bluuebunny/crossref_metadata_2025 huggingface dataset:
Quantitative Assessment of Anisotropic Elastic Properties of Rock Using Disk Specimens
Authors: Kim D. Y. , Kim S. S. , Seo S. M. , Diaz M. B. , Kim K. Y. , Kim H. , Park E. S.
Published: August 2024

Rocks exhibit anisotropy in various properties, including ultrasonic waves and diametrical deformation in the radial direction. While previous studies have attempted to analyze rock failure and anisotropy using several cores, the aim of this research is to evaluate the properties and anisotropic characteristics of rock materials through non-destructive experimental analysis of a single core. This analysis focuses on three physical properties: diametrical change, P-wave velocity, and compressive elastic modulus. The specimens were shaped into disks, with a diameter of 50 mm and thickness ranging from 26 to 29 mm. They were subjected to mechanical loading, ultrasonic waves, and diameter measurements in rotating radial directions to quantify anisotropic traits. These experiments revealed significant variations in diameter change, elastic modulus, and P-wave velocity across different orientations, indicating a substantial degree of anisotropy in the rock specimens. Consequently, this research performed non-destructive analysis of rock cores, characterizing physical properties, and analyzing anisotropy, confirming that the diameter and P-wave velocity maintain nearly perpendicular angles. Additionally, X-ray CT analysis has shown that rocks with lower homogeneity exhibit more pronounced anisotropy.

1. INTRODUCTION

Rock anisotropy is primarily influenced by structural features such as cleavage, foliation, bedding planes, schistosity, along with various joints and fissures ranging from micro to macro in size, and is also determined by two main mechanisms when subjected to structural loads: the alignment of minerals and particles, and the arrangement of pores and microcracks, which vary based on the rock's composition (Phillips and Phillips, 1980). Rocks possess hydraulic, mechanical, thermal, dynamic, and physical properties, and these anisotropy of structural properties causes these characteristics to vary depending on the direction (Johnston and Christensen, 1995; Amadei, 1996). Understanding the importance of rock anisotropy is essential, as it directly influences different applications such as seismic wave propagation in oil and gas exploration, fault stress distribution, which is critical to earthquake preparedness, impacts the structural integrity of dams, dictates the containment of radioactive materials, affects heat extraction rates in geothermal operations, and guides excavation strategies in tunnel construction.
Key Design Parameters for Enhanced Geothermal System (EGS)
Authors: Dehdouh Abdesselem , Mellal Ilyas , Bettir Nassima , Abdelhamid Cilia , Allam Lotfi , Rasouli Vamegh , Gyimah Emmanuel
Published: August 2024

Enhanced Geothermal Systems (EGS) offer significant potential for clean energy extraction from hot rock formations. However, maintaining high flow circulation, maximizing heat extraction, and minimizing thermal losses remain major challenges. This study presents a modeling investigation of key physical processes and design considerations for a multistage hydraulically stimulated EGS, utilizing a fully integrated reservoir model. We analyzed the influence of the key fracturing design parameters (injection rates, cluster spacing, well spacing, and fracture conductivity) alongside formation temperature. The findings demonstrate that multistage stimulation within a single cluster enhances well flow performance, delays thermal breakthrough, and enables higher circulating flow rates. Sensitivity analyses reveal a substantial impact of each parameter on EGS performance. Additionally, net present value (NPV) calculations are incorporated to evaluate the economic feasibility of each scenario. This comprehensive study offers valuable insights for appraising and selecting EGS candidates, paving the way for advancements in sustainable and efficient geothermal energy production.

1. INTRODUCTION

The increasing demand for clean and sustainable energy has intensified the search for alternative sources. Among these, geothermal energy stands out, harnessing the Earth's internal heat to generate electricity and provide direct heat with very low greenhouse gas emissions (Lu, 2018). Around 11000 MWe of electricity is being produced worldwide from geothermal sources, and the DOE goal is to reach 90.000 MWe by 2050 (Huttrer, 2021). High temperatures of more than 120 C and high flow rates are required for an economic project (Blackwell. D et al., 2011). Conventional geothermal sources are limited, often concentrated in regions with naturally fractured and permeable rock formations to maintain high flow rates (Gyimah et al., 2023). This is where Enhanced Geothermal Systems (EGS) emerge as a game-changer, unlocking the vast potential of hot, impermeable rock formations that were previously inaccessible(Okoroafor et al., 2022).

