N2-foam and gel for cyclic-steam-stimulated horizontal wells. A database consisting of 1000 data points was constructed using numerical simulations based on the geological and fluid properties of the heavy oil reservoir in the Chunfeng Oilfield, which was then used for training and validating the XGBoost model. Results show that the XGBoost model is capable of estimating the incremental oil production with relatively high accuracy. The mean absolute errors (MAEs), mean relative errors (MRE) and correlation coefficients are 12.37/80.89 t, 0.09%/0.059% and 0.99/0.98 for the training/validation sets, respectively. The validity of the prediction model was further confirmed by comparison with numerical simulations for six real production wells in the Chunfeng Oilfield. The permutation indices (PI) based on the XGBoost model indicate that net to gross ratio (NTG) and the cumulative injection of the plugging agent exerts the most significant effects on the enhanced oil production. The proposed method can be easily transferred to other heavy oil reservoirs, provided efficient training data are available. Full article
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18 pages, 4122 KiB  
Article
A New Anelasticity Model for Wave Propagation in Partially Saturated Rocks
by Chunfang Wu, Jing Ba, Xiaoqin Zhong, José M. Carcione, Lin Zhang and Chuantong Ruan
Energies 2021, 14(22), 7619; https://doi.org/10.3390/en14227619 - 15 Nov 2021
Cited by 2 | Viewed by 1610
Abstract
Elastic wave propagation in partially saturated reservoir rocks induces fluid flow in multi-scale pore spaces, leading to wave anelasticity (velocity dispersion and attenuation). The propagation characteristics cannot be described by a single-scale flow-induced dissipation mechanism. To overcome this problem, we combine the White [...] Read more.
Elastic wave propagation in partially saturated reservoir rocks induces fluid flow in multi-scale pore spaces, leading to wave anelasticity (velocity dispersion and attenuation). The propagation characteristics cannot be described by a single-scale flow-induced dissipation mechanism. To overcome this problem, we combine the White patchy-saturation theory and the squirt flow model to obtain a new anelasticity theory for wave propagation. We consider a tight sandstone Qingyang area, Ordos Basin, and perform ultrasonic measurements at partial saturation and different confining pressures, where the rock properties are obtained at full-gas saturation. The comparison between the experimental data and the theoretical results yields a fairly good agreement, indicating the efficacy of the new theory. Full article
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19 pages, 10910 KiB  
Article
Creep Rupture and Permeability Evolution in High Temperature Heat-Treated Sandstone Containing Pre-Existing Twin Flaws
by Sheng-Qi Yang, Jin-Zhou Tang and Derek Elsworth
Energies 2021, 14(19), 6362; https://doi.org/10.3390/en14196362 - 5 Oct 2021
Cited by 8 | Viewed by 1982
Abstract
Utilizing underground coal gasification cavities for carbon capture and sequestration provides a potentially economic and sustainable solution to a vexing environmental and energy problem. The thermal influence on creep properties and long-term permeability evolution around the underground gasification chamber is a key issue [...] Read more.
Utilizing underground coal gasification cavities for carbon capture and sequestration provides a potentially economic and sustainable solution to a vexing environmental and energy problem. The thermal influence on creep properties and long-term permeability evolution around the underground gasification chamber is a key issue in UCG-CCS operation in containing fugitive emissions. We complete multi-step loading and unloading creep tests with permeability measurement at confining stresses of 30 MPa on pre-cracked sandstone specimens thermally heat-treated to 250, 500, 750 and 1000 °C. Observations indicate a critical threshold temperature of 500 °C required to initiate thermally-induced cracks with subsequent strength reduction occurring at 750 °C. Comparison of histories of creep, visco-elastic and visco-plastic strains highlight the existence of a strain jump at a certain deviatoric stress level—where the intervening rock bridge between the twin starter-cracks is eliminated. As the deviatoric stress level increases, the visco-plastic strains make up an important composition of total creep strain, especially for specimens pre-treated at higher temperatures, and the development of the visco-plastic strain leads to the time-dependent failure of the rock. The thermal pre-treatment produces thermal cracks with their closure resulting in increased instantaneous elastic strains and instantaneous plastic strains. With increasing stress ratio, the steady-state creep rates increase slowly before the failure stress ratio but rise suddenly over the final stress ratio to failure. However, the pre-treatment temperature has no clear and apparent influence on steady creep strain rates. Rock specimens subject to higher pre-treatment temperatures exhibit higher permeabilities. The pre-existing cracks close under compression with a coplanar shear crack propagating from the starter-cracks and ultimately linking these formerly separate cracks. In addition, it is clear that the specimens pre-treated at higher temperatures accommodate greater damage. Full article
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21 pages, 42283 KiB  
Article
Analysis of Natural Hydraulic Fracture Risk of Mudstone Cap Rocks in XD Block of Central Depression in Yinggehai Basin, South China Sea
by Ru Jia, Caiwei Fan, Bo Liu, Xiaofei Fu and Yejun Jin
Energies 2021, 14(14), 4085; https://doi.org/10.3390/en14144085 - 6 Jul 2021
Cited by 8 | Viewed by 2688
Abstract
The Yinggehai Basin is an important Cenozoic gas bearing basin in the South China Sea. With the gradual improvement of gas exploration and over-development in shallow layers, deep overpressured layers have become the main target for natural gas exploration. There are no large-scale [...] Read more.
