bluuebunny/crossref_metadata_2025 huggingface dataset:
Multi-Interbedded Continental Shale Reservoir Evaluation and Fracturing Practice
Authors: Xiaobing Bian , Yuan Gao , Shuangming Li , Bo Xiao , Shikun Zhang , Xing Liu , Luo Zuo , Jiawei Kao
Published: August 2024

Continental shale oil resources are abundant in Sichuan Basin in China, according to multiple limestone interbeds and variable longitudinal stress characteristic, continental shale reservoir characteristic evaluation method including radar map, geological sweetness and fracability index, were established considering multi-thin interlayer, horizontal stress difference, etc. The continental shale in area A demonstrates the characteristics of high clay mineral content with strong plasticity and serious heterogeneous. The fracture geometry inverse analysis is established based on the results of microseismic monitoring and numerical simulation, the continental shale has simple fracture geometry and lower fracture length propagation rate. Further reducing cluster spacing and increasing fluid intensity are the technological directions to improve the SRV of fractured wells in area A. The study could provide guidance for the shale evaluation and fracturing program design of continental shale oil wells.

1. GEOLOGICAL CHARACTERISTICS OF CONTINENTAL SHALE

There are abundant shale oil and gas resources in the Sichuan Basin in China. At present, the marine shale gas have developed commercially especially in Wufeng & Longmaxi formation, and the continental shale oil resources are important strategic replacement resources (Wei et al, 2022; Shu et al, 2023; Guo et al, 2023; Jin et al, 2023; Zhao et al, 2023; Yu et al, 2023; Wang et al, 2023). Compared with the marine shale oil and gas reservoirs, continental shale in area A is of strong heterogeneity and low oil & gas abundance, the formation is strong plastic with clay mineral content more than 50%, and multiple limestone interbeds exist resulted in variable longitudinal stress, the higher shale thermal evolution leads to complex fluid properties and aggravates the difficulty of multiphase flow (Jiang et al., 2023; Shu et al, 2023; Sun et al, 2023; Wang et al, 2023). Because the huge geological charactristic between marine shale and continental shale, bottleneck problems such as fracture height and fracture complexity limit emerge in conventional hydraulic fracturing technology, which greatly restricts the effective exploration and economic development of continental shale oil & gas (Jiang et al, 2023). Thus, the reservoir characteristic and fracturing effect evaluation method should be developed according to the field cases.

2. COMPREHENSIVE EVALUATION OF RESERVOIR PARAMETERS

2.1. Radar map of key geological parameters

Key geological parameters such as clay minerals, interlayer thickness, natural fracture, etc., and rock mechanics parameters such as horizontal stress difference and Poisson's ratio, etc., were selected as evaluation data set to draw a non-dimensional rose map of the above 9 parameters to qualitatively show the differences in geological parameters among wells.
Axial Stiffness Evolution of Rock Observed During Uniaxial and Triaxial Compression Strength Tests
Authors: Gaines S. , Diederichs M. S.
Published: August 2024

Elastic properties, including Young's modulus and Poisson's ratio, are commonly reported parameters estimated from standard compression strength tests of intact rock core. Although it is recognized that axial stiffness is not constant during loading, Young's modulus is often estimated at a pre-defined portion of the stress-strain curve, typically at approximately 40-50% of the peak strength. Poisson's ratio, which is an indication of the axial to lateral strain response, is also highly non-linear and varies throughout the loading cycle as damage is initiated and accumulates in the rock specimen. While this method of interpreting elastic properties is sufficient for some rock materials under specific test conditions, plotting the instantaneous Young's modulus, or tangent modulus, as a function of axial stress reveals that the maximum stiffness and/or most linear region of elastic response does not necessarily fall within the typically applied fixed range of the stress-strain curves often used for interpretation. A series of uniaxial and triaxial compression tests were completed on Westerly granite, Stanstead granite and Indiana limestone to investigate the relationship between axial stiffness and stress during loading. It was found that while the maximum tangent modulus (i.e. maximum stiffness) for both the Westerly and Stanstead granites is reached between 45-55% of the peak axial strength during UCS tests, as confinement is applied the range decreases to approximately 20-25% and 30-40% of peak axial strength under 30 MPa confinement, respectively. Therefore, applying a standardized fixed range as a percentage of peak stress for calculating elastic properties under various confinement levels can be problematic. An improved strategy is to determine the maximum tangent modulus and select a range of stress or percentage around the stiffness inflection point to estimate Young's modulus.

