bluuebunny/crossref_metadata_2025_split huggingface dataset:
Numerical Study on Mode I Fracture Characteristics of Rock Under Confining Pressure
Authors: Luo Senlin , Zhang Guangqing , Sun Bin , Qiu Renyi , Ling Yansen 
Published: August 2024

ABSTRACT:
Hydraulic fracturing is a key technology for the efficient exploitation of deep oil and gas resources. Mode I fracture is the main fracture type of hydraulic fracturing. However, it is difficult to consider the process of mode I fracture under confining pressure in laboratory experiments. Based on numerical methods, this study aims to reveal the influence of confining pressure on mode I fracture of rock. A three-point bending numerical model was established by discrete particle method to monitor the tensile stress concentration zone at the fracture tip and analyze the fracture morphology and fracture parameters under different confining pressures. The results show that the tensile stress concentration zone changes from a horizontal narrow band to a semi-ellipse as confining pressure increases, and its distribution range becomes smaller. Confining pressure enhances the actual tensile strength and actual fracture toughness of the rock. The mode I fracture of rock under confining pressure needs to overcome the confining pressure and rock cohesion resistance. The compaction effect of confining pressure on the rock will strengthen the fracture performance of the rock itself. This strengthening mechanism is affected by the critical confining pressure. This work provides a theoretical basis for studying the mode I fracture of hydraulic fracturing.

1. INTRODUCTION
Hydraulic fracturing is a widely stimulation method to increase oil and gas production (Detournay, 2016). Fracture mechanics has proven to be a useful tool for the investigation of brittle failure of rocks during hydraulic fracturing (Janiszewski et al., 2019; Yue et al., 2020). Generally, the fracture can be divided into three fundamental modes, tensile (mode I), in-plane shear (mode II), anti-plane shear (mode III), and mixed mode. The fracture is constrained by the in-situ stress state in the process of hydraulic fracturing and mode I fracture is the main fracture mode. However, it is difficult to conduct traditional fracture mechanics experiments under confining pressure, such as three-point bending, four-point bending and semi-circular three-point bending (Kuruppu et al., 2014; Wang and Hu, 2017; Zhu and Deng, 2023). It is of great significance to reveal mode I fracture rule of rock under confining pressure for further understanding of hydraulic fracturing mechanism.
In-Situ Horizontal Stress Inversion and Error Analysis Based on Borehole Deformation Monitoring
Authors: Chen Jie , Ma Tianshou , Liu Yang , Xu Wen 
Published: August 2024

ABSTRACT:
In-situ measurement is one of the most accurate and effective methods, but it is challenging to measure directly in the deep well. Therefore, a new horizontal in-situ stress inversion method was proposed based on the relationship between the borehole deformation and the in-situ stress state under nonuniform horizontal in-situ stress. The accuracy of the inversion model is validated by finite element simulation. The sources of measurement errors and their control method were discussed. The results indicated that the inversion error for the maximum and minimum horizontal principal stress is 0.001% and 0.052%, respectively. The results and errors of the horizontal principal stress inversion are significantly influenced by the measurement deviation of the borehole diameter, the borehole deformation, the wellbore pressure, and the elastic parameters of the rock. If the deviation from the borehole diameter measurement is less than 0.5 cm or the percentage deviation from the borehole deformation, the wellbore pressure and the elastic parameters of the rock is within ±5%, the horizontal stress inversion error is less than 4%, and thus the horizontal principal stress inversion results are deemed acceptable. This study can provide new ideas for in-situ stress measurement in the petroleum industry.

1. INTRODUCTION
In-situ stress is a natural stress that exists in formation without engineering disturbance. Accurate measurement of the magnitude and direction of in-situ stress is a prerequisite for determining the mechanical properties of engineering rock, for analyzing the stability of the surrounding rock, and for the scientific design and decision-making of rock engineering excavations (Cai, 1993; Wang et al., 1991; He et al., 2000).
In petroleum exploration and development, in-situ stress plays a very important role (Ma et al., 2022a, 2022b). In-situ measurement is one of the most accurate and effective methods (Wu, 2012). Currently, the hydraulic fracturing method has been widely used in petroleum, mining, and underground engineering. However, it can only directly measure the minimum horizontal principal stress (Duan et al., 2023). In mining and underground engineering, some in-situ measurement methods can directly measure both the minimum and maximum horizontal principal stresses, but it is very difficult to measure them directly in the deep well.
Coupled Numerical Simulation of Coal Failure Caused by Thermal Impacting in Underground Coal Gasification Zone
Authors: Zhao Yufeng , Zhang Mengyuan , Chen Yanpeng , Dong Zhen , Chen Hao , Chen Shanshan , Xue Junjie , Konietzky Heinz 
Published: August 2024

