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Special Issue "Unconventional Natural Gas (UNG) Recoveries"

A special issue of Energies (ISSN 1996-1073).

Deadline for manuscript submissions: closed (15 December 2016)

Special Issue Editor

Guest Editor
Prof. Ranjith Pathegama Gamage

Department of Civil Engineering, Monash University, Victoria 3800, Australia
Website1 | Website2 | E-Mail
Interests: geomechanics; CO2 sequestration; shale gas; coal seam gas; hydro-fracturing; stimulations

Special Issue Information

Dear Colleagues,

Our energy resources are crucial; but they are under pressure from growing populations, urbanisation, and economic prosperity. By 2040, global demand for energy will increase by 40% over present levels. By then, it is forecast that approximately 75% of energy demand will still be met by fossil fuels—but oil and coal will have lessened in importance due to tight regulation and advances in more environmentally friendly alternatives. Which newer fossil fuel will emerge to dominate in the next few decades? There is one clear answer: Natural gas

Despite the challenges, climate change legislation and regulation to limit greenhouse gases have already allowed natural gas to compete favourably with oil and coal. It is now ranked third among the world’s major energy sources; and it is the cleanest and the richest in hydrocarbons, offering high energy-conversion efficiencies for power generation.

Conventional gas is easy to harvest. It has been produced across the world in the last few decades, so its reserves are now rapidly depleting. In contrast, the earth holds huge untapped reserves of unconventional—but locked within compact rocks, such as shale, coal seams, and tight sandstones.

The greatest challenge for exploitation of unconventional natural gas (UNG) is very low recovery rates, because of low permeability in its deep reservoirs. Reducing the complexity in exploration targets, using suitable exploration and production technologies (such as new stimulation technologies to prompt release of the gas), has potential to release vast quantities of UNG from highly impermeable format­ions. However, unique conditions in UNG reservoirs call for innovative technologies founded securely on new science. This proposal calls for papers in the areas of new sciences developed to enhance the recovery process of gas from coal seams, shale, and tight formations. 

We will therefore especially welcome submissions on the following topics:

  • Gas flow and diffusions of coal seams, shales, tight gas reservoirs
  • Techniques to enhance gas recoveries, such as hydro fracking and innovations in well drilling
  • Caprock integrity and associated environmental issues
  • Coupled hydro-chemico-mechanical processes
  • Reservoir geomechanics, and wellbore and drilling mechanics
  • Constitutive modelling and numerical methods
  • Numerical modelling of THM
  • Adsorption/desorption characterises of rocks
  • Case studies of international interest
  • Any other relevant research areas in Unconventional Natural Gas domain

Prof. Ranjith Pathegama Gamage
Gues Editor

Manuscript Submission Information

Manuscripts should be submitted online at www.mdpi.com by registering and logging in to this website. Once you are registered, click here to go to the submission form. Manuscripts can be submitted until the deadline. All papers will be peer-reviewed. Accepted papers will be published continuously in the journal (as soon as accepted) and will be listed together on the special issue website. Research articles, review articles as well as short communications are invited. For planned papers, a title and short abstract (about 100 words) can be sent to the Editorial Office for announcement on this website.

Submitted manuscripts should not have been published previously, nor be under consideration for publication elsewhere (except conference proceedings papers). All manuscripts are thoroughly refereed through a single-blind peer-review process. A guide for authors and other relevant information for submission of manuscripts is available on the Instructions for Authors page. Energies is an international peer-reviewed open access monthly journal published by MDPI.

Please visit the Instructions for Authors page before submitting a manuscript. The Article Processing Charge (APC) for publication in this open access journal is 1500 CHF (Swiss Francs). Submitted papers should be well formatted and use good English. Authors may use MDPI's English editing service prior to publication or during author revisions.

