bluuebunny/crossref_metadata_2025 huggingface dataset:
Stress Variations Controlled by Rock Composition in an Appraisal Well in the US: Evaluating Lithology, Stress Measurements, and Methods
Authors: Cruz Leonardo , Jimba Solomon , Affinito Ralph , Husain Zoya
Published: August 2024

We evaluate stress and lithology relationships, compare least-stress test methods, and assess stress-from-logs empirical equations, from a recently drilled and tested appraisal vertical well in the continental US. We perform multiple least-stress tests in several intervals spanning multiple lithologies, and including containment, injection, and producing zones. The presence of carbonates, including limestones and dolomites, is a better predictor of high fracture gradient than any other lithology. The least-stress magnitude in the salts is very close to the overburden stress. The shales exhibit the lowest least-stress magnitude among all lithologies, and when the shale intervals are compared to each other, the carbonate content seems to control the trend. The least-stress magnitude in sandstones is highly variable. The commonly used Eaton's equation performs relatively poorly predicting stresses in the non-shale lithologies, but the extended Eaton's equation, using Biot's coefficient equal to one, compares adequately to the stress calibration points in most lithologies. The relatively recent viscoelastic stress relaxation equation applied to shales performed slightly better than the Eaton's equation but laboratory data or applicable correlation data from similar lithologies/basins are needed to confirm this trend. Differences in the stress magnitudes from the least-stress test methods are probably related to poroelastic effect, test order, and operational related issues.

1. INTRODUCTION

Predicting in-situ stresses in sedimentary basins has significant value to the Petroleum and Carbon Capture and Storage (CCS) industry. In particular, knowledge of the magnitude of the least principal stresses has been used to prevent fracturing the cap rock in CCS projects, identify hydraulic fracture containment in Unconventional plays, and reduce wellbore stability issues during drilling operations, among other applications. Stress contrast, in sedimentary basins, is often observed from layer to layer within intervals where the mineral composition changes. These interlayer principal stress variations have been measured and reported before (i.e., Anderson et al. 1973, Hickman et al. 1985, Warpinski 1989, Ma and Zoback 2020), and at least two explanations have been proposed: elastic properties contrast and stress relaxation within shales and clay-rich intervals. Interestingly, in some basins, shale and clay-rich formations show higher stresses than adjacent formations such as the Piceance basin in Colorado (Warpinski et al., 1985) and the Paris basin in France (Magnenet et al., 2017), but in others, the stresses in shales are comparatively low such as the Canning basin in Australia (Bailey et al., 2021).
Rockburst Damage Characterization in a South African Gold Mine Subjected to High Horizontal Stresses
Authors: Masethe R. , Wienand G. , Adoko A. C. , Zvarivadza T.
Published: August 2024

Rockburst risk can often be reduced by selecting appropriate mining or excavation methods, sequences, strategically placing developments, and other infrastructure. However, owing to the uncertainties in rock mass properties and boundary conditions, rock engineering design necessitates dependence on ground control measures reinforced with burst-resistant rock support to ensure safety at the workplace. High horizontal stress at depth aids the rockmass to be self-supporting by clamping the rockmass. It reduces the gravity-driven damages that could be experienced at the excavation, as stress-induced damages might become significant when mining goes deeper. Significant seismic events (ML≥0.9) are usually associated with large damages near any excavation in the underground workings. We analyzed the damage mechanisms of large mining-related seismic events (0.9≤ML0≤ 3.0) located within the shaft. The study indicates that with the large energy release, damages were only observed with fewer stopes. This is not only influenced by the high horizontal stresses but also by the rockmass mineralogical and contrasting geomechanical properties. The rockburst analysis within active mine infrastructure indicates that 63% large seismic events did not cause any damage to the mine infrastructure or cause injuries within the WAF.

1. INTRODUCTION

Rockburst is a significant occurrence that has impacted numerous deep underground mines worldwide. Understanding this phenomenon is essential for managing such catastrophic events, which can lead to substantial increase in investment risk and loss of lives. It results from an overstressed rock mass or an intact, brittle rock and occurs when the stresses exceed the material's compressive strength. It is associated with excavation damage and is characterized by its destructiveness, suddenness, and complexity (Feng et al., 2012). However, due to uncertainties in rock mass properties and boundary conditions, rock engineering design must rely on ground control measures with burst-resistant rock support to ensure workplace safety. This ground control strategy is a norm in the deep gold South African mines. The rockburst phenomenon is driven by various factors, including mining depth, rockmass conditions, geological structures, and mining elements (e.g., abutments, pillars, etc.). In recent years, research has been increasingly addressing the global topic of reducing the challenges of rockburst. Its mitigation or prevention remains as one of the most crucial problems to be solved in underground rock engineering.
Analyzing Shale Gas Well Casing Deformation in Pittsburgh Seam Longwall Chain Pillars: A Case Study Integrating Numerical Methods and Field Monitoring
Authors: Tulu I. B. , Zhang P. , Su D. , Khademian Z. , Kim B. H.
Published: August 2024

