bluuebunny/crossref_metadata_2025 huggingface dataset:
Shear Band Development and Permeability Anisotropy Evolution in Fault Gouge
Authors: Li Q. , Chen J. Y. , Zhang C. Y. , Zhang F. S. , Gan Q. , Elsworth D.
Published: August 2024

The fault zone generally undergoes grain breakage during shear slip events, resulting in the change of shear mode and pore structure in fault gouge. We establish a discrete elemental model of shear tests of granular fault gouge under different normal stresses at the constant velocity, to investigate the effects of grain breakage on fault friction strength evolution and shear band development as well as permeability. With the increase of normal stress, the fault friction strength increases by about 20% due to grain contact area evolution, accompanied by many small slip events triggered by grain breakage. Additionally, the fault gouge has a high tendency to dilation under low normal stress, causing an increase in permeability; while the fault gouge compacts rapidly due to grain breakage under high normal stress, leading to the decrease in permeability. Consequently, a constitutive model about porosity evolution is developed through relating to volumetric strain. Further analyses show that the heterogeneous grain breakage leads to the local reduction in porosity, promoting shear band development and changing the microstructural characteristics: P shears dominate microstructure of shear bands at low normal stress. With the increase of normal stress, more R1 shears with larger angle develop.

1. INTRODUCTION

A large amount of fine-grained gouge materials usually exists in the fault cores, which are mainly evolved from the abrasion and fragmentation of the rock mass on both sides of the fault during the fault sliding process (Billi et al., 2003). The presence of fault gouge affects the friction strength magnitude and stability of the fault. In mature fault zones, most of the deformation from sliding locates at the fault gouge, with undergoing a long-term process of grain breakage (Scuderi et al., 2017). This continuous evolution of grain size distribution and morphology during the shearing process profoundly alters the friction mechanical properties and permeability of the fault (Zhang et al., 2020). This also impacts the microstructure of the faults and induces the development of shear bands within the fault gouge (Balsamo and Storti, 2010). These macro-micro features resulting from grain breakage are often correlated with the deformation and strength of the fault and can be used to further understand the slip behavior of the fault (Kuo et al., 2014).
Mechanical Performance of One Latex Cement System for Thermal Storage Wells
Authors: Meng M. , Pytiana T. , Frash L. , Bijay K. , Madenova Y. , Uwaila I. , Li W. , Zhang W.
Published: August 2024

To minimize heat losses in the high temperature thermal storage wells, Brookhaven National Lab introduced one type of hydrophobic lightweight thermally insulating cement with a thermal conductivity three times lower than that of regular cement. However, the repeated thermal shock conditions that are the major concern for well integrity can be especially difficult for lightweight cements. This study provides an in-depth evaluation of the latex cement's mechanical performance under in situ conditions, including pressures up to 10 MPa and temperature as high as 200 °C. Before being subjected to thermal shock, the cement showed an elastic modulus ranging from 0.3 to 1.9 GPa, a Poisson ratio between 0.05 to 0.19, and a strength between 4.7 and 27.5 MPa. However, after thermal shock, there is a noticeable increase in its mechanical properties: the elastic modulus spans from 1.6 to 3.7 GPa, the Poisson ratio becomes 0.12 to 0.36, and the strength increases to 6.3 to 27.2 MPa. Although these observations indicate excellent cement performance after thermal cycling, its resistance to cyclic loading decreased after thermal shock tests. Additionally, this formulation shows strong plasticity, and our well integrity simulation shows potentially pose risk to the bond between the cement and the casing.

1. INTRODUCTION

High Temperature Reservoir Thermal Energy Storage (HT-RTES) is a promising solution for large-scale energy storage that can stabilize the electric grid, increase its flexibility, and provide energy on demand. HT-RTES systems are designed to store large amounts of energy over the long term and contribute to the overall affordability and sustainability of the current energy system (Dobson et al., 2023; Jin et al., 2023). To ensure its high efficiency and low cost, we need effective strategies to prevent heat losses in the system. The wellbore, serving as the conduit connecting subsurface reservoirs to surface heat sources, plays a crucial role in the heat exchange process via heat transfer fluids. Without proper design, significant heat loss could occur across the wellbore, diminishing system efficiency and potentially damaging the cement behind casing, leading to well leakage and total loss of the well (Meng et al., 2023).
Geomechanical and Petrophysical Effects of Long-Term Hydrogen Exposure on Berea Sandstone – An Experimental Study
Authors: Thiyagarajan S. R. , Emadi H. , Hussain A. , Maury Fernandez D. , Ispas I. , Henderson S. K. , Watson M.
Published: August 2024

Hydrogen is gaining traction on a global scale as a future low-carbon energy carrier. Underground Hydrogen Storage (UHS) is critical to successfully implementing the hydrogen economy. UHS requires many lab-scale and large-scale studies to understand the behavior of hydrogen in the subsurface before they can be incorporated into the energy system. So far, most of the experimental investigations conducted on UHS focus on the microbial, hydrogeological, and geochemical effects of hydrogen storage in the subsurface. However, studies focused on the geomechanical effects are scarce. In this work, eight Berea sandstone cores were exposed to brine, pure hydrogen, and hydrogen in the presence of brine for 90 days at reservoir conditions of 10 MPa and 60°C to study the geomechanical effects of hydrogen on the samples. Tests for the Berea sandstone's properties for petrophysical (permeability and porosity) and geomechanical (strength, Young's modulus, and Poisson's ratio) properties. The results demonstrate that long-term interactions with hydrogen cause changes in all the properties of the cores. The results also demonstrate changes in the geomechanical properties of the reservoir after three months of hydrogen exposure, which has not been extensively studied, and the results can be used in simulation, site selection, and risk assessment studies.

