bluuebunny/crossref_metadata_2025 huggingface dataset:
Numerical Evaluation of Viscous Fingering Behavior During Underground Hydrogen Storage in Saline Aquifers
Authors: Cunqi Jia , Jinchuan Hu , Kamy Sepehrnoori
Published: August 2024

Underground hydrogen storage (UHS) is a promising option for fully realizing hydrogen potential as an alternative energy source to balance seasonal availability with irregular demand. This study is motivated to investigate and evaluate the viscous fingering behavior during underground hydrogen storage. The specific numerical simulations are implemented using a component simulator, incorporating input parameters such as density, viscosity, salinity, and relative permeability curves, all derived from experimental data. The study findings are first validated through a comparison with oil-water two-phase lab-based flat-plate displacement experiments. Numerical simulation schemes with different grid sizes are employed for comparison with experimental results, validating and optimizing the grid independence to determine the optimum fine grid size. Utilizing the same physical model, a hydrogen-aquifer water two-phase system is subsequently adopted to study the phenomenon of viscous fingering during the hydrogen injection process. The results show that in general, for the studied cases, there is no significant difference between the two mesh sizes, especially in the water recovery factor and hydrogen mass ratio. Based on the current numerical model and results, fingering behavior has a relatively minor impact on the underground hydrogen storage process.

1. INTRODUCTION

As the global climate crisis intensifies, transitioning to a low-carbon economy has become an urgent global mission (Zhao, Jia 2023, He, Liu 2024, Wang, Liu 2024). To reach such an ambitious target, a multitude of solutions are being explored and developed. Hydrogen, as a clean energy carrier, has garnered widespread attention (Zhang, Jia 2024, Bai, Jia 2024). Hydrogen not only possesses various advantages, such as high energy density and zero emissions, but it also plays a significant role in supporting renewable energy systems. When there is an overproduction of renewable energy, hydrogen can be produced through the electrolysis of water, thus forming a closed-loop system for a low-carbon transition. However, the application of hydrogen currently encounters a series of challenges, particularly with the storage of hydrogen. The interaction between hydrogen and metal ions, for example, can cause hydrogen embrittlement. In response, underground hydrogen storage technology has been developed. Utilizing the natural advantages of underground spaces, such as consistent environmental conditions, capacity for large-scale storage, and increased safety, the technology provides a promising approach to enable extensive, secure, and dependable storage of hydrogen.
Experimental Study on Fracture Propagation Characteristics of CO2 Pre-Fracturing in Shale Oil Reservoir
Authors: Wu Hua , Wang Xiaoqiong , Ge Hongkui , Song Jiaxin , Hou Shuoyang , He Jixiang , Wang Zhenyu
Published: August 2024

CO2 pre-fracturing offers the dual advantages of enhancing oil recovery and generating complex fractures, demonstrating positive application effects in the field. However, there is a scarcity of research on the characteristics of fracture propagation associated with CO2 pre-fracturing. This paper presents a comparative experimental study between CO2 pre-fracturing and hydraulic fracturing, utilizing a true triaxial fracturing simulation system. The study examines the fracture propagation patterns of CO2 pre-fracturing through the integration of acoustic emission (AE) monitoring and CT scanning techniques. The findings indicate that CO2 pre-injection significantly reduces the breakdown pressure in the subsequent hydraulic fracturing phase. After the pre-injection of CO2, the pressure curve during the hydraulic fracturing process exhibited a distinct zigzag pattern, with higher pressure fluctuation frequency, AE rate, and cumulative AE counts, indicating that the CO2 pre-fracturing had generated a large number of micro-fractures. CT scan results before and after the fracturing process demonstrate that the fractures formed following CO2 pre-fracturing have a larger fractal dimension, indicating a more complex fracture network. Additionally, the pre-injection of CO2 activates the beddings within the formation, which subsequently influences the direction of fracture propagation during the following hydraulic fracturing. This study confirms that CO2 pre-fracturing provides superior stimulation effects compared to conventional hydraulic fracturing methods. Consequently, the application of CO2 pre-fracturing technology is recommended for the development of shale oil.

1. INTRODUCTION

Shale oil is one of the most important unconventional oil and gas resources in China, and due to its tight reservoir and poor crude oil mobility, it needs to be stimulated by large-scale fracturing to realize the effective mobilization of crude oil. Shale oil wells are developed through large-scale fracturing, but problems such as rapid depletion of formation energy and rapid decline in production often occur during the production process. Some oilfields have conducted CO2 pre-fracturing tests, effectively alleviating the above problems. The results of production data analysis show that after the implementation of CO2 pre-fracturing, the oil well pressure maintenance level was greatly improved, the production decline rate was significantly reduced, and the oil recovery rate was effectively improved.
Effect of Shear Thickening Fluid on the Hydraulic Fracturing Behavior
Authors: Mukuhira Y. , Watanabe N. , Sueyoshi K. , Takuma K. , Zhang R. , Tomai T. , Arai Y. , Uno M. , Ito T. , Goto R. , Tian T. , Sokolovski V. , Naoi M.
Published: August 2024