In the Northern Appalachian region of the U.S., the coexistence of longwall coal mining and shale gas operations is a prominent feature. The gas wells in the vicinity of the longwall operations entail the potential for casing deformations arising from mining-induced subsidence and abutment stresses. Researchers at the Pittsburgh Mining Research Division (PMRD) of the National Institute for Occupational Safety and Health (NIOSH) have been employing a combination of field monitoring and calibrated numerical modeling methods to formulate engineering strategies for assessing the effects of longwall mining operations on gas well casing integrity and implementing measures to mitigate potential damage. This paper introduces the numerical modeling approach developed by PMRD researchers and verifies it by comparing the model's predictions of longwall-induced casing deformations with measurements from shale gas wells located in the gateroad chain pillar of the Pittsburgh coal seam longwall operation. The longwall panels adjacent to gas well chain pillars operate at a depth of approximately 313 meters, with a mining height of 2.1 meters and panel widths ranges between 450 to 480 meters. The gas well is constructed with fully cemented surface and intermediate casings, with the production casing left uncemented from the surface down to below the Pittsburgh coal seam. The model predicted that, following the first panel mining, the production casing would remain deformation-free, as the maximum predicted intermediate casing deformation falls below the annular space between the intermediate and production casing. This model prediction was subsequently confirmed through casing deformations monitored using a multi-finger caliper survey following the mining of the first panel.

1. INTRODUCTION

Pennsylvania is the second largest natural gas producer in the U.S., and since the 2005 shale gas boom, thousands of unconventional gas wells have been drilled through the Pittsburgh coal seam where the coexistence of longwall coal mining and shale gas operations is a prominent feature (Zhang et al., 2023). Pittsburgh coal seam longwall mines on average extract 460-m-wide and 4,000-m-long panels. The gas wells in the vicinity of these longwall operations entail the potential for casing deformations arising from mining-induced subsidence and abutment stresses, and if the deformations would be large enough to damage casing, that would lead to potentially hazardous conditions for the mine workers. The Pittsburgh Mining Research Division (PMRD) of the National Institute for Occupational Safety and Health (NIOSH) started a research project in 2016 to formulate engineering strategies for assessing the effects of longwall mining operations on gas well casing integrity and implementing measures to mitigate potential damage.
Experimental Assessment of the Energy Dissipation and Deformation Capacity of Ground Support Systems Under Dynamic Loading: Insights from LaRonde Mine Dynamic Drop Test Program
Authors: Durham C. , Caron M-E. , Falmagne V. , Brändle R.
Published: August 2024

Over its 30-year history, the design of ground support schemes at the LaRonde mine has evolved significantly to meet the demands of the rock mass while mining from surface to the current depth of over 3000 m. Over the past decade, seismicity and dynamic loading on ground support systems have become a focus of ground control management and one of the major challenges at LaRonde mine. In some rare occurrences, dynamic loading from seismic events have exceeded the capacity of ground support schemes. Furthermore, the ground control practices implemented to safely manage seismic conditions in development drives have been largely successful despite the operational challenges of a deep mine. Thus, as the mine progresses ever deeper, there is a continuous need to implement ground support schemes that are not only tougher but also more efficient to install considering the mine configuration and available equipment. As part of this endeavor, a dynamic testing program was developed at Geobrugg's testing facility, in association with Geobrugg and the Kittilä mine. The test program involved various elements of ground support schemes from LaRonde and Kittilä mines. This paper presents the testing program, instrumentation set up and results analysis process as well as testing results for three of the LaRonde tests. The testing program is intended to guide improvements in ground support schemes designs for deep mining environments.

1. INTRODUCTION

Deep underground mining environments present unique challenges for the design of ground support schemes, particularly in zones susceptible to stress-induced rock failure and rockbursting. Traditional ground support schemes, designed primarily to control gravity-induced rockfalls in blocky rock masses, have been found to be unsuitable for such environments. For the sake of consistency, the authors have adopted the terminology proposed by Windsor and Thompson (1992) as presented in Villaescusa et al (2023). A Ground Support Scheme is therefore defined as a combination of a surface support system, composed of plates, mesh, liner or shotcrete, with an internal reinforcement system consisting of rockbolts and cable bolts. In blocky ground, under static conditions, the design of a ground support scheme aims primarily at meeting the load demand and minimizing deformation to maximize the reinforcement function. In weak and highly deforming ground, ground support schemes require increased ductility to meet the deformation demand while retaining load capacity. Under dynamic conditions, the ground support schemes must also satisfy energy dissipation, rapid rock mass deformation demands and ensure load transfer from the rock mass to the reinforcement, and to the surface support. The ground support elements' load and deformation characteristics must be matched to enable this load transfer.
Fracture Initiation and Propagation Characteristics of SC-CO2 Fracturing in Hot Dry Rock: A Coupled Thermal-Hydraulic-Mechanical-Damage Numerical Model Based on Phase Field Method
Authors: Zhou Xiaoxia , Wang Tianyu , Ma Zhengchao , Zhang Xu , Hsu Maoya , Sun Yaoyao , Tian Shouceng
Published: August 2024