EGS technology creates an artificial flow path by stimulating the underlying rock through hydraulic fracturing (Gyimah. E et al., 2024). Multiple producer and injector wells were drilled with close spacing to circulate the continuously injected fluid. The injected fluid gets its thermal energy from the formation and then produced to the surface (Huenges, 2016). This enhances permeability, allowing fluid circulation and heat extraction. While still in its developmental stages, EGS holds immense promise for significantly expanding geothermal energy's reach and potential (Merzoug, Okoroafor, et al., 2023).
Experimental and Numerical Investigation on the Fracture Characteristics of Different PDC Cutters Penetrating Limestone
Authors: Fu Xinkang , Huang Zhongwei , Zhang Zhuangzhuang , Ji Guodong , Zhang Jiawei
Published: August 2024

Carbonate rocks are an important oil and gas reservoir rock. Conical PDC (Polycrystalline Diamond Compact) cutter, as a special shaped PDC cutter, can significantly improve the rate of penetration (ROP) in hard formations. However, the breakage characteristics and failure mode of the conical PDC cutter penetrating carbonate rock is unclear. A series of single cutter penetration tests were carried out with the conical and conventional PDC cutters. The surface morphology of broken pit and specific energy were analyzed. The conical cutter caused brittle damage on the rock around the broken pit at the rake angle of 10 and 20 degrees. The results of the numerical model based on the cohesive element method show that the conical cutter produce the significant tensile damage on the rock at the rake angle of 10 and 20 degrees. The tensile capacity of the conical cutter is stronger than that of the conventional cutter. The research findings of this work offer insights into the breakage process and failure mode of carbonate rock by the conical cutter.

1. INTRODUCTION

Carbonate rocks are an considerably important oil and gas reservoir rock (Wenzhi et al., 2014). The carbonate rocks store 70% of the oil and gas resources. Nearly 50% of proven recoverable oil and gas resources are stored in deep and ultra-deep carbonate formations worldwide (Al-Shargabi et al., 2022). Therefore, carbonate oil and gas resources have great exploration and development potential.

The carbonate formations in China are concentrated at depths ranging from 5000 to 7000 meters (Ma et al., 2017). These formations consist of challenging rocks that are hard, and poorly drillable, resulting in a low rate of penetration (ROP) (Zeng et al., 2023).

PDC bits dominate the global drilling market (Rostamsowlat et al., 2022). In recent years, a large number of special-shaped cutter, including conical cutter, have flooded into the drilling market (Izbinski et al., 2019). In 2010, Novatek Company invented conical PDC cutter (Durrand et al., 2010).
Deciphering the Tensile Fracturing Process Using Acoustic Emission
Authors: Wu Shan , Yang Hui , Zhao Qi , Su Boyang , Cai Guanglei
Published: August 2024

We simulated the tensile fracturing process, capturing both microseismic events (acoustic emissions) and the dynamics of fracture propagation through high-speed photography. To record the millisecond-level fracture propagation, a synchronization circuit was employed, designed to align the 10 MHz acoustic emission recordings with high-speed images taken at a rate of 200,000 frames per second. This setup enabled us to precisely record fracture propagation within a 1.5-microsecond period. Our findings show that the relocated acoustic emission events align closely with the locations of macroscopic tensile fractures. We observed a microsecond-level time delay between the fracture propagation process and the clustering of acoustic emissions. This delay indicates that the clustering of acoustic emissions is related to the fracture nucleation process, while the high-speed photography records the coalescence and growth of fractures, reflecting the development of macroscopic cracks. Considering the temporal and spatial alignment between the acoustic emissions and the propagation of tensile fractures, it is evident that microseismic monitoring primarily captures the early stages of crack formation. Our research indicates that current microseismic monitoring results lack interpretation of the relationship between the nucleation stage and macroscopic fractures, as well as the relationship between microseismic clouds and fracture networks.

1. INTRODUCTION

Microseismic monitoring represents a field technique in observing hydraulic fracturing cracks(Baig and Urbancic, 2010; Cipolla et al., 2012). While it is understood that hydraulic fracturing generates a mix of tensile and shear fractures(Busetti et al., 2014; Fischer and Guest, 2011; Wu et al., 2019), tensile fractures play a more significant role due to their greater contribution to permeability enhancement. This fact highlights the need to better understand tensile fractures. However, most established theories on microseismic inversion are based on traditional seismology(Eisner et al., 2011; Vavryčuk et al., 2008), which predominantly concentrates on shear fractures or slips. Such a concentration restricts the application of microseismic data in interpreting hydraulic fractures. Key challenges include determining whether microseismic events originate from hydraulic tensile fractures(Maxwell, 2011; Warpinski et al., 2013), understanding the relationship between the spatial extent of microseismic clouds and hydraulic fracture zones(Mayerhofer et al., 2010; McKean et al., 2019), and correlating microseismic event magnitudes with the geometric parameters of fractures(Wu et al., 2019).
Optimizing Recovery in Unconventional Reservoirs by Advancing Fishbone Drilling Technology in the Bakken Formation, Williston Basin
Authors: Dehdouh A. , Bettir N. , Khalifa H. , Kareb A. , Al Krmagi M.
Published: August 2024