Hydraulic fracturing nucleates fractures along the orientation of maximum stress in the normal plane to the borehole. Meanwhile, standard hydraulic fracturing cannot nucleate several fractures in different directions from a borehole and achieve permeability enhancement in another direction other than maximum stress. We conducted the hydrofracturing experiment using shear thickening fluid (STF) as a fracturing fluid in the uniaxial loading condition. We expected that STF could affect the behavior of fracturing since STF has the ability to plug the flow path by changing its viscosity. Hydraulic fracturing occurred at around 38 MPa, which was significantly higher than the tensile strength of the granite. Simultaneously, borehole pressure dropped, and a number of AEs were observed as standard fracturing phenomena. However, sub-consequently, borehole pressure built up again, and then we observed several repeating breakdowns accompanied by AEs. Surprisingly, from the visual observations of the specimen after dismounting from the apparatus, we observed several fractures that propagated in different directions from the borehole. So, the rock was fractured in multi-direction by the effect of STF.

1. INTRODUCTION

Hydraulic fracturing has been used widely to generate fractures in intact or low-permeable rock formations for permeability enhancement in various subsurface resource development fields. A new fracture is nucleated along the orientation of maximum stress in the normal plane to the borehole since fracturing to the direction of maximum stress requires minimum energy to nucleate new fractures and open them. Nucleated fractures drastically increase the permeability around the borehole, especially in the direction of maximum stress, so that the energy fluid exploitation will be improved.

Hydraulic fracturing needs high borehole pressure to yield the tensile strength and stress concentration around the borehole,

(equation)

where Pb is breakdown pressure, Sh is minimum horizontal stress, SH is maximum horizontal stress, Pp is

borehole pressure, and St is the tensile strength of the rock mass. Note that Eq. (1) is the case of vertical wells. As soon as the hydraulic fracture is nucleated, the borehole pressure drops immediately due to the simultaneous increase of space. This pressure drop is regarded as evidence of fracture nucleation, and the maximum borehole pressure is recorded as breakdown pressure (Pb). After the first breakdown, borehole pressure never increases as high as Pb in general because significant permeability has been achieved by hydraulic fracturing, and the borehole cannot be sealed anymore. Pore pressure is used for the extension of fracture. Meanwhile, in the laboratory hydraulic fracturing, nucleated fracture often reaches the edge of the specimen. In such cases, borehole pressure would not build up significantly since the other side of the fracture is connected to open air.
Permeability Evolution of Fractures in Shale in the Presence of High Salinity Water
Authors: Bai T. , Melkoumian N. , Hashemi S. , Badalyan A. , Zeinijahromi A.
Published: August 2024

Shale reservoirs are often associated with hydraulic fracturing, which involves interactions of invading fluids and shale minerals. Formation damage caused by injection impairment is a well-known challenge in the exploration and development of unconventional reservoirs. In this study, the impact of loading history and high salinity water (HSW) on the permeability of fractured shale has been investigated via laboratory studies. The fractured shale sample was loaded and unloaded under different effective stresses with nitrogen gas injection, resulting in a decrease in fracture permeability due to inelastic compaction. Then, the shale sample was stepwise loaded with HSW injection and further decrease in its permeability was registered. This phenomenon became more expressed after extending the imbibition duration to more than ten days, suggesting that shale swelling was caused by fluid-shale interactions. Furthermore, sudden drops in the differential pressure were observed during the process, implying fracture generation. This study shows that the chemistry mismatch between treating fluids and shale can lead to permeability impairment, which can have negative impacts on well injectivity in field applications.

1. INTRODUCTION

Unconventional resource extraction methods are of great interest to the petroleum industry due to their potential in supplying abundant resources to the market. It has been estimated that oil production from tight oil reservoirs can exceed 12 million barrels per day (Aloulou and Cook, 2017). To effectively extract oil and gas resources in deeper, tighter, and more depleted conditions, hydraulic fracturing is necessary to improve well productivity (Xu et al., 2016). Shale formation in the vicinity of wellbores can experience cyclic loading during drilling, completion, and production activities. Therefore, it is crucial to gain a better understanding on how shale fracture permeability responds to repeated stress-relief process over time. Applying loading-unloading cycles under various effective normal stress conditions is a conventional method used to examine the response of shale fracture permeability and its deformation. Three factors influence this permeability changes: changes in fracture roughness, changes in the contact area, and changes in fracture aperture (Zimmerman and Bodvarsson, 1996; Witherspoon et al., 1980). The main stages of the permeability change can be described as decrease in the permeability with the increase in the axial stress and partial recovery of the permeability after the stress relaxation. Experiments conducted on fractured shale samples by Gutierrez et al. (2000) demonstrated that fracture permeability after loading was always lower than before it. Hashemi and Zoback (2021) stated that the irreversible fracture permeability is likely caused by fine-grain displacement and compliant elements such as clays and total organic carbon within shale.
Non-Darcy Coefficient in Fractured Rocks
Authors: Cabrera D.
Published: August 2024