1. INTRODUCTION

An understanding of the physical and mechanical properties of rock material is critical to safely designing structures involving earth materials. This includes surface or near-surface geo-engineering projects such as open pit mines or foundations, and deeper underground projects such as tunnels, mines, or waste repositories. Furthermore, knowledge of material properties is important to effectively design borehole drilling programs and model subsurface behavior for large scale subsurface storage operations/projects.
Correlations Between Microscale Dimensions and Elastic Anisotropy in Shale
Authors: Zhu Junfeng , Zhang Guangqing , Zhang Min , Xu Chuangchao , Chen Houze
Published: August 2024

Shale exhibits significant anisotropy on both macroscopic and microscopic scales, which can influence the efficacy of the fracturing process. Existing research lacks investigations on the laws governing the variation of mechanical anisotropy at the microscopic scale relative to size This study focus on the variation of mechanical anisotropy in shale within the size range of 50-2000 nm. Through nano-indentation experiments conducted on shale samples aligned parallel and perpendicular to the bedding planes under varied loading conditions, we have discerned that the micro-elastic mechanical properties of shale exhibit distinct variations across different sizes. Our findings are as follows: (1) The anisotropy ratio (bed-parallel to the bed-perpendicular) of the Young's modulus in shale is substantial, with minimal fluctuation across sizes, decreasing from 1.61 to 1.50 as the indentation radius increases from 1.1μm to 3.5μm; The anisotropy ratio of the hardness is lower and demonstrates significant variation with size, decreasing from 1.46 to 1.18; (2) The variation in micromechanical properties of shale with respect to indentation depth requires a holistic analysis that incorporates mineral distribution and the indentation process; (3) The microscopic orientational arrangement of clay minerals plays a pivotal role in the emergence of anisotropic characteristics and their variation with size.

1. INTRODUCTION

Shale oil and gas represent a crucial component of the anticipated growth in the production of hydrocarbon resources. These resources are found within shale formations rich in organic material, typically characterized by their low permeability and narrow hydrocarbon-bearing layers (Li et al., 2022; Zhijun et al., 2021). The exploitation of such formations necessitates the application of hydraulic fracturing technologies to enhance extraction outcomes (King, 2012). Shale, a sedimentary rock, emerges from the consolidation of clay minerals, carbonates, quartz, and other minerals, showcasing prominent bedding features. The interlayer bonding is notably weak, and its compressibility is significant, leading to pronounced anisotropy in its mechanical properties, such as elastic modulus and compressive strength. This anisotropy influences the magnitude of in-situ stresses, potentially triggering uncontrollable fractures during hydraulic fracturing processes, thereby compromising reservoir stimulation efficiency (King, 2012).
Seismic Monitoring Systems in Mines, where are we Today?
Authors: Meyer S.
Published: August 2024

Seismic monitoring systems have long been identified as a key tool for engineers to observe, quantify, and manage the seismic rockmass response to mining. The design of the seismic system, in terms of sensor layout, spacing, and type, is an important first step and is dependent on the specific objectives of seismic monitoring at the mine in question. Once installed and operational, the systems expand and evolve to better meet the requirements of the mine. This paper provides an overview of modern microseismic monitoring systems in mines. We examine trends in the configuration and performance of seismic systems over the years and how advances in technology, processing methods, and computational power have lead to larger and richer datasets than ever before. Differences in geological setting and mining method may inform the monitoring approach and are discussed. The results from this review provide engineers with a way to compare their system configuration and sensitivity to other mines, or for new mines to make an initial estimate of their seismic system monitoring requirements.

1 INTRODUCTION

Microseismic monitoring in mines has become ubiquitous in deep, hard rock mines, being one of the fundamental tools used by engineers to identify, quantify, and manage ground control hazards. The earliest examples of seismic monitoring in mines can be traced back to South Africa, Canada, and Australia, going back many decades to the 1970s and 1980s.

Over the recent decades, there have been monumental changes in seismic monitoring in mines. These changes are driven by multiple factors. Some of the earliest and potentially most significant improvements were due to technological advances in data transfer speeds. Around the early 2000s, seismic stations started becoming compatible with ethernet, a significant upgrade from serial ports. This provided a massive boost in data transfer speeds, from a maximum of 115 kb/s over serial ports to 100 Mb/s with ethernet. Upgrading existing in-mine infrastructure to the newest technology can be a difficult and time-consuming process, and newer and smaller mines were often the first to benefit from such new technologies. This meant that while new technologies were available, there was often some delay before the benefits could be realized.
Study on Stress Sensitivity Damage Evaluation in Tight Low-Permeability Gas Reservoirs
Authors: Zhai Hengli , Yu Xing , Zhu Qingjiangpeng , Liang Tongyao , Huang Qirui , Wang Haizhu , Wang Bin , Stanchits Sergey , Cheremisin Alexey
Published: August 2024

Current stress sensitivity evaluation experiments lack accuracy due to the absence of considering the dynamic impact of the slip effect. This paper uses a correction method for Klinkenberg permeability that accounts for the dynamic variation of the slip effect. Gas measurement evaluations are conducted using varying confining and pore pressure methods under in situ reservoir conditions, fitting permeability and effective stress, and establishing a stress-sensitive gas well productivity equation that considers eliminating the slip effect. The results indicate that as effective stress increases, the gas slip effect intensifies. The average damage rate of corrected permeability increases by 0.9% and 12.09% in the two methods, with the pore pressure variation method being closer to field conditions. The greater the stress sensitivity, the faster the decline in gas well productivity, and both excessively high or low bottom hole flowing pressure are detrimental to gas well production. By calculating data from a gas well in Linxing, the reasonable range of bottom hole flowing pressure is determined to be 13.07 MPa to 13.99 MPa, while the reasonable production pressure difference range is 1.29 MPa to 2.21 MPa. This provides effective guidance for the development of low permeability reservoirs and reservoir protection.