Hydraulic fracturing is a pivotal technique for harnessing geothermal energy from HDR. Conventional hydraulic fracturing encounters formidable challenges, including high fracture initiation pressure and simplistic fracture geometry. In contrast, SC-CO2 fracturing represents a novel and promising stimulation method capable of mitigating these challenges. However, limited work has been conducted on studying the fracture initiation and propagation of SC-CO2 fracturing. To have a clear understanding of how geological engineering parameters affect SC-CO2 fracturing, numerical simulations were conducted under some key parameters (horizontal stress difference, permeability, elastic modulus, etc.) based on the THM-D phase-field model. Furthermore, using breakdown pressure as the evaluation criterion, the primary control factors were determined. Results showed that the breakdown pressure in SC-CO2 fracturing is dominated by pore water pressure, thermal stress, and SC-CO2. Thermal stress and SC-CO2 could induce multi-directional fractures and reduce breakdown pressure. Decreasing the horizontal stress differences, permeability, injection temperature, and fluid injection rate is conductive to inducing more complex fracture morphology. The growth in coefficient of thermal expansion and elastic modulus promotes the number of the fractures. Horizontal stress difference and injection temperature play a more important role in fracture initiation. The study aims to uncover the potential of SC-CO2 fracturing in HDR.

1. INTRODUCTION

Hot Dry Rock (HDR) is a kind of geothermal resource where no water or only a small amount of fluid exists inside the rock and the average temperature of HDR is typically above 180°C. HDR geothermal resources represent a stable, efficient, and clean renewable energy source unaffected by seasonal variations, applicable across power generation, heating, agriculture, and other sectors (Zhang et al., 2018; Zhou et al., 2020). Enhanced Geothermal Systems (EGS) play a pivotal role in harnessing HDR, employing hydraulic fracturing to create artificial fractures within the HDR reservoir, establishing high-conductivity pathways, and enabling stable and efficient heat extraction. However, conventional hydraulic fracturing techniques encounter challenges such as limited fracture propagation, reservoir hydrothermal mineralization reactions, and pipeline blockages. Consequently, Supercritical Carbon Dioxide (SC-CO2) emerges as a promising, environmentally friendly, and efficient fracturing medium (Xu et al., 2022; Li et al., 2021; Chen et al., 2021). SC-CO2 not only enhances reservoir transformation outcomes but also facilitates carbon utilization and storage during the fracturing process.
A Feasibility Study on the Pressure Monitoring Above the Injection Zone for CO2 Geological Storage in the Uinta Basin, USA
Authors: Bakelli Omar , Xiao Ting , McPherson Brian , Vega-Ortiz Carlos , Moodie Nathan , Chellal Hichem A. K. , Mellal Ilyas
Published: August 2024

Geological Carbon Storage (GCS) offers a promising solution to mitigate climate change, but ensuring safe and permanent CO2 confinement demands reliable monitoring techniques. This study investigates the feasibility of using pressure changes above the CO2 injection zone (AIZ) for leak detection and storage assessment in the Uinta Basin, Utah. Focusing on the Cretaceous Dakota Formation as a potential storage reservoir, we employed a coupled fluid-flow and poroelasticity model to simulate various leakage scenarios. These scenarios included both the injection well and a nearby legacy well, each experiencing varying leakage rates, pressure changes was analyzed at two monitoring points (AIZ-1 and AIZ-2) situated above the injector and legacy wells, respectively. The simulations revealed minor to significant pore pressure changes (ranging from 0.1 kPa to 2790 kPa) above the injection zone, directly influenced by the leakage rate. Notably, pressure changes were found to be more sensitive to leaks compared to gas saturation changes, enabling early detection, especially at higher leakage rates. While the AIZ-1 sensor exhibited larger and earlier pressure changes due to its proximity to the injection point, the wider monitoring coverage offered by the AIZ-2 sensor necessitates further consideration for optimal monitoring strategies. These findings demonstrate the promising potential of AIZ pressure monitoring as a valuable tool for CO2 leak detection and storage assessment in the Uinta Basin. However, successful implementation requires high-precision pressure measurements for accurate data acquisition and analysis. This study contributes significantly to a more comprehensive understanding of CO2 plume behavior and leakage potential, ultimately paving the way for developing safer and more reliable GCS practices.

1. INTRODUCTION

In the task to mitigate climate change and achieve the Paris Agreement's net-zero emissions goal by 2050, Geological Carbon Storage (GCS) has emerged as a potentially significant tool (IPCC, 2005). This technology captures carbon dioxide (CO2) from industrial processes (point-source emissions) or directly from the air (direct air capture) and safely stores it in deep underground porous rock formations sealed by impermeable layers (Boot-Handford et al., 2014).

Monitoring underground CO2 migration and containment is crucial for the safe and responsible implementation of GCS projects (Xiao et al., 2024). A diverse toolkit of techniques exists, each offering unique insights (Hannis, 2012). For instance, seismic surveys map the plume's physical extent and potential leakage pathways through changes in acoustic properties (Ma et al., 2016).
Back to Basics: Experimental Measurement of Liquid and Solid-Liquid Properties to Predict Hydraulic Fluid Behavior in Fractures
Authors: AlDajani O. A. , Germaine J. T. , Einstein H. H.
Published: August 2024

The primary mechanisms to initiate hydraulic fractures in rock include overcoming the tensile resistance of the rock, overcoming its shear resistance, or a combination of both. The focus of this study is the solid-liquid interaction in hydraulic fractures using basic measurements and models to aid in investigating these mechanisms. The materials of study were Opalinus Shale and hydraulic oil. The basic measurements included liquid density, viscosity, surface tension, and contact angle with the rock. These measurements were used to determine the characteristic capillary length, capillary number, spreading, and imbibition parameters. Two fracture scenarios were considered for a range of apertures:

A- liquid inside a pre-existing fracture was modelled as parallel plates to determine the Laplace pressure and adhesion force.

