Experimental Study on Crack Extension Rules of Hydraulic Fracturing Based on Simulated Coal Seam Roof and Floor

Hydraulic fracturing can increase the fracture of coal seams, improve the permeability in the coal seam, and reduce the risk of coal and gas outburst. Most of the existing experimental specimens are homogeneous, and the influence of the roof and floor on hydraulic fracture expansion is not considered. Therefore, the hydraulic fracturing test of the simulated combination of the coal seam and the roof and floor under different stress conditions was carried out using the self-developed true triaxial coal mine dynamic disaster large-scale simulation test rig. The results show that (1) under the condition of triaxial unequal pressure, the hydraulic fractures are vertical in the coal seam, and the extension direction of hydraulic fractures in the coal seam will be deflected, with the increase of the ratio of the horizontal maximum principal stress to the horizontal minimum principal stress. The angle between the extension direction of the hydraulic fracture and the horizontal maximum principal stress decreases. (2) Under the condition of triaxial equal confining pressure, the extension of hydraulic fractures in the coal seam are random, and the hydraulic fracture will expand along the dominant fracture surface and form a unilateral expansion fracture when a crack is formed. (3) When the pressure in one direction is unloaded under the condition of the triaxial unequal pressure, the hydraulic fractures in the coal seam will reorientate, and the cracks will expand in the direction of the decreased confining pressure, forming almost mutually perpendicular turning cracks.

Novel Four-Cell Lenticular Honeycomb Deployable Boom with Enhanced Stiffness

Composite thin-walled booms can easily be folded and self-deployed by releasing stored strain energy. Thus, such booms can be used to deploy antennas, solar sails, and optical telescopes. In the present work, a new four-cell lenticular honeycomb deployable (FLHD) boom is proposed, and the relevant parameters are optimized. Coiling dynamics analysis of the FLHD boom under a pure bending load is performed using nonlinear explicit dynamics analysis, and the coiling simulation is divided into three consecutive steps, namely, the flattening step, the holding step, and the hub coiling step. An optimal design method for the coiling of the FLHD boom is developed based on a back propagation neural network (BPNN). A full factorial design of the experimental method is applied to create 36 sample points, and surrogate models of the coiling peak moment (Mpeak) and maximum principal stress (Smax) are established using the BPNN. Fatigue cracks caused by stress concentration are avoided by setting Smax to a specific constraint and the wrapping Mpeak and mass of the FLHD boom as objectives. Non-dominated sorting genetic algorithm-II is used for optimization via ISIGHT software.

A Fast Calculation Method for Natural Fractures Activation Considering Stress Shadow and Study on the Law of Natural Fractures Activation State Changes

During hydraulic fracturing process of the Permian Basin in North America, the cluster spacing has been shortened to 3m, and stress shadow can no longer be ignored. Many scholars have studied the influence of stress shadows to optimize cluster spacing. For reservoirs with natural fractures, how to activate more natural fractures through hydraulic fracturing has become the purpose. However, few scholars have studied changes in the activation law of natural fractures under stress shadow conditions. This paper establishes stress change value around single fracture according to Sneddon formula, and calculates the maximum and minimum principal stress according to plane principal stress calculation formula. Considering attenuation of net pressure, stress field of multiple fractures is established, and influence of various factors on stress re-orientation is studied. Finally, considering attenuation of net pressure with distance, according to discriminant formulas of tension & shear activation, the proportion of natural fractures that are easily activated is calculated. By designing orthogonal experiments, the influence of different factors on the proportion of activated natural fractures was studied. The stress increase in the direction of the minimum principal stress is much greater than the increase in the direction of the maximum principal stress. The stress increases in the direction of the maximum principal stress at the tip of the hydraulic fracture. The tip position between hydraulic fractures is "neutralized" due to the superposition of shear stress. Stress-fracture angle and the in-situ stress difference are the common main influencing factors for both tensile and shear activation, but the net pressure has little effect on the tensile activation of natural fracture. The fracture spacing has little effect on the activation of natural fractures. When formulating the fracturing scheme, we should pay more attention to the net pressure rather than the fracture spacing. This article provides a fast calculation method for the activation state of natural fractures considering the stress shadow, which provides a reference index for activating more natural fractures and increasing the production of a single well.

