bluuebunny/crossref_metadata_2025 huggingface dataset:
Damage Mechanism and Parameter Optimization of Rock Mass Blasting Under High In-Situ Stress
Authors: Zhang Xin , Xia Xiang , Li Haibo , Fu Shuaiyang
Published: August 2024

The response of blasting effects during deep rock excavation processes is significantly influenced by high in-situ stress conditions. It plays a pivotal role in guiding both the preliminary design and subsequent implementation of support systems. This paper utilizes numerical simulation methods to simulate blasting under high in-situ stress, wherein the detonation process of high-energy explosives was simplified as instantaneous loading applied to the borehole walls. Through a combination of numerical simulations and on-site experiments, the study explores the evolution of blasting-induced damage. Adjustments and optimizations are applied to the blasting sequence, blasthole layout and explosive charge weight. The findings indicate that the scope of rock damage is reduced with the increasing of in-situ stress, there exists a transition of rock failure mode from tensile-shear to compressive-shear, and the detrimental effect of blast on tunnel safety matters less and less. In addition, excessively large spacing hinders effective penetration, while too small spacing will reduce the blasting efficiency. Under the circumstance with a constant borehole layout, the explosive charge governs the evolution of the blasting damage zone and the pre-blast area exhibits cyclic cumulative damage.

1. INTRODUCTION

Blasting in rock masses holds significant importance for engineering construction. In the past time, researchers have concentrated on the entire process of rock damage (initiation, propagation, and temporal evolution), as well as the mechanism of crack propagation with respect to the distance from the blast hole experimentally and simulation-based. The effectiveness of rock breaking primarily manifests through the action of detonation gases generated by explosive initiation and the propagation of stress waves that cause damage to the rock mass. Under the influence of blasting gases, rock damage near the blasthole is predominantly in the form of tensile failure (Pramanik & Deb, 2015), while at a distance, shear failure prevails (C. Ding et al., 2023). The extent of crack propagation depends on the surrounding rock conditions near the blasthole (Kutter & Fairhurst, 1971; Hagan, 1980; Trivino & Mohanty, 2015).
Deformation and Failure Mechanism of Ultra-Deep Dolomite and FDEM Numerical Simulation
Authors: Li Hongda , Lu Yunhu , Li Ji , Zeng Ping
Published: August 2024

The mechanism of rock deformation and failure in ultra-deep reservoirs is unclear, so this article takes ultra-deep dolomite as the research object and conducts rock mechanics experiments. Based on the damage variable theory model of Weibull distribution, the combined finite-discrete element method (FDEM) is used to numerically simulate the deformation and failure of dolomite. The results show that: (1) The compressive strength of ultra-deep dolomite is relatively high, and the failure is mainly brittle shear failure. Under high temperature and low confining pressure, the failure of dolomite exhibits a high angle shear fracture, accompanied by the development of secondary fractures. Under high temperature and high confining pressure, the failure mode of dolomite is often accompanied by other shear fractures in addition to one main shear fracture. (2) In the initial deformation stage, the linear characteristics of ultra-deep dolomite are more obvious. As the compression deformation deepens, the internal damage of the rock intensifies, gradually transitioning from elastic deformation to plastic deformation. The maximum damage rate occurs near the peak failure point and the fractures are formed at the macro level. In the post peak stage, the internal damage rate of the rock rapidly slows down and gradually reaches the residual strength.

1. INTRODUCTION

The first ten thousand meters geological exploration well in China, "Deep Taco 1 Well," was drilled on May 30, 2023 in the Tarim Oilfield in the hinterland of the "Sea of Death" Taklamakan Desert. However, the current drilling technology for ultra-deep and complex wells remains one of the global drilling challenges (Li et al., 2018; Li et al., 2020). The ultra-deep formation has characteristics such as high temperature and high pressure, brittle transition of rock mechanical properties, poor wellbore stability, poor drillability of the formation, and low drilling production efficiency. Among them, obtaining the mechanical characteristics of ultra-deep rock and studying the deformation and failure mechanism of deep rock are important theoretical and technical foundations to solve these problems, and basic research needs to be carried out in advance.
Productivity Prediction of Multilateral Horizontal Wells in Low-Permeability Gas Reservoirs Under Two-Phase Flow Conditions
Authors: Guo Xinyang , Cheng Shiqing , Bai Wenpeng , Cai Dingning , Xu Zexuan , Wang Yang
Published: August 2024

The productivity of multilateral horizontal wells will decrease rapidly after water breakthrough, which brings challenges to accurately predict production. In this paper, a semi-analytical model for predicting the productivity of multilateral horizontal wells considering two-phase flow is established based on the principle of conformal transformation and superposition of potential functions. The model considers the effects of various factors such as gas slippage effect and reservoir stress sensitivity. Compared to the conventional productivity prediction model, the model can not only consider variable mass flow in horizontal wellbore, but also different combinations of multiple branches. The results of field case and sensitivity analysis show that the productivity of multilateral wells decreases with the increase of gas-water volume ratio and reservoir stress sensitivity coefficient, and increases with the increase of gas slippage coefficient and the angle between branch wellbore and main wellbore. The presented model can help to better understand the flow characteristics of water-producing multilateral horizontal wells and accurately calculate the productivity, which is of great significance for the development of low-permeability gas reservoirs.