B- liquid penetrating a newly created fracture was modelled as a capillary to determine the dynamic and equilibrated rise.

This study provides a theoretical framework to assess solid-liquid interaction effects in fractures and introduces the capillary draw amplification coefficient. The results of this study provide insight into the lag or lack thereof between the liquid front and the fracture tip during hydraulic fracturing and how the liquid interacts with the fracture after it has been created. These, in turn, have implications on the mechanism of fracturing and potential operational considerations.

1. INTRODUCTION

Several mechanisms play a role in the hydraulic fracturing process. There are two main ways to initiate hydraulic fractures in rock:

1. increasing the liquid pressure beyond the tensile resistance of the rock will result in tensile fractures.

2. decreasing the effective stress (by increasing the pore pressure) to have the Mohr circle tangent to the shear failure envelope will result in shear fractures.

After initiation, the propagation of hydraulic fractures depends on many factors. One of these factors that affect hydraulic fracture behavior is the solid-liquid interaction. A higher viscosity liquid pressurizing rock may result in different behavior than a lower viscosity liquid.

The motivation of this study was to investigate the solid-liquid behavior that occurs in the hydraulic fracturing, specifically in the experiments conducted by AlDajani (2022). These hydraulic fracture experiments are beyond the scope of this paper, but the materials used in the study are the same. The liquid used as a fracturing fluid was hydraulic oil and the rock being hydraulically fractured was Opalinus Shale.
Continuous in Situ Characterization by AFM of Surface Mechanical and Electrical Features of Shale Organic Matter Under Different Atmospheres
Authors: Kang Ying , Ning Zhengfu , Lyu Fangtao , Jia Zejiang
Published: August 2024

Unconventional reservoirs, which are the mainstay of CO2 enhancement and carbon sequestration, are not well characterized in terms of micro-surface electrical properties. In this study, the micro-surface morphology and potential of organic matter (OM) under three atmospheres: dry, water-wet, and ScCO2 were characterized in detail using atomic force microscopy (Tapping mode, KPFM). The results showed that the micro-surface morphology of the OM fluctuated in the range of less than ten nanometers, and the micro-surface roughness under dry and ScCO2 conditions was higher than that under water-wet conditions, which indicated that the ScCO2 extraction and induced swelling were effective. In addition, the contact potential difference of the organic surface under water-wet and ScCO2 conditions was positive, and there was no correlation with the change of surface morphology. Combined with the reduction of Young's modulus, the difference of organic surface potential and the reduction of Young's modulus provide a good physical basis for the emergence of micro-cracks and other defects under the external forces, and provide a new idea for revealing the adsorption characteristics of unconventional reservoirs and realizing the long-term and stable carbon geologic sequestration.

1. INTRODUCTION

Due to the decrease of conventional oil and gas resource production rates and the impact of the greenhouse effect in recent years, unconventional oil and gas reservoirs, such as shale, have become an important option for enhancing resource production rates and realizing carbon geologic sequestration (Farokhpoor et al., 2013; J. Zhang et al., 2023). A large number of studies have been carried out on rock damage, fracture generation and other related phenomena, but most of these studies have focused on the macro-scale level and lacked detailed analysis of related changes at the micro- and nano-scale (J. He et al., 2020; Yin et al., 2023; Gu et al., 2023). For unconventional reservoirs such as shale, which is both a key target for the implementation of CO2 enhanced recovery measures and an ideal place to realize efficient and stable carbon geological storage. Direct surface characterization of them can provide a better understanding of the CO2 action mechanism.
Simulation of Coupled THMC Processes in Fractured Rocks Using a Dual-Media Approach
Authors: Mosley K. , Hartley L. , Turnbull R. , Cottrell M. , Bym T. , Mas Ivars D.
Published: August 2024

Simulation of coupled thermo-hydro-mechanical-chemical processes in fractured rocks is relevant to many areas of applied geoscience. Groundwater flow and reactive transport (HC processes) in hard rocks are primarily controlled by the network of rock fractures, and therefore best modelled in a discrete fracture network. By contrast, heat and stress (TM processes) are mediated via the rock mass, and thus best represented as a continuum. Here, we present an approach using dual, coincident meshes, each simulating the relevant processes in their ‘natural’ media. A sequential coupling between TM and HC processes is achieved by computing the effective stress on fractures and updating their hydraulic properties.

The simulated processes are modelled in PFLOTRAN, using a linear elastic constitutive model for rock deformation. Several updates to the PFLOTRAN code were required to enable this, as verified against analytical cases and other numerical codes. We present an example application to a model of a nuclear waste repository in Finland, which simulates the evolution of thermal stress due to radiogenic heating from spent fuel canisters. The results illustrate how continuum-based TM processes influence near-field flows around the repository that can only be accurately captured in a DFN, thus demonstrating the advantages of a dual-media approach.

