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Accurate P-phase first arrival time is a premise for improving accuracy of seismic source localizations and achieving hazard warning. Traditional algorithms failed to meet the requirements of high precision and accuracy for microseismic (MS) monitoring in deep geological engineering. In this study, a multi-step method: convolutional neural network combined with K-means and AIC (CNN-KA) for picking up arrival time of P-phases is proposed. Firstly, convolutional neural network (CNN) technique is used to recognize waveforms of rock fractures instead of manual. Secondly, maximum overlapping discrete wavelet transform and multi-resolution analysis are combined to denoise. Finally, a new picker was developed by introducing K-means clustering algorithm, which was used to extract the target time window where the P-phase was located. It compensates for inherent shortcomings of AIC when applied to field data itself. Experiments and engineering applications show that the average absolute error of the proposed method (CNN-KA) is 0.0915s at frequency of 200Hz, which is 86.65% lower than STA/LTA algorithm. Automatic location error of rock fracture MS events is reduced from 37.33m to 10.89m. CNN-KA was able to warn a potential geological hazard in a coal mine of Anhui Province, China. The in-situ mine pressure data validated the validity of CNN-KA. The proposed workflow greatly improves accuracy of P-phases and identification of rock fracturing events in geo-engineering. The computed results can be used further for calculating precise parameters of MS sources and early warning of engineering geohazards.

The hydraulic fracture (HF) morphology and corresponding stimulated reservoir volume (SRV) are significantly dependent on the geomechanical factors of the formation. A better understanding of the hydraulic fracturing mechanism under different reservoir attributes is crucial for fracability evaluation and fracturing treatment optimization. In this work, the geomechanical controls on hydraulic fracturing in a heterogeneous formation and its fracability are investigated using a three-dimensional (3D) fully coupled hydraulic–mechanical–damage (HMD) model. Rock heterogeneity, which causes nonlinear progressive failure behavior, is considered in this model by assuming that the mechanical parameters of elements follow a Weibull distribution. The elastic damage mechanics and Darcy’s law describe the damage process and fluid flow in elements, respectively. The element permeability is dependent on its state, which describes the effect of stress on the seepage field. The HF width is conceptually represented by the aperture of fractures, which depends on the failure mechanism of the damaged element. The coupled equations are solved numerically using the finite element method. The model is verified with experimental results of HF network propagation and multi-fracture interference. Then, a series of numerical simulations were performed to investigate the geomechanical controls of HF geometry and SRV in heterogeneous formations. At last, the optimal conditions for the formation of a complex HF network are further discussed according to the numerical results, based on which an improved fracability index is established. The results show that the numerical model can capture the 3D nature of the HFs and reproduce the HF network propagation and multi-fracture interference process. The complex HFs are more likely to generate in formations with high brittleness, large natural fracture (NF) density, small horizontal stress difference, and small fracture toughness. This study provides a reliable numerical method for hydraulic fracturing simulation and offers some reference for the fracability evaluation and fracturing treatment design in heterogeneous formations.

Tight glutenite reservoirs are widely developed in Bohai Bay Basin, East China. They are mostly huge thick and rely on hydraulic fracturing treatment for commercial exploitation. To investigate the propagation behavior of hydraulic fractures in these glutenite reservoirs, the geological feature of reservoirs in Bohai Bay Basin is studied firstly, including the reservoir vertical distribution feature and the heterogeneous lithology. Then, hydraulic fracturing treatments in block Yan 222 are carried out and the fracturing processes are monitored by the microseismic system. Results show the hydraulic fractures generated in the reservoirs are mostly in X shape. The cause of X-shaped hydraulic fractures in this study is mainly ascribed to (I) the reservoir heterogeneity and (II) the stress shadow effect of two close hydraulic fractures propagating in the same orientation, which is confirmed by the following numerical simulation and related research in detail. This study can provide a reference for the research on the fracturing behavior of the deep thick glutenite reservoirs.