1. INTRODUCTION

Multilateral wells are widely used in complex reservoirs with the improvement of drilling and completion technology (Ouyang and Aziz, 2001; Tabatabaei and Ghalambor, 2011; Hassan et al., 2017). Compared to conventional vertical wells, multilateral horizontal wells can create more contact area with reservoir and effectively increase gas well productivity, making it an attractive option for efficient exploitation of low permeability reservoirs (Wang et al., 2015; Zhou et al., 2016).

The research on horizontal well productivity began in the 1950s. Many attempts have been made to characterize the productivity of horizontal wells, and several models have been presented. Analytic models are widely accepted because they are very convenient to use. Borisov (1984) proposed a production capacity model for horizontal wells in steady state. The assumptions of the model are: the formation is homogeneous and isotropic, the drain boundary is elliptical, the fluid is incompressible, and the formation damage caused by drilling and completing the well is ignored. The analytical model has more assumptions and is not realistic, so its use is limited.
Prediction of Liquid Loading in Changning Shale Gas Well
Authors: Li Jia , Jiang Zeyin , Liu Shuang , Wen Ming , Li Na
Published: August 2024

Addressing the technical challenge of fluid accumulation in Changning shale gas wellbores, we employed the Beggs Brill model to establish a tubing pressure drop calculation model. By integrating this model with the calculation model for annular static gas column pressure and fitting the casing pressure and bottom hole flow pressure production data, we derived a comprehensive calculation model for determining the depth of fluid accumulation in both the tubing and annulus. The accuracy of the model was validated through on-site assessments involving flow pressure, static pressure, and liquid accumulation depth testing. Utilizing Java language and a web development framework, we developed a wellbore fluid accumulation prediction software. This software enables production performance analysis and prediction of wellbore fluid accumulation, providing essential technical support for accurately forecasting fluid accumulation in Changning shale gas wells.

1. INTRODUCTION

During the middle and later stages of exploitation, the decline in gas production from Changning shale gas wells can be attributed to insufficient formation energy. Inadequate liquid carrying capacity in the production of gas wells during these stages is a consequence of the backflow of fracturing fluid retained from the early stages, leading to a susceptibility to wellbore fluid accumulation. [1-3] Consequently, predicting wellbore fluid accumulation becomes paramount for ensuring the stability of the shale gas well production process.

To address this, scholars have extensively researched methods for predicting wellbore fluid accumulation. Classic empirical calculation models, such as those proposed by Duns-Ros, Hagedorn-Brown, Orkiszewski, and Beggs-Brill et al., [4-7] are based on a wealth of experimental data and empirical formulas. In a later stage, Hasan et al.[8] introduced a new expression for liquid holdup grounded in the drift flow model. However, the empirical coefficients of traditional models have poor calculation accuracy in the Changning block, these models have limitations when applied to the conditions of calculating wellbore fluid accumulation in Changning shale gas wells, making it challenging to accurately predict the characteristics of wellbore fluid accumulation.[9]

To tackle the challenge of accurately predicting fluid accumulation, a novel pressure drop calculation model for gas-liquid two-phase flow in the Changning shale gas wellbore was developed. The accuracy of the model was validated through on-site assessments involving flow pressure, static pressure, and liquid accumulation depth testing. Additionally, corresponding software for predicting wellbore fluid accumulation was created, ensuring an accurate forecast of fluid accumulation in the wellbore.
Dynamic Rock Testing and Implications for Blast-Induced Ground Vibration Mitigation
Authors: Tetteh H. N. , Rinehart A. J. , McLemore V. T. , Li W. F.
Published: August 2024

mining requires continuous production through rock fragmentation by drilling and blasting. Drilling and blasting are essential to productivity but can result in blast-induced ground vibration issues in sensitive communities. Civil and geotechnical strategies considering rock structure and behavior under cyclic loading conditions can be used to predict ground vibration behavior and mitigate its effect. This study investigates key physics in cyclic dynamic wave propagation in rocks and provides fundamental results on dynamic wave propagation and attenuation in phyllite and granite under field-representative cyclic loading conditions. Supporting tests – x-ray diffraction (XRD) and porosity measurement are designed to characterize the physical and mineralogical behavior of the rocks. Uniaxial Compressive Strength (UCS) tests established the strong mechanical heterogeneity for both rocks and form the basis of the proposed mathematical model to predict each's monotonic rock strength. The mathematical model was developed based on the UCS to verify indicator points for subsequent cyclic experiments. Future work includes: (1) application of the mathematical model; (2) determination of wave propagation and attenuation in rocks under cyclic loading; and (3) investigation of coupling between rock damage accumulation and wave propagation. The experimental results are crucial to understand the fatigue process and predict rock behavior over time.