1. INTRODUCTION

Simulation of coupled thermo-hydro-mechanical-chemical (THMC) processes in fractured rocks is relevant to many areas of applied geoscience and geo-engineering, including in nuclear waste disposal, enhanced geothermal systems, CO2 sequestration and mining applications. The ability to accurately predict the influence of coupled processes in the context of fractured media has potentially far-reaching economic and environmental consequences. In this study, we introduce a novel workflow for simulation of such processes, which employs a ‘dual-media’ representation of the modelled rock volume.

In any numerical THMC model, a range of different couplings to geomechanical processes can feasibly be implemented. These include thermo-mechanical, hydro-mechanical and chemo-mechanical coupled processes. For the purposes of this study, the focus was primarily on the thermo-mechanical coupling between the rock's temperature and its stress state. The fundamental mechanism by which this is governed is the thermal expansion/contraction of the rock (and other modelled materials), as determined by the thermal expansion coefficient. Such expansion induces deformations, or ‘strains’, in the rock mass, which in turn bring about alterations in the stress field. A coupling to hydraulic processes (i.e. groundwater flow) can also be incorporated by computing the impact of thermal and mechanical strains on the rock's hydraulic properties – whether that be on the porosity field in the case of porous materials, or indeed on fracture apertures and transmissivities in the case of fractured rocks.
Computational-Rock Mechanics in Pedagogy and Practice
Authors: Kayen R. , Russo B. M.
Published: August 2024

Point cloud modeling of rock slopes using LIDAR and Structure-from-Motion digital stereophotogrammetry provides, at a minimum, thousands of facets and facet normals that can be used to identify the densities of orientations of rock mass discontinuities, the geometries of potentially removable blocks, and the character of the excavation face. As part of the Engineering Geology graduate curriculum at the Civil and Environmental Engineering program at the University of California, Berkeley we teach graduate students an integrated methodology for [a] gathering point cloud information be laser or camera; [b] computing facets and facet normals form point clouds for stereonet presentation and geometric analysis of block dimension; [c] extract rock mass discontinuities from stereonet data to analyze key blocks, assess discontinuous deformation analysis (DDA) behavior, and model rock slope stability. These new methods require a suite of different software tools discussed in the paper to move through the workflow process. Computational rock mechanics provides data sets that are orders of magnitude richer in detail and result in better understanding of rock slope and tunnel key block behavior. Full application of computational rock mechanics methods should reduce the cost of bolting by identifying critical support orientations and design loads.

1. INTRODUCTION

This paper is intended to serve as an instruction manual for teachers who include digital methods in rock mechanics. Innovations in digital methods for computing surface models have advanced such that models can be used for quantitative rock mechanics analyses. An array of new remote sensing tools utilizes laser and photogrammetric means to build digital twins of a surface at millimeter-to-centimeter accuracy by processing a point cloud of the target. The oldest of these technologies, Terrestrial Laser Scanning (TLS), has transformed the ability of engineers to document geotechnical sites.

Though TLS remains an expensive technique for capturing point clouds of data needed to model surfaces, the cost has dropped in the last decade, and is now widely available on common cell phones. A remarkable new computer vision-based method, Structure-from-Motion (SfM), allows engineers to build complex pointcloud visualizations with digital cameras (airborne or handheld) at a fraction of the cost. In the sections below, we outline a procedure for gathering data and moving through the analysis process to extract meaningful rock mechanics information from point cloud data. The procedural steps are outlined in Figure 1.
Stress Distribution in Accreting Sediments: A Geomechanical Study of Upper-Plate Faults
Authors: Lopez-Campos G. , Nikolinakou M. A. , Flemings P. B. , Saffer D. M.
Published: August 2024

We study the influence of an upper-plate fault on the stress state of accreting sediments under large-scale deformation. We develop drained evolutionary geomechanical models using the Finite Element program Elfen. We simulate sediments as porous-elastoplastic material, and we model the fault as a pre-existing contact surface with a varying frictional strength that is lower than the intact sediment. The weaker fault results in a decrease in sediment differential stress near and especially seaward of the fault. A significant section of the wedge is affected by this stress variation. In contrast, the stress ratio is that of Coulomb failure further away from the fault. We also show that the maximum principal stress the sediments can support decreases with decreasing fault strength. This study offers a significant improvement over previous models of continuum wedge sediments that predict Coulomb failure throughout the wedge. Our results improve our understanding of near-fault stress state, hence improving our understanding of seismic hazards in subduction zones and providing practical insights for reservoir quality and the design of safe and economic well trajectories.

1 INTRODUCTION

Accretionary wedges are geological structures that develop at convergent plate boundaries, particularly at subduction zones. They form as a result of offscraping sediments from the subducting oceanic plate, which then accumulates on the leading edge of the overriding plate (e.g., Moore et al., 2011; Buiter et al., 2016; Gao et al., 2018).