Research on In Situ Stress Distribution of the Railway Tunnels in Southwest China Based on the Complete Temperature Compensation Technology

In situ stress is the natural stress existing in the stratum without engineering disturbance, also known as initial stress, absolute stress, or original rock stress. In order to master the in situ stress of the Manmushu Tunnel and Mamo Tunnel in Southwest China, the casing stress solution method is adopted in this paper. Through the combination of field measurement and laboratory test, the basic data such as initial strain during stress relief are collected, and the in situ stress values are analyzed in combination with indoor temperature compensation test, confining pressure calibration, and relevant rock mechanics tests. The measured results show the following: (1) the maximum horizontal principal stress σh.max ranges from 6.44 MPa to 19.74 MPa; the vertical principal stress σ v ranges from 4.11 MPa to 13.48 MPa; and the minimum horizontal principal stress ranges from 4.32 MPa to 11.22 MPa. (2) The maximum horizontal principal stress directions of the five measuring points are all located in the NW direction, which is basically consistent with the maximum principal stress direction of the regional tectonic stress field. The maximum horizontal principal stress (σh.max), the minimum horizontal principal stress (σh.min), and the vertical principal stress ( σ v ) all increase with the increase of buried depth, and the relationship is approximately linear. It is suggested that, in the actual construction process, the construction method and construction parameters should be optimized scientifically and reasonably to reduce the disturbance of blasting on the tunnel surrounding rock. After tunnel excavation, support measures should be taken quickly, timely, and scientifically to reduce and control the deformation of the surrounding rock.

Dynamic Evolution Law of Overburden Rock in Shallow-Buried Super-High Fully Mechanized Working Face and Determination of Support Strength

The objective of this study was to reveal the law of overburden movement and stress evolution during the mining of super-high fully mechanized mining faces. Based on the 12401 fully mechanized mining face of Shangwan Coal Mine in Shendong, this study conducted research and analysis using the methods of similarity simulation experiment, numerical simulation, and field measurement. The results showed that the maximum and minimum principal stresses in the coal seam in front of the working face are concentrated with the advance of the working face. The degree of stress concentration increases with the increase in the advancing range, and the concentration degree of the maximum principal stress and the change gradient is greater than that of the minimum principal stress. But the range of the peak lead coal wall is lower than that of the minimum principal stress of the peak lead coal wall. The phenomenon of stress recovery exists in the goaf. With the increase in the advancing range of the working face, the degree of stress recovery gradually increases, and the degree of maximum principal stress recovery is higher than that of the minimum principal stress recovery. The large fractures observed near the working face are closely related to the underground pressure, relatively large fractures appear on the surface, and the fractures become narrower near the two pathways. Only caving and fissure zones exist in the thin bedrock overburden, and the bending subsidence zone changes with the bedrock thickness. The support strength of the hydraulic support should not be less than 1.47 MPa. This research on the overburden movement and stress evolution law of a super-high fully mechanized mining face can provide theoretical guidance for the exploitation and utilization of extrathick coal seam resources. It has broad engineering prospects.

Study on the Rock Fracture during Fluid Injection Using a Coupled Flow-Stress-Damage (FSD) Model: Insight into the Stress Shadow Effect

In order to investigate the influence of pore pressure on hydraulic fracturing behavior in the local and whole model, the coupled flow-stress-damage (FSD) analysis system RFPA-Flow was used to study the influence of rock heterogeneity, natural stress ratio, double-hole spacing, and water pressure gradient on the stress shadow effect. The numerical results show that the tensile crack induced by pore water pressure is significantly affected by the pore water pressure and water pressure gradient. The larger the pore pressure gradient is, the more asymmetrical the crack development pattern and the smaller the instability pressure of the model. In addition, the shape of hydraulic fracture becomes much more irregular with the increase in rock heterogeneity. The number and shape of tip microcracks under the influence of local water pressure are closely related to the homogeneity of rock. Moreover, when the natural stress difference is large, the hydraulic fracture propagates parallel to the maximum principal stress; when the stress field is close and the spacing between two holes is less than 5 times the diameter, the propagation direction of hydraulic fractures between holes is perpendicular to the maximum principal stress. It is found that no hydraulic fractures occur between the two holes when the distance between holes is greater than 5 times the diameter.

