Search Results (11)

The exploration of oil and gas resources is shifting toward ultra-deep and extra-deep reservoirs, including in the Tarim Basin, where various types of carbonate rocks are buried. Owing to the extreme burial depths, the mechanical behavior of these rocks under extra-deep conditions differs significantly from that of shallow formations, making it essential to understand their mechanical responses. This study investigated the mechanical properties and failure modes of carbonate rocks, specifically dolomite, argillaceous limestone, and pure limestone. Samples from extra-deep formations were initially analyzed for mineral composition and microstructure, after which uniaxial and triaxial compression tests were conducted to evaluate strength, static elastic modulus, and axial strain at peak stress. The results indicate that dolomite exhibits the highest mechanical strength and stiffness among the three lithologies. Under conditions of a high confining pressure of 100 MPa and a temperature of 160 °C, its elastic modulus and triaxial compressive strength are 65.4 GPa and 611.2 MPa, respectively, compared with 52.8 GPa and 444.2 MPa for limestone. Strength increases with confining pressure for all lithologies, with dolomite showing the most pronounced strengthening response. Although elevated temperature reduces rock strength, its effect is weaker than that of confining pressure. The failure mode is strongly controlled by confining pressure. At low confining pressures, failure is dominated by localized shear bands and brittle fracturing, whereas increasing confining pressure promotes a transition toward quasi-brittle deformation or ductile plastic flow. Dolomite predominantly maintains a quasi-brittle failure mode, argillaceous limestone exhibits a clear brittle-to-ductile transition, and pure limestone shows the greatest tendency to develop ductile plastic flow under high confining pressure conditions. The results further demonstrate that mineral composition and microstructural characteristics play critical roles in controlling the deformation and failure mechanisms of carbonate rocks. High-calcite pure limestone can exhibit ductile-like deformation behavior due to cataclastic processes, allowing significant strain without localization failure. These insights enhance understanding of carbonate rock behavior under extra-deep formations, informing practical applications in geology science. Full article

The cyclic material properties are crucial for accurate assessment of structural response; however, conventional numerical methods often fail to replicate the mechanical degradation caused by ultra-low cycle fatigue (ULCF) damage accumulation. This paper focuses on a novel constitutive model characterizing the continuous degradation in yield stress and stiffness under large-strain tension–compression cyclic loads. A series of 32 cyclic tests on circular notched specimens covering a range of stress triaxialities are first revisited to systematically identify the cyclic softening behavior. The corresponding limitations of the widely applied classical Chaboche theory are examined, and an enhanced elastoplastic constitutive framework is proposed accordingly. On this basis, the stress-weighted ductile fracture model (SWDFM), which is developed from micro-damage mechanics, is then employed as the internal damage criterion of the proposed constitutive model. Explicit functional relationships linking the evolved yield stress and Young’s modulus to the imposed damage index are established and calibrated against the test data. Based on these findings, a new cyclic ductile constitutive model is developed, which is programmed into the UMAT subroutine compatible with ABAQUS/Standard. Numerical validations against the experimental cyclic responses exhibit good agreement, as indicated by the considerably low average errors of 5% for both strength and stiffness, demonstrating its validity in terms of the hysteretic behavior simulation for metals subjected to progressive ULCF damage. Full article

The ultra-deep carbonate reservoirs of the Fuman Oilfield in the Tarim Basin are characterized by intense fracture development. The coupled effects of high in-situ stress and fracture structures significantly deteriorate the mechanical properties of the rock mass, thereby constraining wellbore stability evaluation and safe drilling and completion operations. Existing studies have primarily focused on medium- to low-confining-pressure conditions and isolated fracture parameters, making it difficult to characterize the mechanical response of fractured rock masses under the high-stress conditions of ultra-deep reservoirs. To address this issue, limestone from the Yingshan Formation of the target reservoir was selected as the research object, and fractured specimens with varying fracture angles, widths, and densities were prepared. Uniaxial compression tests and triaxial compression tests under high confining pressures of 90 MPa and 120 MPa were conducted to systematically reveal the evolution of rock strength, deformation parameters, shear strength parameters, and failure modes under the coupled influence of fracture geometric parameters and confining pressure. On this basis, a confining-pressure–fracture coupled damage prediction model was established, and wellbore stability around the reservoir was analyzed using Finite Difference Method. The results indicate that fracture angle causes the peak strength and Young’s modulus to first decrease and then increase, with an inclination angle near 45° representing the most unfavorable fracture orientation. Increases in fracture width and density lead to continuous degradation of strength and stiffness. Although high confining pressure can close fractures and enhance load-bearing capacity, it cannot eliminate the controlling influence of fractures on failure pathways. Sensitivity analysis shows that the Young’s modulus and Poisson’s ratio are most sensitive to fracture width; cohesion is mainly governed by fracture width and density; and the internal friction angle is most sensitive to fracture density. Numerical simulations of wellbore stability further confirm that medium-inclination, large-aperture, and high-density fractures significantly increase the risk of wellbore instability. The findings provide experimental and theoretical support for mechanical-parameter correction, wellbore stability assessment, and construction-risk control in ultra-deep fractured carbonate reservoirs. Full article