The tight heterogeneous glutenites are typically characterized by highly variable lithology, low/ultra-low permeability, significant heterogeneity, and a less-developed natural fracture system. It is of great significance for economic development to improve hydraulic fracture complexity and stimulated reservoir volume. To better understand the hydraulic fracturing mechanism, a large-scale experimental test on glutenite specimens was conducted and the hydraulic fracture propagation behaviors and focal mechanism were analyzed. A three-dimensional numerical model was developed to reproduce the hydraulic fracture evolution process and investigate the effects of operating procedures on hydraulic fracture geometry and stimulated reservoir volume. A simultaneous variable injection rate and fluid viscosity technology was proposed to increase the hydraulic fracture complexity and stimulated reservoir volume. The results indicate that four fracturing behaviors can be observed, namely, penetration, deflection, termination, and bifurcating, in the laboratory experiment. Tensile events tend to appear during the initiation stage of hydraulic fracture growth, while shear events and compressive events tend to appear during the non-planar propagation stage. The shear and compressive mechanisms dominate with an increase in the hydraulic fracture complexity. The variable injection rate technology and simultaneous variable injection rate and fluid viscosity technology are effective techniques for fracture geometry control and stimulated reservoir volume enhancement. The key to improve hydraulic fracture complexity is to increase the net pressure in hydraulic fractures, cause evident pressure fluctuations, and activate or communicate a wide range of natural discontinuities. The results can provide a better understanding of the fracture geometry control mechanism in tight heterogeneous glutenites, and offer a guideline for treatment design and optimization of well performance.

The evolution of water flow channels typically includes a micro-fracture precursor, especially during deep mining. However, this precursor has not been well characterized. To get more insight into this precursor and its development, the no. 22517 panel in the Dongjiahe coal mine was studied using microseismic monitoring. The energy density of microseismic events was used to identify the spatial location and formation process of the water flow channels. Microseismic focal parameters, such as seismic energy, seismic potency, apparent stress, seismic moment, apparent volume, energy index, and the Gutenberg-Richter b value were determined for the micro-fracture precursors of the water flow channels. The cumulative apparent volume increased significantly and the Gutenberg-Richter b value decreased rapidly, corresponding to the micro-fracture precursor of the water flow channel. Based on the results, a Gutenberg-Richter b value of 0.7 may be an early warning threshold for the formation of a water-flow channel. Finally, a real-time early warning method for water inrush disasters in floor was established based on microseismic monitoring, geophysics, and the water inrush coefficient.

The evolution characteristics of high-energy and low-energy microfracture events play an important role in the brittle failure mechanism of rock and reasonable microseismic (MS) monitoring and acoustic emission (AE) monitoring. The bimodal distribution (BMD) model is commonly used to observe the evolution characteristics of high-energy and low-energy MS events; however, its precise mechanism remains unclear. The evolution characteristics of high-energy and low-energy microfracture events are assessed in this study based on a BMD model. MS monitoring results from the No. 22517 working face of the Dongjiahe Coal Mine are studied, and AE monitoring results of a biaxial compression experiment of a granite specimen are analyzed. High-energy MS events in the No. 22517 working face are found to be generated by an increase in the failure scale of the overlying rock mass upon exiting the insufficient mining stage and entering the sufficient mining stage. The change characteristics of the high-energy AE hits are positively correlated with crack evolution characteristics in the granite specimen and negatively correlated with changes in the Gutenberg-Richter b value. A precise high-energy and low-energy AE hit evolution mechanism is analyzed based on the microscopic structure of the granite specimen. Similarities and differences between high-energy MS events and low-energy AE hits are determined based on these results. Both are found to have bimodal characteristics; an increase in the failure scale is identified as the root cause of the high-energy component. The bimodal distribution of AE hits is far less obvious than that of MS events.

In this study, a numerical model for long-term deformation and progressive failure of rock slope is presented. The model accounts for both rock heterogeneity and the initiation, activation, nucleation, and coalescence of cracks in rock slope through a stochastic local stress field and local rock degradation by using an exponential softening law. The time-dependent behaviour of rocks is taken as a macroscopic consequence of damage evolution and strength degradation in microstructure. A series of demonstrative slope cases containing preexisting joints are constructed and investigated. The slope instability occurs at a particular point in time when the rock strength is reduced to a certain value. The temporal and spatial evolution of joint linkage structures is numerically obtained, which clearly shows how the local stress field and damage evolution within the joint network contribute to the fracture pattern and the long-term instability. Then, a practical slope case in jointed and layered rock formations in Yunyang city is studied. The prevailing failure phenomena of the slope, including gradual surface scaling, sliding collapses, and block falling, are numerically reproduced, with an emphasis placed on the slope failure process and development tendency. There is a good agreement on the failure mode and instability time between the numerical simulations and the field observations.

The AE energy radiated from rock microfractures is widely used to understand the rock failure process. However, the radiation energy of AE sources is usually inaccurately quantified because of imprecise knowledge of the radiation pattern associated with the tensile angle. Based on theoretical calculations, we quantify the radiation energy of the AE source more accurately by considering the influence of the tensile angle on the radiation pattern and then analyze the radiation energy evolution during the rock failure process. The energy radiation pattern and average energy radiation pattern coefficients of the P and S waves change significantly with the tensile angle. During the failure process of the granitic gneiss specimen, the radiation energy release of the rock specimen is characterized by a sudden intermittent increase in the time domain. The sudden increase is mainly due to the occurrence of large-energy AE sources rather than many low-energy AE sources in a short time. The main microfracture mechanism at the low stress level is shear compression; as the stress increases the main microfracture mechanism changes to shear. When the specimen is at failure, the shear microfractures account for > 70%.