In the present work, an experimental methodology was developed for measuring the high-velocity coefficient in fractured rocks. The measurements were carried out at reservoir conditions (high pressure and temperature), in four full-diameter (whole) samples of a naturally fractured gas and condensate reservoir, with an approximate depth of 6500 m and an average temperature of 160 °C. The results of calculated inertial coefficients show a dependence on the effective stress applied to the rock, due to the effect of the compaction of the pore volume of the core and the morphology change in the the rock. Also, variation effect with water and condesate are described. These results are applicable for estimating the high velocity skin factor in gas reservoirs. This work presents a new correlation for estimating the coefficient of inertial resistance from permeabilityfor fractured rocks. This correlation was compared with several published in the literature.

1. INTRODUCTION

The non-Darcy coefficient (inertial, non-laminar, turbulence, high velocity) is essential for characterizing a gas-condensate reservoir because its effects occur near the well due to the high-velocity gas flow.

In the case of naturally fractured reservoirs, the effects are amplified due to the properties of the fracture network present in the reservoir and the geomechanical impact that can be caused during reservoir exploitation.

For example, the compaction generated by the closure of fractures during the pressure drop of the reservoir is an adverse effect that produces a considerable increase in the inertial coefficient.

Given the importance of the inertial coefficient, it is proposed to experiment with reservoir conditions to evaluate said petrophysical parameter.

Four full-diameter samples were used from a terrestrial carbonate gas and condensate reservoir in southern Mexico to carry out an experiment representative of the behavior of naturally fractured reservoirs.

The samples present a natural fracture network representative of the reservoir. Furthermore, the behavior of the inertial coefficient was analyzed as a function of the effective stress applied to the rock. Nitrogen gas injection was used, and an average rock temperature of 150°C. The effective applied stress range is 500 psi – 7000 psi.

These experimental pressure and temperature conditions generate a ductile constitutive behavior in the carbonates, allowing a greater range of stress application in the rock without reaching induced fracturing.
Laboratory Study of Fracture Propagation Behaviors in Fractured-Cavity Carbonate Reservoirs in Shunbei Oilfield
Authors: Luo Pandeng , Li Chunyue , Sun Yaoyao , Huang Zhongwei , Wu Xiaoguang , Zhang Xu , Yang Ruiyue , Wang Zikang
Published: August 2024

In order to determine the interaction behaviors between artificial fracture and nature fracture-cavity, we performed a laboratory true tri-axial fracturing experiment on natural carbonate outcrops with Mico-fracture and pores, and the natural fracture-cavity are simulated by pre-existing fracture-cavity. The propagation of artificial fractures is investigated by changing the approach angle for fracture and maximum horizontal stress, and arrangement of pre-existing cavity. Moreover, re-injection experiments are used to evaluate the conductivity of fractures. The results indicate that the extension of fractures in pre-existing fractured-cavity carbonate rocks is random. The extension of artificial fracture is steering when it encounters pre-existing cavity. Three interaction modes occur between artificial fracture and pre-existing fracture, and the large approach angle allows the artificial fracture to pass through the pre-existing fracture. Artificial fracture is always captured by pre-existing fracture, and the tortuosity is greater in the process of propagation. The breakdown pressure of pre-existing fracture-cavity carbonate rocks is about 50% lower compared to that of tight carbonate rock. Pre-existing fracture-cavity make the conductivity greater of effective fracture, but the length of effective fracture is not conducive to the conductivity. This research may contribute to the field application of hydraulic fracturing in carbonate formation.

1. INTRODUCTION

The increase of burial depth causes the gradual decrease of carbonate porosity, which is a past view on the lack of high-quality reservoirs in deep carbonate rocks (SCHMOKER J W et al., 1982; EHRENBERG S N et al., 2009). In recent years, 60 % of the world ‘s new oil and gas reserves come from deep strata, and the exploration potential is huge (Wei C et al., 2017; Sun K et al., 2022). The deep strata of oil and gas reserves and production of marine carbonate rocks in Tarim Basin, Sichuan Basin and Ordos Basin have been increasing rapidly year by year, which has attracted wide attention from the industry. In the proven deep strata of carbonate reservoirs, two-thirds belonging to the type of fractured-cavity reservoirs (Guo W et al., 2022). Complex spatial structure is the characteristic of fractured-cavity reservoir, mainly including matrix pores, karst cavity, and natural fractures with different development degrees (Durrani M Z A et al., 2021; Tian F et al.,2019; Sun, K et al., 2021). Due to the extreme heterogeneity and complex flow environment of facture-cavity carbonate formation, that is a challenge to increase oil and gas extraction rate (Tian F et al.,2016; Dmitriy A. Martyushev et al., 2022).
Evaluating Geological Seals for Underground Hydrogen Storage: A Comprehensive Assessment
Authors: Galvis-Silva Henry , Okoroafor Esuru R. , Liu Kai-Wei
Published: August 2024