Keywords

  • shale gas
  • coal seams gas
  • tight gas
  • geomechanics
  • adsorptions
  • desorption
  • stimulations  

Published Papers (7 papers)

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Research

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Open AccessArticle Failure Mechanical Behavior of Australian Strathbogie Granite at High Temperatures: Insights from Particle Flow Modeling
Energies 2017, 10(6), 756; doi:10.3390/en10060756
Received: 30 November 2016 / Revised: 4 May 2017 / Accepted: 15 May 2017 / Published: 28 May 2017
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Abstract
Thermally induced damage has an important influence on rock mechanics and engineering, especially for high-level radioactive waste repositories, geological carbon storage, underground coal gasification, and hydrothermal systems. Additionally, the wide application of geothermal heat requires knowledge of the geothermal conditions of reservoir rocks
[...] Read more.
Thermally induced damage has an important influence on rock mechanics and engineering, especially for high-level radioactive waste repositories, geological carbon storage, underground coal gasification, and hydrothermal systems. Additionally, the wide application of geothermal heat requires knowledge of the geothermal conditions of reservoir rocks at elevated temperature. However, few methods to date have been reported for investigating the micro-mechanics of specimens at elevated temperatures. Therefore, this paper uses a cluster model in particle flow code in two dimensions (PFC2D) to simulate the uniaxial compressive testing of Australian Strathbogie granite at various elevated temperatures. The peak strength and ultimate failure mode of the granite specimens at different elevated temperatures obtained by the numerical methods are consistent with those obtained by experimentation. Since the tensile force is always concentrated around the boundary of the crystal, cracks easily occur at the intergranular contacts, especially between the b-b and b-k boundaries where less intragranular contact is observed. The intergranular and intragranular cracking of the specimens is almost constant with increasing temperature at low temperature, and then it rapidly and linearly increases. However, the inflection point of intergranular micro-cracking is less than that of intragranular cracking. Intergranular cracking is more easily induced by a high temperature than intragranular cracking. At an elevated temperature, the cumulative micro-crack counts curve propagates in a stable way during the active period, and it has no unstable crack propagation stage. The micro-cracks and parallel bond forces in the specimens with elevated temperature evolution and axial strain have different characteristics than those at lower temperature. More branch fractures and isolated wider micro-cracks are generated with increasing temperature when the temperature is over 400 °C. Therefore, the total number of cracks is almost constant when the temperature is below 400 °C; next, it linearly increases when the temperature is over 400 °C. This trend is the same as that observed by experimentation. Full article
(This article belongs to the Special Issue Unconventional Natural Gas (UNG) Recoveries)
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Open AccessArticle Methane Adsorption Rate and Diffusion Characteristics in Marine Shale Samples from Yangtze Platform, South China
Energies 2017, 10(5), 626; doi:10.3390/en10050626
Received: 6 January 2017 / Revised: 13 April 2017 / Accepted: 21 April 2017 / Published: 4 May 2017
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Abstract
Knowledge of the gas adsorption rate and diffusion characteristics in shale are very important to evaluate the gas transport properties. However, research on methane adsorption rate characteristics and diffusion behavior in shale is not well established. In this study, high-pressure methane adsorption isotherms
[...] Read more.
Knowledge of the gas adsorption rate and diffusion characteristics in shale are very important to evaluate the gas transport properties. However, research on methane adsorption rate characteristics and diffusion behavior in shale is not well established. In this study, high-pressure methane adsorption isotherms and methane adsorption rate data from four marine shale samples were obtained by recording the pressure changes against time at 1-s intervals for 12 pressure steps. Seven pressure steps were selected for modelling, and three pressure steps of low (~0.4 MPa), medium (~4.0 MPa), and high (~7.0 MPa) were selected for display. According to the results of study, the methane adsorption under low pressure attained equilibrium much more quickly than that under medium and high pressure, and the adsorption rate behavior varied between different pressure steps. By fitting the diffusion models to the methane adsorption rate data, the unipore diffusion model based upon unimodal pore size distribution failed to describe the methane adsorption rate, while the bidisperse diffusion model could reasonably describe most of the experimental adsorption rate data, with the exception of sample YY2-1 at high pressure steps. This phenomenon may be related to the restricted assumption on pore size distribution and linear adsorption isotherm. The diffusion parameters α and β/α obtained from the bidisperse model indicated that both macro- and micropore diffusion controlled the methane adsorption rate in shale samples, as well as the relative importance and influence of micropore diffusion and adsorption to adsorption rate and total adsorption increased with increasing pressure. This made the inflection points, or two-stage process, at higher pressure steps not as evident as at low pressure steps, and the adsorption rate curves became less steep with increasing pressure. This conclusion was also supported by the decreasing difference values with increasing pressures between macro- and micropore diffusivities obtained using the bidisperse model, which is roughly from 10−3 to 100, and 10−3 to 10−1, respectively. Additionally, an evident negative correlation between macropore diffusivities and pressure lower than 3–4 MPa was observed, while the micropore diffusivities only showed a gentle decreasing trend with pressure. A mirror image relationship between the variation in the value of macropore diffusivity and adsorption isotherms was observed, indicating the negative correlation between surface coverage and gas diffusivity. The negative correlation of methane diffusivity with pressure and surface coverage may be related to the increasing degree of pore blockage and the decreasing concentration gradient of methane adsorption. Finally, due to the significant deviation between the unipore model and experimental adsorption rate data, a new estimation method based upon the bidisperse model is proposed here. Full article