Slurry grouting plays an important role in water plugging for hydrogeological engineering and there are multiple factors that restrict the slurry flow. This study investigates the restriction mechanisms, based on a single inclined rough fracture, employing a space stepwise method (SSM) using multi-direction sectors (MDS) for calculating the diffusion of variable-viscosity slurry under constant flow. The specifically developed numerical algorithm is validated by experimental results. The spatio-temporal effects of major factors on the flow, and the rock hydraulic properties, were analyzed for cement-based slurry. The results show that the slurry flow calculated by MDS-SSM exhibits strong fitting with experimental results. The diffusion pattern no longer exhibits a circular plane, but rather a double semi-elliptical distribution. The fracture direction angle, especially for the upward vertical flow, must be considered in the case of a larger dip angle. The main controlling factor of the pressure field is the higher slurry viscosity in regions far from the injection hole, and the influence of dip angle is significantly greater closer to the hole. Roughness has an influence on the fracture hydraulic properties, and the required grouting pressure increases with an increase in roughness. The fracture aperture, viscosity coefficient, and diffusion radius are three key factors with strong sensitivity affecting the forecasting accuracy of grouting parameters. The role of the fracture dip angle can be neglected only in narrow fractures and long-distance diffusion with higher-viscosity slurry or larger pressure. The key threshold value can be obtained with a fitting formula under given conditions.

The sandstone-mudstone interbedded reservoirs in Bohai Bay Basin, eastern China are typically characterized by high in-situ stress difference, low permeability, and significant heterogeneity. A better understanding of hydraulic fracturing mechanisms and optimization strategies of fracturing treatments is crucial for stimulated reservoir volume (SRV) enhancement and efficient oil recovery. In this study, laboratory tests were conducted on the sandstone and mudstone cores to analyze the mechanical and structural properties. A finite element method (FEM)-based numerical model was developed to simulate the 3D nature of complex hydraulic fracture (HF) propagation. Rock heterogeneity and seepage-stress-damage coupling were both considered to reproduce the fracturing behaviors. A novel approach for SRV simulation was proposed and implemented in the model, which can be used to comprehensively reflect the fracturing performance. A series of numerical simulations were performed to explore the effect of reservoir thickness and treatment parameters on hydraulic fracturing performance. Furthermore, a simultaneous variable injection rate and alternate fluid injection technology were proposed, and the effects of operation procedures on hydraulic fracturing performance were investigated. The results show that HFs tend to present more complex morphology in thick interbedded layers than that in thin interbedded layers under a certain treatment condition. In thin layers, complex HFs can be created when the injection rate is greater than 10 m3/min, while in thick layers, complex HFs can be created when the injection rate is greater than 7 m3/min. With the increase of injection rate, the HF complexity, SRV, and fracture height increase. With the increase of fluid viscosity, the SRV decreases, while the fracture height increases. The simultaneous variable injection rate and alternate fluid injection technology could improve SRV by 19.6%, which appears to be an effective technique to achieve massive fracturing stimulation and fracture height containment for the heterogeneous interbedded reservoirs. The results can provide an insight into the fracturing mechanisms and offer a guideline for fracturing design and treatment optimization in tight sandstone-mudstone interbedded reservoirs.

The stability of the slope is directly related to the safety operation of the open-pit and the life safety of personnel. Effective evaluation of slope stability can reduce the loss caused by slope disasters such as the Fushun West Open-pit Mine. In recent years, some damage has occurred in the south slope of the mine. Therefore, a progressive research method combining the rock mass classification system, slope mass rating method and numerical method is proposed in this study to analyze the possible failure mode and deformation mechanism of the slope. The digital photogrammetry technology, borehole television technology, infrared scanning technology, and on-site monitoring are combined to obtain the relevant data needed for the method. Based on the rock mass characteristics and the survey on-site, the quality of the rock mass of the slope was evaluated through the rock mass classification system and the borehole TV technique. The slope mass classification system with essential adjustment factors considering the discontinuities and excavation methods are a feasible method to estimate rock slope stability. Thereafter the stability of the south slope was estimated by using the slope mass rating method. The results indicate that the stability of the slope is controlled by the weak bedding plane prone to planar failure. Additionally, the deformation mechanism and potential failure dimensions were analyzed through numerical method. Incorporating the empirical result with the numerical result indicates that the south slope is in limit equilibrium state and susceptible to failure along bedding planes. The resulting of the slope stability was validated using the monitoring data that obtained by the GPS devices in situ. According to the deformation characteristics, the corresponding reinforcement methods were established and achieved good result. The progressive methodology provides an effective way to assess the stability and serves for the formulation reinforcement scheme of the rock slope in the complex geological conditions.