1. INTRODUCTION

In order to liberate minerals in the mining process, blasting is conducted in a measured and controlled manner to expose orebodies for mechanical excavation (Darling, 2011). In mineral projects, the mining methods usually dictate the tools and explosives used to accomplish blasting. The mechanism of blasting breaks the continuum of the rocks for various mining purposes. In the instance of hard rock mining operations, production holes of a specific diameter and depth are drilled according to the mine plan and orientation of the orebody. Explosives used in blasting operations are selected based on several factors including the type and size of the ore zone, rock hardness, and the type of fragmentation required by the processing plant.

Generally, drill and blast activities require the design of a drill plan comprising a boring of specified bedding and spacing for production holes in panels; loading of explosives (priming, stemming, emulsifying); attachment of detonators and delays, and subsequent detonation (manual or electric).

The energy from blast detonation is primarily expended in rock breakage but some of the energy is wasted due to factors such as overcharging. This wasted energy travels from the blast site in the form of ground vibration and airwaves and dissipates in various forms including ground vibration, noise, dust, heat, and noxious gasses – giving rise to some undesirable environmental effects if not mitigated (Gad et al., 2005).
Wellbore Stability Analysis of Wells in Paradox Play in Southeastern Utah: Investigating the Impact of Salt Mobility
Authors: Xing Pengju , McCormack Kevin , Edelman Eric , List Dave , Maxwell Gregor , McLennan John
Published: August 2024

This study investigated two incidents of pipe sticking that occurred during the drilling of well State 36-2LNW-CC in the Paradox Play in Utah. The study indicates that the pipe sticking could be associated with a combination of several mechanisms, all potentially active to some degree - wellbore breakout/limited caving and kinematic pipe constraint, fault slip, and salt creep. These three failure mechanisms might work together or one of them could dominate the pipe sticking. A more comprehensive analysis of the three mechanisms was conducted for the second stuck-pipe incident. The results show that increasing the mud weight from 13.0 ppg (used during the second stuck-pipe incident) to even 14.0 ppg could mitigate the risk of wellbore failure and stuck pipe. In addition, while back-reaming should be avoided (damage to gauge on PDC cutters), drilling ahead incrementally and pulling back periodically may be advisable in salt/clastic sequences.

1. INTRODUCTION

The Paradox Hydrocarbon Play in southeastern Utah could be among the most significant unconventional plays in the western USA. However, due to the unique geomechanical setting of salt/clastic cycles, it has witnessed several drilling challenges that may impact its future development. These include pipe sticking and well control events due to the high formation pressure in the reservoir. We conducted a comprehensive investigation into certain pipe sticking incidents, such as two stuck-pipe incidents in well State 36-2LNW-CC, using drilling data, wireline logs, and laboratory core test data. Several issues were identified as possible causes including wellbore stability associated with extensive salt interbedding between clastic layers. Multiple primary modes of salt mobility were identified, including creep, fault slippage in either the evaporites or the clastics, and more conventional wellbore instability (breakout or caving), any or all of which could have contributed to pipe sticking.

Understanding these mechanisms, as presented in this study, is crucial in mitigating future drilling challenges and ensuring the efficiency of operations within the Paradox Hydrocarbon Play.

The paper first describes the two stuck-pipe incidents during the drilling of well State 36-2LNW-CC and discusses the possible reasons. Then, a more comprehensive analysis of the three mechanisms (wellbore breakout, fault slip, and creep) was conducted for the second stuck-pipe incident. Finally, recommendations are proposed to mitigate the drilling challenges.
Numerical Evaluation of Mass Transfer Process on Acid Reactive Flow in Carbonate Rocks
Authors: Cunqi Jia , Saif Alkaabi , Kamy Sepehrnoori
Published: August 2024

Acid stimulation in carbonate rocks represents a quintessential application of reactive flow in subsurface porous media. Unlike chemical reactions that occur in solution, the chemical reactions involved in acid stimulation are essentially heterogeneous reactions. During the reaction process, the acid fluid and the solid matrix are accompanied by the mass transfer of reactants and products. This work is motivated to investigate the effect of the mass transfer process on the acid stimulation process. The two-scale continuum model is employed for capturing mass and momentum transfer, updating rock property parameters and acid fluid transport and reaction process parameters in response to pore structure changes. A comprehensive parametric study is conducted to examine the effect of the mass transfer rate on the acid breakthrough volume. The results show that compared with the influence of velocity and viscosity of the fluid phase, the ratio of surface reaction rate and mass transfer rate of the acid fluid is more sensitive to changes in pore radius and molecular diffusion rate. A smaller breakthrough volume is calculated with consideration of acid mass transfer compared to that calculated without consideration of acid mass transfer and less solid matrix being dissolved.