The integration of multi-stage hydraulic fracturing and extended horizontal drilling has markedly advanced oil extraction from the Bakken tight oil reservoir, a prominent unconventional reservoir in the United States. Despite these technological strides, the primary recovery rates from this tight formation have remained suboptimal, leaving a considerable volume of oil unrecovered. This challenge underscores the necessity for innovative Enhanced Oil Recovery (EOR) methods tailored to the unique conditions of the Bakken formation. While CO2 injection has been explored, its effectiveness is limited by the formation's heterogeneity. This study introduces an innovative approach through the application of Fishbone Drilling (FbD) technology, aimed at enhancing recovery efforts in the Bakken formation. Our investigation reveals that FbD, characterized by multiple minor holes branching from the main wellbore, significantly improves hydrocarbon recovery by enhancing reservoir contact and exploiting the existing natural fractures more effectively than traditional methods. The findings indicate that FbD not only promises an increase in hydrocarbon flow but also, through optimization of design parameters, aligns with economic viability over time despite higher initial costs. This paper presents a comprehensive analysis of FbD's optimal design and its efficacy in the Bakken formation, supported by data-driven insights that validate the technology's potential to revolutionize EOR strategies in unconventional reservoirs. Through detailed numerical simulations and comparative analyses, this study highlights the superior performance of FbD over conventional drilling and fracturing methods, proposing a promising avenue for future exploration and development within the unconventional reservoirs.

1. INTRODUCTION

The quest for enhanced oil recovery (EOR) techniques in unconventional reservoirs has been a pivotal focus within the energy sector, driven by the imperative need to optimize hydrocarbon extraction from formations like the Bakken tight oil reservoir (Almousa et al., 2023a). Located in the Williston Basin, one of the United States' most expansive unconventional reservoirs, the Bakken formation presents a unique set of challenges and opportunities for EOR strategies (Helms et al., 2023). Hydraulic fracturing and re-fracturing contributed heavily in the oil and gas production in the Bakken as reported by (Dehdouh et al, 2023a). Despite the significant strides made through the integration of multi-stage hydraulic fracturing and extended horizontal drilling, these conventional methods have only marginally improved the primary recovery rates in such tight formations. This scenario underscores the substantial volume of oil that remains untapped, highlighting the critical need for innovative approaches to EOR as detailed by Merzoug et al. (2024).
Quantifying Anisotropic Viscoelastic Parameters Using In-Situ Pressuremeter Testing in Shale
Authors: Liu L. , Trzeciak M. , Xie X. , Chalaturnyk R.
Published: August 2024

Shale exhibits a certain degree of anisotropy in its deformation properties. This anisotropy is important to consider in rock engineering problems. Pressuremeter (or dilatometer) testing, as an in-situ method to determine geomechanical parameters, has revealed its potential in characterizing this anisotropy with the borehole deformation measured by calipers oriented at multiple diametric axes. In this work, a transversely isotropic viscoelastic model is proposed to predict the anisotropic borehole deformation for pressuremeter testing in the boreholes drilled perpendicular and parallel to the bedding of the shale. The determination of the model parameters is demonstrated using the data obtained from two pressuremeter test cases in Opalinus Clay, a shale with strong anisotropy. The elastic parameters of the model are determined with the model fit on the data from the unloading step while the stiffness relaxation parameters are determined using the model fit on the borehole creep deformation measured during the pressure hold step. The stiffness relaxation parameters determined from the two test cases are considerably different, implying the altered viscoelastic properties in the local borehole damage zone induced in the test case when the borehole is drilled parallel to bedding.

1. INTRODUCTION

Shales play a critical role in various subsurface engineering projects, including petroleum extraction, geological sequestration of CO2, and construction of underground gas storage and radioactive waste disposal facilities. Accurate measurement of their geomechanical properties is important for the successful design and implementation of these projects. A key aspect of shale behavior under subsurface conditions is its tendency to undergo creep deformation when subjected to loading or unloading (Sone & Zoback, 2013). The principle of linear viscoelasticity has been used to model the creep of shale (Trzeciak et al., 2018). Due to its fine-grained texture and laminated structure, shale also exhibits a certain degree of anisotropy in its geomechanical properties. This anisotropy is important to consider in predicting and managing the behavior of shale formations in various engineering applications such as wellbore stability and caprock integrity assessment.
An Improved Time-Dependent Convergence Confinement Method for Estimation of Tunnel Support Loads in Squeezing Ground Conditions
Authors: Arora K. , Gutierrez M.
Published: August 2024

In tunnel construction, the total tunnel wall displacement increases due to face advancement and the time-dependent behavior of the surrounding rock mass. The convergence confinement method (CCM) is one of the approaches used to analyze the interactions between tunnel wall displacement and support load. CCM utilizes either analytical closed-form solutions or empirical Longitudinal Displacement Profiles (LDP); however, it usually neglects the influence of time-dependency of ground response and associated gradually increasing deformations even after the excavation stage. Designing the tunnel support system without consideration of the time-dependent deformation may result in a false selection of the installation time and the support system type, causing safety issues, cost overruns, and project delays. This paper focuses on discussing a revised CCM to estimate the tunnel's support system loads in a geomaterial with squeezing characteristics and tunnel wall converges significantly with time. The proposed methodology is based on the outcomes of the physical model test results and observations from the tunnels that have experienced squeezing in different parts of the world. The initial section of this paper elucidates the Convergence Confinement Method (CCM) and outlines enhancements aimed at broadening its applicability to squeezing tunnels. The subsequent portion of the paper details the validation process of proposed method, followed by a systematic approach to designing tunnel support systems in squeezing ground.