Rock brittleness is a crucial mechanical property and essential for fracability evaluation and fracturing scheme design in unconventional reservoirs. However, the influence of inherent anisotropy on deep laminated sandstone’s mechanical properties and brittleness characteristics is rarely investigated. The energy transformation and damage evolution reflected by complete stress-strain curves are analyzed during the entire process of rock rupture under compressions. A new brittleness index is established based on energy evolution during sandstone failure. Its advantages involve comprehensively considering the energy transformation characteristics at both pre-peak and post-peak stages and the capability to characterize the effect of confining pressure and bedding plane (BP) geometry on sandstone brittleness. The triaxial compression tests on sandstones are conducted to validate the reliability and accuracy of the new brittleness index. Numerical simulations are then performed to further investigate the manner in which BP angle, BP density, and confining pressure control the brittleness anisotropy of deep laminated sandstones based on the finite element method. Then the acoustic emission (AE) characteristics of anisotropic sandstone and correlations between AE mode and brittleness index are discussed. The results indicated that the anisotropy of mechanical properties and brittleness of deep laminated sandstones were significantly affected by BP angle, BP density, and confining pressure. With the increase of BP angle, the brittleness index of deep laminated sandstone decreases first and then increases, showing a U-shape variation law, whose maximum and minimum values are obtained at φ =0° and φ =45°, respectively. The AE characteristics were closely related to rock brittleness, which was jointly controlled by BP geometry and confining pressure. The results provide a basis for the brittleness and fracability evaluation and optimum hydraulic fracturing design in deep laminated sandstones.

This paper proposed a coupled hydraulic-mechanical-damage (HMD) geotechnical model for simulating the entire failure process in geological media. In this model, elastic damage mechanics describes the damage process of elements. Darcy's Law and unsteady seepage flow equation describe fluid flow in elements, where compression of fluid and rock are considered. The coupled process is fulfilled by Biot's consolidation theory describing how pore-pressure affects the stress field and the stress/aperture/mesh-dependent permeability that represents the influence of stress on seepage field. The HMD model is implemented in the calculation module of Rock-Failure-Process-Analysis on Petroleum-problems (RFPA-Petrol) codes. The RFPA suite of software codes is based on the idea that the heterogeneity leads to non-linearity and causes progressive failure behavior observed in brittle rock. The capability of RFPA-Petrol simulator is demonstrated by a typical coupled problem related to double fractures propagation during hydraulic fracturing. This simulation successfully reproduces the asynchronous initiation, asymmetric growth and non-equal deflection of fractures in the numerical model. Then this simulator was used to investigate how fracture spacing and horizontal stress ratio influence propagation and reorientation of three hydraulic fractures from a horizontal well. At last, a horizontal well drilled by SHENGLI oilfield is simulated to certificate RFPA-Petrol's ability on field-scale models. It is found that larger fracture spacing and higher initial fracture height will form longer and wider fractured zones. HMD-based RFPA-Petrol is suitable for simulation of laboratory scale and field scale fracture propagation problems.

Hydraulic fracturing performance, affected by multiple factors, was essential to the economic exploitation of oil and gas in heterogeneous unconventional reservoirs. Multifactor analysis can gain insight into the fracturing response of reservoirs and in turn optimize the treatment design. Based on characterizations of the geological setting of a heterogeneous glutenite reservoir, the hydraulic fracture (HF) initiation and propagation process, as well as the stimulated reservoir volume (SRV), were simulated and analyzed using a coupled hydraulic-mechanical-damage model. The Weibull distribution was employed to describe rock heterogeneity. The numerical model was verified with microseism (MS) interpretation results of HF geometry. A multifactor analysis and optimization workflow integrating response surface methodology, central composite design (CCD), and numerical simulations was proposed to investigate the coupling effects of multiple geomechanical and hydrofracturing factors on SRV and identify the optimum design of fracturing treatment. The results showed that the horizontal stress difference and injection rate were the most significant factors to control the SRV. Increasing the injection rate and reducing fluid viscosity may contribute to improving the SRV. It is more difficult to increase the SRV at higher horizontal stress difference than at lower horizontal stress difference. The multifactor analysis and optimization workflow introduced in this work was a practical and effective method to control the HF geometry and improve the SRV. This study provided a deep understanding of the hydraulic fracturing mechanism and possessed theoretical significance for treatment design.