Underground Hydrogen Storage in porous media offers a viable solution for long-term, large-scale energy storage. Yet, challenges such as low storage efficiencies and potential losses persist, with a key issue being the sealing capacity and integrity of the storage reservoir. This study specifically addresses two primary hydrogen leakage mechanisms in caprocks: capillary leakage and faults or fractures in the caprock. We conducted static reactivity tests on various shale samples, at a constant pressure of 3.4 MPa and exposed to hydrogen in the presence of brine under different temperature conditions over 30-day periods.

The findings generally indicate low reactivity of hydrogen with the shale samples. Minor compositional changes were observed, which were determined through XRD analysis. However, significant alterations in petrophysical properties were observed in all the plug samples, including more than a 100% increment in porosity and even greater changes in permeability. Additionally, uniaxial compressive strength tests were conducted, revealing that the shales exhibited a reduction in all mechanical properties. Micro-CT scans confirmed the increases in permeability and exposed the presence of small microcracks.

These findings underscore the potential for hydrogen storage to modify shale structure by reopening microcracks, emphasizing the need for further research to fully understand the geochemical interactions at play.

1. INTRODUCTION

The urgency of transitioning to a clean, low-carbon energy future underscores the pivotal role of energy storage technologies. These technologies are crucial for integrating intermittent renewable energy sources, such as wind and solar, into the electrical grid to ensure a consistent energy supply. Particularly, large-scale energy storage is instrumental in facilitating the shift towards more sustainable energy systems, a topic that has gained prominence in recent research (Krevor et al., 2023).

A primary challenge in the renewable energy sector is the mismatch between the periods of energy production and peak demand times (Matos et al., 2019), as depicted in Figure 1. This discrepancy underscores the critical need for efficient energy storage solutions that can store excess energy during high production periods and release it during peak demand times. The transition towards renewable energy sources amplifies the urgency for such solutions to combat climate change.
Use of Combined Analytical and Numerical Sand Production Prediction Methods to Define Simple Cut-Off for Sand Control and Well Completion Selection
Authors: Khaksar A. , Younessi A. R. , Likrama F. , Gui F.
Published: August 2024

There are several sanding prediction techniques commonly used by well engineers and geomechanics specialists. The most desirable ones are empirical models with simple rules but these need a vast amount of field observations and historical production data to derive the appropriate cut-offs. We introduce a geomechanical workflow in which field-calibrated geomechanical models are used to define simple cut-offs, e.g. rock porosity or strength, for sand production using a combination of advanced analytical and numerical sanding models. These simple cut-offs are then incorporated in decision flowcharts to assist well engineers for completion and sand control decisions considering well-life sanding risks by following a quick, cost-effective and holistic approach. Pros and cons of pragmatic cut-offs are listed and the residual risks involved with such approaches are also highlighted. The workflow is shown in a gas field and the validity of the workflow and sand control decisions are confirmed after several years of production.

1. INTRODUCTION

Sand or solids production, also called sanding, along with hydrocarbon, water and scales is a natural component of production systems in oil and gas wells. The root cause of sanding is failure of formation rock around the wellbore due to effective in situ stresses acting on the rocks exceeding the mechanical strength under production conditions. Sanding is manageable, it can be avoided, mitigated, controlled or handled depending on its timing (i.e. reservoir pressure depletion and bottom hole flowing pressure), interval (sanding zone) and its severity (volume of produced solids, impacts of sanding on well components and well productivity). Essential parts of integrated and holistic sand management practices include; competent sanding prediction which require understanding of field geomechanical conditions, followed by sand control measures for limiting sand production to an acceptable level for both well productivity and integrity, and safety and environmental constraint. It also included consideration of sand handling and monitoring at both the surface and downhole. Sand production prediction studies are an integral part of overall hydrocarbon field development planning so that sanding potentials can be assessed and appropriate completion designs are considered.
AI-Powered Semi-Automated Trace Detection Techniques on 3D Digital Rock Mass Models
Authors: Mehrishal S. , Kim J. , Song J.-J.
Published: August 2024

Estimating rock mass structure relies heavily on deciphering trace lines on exposed rock surfaces. Manual detection of traces on 3D digital models can be tedious and time-consuming. This study proposes a novel AI-driven approach to enhance 3D trace surveying, overcoming potential limitations. A custom pixel-wise texture mapper algorithm transforms 3D rock models into virtual 2D images, eliminating access restrictions. DeepLabV3+, a deep-learning network, efficiently detects trace lines within these virtual images. The detected traces are transformed into 3D data through a multi-step process involving point cloud integration, thinning and segmentation, and re-projection. This meticulous reconstruction culminates in a 3D point cloud representing a trace network. This AI-powered method promises several advantages over traditional methods. Automating trace detection and minimizing human subjectivity enhances accuracy and reduces surveying time. Additionally, virtual image generation allows for surveying difficult-to-access outcrops or overcoming camera positioning challenges. Beyond the proposed methodology, the study will compare the performance of AI and human experts in trace pattern detection. Furthermore, it will explore potential applications of this technology in various geological contexts, paving the way for future advancements in rock structure analysis.