1. INTRODUCTION

With the sustained and stable development of Chinese petroleum industry, the development focus has gradually shifted towards low-permeability oil and gas reservoirs as middle to high permeability reservoirs are gradually being developed and the proportion of low-permeability oil and gas reserves continues to increase. Currently, low-permeability oil and gas reservoirs account for a relatively high proportion of petroleum reserves in China and are the main source of petroleum and natural gas production at this stage. During the production of tight low-permeability gas reservoirs, the pore pressure in the reservoir decreases, causing deformation of the reservoir rock and compression of the internal flow channels within the rock, resulting in damage to reservoir permeability, which is one of the reasons for the decrease in production well capacity. The tight sandstone gas reservoirs in Linxing area are highly heterogeneous, with low porosity and permeability, and their high clay content makes them vulnerable to permeability damage caused by stress changes. Therefore, understanding the stress sensitivity of low-permeability reservoirs is of great significance for formulating reservoir protection measures and guiding the efficient development of oil and gas reservoirs.
A Study of Rock Breakage Under Extreme Submerged Confining Pressure: Can DTH Hammer Drilling Deliver?
Authors: Gerbaud L. , Velmurugan N. , Aising J. , Chambres C. , Naderi S. , Latham J. P. , Xiang J. , Yang X.
Published: August 2024

Drilling cost is one of the main obstacles hindering the development of deep geothermal energy, especially in hard granite formations at great depths. In ORCHYD, an H2020 project, we develop an innovative drilling technology that merges percussion drilling and high-pressure water jetting for drilling hard crystalline rocks. In laboratory, we have demonstrated a fourfold improvement in drilling performance when contrasted with conventional techniques. This article delves into the study of impact of realistic operational conditions, characterized by high in-situ confining stress and hydrostatic pressure, on the drilling technique.

We study variables affecting the mud hammer's performance. Experimentally, our drilling test laboratory houses a set-up designed to dissect the mechanisms of rock fracture during indentation under realistic downhole conditions. Numerically we harnessed our in-house rock fracture software, SOLDITY that leverages a hybrid FDEM method to simulate the intricate interactions of rocks under confining pressures.

Our analysis of experimental and numerical data centered on rock fractures. Our findings show that breaking rock can be 2-3 times more challenging under higher confinement compared to lower confinement conditions. This insight has significant implications for optimizing ORCHYD techniques, improving the efficiency of rock destruction.

1. INTRODUCTION

Geothermal energy is positioned as one important component in the drive to achieve Net Zero Emissions (NZE) by 2050, offering a clean, sustainable, and reliable source of renewable energy. Nevertheless, the high costs associated with geothermal drilling pose a significant obstacle to its widespread adoption and scalability (Angelone, 2014). The primary cost driver in deep geothermal drilling stems from the necessity to penetrate deep and dense crystalline rock formations, such as granites, typically located at depths exceeding 4 km. Deeper drilling not only escalates operational expenses but also demands a substantial increase in the energy required to break rocks under extreme confining stresses and hydrodynamic pressures.

Conventional rotary drilling, such as roller cone and polycrystalline diamond carbide (PDC) bits, encounter difficulties penetrating hard formations like granite, resulting in sluggish rate of penetration (ROP) (Cardoe et al., 2021; Baujard et al., 2017). Percussion drilling, characterized by its effectiveness in penetrating hard formations, has garnered considerable attention in deep geothermal industry, specifically for basement drilling. Despite its advantages, such as enhanced rock penetration and reduced drilling costs, percussion drilling encounters limitations, notably in achieving higher ROP under high confining conditions dictated by geological stresses and mud pressure in the wellbore (Gnirk and Cheatham, 1965; Han et al., 2006).
Understanding Cement Hydration in CO2 Reservoirs: Geochemical Modeling in the Powder River Basin
Authors: Allam L. , Malki M. L. , Mellal I. , Kolla N. , Rasouli V. , Dehdouh A. , Abdelhamid C.
Published: August 2024

The Powder River Basin is one of the primary hydrocarbon-bearing basins in the state of Wyoming. This provides consequential knowledge on the basin's characteristics, which allows effective identification of potential candidates for CO2 storage. In this paper, we aim to characterize the hydration of Portland cement in CO2 injection wells. Properties of the Minnelusa formation in the Powder River Basin serve as a base case scenario. From this, we will run a sensitivity analysis to determine the influence of various parameters on cement hydration. These parameters consist of pressure, temperature, formation water fraction, and water-to-cement ratios. The findings will allow us to determine the most influential parameters affecting cement hydration, hence threatening the integrity of CCUS operations. For this purpose, we used the geochemistry modeling software PHREEQC. The CEMDATA thermodynamic databases were used to supplement the default PHREEQC database with missing mineral phases involved in cement hydration. For our simulations, initial data include formation water compositions, formation mineral fractions, clinker phases fractions and downhole conditions. The thermodynamic process of solids precipitation was then simulated for each of the base cases, then at various conditions. The obtained simulation results include phase assemblage at equilibrium for each case. From a well integrity perspective, predicted phase assemblage of the cement sheath allows for tailored cement degradation analysis on a well-to-well basis when activated by carbonic acid. In addition, the phase assemblage can be used in combination with time-dependent diffusivity and convection models for higher precision characterization.