Factors influencing gas well productivity in fractured tight sandstone reservoir

There are many factors which influence the absolute open flow potential (AOFP) of gas well. One of them is the angle between maximum principal stress direction and natural fracture strike in gas reservoir. In order to find out how the angle influences the AOFP of gas well. A lot of data related to gas well productivity of 14 wells located in gas reservoir T were collected and collated. Influential intensity of each factor on the AOFP before and after reservoir modification was investigated through grey relation analysis method. Results indicated that the AOFP of gas well before and after reservoir modification was governed by 10 factors. The five central factors influencing the initial AOFP are natural fracture density, porosity, permeability, elevation of geological top surface, and gas saturation, respectively. The five central factors influencing the AOFP of hydraulically fractured gas well are porosity, gas saturation, elevation of geological top surface, minimum principal stress, and permeability, respectively. Angle between maximum principal stress direction and natural fracture strike was not the central factor influencing gas well productivity. Reservoir modification can greatly improve gas well productivity in fractured tight sandstone reservoir. Natural fracture density was the strongest influencing factor of the initial AOFP. Minimum principal stress was one of the central factors influencing the AOFP of hydraulically fractured gas well. Research results can be used to guide well deployment and gas productivity investment projects of fractured tight sandstone reservoir.

Numerical Simulation of the Stress Field in Repeated Mining of Coal Seams Based on In Situ Stress Measurement

We established an evaluation index of the rock mass stress state for underground coal mines using the strength-stress ratio based on the measured in situ stress and the generalized Hoek–Brown strength criterion. Three in situ stress states, σcm/σ1m < 1.4 (high), 1.4 < σcm/σ1m < 3.6 (medium), and σcm/σ1m > 3.6 (low), were established based on the value of the unconfined compressive strength (σcm) and the maximum principal stress of the rock mass (σ1m). This index classifies the Burtai mine as a medium-high in situ stress field, which is in agreement with the on-site situation, establishing the reliability of the index. The working face was a three-dimensional geological model based on the log sheets. The initial conditions for the model were determined using the combined measurements of the in situ stress regression model. We performed numerical simulations of the roof stress field distribution under repeated mining. Mining the overlying coal seam leads to significant variation in the value and direction of the main roof, σ1, within the range of the front abutment pressure under the pillar and gob. Along the main roof strike direction, σ1 under the pillar is 1.5 times that under the gob, and the σ1 direction under the pillar is deflected by 5°, which is 30° smaller than that under the gob. This provides a reference for optimized underground coal mining.

Design and Analysis of CFETR CS Coil and Pretension Structure

The coil and the pretension rod play important roles in the central solenoidal coil system of Tokamak. Therefore, the analysis of the coil and the pretension rod in the normal operating environment is essential for the facility’s construction. In this paper, the 3D model of the two parts mentioned above is built based on SolidWorks. Subsequently, the simulations are carried out to revert the real operation environments following the fields, loads, and boundary conditions. Then, we compare the official analysis results of the corresponding parts in the central solenoidal system of the China Fusion Engineering Test Reactor (CFETR) based on the results from COMSOL. Compared with the official analysis, the error is about 1.3% for the maximum magnetic field and about 0.53% error for the outer part, respectively. As for the pretension rod, simulations are carried out based on the finite element analysis software ABAQUS. The Von mises and the maximum principal stress are 140.1 MPa and 168.4 MPa, which is much lower than the material’s yield stress (205 MPa). The safety factor is 1.78, larger than the critical failure value 1. According to our results, the rationality of the CFETR central solenoidal coil system design is re-examined and verified.

Exploratory Finite Element Analysis of Monolithic Toughened Glass Panes Subjected to Hard-Body Impact

The paper reports the results of an extensive experimental campaign, in which simply supported toughened glass samples with dimensions of 500 × 360 mm2 and three thicknesses (6, 8 and 10 mm) were subjected to hard-body impact. A steel ball (4.11 kg) was released from different drop heights, starting from 10 cm above the sample and increasing by 10 cm in each step until glass breakage occurred. In this way, for all samples a critical drop height (causing fracture of glass) was determined. Experiments were carried out for 35 samples for each thickness; thus 105 samples were tested in total. A 3D numerical model of the experimental setup was developed using the commercial finite element analysis (FEA) software ABAQUS and Implicit Dynamic solver. The numerical study was aimed at numerical reproduction of the experiments and determination of the maximum principal stress in the glass that occurs during the impact. To reduce the number of FEs and increase the computational efficiency of the simulations, only a quarter of the nominal geometry with appropriate boundary conditions were modelled. The simulations were performed for a given weight of the steel impactor, glass thickness and the corresponding critical/breaking drop height found in the experimental campaign. In this way, an impact strength of the toughened glass was retrospectively evaluated. The simulations were used to investigate the impact history in terms of stress in glass, acceleration and velocity. Moreover, the resulting history of impact force was determined.