Geomechanics and Geophysics for Geo-Energy and Geo-Resources最新文献

Lithofacies identification plays a pivotal role in understanding reservoir heterogeneity and optimizing production in tight sandstone reservoirs. In this study, we propose a novel supervised workflow aimed at accurately predicting lithofacies in complex and heterogeneous reservoirs with intercalated facies. The objectives of this study are to utilize advanced clustering techniques for facies identification and to evaluate the performance of various classification models for lithofacies prediction. Our methodology involves a two-information criteria clustering approach, revealing six distinct lithofacies and offering an unbiased alternative to conventional manual methods. Subsequently, Gaussian Process Classification (GPC), Support Vector Machine (SVM), Artificial Neural Network (ANN), and Random Forest (RF) models are employed for lithofacies prediction. Results indicate that GPC outperforms other models in lithofacies identification, with SVM and ANN following suit, while RF exhibits comparatively lower performance. Validated against a testing dataset, the GPC model demonstrates accurate lithofacies prediction, supported by synchronization measures for synthetic log prediction. Furthermore, the integration of predicted lithofacies into acoustic impedance versus velocity ratio cross-plots enables the generation of 2D probability density functions. These functions, in conjunction with depth data, are then utilized to predict synthetic gamma-ray log responses using a neural network approach. The predicted gamma-ray logs exhibit strong agreement with measured data (R² = 0.978) and closely match average log trends. Additionally, inverted impedance and velocity ratio volumes are employed for lithofacies classification, resulting in a facies prediction volume that correlates well with lithofacies classification at well sites, even in the absence of core data. This study provides a novel methodological framework for reservoir characterization in the petroleum industry.

Abstract

The stress paths of the cylindrical specimen in the p–q stress space by controlling the ratio of the axial and the radial loading is guaranteed to be consistent with the cuboid specimen, a novel method for imitating true-triaxial stress path by conventional triaxial apparatus was presented. Under the condition that p and q were variables and b was constant, the true-triaxial stress paths were realized by conventional triaxial apparatus strictly and easily. Under the condition that b and p were invariants, the b was used to control the ratio of axial and radial loading to ensure p constant, the method can be used to measure the strength on the π plane. If the tests were conducted at the different p with the same b, the critical state line of different b could be obtained. Under the condition that p and q were constant, the proposed method of nonlinear loading with b as a parameter could be used to design the various stress paths of true-triaxial under the condition of deviatoric stress consolidation, and which could be used to determine the deformation and the plastic flow of soil in 3D space. The proposed method could be used to achieve the equivalent stress path in the p–q stress space to obtain the 3D mechanical properties, and the stress path controlled by stress, strain, and a hybrid of stress and strain. Once the software of conventional triaxial apparatus was developed by the novel method, the measuring range of stress paths could be expanded greatly.

The Da’anzhai Member limestone in the central Sichuan Basin holds significant importance as a tight oil-producing formation. Despite its crucial role, the intricate patterns of hydrocarbon enrichment and the elusive geological factors dictating high-yield production have impeded tight oil exploration and development in the Sichuan Basin. This study delves into the geological characteristics of tight oil and identifies key factors influencing high-yield production, utilizing comprehensive data derived from cores, thin sections, well logging, seismic studies, and production tests of the Da’anzhai Member in the western Gongshanmiao within the central Sichuan Basin. Our findings reveal that the primary productive strata for tight oil are the Da 1 (1st Submember of the Da’anzhai Member) and Da 3 (3rd Submember of the Da’anzhai Member) Submembers, characterized by high-energy and low-energy shell beach microfacies. The kerogen type is sapropelic, ranging from mature to highly mature, positioning it as a moderately good hydrocarbon source rock. The predominant lithology of the reservoir consists of coquina and argillaceous coquina, with secondary dissolved pores, fractures, and nano-scale micropores serving as the predominant reservoir spaces. The overall lithology represents a dense limestone reservoir of the pore-fracture type, featuring low porosity and permeability. Critical controlling factors for achieving high-yield production of tight oil encompass lithological composition, fracture development, tectonic position, and source-reservoir configuration. Notably, substantial coquina thickness, fracture development, and the strategic relationship between the lower reservoir and upper source rocks contribute significantly to unlocking high tight oil yields. Additionally, thin-layer coquina emerges as a potential area for realizing increased oil and gas production capacity during later stages of development. This comprehensive analysis sheds light on the intricate dynamics governing tight oil production in the Da’anzhai Member, offering valuable insights for advancing exploration and development strategies in the Sichuan Basin.