In order to investigate the hydraulic fracture development of the rock and simulate the cracks driven by fluid flow in rocks, a statistical meso-damage mechanical method is developed, based on the statistical and continuum flow-stress-damage model and the finite element method. The constitutive law of this model considers strength and stiffness degradation, stress-dependent permeability for the pre-peak stage, and deformation dependent permeability for the post-peak stage. The damage evolution and accumulation of mesoscopic representative volume element is used to reflect the macroscopic failure characteristics of rock. Three coupled hydro-mechanical processes are considered: (1) mechanical deformation of the solid medium induced by the fluid pressure acting on the fracture surfaces and the rock skeleton, (2) fluid flow within the fracture, and (3) propagation of the fracture. The global flow, stress and strain fields are solved by the finite element method. An element represents a representative volume element, the initiation and propagation of meso-macroscopic trans-scale cracks and their interaction are manifested by removing the failed elements. The above ideas are formulated in the framework of the coupled fracture modelling, which is encoded and implemented into a self-developed software Rock Failure Process Analysis code (RFPA).

Multi-cluster hydraulic fracturing of long-range horizontal wells is an approach for enhancing the productivity of low-permeability shale reservoirs. In this study, RFPA-Petrol (rock failure process analysis on petroleum problems) is applied for modeling hydraulic fracture propagation in multilayered formations. RFPA-Petrol based on coupled hydraulic-mechanical-damage (HMD) modeling was first tested by modeling a laboratory scale experiment on a physical (cement) model with a single completion. The modeling demonstrated the capability of RFPA-Petrol for simulating hydraulic fracture propagation. Then, we used RFPA-Petrol to investigate how the difference in material properties between oil-bearing layers and interlayers and the fracturing fluid properties influence the propagation of dual fractures in multilayered laboratory-scale models. In this case, the models with geological discontinuities in the vertical direction are strongly heterogeneous and RFPA-Petrol simulations successfully modeled the fracture configurations.

Hydraulic fracturing is an effective stimulation technology for enhancing recovery in deep reservoirs. Multi-factor analysis and fracturing design optimization are essential for the efficient development of deep naturally fractured sandstones. A three-dimensional flow-stress-damage (FSD) coupled model was presented to simulate the hydraulic fracture (HF) propagation and stimulated reservoir volume (SRV). The numerical model was validated with experimental results of the HF-natural fracture (NF) intersection. The sensitivity analysis is conducted to screen the significant factors affecting HF geometry and SRV. The response surface method was employed to investigate the coupling effects of multiple geomechanical and hydraulic factors on SRV by integrating Box-Behnken design and numerical modeling. Subsequently, the SRV was optimized by identifying the optimum combinations of uncertain parameters based on the established response surface model (RSM). The results indicated that the injection rate, NF density, fluid viscosity, and horizontal stress difference are the key factors controlling SRV. It is more difficult to improve SRV by increasing injection rate at higher horizontal stress difference than at lower horizontal stress difference. The proposed method is effective for enhancing the artificial ability to optimize the HF geometry and SRV. The results can provide insight into the fracture geometry control mechanism in deep naturally fractured sandstones, and offer a guideline for treatment design and optimization of well performance.

The south slope of the Fushun west open-pit mine in China is geologically complex and is a high-steep rock slope. In recent years, a larger deformation has appeared in the south slope, with the gradual formation of a potential landsliding block that is estimated to be over 300 m3. The east and south boundaries of the block have been identified using different monitoring techniques. However the west boundary has not yet been clearly delineated, a situation which undermines safety assessments and the slope management of the mine. The focus of this was on how the microseismic technology, DInSAR technique and the numerical method are used to determine the shape and formation of the west boundary of the potential sliding block. We found that the upper part of west boundary as determined by numerical simulation is concordant with that determined by microseismic monitoring. The surface deformation area coincides with the DInSAR monitoring results and damage area based on in situ observations. Numerical simulation provides supplementary information on the distribution of stress and the formation process of the west boundary that cannot be observed directly on-site. The microseismic monitoring system was able to capture the progressive rupture of the west boundary during the deformation of the block. The cooperative monitoring strategy together with the numerical simulation can delineate the sliding surface and provide a method for a comprehensive assessment of the slope stability of other similar open-pit mines.