1. INTRODUCTION

The global imperative to address climate change has intensified the focus on carbon capture and storage (CCUS) technologies, particularly in the context of ensuring the integrity of CO2 storage injection wells (IPCC, 2005). As the world grapples with the urgent need to reduce greenhouse gas emissions, CCUS has emerged as a key strategy to mitigate climate change (IPCC, 2005). The reliability and safety of CO2 injection wells are central considerations, with challenges ranging from corrosion to cement degradation and scale formation that can compromise the effectiveness of CCUS initiatives (Bai et al., 2016; Carroll et al., 2016). Therefore, having a proper understanding of the evolution of well integrity components offers means of preemptive maintenance (Le Guen et al., 2008).
Thembelani and Khuseleka Mines in the Bushveld Igneous Complex's Western Limb in South Africa
Authors: Masethe R. , Durapraj S. , Modika M. , Ngwenya M. , Adoko A. , Zvarivadza T.
Published: August 2024

The Merensky (MER) orebody in the Bushveld Complex in South Africa has been mined extensively. Regional and local pillars, zones of unmined ground, potholes, and remnants were left in-situ. 15 to 400 m beneath the MER orebody, the UG2 chromitite orebody is currently being exploited. Due to the location of these two orebodies, it is postulated that the unmined ground in the MER above may have an influence on the current UG2 ground conditions. Although many mining variables could have impacted the successful exploitation of the UG2, the in-situ blocks of ground on the MER reef horizon seemed to present an additional risk, which had not been extensively researched before. Of concern is that, this phenomenon is occurring at different depths and different middling between the two orebody packages. In some cases, it was found that relatively shallow UG2 reef mining could encounter conditions like those experienced in deep mining environments (e.g., spalling, scaling pillars, and rockburst hazard). Source mechanisms of significant mining-related seismic events (ML≥1.0) were analyzed and facilitated the comprehension of the dominant modes of failure. Much of the work focused on seismic events triggered by movement along planes of weakness and the closure rate experienced by the stopes in both the MER and UG2.

1. BACKGROUND

The Merensky Reef is a regular, persistent tabular orebody with various layers: pegmatoid, chromitite, and pyroxenite (Lougher and Mellowship, 1991). It is generally composed of an upper feldspathic pyroxenite assemblage near the top of the Upper Group of the Upper Critical Zone of the Bushveld Complex. It is overlying a thin basal chromitite stringer, followed by an anorthosite to norite footwall and a thin anorthosite layer (0.1 – 0.2 m thick), which is underlain by norite (Fig. 1). Locally, this is termed a "pyroxenite reef".

The UG2 chromitite layer comprises chromitite and pegmatoid units. It is well renowned for its short-distance undulation due to its conformable interaction with the footwall. The UG2 Reef is made up of a coarse plagioclase-orthopyroxene pegmatoid layer that can grade into coarse pyroxenite (Leeb-Du Toit, 1986). The reef's thickness ranges from 0.2 to 2 m, with an average of about 0.7 m. The absence of coarse pyroxenite beneath the UG2 chromitite layer suggests that the stratum is potholed (Lougher and Mellowship, 1991). The UG2 Reef consists of a primary chromitite layer, which contains the majority of the PGM and base metal mineralization, followed by a poorly mineralized pegmatoidal pyroxenite footwall (Fig. 1). The hanging wall to the main chromitite layer is feldspathic pyroxenite with up to four thin, weakly mineralized chromitite layers.
New Method of XRF/XRD Based Mineralogy for Estimating Matrix-Modulus, Biot's-Coefficient and Effective-Stress Using VRH and HS Bounds of Rock Physics Principles
Authors: Prasad Umesh , Alarcon P. Nora , Franquet Javier
Published: August 2024

The concept of ‘effective stress’ is the foundation of any subsurface investigations which controls the deformation, fracture, failure, and production decline characteristics including dynamic acoustic wave propagation or specifying confining pressures in laboratory testing. Effective stress (σ′) is the difference between total stress (σ) and existing pore pressure (Pp) multiplied by Biot's coefficient, α. The ‘α′ is expressed in terms of bulk modulus (Kb) and matrix bulk modulus (Kma) as in 1 − Kb/Kma. The Kb is estimated by acoustic logs as well as measured by triaxial test. However, Kma is difficult to measure, especially in low porosity or low permeable shale/caprock or crystalline rocks. Current advancement in hardware and software enabled use of portable X-ray fluorescence (XRF) and X-Ray diffraction (XRD) tools to get inorganic elements and successively mineralogy for estimating Kma and further constrain α using rock-physics models (VRH or HS) for better confidence in modeling and simulation.