1. INTRODUCTION

Tunnels in squeezing ground experience significant deformations even after the excavation (Barla, 1995). The load the converging rock mass/soil imposes on the tunnel support system also increases gradually. The support required to stabilize the tunnel boundary in squeezing ground can be two to three times the thickness/stiffness of the support for the same tunnel in non-squeezing conditions (Jethwa et al., 1984). Estimating the deformations due to squeezing beforehand is vital to ensure safe, efficient, and economical tunnel construction (Barla, 2001).

The primary reason for the squeezing ground conditions is the presence of clay minerals in a geological unit with a low strength ratio to the in-situ stress at the tunnel location (Terzaghi, 1946; Wood, 1972). Some of the notable examples of such tunnels include the John Street Pumping Station Tunnel in Canada (Lo et al., 1987), the Still Water Tunnel in the United States (Phien-wej & Cording, 1991), the Uluabat Project Tunnel in Turkey (Bilgin & Algan, 2012), and the Laodongshan Tunnel in China (Cao et al., 2018).
Well Integrity Analysis During CO2 Injection in a Depleted Chalk Field
Authors: Amour F. , Hajiabadi M. R. , Hosseinzadehsadati S. , Nick H. M.
Published: August 2024

The present study is part of the Bifrost project and aims at assessing the feasibility of storing CO2 in chalk reservoirs by using the depleted Harald East gas field located in the northern part of the Danish North Sea as a showcase. Chalk is a singular rock with a high compressibility and the resulting pronounced compaction during hydrocarbon exploitation and possibly during CO2 storage raises questions on the integrity of cement surrounding the legacy, monitoring, and injector wells. In this context, the study carries out a well stability analysis of a well present in the field. The strain and stress distribution computed by a full-field coupled thermo-hydro-mechanical model is used as input data in a finite element model, which captures the properties and mechanical behavior of a near wellbore area (formation, cement, and casing) located in the seal interval of the field. The results show negligible deformation in the cement during hydrocarbon production and CO2 storage, thereby suggesting low risk of leakage along the studied well.

1. INTRODUCTION

Complying with the ambitious objectives of the intergovernmental panel on climate change in terms of amount of greenhouse gas emission reduction requires the quick implementation of multiple carbon capture storage (CCS) projects worldwide (IPCC, 2005). In this context, the re-use of pre-existing oil and gas infrastructures and fields that were thoroughly studied for decades represents a time- and cost-efficient opportunity associated with a relatively low risk of leakage due to a proof of containment at least at pre-production time (Schmelz et al., 2020). In the Danish North Sea, over 800 Mt of CO2 storage capacity was estimated within mature chalk fields (Bonto et al., 2021). Assessing the possibility of storing CO2 in chalk deposits is currently based on laboratory studies such as batch-reactor and flow-through experiments combined with triaxial tests. Exposures of the rock to CO2 in supercritical and gas phase under temperature and pressure conditions relevant for the subsurface of the North Sea indicate no to minor mechanical alteration of the elastic, shear, plastic, and time-dependent properties (Alam et al., 2014; Ditlevsen & Christensen, 2010; Liteanu et al., 2013; Schroeder et al., 2001).
Probabilistic Analysis of Compressed Air Energy Storage Cavern Stability Considering Rock Mass Property Uncertainties
Authors: Lu Hui
Published: August 2024

Compressed Air Energy Storage (CAES) in caverns is gaining prominence for its role in ensuring grid stability by storing surplus energy and releasing it as needed, thus addressing the intermittent nature of renewable energy sources. However, existing research on CAES caverns stability often overlooks inherent uncertainties in rock masses, approaching stability evaluations deterministically. This study addresses this gap by incorporating uncertainties in rock mass properties, conducting a probabilistic analysis of the mechanical responses during both the excavation and air-charging stages using numerical modeling with the point estimate method. Specifically, rock mass yield zone development and cavern displacement distributions have been obtained to assess cavern stability. Additionally, the effects of cavern depth, cavern size and the COV of rock mass deformation modulus on cavern stability have been investigated. The probabilistic analyses shed light on the impact of rock mass property uncertainties on cavern stability, offering valuable insights for a more systematic and rational approach to the CAES cavern design.