1. INTRODUCTION

Carbonate reservoirs contain a significant portion of the world oil and gas resources (Fredd and Fogler 1999, Panga, Ziauddin 2005). The complex nature of carbonates, including strong heterogeneity and the presence of fractures and vugs, represent technical challenges (Jia, Huang 2021, Jia, Sepehrnoori 2021). During oil and gas production, eroded particles accumulate near the wellbore and impede flow, thus diminishing the connection between the well and the reservoir and impacting productivity (Jia, Alkaabi 2023, Cunqi, Saif 2023). To combat this, acid is injected into the formation to dissolve carbonate minerals and form wormholes in a process known as matrix acidizing. This process provides higher conductivity pathways between the well and reservoir, bypassing formation damage and stimulating productivity (Dong 2018, Dong, Zhu 2018).
Study on Microscopic Pore-Fracture Structure of Metamorphic Buried Hill Reservoirs
Authors: Wang Xifeng , Huang Shijun , Zhao Fenglan , Liu Xinrong
Published: August 2024

The reservoir space of metamorphic buried hill reservoirs are complex and the heterogeneity is strong. It is difficult to fully characterize the micro pore and fracture structure of single resolution digital core. In this paper, multi-resolution CT and FIB-SEM scanning were performed on the cores of metamorphic buried hill reservoirs. The image registration method is used to match the core images of different resolutions. The union of pore structure at each resolution is taken to characterize the distribution law of pore and fractures quantitatively. The results showed that multi-scale digital core can more fully reflect the multi-type and multi-scale pore and fracture structure of metamorphic buried hill reservoir; The total porosity of the core is 1.25 %. The fracture is the main storage and seepage space. This method realizes the multi-scale characterization of the microscopic pore and fracture structure of metamorphic buried hill reservoirs, and provides a reference for similar complex reservoir research.

1. INTRODUCTION

A large number of buried hill reservoirs have been recognized worldwide, such as Zeit Bay, Bach Ho, Bozhong 19-6 fields, and so on (Mi et al., 2023; Trice et al., 2022). A buried hill is defined as a hydrocarbon accumulation in younger sedimentary formations in buried hill traps. The lithologies of buried hill reservoirs include metamorphic, igneous, and sedimentary rocks (Han et al., 2020). According to statistics, metamorphic rocks account for 40% of the world's total buried hill reservoirs, and oil and gas reserves account for 75% of the total reserves (Zhang et al., 2023). However, the metamorphic buried hill reservoir has developed fractures, complex spatial structure, and strong heterogeneity (Liu et al., 2020). An accurate understanding of the microstructure of metamorphic buried hill reservoirs is the key to the efficient development of such reservoirs.

The digital core has the unique advantages of non-destructive and three-dimensional characterization, which greatly retains the original characteristics of reservoir pores and fractures, and is an effective means of reservoir microstructure research. However, there is a contradiction between the field of view and the resolution of the digital core (Lin et al., 2019). When the resolution is low, a larger core size can be obtained, but at the same time, some microstructure information will be lost. When the resolution is high, the microscopic characteristics of the core are more refined, but there may be a problem that the representative volume unit (Gitman et al., 2007; Kanit et al., 2003) cannot be obtained. Therefore, it is necessary to integrate multi-scale information to describe the core structure.
An Autoencoder-Based Deep Learning Model for Enhancing Noise Characterization and Microseismic Event Detection in Underground Longwall Coal Mines Using Distributed Acoustic Sensing Monitoring
Authors: Tourei A. , Martin E. R. , Ankamah A. T. , Hole J. A. , Chambers D. J. A.
Published: August 2024

The longwall mining method is designed to optimize coal extraction through controlled roof caving, which inevitably induces seismicity. This research employs a distributed acoustic sensing (DAS) system incorporating a fire-safe fiber-optic cable strategically installed underground within an operational longwall coal mine. Despite lower sensitivity than traditional seismometers, DAS sensing technology benefits from dense sensor spacing and close proximity to the active face, where many microseismic events occur. To automatically detect seismic events within the voluminous DAS data records, we employ convolutional autoencoder deep learning models that can be used for anomaly (potential seismic event) detection in power spectral density (PSD) images of DAS recordings. The kernel density estimation (KDE) technique is used to calculate the probability density function (PDF) for the density scores of the latent space (representation of compressed data). We then use this calculated parameter as a threshold to distinguish between the PSD associated with background noise and with potential seismic events. The DAS monitoring system in conjunction with the developed deep learning model could enhance longwall coal mining safety and efficiency by offering valuable data from its densely deployed multichannel sensors near mining operations.