1. INTRODUCTION

Renewable energy production is viewed as an alternative to environmentally detrimental fossil fuel energies (coal, oil, and natural gas), contributing to global net-zero carbon emissions by 2050(Perera, 2023; Ugarte et al., 2023). According to a study by the International Renewable Energy Agency (IRENA), renewable energy will become the primary energy source, increasing worldwide share from 16% in 2020 to 77% by 2050 (IRENA, 2023). While there is a rising global interest in renewable energy, wind and solar energy have limitations: they depend on unpredictable factors such as weather. Renewable energy's intermittent generation creates a supply-demand imbalance, making renewable energies unreliable since they cannot maintain a continuous supply (Abdellatif et al., 2023; Heinemann et al., 2021; Kanaani & Sedaee, 2022; Muhammed et al., 2022; P. & P.G., 2023). To balance supply and demand, surplus energy from wind and solar energies can be converted to hydrogen through electrolysis, stored in the energy storage system, and subsequently utilized when demand arises (Okoroafor et al., 2022; Zamehrian & Sedaee, 2022).
Predicting Scour in Unlined Rock Spillways: A Coupled Analytical-Numerical Modeling Approach
Authors: Weidner L. , George M. F. , Clohan D.
Published: August 2024

Hydraulic plucking of rock blocks bounded by discontinuities is one of the primary modes of scour in unlined spillways. The methods used in the current state-of-practice either model a single block in considerable detail or introduce many simplifications to reduce model runtime and complexity. We present a method for simulating block scour at the spillway scale using an open-source discrete fracture network and block slicing program coupled with FLOW-3D. We built a custom implementation of limit equilibrium block theory for this task which enables eroded blocks to have any convex polyhedral shape. Preliminary results from this method highlight that block shapes introduce significant anisotropy to erosion behavior, which is not captured in other methods.

1. INTRODUCTION

Hydraulic plucking of rock blocks bounded by discontinuities is known to be one of the primary modes of scour in rock masses, resulting in channelization and head cutting. As such, understanding this process has immense implications for the design and maintenance of engineering structures such as spillways and dam foundations interacting with water. Modeling block scour can be accomplished using empirical relationships (Annandale, 1995), physics-based analytical stability solutions (Bollaert & Schleiss, 2002; George & Sitar, 2016; Goodman & Shi, 1985), or numerical simulations using coupled fluid-solid numerical methods (e.g., Gardner, 2023). Empirical methods are fast to apply at a spillway scale but are based on simplified inputs. Analytical methods apply directly to individual blocks but require many inputs of the forces acting on each block face - which can be time consuming to estimate.

Three-dimensional numerical methods currently in development can model scour and plucking of individual blocks directly by modelling fluid-solid interactions, but they are typically impractical at the spillway scale due to prohibitively large computational requirements (Gardner, 2023; Gardner & Sitar, 2019; Teng et al., 2023). Existing numerical models used in practice generally simplify the geometry to two-dimensional cross sections (Bollaert & Schleiss, 2005). Current methods are unable to both explicitly model hydraulic plucking mechanisms at the spillway scale and do this in three dimensions with arbitrary block shapes. Using the actual 3D block geometry is required to represent the block kinematics which is widely known to be a predominant factor in stability of rock blocks. Simplifications using rectangular blocks are unable to account for anisotropy caused by oblique joint orientations, and the stability of more irregular polyhedrons may vary significantly from a regular cube (George & Sitar, 2016).
Deformation Characteristics and Damage Evolution Mechanisms of Dry and Saturated Argillaceous Siltstone Under Cyclic Loading
Authors: Wu P , Chen J , Fu X. D , Huang J. H
Published: August 2024

Argillaceous siltstone is a common hydrophilic rock in geotechnical engineering, and its mechanical properties undergo significant changes under the combined influence of water and cyclic loading. Therefore, systematically investigating the mechanical behavior of argillaceous siltstone under cyclic loading in different moisture conditions is of great significance for guiding engineering design and improving the accuracy of disaster prediction. In this study, triaxial cyclic loading tests were conducted on dry and saturated specimens of argillaceous siltstone from Shiyan, Hubei Province, under different frequencies and confining pressures. A comparative analysis was performed on the strength and deformation characteristics of dry and saturated specimens, as well as the influence of frequency and confining pressure on their mechanical properties. The evolution of damage in argillaceous siltstone under cyclic loading was elucidated. The results indicate that: (1) Compared to dry specimens, the strength of saturated specimens significantly decreases, and the movement of the hysteresis loop towards larger strains is more pronounced; (2) Deformation of dry specimens shows minimal variation with increasing frequency, while an increase in frequency significantly accelerates the degradation of saturated specimens; (3) Lower confining pressures can restrain crack development in saturated specimens, but excessively high confining pressures can accelerate their failure; (4) After saturation, damage variables are noticeably influenced by frequency, showing a trend of initially decreasing and then increasing with changes in confining pressure.