1. INTRODUCTION

In any civil, mining, or petroleum application the concept of stress (or strain) under the influence of load or displacement is the foundation of geomechanical investigations. However, since the subsurface geological formation consists of pore, voids and fluid pressure, it is the ‘effective stress’ and not the ‘total stress’ which controls the deformation, fracture, failure, and production decline characteristics including dynamic acoustic wave propagation in rock formations.

Fig 1 shows the elastic behavior of volumetric strain change for dry and saturated sandstone under laboratory testing environment while changing the confining and pore pressure (Pp). While ignoring the Pp, change in elastic behavior is not explained, by using the concept of Terzaghi (1934) effective stress the response in elastic behavior is better explained. However, while incorporating the concept of Biot (1941), full elastic behavior is best explained [modified after Nur, 1971].

Effective stress is also useful in any triaxial core testing in the laboratory for calibrating log-based estimations. Effective stress (σ′) is expressed as difference between total stress (σ) applied and existing pore pressure (Pp) multiplied by Biot's coefficient, α as postulated by Terzaghi, (1934) and Biot (1941), Eq. (1). The ‘α′ for highly porous rocks is unity where load applied is counteracted equally by grain-matrix and pore-pressure. However, for low porosity and low permeable tight rocks as encountered in unconventional reservoirs, CCUS cap-rocks, or geothermal crystalline rocks, only a fraction of load is shared by pore fluid and the ‘α’ is much smaller than unity.
Reliability-Based Design (RBD) Updating by Sample Reweighting for Rock Slope Design
Authors: Aladejare A. E. , Idowu K. A. , Ozoji T. M.
Published: August 2024

Rock slope design comprises systematic evaluation of geological, geotechnical, and engineering parameters affecting safety and stability of slopes in rockmass. However, it is common to have several design possibilities for a single slope in rockmass. In the forementioned instance, reliability-based design (RBD) could provide insights on the most feasible slope design for rockmass. However, this generally involves several computational runs for each slope design. In this study, combined Monte Carlo simulation (MCS) and sample reweighting method was explored with the aim of obtaining parameters for rock slope design during a single computational run. With this approach, the repeated run of MCS for different design possibilities are avoided. One key advantage of the method is that it saves computational time by using a single run of simulation since there is possibility of a great number of design scenarios at a rock slope. To evaluate the performance of this approach, the proposed method was applied to a real-life rock slope. Results reveal that the proposed approach for rock slope design is both reliable and advantageous by providing sufficient failure samples for reliability estimation of the final designs for various design possibilities. In addition, the approach proposed in this study can link site characterization processes to design, and significantly reduces the time and efforts required for the processes. Through the proposed approach, there is improved knowledge on the contribution of uncertain rock parameters and slope conditions involved in design to the rock slope.

1. INTRODUCTION

Mining engineering is faced with several uncertainties, ranging from variability of rock conditions, uncertainties in rock parameters that are affected by geological processes, limited number of test values or site mapping, and model uncertainties (Baecher and Christian, 2003; Qi and Li, 2018; Aladejare and Wang, 2017). The uncertainties affect mining designs, and incorporating the uncertainties into mining design procedures in a quantitative manner is crucial to obtaining reliable designs of structures in mining engineering. A reliability-based framework can be used for such incorporation. In the past few decades, several studies have been conducted on reliability analysis of slope stability (Jimenez-Rodriguez et al. 2006; Hoek 2007; Tang et al., 2013; Dadashzadeh et al., 2017; Aladejare and Wang, 2018; Aladejare and Akeju, 2020).
Slope Stability Finite Element Modeling of El Misti Volcano, Peru
Authors: Hollander J. A. , Walton G. , Villeneuve M. , Salas G. , Luza C.
Published: August 2024

El Misti is a 5822 m elevation symmetrical, andesitic stratovolcano located near the city of Arequipa in Peru. Numerous historic debris and avalanche deposits indicate that the volcano slopes could be unstable, and that seismic or volcanogenic activity might have the potential to trigger movement that could threaten Arequipa. To evaluate this potential, a cross-section was first developed along a transect from the summit southwest towards Arequipa based on available geologic information. The stability of the southwestern slope of El Misti approaching Arequipa was then investigated using 2D finite element homogenized continuum modeling. The shear strength reduction (SSR) method was used to find critical strength reduction factors (SRF) at which the slope is marginally stable (SRF ∼ 1) given a pseudo-static seismic loading based on the peak ground acceleration (PGA) for Arequipa. Possible mechanisms of slope failure for different assigned subsurface conditions were investigated.