1. INTRODUCTION

Longwall mining is an efficient underground mining method for extracting a variety of stratified resources including coal, potash, and soda ash and represents a considerable advancement over conventional methods (Peng, 2019). A modern longwall primarily consists of hydraulic shields that support the roof and floor, a cutting device (e.g., a shearer or plow) that travels along the face extracting slices of coal, and an armored conveyor belt, which transports the resource to a larger mine haulage system. Normally longwall mining is safe and efficient, but a variety of ground control-related hazards are possible, especially in deep mines. One of the most significant of these hazards is a class of dynamic failures associated with induced seismicity and damage to mine workings, generally referred to as coal bursts or mine bumps. Much like tectonic earthquakes, mining-induced seismicity is difficult to predict and can have devastating consequences. For example, over the past several decades, coal bursts have killed hundreds of miners (Zhang et al., 2017).
Parameter Study on Crystalline and Sedimentary Rock for High-Level Radioactive Waste Disposal Considering the Geological Characteristics of the Korean Peninsula
Authors: Cheon D. S. , Park J. Y. , Jin K. W. , Lee H.
Published: August 2024

The selection of bedrock for deep geological disposal of high-level radioactive waste (HLW) typically depends on the geological characteristics of each country. In Korea, plutonic rock, gneiss, and sedimentary rock constitute 30%, 30%, and 25% of the geological composition, respectively, and the bed rock for deep geological repository has not yet been determined. In this study, 750 m deep drilling was conducted on several rock types. The drilling location was selected in consideration of the tectonic structure and the rock types. Several field tests were conducted using boreholes in granite and sedimentary rocks, as well as laboratory tests using cores. Among various evaluation parameters, uniaxial compressive strength, in-situ stress, hydraulic conductivity, fault development etc. were obtained and compared between granite and sedimentary rock. Through a series of analyses, it was found that in the case of crystalline rocks in Korea, granite from the Mesozoic Jurassic and in the case of sedimentary rocks, Jinju and Jindong formation from the Mesozoic Cretaceous period are likely candidate rock types. However, since it is the result of utilizing a small number of boreholes, it is expected that numerous in-depth investigations and mineralogical analyzes will be required.

1. INTRODUCTION

Korea began site selection for nuclear power plants in 1964, and Gori Unit 1 began commercial operation for the first time in 1978. In 1978, the first year of nuclear power generation, it generated 2,324 GWh, accounting for 7.4% of Korea's total power generation, but in 2022, it generated 176,054 GWh, accounting for 29.6% of the total power generation, playing an important role in Korean industry. However, like any other country that uses nuclear power, the issue of disposal of spent nuclear fuel generated from nuclear power generation is an important task to resolve. Currently, Korea temporarily stores high-level radioactive waste within nuclear power plants.

In December 2021, the government announced the second high-level radioactive waste management master plan and presented basic policies for high-level radioactive waste, facility plans such as site selection, technology development, and investment strategies. A step-by-step site selection procedure was presented to establish a deep disposal facility for high-level radioactive waste, and objectives and major activities for site suitability investigation were presented. According to the 2nd management master plan, site selection is scheduled to be carried out over a period of 13 years after the start of the project (Fig. 1)
An Assessment of Geomechanical Considerations in Thermal Energy Storage Systems Using 3D Coupled Thermo-Hydro-Mechanical Models
Authors: Mutlu U. , Ghassemi A. , Boitnott G.
Published: August 2024

Thermal Energy Storage (GeoTES) systems are designed to store heated or chilled brine to be produced when needed for power generation or district heating and cooling. High permeability and porosity sedimentary reservoirs have the potential to store large quantities of brine. In this study we present a geomechanics workflow that integrates subsurface data, from a US Texas Gulf Coast field site, with a fully coupled 3D Thermo-Hydro-Mechanical (THM) model. Our geomechanical modeling approach incorporates THM coupled solutions to simulate flow through porous media and heat transfer while considering progressive formation failure. 3D geomechanical model results show that THM loading conditions can lead to progressive damage around the wellbore and mechanical/flow alteration. Results also suggest that operational parameters can be optimized to minimize these alterations with direct implications to flow channeling and fines migration. Furthermore, preliminary Thermo-Hydro-Chemical (THC) simulations explore the scaling potential of the same thermal storage system using brine chemistry from offset wells. In the model, brine with measured concentrations of the major chemical constituents from the nearby well is heated and cooled (under pressure) to simulate injection of the brine under temperature and pressure. The relative changes in concentrations of various minerals, quantified from THC models, suggest that chemistry can play a significant role in thermal storage systems.