Understanding the state of stress in accretionary wedges is crucial for the study of earthquake mechanics in subduction zones (e.g., Brodsky et al., 2017; Huffman and Saffer, 2016; Suppe, 2007). It provides insights for earthquake occurrence, large or slow-slip events (e.g., Kodaira et al., 2012; Tobin et al., 2022; Liu and Rice, 2007) and tsunami potential (e.g., Dean et al., 2010; Riedel et al., 2016). In addition, estimates of mean and differential stress, as well as porosity evolution can help improve the assessment of reservoir quality and the design of safe and economic well trajectories (e.g., Morley et al., 2011).

The accumulation of sediments in an accretionary wedge resembles a bulldozer gathering snow as it moves. Hence, wedge sediments are often assumed to be at compressional failure (e.g., Davis et al., 1983; Dahlen et al., 1984; Flemings and Saffer, 2018). Several field observations of faulting, folding and lateral compression as seen in seismic images and drilling measurements (e.g., Flemings and Saffer, 2018; Henry et al., 2003; Moore et al., 1990; Westbrook et al., 1988) provide evidence that the accretionary wedge is at a state of failure in compression.
Optimal Camera Angle for Measuring Joint Roughness Using UAV-Based 3D Photogrammetry
Authors: Leem J. , Choi J. , Kang I. S. , Song J.-J.
Published: August 2024

Joint roughness strongly affects rock mass engineering properties, including shear strength and hydraulic behavior. Integrating 3D photogrammetry with unmanned aerial vehicle (UAV) technology rapidly generates 3D roughness profiles from 2D images. However, inaccuracies in roughness measurements can occur due to survey errors, and the directional nature of roughness complicates the analysis of how these errors affect the inaccuracies. This study simulates joint roughness coefficient (JRC) measurement distortion due to UAV-based 3D photogrammetry errors, considering image distortion, joint plane orientation, and JRC direction. We measured JRC values through 3D photogrammetry on joint planes of a rock mass at Mt. Gwanak, Seoul, and compared them with JRC values distorted through the simulation. The comparison result shows that the simulation has a high performance in predicting JRC errors with an RMSE of 1.23. Based on the simulation, we propose guidelines for camera angle and UAV movement direction to minimize JRC measurement errors. The study recommends that flying the UAV along the strike of a joint plane with a camera angle orthogonal to the plane can minimize JRC measurement errors when the roughness of interest corresponds to the dip vector of the joint plane.

1. INTRODUCTION

Joint roughness has a significant impact on both the mechanical and hydraulic behavior of rock masses (Paixão et al., 2022), and it should be a critical factor in designing rock structures. The Joint Roughness Coefficient (JRC), one of the most commonly used roughness parameters in practice (Barton, 2023), can be physically assessed through tilt tests (Barton and Choubey, 1977) or estimated visually from roughness profiles. Since measuring JRC values from visual inspection can be subjective, research has also been conducted using statistical parameters such as fractal dimension (Tse and Cruden, 1979), maximum amplitude of the profile (Barton and Bandis, 2017), and the standard deviation of profile gradients Z2 (Myers, 1962).

The methods of obtaining roughness profiles can be categorized into two methods: contact and non-contact methods (Ge et al., 2014). Contact methods such as linear profiling and stylus profilometry are advantageous over non-contact methods in terms of cost-effectiveness, but they are time-consuming and unsuitable for surveying large areas (Paixão et al., 2022). When investigating large rock surfaces, non-contact methods like 3D photogrammetry or LiDAR (Light Detection and Ranging) technology are preferred for acquiring 3D roughness profiles due to their efficiency compared to the time-consuming contact methods.
New Insights for Stress Conditions of Laboratory Shear Tests
Authors: Chai Shupeng , Zhao Qi
Published: August 2024

Numerous laboratory tests have been performed to investigate the shear behavior of rock discontinuities using various experimental setups. Stress heterogeneity on rock discontinuities can significantly influence the shear behavior, however, it has seldom been specified clearly. Instead, the macroscopic or average stress on the joints other than the local stress distribution was usually analyzed in most laboratory shear tests. In this study, numerical simulations considering a simple linear velocity weakening law are conducted on smooth rock joints in various laboratory setups to analyze the stress distribution before and during slip. The results show nonuniform normal and shear stress distribution in almost all the laboratory tests and significant local stress concentration near the edges of the shear surface. The estimated sample-average friction coefficient is slightly higher than the dynamic friction coefficient but lower than the static friction coefficient. Furthermore, a sudden release of the shear stress, especially near the stress-concentrated edges, can be observed in the slip phase during a stick-slip cycle. This study provides a critical review of laboratory shear tests on rock joints and contributes to our understanding of stress distribution and variation during stick-slip shear.

1. INTRODUCTION

Rock discontinuities including joints, fractures, faults, and bedding planes commonly exist in rock masses (Elsworth et al. 2016; Marone 1998). Shear failure on rock discontinuities can occur in various engineering cases, such as underground excavations including tunneling, carven and mining (Chai et al. 2023; Wu et al. 2023), slopes and dams (Chai 2023), fluid-injection projects including oil and gas extraction (Elsworth et al. 2016), enhanced geothermal systems (Zhao et al. 2020), and CO2 or waste storage (Elsworth et al. 2016), etc. The shearing of rock discontinuities has also been reported to affect earthquake nucleation (Kaproth & Marone 2013).