1. INTRODUCTION

Manual rock discontinuity surveys, typically relying on handheld devices, face inherent drawbacks: difficulty, time consumption, safety hazards, and surveyor subjectivity (Ferrero et al., 2009; Franklin et al., 1988; Priest & Hudson, 1976). In response, remote surveying techniques emerged, generating 3D models through dense point clouds or textured mesh models (Assali et al., 2014). Recent advancements in computational methods have empowered the analysis of these models, promising a more accurate and detailed understanding of rock mass geometry (Battulwar et al., 2021).

Algorithms leveraging 3D rock mass models offer solutions to traditional survey limitations, including challenging access, lengthy procedures, and surveyor bias. They enable significantly increased measurement volumes on rock outcrops, covering inaccessible areas with minimal effort (Ferrero et al., 2009). Moreover, automated analysis of rock faces using 3D models presents cost-effectiveness in terms of operational expenses and time compared to traditional surveying (Slob et al., 2005).
AI-Driven Predictive Maintenance for Enhanced Reliability of Top Drive Thrust Bearings
Authors: Benarbia A. , Ghachi S. , Khalifa H. , Tomomewo O. S.
Published: August 2024

The maintenance of the thrust bearing in the Top Drive system is important for modern drilling operations. The bearing's role is to support the drill string and ensure efficient drilling processes. However, Operational stress often leads to wear and tear, resulting in significant consequences including unexpected downtimes and compromised safety, as well as bearing failures that come with costly repairs and operational delays. These issues raise the critical need for regular and proactive maintenance strategies to mitigate risks and ensure safety. Despite the recognized need for meticulous maintenance, there exists a gap in effectively predicting the health of these bearings to prevent failures. To address this gap, our study aims to evaluate the effectiveness of machine learning algorithms, using XGBoost and Random Forest, in predicting the health status of the thrust bearings. We conducted simulations to gather data to be able to study the potential of these algorithms to facilitate automatic monitoring and detect defects. The results indicate that machine learning can significantly enhance the predictive maintenance of thrust bearings, with XGBoost achieving a leading accuracy of 97.17%, followed by Random Forest at 93.09%. The implications of our findings are profound, offering a pathway to more reliable and efficient drilling operations through enhanced preventive maintenance that implement machine learning models.

1. INTRODUCTION

A top drive is a mechanical device on a drilling rig that provides clockwise torque to the drill string to drill a borehole. It is an alternative to the rotary table and Kelly drive (Boyadjieff, 1986). The top drive is located at the swivel's place below the traveling block and moves vertically up and down the derrick.

The role of a top drive in drilling is multifaceted. King and Douglas (1995) highlight the efficiency gains and safety improvements achieved with the adoption of Top Drive systems in modern drilling operations, they enhance drilling efficiency by allowing the drilling rig to drill longer sections of a stand of drill pipe in one operation. A rotary table type rig can only drill 30-foot sections of drill pipe whereas a top drive can drill 60–90-foot stands, depending on the drilling rig size (Mahmud, and Elmabrouk, 2016).
Changes in Seismicity, Stress, and Stress Proxies Before and After the Large Seismic Event with Mw 4.2 on May 18, 2020 in Kiirunavaara Mine (Sweden)
Authors: Dineva S. , Dahner C. , Beck D. , Agostinetti N. P. , Mihaylov D. , Guclu I. E.
Published: August 2024

An unusually large event with moment magnitude MW 4.2 occurred on May 18, 2020 in Kiirunavaara Mine (Sweden), followed by very intense aftershock activity for about 2 weeks. A comprehensive interdisciplinary study found that the event was the result of a combination of stress, rock stiffness, strength, structural and mining factors.

In this paper, we concentrate on the observed patterns of seismic source parameters, inferred seismic velocity, and inferred stress and strain changes before and immediately after the large event, to appreciate potential warning signs for future similar events. The observations of the relation between investigated source parameters - apparent stress, energy index and seismic velocity changes - and calibrated stress and strain change from a calibrated mine scale numerical model, add to the understanding of system changes during the event and may assist in future to infer the stability state of parts of the mine, and potentially to identify some emerging instability hazards.