1. INTRODUCTION

Solar and wind power are key in our transition to a net-zero carbon future. While abundant and cost-competitive, they are variable, intermittent and their energy supply not always optimized to meet demand needs. For example, the shift to a clean, efficient, and modern grid is essential to California's economy and its environment. This transition to a low-carbon grid provides challenges and opportunities, as the state incorporates increasing amounts of renewable energy into the electric system.

When renewable resources generate more electricity than is needed, markets automatically reduce the production of energy from renewable resources, or "curtail" generation. While curtailment is an acceptable operational tool, as increasing amounts of renewable resources, oversupply conditions are expected to occur more often. System Operators are seeking solutions to avoid or reduce the amount of curtailment of renewable power to maximize the use of clean energy sources.
FDEM Modeling of PDC Cutter-Rock Interactions for Geothermal Drilling Applications
Authors: Heilman E. , Euser B. , Frash L. , Meng M. , Li W. , Lei Z. , Rougier E.
Published: August 2024

We aim to model PDC cutter-rock interactions in 3-D through numerical study to find an optimal arrangement of parameters for efficient geothermal drilling. Using the combined finite discrete element method (FDEM), we model realistic pressure conditions and rock fracture and fragmentation. FDEM can model both continuum and discontinuous mechanical behavior, which allows for modeling of fracture in targeted rock at downhole conditions. We simulate PDC cutter-rock interaction in 3-D for a range of back rake angles, cutting speeds, and depth of cuts using granite as our target rock material. These parameters are varied to investigate the impact on the cutting force and fracture dynamics in an effort to benchmark performance. We reproduce results from numerical rock cutting tests and qualitative results from laboratory experiments. We report on the progress of the numerical analysis for this complex multi-physics problem.

1 INTRODUCTION

Geothermal energy is an important form of clean energy in the United States and other parts of the world. Geothermal energy drilling involves deeper, hotter, and harder rock sources than conventional/unconventional oil and gas targets (Dwivedi et al., 2008; Kumari et al., 2017). Understanding the mechanics of geothermal energy drilling is essential to reducing the overall cost, of which drilling can be over half of the total cost (Pierce and Livesay, 1994). A commonly used drill bit due to its strength and resistance to abrasive wear is the polycrystalline diamond compact (PDC) cutter bit (Dougherty et al., 2014; Yang et al., 2018). The combination of deep, hard rock means the forces in geothermal drilling are typically larger and drilling performance needs to be increased. The rate of penetration (ROP) determines the efficiency of drilling, as in how much rock material can be removed. A high ROP would remove more rock material, however, higher ROP means higher forces on the drill bit. Determining the forces on the drill bit under a geothermal drilling interaction allows us insight into drilling performance.
Physical Models of Dry Masonry Retaining Walls
Authors: R. Alejano, L. , I. Pérez-Rey, , Muñiz-Menéndez M. , Ding B. D.
Published: August 2024

Physical models are a sound tool for verifying some instability mechanisms. The authors apply these physical models to check the stability analysis of traditional dry granite masonry retaining walls as built in NW Spain. Unlike concrete retaining walls, dry masonry ones should be designed to ensure not only general stability but also an intrinsic one, since they can fail due to sliding through masonry contacts or to toppling mechanisms of parts of the wall. Accordingly, first, unfilled scaled retaining walls made up of small granite slabs are tilted until failure is observed, in this case due to toppling, following a stepped path. Secondly, simple retaining walls made of granite slabs are filled with sand of known friction angle until failure. In both cases, analytical expressions are applied to the model at stake, showing a rather good accuracy in predicting the observed phenomena. Some ideas are finally provided suggesting aspects that need a rigorous knowledge when applying the formulation to large size real cases.

1. INTRODUCTION

In the words of Professor Einstein (2023), physical models are very suitable tools for verifying analytical models and the actual response of rock against specific failure mechanisms. Once verified, numerical approaches can help to expand the variation of parameters tested and carry out advance studies (back-analysis or sensitivity). Popular in the past, physical models are now hardly used by the rock mechanics community, probably due to cheaper numerical modeling options. However, they present some advantages in relation to modern numerical approaches.

In this brief study, the authors present some rather simple physical lab-scale models of dry masonry retaining walls that were used first to identify the failure mechanism at stake and then to provide a verification of analytical formulations proposed elsewhere.