1. INTRODUCTION

In China, red-colored soft rock formations, commonly referred to as the "red layer" in engineering contexts, are widely distributed. Among these formations, argillaceous siltstone stands out as a crucial component. Characterized by minerals such as montmorillonite and kaolinite, argillaceous siltstone exhibits hydrophilic properties, resulting in significant mechanical changes upon immersion in water. This type of rock is particularly prevalent in the southern regions of China, notably in the southwest and south-central areas, where heavy precipitation is common, rendering water presence inevitable in various construction projects.
A Training Experiment on the Cost and Benefit of Training U-Net for Laboratory Signal Processing
Authors: Kong A , Wang C. , Lin Q. , Qiquan X. , Opperman N. , Wu C. , Knippel E. P. , Hampton J. C.
Published: August 2024

U-Net is used by seismologists for picking phases of seismic waveforms. Applying U-net to process laboratory acoustic emission (AE) signals is useful for lab-scale experiments. In this paper, we document experiments on training two different U-Nets with a dataset containing 6532 manual pickings. This is to prepare for optimizing (or challenging) the maximum phase picking accuracy in the next step. Two label options (i.e., manual picking versus AIC auto-picking), two mask options, and two loss function options are considered in the training experiments. We observe the accuracy of the U-Nets, the shape of the output probability string, and the training and validation loss curves. The training time for U-Net-Two is shorter than that of U-Net-One. The binary-cross-entropy loss function is, as expected, proper for the initial stage of training. Applying the alternative option of the loss function may be useful for fine-tuning the model. We find that the length of the non-zero probability points in the label should be adjusted according to the frequency of the waveform. Shifting the signals and labels simultaneously will not affect the training. In the future this "shifting treatment" will be used for data augmentation.

1. INTRODUCTION

While originally developed for biomedical imaging segmentation [1], U-Net is useful for many other tasks that require classification at the data-point-scale. In recent years, the application of U-Net for picking different phases of seismic waveforms has received broad attention [2]. Such a technique for processing seismic waveforms has immediate application to laboratory AE signal processing. However, training a U-Net for processing laboratory AE signals faces many challenges. First, environmental noise in laboratory AE signals can be catalog based, so the usefulness of conducting transfer learning using a well-trained U-Net is limited. Second, the training of a U-Net demands a large amount of manually labeled P-wave arrivals. Previously successful training of a U-Net for seismic waveform processing utilized an extensive data pool consisting of over 700,000 manually labelled P and S arrival times picked from over 30 years of earthquake recordings provided by the Northern California Earthquake Data Center [2].
An Experimental Study on the Influence of Cyclic Loading and Eccentric Casing Geometry on Cement Sheath Integrity
Authors: Zheng Danzhu , Ozbayoglu Evren , Miska Stefan , Baldino Silvio
Published: August 2024

Zonal isolation is essential for the longevity and productivity of any oil or gas well. Losing zonal isolation can lead to environmental problems such as contamination of freshwater tables, sustained annular pressure buildup (SAP), and extensive and expensive remedial work. In this study, we studied the fatigue failure of cement through cyclic loading experiments on thick-wall-cylinder samples. To simulate the actual wellbore conditions, we designed a two-layered sample, incorporating casing and cement. A dedicated tri-axial facility was modified to introduce cyclic internal casing pressure. Low and high-impact cyclic loading tests were conducted for both concentric and eccentric thick-wall-cylinder samples. The fatigue behavior of Class H cement and CNTs-reinforced Class H cement was studied. Besides the study of fatigue behavior, the influence of casing eccentricity was studied by analyzing the first-cycle failure. The Mohr-Coulomb failure criterion was employed to analyze the experimental data, supporting the validity of the analytical eccentric casing model established in prior research.

The experimental results demonstrate the fatigue properties of the cement sheath and validate the impact of an eccentric casing geometry. This outcome is significant for enhancing cement system design and optimizing wellbore operational strategies, including applications like hydraulic fracturing and enhanced oil recovery processes.