1. INTRODUCTION

Wind and solar energy are emerging renewable energy sources being developed globally. However, wind and photovoltaic power generation exhibit intermittent and random fluctuations, urgently necessitating large-scale energy storage technologies to absorb renewable energy and to "smooth out" peaks and troughs in energy demand, thereby addressing technical bottlenecks in the development of renewable energy sources. Pumped hydro storage technology is mature and highly efficient, yet its implementation is constrained by factors such as site selection, ecological and environmental protection, and lengthy construction periods. Compressed air energy storage (CAES) is a promising technology solution that can store energy generated at one time for use at another time using compressed air. The CAES system operates by utilizing surplus electrical energy from the grid during periods of low demand to drive compressors that store air in storage containers. During peak electricity demand, this compressed high-pressure air is released from storage and expanded through a turbine to perform work, driving a generator to produce electricity for grid use. It can offer advantages including energy storage, grid stability and renewable integration by storing surplus energy from wind and solar and releasing it for later use during peak demand period (Qin et al., 2023, 2024).
Where Does the Energy Go in Percussion Drilling? FDEM's Answer
Authors: Yang X. , Xiang J. , Naderi S. , Wang Y. , Latham J. , Aising J. , Gerbaud L. , Ugarte I.
Published: August 2024

A drilling system incorporating percussive action is generally preferred for very hard rock to propagate significant cracking and fragmentation. Therefore, improving energy utilization efficiency becomes crucial for increasing the drilling rate. In this study, 3D FDEM simulations were employed to model the piston-bit-rock interaction in percussive drilling, revealing how the impact energy is partitioned and consumed during the impact process. To ensure the accuracy of energy analysis, rigorous validation against experiments was conducted, comparing the bit indentation and rebound velocity, fragment mass, and the range of rock cracking after impact. The results revealed that, for both St Anne limestone and Rhune sandstone, during a single impact, only 2.4-2.6% of the energy was utilized for crack propagation, while 30-70% of the energy was consumed by friction between fragments. As the piston impact energy increases, there is no significant change in the proportion of fracture energy, while the energy lost to friction gradually increases, then remains unchanged.

1. INTRODUCTION

With the increasing demand for underground mineral resources and green energy (i.e., geothermal energy), efficient drilling technologies are crucial for reducing the cost of drilling (Zappa et al., 2019). When drilling into hard brittle rock formations, impact-assisted drilling methods achieve faster drilling rates compared to purely cutting drilling technologies. Therefore, for percussion drilling, achieving faster drilling rates to reduce drilling costs has become a focus of many engineers and researchers (Aldannawy et al., 2022; Kahraman et al., 2003). It also becomes crucial to fully understand the cracking mechanism and energy transition during the multi-body interaction (i.e., piston-bit-rock interaction), to improve the drilling rate and the energy utilization efficiency.

Several experimental studies have been conducted at various scales to study the rock fragmentation efficiency (i.e., fragment mass per impact) of percussion drilling. Factors such as insert geometry (Qin et al., 2014), impact energy (Saksala et al., 2014), "weight on bit" (Aldannawy et al., 2022) and rock confinement pressure (Li et al., 2021) have been studied for their impact on the rock fragmentation process.
Machine Learning (ML) Model for Blasting and Seismic Event Classification on Chilean Copper Mines
Authors: Otaíza J. R. , Jimenez O. , Vera M. , Sartori R.
Published: August 2024

In this paper, the support vector machine model (SVM) was used to classify the earthquakes and blasting from the files recorded by an open pit copper mine seismic network. The database cleaning of false positive records (vibrations records not from blasting) must be supervised. This implies spending several hours of highly skilled engineers doing this routine task. Changing to an unsupervised classification method, based on AI, has shown an extremely high overall accuracy (as high as 0.97 over the validation dataset), allowing to perform the database filtering on real time while the vibration events are recorded by the seismographs.

1 INTRODUCTION

In open pit mining, it is a very common practice to implement a seismographers network into the pit to monitor 24/7 any vibration on the ground that exceed a certain limit and especially to record the vibrational event generated by blasting. These records are extremely important to understand and model the vibrational behavior of the rock mass and guarantees the operation safeness for workers and equipment. Due to the heterogeneously of the rock mass and the pit it self, the existence of unstable areas in the pit are a extremely likely scenario. It must be watched carefully and the vibrations generated by the blasting reported on real time to avoid risks.

To model the rock mass vibrational response, the "tendency behavior" is widely used that means to model the Peak Particle Velocity (PPV) as function of "Scalar distance", the distance from the sensor to the closest blasthole, scaled by the explosive load of the blasthole.

On the other hand, the database housekeeping is a highly time consuming process that must be performed by a well trained engineer and the main cause of that time expending is the matching between the blasting information given by the mining company (date, time and blast pattern position) to guarantee that all registered database is in fact blasting induced vibrations. As above mentioned, it is quite important to differentiate the vibration from blasting or by any other source, in particular seismic events.
A Casing Force Prediction Method with a Missing of Cement Sheath Based on Cementing Displacement Efficiency
Authors: Liu Jinlu , Li Jun , Li Yingjie , Li Hui , Yang Hongwei , Liu Gonghui
Published: August 2024

Due to the low cementing displacement efficiency(CDE), the cement sheath around the casing is prone to be missing, and the casing deformation problem occurs frequently during the subsequent hydraulic fracturing process. Aiming at this problem, a casing force prediction method is proposed in this paper. Firstly, a prediction model of replacement efficiency was established to analyze the influence of different factors on CDE, which can predict the geometry of the cement sheath at different well depths, and is used to characterize where and to what extent the cement sheath is missing in the wellbore. Subsequently, a casing-cement sheath-formation finite element model was established to study the distribution law of casing stress at different degrees of cement sheath missing and calculate the corresponding casing safety coefficient. Based on the safety coefficient, it can be judged whether the casing at the depth of the current well meets the requirements for safe production. Taking well A in Changqing oilfield in China as an example, the method was utilized. The results show that the upper well section is highly susceptible to casing deformation due to the low CDE, the large degree of missing casing, and the low safety coefficient. According to the method proposed, field workers can predict the casing stress in advance.