Hydraulic fracturing is an extensively used technique for development of oil and gas resources. In the shale reservoir, rock brittleness plays an important role during hydraulic fracturing. In this paper, a numerical code known as RFPA (Rock Failure Process Analysis) is introduced and the embedded digital-image-based (DIB) technique is illustrated in detail. Based on this integration, the effects of rock brittleness on the failure mode and stress-strain characteristic of the shale specimens are numerically investigated. It is found that the brittle shale specimen is more likely to rupture with multi crossed failure planes while the ductile specimen is more likely to rupture with a penetrating failure plane, from which we deduce the brittle shale is easier to develop more natural fractures than the ductile shale. The influence of natural fractures on complex hydraulic fracture network is further investigated through numerical simulation and the positive effect of rock brittleness is indirectly verified. It is found that hydraulic fractures are preferable to propagate in brittle minerals, i.e. the hydraulic fractures always choose the brittle minerals as the favorite path to propagate or choose a thin or weak part of ductile minerals to penetrate and is blocked by the ductile minerals. Moreover, the hydraulic fractures generated in the brittle shale are tortuous and appear with multi branches, which is much beneficial to form hydraulic fracture network in contrast to the smooth hydraulic fracture generated in the ductile shale. This is probably one of the causes of that the required treatment pressure in ductile shale layer is higher than that in brittle shale layer.

Cement as the main grouting material is usually used in the plugging and reinforcement of tunnels and rock slopes. The flow and diffusion mechanism of grout in fractures plays a key role in guiding the design. Based on the geological fractures and Navier-Stoke equations, the coupling model of cement flow considering the mutual interaction between fractures is established. The fracture-fracture perturbation factor (FPF) and perturbation intensity factor (PIF) are defined. Considering the coupling effect under the diversion, the disturbance in the cement flow and the grouting parameters are discussed with a case of XW coal mine as background. The results show that, in cement diffusion, the existing fractures can weaken the flow capacity of cement in another fracture, resulting in a power function decline of pressure drop, velocity and its related FPF with coefficient of

Collapse columns are geological features formed by chimney caving of karst voids. Collapse columns can cut through the rock mass and introduce vast geological complexity in coalfields. When a collapse column becomes accessible to a confined aquifer, water inrush disasters can be triggered. In northern China, inrushes related to collapse columns occur in regions where extensive Ordovician limestone underlies the coal measures. This limestone is strongly karstified and holds large quantities of water. The piezometric surface can be considerably higher than the coal extraction level. Numerical simulation was used to explore how the water inrush pathway forms, based on a demonstration model. First, the evolution of the stress field, damage field, and flow volume within the coal floor was analysed. Then factors such as the column height, mechanical strength of the coal floor, and hydraulic pressure in the aquifer were considered. In the demonstration models, if the height of the collapse column exceeded 35 m and the initial thickness of the aquiclude was less than 15 m, an inrush definitely occurred, even when the strength of the floor strata was relatively high. Finally, a practical case of water inrush in the Fangezhuang coal mine related to a collapse column was numerically investigated. Modelling of the fracturing process provided insight into evolution of the fracture zone and inrush processes that cannot be observed in the field and are difficult to evaluate using static stress analysis.

Natural fractures play a key role in shale gas production. In this study, a digital-image-processing (DIP) technique is employed to characterize the digital images of shale samples with complex natural fractures. Incorporated the DIP technique with FEM code, the propagation behavior of hydraulic fracture in fractured shale reservoirs is numerically investigated. Three digital images of in-situ shale samples are applied for numerical analysis. Numerical results reproduce the propagation of hydraulic fractures. The hydraulic fracturing process can be divided into three stages: (I)stress accumulation stage, (II)fracture steady propagation stage and (III) fracture unsteady propagation stage. The hydraulic fracture morphology is closely dependent on the natural fracture characteristics. More complex hydraulic fracture can be produced in reservoirs with denser natural fractures. Further numerical models are subsequently established to investigate the impact of the in-situ stress anisotropy and rock brittleness on the hydraulic fracture behavior in shale. Reservoirs with lower in-situ stress anisotropy tend to produce more complex hydraulic fractures. Brittle minerals in the shale are more inclined to induce extensive fracture failure and require lower propagation pressure compared to the ductile mineral. The result can provide a reference to the research on the fracture propagation behavior of the shale reservoir during fracturing stimulation.

The right bank slope of Dagangshan hydropower station in China has complex geological conditions and is subjected to high in situ stress. Notably, microseismic activities in the right bank slope occurred during reservoir impounding. This paper describes the microseismic monitoring technology, and three-dimensional (3D) finite element analysis is used to explore the microseismic activities and damage mechanisms in the right bank slope during reservoir impounding. Based on data obtained from microseismic monitoring, a progressive microseismic damage model is proposed and implemented for 3D finite element analysis. The safety factor for the right bank slope after reservoir impoundment obtained from the 3D finite element analysis, which included the effects of progressive microseismic damage, was 1.10, indicating that the slope is stable. The microseismic monitoring system is able to capture the slope disturbance during reservoir impounding in real time and is a powerful tool for qualitatively assessing changes in slope stability over time. The proposed progressive microseismic damage model adequately simulates the changes in the slope during the impoundment process and provides a valuable tool for evaluating slope stability.