1. INTRODUCTION

El Misti is a 5820 m elevation composite stratovolcano with an extensive history of explosive eruptions, and lava and debris flows (Thouret et al., 2001). It is constructed of a composite of accumulated material from several sequences of eruptions and collapses. It lies to the northeast of the city of Arequipa, Peru. Arequipa has seen rapid growth in recent years, with a population of 1.38 million people per the 2017 national census, up from 700k in 1980 (CEIC, 2021).

Stratovolcanoes are built rapidly compared to non-volcanic mountains, leading to potentially unstable flanks and edifices (van Wyk de Vries and Francis, 1997). Collapse of these deposits can lead to large rock avalanches and debris flows with extensive runout lengths (Reid et al., 2000). Debris flow and collapse deposits can be seen in the historic geologic record of El Misti and pose a threat to the city of Arequipa and surrounding areas (Acocella, 2005; Thouret et al., 2001). Seismic activity, such as is common in the tectonic environment of Arequipa, or eruptions can trigger such massive collapses (Reid, 2004; Voight et al., 1983). Therefore, understanding the current stability of El Misti against seismic influences is vital to evaluating its long-term stability and the safety of residents of Arequipa.
Assessment of Leakage Risk of Legacy Wells Based on Wellbore Schematics Using a Systematic Approach
Authors: Anwar I. , Meng M. , Carey W. J. , Stauffer P. , Lackey G.
Published: August 2024

A systematic approach is used to assess the leakage risk of legacy wellbores using available wellbore information from a site in Louisiana, USA. The proposed approach assesses potential risk of groundwater contamination resulting from fluid leakage in legacy wells associated with carbon sequestration operation. The assessment incorporates several key factors, such as leakage through open borehole pathways, leakage through annular spaces between the casings, inadequate cement design, presence of fish in wellbore, and complex sidetracks. The approach integrates geological information with well construction data to form wellbore schematics, contributing to a richer understanding of 10 legacy wells located near a possible sequestration site. In addition, we have established a qualitative risk scoring system that prioritizes wellbores susceptible to CO2 leakage for remediation or leak identification/monitoring. From assessing the well features, accumulated qualitative risk scores are obtained. As per the current assessment described in this study, a combined risk score exceeding 20 for any legacy well within the area of review is indicative of a potential leakage risk.

1. INTRODUCTION

Wells, or wellbores are subsurface structures that are typically constructed of multiple steel casings surrounded by an oil-well cement or Portland cement sheath to create a seal in the annular space between the host formation and the steel casing or between the casings (Figure 1). Various categories of both documented and undocumented wells, including abandoned wells, orphaned wells, idle wells, and inactive wells, are found all over the world. For simplicity, we have termed all the above-mentioned wells as legacy wells in this article. A properly designed well can prevent leakage of fluid from an underground reservoir during operational phases, and if effectively plugged and abandoned, the integrity of the well can be maintained, thereby preventing upward leakage or movement of fluids between geologic layers for an extended period. However, the old legacy wells may have construction flaws, substandard workmanship, improper well abandonment, and much more, leading to a likely compromise in their integrity. In this article, we have used the terms "well" and "wellbore" interchangeably.
Failure Mechanisms in Jointed Rock Slopes Concerning the Dip Angle of Continuous Joints
Authors: Shandilaya Shivesh , Roshankhah Shahrzad
Published: August 2024

Jointed rock slopes surround critical infrastructure and communities, such as dams, roads, tunnels, bridges, mines, and buildings. The mechanical behavior of jointed rock masses depends on the complex interactions between joints, rock matrix, boundary conditions, and loading states. When a jointed rock mass undergoes environmental stressors, the generated tensile and shear cracks through intact rock blocks and pre-existing discontinuities determine the mechanisms and implications of the consequent slope failure. However, predicting fracture evolution, corresponding stress transfer mechanisms, and consequent failure states are challenging due to the complex interactions between the slopes' structural components. This study aims to understand the effects of such interactions on the overall response of slopes concerning the dip angle of continuous natural fractures when a local failure is triggered in fractures close to the slope face. This study employs the combined finite discrete element method (FDEM) to examine the slopes' mechanical response, including the progressive failure mechanisms. The results of this quantitative analysis are processed and visualized through graphical representations depicting the distinct failure modes observed in jointed rock slopes with various dip angles.

1 INTRODUCTION

Failure of rock slopes ranks among the most dangerous geological hazards, causing yearly fatalities, significant disruptions to infrastructure, environmental challenges, and adverse socioeconomic impacts. Thus, it is crucial to detect the early signs of failure by understanding the mechanisms that drive new fractures, activate pre-existing fractures, and finally cause a global failure. Failure of rock slopes occurs for various reasons that contribute to either an increase in shear stresses applied on the slope or a decrease in the shear strength of the rock mass. The latter may happen due to weathering, water infiltration, and anthropogenic activities. Studies show that the impact of shear strength reduction on the failure mechanism of jointed rock slopes depends on factors like joint or discrete fracture network (DFN) density (Wang et al., 2003), roughness (Ban et al., 2020, Rullière et al., 2020) and orientation of the joints (Bahaaddini et al.,2013), the properties of the infill material within the joints (Jiang et al., 2015), the degree of saturation of the infill (Indraratna et al., 2014), and the average normal stress acting on the joints (Mišcˇević et al., 2014). The arrangement of fractures within a rock mass can also determine the potential for block detachment, toppling, and sliding. Highly connected fracture networks can delineate potential sliding or toppling blocks, significantly influencing slope stability (Romer & Ferentinou, 2019).
Solving Coupled Partial Differential Equations in Porous/Fractured Geomaterials
Authors: Cui X.
Published: August 2024