ABSTRACT:
Underground Coal Gasification (UCG) is an environmentally friendly way to utilize coal resources. The cavity evolution process, especially the thermal impact on coal rock, is an important topic of UCG. The thermally induced coal rock failure processes in gasification environment are rarely investigated, but they are important to the gas production space and the flow from the injection well to the production well. In this study, a coupled numerical simulation program based on PFC3D and FLAC3D was established to realize multi-field coupled thermal impact conditions. The numerical model of burning-induced coal surface failure was established, and the basic cavity evolution mechanism was revealed. The main results show that (1) the redistributed temperature field in the coal rock is the most important parameter for the coupling model. The mechanical properties of the model element are assigned in accordance with its temperature state. (2) Taking into account the effects of heating, microscopic fracture and deformation are generated through PFC3D model, inhomogeneous temperature distribution is simultaneously achieved through FLAC3D model. (3) The evolution law of the expansion of the gasification cavity has been obtained by the coupled numerical simulation. (4) Eventually, the height of the gasification cavity increases, and the upper coal seams expand to an approximate equilibrium, the height affected by the temperature field becomes stable, and the vertical expansion rate of the gasification cavity gradually decreases.

1. INTRODUCTION
Underground Coal Gasification (UCG) is a revolutionary technology for clean coal utilization that integrates the three key processes of well construction, coal mining and gasification for efficient gas production. This technology allows coal to undergo controlled in-situ combustion, pyrolysis and gasification reactions to produce syngas with methane, hydrogen, carbon monoxide, carbon dioxide and other components for surface recovery by injecting air, oxygen-enriched air, oxygen/water and other gasification agents into underground coal seams[1–4].
Currently, the underground gasification of middle-deep coal seams has no practical experience, especially for the dynamic study of the deforming and destroying process of the surrounding stone (coal) in the gasification cavity under high temperature and high pressure. The numerical simulation technology of gasification cavity development dynamics is still in the initial stage. The simulation technology of gasification cavity controllable development control is not deep enough, and the analysis of the effect of dynamic process parameters in the process of cavity formation is still insufficient.
Simulation of Elastic Waves Transmission and Attenuation Considering Fluids Properties Change and Radiator of Branches Well
Authors: Tan Yawen , Tian Shouceng , Zhang Yiqun , Wang Tianyu , Wang Fei 
Published: August 2024

ABSTRACT:
Crude oil production is promoted by means of physical vibration, such as the use of high-power acoustic waves, ultrasonic waves, electro-hydraulic pulse waves, and high-pressure shock waves of explosive gases to stimulate the reservoir. And the propagation speed, distance, and attenuation of the elastic waves generated by physical vibration are the main factors affecting the effect of production enhanced by physical vibration. In this paper, the Biot model is improved. Firstly, the influence of elastic wave on reservoir physical properties is considered, and the change of viscosity and permeability field under the elastic wave is obtained by numerical superposition method. Then, the branch of horizontal wellbore is equivalent to a finite linear radiator, the wave field constructed as a superposition of fields generated by branches in all. And we simulated for different branches cases, the propagation and attenuation of pressure wave in oil reservoirs are obtained. When the lateral level is used as the vibration source, the fast attenuation area and the slow attenuation area of the pressure fluctuation can be formed in the reservoir. The size and range of slowly decreasing pressure wave can be controlled by optimizing the structure of branch horizontal well pattern.

1. INTRODUTION
Improving the recovery rate of oil and gas resources is a crucial subject in the petroleum industry. Stimulating reservoirs through pulse injection is one of the important methods. The elastic waves generated during the pulse injection process can facilitate the flow of oil and gas, thus increasing the production yield of oil and gas.
Heavy oil is an important fossil energy source(Zheng, Li, Sun, & Yang, 2016; Zhou, Yuan, Peng, Zeng, & Zhang, 2018). Due to its high viscosity and poor flowability, the cost of extracting heavy oil is higher than conventional oil and gas resources. Heavy oil extraction typically requires methods such as steam injection or chemical viscosity reduction. Supplementing existing techniques with physical vibrations can further reduce crude oil viscosity and increase production yields.
During pulse injection, elastic waves primarily propagate in the form of pressure waves (longitudinal waves). This study simulates the propagation velocity and attenuation of waves in heavy oil reservoirs, considering the coupled effects of fluid and solid phases in the reservoir, particularly the influence of liquid phase viscosity. The wave field is simulated using staggered-grid finite difference method. Ultimately, the transmission characteristics of elastic waves are obtained.
Damage Evolution of Sandstone Under Cyclic Loading for Underground Gas Storage
Authors: Zhang Wenhong , Tian Shouceng , Weng Jintao , Sheng Mao , Wang Tianyu , Ma Zhengchao 
Published: August 2024