1. INTRODUCTION

The main goal of cementing is to provide reliable zonal isolation throughout the life of a well. The integral of cement protects the casing and prevents the formation fluid from migration. Cement quality is influential to wellbore integrity and zonal isolation. Zonal isolation is essential for the longevity and productivity of any oil or gas well. Nowadays, most wells around the world are extending deeper and deeper. Downhole conditions become increasingly complicated. Changes in downhole conditions, in terms of temperature and pressure, expose cement sheath to deductive stress that impacts the integrity (Goodwin & Crook, 1990). There are many reports about cement sheath failure in deep wells where high pressure and temperature exist. Cement failure is manifested by interzonal annular fluid movement and abnormally high annular pressure at some point behind the casing up to and at the surface (Nelson, 2006). Subsurface geo-mechanical changes can also cause severe defects in oil and gas wells' cement barriers. Damages in cement sheath, such as voids, cracks, and channels or debonding between cement and casing, caused by either a poor cementing operation or due to the thermal/pressure fluctuations in a well, can threaten the integrity of cemented sheath and compromise zonal isolation.
Laboratory Study on the Strength Deterioration Characteristics of Solid Potash Ore Samples in Moist Environment
Authors: Zhang J. W. , Wang S. , Yang S. L. , Chen X. , Wei W. J. , Li L. H.
Published: August 2024

Potassium salt rock is a typical discontinuous heterogeneous material formed naturally, which is mainly composed of carnallite, sylvite, halite and so on. Potassium salt is widely used in various industry fields (such as agriculture, chemical industry, medicine, metallurgy, electroplating, printing and dyeing, glass, etc.) and has a large demand. One of the important sources is the mining of solid potassium salt mines. Through the laboratory test of solid potassium salt ore samples obtained from an underground potassium salt mine in a mine in Laos, various mechanical parameter data of potassium salt ore deformation and failure can be obtained intuitively and effectively, so as to provide theoretical and data basis for the design of specific engineering parameters such as pillar strength calculation and size optimization. In this paper, the large potassium salt ore obtained from the underground mining site of a potassium salt mine in Laos was cut and cored, and the cylinder and disc standard specimens of the potassium salt rock were obtained. By simulating the underground moist environment under different humidity conditions, potassium salt cylinder specimens with different water contents were prepared in the laboratory. Then, the uniaxial compressive strength test and other indoor tests were carried out on these potassium salt specimens, and the test results were obtained and analyzed in depth. The results show that the potassium salt ore sample exhibits typical plastic rheological properties under the condition of unconfined uniaxial compression, and its uniaxial compressive strength in the natural state is roughly between 6.7MPa -9.3MPa; and within a certain range of water content, the uniaxial compressive strength of potash ore samples gradually decreases with the increase of water content. Through the analysis of strength deterioration characteristics of solid potash ore, it can provide scientific guidance for the optimization of pillar size in underground mining of solid potassium salt ore with different moist environments, and then lay a theoretical foundation for improving the underground mining recovery rate of potassium salt ore and reducing the waste of resources.

1. INTRODUCTION

Potassium salt is a non-metallic mineral with potassium chloride and sulfate minerals as the main components (Bai et al., 2014). The main potassium salt minerals are sylvite, carnallite, potassium magnesium alum, anhydrous potassium magnesium alum, polyhalite, potassium magnesium alum, soft potassium magnesium alum and so on. In its raw state, potassium salt is an important industrial and agricultural mineral, which has important economic value, (Cheng, 2007). It is mainly used as potassium fertilizer, which is an important agricultural fertilizer resource and one of the three essential fertilizers for crops. Therefore, it is of great significance to study the mining of underground solid potash ore.
Mechanism Transition for Fracture Energy Dissipation of Shale Under Crack-Parallel Compression
Authors: Zhang Min , Zhang Guangqing , Ling Yansen , Yang Feng , Zhong Anhai , Zheng Xuelin
Published: August 2024

The microcracking processes in front of the rock fracture tip could determine the macro-fracture propagation. Understanding the effect of crack-parallel stress on the microcracking processes could facilitate characterization of fracture propagation in shales and guide hydraulic fracturing. The gap test is improved by replacing elastoplastic pads with a micro servo pump for applying precise crack-parallel stresses. The fracture propagation with crack-parallel stress is captured, where microcracking processes are identified unifying digital image correlation (DIC) and acoustic emission (AE) method. Based on the crack cohesive model, the influence of crack-parallel stress on fracture energy is characterized. Experiments show that the parallel stress of 10MPa increases the fracture energy of shale by about 25%, due to more shear type microcracks forming macro-fracture. The microcracks are spread over a wider damage zone rather than concentrated in fracture processes zone (FPZ) because of the crack-parallel compression. Hence the whole damage zone instead of FPZ dominates fracture extension. This work provides fundamental understandings in propagation of hydraulic fracture in shale under reservoir conditions.

1. INTRODUCTION

Hydraulic fracturing is a key technology increasing reservoir permeability by injecting high-pressure fluids to form fractures(Xing, Zhang, Luo, et al., 2019). Essentially, hydraulic fracture extends in the reservoir under the crack parallel compression. There are various standard specimens to characterize fracture resistance of rock, such as three-point-bend and compact tension, but all the standard tests almost ignore the effect of crack parallel stress.