The authors presented a study on the behavior and stability of traditional retaining drystone granite walls as traditionally built in Galicia, the northwest region of Spain (Alejano et al. 2012a and b). These structures are still commonly in use in this region for building and road construction, and occasionally, they achieve heights well over 10 m. They are generally cheaper and less environmentally aggressive than concrete walls and present an excellent landscape integration (Pozo et al., 2013), generating a low environmental impact, since the material used often comes from waste or low-price material from granite quarries producing large blocks for the ornamental stone market in the region.
Analysis of Shear Slip Ahead of Hydraulic Fracture Along a Frictional Interface Not Aligned with Principal Stresses and Conditions for Secondary Branching
Authors: Weng X.
Published: August 2024

Stress field at the trailing edge of the shear-slip zone ahead of a hydraulic fracture (HF) traveling along a natural fracture (NF) not aligned with maximum horizontal stress is studied in the context of its potential for inducing secondary fracture branching from the NF. The size and amount of shear-slip and the stress parallel to the NF wall are computed. Localized tensile stress occurs at the trailing edge of the shear-slip zone, with a logarithmic singularity which is similar to the elastic solution for a half-space subjected to a surface shear traction. The induced tensile stress may lead to a small surface crack being nucleated and HF branching away from NF. From the stress solution, maximum stress intensity factor K1max at the tip of a prospective crack and its corresponding length are estimated. If K1max is greater than fracture toughness, a crack can be nucleated. Example cases show that crack initiation strongly depends on HF length, NF angle, coefficient of friction and stress anisotropy. Crack initiation occurs only when HF reaches a sufficient length since the shear-slip zone length is proportional to HF length, which in turn affects K1 at the crack tip.

1. INTRODUCTION

Interaction of a hydraulic fracture (HF) and a weak interface, which can be an open or poorly cemented natural fracture (NF) or a bedding plane and is hereafter referred to as a NF in general, plays an important role in fracture stimulation of unconventional formations. When a HF intersects a NF, the interaction may result in HF crossing NF directly or with an offset (also referred to as a step-over), or being arrested by the NF. In the latter case, HF can dilate and propagate along the NF if fluid pressure exceeds the normal stress on the NF. The crossing/arrest behavior at the moment of fracture intersection has been extensively studied. Various empirical/theoretical criteria had been investigated and proposed (Warpinski and Teufel (1987), Blanton (1982, 1986), Renshaw and

Pollard (1995), Gu and Weng (2010), Gu et al. (2011), Chuprakov et al. (2013)) and been incorporated in complex fracture network models (Weng et al. (2011), Weng et al. (2016)). The understanding of a HF crossing a weak interface with an offset is somewhat limited. Cooke and Underwood (2001) studied numerically the stress parallel to the interface on the opposite wall ahead of the tip of a vertical HF approaching a weak bedding plane. They attributed the initiation of offsetting fracture to tensile peaks in the stress on both sides of the intersection point due to local interfacial opening and slippage near the intersection point for "moderate-strength" interfaces. These peaks move closer together and their magnitude increases as the distance between the fracture tip and the interface becomes small, and they occur in the shear-slip region ahead of the opening segment. Fracture offset has also been studied numerically by Thiercelin and Makkhyu (2007), and Chuprakov et al. (2010).
Hyper Fast THMC Well Integrity Simulator for Powering Optimization, Sensitivity and Risk Analysis
Authors: Kostorz W. , Le Blevec T. , Joulin C.
Published: August 2024

A semi-analytical approach to three-dimensional Thermo-Hydro-Mechanical-Chemical (THMC) well integrity simulations is discussed. The method extends previous plane-strain thermo-poroelastic models under a set of realistic assumptions and combines analytic and numerical solutions in a single framework. Non-analytical components of the model are resolved via one-dimensional Finite Difference schemes, and coupled with pen-and-paper solutions via simple numerical integration schemes. Time-dependent material properties of hydrating cement are taken into account in the incremental problem formulation and all model components are fully coupled. The proposed formulation is demonstrated to be in very good agreement with sophisticated FEM software, while the simulation run times are reduced by several orders of magnitude. Natural applications of the methodology proposed include high-volume workflows relying on thousands or more simulation runs (sensitivity studies, optimisation, Monte Carlo methods etc.).

1 INTRODUCTION

Well integrity is essential in many engineering applications, including CO2 storage, well plugging & abandonment, geothermal energy, oil & gas and gas fuel storage (CH4 and H2). There is a clear need from the industry for a computational well integrity assessment tool which is efficient, robust, can work with limited data sets and/or high uncertainties, and fits well within a risk-based statistical approach.

From the point of view of computational mechanics, well integrity is primarily controlled by the internal stresses within different materials comprising the well structure; these typically include metal casings surrounded by cement sheaths and the formation. Historically, two primary approaches have been undertaken to model the relevant stress responses. On one hand, the Finite Element Method (FEM) is widely used due to its flexibility and generality, in particular the natural capability of working with realistic geometries. Alternatively, idealised axisymmetric analytic models (often limited to two dimensions) have been studied by multiple authors Onaisi et al. (2017); Gholami et al. (2016); Joulin et al. (2022); Li et al. (2016); Bai et al. (2015). FEM and two-dimensional analytic models mark two ends of a spectrum; FEM is general and flexible but computationally expensive, while analytic models are ‘near-free’ computation-wise but oversimplified.
Methodology for Constructing Three-Dimensional DEM Model of Rock Specimen from CT Scan
Authors: He Changdi , Mishra Brijes
Published: August 2024

Developing numerical simulation models that accurately represent actual features of rocks, including the distribution of minerals, voids, and fractures, is crucial. The advancement in non-destructive evaluation methods, especially X-ray computed tomography (CT), allows for detailed analysis of the internal structures of rocks. This study uses the Res-VGG-UNet deep learning model to segment CT images, distinguishing between components that appear similar but are distinct. Integrating the findings from deep learning into PFC3D software enables the creation of rock models that truly mimic the features of actual samples. The simulation results obtained from these PFC3D models show a high degree of agreement with experimental outcomes, highlighting the effectiveness of these methods in developing accurate rock numerical models.