ABSTRACT:
Underground gas storage plays an important component of emergency gas supply and peak shaving capabilities in winter. It plays a vital role in ensuring the security of natural gas supply. Pressure variations and frequency changes in gas storage can easily cause fatigue damage to formation rocks. The M reservoir is currently in the initial phases of development, and conducting a thorough analysis of reservoir damage deformation under various storage conditions, including storage frequency and fluid pressure fluctuations, is crucial for ensuring safe and efficient operation. In this study, uniaxial compression tests and cyclic loading mechanical experiments are conducted on sandstone samples from M gas storage in Junggar Basin. These experiments combined with the operational data of M gas storage investigate the influence of different frequencies and amplitudes by stress-strain curves and hysteresis loop. Damage variables D are introduced to quantitatively analyze the damage behavior of sandstone at different frequencies and amplitudes. The results show that the compressive strength of sandstone is affected by amplitude and frequency. On the one hand, the higher the amplitude, the lower the rock strength, the fuller the hysteretic loop. On the other hand, the lower the frequency, the lower the rock strength. From the point of view of damage variable, the increase of amplitude prolongs the development cycle of inelastic strain. The higher the amplitude, the greater the final damage. When the amplitude is constant, the increase in frequency reduces the interaction time between the stone particles and the degree of rock damage decreases. The study provides a reference for the construction and safe operation of gas storage reservoirs.

1. INTRODUCTION
Underground gas storage is an important infrastructure for the storage of natural gas resources and the peaking of natural gas supply and demand(He,A., Ma,Q., Pang,Y. et al., 2023). Around the world, there are four main types of gas reservoirs: porous reservoirs, salt caverns, caverns, and depleted reservoirs. Depleted reservoir type gas storage is the most dominant type, with over 80% of the total volume ratio dedicated to working gas. By 2023, China had built 28 gas storage reservoirs with a peaking capacity of 165×109 m3. However, their working capacity only represents 4.4% of China's natural gas consumption, which is far below the minimum requirement of 12% set by the International Gas Union(Changyin, D, Chen, C, Lizhi, W, 2022). Thus, the conversion of depleted gas reservoirs into gas storage has been identified as a highly promising tool. The attention of academic institutions, governments, and oil companies has been drawn to accelerating the construction of gas storage.
Strength Characterization and Empirical Correlations for Carbonate Rocks from the Brazilian Pre-Salt Reservoirs
Authors: Sampaio R. V. , Righetto G. L. , Fontoura S. A. B. , Lima C. , Naumann M. 
Published: August 2024

ABSTRACT:
The pre-salt regions in Brazil, especially the Campos and Santos Basins, are responsible for most of the country's hydrocarbon production. Although there has been much discussion about the geological and petrophysical aspects of the carbonate reservoirs found in these areas, information about their mechanical properties is still limited. This study aims to help fill this gap by characterizing the strength of well plugs from four different wells located in the Santos Basin and developing empirical correlations. The samples are lacustrine carbonates of varying lithologies from the Barra Velha Formation, with porosity ranging between 0.7% and 20.5%, and permeability ranging from near-zero to 22.1 mD. The results showed cohesion (c') varying from 11.1 to 43.1 MPa, internal friction angle (φ') between 12.7° and 39.2°, and UCS between 29.2 and 171.7 MPa. The average Poisson's ratio (ν50) was 0.26 ± 0.1, and the Young's modulus (E50) results were between 8.7 and 72.4 GPa. The established empirical correlations between porosity and E50, UCS, c', and φ' were exponential and power equations that provided suitable fitting of the data and could be applied to other carbonates from the Barra Velha Formation.

1. INTRODUCTION
The worldwide exploration and production of carbonate reservoirs for oil and gas have been a significant driving force for the study of these rocks. In the offshore basins of Southeastern Brazil, the lacustrine carbonates are a major challenge for the industry. These rocks are known for being complex and heterogeneous, requiring a wide understanding of the interplay of their geological, petrophysical, and mechanical aspects. Even though most of the country's production comes from pre-salt reservoirs, obtaining well samples for mechanical characterization is expensive and, therefore, rare. Thus, there is a lack of information about the mechanical behavior of these lacustrine carbonates.
Considering this scenario, many authors have been addressing their efforts to develop empirical correlations to be used with parameters such as porosity, sonic wave's transit time, and density, which are easily obtained from well logs. Some of these correlations available in the literature were gathered by Santos and Ferreira (2010). However, due to the complexity of these carbonate rocks, it is important to verify the suitability of the correlations. In this paper, some well plugs from Santos Basin were used in triaxial tests to obtain the Mohr-Coulomb failure envelopes and elastic parameters. In addition, empirical correlations were developed based on the tests' results and the porosity of the specimens.
Shear Strength and 1D Compressibility Characteristics of a Plastic and Non-Plastic Soil Exposed to Wet-Dry Weathering
Authors: Tohm Calvin , Lingwall Bret 
Published: August 2024