(This article belongs to the Special Issue Unconventional Natural Gas (UNG) Recoveries)
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Open AccessArticle Exploring the Micromechanical Sliding Behavior of Typical Quartz Grains and Completely Decomposed Volcanic Granules Subjected to Repeating Shearing
Energies 2017, 10(3), 370; doi:10.3390/en10030370
Received: 15 January 2017 / Revised: 12 March 2017 / Accepted: 13 March 2017 / Published: 15 March 2017
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Abstract
The micromechanical behavior at grain contacts subjected to tangential and normal forces is of major importance in geotechnical engineering research and practice. The development of the discrete element method (DEM) over the past three decades necessitated a more systematic study on the experimental
[...] Read more.
The micromechanical behavior at grain contacts subjected to tangential and normal forces is of major importance in geotechnical engineering research and practice. The development of the discrete element method (DEM) over the past three decades necessitated a more systematic study on the experimental grain contact behavior of real soil grains as DEM simulations use as input tangential and normal load–displacement relationships at grain contacts. In this study, experimental results conducted at the City University of Hong Kong are presented exploring the tangential load–displacement behavior of geological materials. The focus of the study is to explore the possible effect of repeating the shearing test to the same grains on the inter-particle coefficient of friction accounting for the level of the applied normal load. Additionally, the study reports on the frictional behavior of different geological materials including quartz sand grains, denoted as Leighton Buzzard sand (LBS) in the study and completely decomposed volcanic granules denoted as CDV. Quartz grains may find applications as proppant in petroleum engineering, whilst the CDV granules consisted of a material taken from a recent landslide in Hong Kong, whose applications are related to debris flow. Through the micromechanical sliding experiments, the inter-particle coefficient of friction was quantified following shearing paths of about 60 to 200 microns. While at the smallest vertical load of 1 N, there was not observed a notable effect of the repeating shearing for the LBS grains, it was noticed that for small to medium vertical loads, between 2 and 5 N, the repeating shearing reduced the friction at the contacts of the LBS grains. This trend was clear between the first and second shearing, but additional cycles did not further alter the frictional response. However, at greater vertical loads, between 7 and 10 N, the results showed a continuous increase in the dynamic inter-particle friction for the LBS grains with repeating shearing. It was also noticed that at 7 and 10 N of vertical load, there was absence of a peak state in the tangential force–displacement plot, whereas a peak state was observed at smaller loads particularly for the first shearing cycle. These observations might be explained by the possible plowing effects at greater vertical loads which resulted in an increase of the inter-particle coefficient of friction when the shearing test was repeated. For the CDV granules, only the first shearing cycle gave a peak state and, in general, the effect of repeating the shearing was small but with an increase of the inter-particle friction from the first to the second cycle. Overall, during the repeating shearing the LBS grains had a dynamic inter-particle coefficient of friction that ranged between about 0.18 and 0.38, but the CDV granules exhibited much greater friction with values that corresponded to the steady state sliding that ranged between 0.54 and 0.66 . The observed trends in the study might be due to mechanisms that take place at the atomic level and the possible more pronounced distortion of the surfaces for the CDV granules which are much softer than the LBS grains. Full article
(This article belongs to the Special Issue Unconventional Natural Gas (UNG) Recoveries)
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Open AccessArticle Study on the Progressive Failure Characteristics of Longmaxi Shale under Uniaxial Compression Conditions by X-ray Micro-Computed Tomography
Energies 2017, 10(3), 303; doi:10.3390/en10030303
Received: 12 January 2017 / Accepted: 28 February 2017 / Published: 3 March 2017
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Abstract
To investigate the deformation-failure process of Longmaxi shale under uniaxial compression conditions from the mesoscopic and macroscopic points of view, novel X-ray microComputed Tomography (micro-CT) equipment combined with unique loading apparatus was used. Cylindrical shale samples (4 mm in diameter and 8 mm
[...] Read more.