Various laboratory shear setups have been developed to investigate the shear behavior of rock discontinuities. They include direct shear (DS) tests (Petit 1988), true triaxial direct shear tests (Zhao et al. 2023), single direct shear tests (Rubinstein et al. 2004), double direct shear tests (Marone 1998), triaxial direct shear tests (Frash et al. 2016), triaxial shear (TS) tests (Thompson et al. 2005), biaxial shear tests (Buijze et al. 2020), axial shear tests (Rubino et al. 2022), oblique shear tests (Ghazvinian et al. 2013), and rotary shear (RS) tests (Zhao et al. 2018). Among these test setups, the DS, TS, and RS tests are commonly used by the rock mechanics community.
The Mechanical Response and Sealing Characteristics of Caprock During CO2 Injection in Depleted Reservoirs
Authors: Ren Qianqian , Yang Bing , Huang Hai , Chen Junbin , Zheng Yong , Zhou Dongqiang
Published: August 2024

Carbon Capture and Storage (CCS) is globally recognized as the most effective method for achieving the ‘dual-carbon’ goals. The depleted oil reservoirs in the Ordos Basin of China have significant potential for carbon storage. In the process of CO2 geological burial, caprock, serving as a natural barrier for CO2, is crucial element in preventing CO2 from escaping to the upper layers and ensuring the safety of carbon sequestration. After CO2 injection into the reservoir, it affects the mechanical properties of the caprock and alters it sealing ability. Therefore, it is necessary to conduct relevant research to explore the evolution characteristics of the mechanical properties and sealing ability of the caprock during CO2 injection. This paper has conducted experiments on the evolution of the mechanical property of caprock before and after CO2 exposure. Furthermore, it delves into the research on the breakthrough pressure of CO2 in the caprock, aiming to determine the CO2 sealing characteristics of caprock. The results indicate that CO2 injection causes a slight change in the mechanical properties of the caprock. Compressive and tensile strength show a decreasing trend, and the elastic modulus experiences a decrease, which mainly caused by partial dissolution of mineral and alterations in rock pore structure observed by SEM. The breakthrough pressure of CO2 in target caprock range from 2.08 to 10.27 MPa. With increasing permeability, the CO2 breakthrough pressure significantly reduces, while CO2 breakthrough pressure averagely decreases by 9.7% under SC-CO2 exposure. The above research showed that CO2 injection will not only cause changes in the mechanical properties and a decrease in breakthrough pressure for caprock. This study can provide theoretical guidance for carbon sequestration in depleted oil reservoirs and the sealing feasibility analysis of the caprock against CO2.

1. INTRODUCTON

Carbon capture and storage (CCS) is widely recognized as one of the main pathways for reducing carbon emissions, playing a crucial role in supporting China's achievement of its "dual carbon" goals. The mature development and scaled application of CO2 storage technology are of strategic importance [1-4]. CO2 storage primarily involves geological structures such as depleted/exhausted oil and gas reservoirs, deep saline aquifers, and unmineable coal seams [5-7]. Depleted/exhausted oil and gas reservoirs, due to their well-developed geological structures and infrastructure, make CO2 storage more straightforward and economically viable. Additionally, injecting CO2 into depleted/exhausted oil and gas reservoirs not only helps maintain the integrity of the reservoir's pressure system but also holds potential for enhancing the displacement of residual oil. Therefore, utilizing depleted/exhausted oil and gas reservoirs for carbon storage is not only significant in terms of economic prospects but also presents positive development [8, 9].
Stress-Permeability Characterization and Proposal of Creep Model for Pre-Salt Carbonates Under Lab-Scale Stepwise Hydrostatic Loading and Unloading
Authors: Garcia P. F. V. , Santos E. S. R. , Rossi D. F. , Ferreira F. H. , Borba A. M.
Published: August 2024

In an attempt to expand the studies of rock creep deformation from the classic unidimensional creep theory, this study presents results obtained from geomechanical lab-scale tests of pre-salt carbonate cylindrical samples subjected to stepwise hydrostatic loading and unloading ramps, from 30 to 80 MPa and back, followed by short-term 2.5-hour periods of stress hold. All samples were simultaneously subjected to an axial flow of monophasic oil with upstream pressure measurements. By using Darcy's law, it was possible to obtain an estimate of absolute permeability variation as function of measured axial and radial deformation. It was possible to characterize a power fit nondimensional coefficient of the relation between permeability and porosity variations during loading conditions. Such coefficient, despite presenting higher values during stress hold steps (order 10 to 100) than during load ramps (order 0.01 to 0.1), its mean value was observed to be similar to the ones found in the literature (order 1). At unloading stage, most of the samples did not present permeability recovery, indicating a high level of plastic deformation. The authors also propose volumetric creep models for each sample by taking into account their porosity and their variation with each stress step that they were subjected to.

1. INTRODUCTION

It is known that reservoir rock permeability varies during the oil and gas exploitation process. This variation occurs due to increase/decrease of fluids pressure and/or rock stress state. In order to better emulate the variation of permeability in geomechanics and production simulators, lab-scale experiments can be performed on reservoir rock samples to obtain relations between permeability and stress states.