The Yinggehai Basin is an important Cenozoic gas bearing basin in the South China Sea. With the gradual improvement of gas exploration and over-development in shallow layers, deep overpressured layers have become the main target for natural gas exploration. There are no large-scale faults in the strata above the Meishan Formation in the central depression, and hydraulic fracturing caused by overpressure in mudstone cap rocks is the key factor for the vertical differential distribution of gas. In this paper, based on the leak-off data, pore fluid pressure, and rock mechanics parameters, the Fault Analysis Seal Technology (FAST) method is used to analyze the hydraulic fracture risk of the main mudstones in the central depression. The results show that the blocks in the diapir zone have been subjected to hydraulic fracturing in the Huangliu cap rocks during the whole geological history, and the blocks in the slope zone which is a little distant from the diapirs has a lower overall risk of hydraulic fracture than the diapir zone. In geological history, the cap rocks in slope zone remained closed for a longer time than in diapir zone and being characterized by the hydraulic fracture risk decreases with the distance from the diapirs. These evaluation results are consistent with enrichment of natural gas, which accumulated in both the Yinggehai Formation and Huangliu Formation of the diapir zone, but it only accumulated in the the Huangliu Formations of the slope zone. The most reasonable explanation for the difference of the gas reservoir distribution is that the diapirs promote the development of hydraulic fractures: (1) diapirism transfers deep overpressure to shallow layers; (2) the small fault and fractures induced by diapir activities weakened the cap rock and reduced the critical condition for the natural hydraulic fractures. These effects make the diapir zone more prone to hydraulic fracturing, which are the fundamental reasons for the difference in gas enrichment between the diapir zone and the slope zone. Full article
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14 pages, 5131 KiB  
Article
Study on Asymmetric Failure and Control Measures of Lining in Deep Large Section Chamber
by Yubing Huang, Bei Jiang, Yukun Ma, Huayong Wei, Jincheng Zang and Xiang Gao
Energies 2021, 14(14), 4075; https://doi.org/10.3390/en14144075 - 6 Jul 2021
Cited by 3 | Viewed by 1826
Abstract
Lining is often used as the last line of defense in deep large section chamber. Under the asymmetric load, it is easy to damage, resulting in the overall repair of the chamber. Aiming at this problem, taking the pump house in Wanfu Coal [...] Read more.
Lining is often used as the last line of defense in deep large section chamber. Under the asymmetric load, it is easy to damage, resulting in the overall repair of the chamber. Aiming at this problem, taking the pump house in Wanfu Coal Mine under construction in China as an engineering example, we analyzed the asymmetric failure of pump house lining caused by construction disturbance, established the lining mechanical model and quantitative evaluation indexes, such as bending moment change rate, bending moment balance rate, displacement change rate and displacement balance rate, studied the influence mechanism of asymmetrical coefficient, section size and lining thickness on the mechanical behavior of lining, and proposed the control measures of deep large section chamber with the core of “strengthening asymmetric support, reducing section size and improving lining strength”. The field monitoring shows that the asymmetric deformation of the pump house is effectively controlled, and the maximum displacement is only 7.3 mm, which ensures the long-term stability of the chamber. Full article
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13 pages, 2909 KiB  
Article
Characteristics of Airflow Reversal of Excavation Roadway after a Coal and Gas Outburst Accident
by Junhong Si, Lin Li, Jianwei Cheng, Yiqiao Wang, Wei Hu, Tan Li and Zequan Li
Energies 2021, 14(12), 3645; https://doi.org/10.3390/en14123645 - 18 Jun 2021
Cited by 9 | Viewed by 1802
Abstract
Determining the influence scope of the airflow disorder is an important problem after coal and gas outburst accidents in ventilation systems. This paper puts forward the indexes of airflow disorder, including the length of the excavation roadway, the outburst pressure, the pressure difference [...] Read more.
Determining the influence scope of the airflow disorder is an important problem after coal and gas outburst accidents in ventilation systems. This paper puts forward the indexes of airflow disorder, including the length of the excavation roadway, the outburst pressure, the pressure difference of the air door, and the air quantity of the auxiliary fan. Using the orthogonal table of L9 (34) and numerical simulation method, the characteristics of airflow reversal are studied, and the outburst airflow reversal degree is calculated should the ventilation facility fail. Furthermore, on the basis of fuzzy comprehensive optimization theory, the comprehensive evaluation model of the airflow disorder is established. The results show that the length of the excavation roadway is the most important factor affecting the stability of the ventilation system, followed by the outburst pressure, pressure difference of the air door, and air quantity of the auxiliary fan. The influence of a gas outburst accident on the return air system is greater than that on the inlet air system, and a larger air velocity has a greater impact on the ventilation system, especially the air inlet part. Moreover, the airflow reversal degree of the ventilation system increases with the increase of the outburst pressure or decreases with the length of the excavation roadway. This paper provides a basis for the prevention of gas outburst accidents. Full article
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