The crack parallel stress is part of triaxial stresses from confining pressure, which is parallel to the fracture propagation. Some fracture tests under confining pressure have checked the effect of triaxial stresses. The straight notched round bar was proposed to obtain mode I fracture toughness of shale influenced by confining pressure(Antinao Fuentealba et al., 2024), using linear fracture mechanics. Experiments present that fracture toughness is double when confining pressure is increased to 70 MPa. The Brazil splitting tests with confining pressure indicate that the tensile strength of shale increases by up to 58.5%(Guo et al., 2022), as well as the sandstone and limestone samples. It indirectly shows obvious effect of confining pressure on rock fracture toughness because fracture toughness is proportionate to tensile strength(Dutler et al., 2018).
Experimental Study on the Size Effect of Fracture Process Zone of Mixed Mode I-II Crack in Sandstone
Authors: Sun Bin , Zhang Guangqing , Luo Senlin , Cai Dingshan , Lin Yansen , Zhou Dawei
Published: August 2024

Rock cracks tend to grow in the form of tensile and shear failure, i.e., mixed mode I-II crack. Size effect of fracture process zone (FPZ) should be highlighted due to it may alter the fracture growth. To study the size effects of FPZ on the development characteristics of mixed mode cracks, three-point bending experiments with off-center initial crack were carried out on sandstone with ratio of length and height of 3. Combined with digital image correlation (DIC) method, the variation of deflection angle and displacement field around the crack tip were analyzed. The results show that the FPZ length increases with the specimen size while the normalized length of the FPZ decreases. The FPZ begins to develop at 60-70% pre-peak and is fully developed at 90% post-peak. The cracks of different sizes begin to deflect when the FPZ is fully developed, and the deflection is completed at 60% post-peak. The deflection angle of FPZ decreases with the sample size. To ensure that the local tensile stress is the maximum, the cracks will deflect and cause the local shear stress to decrease and disappear, which is more obvious in small-size specimens. This study further revealed the fracture mechanism of mixed mode I-II crack under different sizes.

1. INTRODUCTION

Cracks are common internal structure in rock materials, due to different external forces, there will be different forms of expansion, generally can be divided into three modes: mode I (open) cracks, mode II (slip) cracks and mode III (tear) cracks (Surberg and Tschegg, 2001).

In view of different shapes of prefabricated cracks, Lin and Labuz, (2013) et al. studied mode I fractures of sandstone. The results show that the critical crack opening displacement does not change with the prefabricated crack shape. The FPZ length is about 10 times the maximum particle size of the sample. The critical crack opening displacement and FPZ length in mode II cracks are larger than those in mode I cracks (Lin et al., 2014; Moazzami et al., 2020).

Because cracks in actual rock structure often do not exist in a single fracture mode, such cracks are called mixed mode cracks. In practical engineering, mixed mode I-II cracks (hereinafter referred to as mixed mode cracks) are the most common (Aliha and Ayatollahi, 2013). In the mode I fracture, the fracture receives only tensile stress (Meirong. et al., 2024). Compared with mode I cracks, mixed mode cracks are not only subjected to tensile stress, but also to shear stress (de Moura et al., 2016). Since the shear resistance of rock materials is stronger than the tensile resistance, cracks generally start and expand in the direction perpendicular to the maximum tensile stress, that is, tensile failure is still the main type of mixed mode cracks (Song et al., 2018). Due to the existence of shear stress, there is a deflection Angle in the initiation of mixed mode cracks compared with mode I cracks.
Investigation on the Damage Mechanisms of Unconsolidated Sand, Tight Sand and Shale Induced by Cryogenic Temperature
Authors: Zheng Xuelin , Lu Mingjing , Wang Dongying , Qian Qin , Zhong Anhai , Zhang Liaoyuan , Yang Feng , Li Yuan , Zhang Yuzhe , Su Quansheng , Zhou Dawei , Zhang Min , Zhang Guangqing
Published: August 2024

Liquid nitrogen (LN) fracturing is a new approach that utilizes the LN cryogenic temperature to induce complex fracture networks in tight reservoirs. Under the cryogenic treatment of LN, the damage and fracture characteristics of different lithologies such as shale, tight sand, and unconsolidated sand are different, and there are currently few scholars conducting relevant research. This article innovatively designed a visualization experimental method for the continuous propagation of fractures induced by cryogenic LN. Based on digital image correlation (DIC) methods, it explored the differences in the damage mechanisms of unconsolidated sand, tight sand and shale induced by LN cryogenic temperature. The results showed that there were no macro fractures in tight sand and unconsolidated sand, only discrete local deformation occurred, while macro induced fractures appeared in shale. Based on damage mechanics, the numerical simulation method and model of damage induced by LN cryogenic temperature for different lithologies were established by COMSOL to simulate the thermal stress-induced damage characteristics of different lithologies. The results show that the cementation mode of rock skeleton is the main reason for the difference of damage mechanism of different lithologies induced by LN cryogenic temperature. The conclusion is that cryogenic LN can effectively induce macro fractures in shale, while inducing micro damage in unconsolidated sand and tight sand. The research results provide a theoretical basis for the design and feasibility of LN fracturing.