1. INTRODUCTION

The Kiirunavaara Mine is a Sub-Level Caving (SLC) mine located in the north of Sweden, currently producing 25 Mtones crude iron ore per year. The production areas span ∼4 km along strike with footwall to hangingwall width varying from 30 to 150 m (Figure 1). The mine is divided into 10 production areas (blocks) along the strike, numbered by the central Y-coordinate.

The mine was declared significantly seismically active after a 2008 seismic event in Block 19 (22) which caused significant damage across sub-levels from 907-964 m. Today, the mine has one of the world's largest underground seismic systems with approximately 250 geophones (Figure 1) covering the production areas with a typical magnitude of completeness of the seismic system of around ML -1.5 (Dineva and Boskovic, 2017).

This study aims to continue the seismological investigation of the M 4.2 event, looking at long- and short-term changes in patterns of seismicity and quantitative seismological measures such as energy index, cumulative seismic moment and apparent stress, and relate them to numerical modeling results. The work also aims to progress the integration, in the mine's workflow for observational management of seismic hazard, of calibrated numerical modelling forecasts of stress with quantitative seismological parameters and seismic wave velocity changes inferred via passive tomography.
A New Finite Element Code for Thermo-Hydro-Mechanical Coupling in Geomechanics: Application to Deep Borehole Disposal of High-Level Radioactive Waste
Authors: Yoo H. , Min K.-B.
Published: August 2024

A new finite element code was developed for modeling thermo–hydro–mechanical coupled processes in continuum-based geomechanical applications. The development was conducted using open-source software and libraries for input of model geometry and mesh, numerical calculation, and data management to take advantage of the accessibility and flexibility of the source code for updates and revisions. Implemented primarily with the PETSc library for parallel mesh discretization and numerical solvers in C++, the code incorporates physics models including heat transfer, Darcy's flow and storage, and linear elasticity and their coupled processes. The physics models were verified by comparing them with analytical solutions and a commercial FEM code. The code was applied to the simulation of deep borehole disposal for high-level radioactive waste, modeling the generated thermal stress due to decay heat emitted from spent nuclear fuels. The simulated results of changes in coulomb failure stress and ground uplifts in the deep borehole disposal model were compared with those of the reference disposal concept. This study demonstrates the applicability and future extensibility of the new finite element code to field-scale geomechanical applications.

1. INTRODUCTION

Thermo-hydro-mechanical (THM) coupled processes are a key behavior in geomechanical applications such as nuclear waste disposal in geological media, underground CO2 sequestration, and enhanced geothermal systems. Understanding the complex interaction between heat transfer, fluid flow, and mechanical responses in porous media is essential for accurately analyzing and predicting the long-term performance and safety in subsurface environments. Numerical modeling can improve the understanding of complicated coupled processes by capitalizing on the benefits of advanced computer technology.

The active development of open-source software for numerical computation as well as pre- and post-processing has facilitated the emergence of large-scale projects aimed at developing numerical simulators for coupled processes in subsurface environments. Notable examples include OpenGeoSys (Bilke et al., 2022), MOOSE (Lindsay et al., 2022), PFLOTRAN (Lichtner et al., 2020), and PyLith (Aagaard et al., 2023). The development of open-source frameworks offers several advantages, including easy access to the source code and, thus, flexibility in writing and revising the code suitable for specific needs. Furthermore, another significant advantage is the ability to obtain independent simulators from commercial software.
Research on Fracture Characteristics of Supercritical CO2 Fracturing Hot Dry Rock Based on THMD Coupling
Authors: Liu Mingsheng , Wang Haizhu , Zhang Xu , Fan Xuhao , Wang Bin , Tian Ganghua , Yi Yonggang , Stanchits Sergey , Cheremisin Alexey
Published: August 2024

Supercritical CO2 has excellent thermal properties and flowability, making it a potential replacement for water in efficient and eco-friendly fracturing of hot dry rock (HDR). This study develops a coupled model for fracture propagation, considering wellbore stress superposition, thermal-poroelastic effects, elastic-brittle failure criteria, and varying matrix porosity and permeability due to damage. The model's accuracy is validated through comparisons with analytical solutions and experiments. Research finds that supercritical CO2 fracturing creates more complex fractures with a wider temperature field influence. Thermal stress reduces the breakdown pressure by 55.53%, while CO2 contributes 5.89%. Breakdown pressure in HDR increases with CO2 injection rate due to pressure transmission, coupling, and filling effects. Fracture initiation and propagation can be divided into three stages: (1) Early stage dominated by thermally induced stress from; (2) Intermediate stage with competition between in-situ stresses and stress shadow effects; (3) Later stage controlled by in-situ stresses, extending along the direction of maximum principal stress.