Tight glutenite reservoirs are typically characterized by highly variable lithology and permeability, low and complex porosity, and strong heterogeneity. Glutenite brittleness is an essential indicator for screening fracture targets, selecting technological parameters, and predicting the hydraulic fracturing effect of tight glutenite reservoir exploitation. Glutenite formations with high brittleness are more likely to be effectively fractured and form complex fractures. Accurate evaluation of glutenite brittleness facilitates the recovery of oil and gas in a tight glutenite reservoir. Accordingly, two brittleness indexes are proposed in this paper based on energy balance and damage evolution analysis of complete stress&ndash, strain curves to evaluate the brittleness of glutenite. Uniaxial and triaxial compression tests of glutenite specimens were carried out and the brittleness indexes were verified by comparison with other existing indexes. The relationships between the mechanical properties and brittleness of glutenite under confining pressure were analyzed based on experimental results and the effects of mechanical and structural parameters on glutenite brittleness are investigated with a numerical approach. The brittleness of glutenite increases with the increase of gravel size and/or volume content. During hydraulic fracturing design, attention should be paid to the brittleness of the matrix and the size and content of gravel. This paper provides a new perspective for glutenite brittleness evaluation from the perspectives of energy dissipation and damage evolution. Our results provide guidance for fracturing layer selection and may also facilitate field operations of tight glutenite fracturing.

The no-pillar mining method is widely used in coal mining engineering because of its superiority in resolving mine pressure hazards and protecting natural resources. In view of the geological conditions of stable strata in a coal mine of the Shandong Coal Zone, a novel segmented roadside plugging-filling mining method is proposed by introducing the filling coefficient into roadside filling. The operation process is designed with a new grouting filling forming device. Based on the relationship between the theoretical deflection of a cantilever beam and geometric settlement and parallel settlement models, strength formulas applicable to the segmented roadside plugging-filling (SRPF) method at different migration stages are obtained, and the deformation formulas of a roadway toward a rock slab are solved by an elastic equivalent model. Further, the determination procedure of the filling mode under the mining method is clarified. The SRPF method was implemented on a test stope, and the test results indicated that the theoretical deformation produced by the SRPF method was small and can meet the mining requirements. Through on-site test monitoring, the deformation of surrounding rock was 0–160 mm and the filling body under the SRPF method could maintain its own strength and the stability of the surrounding rock. In addition, entire successful mining been completed in the working stope, which further proves the applicability of this method. The backfilling cost of the gob-side filling was reduced by approximately 50%, and the backfilling efficiency was improved in the mine. The description of the novel mining method may provide theoretical and practical guidance for coal mining in similar geological conditions.

The time-varying and creep failure of rocks affect the long-term stability of support structures in deep rheological tunnels. Structure instability is the macroscopic result of damage evolution and strength degradation in microstructures. Hence, the calculation model of long-term deformation and failure of soft rock tunnels is established. The model explains the heterogeneity of rocks by a random local stress field, and explains the local degradation of rocks by an exponential softening law. The effect of stress field distribution on failure mode and long-term instability is revealed. The deformation characteristics of different structure sections are analyzed, and the damage evolution process, failure, and instability properties are further clarified in rock tunnels. The results indicate that the coefficient of lateral pressure determines the long-term creep and damage properties of the surrounding rock, and the rocks show a strong time effect from a large to small deformation rate. The stability of curve shapes is better than that of straight ones in deep tunnels. The shoulder and foot of the structure are always damaged firstly, and continue to evolve in an arc until fracture. A mechanical mechanism is applied to the design of a stable structure, and the control principle of soft rocks is proposed: stress–deformation applicability and space–time applicability. The failure mechanism, control principle, and stability structure “yielding bolt + W steel strip + shotcrete” are demonstrated with the soft rock tunnels of cross mining in Baijiao Coal Mine. Finally, results of this study promote the utilization of soft rock tunnels and provide a new control structure for engineering stability in deep-cross mining.