Petunin et al. (2011) proposed a power relation between the ratios of absolute permeability k and porosity φ to their respective reference state for sandstone, carbonate and fractured rock samples tested with hydrostatic compressions and simultaneous porosity and permeability measurements. Santos et al. (2014) successfully reproduced Petunin's k−φ relation on limestone and dolomite samples by testing them on robust compression test machine and indirectly measuring φ from volumetric deformation εv and estimating k from Darcy's law by imposing axial oil flow on the samples and measuring flow pressure.
Permeability Test on Opalinus Clay Shale with a Preexisting Discontinuity
Authors: Arzuaga-García I. M. , Einstein H. H. , Casagrande C. , Celleri H. M. , Sánchez M.
Published: August 2024

The interaction between Hydraulic Fractures (HF) and Preexisting Discontinuities (PD, like bedding planes and natural fractures, is affected by the coefficient of friction of the PD, the state of stress around the interacting fractures' tips, and the angle between the approaching HF and the PD. Recent formulations (Kresse et. al, 2013; Chuprakov and Prioul, 2016), also consider the effect of flow rate, fluid viscosity, and PD permeability in the crossing interaction. In this study, a series of permeability tests on cylindrical Opalinus Clayshale specimens were conducted in a triaxial pressure cell. The specimen of 1.5 in diameter and 3 in height includes a throughgoing longitudinal cut, representing the PD. Different confining pressure (CF) and flow rate (FR) conditions were tested for the same fluid (constant fluid viscosity) and temperature. The results obtained are comparable with other authors, with the Opalinus clayshale showing a higher fracture conductivity than Barnett shale (Zhang et al., 2013)), and Fayetteville shale (Jansen et al., 2015).

1. INTRODUCTION

Hydraulic permeability of rock discontinuities is widely investigated in geotechnical engineering and geomechanics. This is mainly due to the influence of mechanical discontinuities, such as fractures and faults, on the permeability of rock formations. Permeability is a critical parameter in the evaluation of numerous geotechnical and environmental aspects, including slope stability, underground fluid flow, and storage, exploitation of hydrocarbon resources, etc. Preexisting discontinuities (PD) act as preferential channels for flow, and the characterization of their permeability is vital to accurately model flow. Furthermore, in the context of geological storage and carbon sequestration, the permeability of discontinuities is essential to evaluate the long-term integrity of the host formations. Many authors have studied the effect of permeability of discontinuities. For instance: Carlsson and Olsson (1993) analyzed the increase in conductivity in fractures due to shear displacement. Makurat (1996) showed that the permeability of natural fractures is affected, among other factors, by tortuosity, roughness, and wall strength.
Update on Rock-Type Specific Hoek-Brown Constant “s”
Authors: Khamitova A. , Suorineni F. T. , Madenova Y.
Published: August 2024

Martin (1993) established the m-zero failure criterion following extensive field and laboratory testing at the Atomic Energy of Canada Limited Underground Research laboratory. The m-zero criterion is widely used for assessing excavations in rockmasses with a Geological Strength Index ≥75. Based on the m-zero criterion stopes in massive sulfides in underground nickel mines that were expected to fail did not fail. This observation prompted further studies on the m-zero criterion at the Geomechanics Research Centre. According to the m-zero failure criterion, damage in massive rockmasses is caused by destruction of the Hoek-Brown failure criterion constant "s" with the frictional constant "m" playing no role. The value of the constant "s" to initiate damage was established as 0.11. However, this value was based on studies on the La du Bonnet granite. Suorineni and Kaiser (2002) argued that the "s" value of 0.11 should vary depending on the rock type, and hence the need for rock type-specific "s" values. Suorineni et al. (2009) presented a procedure for the determination of rock type specific "s" values. This paper presents "s" values for different rock types from Kazakhstan in a continuing effort to provide users of the criterion with a database of rock type-specific "s" values.

1. INTRODUCTION

There are various failure criteria that exist in rock engineering for the evaluation of the performance of underground excavations in rock. Two frequently used failure criteria are the Mohr-Coulomb (Mohr, 1900; Coulomb) and Hoek-Brown (1980) failure criteria. This paper focuses on the latter, given its history. Given the long history of the Mohr-Coulomb failure criterion which dates to the 1700s, one would ask why there was the need for another failure criterion in the 1980s? The Mohr-Coulomb failure criterion has its foundation in soil mechanics. Hoek and Brown (1980) provide the key reasons why the Hoek-Brown failure criterion was necessary for rocks. These authors noted that to utilize the knowledge of stresses induced around underground excavations it was necessary to have available a criterion or a set of rules which will predict the response of a rockmass to a given set of induced stresses. The search for such a criterion for rockmasses had been challenging because of the complexity of the rockmass compared to a soil in that the behaviour of the rockmass transitions from an intact rock to a heavily jointed rockmass as shown in Fig. 1. All the rockmass components in Fig.1 are of concern to the underground excavation designer or rock engineer. The stability of the entire system of underground openings which make up a mine or any underground excavations network depends upon the behaviour of the entire rock mass surrounding these openings, including all the various components of the rockmass in Fig. 1. This observation implies that a failure criterion developed for soils is unlikely to be applicable to a rockmass, and hence the need for a more suitable failure criterion for rockmasses.