Abstract

Roadway support can effectively improve the stability of roadway excavation and ensure the safety of underground mining. This study investigates the secondary support time and parameter optimization of combined support for a deep roadway in the stage of resource replacement in the Huize lead–zinc mine in Yunnan Province, China. The aim of this study is to increase the stability and safety of the roadway and decrease the cost of support. Research on support methods and failure modes has shown that under the action of high in-situ stress in deep mining, the surrounding rock of the roadway exhibits obvious rheological phenomena. The change in the radial displacement of the roadway is combined with creep tests of the main exposed surrounding rock to determine the secondary support time. Numerical simulations and orthogonal tests are utilized to optimize the support parameters in terms of the roof subsidence, floor heave displacement, side displacement, and plastic zone by analyzing the effects of the sprayed concrete thickness, bolt length, bolt row spacing, and bolt diameter on the support results. The proposed secondary support time and combined parameters can provide a reference for roadway support in similar strata.

Using unique experimental equipment on large bench-scale samples of Polymethylmethacrylate, used in the literature as an analogue for shale, we investigate the potential benefits of applying cyclical hydraulic pressure pulses to enhance the near-well connectivity through hydraulic fracturing treatment. Under unconfined and confined stresses, equivalent to a depth of up to 530 m, we use dynamic high-resolution strain measurements from fibre optic cables, complemented by optical recordings of fracture development, and investigate the impact of cyclical hydraulic pressure pulses on the number of cycles to failure in Polymethylmethacrylate at different temperatures. Our results indicate that a significant reduction in breakdown pressure can be achieved. This suggests that cyclic pressure pulses could require lower power consumption, as well as reduced fluid injection volumes and injection rates during stimulation, which could minimise the occurrence of the largest induced seismic events. Our results show that fractures develop in stages under repeated pressure cycles. This suggests that Cyclic Fluid Pressurization Systems could be effective in managing damage build-up and increasing permeability. This is achieved by forming numerous small fractures and reducing the size and occurrence of large fracturing events that produce large seismic events. Our results offer new insight into cyclical hydraulic fracturing treatments and provide a unique data set for benchmarking numerical models of fracture initiation and propagation.

Lake Fuxian is one of the deepest tectonic plateau freshwater lakes in the southeastern Tibetan Plateau, China. However, questions such as how old the lake is, how deep the total sedimentary thickness sequences are, and what landscape of the lake basin settings and geological structures are unknown. Here, based on fifteen seismic reflection profiles, we applied seismic facies and seismic sequence stratigraphic analyses to interpret the lake sequences. The results of the seismic response reveal that the maximum thickness of the sedimentation is ca. 1238 m and lies toward the NNE region of the lake basin on the L10-2 survey line. Lake sediments can be categorized into five seismic sequences and six seismic horizons. The oldest clinoforms in the deepest sequence (Sq-5) show that the depositional center was shifted to ~ 19 km from the NNE region to the SSW modern location and was ~ 930 m lower than the current lake floor. Multiple and complex tectonic activities strongly impacted on the lake basin, and a series of normal faults created an overall crustal extensional regime, resulting in the formation of many horst and graben structures.

A new CTOD calculation method is investigated in this study, considering the FPZ and the effective Young’s modulus. The calculated CTOD values from four theoretical models are compared with the measured CTOD values from the three-point beam experiments, and the differences between them are analyzed. The measured CTOD consists of two parts: (1) the displacement generated by the elastic–plastic deformation in the crack tip region, and (2) the displacement generated by micro-damage in the FPZ. CTOD value caused by micro-damage in the FPZ accounts for 81–92% of the overall CTOD. Thus, the FPZ and the effective Young’s modulus are introduced to modify the models for calculating CTOD. The result indicates that the modified plastic zone model is better than the strip-yield model, the plastic zone model and the modified strip-yield model in calculating CTOD, and CTOD error is reduced from 81 to 90% between the plastic zone model and the experiment to 4–34% between the modified plastic zone model and the experiment, with nearly half of the specimens having an error of less than 10%.