Exploring efficient and robust algorithms to solve the simultaneous Partial Differential Equations (PDEs) is essential to model the prevalent multiphysics processes in deep rock engineering activities, such as the thermo-hydro-mechanical coupling in nuclear waste disposal and geothermal exploitation. In this study, the staggered and monolithic solution schemes are developed in the context of poroelastic and fractured geomaterials, and the applicability of the two solution schemes is analyzed in detail. It is found that the degree of coupling between primary variables plays a pivotal role in determining the performance of the staggered and monolithic solution schemes. Two-way coupling can only be tackled by the monolithic solution scheme, while the staggered solution scheme is very robust and efficient to deal with one-way coupling. This study provides an overarching principle to solve simultaneous PDEs: Understand the degree of coupling between primary variables, and use the staggered and monolithic solution schemes to address one-way and two-way couplings, respectively.

1. INTRODUCTION

Rock mechanics, as an applied geoscience, has its roots deeply entangled with various rock engineering activities. The pioneers and early generations of rock engineering practitioners established the framework and theory of rock mechanics mainly from on/near ground surface projects, such as rock foundations, rock slopes, and shallowly buried tunnels. In the last two decades, rock engineering activities started to go deeper and deeper to serve increasingly complex engineering purposes. Examples include nuclear waste disposal, hydraulic fracturing, geothermal exploitation, CO2 sequestration, and injection-induced earthquakes. The above new rock engineering activities pose great challenges to the existing rock mechanics theory, but at the same time, they provide a fertile ground for rock mechanics to extend and prosper. Future rock mechanics will interact more closely with computer science, seismology, petroleum engineering and many other disciplines to further highlight its interdisciplinary attributes.

A common feature of rock engineering at a great depth is the potential coexistence of in-situ stress, high temperature, fluid pressure in pores/fractures, and chemical reactions. These factors are coupled together, and each of them plays a considerable role in the entire system. Therefore, deep rock engineering is a typical multiphysics system. It is not trivial to replicate such a complex working environment in the laboratory, neither can the field tests be readily conducted at such a depth. Numerical simulation, however, provides an insightful, economical and effective approach to understand the multiphysics system.
Effects of Interactions Between Produced Formation Fluid and Rock Matrix on Pore Structure of Caney Shale, Southern Oklahoma
Authors: Xiong Fengyang , Awejori Gabriel , Radonjic Mileva
Published: August 2024

Rock-fluid interactions change properties of shales during exploitation. To investigate effects of rock-fluid interactions on pore structure of shales matrix after hydraulic fracturing, powder samples from two late Mississippian Caney Shale cores in the Ardmore Basin, southern Oklahoma, were used to react with formation produced fluid from the field in the batch reactor analysis. X-ray diffraction for mineralogy and Low-pressure nitrogen adsorption isotherms for pore structure were measured for original, after-7days, and after-30days samples. Results show that the samples consist mainly of quartz, followed by clay minerals, carbonates, and feldspar. The pore sizes of micropore (<2 nm) and mesopore (2-50 nm) increase 14%-233% due to dissolution of pyrite, feldspar, and carbonates after 7 days. Due to the transformation from smectite to illite and the increase of pore size, the specific surface area (SSA) decreases after 7-days interactions. After 30-days interactions, the micropore volume slightly increases and the mesopore and macropore volume decreases. Due to the decrease of pore size, the SSA of 30-days reacted samples increases correspondingly and is lower (for the clay-rich sample) or higher (for the calcareous sample) than that of the unreacted samples. Findings improve our understanding of dynamic alteration of shale properties during production.

1. INTRODUCTION

Energy demand will continuously grow owing to the increasing global population as well as energy consumption (EIA, 2023). On the other hand, shale gas and oil reshaped the energy market in the United States, enabling the United States to become a net-export of natural gas country in 2017 (EIA, 2023). However, shale reservoirs are challenging tight formations that are still poorly understood in the extraction and production of hydrocarbons (Ross and Bustin, 2009; Curtis et al., 2012; Xiong et al., 2015, 2021a; Li Y. et al., 2016; Gong et al., 2019a; Benge et al., 2021; Awejori et al., 2022; Huang et al., 2022). One of the most challenging topics is the rock-fluid interactions post hydraulic fracturing and its subsequent impacts on the pore structures of fractured formation matrix.
Experimental Study of Fluid Rheological Properties on the Friction of Cuttings Bed in the Horizontal Well
Authors: Zhou Mengmeng , Wang Huanhao , Xiao Hao , Song Xianzhi , Qi Yiqun , Wang Haobo , Han Liang , Duan Shiming , Xu Zhengming , Zhu Zhaopeng
Published: August 2024