To investigate the deformation-failure process of Longmaxi shale under uniaxial compression conditions from the mesoscopic and macroscopic points of view, novel X-ray microComputed Tomography (micro-CT) equipment combined with unique loading apparatus was used. Cylindrical shale samples (4 mm in diameter and 8 mm in height) were produced to perform a series of uniaxial compression tests. CT scanning images at different time points during the loading process were obtained to study the characteristics of the progressive failure. In addition, stereograms were reconstructed and vertical slices were selected to explain the failure mechanism. From the results of the testing the low-density area, local per-peak cracks, numerous post-peak cracks and secondary cracks consecutively appeared in the CT images. Vertical and inclined fissures in the samples could be observed from the stereograms’ surfaces and from internal slices. The cracking indicates that the failure process of shale is progressive and the failure mechanism of shale under uniaxial compression is mainly tension destruction or comprehensive tension-shear destruction. Full article
(This article belongs to the Special Issue Unconventional Natural Gas (UNG) Recoveries)
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Open AccessArticle Influence of Water Saturation on the Mechanical Behaviour of Low-Permeability Reservoir Rocks
Energies 2017, 10(2), 236; doi:10.3390/en10020236
Received: 5 October 2016 / Revised: 24 December 2016 / Accepted: 9 January 2017 / Published: 16 February 2017
Cited by 1 | PDF Full-text (6044 KB) | HTML Full-text | XML Full-text
Abstract
The influence of water on the mechanical properties of rocks has been observed by many researchers in rock engineering and laboratory tests, especially for sedimentary rocks. In order to investigate the effect of water saturation on the mechanical properties of low-permeability rocks, uniaxial
[...] Read more.
The influence of water on the mechanical properties of rocks has been observed by many researchers in rock engineering and laboratory tests, especially for sedimentary rocks. In order to investigate the effect of water saturation on the mechanical properties of low-permeability rocks, uniaxial compression tests were conducted on siltstone with different water contents. The effects of water on the strength, elastic moduli, crack initiation and damage thresholds were observed for different water saturation levels. It was found that 10% water saturation level caused more than half of the reductions in mechanical properties. A new approach is proposed to analyze the stress-strain relations at different stages of compression by dividing the axial and lateral stress-strain curves into five equal stress zones, where stress zones 1–5 refer to 0%–20%, 20%–40%, 40%–60%, 60%–80% and 80%–100% of the peak stress, respectively. Stress zone 2 represents the elastic range better than stress zone 3 which is at half of the peak stress. The normalized crack initiation and crack damage stress thresholds obtained from the stress-strain curves and acoustic emission activities averaged 31.5% and 83% of the peak strength respectively. Pore pressure is inferred to take part in the deformation of low-permeability siltstone samples, especially at full saturation levels. A change of failure pattern from multi-fracturing to single shear failure with the increase of water saturation level was also observed. Full article
(This article belongs to the Special Issue Unconventional Natural Gas (UNG) Recoveries)
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Open AccessArticle Numerical Modeling and Investigation of Fluid-Driven Fracture Propagation in Reservoirs Based on a Modified Fluid-Mechanically Coupled Model in Two-Dimensional Particle Flow Code
Energies 2016, 9(9), 699; doi:10.3390/en9090699
Received: 17 June 2016 / Revised: 4 August 2016 / Accepted: 22 August 2016 / Published: 2 September 2016
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Abstract
Hydraulic fracturing is a useful tool for enhancing rock mass permeability for shale gas development, enhanced geothermal systems, and geological carbon sequestration by the high-pressure injection of a fracturing fluid into tight reservoir rocks. Although significant advances have been made in hydraulic fracturing
[...] Read more.
Hydraulic fracturing is a useful tool for enhancing rock mass permeability for shale gas development, enhanced geothermal systems, and geological carbon sequestration by the high-pressure injection of a fracturing fluid into tight reservoir rocks. Although significant advances have been made in hydraulic fracturing theory, experiments, and numerical modeling, when it comes to the complexity of geological conditions knowledge is still limited. Mechanisms of fluid injection-induced fracture initiation and propagation should be better understood to take full advantage of hydraulic fracturing. This paper presents the development and application of discrete particle modeling based on two-dimensional particle flow code (PFC2D). Firstly, it is shown that the modeled value of the breakdown pressure for the hydraulic fracturing process is approximately equal to analytically calculated values under varied in situ stress conditions. Furthermore, a series of simulations for hydraulic fracturing in competent rock was performed to examine the influence of the in situ stress ratio, fluid injection rate, and fluid viscosity on the borehole pressure history, the geometry of hydraulic fractures, and the pore-pressure field, respectively. It was found that the hydraulic fractures in an isotropic medium always propagate parallel to the orientation of the maximum principal stress. When a high fluid injection rate is used, higher breakdown pressure is needed for fracture propagation and complex geometries of fractures can develop. When a low viscosity fluid is used, fluid can more easily penetrate from the borehole into the surrounding rock, which causes a reduction of the effective stress and leads to a lower breakdown pressure. Moreover, the geometry of the fractures is not particularly sensitive to the fluid viscosity in the approximate isotropic model. Full article
(This article belongs to the Special Issue Unconventional Natural Gas (UNG) Recoveries)
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Review