ABSTRACT:
Seasonal weathering cycles such as wet-dry desiccation weathering can alter geomaterial (weak rock to soil) structure, particle arrangement (i.e. fabric), shear strength, 1D compressibility, and other engineering properties. During the drying and rehydration process associated with wet-dry weathering cycles geomaterial intra-particle forces are altered. These micro-scale changes to rock structure and soil fabric facilitate macro-scale responses in the soils engineering properties. As the number of wetting-drying cycles increases its expected to see the geomaterial mass become fragmented and disjointed due to the formation of drying tensile cracks. Upon rewetting the tensile cracks within the soil mass may partially recover but will still result in altered soil properties. This study sought to investigate how this phenomenon alters a soils shear strength and 1D compressibility properties. To accomplish this a plastic (MH) weathered shale residual and non-plastic (SC) soil was molded at its optimum moisture content (OMC) in accordance with ASTM D698 then exposed to 0, 1, 2, and 10 wetting-drying cycles. It was found that the wetting-drying cycles had a large impact on the MH material. This is most likely a result of the higher swelling clay content in the MH material, resulting in larger amounts of shrink swell that occurring during drying and rehydration of the samples. The SC soil contained relatively little amounts of swelling clay which resulted in smaller amounts of sample alteration that occurred during the weathering cycles.

1. INTRODCUTION
Seasonally driven weathering cycles act as external environmentally driven stresses on near surface soil, rock and intermediary geomaterial (IGM) masses. Seasonal weathering cycles gradually alter geomaterial engineering properties by gradually degrading the material fabric. Seasonal weathering cycles (freeze-thaw and wetting-drying desiccation) interact with the topmost layer of soil or rock in the upper 2 to 5-m and develop an active zone in this depth range in the slope as the degree of weathering over time increase. This ultimately results in degraded soil, weak rock or IGM that becomes fragmented and disjointed at the degree of degradation increases unless repaired through mechanical compaction, grouting, or anchoring.
High-Performance Modeling of Coupled Hydro-Mechanical Behavior with Graphical Processing Units
Authors: Bondarenko N. , Makhnenko R. , Podladchikov Y. 
Published: August 2024

ABSTRACT:
Proper consideration of the complexity of coupled processes is essential for accurate assessment of outcomes of geoenergy projects. The potential applications include evaluation of the sealing capacity of caprock, storage potential of the reservoir formation, fluid transport, and induced seismicity risks. The numerical implementation of the governing equations for thermo-hydro-mechanical-chemical response is challenging due to the substantial contrast between the characteristic times of coupled processes. Furthermore, addressing the heterogeneity and anisotropy of geological formations requires development of representative three-dimensional models, which can reach billions of grid points. These challenges are addressed by implementation of pseudo-transient iterative method with the use of graphical processing units (GPU) to achieve low-level parallelization. The implemented numerical codes are formulated in consistent modular structure allowing to enable modules based on the desired applications and are presented for the case of hydro-mechanical coupling. Hydro module handles single- and two-phase flow coupled with the mechanical component. The foundational constitutive framework of mechanical response is Biot's poroelasticity that can be expanded to incorporate viscous- and plastic-deformation. Resolving the governing equations of coupled hydro-mechanical (HM) response with high resolution is a bandwidth-limited problem, highlighting that effective management of memory access is essential for scalability of parallel computations. All the modules are implemented in the single executable file, allowing to limit unnecessary memory access, and resulting in up to 50x performance increase in single-GPU implementation compared to single CPU. The accuracy of HM solver is benchmarked with known analytical solutions for uncoupled and coupled processes.

1. INTRODUCTION
The complex environment of the subsurface frequently involves non-equilibrium mechanical (M), hydraulic (H), and other processes. These processes are inherently interrelated, and their independent consideration requires specific conditions. Precise control over the environmental conditions preserved in laboratory experiments opens way for measurements of material properties required for characterization of the coupled processes. However, laboratory approaches are usually limited in spatial- and temporal-scales, failing to directly address the material response at field scales and potential heterogeneity of geological formations.