Fracability evaluation is essential for hydraulic fracturing interval selection and stimulation optimization in ultra-low permeability sandstone reservoirs. Conventional brittleness-based methods derived from shale reservoirs are insufficient for characterizing fracture initiation difficulty, fracture propagation resistance, natural fracture interaction, and post-fracture conductivity in tight sandstone formations. In this study, the Chang 6 ultra-low permeability sandstone reservoir in Block H was investigated by integrating triaxial rock mechanical testing, Kaiser acoustic emission stress measurement, FMI/MCI image-log interpretation, and logging-based dynamic-to-static mechanical parameter conversion. The results show that the reservoir is characterized by relatively high stiffness and strength, with an average static Young’s modulus, Poisson’s ratio, and compressive strength of 24.05 GPa, 0.21, and 131.97 MPa, respectively. The all-sample average maximum and minimum horizontal principal stresses are 35.70 MPa and 29.91 MPa, respectively. After excluding the anomalous C6-19 stress-memory response, the representative average ${\sigma }_H$ and ${\sigma }_h$ are 37.06 MPa and 30.95 MPa, respectively, with a representative stress difference of 6.12 MPa. A multi-factor integrated fracability index was established by considering brittleness, natural fracture development, compressive strength, equivalent fracture propagation resistance, and effective confining pressure. The average fracability indices of Wells L7 and L26 are 0.624 and 0.596, respectively, indicating relatively favorable fracturing potential. The proposed workflow provides a geomechanically constrained method for relative sweet-spot ranking and preliminary hydraulic fracturing design in ultra-low permeability sandstone reservoirs. Full article

With the continuous development of drilling and reservoir stimulation technologies, the drilling depth of enhanced geothermal system projects is getting deeper and deeper, and the surrounding rock stress of dry hot rock reservoirs is also increasing. Therefore, it has become an inevitable demand for geothermal exploitation to study the evolution law of fracture seepage characteristics of granite under high temperature and ultra-high pressure. To reveal the evolutionary patterns of seepage characteristics in deep-seated hot dry rock fractures, an independently developed ultra-high pressure rock triaxial mechanical testing system was employed to investigate the seepage characteristics of fractured granite under varying temperatures (25–150 °C) and triaxial stresses (50–100 MPa). The study explores the influence of temperature on the seepage characteristics of granite fractures under ultra-high triaxial stress conditions. The results indicate that: (1) In the temperature range of 25–125 °C, as the rock temperature increases, the permeability of the Specimens showed a continuously decreasing trend due to the effect of thermal expansion. (2) In the temperature range of 125–150 °C, as the rock temperature increases, the permeability continues to decrease under low triaxial stress (50 MPa). However, under high triaxial stress (75 MPa) and extremely high triaxial stress (100 MPa), the permeability shows a slight increase instead. This phenomenon is attributed to free surface dissolution. (3) Quantitative analysis of the mesoscopic morphological data of the rock fracture surfaces after testing, combined with SEM images from scanning electron microscopy, confirms that within the high-temperature range of 125–150 °C, the differing levels of triaxial stress determine the variation in the dominant mechanism governing the evolution of the Specimen fracture surfaces, which in turn leads to the divergence in the trend of their permeability changes. Full article

Fatigue damage is critical for 0Cr17Ni4Cu4Nb stainless-steel components that may operate near yield under stress-controlled cycles and occasional peak holds. This work investigates the cyclic response of 0Cr17Ni4Cu4Nb stainless-steel under near-yield-stress-controlled (NYSC) loading and proposes a unified damage framework that bridges monotonic ductile fracture, near-yield stress-controlled fatigue. Building on the Enhanced Lou-Yoon model, an elastic-damage term is introduced and embedded within a continuum damage mechanics framework, allowing elastic (sub-yield) and plastic (post-yield, Ultra-Low-Cycle-Fatigue/Low-Cycle-Fatigue (ULCF/LCF)) damage to be treated in a unified, path-averaged stress-state description defined by stress triaxiality and the Lode parameter. Five stress-controlled test groups are examined, with applied load amplitudes from 20.6 to 25.1 kN (equivalent stress amplitudes 858~1044 MPa) yielding fatigue lives ranging from 32 to 13,570 cycles. The extended model captures the evolution of damage origin mechanisms from elasticity-dominated to plasticity-dominated as loading severity increases, demonstrating a unified elastic-plastic damage modeling approach. As a result, it accurately predicts fatigue lives spanning two orders of magnitude with an average absolute percentage error of approximately 14.5% across all conditions. Full article

Conventional methods for studying the plastic behavior of materials involve uniaxial tension and uniaxial compression. However, in the metal rolling process, the deformation zone undergoes a complex loading of multidirectional compression and shear. Characterizing the corresponding plastic evolution process online poses challenges, and the existing specimen structures struggle to accurately replicate the deformation-induced loading characteristics. In this study, we aimed to design a compression–shear composite loading specimen that closely mimics the actual processing conditions. The goal was to investigate how the specimen structure influences the stress–strain response in the deformation zone. Using commercial finite element software, a compression–shear composite loading specimen was meticulously designed. Five 304 stainless steel specimens underwent uniaxial compressive loading, with variation angles between the preset notch angle (PNA) of the specimen and compression direction. We employed digital image correlation methods to capture the impact of the PNA on the strain field during compression. Additionally, we aimed to elucidate the plastic response resulting from the stress state of the specimen, particularly in relation to specimen fracture and microstructural evolution. Full article