1. INTRODUCTION

Hot dry rock (HDR) refers to a type of rock formation that lacks or contains minimal stagnant water and has a high temperature (usually above 180°C). It is an important component of geothermal resources (Zhu et al., 2015). HDR contains significant geothermal energy that can be harnessed for power generation and heating, serving as a renewable and environmentally friendly energy source. In comparison to other renewable energy sources like wind and solar, HDR geothermal resources offer advantages such as stability and freedom from seasonal and diurnal limitations (Armstrong et al., 2016). Therefore, the efficient and safe development of HDR plays a crucial role in achieving sustainable energy. HDR mainly consists of metamorphic or granitic rocks characterized by dense structures and extremely low permeability, making it challenging to exploit thermal energy through natural fractures and rock matrix permeability (Olasolo et al., 2016). Hence, engineering methods are required to transform the HDR reservoir and create a complex network of fractures, providing efficient pathways for heat transfer to the working fluid.
Molecular Dynamics Modeling of Effects of Water and Temperature on Rock Strength
Authors: Liu Li , Li Xinlong , He Wenhao , Zhang Runqing , Li Yongxin , Fan Gaojian , Ji Ran , Shi Huaizhong , Xiong Chao
Published: August 2024

The mechanical properties of reservoir rocks are of great significance to the study of rock mechanics, hydraulic fracturing design and reservoir stimulation. The failure mechanism and characteristics of rock under the condition of high temperature hydration have been one of the difficult problems for many scholars. In order to solve this problem, a Molecular dynamics model is used to analyze the mechanism of rock strength degradation. The compressive strength, tensile strength, Young's modulus and Poisson's ratio of quartz crystals with different water content and temperature were quantified using uniaxial tensile and uniaxial compression simulation experiments based on the established single crystal molecular model of quartz. The simulation results show that hydration and high temperature degradation have significant effects on the mechanical strength of rock, and hydration is a major factor leading to the decline of rock strength. In addition, temperature and external loading force make the internal structure of the crystal looser by changing the bond length between Si and O atoms and the angle of Si-O-Si bond. It is also found that the presence of tensile stress will change the distribution of water molecules in the interlayer, making water molecules adsorbed on both sides of the quartz surface.

1. INTRODUCTION

With the continuous improvement of oil and gas exploration and development technology, oil and gas exploration and development gradually changed from conventional oil and gas reservoirs to unconventional oil and gas reservoirs[1-2]. As an important unconventional oil and gas resource, tight sandstone reservoir has become a new field of increasing oil and gas reserves and production. Tight sandstone generally has the characteristics of low porosity and low permeability[3], and is more compact than ordinary sandstone, so it needs to be reformed by hydraulic fracturing[4]. Previous studies have shown that the mechanical properties of reservoir rocks are the key factors to determine the fracturing effect besides fracturing parameters and reservoir temperature[5-6]. Therefore, understanding the mechanical properties of reservoir rocks is the key to increase oil production. Previous studies have shown that rock macro-mechanical parameters have an important impact on the study of mechanical properties of rock. However, they can not explain the fracture initiation, fracture propagation and anisotropy in the rock, so it is necessary to further study the characteristics of the rock from microscale or nanoscale[7-8].
Appraisal of Boreability Characteristics of Rocks to Estimate the TBM Advancement
Authors: Yagiz S. , Yazitova A. , Smirnov G. , Thyagarajan Vinayak M. , Hassanpour J.
Published: August 2024

The production rate of mechanical excavators such as tunnel boring machine (TBM) and roadheader depend on the geotechnical properties of rock, machine specifications and operational parameters. TBMs are being utilized for the construction of various tunnels for the purpose of access, conveyance of ore and waste, drainage, exploration, water supply and diversion in mining and civil projects. To obtain the maximum production with optimum power from any mining or civil operation, the excavator should be chosen carefully depending on the ground condition and mass rock characterization. Examining those rock characteristics, evaluating the geological structures in the field, and choosing the right TBM for excavation are the main geotechnical and technological investigation steps to conduct the project. In this paper, intact rock properties e.g., strength, density, brittleness and mineralogical features of rock, rock mass characteristics such as frequency and orientation of discontinuities are examined and their influence of TBM advancement are highlighted.