Discontinuous joints are prevalent in engineered rock masses and play a significant role in the stability of the rock mass. This study aims to analyze the impact of the inclination angle and number of prefabricated flaws on the crack evolution and failure pattern of sandstone specimens. Uniaxial compression tests, along with acoustic emission technology and digital image technology, were employed to monitor and analyze the effects. The findings indicate that: (1) With the increase in the flaw inclination angle, the damage mode of the specimen transitions from tensile to compressive-shear failure. The localized high-strain region on the surface of the specimen predicts the propagation path for the formation of macroscopic cracks. (2) When the number of prefabricated flaws is small, the flaws mainly expand through tensile wing cracks. As the number of flaws increases, the inner flaw tip does not produce cracks. Instead, the failure of the entire specimen occurs along the direction of the outer flaw's tensile wing crack, with the inner flaw running through it. (3) The winged tensile crack is the first crack to appear in all rock samples, regardless of the flaw initiation angles. Finally, the stress intensity factor at the crack tip under uniaxial compression conditions, without considering the closure effect, was expressed based on fracture mechanics theory. The crack initiation angle was then calculated. The results of the theoretical calculation of the initiation angle were found to be consistent with the test results. These research findings can serve as theoretical references and provide insights into the failure mechanisms of cracked rocks and the development of disaster control methods in rock engineering.

The landslide of mine is of great harm and wide influence, which can easily cause huge economic losses and endanger the life safety of workers. Therefore, landslide failure mechanism and more efficient landslide treatment methods have been the focus of landslide research. Laoyinzui landslide with a volume of 250,000 m³ occurred along the gently inclined weak interlayer at 6:00 (UTC + 8) on 5 January 2019 in Huangshan Limestone Mine, Emei City, Sichuan Province, China. The deformation history and failure mechanism of the landslide were analyzed based on the field investigation and geological conditions of landslide area. The treatment method of using excavators to remove all sliding body within the arm length by excavating the small-bench in the bedrock was proposed. The slope stability after treatment was analyzed based on the monitoring data. The results showed that the landslide was triggered by rainfall and earthquake after long-term creep deformation under the action of various factors. Weak interlayer was the potential sliding surface of landslide. The tensile cracks at the back edge of the landslide and the joint fissures and karst caves of the upper limestone provided convenient conditions for rainwater infiltration. Mining activities, including excavation and blasting, resulted in deterioration of mechanical properties of rock mass. Rainfall was the main trigger for the landslide. Water accumulated in weak interlayer, leading to increase of pore water pressure and decrease of anti-sliding force. Earthquake was the trigger for the landslide, which resulted in the reduction of rock mass structural strength. The Laoyingzui landslide consisted of two stages. First, a traction landslide of + 825 m–915 m occurred, and then a push landslide of + 725 m–+ 825 m occurred under the compression of the upper rock mass. The slope displacement was small and the deformation tended to be stable. The treatment method was safe and efficient. This paper can provide reference for the failure mechanism research and treatment of similar landslides.

Abstract

The study of the mechanism of thermal shock directional fracture of rocks under bidirectional horizontal stress is important for the application of directional thermal shock fracture technology. With the engineering background of the thick igneous roof overlying the coal seam, we conducted high temperature thermal shock directional fracture tests on granite under different horizontal loads to investigate the fracture mechanism. The results show that during the directional thermal shock of granite, the heating rate of borehole surrounding rock experienced three stages of rapid increase, rapid decrease and slowly decrease. AE tests were used to characterize the typical features of rocks during thermal shock fracture: the appearance of macrocracks in the specimen was accompanied by sharp increases in the cumulative AE count and the sudden drops in b-value. The experimental results show that thermal shock can create macroscopic directional fractures within the rock. Within a certain range of horizontal stress difference, the expansion direction of thermal shock cracks could be released locally from geological stress control, i.e. expanding along the direction of the minimum horizontal dominant stress. This provides completely new thinking for the cutting of hard roof and the directional fracturing of rock. In addition, directional thermal shock caused modifications in the distribution of stress in borehole surrounding rocks. We have established a model for stress distribution around the borehole rock and given the calculation formula for the initiation stress of the rock. The studies provide significant theoretical guidance for the industrial application of directional thermal shock fracturing technology.