1. INTRODUCTION

It is well known that cementing quality has a large impact on wellbore integrity (Yan, et al., 2018; Liu, et al., 2022). Due to the low cementing displacement efficiency(CDE) during cementing, the cement ring around the casing is prone to defects, and the casing deformation problem occurs frequently in the subsequent hydraulic fracturing process (Li, et al., 2019; Yan, et al., 2019; Jiang, et al., 2020; Liu, et al., 2023).

Casing eccentricity and low displacement are the main reasons for low CDE. There are more research results on the displacement efficiency, and most of them are calculated by numerical simulation software (Zulqarnain, et al., 2016; Yang, et al., 2021; Xue, et al., 2022). However, fewer research results have been applied to the field of cement ring integrity(Vefring, et al., 1997; Mirjalili, et al., 2017).
Geotechnical Design for Open Pit Coal Mining in Proximity to Electrical Power Lines: Risks, Mitigation, and Regulatory Compliance
Authors: Zvarivadza T. , Ng’andu S. , Zindi L. , Mohutsiwa M. , Masethe R. T. , Adoko A. C. , Onifade M. , Chikande T. , Moyo S. , Firoozi A. A.
Published: August 2024

Open pit coal mining near electrical power lines presents distinct geotechnical challenges and regulatory demands, particularly under Section 17.6 of South Africa's Mine Health and Safety Act. This section requires a minimum 100 m buffer between mines and structures such as power lines unless deemed safe at closer distances. To balance safety with economic considerations, a detailed geotechnical risk assessment was performed to reduce this distance without compromising safety. This practical study is pivotal in optimising coal extraction while ensuring the integrity and safety of nearby power lines through well-considered geotechnical design. Employing empirical methods based on geological and geotechnical data, the study proposes a suitable geotechnical design, incorporating slope stability, ground control, and real-time monitoring to evaluate risks. This approach not only ensures compliance with regulatory standards but also enhances economic feasibility by increasing the minable coal resources. The findings highlight the critical role of comprehensive geotechnical design in allowing mining operations to coexist safely with essential power infrastructure. This research provides valuable insights for engineers, regulators, and industry stakeholders, establishing a framework for responsible mining in complex geotechnical environments. The recommendations from this study also guide future mining projects facing similar challenges, promoting sustainable and compliant mining practices.

1. INTRODUCTION

The extraction of coal through open pit mining is a cornerstone of the global energy landscape, supplying an indispensable resource for power generation (Moloi and Zvarivadza, 2016; Woodworth, 2017; Pouresmaieli et al. 2023). However, when such mining operations occur in close proximity to electrical power lines, a unique set of geotechnical challenges emerges, requiring meticulous attention to safety, regulatory compliance, and economic feasibility. This paper delves into the intricate interplay of these factors, focusing on the geotechnical design considerations that underpin the successful and responsible extraction of coal resources in challenging geological settings near critical power infrastructure.

The slope stability evaluation and design in this study pertain to an open pit mining project in close proximity to power lines. The objective is to find appropriate slope parameters such that the mine is stable, and the pit edge is not close enough to the power lines to cause the instability of the powerlines. This is in fulfilment of the Mine Health and Safety Act requirements as stipulated below:
Assessing the Variability in Direct Shear Testing Interpretation for Sets of Natural Fractures
Authors: Franco M. A. , Potter J. , Meyer B.
Published: August 2024

A direct shear test is a standard geotechnical laboratory test used to estimate the shear strength of rock fractures. This paper builds on previous work conducted by the authors to assess the extent of variation in shear strength parameters resulting from different approaches to interpreting direct shear test data. This interpretation involves selecting shear stresses from a shear stress versus shear displacement plot and utilizing them in the Mohr-Coulomb constitutive model to calculate friction angle and cohesion. The authors navigate through a series of direct shear results on fractures in the same rock type. Furthermore, a Python script was developed to automate the interpretation process, offering analysts an alternative tool. A variation up to 22.9 degrees in friction angle was observed and the effects of area and dilation corrections were analyzed.

1. INTRODUCTION

Direct shear tests (DST) are geotechnical tests designed to assess the shear strength and deformation properties of soils and rock discontinuities (MacDonald et. al. 2023). This test can be conducted either in a controlled laboratory setting using core specimens or in the field under in-situ conditions (Sanei et al. 2015).