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Open AccessReview An Alternative to Conventional Rock Fragmentation Methods Using SCDA: A Review
Energies 2016, 9(11), 958; doi:10.3390/en9110958
Received: 9 October 2016 / Revised: 3 November 2016 / Accepted: 10 November 2016 / Published: 17 November 2016
Cited by 2 | PDF Full-text (4378 KB) | HTML Full-text | XML Full-text
Abstract
Global energy and material consumption are expected to rise in exponential proportions during the next few decades, generating huge demands for deep earth energy (oil/gas) recovery and mineral processing. Under such circumstances, the continuation of existing methods in rock fragmentation in such applications
[...] Read more.
Global energy and material consumption are expected to rise in exponential proportions during the next few decades, generating huge demands for deep earth energy (oil/gas) recovery and mineral processing. Under such circumstances, the continuation of existing methods in rock fragmentation in such applications is questionable due to the proven adverse environmental impacts associated with them. In this regard; the possibility of using more environmentally friendly options as Soundless Chemical Demolition Agents (SCDAs) play a vital role in replacing harmful conventional rock fragmentation techniques for gas; oil and mineral recovery. This study reviews up to date research on soundless cracking demolition agent (SCDA) application on rock fracturing including its limitations and strengths, possible applications in the petroleum industry and the possibility of using existing rock fragmentation models for SCDA-based rock fragmentation; also known as fracking. Though the expansive properties of SCDAs are currently used in some demolition works, the poor usage guidelines available reflect the insufficient research carried out on its material’s behavior. SCDA is a cementitious powdery substance with quicklime (CaO) as its primary ingredient that expands upon contact with water; which results in a huge expansive pressure if this CaO hydration reaction occurs in a confined condition. So, the mechanism can be used for rock fragmentation by injecting the SCDA into boreholes of a rock mass; where the resulting expansive pressure is sufficient to create an effective fracture network in the confined rock mass around the borehole. This expansive pressure development, however, dependent on many factors, where formation water content creates a negative influence on this due to required greater degree of hydration under greater water contents and temperature creates a positive influence by accelerating the reaction. Having a precise understanding of the fracture propagation mechanisms when using SCDA is important due to the formation of complex fracture networks in rocks. Several models can be found in the literature based on the tangential and radial stresses acting on a rock mass surrounding an SCDA charged borehole. Those fracture models with quasi-static fracturing mechanism that occurs in Mode I type tensile failure show compatibility with SCDA fracturing mechanisms. The effect of borehole diameter, spacing and the arrangement on expansive pressure generation and corresponding fracture network generation is important in the SCDA fracturing process and effective handling of them would pave the way to creating an optimum fracture network in a targeted rock formation. SCDA has many potential applications in unconventional gas and oil recovery and in-situ mining in mineral processing. However, effective utilization of SCDA in such application needs much extensive research on the performance of SCDA with respect to its potential applications, particularly when considering unique issues arising in using SCDA in different applications. Full article
(This article belongs to the Special Issue Unconventional Natural Gas (UNG) Recoveries)
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