1. INTRODUCTION

In recent years, with the continuous progress of horizontal wells and segmented fracturing technology, there has been a wave of exploration and development of shale oil and gas resources, especially the shale oil and gas revolution in the United States, which has greatly increased the production of oil and gas resources in the United States, making it a major exporter of oil. The rise of shale oil and gas is profoundly affecting the global petroleum energy landscape. Shale oil and gas reservoirs have extremely low permeability and generally have no natural production capacity. It is well known that stimulated reservoir volume (SRV) has become the key to shale oil and gas extraction in fracturing and transformation. Therefore, how to form more artificial fractures and increase SRV in shale oil and gas reservoir fracturing and transformation is a key and difficult problem (Zoback and Kohli, 2019).
Experimental Study on Shale Mechanical Properties Interacted with Supercritical Carbon Dioxide and Brine
Authors: Feng Jie , Ma Tianshou , Liu Yang
Published: August 2024

Supercritical carbon dioxide (SC-CO2) can be utilized to displace methane in shale gas reservoirs due to its low viscosity, high diffusivity, and high density. However, injection of SC-CO2 can cause a series of physical and chemical interactions with the pore brine and shale rock, leading to irreversible changes in the mechanical properties of the shale. To investigate changes in the mechanical properties of the shale that interacted with SC-CO2 and brine, Longmaxi shale samples were soaked in pure SC-CO2 and SC-CO2 + brine for seven days at 50 ° C and 15MPa. Uniaxial and triaxial compression tests were performed on the soaked samples, and acoustic emission was monitored during the loading process. The results indicated that the Longmaxi shale exhibited a more pronounced reduction in strength under the SC-CO2+brine condition compared to the pure SC-CO2 condition. In the loading process, the acoustic emission signals exhibit a distinct staged characteristic, and the signals are generated primarily during the unstable crack expansion stage. The observed increase in the fractal dimension of the signals at the same circumferential pressure indicates that the shale damage is multifaceted, resulting in a reduction in the stability of the shale. This study may help to elucidate the alterations in the mechanical properties of shale as a consequence of its interaction with SC-CO2. Furthermore, the findings may provide a foundation for the recovery of shale gas using CO2 displacement.

1. INTRODUCTION

With the rapid growth of the global economy and the increasing level of industrialization, the supply of traditional fossil energy has gradually failed to meet the energy needs of today's society (Zou et al., 2016). Shale gas, as one of the most important unconventional oil and gas resources, has attracted a lot of attention because of its abundant reserves, cleanliness, low cost, and long supply cycle (Zhou and Zhang, 2020). Shale reservoirs are typically characterized by low porosity, low permeability, and high clay content (Jin et al., 2012), so hydraulic fracturing is commonly used to enhance shale gas production (Daneshy, 2010). However, it can result in significant water loss, damage to the physical properties of the reservoir, and contamination of surface water with back-draining fluids (Estrada and Bhamidimarri, 2016; Yang et al., 2019; DiStefano et al., 2019). Supercritical carbon dioxide (SC-CO2) is often used to enhance shale gas recovery due to its low viscosity, low surface tension, and high extraction capacity in its supercritical state (Adwani, 2007; Wang et al., 2011; Yang et al., 2008). This method has several advantages, including no damage to the reservoir, high efficiency of jet breakthrough, and the ability to lower the viscosity of crude oil and displace shale gas (Lu et al., 2021; Zhao et al., 2021).
A Comparative Study of Preparation Methods, Weighting Agents, and Temperature on Quality of E-M Compatible Borehole Imaging Fluid
Authors: Abbasian L. , Akindele O. M. , Abugharara A. N. A. , Butt S. D.
Published: August 2024

Imaging fluids plays a crucial role in mitigating the impact of borehole groundwater on borehole E-M imaging results. Such fluid must possess specific characteristics such as low conductivity to minimize electromagnetic wave attenuation, appropriate dielectric permittivity to prevent signal ringing at the fluid-wellbore boundary, higher density than water for settlement at the bottom hole, and long-term stability throughout the imaging process. This study meticulously examines the influence of agitation and temperature on the quality of oil-based imaging fluids, comparing two major preparation methods with distinct grain sizes of the weighting agent. One is the conventional method involving a critical heating step of the emulsifier in 20% of the imaging fluid liquid base, while the second involves dissolving the emulsifier in the liquid base by agitation. Evaluation of the produced fluids encompasses considerations of their stability over time, settlement in various water temperatures, and rheological properties. The results of these experiments reveal the pros and cons of the agitation process compared to the conventional method, weighting agent grain size, and temperature on the overall quality of the produced imaging fluid.

1. INTRODUCTION

High frequency electromagnetic waves are used as the source of Ground Penetrating Radar (GPR) to image and map subsurface geological formations and structures (Jol, 2008). Borehole GPR utilizes high frequency electromagnetic waves for mapping out the downhole subsurface geology. This GPR data quality can be influenced by the presence of borehole water or the media between the antenna and the wellbore Li (2023). Hence, the selection of a proper borehole fluid will help overcome the impact of ground water on imaging data quality. Borehole water should be replaced by an imaging fluid at the bottom hole and cover the E-M antennas while imaging.