A DST involves placing a core sample containing a plane of interest, referred to as a ‘discontinuity,’ onto a shear holder. The top part is displaced in relation to the bottom by applying a force parallel to the discontinuity (shear force) while maintaining a constant force perpendicular to the discontinuity (normal force). The resulting shear forces and corresponding shear displacements are measured and recorded. These forces are converted to stresses by dividing by the surface contact area, resulting in a shear stress vs. shear displacement trace (ASTM 2016). The test is generally performed three to five times on the same or similar samples with increasing applied normal loads. Figure 1 shows direct shear traces of a quartz monzonite fracture. The resulting data allows the interpreter to determine normal and shear stiffness, peak and residual shear strengths, and dilation angles (MacDonald et al. 2023).
Application of Novel Structural Finite Elements Implemented Within Combined Finite-Discrete Element Methods
Authors: Padilla A. , Lei Z. , Munjiza A. , Rougier E. , Knight E. E.
Published: August 2024

Simulating the structural response of concrete and reinforced concrete structures to blast loading conditions is a challenging task, as it requires modeling the fracture and fragmentation processes in this composite material with reasonable computational efficiency for large scale simulations. To address this problem, novel shell formulations have been incorporated within Los Alamos National Laboratory's implementation of the combined finite-discrete element method, called HOSS (Hybrid Optimization Software Suite). In this numerical study, these new finite element formulations are utilized for modeling thin concrete and metal linings for applications in building construction. Both unreinforced and reinforced concrete can be modeled with this method which opens the door for addressing many practical applications.

1. INTRODUCTION

Since its inception, the combined finite discrete element method (FDEM) has become a tool of choice to address a variety of problems involving fracture and fragmentation processes in solids. FDEM combines the strengths of the finite element method (FEM) and the discrete element method (DEM). The key advantage of FDEM is the utilization of finite strain-based deformability combined with suitable constitutive material laws which are then merged with discrete element based transient dynamics, contact detection, contact interaction solutions, and objective discrete crack initiation and crack propagation solutions (Munjiza, 2004). The comparison of DEM and FEM with a schematic visualization of FDEM is shown in Fig. 1.

In FDEM, the solid bodies are modeled as a collection of deformable particles that are bonded with each other (Rougier et al., 2014), Fig. 1. These solid bodies (called discrete elements) are discretized into finite elements where their finite displacements and rotations are assumed a priori (Munjiza, 2004; Munjiza et al., 2011, 2015). The bonding is numerically represented with cohesion points along the boundaries of the deformable particles (Rougier et al., 2014). The cohesion of these bonds is a combination of normal cohesion and tangential cohesion. When an FDEM model experiences fracture, failure, or fragmentation and the material is exposed to high enough levels of stress, these bonds become increasingly strained until they reach their ultimate strengths and eventually break resulting in the adjacent finite elements to also become unbonded (Rougier et al., 2014). The single discrete element domains are transformed into interacting domains upon which discretized contact solutions can be used for both contact detection and contact interaction (Knight et al., 2020).
Crack Initiation (CI) Thresholds from Acoustic Emission and Stress-Strain Data: A Comparison of Values Measured for Rocks from Montana and Idaho
Authors: Humphreys M. C. , Link C. , Berry S. M. , MacLaughlin M. M.
Published: August 2024

Rock mechanics researchers have defined crack initiation (CI) and crack damage (CD) thresholds as the points at which microcracks begin to propagate and coalesce under increasing stress levels. CI and CD correspond to cracking events and can be identified using Acoustic Emissions (AE). AE monitoring is a passive, nondestructive testing method which records the elastic energy released during loading. In this study, CI is determined using the Cumulative Acoustic Emissions Hits (CAEH) method (Zhao et al, 2015), which involves graphing cumulative counts vs. axial stress, implemented as a MATLAB® v7 module. The CAEH method was designed to make the determination of CI less subjective. Augmenting previous studies by others, this study involves using unconfined compression tests in conjunction with AE monitoring to identify CI and CD thresholds for specimens of several different rock types from Montana and Idaho. The unconfined compression tests were performed on a TerraTek load frame using strain rate control, and the AE data were collected and recorded using a Mistras' Express-8 AE system and AEwin Express-8 software. CI values determined from the stress-strain curves ranged between 34% to 60% of the UCS for all but 2 of the specimens. The CAEH CI values were generally higher than the stress-strain CI values, and were sensitive to the selection of the initial point. Data from two AE sensors mounted on the same specimen yielded surprisingly different CI values.