1. INTRODUCTION

Currently, mechanized excavation is one of the most popular and fastest way of rock and soil excavation around the world due to its high mechanization level, great operational efficiency and low environmental impact. However, the application of the method requires an accurate estimation of possible costs in the primary and secondary stages of projects. Due to that, the assessment of TBM performance via an either rate of penetration (ROP) or field penetration index (FPI) and rate of advance (AR) as well as utilization (U %) is one of the most effective factors, both in estimating the cost and the operation time of any project. Accordingly, the incorrect estimation of these parameters can lead to irrecoverable economic and time losses in proposed project. The ROP is one of the most commonly used parameters of TBM boring process; however, utilization and advance rate are also connected with ROP due to that these three indicators are the main targets to obtain for project scheduling. In present, different models and relations have been introduced to predict the penetration rate as a function of intact and mass rock and also machine specifications. Numerous simple and comprehensive methods have been proposed for predicting the TBM performance by researchers (Graham, 1976; Farmer and Glossop, 1980; Hughes, 1986; Rostami and Ozdemir, 1993; Palmstrom, 1995; Bruland, 1999; Barton, 2000; Yagiz, 2002; Bieniawski et al., 2008; Gong and Zhao, 2009; Hassanpour et al., 2009; Delisio and Zhao, 2014; Yagiz, 2017; Goodarzi et al 2021; Yagiz et al., 2024), and machine manufacturing companies (such as, Robins and Herrenknecht) throughout the world. These methods are generally based on field, machine and laboratory dataset obtain from laboratory tests. In general, these parameters can be classified into three main categories:
Characterization of Carbonate Rock Fragmentation Under Torsional Impact Loading
Authors: Xu Zhaohui , Ji Ran , Fan Jinzhao , Shi Huaizhong , Fang Chao , Liu Dong , Wang Yongqi , Song Shaobo
Published: August 2024

In order to study the influence of torsional impact technology on rock stress response, cutting force and other parameters of carbonate rock under different impact amplitude and impact frequency, the crushing test of carbonate rock was carried out by using the method of torsional impact assisted PDC bit, and the single-tooth rock crushing model under torsional impact was established, and the influence of different impact parameters on crushing efficiency was analyzed. The results show that under the condition of reasonable selection of torsional impact frequency and amplitude, torsional impact technology can effectively reduce the rock breaking torque and improve the rock breaking efficiency. The peak Mises stress increases by 42.3%,and the average cutting force of 1.5 times the amplitude is reduced by 10.6%. The research results are expected to provide a scientific basis for accelerating the drilling rate of carbonate rocks.

INTRODUCTION

Geothermal energy, as one of the renewable energy sources, is a resource that can provide low-carbon, renewable loads for the production of electricity and heat, and has great potential for development. Although geothermal energy resources have their unique advantages, taking carbonate reservoirs as an example, the deep geothermal resource reservoirs are characterized by low permeability, high abrasiveness and high hardness, which makes the development of geothermal resources relatively difficult. Geothermal drilling, as the most basic and important part of geothermal development, has been attracting much attention. Expensive drilling equipment, experienced drilling experts and strong financial support are necessary for geothermal energy development. Therefore, reducing the number of drilling days and improving the drilling efficiency are the main ways to achieve the goal of commercializing geothermal energy. Currently, the conventional rotary drilling method is commonly used for drilling wells in carbonate formations, but the low rock crushing efficiency and severe wear and tear of the drill bit of this drilling method, which in turn leads to long drilling cycles and high drilling costs, have severely restricted the large-scale development of geothermal energy in carbonate formations.
Research on the Characteristics of Supercritical CO2 Shock Fracturing Flow Field Based on Numerical Simulation
Authors: Zeng Changhui , Mu Zongjie , Wang Haizhu , Xu Chuanyou , Yi Yonggang , Stanchits Sergey , Cheremisin Alexey , Popov Yury
Published: August 2024

Unconventional oil and gas have significant resource potential and economic value. Based on the technical method of extracting unconventional oil and gas through supercritical carbon dioxide (SC-CO2) shock fracturing proposed by the research team, a SC-CO2 shock fracturing tool has been developed. Based on computational fluid dynamics and the finite element method, a three-dimensional model was established to simulate the flow field of SC-CO2 shock fracturing. Numerical simulations were conducted to obtain the spatiotemporal distribution patterns of pressure, temperature, and velocity. The results show that under impact load, the maximum stress strain and maximum temperature of the tool occur at the nozzle wall, with a maximum equivalent stress of 409.25 MPa and a maximum total deformation of 0.075 mm, meeting the strength requirements. The SC-CO2 fluid is released through the nozzle, resulting in drastic changes in the flow field temperature, pressure, and velocity, causing a backflow phenomenon and continuously impacting the reservoir. The research results can provide a reference for the development of SC-CO2 shock fracturing tools.

1. INTRODUCTION

Currently, with the advancement of global oil and gas resource exploration and development, unconventional oil and gas have shown significant resource potential and economic value (Wang et al., 2016). However, unconventional reservoirs have issues such as high seepage resistance and poor connectivity (Middleton et al., 2017), leading to high development costs. Therefore, it is necessary to enhance the reservoir, and hydraulic fracturing is the most widely used reservoir modification technology (Li et al., 2024). However, in the development of unconventional oil and gas, water-based fracturing fluid systems face technical challenges such as hydraulic fractures being controlled by formation stress; high water demand and low utilization efficiency (Shahbazi et al., 2020); clay swelling due to water sensitivity, blocking reservoir flow channels, and damaging the reservoir (Ahamed et al., 2019); incomplete flowback contaminating groundwater, and high wastewater treatment costs (Li et al., 2015), which restrict the development and utilization of unconventional oil and gas resources (Wu et al., 2022).