Horizontal wells have been frequently used in many oil reservoirs, which has significant advantages in improving oil development efficiency. During horizontal well drilling, the cuttings can easily accumulate in the wellbore to form a cuttings bed at the bottom. The formed cuttings bed would directly increase the contact area so that wellbore friction during drilling is also increased. The increase in friction can cause serious underhole accidents. Hence it is necessary to study the parameters affecting the friction between the drillpipe and cuttings bed. The viscosity of drilling fluid and the rotation speed of drillpipe are important factors affecting the cuttings transport. In this paper, a laboratory experiment was designed to study the effects of fluid viscosity and drillpipe rotation speed on the sliding friction coefficient, then a general correlation model is built according to experimental results. Genetic programming is used here to analyze the effects accurately and optimize the structure and parameters of the model. This new way could greatly improve the model's accuracy and provide guidance for the sliding friction coefficient prediction.

1. INTRODUCTION

Horizontal wells have unique advantages in the development of these reservoirs, which is an important technology to improve the development efficiency of shale oil and gas reservoirs. The long horizontal well may greatly increase the exposed area of the reservoir in the wellbore and effectively improve the oil and gas recovery. However, horizontal wells also face some problems, such as cuttings accumulation during drilling, which will lead to out-of-control wellbore trajectory, excessive friction and torque, stuck drilling and wellbore instability. Borehole cleaning is one of the key problems affecting the safety and efficiency of drilling in horizontal wells. The distribution of cuttings bed in horizontal well has great influence on drillpipe friction, drillpipe position and drillpipe buckling. Cuttings bed accumulation is common in horizontal well, especially in inclined and horizontal well section. The cuttings bed accumulation increases the roughness of the contact surface between drillpipe and wellbore, and the friction coefficient between drillpipe and cuttings bed may increase significantly with the extension of the inclined section and horizontal section length. The excessive friction of drillpipe is easy to affect the drilling speed and wellbore trajectory, aggravate the fatigue damage of drill tool, affect the service life of drill tool, increase the probability of complex downhole conditions, and restrict the extension of horizontal section. Therefore, the frictional characteristics between drillpipe and cuttings bed during horizontal well drilling becomes to a key problem.
Experimental and Numerical Investigations on Fully Grouted Instrumented Rock Bolts Subjected to Pull Load
Authors: Modi J. V. , Deb D.
Published: August 2024

Rebar type rock bolts of 22 mm diameter and length of 600 mm were instrumented with four strain gauges along the length. The embedded length of the bolt was 450 mm grouted with resin inside a cement mortar cylindrical sample of diameter 250 mm. The diameter of the drill hole was varied as 32, 36 and 42 mm. The load-deformation relationships and variation in axial strains was measured with the pull load until debonding occurred. The bond strength and stiffness data revealed that hole diameter should be kept as low as possible for better performance of bolt. The axial strains at different locations along the bolt rod are analyzed and a modified equation based on Farmer (1975) is presented. Numerical studies on the axisymmetric models of the above experiments reveal the debonding phenomena between the bolt-grout and grout-rock interfaces which are evident from the experiments. A cohesion reducing constitutive model in the interfaces is applied to simulate the debonding with applied load and are explained in the paper.

1. INTRODUCTION

Rock bolt is the key reinforcement member used to provide support to rock strata in tunnels, underground mine drives/galleries and other excavations. A bolt enhances the mechanical properties of rock masses and reduces deformation and support time (Pinazzi et al., 2020). A fully grouted rock bolt generates shear stress along the interfaces of grout-bolt and grout-rock and resists the movement of rock by increasing the overall stiffness of the bolted zone (Cai et al., 2004; Deb & Das, 2010). Every year millions of rock bolts are installed worldwide to support and stabilize engineering structures (Benmokrane et al., 1995).

On other hand, the mechanics of shearing along the interfaces of bolt-grout-rock due to pull load was investigated by a few researchers. These studies showed that in the absence of rock movement, shear strain along the bolt rod varies non-linearly having the maximum value at the load side and near zero value at the other end (Farmer, 1975). Li & Stillborg, 1999 showed a similar distribution of strain development on the bolt rod considering the deformation of rock. Since then, several researchers attempted to simulate the shearing mechanism of bolt-grout and grout-rock interfaces using experimental and numerical models. A significant contribution was made to understand and evaluate the mechanism of bolt-grout interfaces (Hyett et al., 1996). Moreover, the extended finite element method (XFEM) was also proposed for analyzing the behavior of grouted rock bolts and applied for simulating reinforcement around tunnels and slopes (Deb and Gujjala, 2018). This experimental study evaluated the effects of grout, hole diameter and rate of loading on bond strength, stiffness and distribution of axial strains along the bolt rod.