1. INTRODUCTION

As compressive stress increases in rock, cracks initiate and coalesce, leading to nonlinear behavior and eventually to failure. Stress and strain data from a traditional unconfined compression test on rock provide values of yield stress and unconfined compressive (peak) strength (UCS) as the specimen is loaded past the point of linear elastic behavior. Rock mechanics researchers have also defined crack initiation (CI) and crack damage (CD) thresholds as the stress levels at which microcracks begin to propagate and coalesce under increasing compressive stress, originally associated with the point of departure from linear on axial stress vs. lateral strain (for CI) and on axial stress vs. axial strain (for CD) plots (Ghazvinian et al., 2012, referring to work by Brace et al., 1966, Bieniawski, 1967, and Lajtai & Lajtai, 1974). Identifying CI and CD can be valuable for understanding the behavior of rock under load, so efforts have been made to associate them with more definitive observable points on the stress-strain data plot. Due to the subjectivity involved in selecting some of these points combined with large variations in behavior observed in rocks with drastically different porosity values, this process is not necessarily a straightforward one.
Methodology Development to Incorporate Anisotropy in Slope Stability Analysis at Los Bronces Mine®
Authors: Córdova M. , Vallejos J.
Published: August 2024

The rock mass strength depends on a combination of non-persistent discontinuities and blocks of rock mass. Currently, the representation of the strength for slope stability analysis includes the joints sets (structural domains) with preferential orientations that generate unfavorable stability conditions. For the limit equilibrium stability analysis of slopes the strength of the anisotropic rock mass is estimated by means of an anisotropic rosette and the respective rock bridge percentage. This work focusses on providing guidelines for Los Bronces Mine to estimate the directional resistance according to the structural domain and slope design. First, it is created a discrete fractures network (DFN) using a robust structural database that represents the probabilistic distribution of each joints characteristic, i.e., orientation, persistence and intensity. Once the DFN has been established, it is possible to characterize the structural condition of the rock mass and estimate the equivalent structure that controls the slope failure. This process is carried out through two-dimensional continuous numerical modeling with explicit structures and the obtained indicators of slope stability. Next, the results of the numerical model are compared with a two-dimensional limit equilibrium analysis, allowing to establish the characterization of the anisotropy rosette and the rock bridge percentage.

1. INTRODUCTION

it is important that when studying rock mass geomechanical behavior in slopes, its characterization includes the effect of discontinuities and their possible effects on stability at both the inter-ramp and global scales in an open-pit mine.

Structural domains play a significant role in this study, representing in some way the strength of the rock mass, whose preferential orientations promote the formation of instabilities. The orientations of the discontinuities create a kind of network that translates into a final rupture surface, where rock bridges define an equivalent structure or possible instability mechanism.

The final rupture surface includes both discontinuities and rock bridges, allowing the definition of an equivalent structure (Fig. 1). Generally, to estimate the shear strength of these equivalent structures (which conditions slope stability), a stepped path simulation is developed considering the properties of structures oriented sub-parallel to the slope (unfavorable).
Enabling Geomechanical Insights from Drilling Data: Automated and Data-Driven Proxy Mechanical Earth Modeling
Authors: Jeong C. , Lin S. , Reyes A. , Sharma V. , Wendt A. S.
Published: August 2024

The energy industry has acknowledged the intricate relationship between mechanical drilling and subsurface geomechanical parameters. However, establishing an accurate connection between these parameters remains technically challenging. Constructing a mechanical earth model (MEM) and utilizing it to predict or monitor mud weight window (MWW) boundaries currently necessitates log data acquisition, processing, and significant expertise in data interpretation, modeling, and correlation identification. To simplify MEM and MWW construction and reduce reliance on log data, an automated and drilling data-driven workflow is proposed. This involves a systematic exploration of the relationship between mechanical specific energy (MSE) and unconfined compressive strength (UCS) to identify the most relevant MSE to UCS model(s). The workflow includes data segmentation for scaling and fine-tuning related parameters such as well inclination, hole conditions, and geological formations, change point detection for transition patterns, and integrative machine learning for optimal correlation modeling. The resulting correlation model(s) can then be utilized for well planning and real-time MWW monitoring, offering a significant advantage in scenarios where LWD or wireline data acquisition is limited.

1. INTRODUCTION

The energy sector has acknowledged the intricate connection between mechanical data obtained during drilling and the geomechanical conditions of the subsurface. This acknowledgment is supported by empirical and physical correlations that explain how subsurface formations react to the drilling process. These correlations have been pivotal in recent decades, leading to the development of effective workflows for deducing rock properties

Mechanical specific energy (MSE) is defined as the amount of energy required to destroy a unit volume of rock and was proposed by Teale (1965) for the rotating drilling systems in the mining industry. MSE measures provide a quantification of the total energy input into drilling divided by the volume of rock destroyed over a given timeframe. This approach was defined in the following formulations (Eq. (1) to (4)).

(equation)

(equation)

(equation)

(equation)

where WOB = weight on bit, RPM = rotational speed, AreaB = borehole area, DB = bit diameter, ROP = rate of penetration, and T = torque.

After Rabia (1985) applied the concept of specific energy for bit selection, various studies were performed to broaden its applicability. Pessier and Fear (1992) sought to quantify drilling problems, and Graves et al. (2002) summarized and tested multiple published MSE equations to enhance drilling methods. In a pilot study, Dupriest and Koederitz (2005) reported that the average rate of penetration (ROP) increased by 133% for six rigs when using MSE surveillance.