For high-steep slopes in hydropower engineering, damage can be induced or accumulated due to a series of human or natural activities, including excavation, dam construction, earthquake, rainstorm, rapid rise or drop of water level in the service lifetime of slopes. According to the concept that the progressive damage (microseismicity) of rock slope is the essence of the precursor of slope instability, a microseismic monitoring system for high-steep rock slopes is established. Positioning accuracy of the monitoring system is tested by fixed-position blasting method. Based on waveform and cluster analyses of microseismic events recorded during test, the tempo-spatial distribution of microseismic events is analyzed. The deformation zone in the deep rock masses induced by the microseismic events is preliminarily delimited. Based on the physical information measured by in situ microseismic monitoring, an evaluation method for the dynamic stability of rock slopes is proposed and preliminarily implemented by combining microseismic monitoring and numerical modeling. Based on the rock mass damage model obtained by back analysis of microseismic information, the rock mass elements within the microseismic damage zone are automatically searched by finite element program. Then the stiffness and strength reductions are performed on these damaged elements accordingly. Attempts are made to establish the correlation between microseismic event, strength deterioration and slope dynamic instability, so as to quantitatively evaluate the dynamic stability of slope. The case studies about two practical slopes indicate that the proposed method can reflect the factor of safety of rock slope more objectively. Numerical analysis can help to understand the characteristics and modes of the monitored microseismic events in rock slopes. Microseismic monitoring data and simulation results can be used to mutually modify the sensitive rock parameters and calibrate the model. Combination of microseismic monitoring and numerical simulation provides a more objective basis for the numerical model and parameters and a solid mechanical foundation for the microseismic monitoring.

Using a true triaxial compression test apparatus, physical model tests are carried out to investigate the fracture propagation in open-hole horizontal wells under hydraulic pressure. The test results indicate that hydraulic fracture propagation is closely related to the direction of the minimum principal stress. When the difference between the maximum and minimum horizontal stresses is maintained at 10.0 MPa and the angle between the horizontal well and the minimum horizontal stress varies from 0° to 90°, the fracturing pattern changes from transversal to longitudinal fractures. To capture the complex hydraulic fractures in heterogeneous rock materials, a three-dimensional numerical code incorporating the seepage-stress-damage coupled model is then employed to study the hydraulic fracture propagation near open-hole horizontal wells. The fracture propagation in horizontal wells under hydraulic fracturing is simulated and the numerical results are in good agreement with the experimental results. The stress ...

The diffusion of pore fluid pressures may create both spatial and temporal effective stress gradients that influence or control the development and evolution of fractures within rock masses. To better understand the controls on fracturing behavior, numerical simulations are performed using a progressive fracture modeling approach that shares many of the same natural kinematic features in rocks, such as fracture growth, nucleation, and termination. First, the pinch-off breaking test is numerically performed to investigate the tensile failure of a rock specimen in a uniform pore pressure field. In this numerical simulation, both mechanical and hydrological properties of a suite of rocks are measured under simulated laboratory conditions. The complete tensional failure process of the rock specimen under pore pressure was reproduced. Second, a double-notched specimen is numerically extended to investigate how the water flow direction or pore pressure gradient influences the fracture growth. An exhaustive sensitivity study is conducted that examines the effects of varying both hydrological and mechanical boundary conditions. The simulation results indicate that local fluid pressure gradients strongly influence the state of stress in the solids and, thereby, fracture growth. Fracture and strength behavior is influenced not only by the pore pressure magnitude on a local scale around the fracture tip, but also by the orientation and distribution of pore pressure gradients on a global scale. Increasing the fracture growth rate increases the local model permeability and decreases the sample strength. The results of this study may provide useful information concerning the degree of hydrological and mechanical coupling action under geologic conditions.

Natural layered rocks subjected to layer-parallel extension typically develop an array of opening-mode fractures with a remarkably regular spacing. This spacing often scales with layer thickness, and it decreases as extension increases until fracture saturation is reached. To increase the understanding of how these opening-mode fractures form in layered rocks, a series of 2D numerical simulations are performed to investigate the infilling process of fractures subjected to different driving forces. Numerical results illustrate that any one of the following could be considered as a driving force behind the propagation of infilling fractures: thermal effect, internal fluid pressure, direct extension loading, or pure compressive loading. Fracture spacing initially decreases with loading process, and at a certain ratio of fracture spacing to layer thickness, no new fractures nucleate (saturated). Both an increase in the opening of the infilled fractures and interface delamination are observed as mechanisms that accommodate additional strain. Interface debonding stops the transition of stress from the neighboring layers to the embedded central layer, which may preclude further infilling of new fractures. Whatever the driving force is, a large overburden stress and a large elastic contrast between the stiff and soft layers (referred to as a central or fractured layer and the top and bottom layers) are key factors favoring the development of tensile stress around the infilled fractures in the models. Fracture spacing is expected to decrease with increasing overburden stress. Numerical results highlight the fracturing process developed in heterogeneous and layered sedimentary rocks which provides supplementary information on the stress distribution and failure-induced stress redistribution., It also shows, in detail, the propagation of the fracture zone and the interaction of the fractures, which are impossible to observe in field and are difficult to consider with static stress analysis approaches.