The ideal borehole fluid (imaging fluid) should have specific properties such as low conductivity (low EM wave attenuation), appropriate dielectric permittivity close to that of the host rock to avoid signal ringing between the fluid-wellbore boundary, higher density than water to enable it settle at the bottom hole, and stability such that it does not discompose while imaging. Following the above characteristics, the imaging fluid is made up of a liquid base with low dielectric permittivity and conductivity, a weighting agent to increase its density than water to about Specific Gravity (SG) of 1.2 and an emulsifier to ensure its stability by preventing the separation and settlement of suspended solids.
Thermal Measurements and Geothermal Gradient from Deep Boreholes in the Revell Batholith, Northwest Ontario
Authors: Kasani H. A. , DesRoches A. , Sykes E. , Parmenter A. , Khorshidi M.
Published: August 2024

The Revell Site, in northwestern Ontario, within the Canadian Shield, is presently being studied as one of the sites for hosting a deep geological repository (DGR) in Canada. Preliminary site investigation included the drilling and coring of six 1-km long boreholes. The thermal characterization involved obtaining temperature logs from multiple sources in boreholes and laboratory thermal properties testing of different rock types. After drilling, coring, and flushing of the borehole, downhole temperature data were collected in this order: (1) continuous logs from geophysical dynamic fluid temperature probe, (2) discrete temperature data measured during hydraulic packer testing, and (3) discrete temperature data collected post installation of a Westbay monitoring system. The hydraulic testing and Westbay data provided the most reliable estimates of in-situ temperature profile with an average geothermal gradient of 9.9°C/km. This gradient is within the lower range of the data from other study sites in the Canadian Shield. The low gradient is consistent with low concentrations of natural uranium, thorium, and potassium in the tested core samples, which results in a low radiogenic heat generation rate. This paper presents a summary of the thermal data collection in the first three boreholes and development of a thermal model for the site, which provides boundary conditions for thermal simulations of the DGR. This initial thermal property dataset and site model also provides a prediction against which data from the latter three boreholes can be expected.

1. INTRODUCTION

Deep geological disposal is an internationally accepted method for the safe and long-term management of used nuclear fuel. Safe long-term performance of a Deep Geological Repository (DGR) spans over the construction, operation, and post-closure phases which includes geological processes such as earthquakes and glacial advance and retreat.

The Nuclear Waste Management Organization (NWMO) is implementing Adaptive Phased Management (APM), the approach approved by the Government of Canada in 2007 for the long-term management of used nuclear fuel. The NWMO site selection process is described in NWMO (2010). As part of APM, the NWMO is conducting preliminary site assessment activities in potential siting areas in: i) crystalline rocks in the northern portion of the Revell batholith within the Canadian Shield, near the Township of Ignace in the traditional territory of Wabigoon Lake Ojibway Nation (WLON) in northwest Ontario (referred to as the Revell Site), and ii) in sedimentary rocks in the Municipality of South Bruce in the traditional territory of Saugeen Ojibway Nation (SON) in southern Ontario (referred to as the South Bruce Site).
Monitoring the Propagating Process of Hydraulic Fracture with Fiber Optic Strain and Strain Rate Technology
Authors: Hao Yalong , Chen Mian , Wang Su , Zhao Changjun , Mao Yu
Published: August 2024

A true tri-axial experiment was conducted to simulated hydraulic fracture propagating process with a 30 cm cubic block cement sample. At the same time, optic fiber in the cement sample recorded strain signals. In this paper, a fiber strain/strain rate calculation method based on wavelet noise reduction algorithm is proposed. The experimental results show that different spatial relationships between the optic fiber and the fracture surface correspond to different optic fiber signal responses. When the fracture surface is perpendicular to the optical fiber, obvious red stretching strips will be formed due to the fracture hitting the optical fiber. However, when the fracture surface is parallel to the optical fiber, the optical fiber will produce a large-scale micro-stretching zone. At the same time, the coupling effect between the fiber and the formation can also affect the signal responses. These results are profitable to understanding Distributed Acoustic Sensing (DAS) signals in the field.

1. INTRODUCTION

Horizontal well fracturing technology in low permeability reservoirs greatly improves the efficiency of unconventional oil and gas development (Warpinski et al., 1991). Distribution and morphology of hydraulic fractures are extremely important for oil and gas production. But it has never been fully understood what the expansion pattern of underground hydraulic fractures actually looks like. Oil companies and service companies have developed technologies such as microseismic, tiltmeters, tracers, etc. for this purpose (Fisher et al., 2005, Maxwell et al., 2009, Magdalene et al., 2022). Due to the complexity of the subsurface, all monitoring techniques can only indirectly reflect some of the characteristics of the hydraulic fracture.

Low-frequency DAS(LF-DAS) signals (<0.05 Hz) are emerging as a new tool for monitoring fracture morphology, providing a completely new approach to hydraulic fracture monitoring (Jin et al., 2017). (Liu et al., 2020 and 2021) used the displacement discontinuity method to simulate the response of a fracture hitting a fiber under single and multiple clusters, and used the fiber data to invert the fracture hitting point location and fracture width information. (Chen et al., 2022) analyzed and summarized the evolution of the strain rate waterfall diagram before and after the fracture hit the fiber. (Smith Leggett et al., 2022) proposed a "zero-strain" localization method for assessing the dynamic location of fracture leading edges by indoor fiber-optic monitoring of fracturing. (Chen et al., 2021) applied machine learning algorithms to fiber optic data processing in an attempt to assist in determining information such as the location of fracture hits.
Numerical Simulation of Elastic Wavefield Evolution in Heterogeneous Fractured Media Based on a Combined Displacement Discontinuity-Discrete Fracture Network Model
Authors: Wang S. F. , Zhang Z. X. , Huang X. , Lei Q. H.
Published: August 2024