1 INTRODUCTION

The mechanical properties of rocks are essential for engineering applications (Jin et al., 2020), including, but not limited to, geological storage of radioactive wastes (Gibb, 1999), enhanced oil recovery (EOR) (Thomas, 2008), and the extraction of deep geothermal energy (Zimmermann and Reinicke, 2010). The determination of these properties typically involves a combination of experimental and numerical methodologies, with numerical simulations being widely used for their cost-effectiveness and time efficiency (Gentilini et al., 2013). Among the numerous techniques available for modeling the mechanical behavior of rock, the finite element method (FEM) and discrete element method (DEM) have emerged as powerful tools.

In the realm of FEM, some researchers have developed ABAQUS numerical models to explore the mechanical behavior of rock bolts bonded with different materials (Małkowski et al., 2023). Others have also applied FLAC3D for simulating both stable and unstable rock failures, implementing a constitutive law that incorporates a softening component, specifically the strain softening model (Kias et al., 2011). From the perspective of DEM, three-dimensional distinct element code (3DEC) bonded block models (BBM) are used to conduct numerical simulations (Corkum et al., 2024). These simulations assessed the behavior of a randomly discontinuous jointed rock mass subjected to a range of isotropic and devia-toric (shear) loading conditions. Particle flow code 3D grain-based models (PFC3D-GBM) integrated the specific mineral composition and grain size distribution of marble, considering the interactions both between and within the mineral grains (Zheng et al., 2023). Moreover, coupled analysis methods are increasingly common, combining the strengths of different simulation techniques for comprehensive analysis. For example, a 3D split Hopkinson pressure bar (3D-SHPB) numerical model was developed through the integration of PFC3D and FLAC3D techniques (Zhou et al., 2024).
Lessons Learned from Recent Longwall Mine-By Cases with Uncemented Casings
Authors: Zhang P. , Su D. , Tulu B. , Kim B. H.
Published: August 2024

Over 2,000 shale gas wells have been drilled in the longwall reserves of the Pittsburgh Coal Seam. When gas wells are in the vicinity of longwall mining, casing deformations are likely to occur due to longwall-induced subsurface movements and abutment stresses. One way to prevent casing deformation is to use a cementing alternative by leaving production casing uncemented. This paper presents three mine-by cases in which shale gas wells were in the chain pillars and the production casings were uncemented. These cases represent mine-by scenarios under shallow, medium, and deep cover. Casing deformations were measured with multi-finger caliper survey and predicted with numerical modeling. These mine-by case studies provide a general demonstration of how uncemented casing can uncouple production casing from longwall-induced stresses and deformations. Based on measurements of casing deformations and numerical modeling, the factors mentioned above are further analyzed to indicate in what conditions a cementing alternative may not eliminate production casing deformations. The knowledge gained from the mine-by case studies would help longwall operators and gas companies to optimize pillar design and casing design, thus minimizing the influence of longwall mining in the stability of shale gas wells in longwall pillars.

1. INTRODUCTION

With the shale gas boom, about 2,000 shale gas wells have been drilled through the current and future Pittsburgh coal reserves over the past 20 years. These shale gas wells, if located in longwall pillars, will be influenced by longwall mining. Longwall-induced subsurface ground movements would induce deformations in gas well casings. The concern is that if casing deformation is excessive, a casing breach could occur, and high-pressure shale gas could leak into the longwall mine with serious results. Because of the safety concern of gas wells in longwall pillars, the National Institute for Occupational Safety and Health (NIOSH) has been conducting research since 2016 to evaluate the effects of longwall mining on casing deformations under various mining depths and geological conditions.

NIOSH research has found that casing deformations are mainly caused by subsurface horizontal movements, especially under shallow and medium cover. NIOSH researchers have done several case studies with test wells in chain pillars under different overburden depths [Su (2016, 2017), Su et al. (2018, 2019), Zhang and Su (2021), and Zhang et al. (2022)]. These studies showed that longwall-induced relative horizontal displacements occur at weak-to-strong rock interfaces and are higher under shallow cover than under deep cover. The studies also showed that longwall-induced horizontal displacements are also affected by surface topography and strata dip.