The generation of dynamic fractures during the process of water injection in ultra-low permeability reservoirs aggravates the heterogeneity of the reservoir, resulting in a rapid rise of water cut and directional flooding of the producers, which affects waterflood sweep efficiency and recovery. A dynamic fracture, as the geological feature of ultra-low permeability reservoirs, has a complex genetic mechanism and is difficult to characterize. Taking the Chang 6 reservoir of the Triassic Yanchang Formation in the Ordos Basin of central China as an example, this paper presents the characterization method and workflow of dynamic fractures. On the one hand, through the analysis of triaxial-compression rock-mechanic experiments and the mineral composition of the core, we evaluated rock brittleness in order to identify the lithology that can easily generate new fractures. On the other hand, beginning with the ancient tectonic stress field and combining the fracture characteristics of core and geological outcrop, the multi-fractal method and the probabilistic neural network were applied to identify the natural fractures and to quantitatively predict the intensity of natural fractures. Based on the rock brittleness evaluation and the natural fracture feature, the intensity of dynamic fractures was characterized by integrating the analysis of the bottom hole pressure, fracture pressure, and production response characteristics. A dynamic fracture is a “double-edged sword” during the waterflooding development of ultra-low permeability reservoirs. The premature activation and generation of dynamic fractures could lead to a worse development status. Nevertheless, the rational control and utilization of dynamic fractures play a positive role in improving oil recovery. Dynamic fractures are of great significance to the optimization and adjustment of well patterns for ultra-low permeability reservoirs. This can provide a reference for similar reservoirs. Full article

Under strong earthquakes, steel structures are prone to undergoing ultra-low cycle fatigue (ULCF) fracture after sustaining cyclic large-strain loading, leading to severe earthquake-induced damage. Thus, establishing a prediction method for ULCF plays a significant role in the seismic design of steel structures. However, a simple and feasible model for predicting the ULCF life of steel structures has not been recognized yet. Among existing models, the ductile fracture model based on ductility capacity consumption has the advantage of strong adaptability, while the loading history effect in the damage process can also be considered. Nevertheless, such models have too many parameters and are inconvenient for calibration and application. To this end, focusing on the prediction methods for ULCF damage in steel structures, with the fragile parts being in moderate and high stress triaxiality, this paper proposes a simplified uncoupled prediction model that considers the effect of stress triaxiality on damage and introduces a new historical-effect related variable function reducing the calibration work of model parameters. Finally, cyclic loading test results of circular notched specimens verify that the proposed model has the advantages of a small dispersion of parameters for calibration, being handy for application, and possessing reliable results, providing a prediction method for ULCF damage of structural steels. Full article

The temperature dependence of tensile characteristics and fracture toughness of the standardly heat-treated low-alloyed steel OCHN3MFA along with three additionally heat-treated grades was experimentally studied. In the temperature range of ⟨−196; 22⟩ °C, all the additional heat treatments transferred the standard steel from a high- to ultra-high strength levels even with improved tensile ductility characteristics. This could be explained by a reduction of the inclusion content, refinement of the martensitic blocks, ductile retained austenite content, and homogenization of the shape ratio of martensitic laths as revealed by metallographic, X-ray, and EBSD techniques. On the other hand, the values of the fracture toughness of all grades were found to be comparable in the whole temperature range as the cause of a high stress triaxiality in the pre-cracked Charpy V-notch samples. The values of the fracture toughness of the standard steel grade could be predicted well using the fracture model proposed by Pokluda et al. based on the tensile characteristics. Such a prediction failed in the case of additionally heat-treated grades due to the different temperature dependence of the fracture mechanisms occurring in the tensile and fracture-toughness tests. While the tensile samples fractured in a ductile-dimple mode at all temperatures, the fracture-toughness specimens exhibited a transition from the ductile to quasi-brittle fracture mode with decreasing temperature. This transition could be interpreted in terms of a transfer from the model proposed by Rice and Johnson to the model of Tvergaard and Hutchinson. Full article

The cyclic void growth model (CVGM) is a micro-mechanical fracture model that has been used to assess ultra-low cycle fatigue (ULCF) of steel structures in recent years. However, owing to the stress triaxiality range and contingency of experimental results, low goodness of fit is sometimes obtained when calibrating the model damage degradation parameter, resulting in poor prediction. In order to improve the prediction accuracy of the CVGM model, a model parameter calibration method is proposed. In the research presented in this paper, tests were conducted on circular notched specimens that provided different magnitudes of stress triaxiality. The comparative analysis was carried out between experimental results and predicted results. The results indicate that the number of cycles and the equivalent plastic strain to ULCF fracture initiation by the CVGM model calibrated by the proposed method agree well with the experimental results. The proposed parameter calibration method greatly improves prediction accuracy compared to the previous method. Full article