The evolution of elastic waves in heterogenous fractured rocks is studied based on numerical simulations. We use the discrete fracture network method to represent the distribution of a natural fracture system and employ the displacement discontinuity method to compute the propagation of elastic waves across individual fractures. The variability of matrix Young's modulus is assumed to obey the Weibull distribution, which is characterized by a homogeneity index m. The rock matrix becomes more heterogeneous as m decreases. The results indicate that the dimensionless angular frequency ῶ = ωZ/κ dominates the wavefield evolution of a plane wave travelling through the heterogeneous fractured rock with less heterogeneous materials (m = 10) or high heterogeneous fractures (ῶ ≥ 1). Here, ω, Z, and κ are the angular frequency, seismic impedance, and fracture stiffness, respectively. As the heterogeneity of the rock materials increases (i.e., m ≤ 5) with small ῶ (say 0.1), both the fracture network and rock matrix jointly dominate the wavefield. An asynchronous arrival phenomenon of wave energy occurs and becomes more significant with an increased ῶ and decreased m. The breakthrough time of elastic wave energy has a positive relationship with ῶ but a negative relationship with m.

1. INTRODUCTION

Rock masses typically contain numerous natural fractures of variable lengths, locations, and orientations (Bour et al. 2002; Davy et al. 2010; Lei et al. 2017; Afshari Moein et al. 2019; Miranda et al. 2023). The heterogeneous matrix with spatially varying mechanical parameters is also commonly present (Tang et al. 2000; Molz et al. 2004; Lei and Gao 2019; Yan et al. 2023). The resultant heterogeneities arising from natural fractures and rock materials may significantly affect the seismic wave transport in the subsurface (Sato and Fehler 2009; Hamzehpour et al. 2016; Zhu et al. 2020; Ma et al. 2023; Wang et al. 2023), which has attracted considerable attention from various disciplines such as rock mechanics, geophysics, seismology, and earthquake engineering (Khoshhali and Hamzehpour 2015; Fan et al. 2018; Feng et al. 2020; Zhang et al. 2021). Thus, comprehending the evolution of waves in heterogeneous fractured rocks and quantifying the relationship between wavefield characteristics and rock mass properties are fundamentally important for characterizing crustal heterogeneities in many rock engineering applications. However, the current studies mainly focus on the onefold impact of fracture- (Shao and Pyrak-Nolte 2016; Lei and

Sornette 2021; Wang et al. 2023) or material-induced (Allaei and Sahimi 2006; Hamzehpour et al. 2016; Ba et al. 2017; Ma et al. 2023) heterogeneity on the wave transport. The competing roles of the fracture network and rock matrix heterogeneity in the wavefield evolution has not been well revealed.
Numerical Simulation of Fracture Propagation on Proactive Utilization of Stress Interference in the Yingxiongling Shale Oil Reservoir
Authors: Chen X. , Xian C. G. , Liu Y. Y. , Wan Y. Y.
Published: August 2024

In the development of shale oil and gas, the tridimensional development model centered around factory drilling, horizontal wells, and hydraulic fracturing has been widely adopted. This development model inevitably triggers both positive and negative hydraulic fracturing interference as well as production interference. Especially during the hydraulic fracturing process, actively controlling stress interference can effectively utilize these interferences to generate positive effects, thereby increasing the complexity of fractures and enhancing the efficiency of oil and gas extraction. This paper develops an analytical expression mathematical model of a spatial stress field, which is used to explore how the coupled design of different well spacings and fracturing layouts proactively affects the spatial stress field. Based on the geoscience & engineering integration software platform, this study combines fracture propagation simulation with finite element numerical simulation of the stress field, clarifying how to proactively utilize and control stress interference. The research indicates that strengthening well interference and implementing staggered fracturing strategies can promote the formation of complex fracture networks at distant ends, while effectively inhibiting excessive extension of the main fractures towards neighboring wells. Through the deployment of a tridimensional well network, and the coupled design of well spacing and fracture spacing, an overall optimization and utilization of the spatial stress field can be achieved.

1. INTRODUCTION

In the global oil and gas industry's exploration and development sector, there is a gradual shift from conventional oil and gas, which accounts for 20% of the total oil and gas resources, to unconventional oil and gas resources, which make up 80% (Zou et al., 2023). As the first country to achieve commercial development of shale oil globally, the United States has effectively driven cost reduction and efficiency improvements through the scaled-up development of shale oil via "super well factories" (Guo et al., 2023). China's shale oil resources are extremely abundant. According to exploration assessments, the recoverable resources are estimated to be about 5 billion tons. However, compared to the marine shale oil in North America, China's continental shale oil exhibits some unique geological characteristics, such as strong reservoir heterogeneity, thin individual layer thickness, and rapid changes in lithology and facies (Li et al., 2023). These characteristics make the use of a tridimensional development strategy particularly important for the overall mobilization of the reservoir.